For Class use only

ACHARYA N. G. RANGA AGRICULTURAL UNIVERSITY

B. Tech (Food Technology)

Course No.: FDEN 222Credit Hours: 3 (2+1)

REFRIGERATION ENGINEERING

AND

COLD CHAIN

STUDY MATERIAL

Prepared by

Dr. S.KaleemullahCollege of Food Science and Technology

Pulivendula - 516 390

Department of Food Engineering

Course No : FDEN-222

Title : Refrigeration Engineering and Cold Chain

Credit hours : 3 (2+1)

Theory Lecture Outlines

L.No. Lecture outline P.No.

1 Refrigeration and air conditioning- Definitions - Necessity of refrigeration and air

conditioning – Factors affecting comfort air conditioning

4

2 Definition of Refrigerant - History of refrigerants - Classification of Refrigerants –

Primary refrigerants – Secondary Refrigerants – Halo carbon refrigerants

6

3 Azeotrope refrigerants - Inorganic refrigerants - Ammomia - Air - Carbon dioxide

- Sulphur dioxide - Water

11

4 Hydro carbon refrigerant – Designation system for refrigerants – Designation

sytem for Dichloro-tetrafluoro-ethane

15

5 Designation system for Dichloro-difluoro-methane - Selection of a refrigerant 17

6 Thermodynamic properties of refrigerants 19

7 Chemical requirements of refrigerants – Physical properties of refrigerants 21

8 Secondary refrigerants – Brines- Applications of various brines 25

9 Types of Refrigerators – Air Refrigerator – Vapour refrigerator – Advantages and

Disadvantage of vapour compression refrigeration system over air refrigeration

system

27

10 Mechanism of a simple vapour compression refrigeration systems 29

11 Evaporators – Capacity of an evaporator – Factors affecting heat transfer

capacity of an evaporator – Types of evaporator

31

12 Bare tube coil evaporator – Finned evaporator – Plate evaporator – Shell and

tube evaporator

34

13 Shell and coil evaporator – Tube in tube – Flooded evaporator – Dry expansion

evaporator

38

14 Natural convection evaporator – Forced convection evaporator – Frosting

evaporator – Non Frosting evaporators – Defrosting evaporators

42

15 Compressor – Classification – Suction pressure, Discharge pressure,

Compression ratio, Suction volume, Stroke volume, Clearance factor,

Compressor capacity, Volumetric efficiency

45

16 Reciprocating compressor - Parts of a reciprocating compressor - Cycle of a

reciprocating compressor

48

17 Rotary compressor – Centrifugal compressor – Advantages and disadvantages

of a centrifugal compressor

52

18 Condensers – Working of a condenser – Factors affecting the condenser

capacity – Heat rejection factor

57

19 Classification of condensers –Air cooled condensers – Water cooled condensers

– Tube in condenser – Shell and coil condenser

61

20 Fouling factor – Difference between air cooled and water cooled condensers –

Evaporative condenser

66

21 Expansion Devices – Types of expansion devices – Capillary tube 69

22 Hand operated expansion valve – Low side float valve – High side float valve 71

23 Ice manufacturing – principle of ice production 74

24 Application of refrigeration in different food products 78

25 Examples of Food processing by refrigeration and storage –Fruits and

vegetables

79

26 Examples of Food processing by refrigeration and storage - Meat products – fish

– poultry products – dairy products

80

27 Food Freezing – Freezing systems – Indirect contact systems 83

28 Plate Freezers – air blast Freezers – Freezers for liquid foods 85

29 Direct contact systems – Air blast - Immersion 89

30 Frozen food properties – Density – Thermal conductivity – Enthalpy – Apparent

specific heat – Apparent thermal diffusivity

91

31 Freezing time – Factors influencing freezing time – Freezing rate – Thawing time 96

32 Quality Changes in Foods during Frozen Storage 99

3

Lecture No. 1

Introduction:

Refrigeration is the process of removing heat from an enclosed space, or from a

substance and moving it to a place where it is Unobjectionable. It also includes the

process of reducing and maintaining the temperature of a body below the general

temperature of its surroundings.

For example, if some space (say in cold storage) is to be kept at - 2°C (271K),

we must continuously extract heat which flows into it due to leakage through the walls

and also the heat which is brought into it with the articles stored after the temperature is

once reduced to - 2°C (271K). Thus in a refrigerator, heat is virtually being pumped from

a lower temperature to a higher temperature. According to Second Law of

Thermodynamics, this process can only be performed with the aid of some external

work. It is thus obvious that supply of power (say electric motor) is regularly required to

drive a refrigerator.

Theoretically, a refrigerator is a reversed heat engine or a heat pump which

pumps heat from a cold body and delivers it to a hot body. The substance which works

in a heat pump to extract heat from a cold body and to deliver it to a hot body is called a

“refrigerant”.

Necessity of Refrigeration and Air Conditioning:

Refrigeration system is used for the manufacture of ice and similar products. It is

also widely used for the cooling of storage chambers in which perishable foods, drinks

and medicines are stored. The refrigeration has also wide applications in submarine

ships, aircraft and rockets.

Air conditioning refers to the treatment of air so as to simultaneously control its

temperature, moisture content, cleanliness, odour and circulation, as required by

occupants, a process or products in the space. The subject of refrigeration and air

conditioning has evolved out of human need for food and comfort. It also deals with the

Refrigeration and air conditioning- Definitions - Necessity of refrigeration and air conditioning – Factors affecting comfort air conditioning

4

conditioning of air for industrial purposes, food processing, storage of food and other

materials.

Factors Affecting Comfort Air Conditioning:

1. Temperature of air: In air conditioning, the control of temperature means

maintenance of any desired temperature within an enclosed space even though the

temperature of the outside air is above or below the desired room temperature. This is

accomplished either, addition or removal of heat from the enclosed space as and when

demanded. It may be noted' a human being feels comfortable when the air is at 21°C

with 56% relative humidity.

2. Humidity of air: The control of humidity of air means the decreasing or

increasing moisture contents of air during summer or winter respectively in order to

produce comfortable healthy conditions. The control of humidity is not only necessary for

human comfort but it increases the efficiency of the workers. In general, for summer air

conditioning, the relative humidity should not be less than 60% whereas for winter air

conditioning it should not be than 40%.

3. Purity of air: It is an important factor for the comfort of a human body. It has

noticed that people do not feel comfortable when breathing contaminated air, even if it is

within acceptable temperature and humidity ranges. It is thus obvious that proper

filtration, cleaning, purification of air is essential to keep it free from dust and other

impurities.

4. Motion of air: The motion or circulation of air should be controlled, in order to

keep constant temperature throughout the conditioned space. Hence, there should be

equi-distribution of air throughout the space to be air conditioned.

5

Lecture No. 2

Introduction:

The development of refrigeration and air conditioning industry depends to a large

extent on the development of refrigerants to suit various applications and the

development of various system components. At present the industry is dominated by

the vapour compression refrigeration systems, even though the vapour absorption

systems have also been developed commercially. The success of vapour compression

refrigeration systems owes a lot to the development of suitable refrigerants and

compressors. The theoretical thermodynamic efficiency of a vapour compression system

depends mainly on the operating costs, safety, reliability and serviceability etc. depend

very much on the type of refrigerant and compressor selected for a given application.

The refrigerant is a heat carrying medium which during their cycle (i.e.

Compression, condensation, expansion and evaporation) in the refrigeration system

absorbs heat from a low temperature system and discards the heat so absorbed to a

higher temperature system.

History of Refrigerants:

The first refrigerant used was ether employed by Perkins in hand operated

vapour compression machine.

In the earlier days, ethyl chloride (C2H5Cl) was used as a refrigerant which soon

gave way to ammonia as early as in 1875. At about the same time, sulphur dioxide (SO2)

in 1874, methyl chloride (CH3Cl) in 1878 and carbon dioxide (CO2) in 1881, formed

application as refrigerants.

During 1910–30 many new refrigerants, such as N2O3, CH4, C2H6, C2H4, C3H8

were employed for low temperature refrigeration. Hydrocarbons were, however, found

extremely inflammable, dichloro–methane (CH2Cl2), dichloro–ethylene (C2H2Cl2) and

monobromomethane (CH3Br) were also used as refrigerants for centrifugal machines.

A great break through occurred in the field of refrigeration with the development

of freons by E. I. du pont de Nemours and Co. Freons are a series of fluorinated

Definition of Refrigerant - History of refrigerants - Classification of Refrigerants – Primary refrigerants – Secondary Refrigerants – Halo carbon refrigerants

6

hydrocarbons, generally know as fluorocarbons derived from methane, ethane etc. as

bases. Although Freon is a trade name, it has come to be commonly used for these

compounds. With fluorine, chlorine and sometimes bromine in their molecule, Freons

form a series of refrigerants with wide range of normal boiling points (boiling point or

saturation temperatures at one atmosphere pressure) to satisfy the varied requirements

of different refrigerating machines. The presence of fluorine in the molecule makes the

compound non–toxic and imparts other desirable physical and physiological

characteristics.

Desirable Properties of an Ideal Refrigerant:

There is no ideal refrigerant which can be used for all type of applications i.e.

there is no ideal refrigerant. A refrigerant is said to be ideal if it has all of the following

properties.

1. Low boiling point

2. High critical temperature

3. High latent heat of vaporisation

4. Low specific heat of liquid

5. Low specific volume of vapour

6. Non-corrosive to metal

7. Non-flammable and non-explosive

8. Non-toxic

9. Low cost,

10. Easy to liquify at moderate pressure and temperature

11. Easy of locating leaks by odour or suitable indicator, and

12. Mixes well with oil

The standard comparison of refrigerants, as used in the refrigeration industry is

based on evaporating temperature of - 15°C and a condensing temperature of + 30°C.

Classification of Refrigerants:

The refrigerants may, broadly, be classified into the following two groups.

1. Primary refrigerants

2. Secondary refrigerants.

7

The refrigerants which directly take part in the refrigeration system are called

primary refrigerants whereas the refrigerants which are first cooled by primary

refrigerants and then used for cooling purposes, are known as secondary refrigerants.

The primary refrigerants are further classified into the following four groups.

1. Halo-carbon refrigerants,

2. Azeotrope refrigerants,

3. Inorganic refrigerants,

4. Hydro-carbon refrigerants.

Halo-carbon Refrigerants:

The American Society of Heating, Refrigeration and Air-conditioning Engineers

(ASHRAE) identify 42 halo-carbon compounds as refrigerants, but only a few of them

are commonly used. The following table 1 gives some of the commonly used

halo-carbon refrigerants.

Table. 1. Commonly used halo-carbon refrigernats

Refrigerant Number

Chemical name Chemical formula

R – 11 Trichloromono fluoromethane CCl3FR – 12 Dichlorodifluoromethane CCl2 F2

R – 13 Monochlorotrifluoromethane CCl F3

R – 14 Carbonatetrafluoromethane CF4

R – 21 Dichloromonofluoromethane CHCl2FR – 22 Monochlorodifluoromethane CHClF2

R – 30 Methylene chloride CH2Cl2R – 40 Methyl chloride CH3Cl

R – 100 Ethyl chloride C2H5ClR – 113 Trichlorofluroethane CCl2FCClF2 or C2Cl3F3

R – 114 Dichlorotetrafluoroethane CClF2CClF2 or C2Cl2F4

R - 115 Monochloropentafluoroethane CClF2CF3

The halo-carbon compounds are all synthetically produced and were developed

as Freon family of refrigerants. Freon is a registered trade mark of E. I. Du Pont de

Nemours and Co., America. Most of the halo-carbon refrigerants, are now available from

8

other manufactures under various trade names such as Genetron, Isotron etc. The first

of the halo-carbon refrigerant i.e. R–12 was developed in 1930 by Thomas Midgley.

1. R-11, Trichloromonofluoromethane (CCl3F): The R-11 is a synthetic

chemical product which can be used as a refrigerant. It is stable, non-flammable and

non-toxic. It is considered be a low-pressure refrigerant. It has a low side pressure of

0.202 bar at -15°C and high side pressure of 1.2606 bar at 30°C. The latent heat at

-15°C is 195 kJ/kg. The boiling point at atmospheric pressure is 23.77°C. Due to its low

operating pressures, the refrigerant is exclusively used in large centrifugal compressor

systems of 200 TR and above. The leaks may be detected using a soap solution, a

halide torch or by using an electronic detector.

R-11 is often used by service technician as a flushing agent for cleaning the

internal parts of a refrigerator compressor when overhauling systems. It is useful after a

system had a motor burn out or after it has a great deal of moisture in the system. By

flushing moisture from the system with R-11, evacuation time is shortened. R-11 is one

of the safest cleaning solvent that can be used for this purpose. The cylinder colour code

for R-l1 is orange.

2. R-l2, Dichlorodifluoromethane (CCl2F2): The R-12 is a very popular

refrigerant. It is colourless, almost odourless liquid with boiling point of -29°C at

atmospheric pressure. It is non toxic, non-corrosive, non-irritating and non-flammable. It

has a relatively low latent heat value which is an advantage in small refrigerating

machines. The large amount of refrigerant circulated will permit the use of less sensitive

and more positive operating and regulating mechanisms. The refrigerant is used in many

different types of industrial and commercial applications such as refrigerators, freezers,

water coolers, room and window air conditioning units etc. Its principal use is found in

reciprocating and rotary compressors, but its use in centrifugal compressors for large

commercial air conditioning is increasing.

R-12 has a pressure of 0.82 bar at -15°C and a pressure of 6.4 bar at 30°C. The

latent heat of R-12 at - 15°C is 159 kJ/kg. The leak may be detected by soap solution,

halide torch or an electric leak detector. Water is only slighted soluble in R-12. The

Solution formed is very slightly corrosive to any of the common metals used in

refrigerator construction. The addition of mineral oil to the refrigerant has no effect upon

9

the corrosive action. The refrigerant is available in a variety of cylinder sizes and the

cylinder colour code is white.

6. R-22, Monochlorodifluoromethane (CHClF2): The R-22 is a man-made

refrigerant developed for refrigeration installations that need a low evaporating

temperature, as in fast freezing units which maintain a temperature of - 29°C to - 40°C. It

has also been successfully used in air conditioning units and in household refrigerators.

It is used with reciprocating and centrifugal compressors. It is also used for low

temperatures refrigeration applications, cold storage, food freezing and storage etc.

The boiling point of R-22 is - 41°C at atmospheric pressure. It has a latent heat of

216.5 kJ/kg at -15°C. The refrigerant is stable and is non-toxic, non-corrosive, non-

irritating and non-flammable. The evaporator pressure of this refrigerant at -15°C is

1.92 bar. This refrigerant has good solubility in oil down to -9°C. The leaks may be

detected with an electronic leak detector. The cylinder colour code for R-22 is green.

10

Lecture No. 3

Azeotrope Refrigerants:

The term “Azeotrope” refers to a stable mixture of refrigerants whose vapour and

liquid phases retain identical compositions over a wide range of temperatures. However,

these mixtures, usually, have properties that differ from either of their components.

Table. 2. Azeotrope refrigerants

Refrigerant No. Azeotropic mixing Refrigerants. Chemical formulaR – 500 73.8% R-12 & 26.2% R-152 CCl F2 / CH3CH F2

R – 502 43.8% R-22 & 51.2% R-115 CHClF2 / CClF2CF3

R – 503 40.1 % R-23 & 59.9% R-13 CHF3 / CClF3

R - 504 48.2% R-32 & 51.8% R-115 CH2F2 / CClF2CF3

R-500: It is an Azeotropic mixture of 73.8% R-12 (CCl2F2) and 26.2% of R- 152

(CH3CHF2) it is non flammable, low in toxicity and non-corrosive. It is used in both

commercial applications but only in systems with reciprocating compressors. It has a

fairly constant vapour pressure temperature curve which is different from the vaporizing

curves for either R- 152a or R- 12.

This refrigerant offers about 20% greater refrigerating capacity than R-12 for the

same size of motor when used for the same purpose. It can be used whenever a higher

capacity than that obtained with R-12 is needed. The solubility of water in R-500 is highly

critical. It has fairly high solubility with oil.

The leakage may be detected by using soap solution, a halide torch, an

electronic leak detector or a colour tracing agent. Water is quite soluble in this

refrigerant. It is necessary to keep moisture out of the system by careful dehydration and

by using driers. The cylinder colour code for this refrigerant is yellow.

The evaporator pressure of this refrigerant is 1.37 bar at -15°C and its

condensing pressure is 7.78 bar at 30°C. It has a boiling point of - 33°C at atmospheric

pressure. Its latent heat at -15°C is 192 kJ/kg.

Azeotrope refrigerants - Inorganic refrigerants - Ammomia - Air - Carbon dioxide - Sulphur dioxide - Water

11

R-502: It is an Azeotropic mixture of 48.8% R-22 (CHClF2) and 51.2% of R-115

(CClF2CF3) it is non flammable, non-corrosive, practically non-toxic liquid. It is a good

refrigerant for obtaining medium and low temperatures. It is suitable where temperatures

from -18°C to -51°C are needed. It is often used in frozen food lockers, frozen food

processing plants, frozen food display cases and in storage units for frozen foods and

ice-cream. It is only used with reciprocating compressors.

The boiling point of this refrigerant at atmospheric pressure is -46°C. Its

evaporating pressure at -15°C is 2.48 bar and the condensing pressure at 30°C is

12.06 bar. Its latent heat at - 29°C is 168.6 kJ/kg.

This refrigerant has all the qualities found in other halogenated (fluorocarbon)

refrigerants. It is non-toxic, non-flammable, non- irritating, stable and non-corrosive. The

leaks may be detected by soap solution, halide torch or electronic leak detector the

cylinder colour code for this refrigerant is orchid.

Presently the most commonly used organic refrigerants are the chloro – fluoro

derivatives of CH4 and C2H6. The fully halogenated ones with chlorine in their molecule

are chloro–fluorocarbons, referred to as CFC’s. Those containing H atoms in the

molecule along with Cl and F atoms are referred to as hydro–chloro–fluoro carbons or

HCFC’s. Those having no chlorine atoms in the molecule are hydro–fluorocarbons or

HFC’s. Simple hydrocarbons are HC’s. We have HC’s, HFC’s, HCFC’s and CFC’s.

CFCs and HCFCs were identified as high C.O.P refrigerants, with many

favourable properties e.g. non - flammability, low – toxicity and material compatibility that

led to their widespread use as refrigerants.

The F atom in the molecule of a refrigerant makes the substances physiologically

more favourable. The Cl atom in the molecule is considered responsible for the depletion

of Ozone layer in the upper atmosphere (stratosphere), thus allowing harmful

ultraviolet radiation to penetrate through the atmosphere and reach the earth’s surface.

The H atoms in the molecule imports degree of flammability to the substance depending

upon the number of these atoms.

Inorganic Refrigerants:

12

The inorganic refrigerants were exclusively used before the introduction of

halocarbon, refrigerants. These refrigerants are still in use due to their inherent

thermodynamic and physical properties. The various inorganic refrigerants are given in

the following table 3.

Table. 3. Inorganic refrigerants

Refrigerant No: Chemical name Chemical formulaR – 717 Ammonia NH3

R – 729 Air -R – 744 Carbon dioxide CO2

R – 764 Sulphur dioxide SO2

R - 118 water H2O

1. R-717 (Ammonia): Ammonia (NH3) is one of the oldest and most widely of all

the refrigerants. Its greatest application is found in large and commercial reciprocating

compression systems where high toxicity is secondary. It is also widely used in

absorption systems. It is a chemical compound of nitrogen and hydrogen and under

ordinary conditions, it is a colourless gas. Its boiling point at atmospheric pressure is -

33.3°C and its melting point from the solid is -78°C. The low boiling point makes it

possible to have refrigeration at temperatures considerably below 0°C without using

pressures below atmospheric in the evaporator. Its latent heat of vaporisation at -15°C is

1,315 kJ/kg. Thus, large refrigerating effects are possible with relatively small sized

machinery. The condenser pressure at 30°C is 10.78 bar. The condensers for R-717 are

usually of water cooled type.

Ammonia It is a poisonous gas if inhaled in large quantities. In lesser quantities, it

is irritating to the, nose and throat. This refrigerant is somewhat flammable and when

mixed with air in the ratio of 16% to 25% of gas by volume, will form an explosive

mixture.

The leaks of this refrigerant may quickly and easily detected by the use of

burning sulphur candle which in the presence of ammonia forms white fumes of

ammonium sulphite. This refrigerant attacks copper and bronze in the presence of a little

moisture but does not corrode iron or steel. Since ammonia is lighter than oil, its

separation does not create any problem. The excess oil in the evaporator may be

removed by opening a value in the bottom of the evaporator.

13

This refrigerant is used in large compression machines using reciprocating

compressors and in many absorption type systems. The use of this refrigerant is

extensively found in cold storage, warehouse plants, ice cream manufacture, ice

manufacture, beer manufacture, food freezing plants etc.

2. R-729 (Air): The dry air is used as a gaseous refrigerant in some compression

systems, particularly in air-craft air conditioning.

3. R-744 (Carbon dioxide): The principal refrigeration use of carbon dioxide is

same as that of dry ice. It is non-toxic, non- irritating and non-flammable. The boiling

point of this refrigerant is extremely low (- 73.6°C) that at -15°C, a pressure of well over

20.7 bar is required to prevent its evaporation. At a condenser temperature of +30°C, a

pressure of approximately 70 bar required to liquify the gas. Its critical temperature is

31°C and triple point is - 56.6°C. Due to its high operating pressure, the compressor of a

carbon dioxide refrigerator unit is very small even for a comparatively large refrigerating

capacity. However, because of its low efficiency as, to other common refrigerants, it is

seldom used in household units, but is used in some industrial applications and aboard

ships.

4. R-764 (Sulphur dioxide): This refrigerant is produced by the combustion of

sulphur in air. It is very stable refrigerant with a high critical temperature and it is

non flammable and non – explosive. It has a very unpleasant and irritating odour.

SO2 is not injurious to food and is used commercially as a ripener and

preservative of foods. It is extremely injurious to flowers, plants and shrubbery. SO2 in its

pure state is not corrosive, but when there is moisture in the refrigerating system be held

to a minimum. SO2 does not mix readily with oil.

R–118 (water): The principal refrigeration use of water is as ice. The high

freezing temperature of water limits its use in vapour compression systems. It is used as

the refrigerant vapour in some absorption systems and in systems with steam jet

compressors.

14

Lecture No. 4

Hydro-carbon Refrigerants:

Most of the hydro-carbon refrigerants are successfully used in industrial and

commercial installations. They possess satisfactory thermodynamic properties but are

highly flammable and explosive. The various hydro-carbon refrigerants are given in the

following table 4.

Table. 4. Hydrocarbon refrigerants

Refrigerant No: Chemical name Chemical formulaR – 170 Ethane C2 H6

R – 290 Propane C3 H8

R – 600 Butane C4 H10

R – 600a Isobutene C4 H10

R – 1120 Tri chloroethylene C2 H4 Cl3R – 1130 Dichloroethylene C2 H4 Cl2R – 1150 Ethylene C2 H4

R – 1270 Propylene C3 H6

Hydro carbon refrigerants are not commonly used now a days.

Designation System for Refrigerants:

The refrigerants are internationally designated as 'R' followed by certain numbers

such as R-11, R-12, R-1l4 etc. A refrigerant followed by a two digit number indicates that

a refrigerant is derived from methane base while three digit number represents ethane

base.

The numbers assigned to hydro-carbon and halo-carbon refrigerants have a

special meaning.

The first digit on the right is the number of fluorine (F) atoms in the refrigerant.

The second digit from the right side is more than the number of hydrogen (H) atoms

present. The third digit from the right is one less than the number of carbon (C) atoms,

but when this digit is zero, it is omitted. The general chemical formula for the refrigerant,

either for methane or ethane base, is given as Cm Hn Clp Fq, in which n+p+q = 2m+2

where

m = Number of carbon atoms,

Hydro carbon refrigerant – Designation system for refrigerants – Designation sytem for Dichloro-tetrafluoro-ethane

15

n = Number of hydrogen atoms,

p = Number of chlorine atoms, and

q = Number of fluorine atoms.

The number of the refrigerant is given by ‘R (m - 1)(n + 1) (q)’.

Let us consider the following refrigerants to find its chemical formula and the number.

1. Dichloro-tetrafluoro-ethane:

In this refrigerant,

Number of chlorine atoms, p = 2

Number of fluorine atoms, q = 4

Number of hydrogen atoms, n = 0

We know that n + p + q = 2m + 2

0+2+4 = 2m+2

∴ m = 2

i.e. Number of carbon atoms = 2

Thus the chemical formula for dichloro-tetrafluoro-ethane becomes C2Cl2F4 and

the number of refrigerant becomes R (2-1) (0+1) (4) or R-114.

16

Lecture No. 5

2. Dichloro-difluoro-methane:

In this refrigerant ,

Number of chlorine atoms, p = 2

Number of fluorine atoms, q = 2

Number of hydrogen atoms, n = 0

We know that n + p + q = 2m + 2

0+2+2 = 2 m+2

∴ m = 1

i.e. Number of carbon atoms = 1

Thus the chemical formula for dichloro-difluoro-methane becomes CCl2F2 and the

number refrigerant becomes R(1-1)(0+1)(2) or R-012 i.e. R-12.

The brominated refrigerants are denoted by putting an additional B and a number

to denote as to how many chlorine atoms are replaced by bromine atoms. Thus R13 B1

is derived from R13 with the replacement of one chlorine atom by a bromine atom. Its

chemical formula is there fore CF3 Br.

In the case of isomers i.e. compounds with the same chemical formula but

different molecular structure, subscripts a, b etc. are used after the designations.

In this manner, there are 15 fluoro– chloro derivatives of methane, 55 derivatives

of ethane, 332 of propane and so on. Besides this, there are brominated ones. There are

also azeotropes, which are mixtures of refrigerants but behave like pure substances.

They are given arbitrary designations e.g. R–502 for a mixture of 48.8% R 22 and 51.2%

R 115.

Unsaturated compounds for which n+p+q=2m, are distinguished by putting the

digit 1 before (m – 1). Thus ethylene is R 1150.

Designation system for Dichloro-difluoro-methane - Selection of a refrigerant

17

In the case of common inorganic refrigerants, numerical designations have been

given according to their molecular weight added to 700. Thus ammonia whose molecular

weight is 17 is designated as R717. Similarly water is designated as R718 and carbon

dioxide as R 744.

Selection of a Refrigerant:

Refrigerants have to be physically non toxic and non flammable.

Thermodynamically there is no working substance which could be called an ideal

refrigerant. In general, a refrigerant may be required to satisfy requirements which may

be classified as thermodynamic, chemical and physical.

The choice of a refrigerant for a given application is governed mainly by the

refrigerating capacity (very small, small, medium or large) and refrigerating temperature

required, such as for air conditioning (5°C), cold storage (-10 to 2°C), refrigerator

(-25°C), food freezing (-40°C) etc.

18

Lecture No. 6

Thermodynamic Properties of Refrigerants:

1. Boiling temperature: The boiling temperature of the refrigerant at

atmospheric pressure should be low. If the boiling temperature of the refrigerant is high

at atmospheric pressure, the compressor should be operated at high vacuum. The high

boiling temperature reduces the capacity and operating cost of the system.

2. Freezing temperature: The freezing temperature of a refrigerant should be well

below the operating evaporator temperature. Since the freezing temperatures of most of

the refrigerants are below -35°C, this property is taken into consideration only in low

temperature operation.

3. Evaporator and condenser pressure: Both the evaporating (low side) and

condensing (high side) pressures should be positive (i.e. above atmospheric) and it

should be as near to the atmospheric pressure as possible. The positive pressures are

necessary in order to prevent leakage of air and moisture into the refrigerating system. It

also permits easier detection of leaks. Too high evaporating and condensing pressures

(above atmospheric) would require stronger refrigerating equipment (i.e. compressor,

evaporator and condenser) resulting in higher initial cost.

The reciprocating compressors are used with refrigerants having low specific

volumes, high operating pressures and high pressure ratios. The centrifugal

compressors are used with refrigerants having high specific volumes, low operating

pressures and low pressure ratios.

4. Critical temperature and pressure: The critical temperature of a refrigerant

is the highest temperature at which it can be condensed to a liquid, regardless of a

higher pressure. It should be above the highest condensing temperature that might be

encountered. If the critical temperature of refrigerant is too near the desired condensing

temperature, the excessive power consumption results.

5. Coefficient of performance and power requirements: For an ideal

refrigerant operating between - 15°C evaporator temperature and 30°C condenser

temperature, the theoretical coefficient of performance for the reversed Carnot cycle is

Thermodynamic properties of refrigerants

19

5.74. R-II has the coefficient of performance equal to 5.09. Which is closest to the

Carnot value of 5.74. Practically, all common refrigerants have approximately the same

coefficient of performance and power requirement.

6. Latent heat of vaporisation: A refrigerant should have a high latent heat of

vaporisation at the evaporator temperature. The high latent heat results in high

refrigerating effect per kg of refrigerant circulated which reduces the mass of refrigerant

to be circulated per tonne of refrigeration.

7. Specific volume: The specific volume of the refrigerant vapour at evaporator

temperature (i.e. volume of suction vapour to the compressor) indicates the theoretical

displacement of the compressor. The reciprocating compressors are used with

refrigerants having high pressures and low volumes of the suction vapour. The

centrifugal or turbo compressors are used with refrigerants having low pressures and

high volumes of the suction vapour.

20

Lecture No. 7

Chemical Requirements of Refrigerants:

1. Flammability: Hydro-carbon refrigerants such as methane, ethane, propane

butane are highly explosive and flammable. Ammonia is also explosive in a mixture with

air in concentrations of 16 to 25 per cent of gas by volume of ammonia. None of the

CFCs is explosive or flammable. Some of the HFCs which are being considered as

alternatives to CFCs because of the ozone layer depletion problem, such as R 152,

R152a, R143 etc, are flammable. However, R134a is neither flammable nor explosive.

2. Toxicity: From the consideration of comparative hazard to life from gases and

vapours, compounds have been divided into six groups by under writers laboratories.

Group six contains compounds with a very low degree of toxicity. It includes CFCs.

Group one at the end of the scale, includes the most toxic substances such as sulphur

dioxide.

Because of flammability and toxicity, ammonia is not used in domestic

refrigeration and air – conditioning. The use of toxic refrigerants its only limited to cold

storages.

3. Solubility of water: Water is only slightly soluble in R-12. At - 18°C, it will hold

six parts, per million by weight. The solution formed is very slightly corrosive to any of

the common metal. The solubility of water with R-22 is more than R-12 by a ratio of 3 to

1. If more water is present than can be dissolved by the refrigerant, the ice will be

formed which chokes the expansion valve capillary tube used for throttling in the system.

This may be avoided by the proper dehydration the refrigerating unit before charging and

by the use of silica gel drier of the liquid line. Ammonia is highly soluble in water. Due to

this reason, a wetted cloth is put at the point of leak to avoid, harm to the persons

working in ammonia refrigerating plants.

4. Miscibility: The ability of a refrigerant to mix with oil is called miscibility. This

property of refrigerant is considered to be a secondary factor in the selection of a

refrigerant. The degree of miscibility depends upon the temperature of the oil and

pressure of the refrigerating vapour. The Freon groups of refrigerants are highly miscible

Chemical requirements of refrigerants – Physical properties of refrigerants

21

refrigerants while ammonia, carbon dioxide, sulphur dioxide and methyl chloride are

relatively non-miscible.

In compressor, some oil is carried by the high temperature refrigerant vapour to

the condenser and ultimately to the expansion value and evaporator. In the evaporator,

as the refrigerant evaporates, a distillation process occurs and the oil separates will

result in a reduced heat transfer coefficient, oil choking in the evaporator due to

restriction caused to refrigerant flow and even blockage and ultimately to oil starvation in

the compressor.

The miscible refrigerants are advantageous from the heat transfer point of view.

They give better lubrication as the refrigerant acts as a carrier of oil to the moving parts.

The miscible refrigerants also eliminate oil-separation problems and aid in the return of

from the evaporator.

5. Effect on perishable materials: The refrigerants used in cold storage plant

and domestic refrigerators should be such that in case of leakage, it should have no

effect on perishable materials.

The freon group of refrigerants have no effect upon dairy products, vegetables,

meat, flowers and furs. There will be no change in colour, taste or texture of the material

when exposed to freon.

Methyl chloride vapours have no effect upon furs, flowers, eating foods or

drinking beverages. Sulphur dioxide destroys flowers, plants and furs, but it does not

effect foods. Ammonia dissolves easily in water and becomes alkaline in nature. Since

most fruits and vegetables are acidic in nature, therefore ammonia reacts with these

products and spoils the taste.

6. Action with material of construction: Ammonia attacks copper and copper

bearing materials. Iron and steel are however, found suitable for use with ammonia. The

recommended material for use piping with halocarbons is copper. Copper continues to

be preferred for construction of heat exchangers in Freon systems.

Physical Properties of Refrigerants:

1. Stability and inertness: An ideal refrigerant should not decompose at any

temperature normally encountered in the refrigerating system. It should not form higher

22

boiling point liquids solid substances through polymerization. Some refrigerants

disintegrate forming non condensable gases which causes high condensing pressure

and vapour lock. The disintegration of refrigerant may be due to reaction with metals. In

order to avoid this, a refrigerant should be inert with respect to all materials used in

refrigerating system.

The freon group of refrigerants are stable upto a temperature of 535°C. Above

this temperature, it decomposes and forms corrosive and poisonous products. The freon

refrigerants are not used with rubber gaskets as it acts as a solvent with rubber. Since

sulphur dioxide does not decompose below 1645°C, therefore it is one of the most stable

refrigerants.

2. Corrosive property: The freon group of refrigerants are non-corrosive with

practically all metals. Ammonia is used only with iron or steel. Sulphur dioxide is non

corrosive to all metals in the absence of water because sulphur dioxide reacts with water

and forms sulphuric acid.

3. Viscosity: The refrigerant in the liquid and vapour states should have low

viscosity. The low viscosity of the refrigerant is desirable because the pressure drops in

passing through liquid and suction lines are small. The heat transfer through condenser

and evaporator is improved at low viscosities

4. Thermal conductivity: The refrigerant in the liquid and vapour states should

have high thermal conductivity.

5. Dielectric strength: The dielectric strength of a refrigerant is important in

hermetically 'sealed units in which the electric motor is exposed to the refrigerant. As the

windiness are cooled by suction vapour. The relative dielectric strength of the refrigerant

is the ratio of the dielectric strength of refrigerant ratio of the dielectric strength of the

refrigerant is the ratio of the dielectric strength of nitrogen and the refrigerant. vapour

mixture to the dielectric strength of nitrogen at atmospheric pressure and room

temperature. The dielectric strengths of R22 is 1.31. This shows that fluorocarbons are

good electrical.

6. Leakage tendency: The leakage tendency of a refrigerant should be low. If

there is a leakage of refrigerant, it should be easily detectable. The leakage occurs due

to opening in the joints or flaws in material used for construction. Since the fluorocarbon

23

refrigerants are colourless, therefore, their leakage will increase the operating cost. The

ammonia leakage is easily detected due to its pungent odour.

The leakage of fluorocarbon refrigerants may be detected by soap solution, a

halide torch or an electronic leak detector. The latter is generally used in big refrigerating

plants. The ammonia leakage is detected by using burning sulphur candle which in the

presence of ammonia forms white fumes of ammonium sulphite.

7. Cost: The cost of refrigerant is not so important in small refrigerating units but it

is very important in high capacity refrigerating systems like industrial and commercial.

The ammonia being the cheapest is widely used in large industrial plants such as cold

storages and ice plants. The refrigerant R-22 is costlier than refrigerant R-12. The cost

of losses due to leakage is also important.

24

Lecture No. 8

Secondary refrigerants - brines:

Brines are secondary refrigerants and are generally used where temperatures

are required to be mainted below the freezing point of water i.e. 0°C. In case the

temperature involved is above the freezing point of water (0°C), then water is commonly

used as a secondary refrigerant.

Brine is a solution of salt in water. It may be noted that when salt is mixed in

water, then the freezing temperature of the solution becomes lower than that of the

water. This is due to the fact that the salt while dissolving in water takes off its latent heat

from the solution and cools it below the freezing point of water.

The mass of the salt in the solution expressed as the percentage of the mass of

solution is known as concentration of the solution. As the concentration of the solution

increases, its freezing point decreases. But if the concentration of the salt is increased

beyond a certain point, the freezing point increases instead of decreasing. The point, at

which the freezing temperature is minimum, is known as eutectic temperature and the

concentration at this point is known as eutectic concentration. The brine used in a

particular application should have a concentration for which the freezing point of the

brine is at least 5°C to 8°C lower than the brine temperature required.

The brines commonly used are calcium chloride (CaCl2), Sodium chloride

(Na Cl ) i.e. common salt and glycols such as ethylene glycol, propylene glycol etc.

The calcium chloride brine has the eutectic temperature of -55°C at salt

concentration of 30% by mass. This brine is primarily used where temperatures below -

18°C are required. It is generally used in industrial process cooling and product freezing.

The chief disadvantages of calcium chloride brine are its dehydrating effect and its

tendency to impart a bitter taste to food products.

The sodium chloride brine has the eutectic temperature of -21.1°C at salt

concentration of 23% by mass. This brine is used in chilling and freezing of meat and

fish.

Secondary refrigerants – Brines- Applications of various brines

25

Both of the above two brines are corrosive in nature for metallic containers which

put limitation on their use. Also the thermal properties of the above two brines are less

satisfactory.

Other water soluble compounds known as antifreeze are also used for

decreasing the freezing point of water for certain refrigeration uses. Ethylene and

propylene glycol have number good properties. Since they are non-corrosive and non-

electrolytic even in the presence of water, these brines are most extensively used as

antifreeze elements.

Table.5. Applications of various brines

Applications Brine used1. Breweries Propylene glycol2. Diaries Sodium chloride, Calcium chloride, Propylene glycol3. Food process Sodium chloride, Calcium chloride, Propylene glycol4. Ice creams Calcium chloride, Propylene glycol5. Ice plant Sodium chloride6. Meat packaging Sodium chloride, Calcium chloride

26

Lecture No. 9

Types of Refrigerators:

Based on working medium, they are classified as

1. Air Refrigerator

2. Vapour Refrigerator

1. Air Refrigerator: In this working agent is air.

2. Vapour Refrigerator: In this working agent are vapours like ammonia, CO2,

SO2, Freon etc.

The main difference between these two classes of refrigeration is the non

condensability of air and condensability & evaporativeness of the above vapours within

the working range of temperatures of refrigerators. This means that for using refrigerator,

air can not be subjected to the change of state (from liquid to vapour and vice versa). In

other words, air behaves as a perfect gas and heat changes are brought about by

changes in temperature only i.e. only sensible heat changes are taking place. Unlike

this, in the case of vapours, heat changes are in the latent heat form and fluid

alternatively changes from liquid to vapour and back to liquid.

There are two types of machine under vapour refrigerators. They are

1. Vapour Compression

2. Vapour Absorption

The main difference between the two is the manner in which the external heat is

added to the vaporised refrigerants.

Simple vapour compression Refrigeration system:

A vapour compression refrigeration system is an improved type of air

refrigeration system in which a suitable working substance, termed as refrigerant, is

used. It condenses and evaporates at temperatures and pressures close to the

atmospheric conditions. The refrigerants usually used for this purpose are ammonia

(NH3), Carbon dioxide (CO2) and Sulphur dioxide (SO2).

Types of Refrigerators – Air Refrigerator – Vapour refrigerator – Advantages and Disadvantage of vapour compression refrigeration system over air refrigeration system

27

The refrigerant used does not leave the system but is circulated throughout the

system alternately condensing and evaporating. In evaporating, the refrigerant absorbs

its latent heat from the brine (salt water – It is used as it has a very low freezing

temperature) which is used for circulating it around the cold chamber. While condensing,

it gives out its latent heat to the circulating water of the cooler.The vapour compression

refrigeration system is therefore a latent heat pump as it pumps its latent heat from the

brine and delivers it to the cooler.

Now–a–days the vapour compression refrigeration system is used for all purpose

of refrigeration. It is generally used for all industrial purposes from a small domestic

refrigerator to a big air conditioning plant.

Advantages and disadvantages of vapour compression refrigeration system over

air refrigeration system:

Advantages:

1. It has smaller size for the given capacity of refrigeration.

2. It has less running cost.

3. It can be employed over a large range of temperatures.

4. The coefficient of performance is quite high.

Disadvantages:

1. The initial cost is high.

2. The prevention of leakage of the refrigerant is the major problem in vapour

compression system.

28

Lecture No. 10

Mechanism of simple vapour compression refrigeration:

Simple vapour compression refrigeration system (Fig. 1) consists of the following

five essential parts.

Figure 1. Simple vapour compression refrigeration system

1. Compressor:

The low pressure and low temperature vapour refrigerant from evaporator is

drawn into the compressor through the inlet or suction valve A, where it is compressed

to a high pressure and temperature. The high pressure and temperature vapour

refrigerant is discharged into the condenser through the delivery or discharged into the

condenser through the delivery or discharge value B. this high pressure must be below

the critical pressure of the refrigerant.

Mechanism of a simple vapour compression refrigeration systems

29

2. Condenser:

The condenser or cooler consists of coils of pipe in which the high pressure and

high temperature vapour refrigerant is cooled and condensed. The refrigerants, while

passing through the condenser, gives up its latent heat to the surrounding condensing

medium which is normally air or water.

3. Receiver:

The condensed liquid refrigerant from the condenser is stored in a vessel known

as receiver from where it is supplied to the evaporator through the expansion valve or

refrigerant control valve.

4. Expansion valve:

It is also called throttle valve or refrigerant control valve. The function of the

expansion valve is to allow the liquid refrigerant under high pressure and temperature to

pass at a controlled rate after reducing its pressure and temperature. Some of the liquid

refrigerant evaporates as it passes through the expansion valve, but the greater portion

is vaporised in the evaporator at the low pressure and temperature. The liquid / gas

mixture leaving the expansion valve is termed “flash gas”.

5. Evaporator:

An evaporator consists of coils of pipe in which the liquid vapour refrigerant at

low pressure and temperature is evaporated and changed into vapour refrigerant at low

pressure and temperature. In evaporating, the liquid vapour refrigerant absorbs its latent

heat of vaporisation from the medium (air, water or brine) which is to be cooled.

Note: In any compression refrigeration system, there are two different pressure

conditions. One is called the high pressure side and the other is known as low pressure

side. The high pressure side includes the discharge line (i.e. piping from delivery value B

to the condenser), Condenser, receiver and expansion valve. The low pressure side

includes the piping from the expansion valve to the evaporator and the suction line (i.e.

piping from the evaporator to the suction valve A). The expansion valve separates the

high pressure region from the low pressure region.

30

Lecture No. 11

Introduction:-

The evaporator is an important device used in the low pressure side of a

refrigeration system. The liquid refrigerant from the expansion valve enters into the

evaporator where it boils and changes into vapour. The function of an evaporator is to

absorb heat from the surrounding location or medium which is to be cooled, by means of

a refrigerant. The temperature of the boiling refrigerant in the evaporator must always

less than that of the surrounding medium that the heat flows to the refrigerant. The

evaporator becomes cold and remains cold d to the following two reasons.

1. The temperature of the evaporator coil is low due to the low temperature of the

refrigerant inside the coil.

2. The low temperature of the refrigerant remains unchanged because any heat it

absorbs is converted to latent heat as boiling proceeds.

Note: The evaporator is also known as a cooling coil, a chilling coil or a freezing coil.

The evaporator cools by using the refrigerant's latent heat of vaporisation to absorb heat

from the medium being cooled.

Capacity of an Evaporator:

The capacity of an evaporator is defined as the amount of heat absorbed by it

over a given period of time. The heat absorbed or heat transfer capacity of an

evaporator is given by

Q = UA (T2 - T1) W or J/s

where

U = Overall heat transfer coefficient in W/m2 °C,

A = Area of evaporator surface in m2,

T2 = Temperature of medium to be cooled (or temperature outside the

evaporator)

Evaporators – Capacity of an evaporator – Factors affecting heat transfer capacity of an evaporator – Types of evaporator

31

°C, and

T1 = Saturation temperature of refrigerant at evaporator pressure (or temperature

inside the evaporator) in °C.

Factors Affecting the Heat Transfer Capacity of an Evaporator:

1. Material: In order to have rapid heat transfer in an evaporator, the material

used for the construction of an evaporator coil should be a good conductor of heat. The

material which is not affected by the refrigerant must also be selected. Since metals are

best conductors of heat, therefore they' are always used for evaporators. Iron and steel

can be used with all common refrigerants. Brass and copper are used with all

refrigerants except ammonia. Aluminium should not be used with Freon.

2. Temperature difference: The temperature difference between the refrigerant

within the evaporator and the product to be cooled plays an important role in the heat

transfer capacity of an evaporator. The following table shows the suggested temperature

difference for some of the products to be cooled.

It may be noted that a too low temperature difference (below g0C) may cause

slime on certain products such as meat or poultry. On the other hand, too high a

temperature difference causes excessive dehydration.

3. Velocity of refrigerant: The velocity of refrigerant also affects the heat

transfer capacity of an evaporator. If the velocity of refrigerant flowing through the

evaporator increases, the overall heat transfer coefficient also increases. But this

increased velocity will cause greater pressure loss in the evaporator. Thus the only

recommended velocities for different refrigerants which give high heat transfer rates and

allowable pressure loss should be used.

4. Thickness of the evaporator coil wall: The thickness of the evaporator coil

wall also affects the heat transfer capacity of the evaporator. In general, the thicker the

wall, the slower is the rate of heat transfer. Since the refrigerant in the evaporator coils is

under pressure, therefore the evaporator walls must be thick enough to withstand the

effects of that pressure. It may be noted that the thickness has only a slight effect on

total heat transfer capacity because the evaporators are usually made from highly

conductive materials.

32

5. Contact surface area: An important factor affecting the evaporator capacity is

the contact surface available between the walls of evaporator coil and the medium being

cooled. The amount of contact surface, in turn, depends basically on the physical size

and shape of the evaporator coil.

Types of Evaporators

1. According to the type of construction

(a) Bare tube coil evaporator,

(b) Finned tube evaporator,

(c) Plate evaporator,

(d) Shell and tube evaporator,

(e) Shell and coil evaporator,

(f) Tube-in-tube evaporator,

2. According to the manner in which liquid refrigerant is fed

(a) Flooded evaporator, and

(b) Dry expansion evaporator

3. According to the mode of heat transfer

(a) Natural convection evaporator,

(b) Forced convection evaporator

4. According to operating conditions

(a) Frosting evaporator,

(b) Non-frosting evaporator,

(c) Defrosting evaporator

33

Lecture No. 12

1.Bare Tube Coil Evaporators:

The simplest type of evaporator is the bare tube coil evaporator, as shown in

Fig. 2, and is also known as prime-surface evaporators. Because of its simple

construction, the bare tube coil is easy to clean and defrost.

Fig. 2. Bare tube coil evaporator.

A little consideration will show that this type of evaporator offers relatively little

surface. Contact area as compared to other types of coils. The amount of surface area

may be increased by simply extending the length of the tube, but there are

disadvantages of excessive tube length. The effective length of the tube is limited by the

capacity of expansion valve. If the tube is too long for the valve's capacity, the liquid

refrigerant will tend to completely vaporise early in its progress through the tube, thus

leading to excessive superheating at the outlet. The long tubes will also cause

considerably greater pressure drop between the inlet and outlet of the evaporator. This

results in a reduced suction line pressure.

The diameter of the tube in relation to tube length may also be critical. If the tube

diameter is too large, the refrigerant velocity will be too low and the volume of refrigerant

will be too great in relation to the surface area of the tube to allow complete vaporisation.

This, in turn, may allow liquid refrigerant to enter the suction line with possible damage

to the compressor (i.e. slugging). On the other hand, if the diameter is too small, the

pressure drop due to friction may be too high, and will reduce the system efficiency.

Bare tube coil evaporator – Finned evaporator – Plate evaporator – Shell and tube evaporator

34

The bare tube coil evaporators may be used for any type of refrigeration

requirement. Its use is, however limited to applications where the box temperatures are

under 0°C and in liquid cooling, because the accumulation of ice or frost on these

evaporators has less effect on the heat transfer than on those equipped with fins. The

bare tube coil evaporators are also extensively used in house-hold refrigerators because

they are easier to keep clean.

2. Finned Evaporators:

The finned evaporator, as shown in Fig. 3, consists of bare tubes or coils over

which the metal plates or fins are fastened.

Fig. 3. Finned evaporator.

The metal fins are constructed of thin sheets of metal having good thermal

conductivity. The shape, size or spacing of the fins can be adapted to provide best rate

of heat transfer for a given application. Since the fins greatly increases the contact

surfaces for heat transfer, therefore the finned evaporators are also called extended

surface evaporators.

The finned evaporators are primarily designed for air conditioning applications

where the refrigerator temperature is above 0°C. Because of the rapid heat transfer of

the finned evaporator, it will defrost itself on the off cycle when the temperature of the

coil is near 0°C. A finned coil should never be allowed to frost because the accumulation

of frost between the fins reduces the capacity. The air conditioning coils which operate at

suction temperatures which are high enough so that frosting never occurs, have fin

spacing as small as 3 mm. The finned coils which frost on the on cycle and defrost on

the off cycle have wider fin spacing.

35

3. Plate Evaporators:

A common type of plate evaporator is shown in Fig. 4. In this type of evaporator,

the coils are either welded on one side of a plate or between the two plates which are

welded together at the edges. The plate evaporators are generally used in house-hold

refrigerators, home freezers, beverage coolers, ice cream cabinets, locker plants etc.

Fig. 4. Plate evaporator.

4. Shell and Tube Evaporators:

The shell and tube evaporator, as shown in Fig. 5, is similar to a shell and tube

condenser. It consists of a number of horizontal tubes enclosed in a cylindrical shell. The

inlet and outlet headers with perforated metal tube sheets are connected at each end of

the tubes.

Fig. 5. Shell and tube evaporator.

36

These evaporators are generally used to chill water or brine solutions. When it is

operated as a dry expansion evaporator, the refrigerant circulates through the tubes and

the liquid to be cooled fills the space around the tubes within the shell. The dry

expansion shell and tube evaporators are used for refrigerating units of 2 to 250 TR

capacity. When it is operated as a flooded evaporator, the water or brine flows through

the tubes and the refrigerant circulates around the tubes. The flooded shell and tube

evaporators are used for refrigerating units of 10 to 5000 TR capacity.

37

Lecture No. 13

1. Shell and coil evaporator:

The shell and coil evaporator, as shown in Fig. 6, are generally dry expansion

evaporator to chill water. The cooling coil is a continuous tube that can be in the form of

a single or double spiral. The Shell may be sealed or open. The sealed shells are usually

found in shell and coil evaporators used to cool drinking water. The evaporators having

flanged shells are often used to chill water in secondary refrigeration system.

Fig. 6. Shell and coil evaporator.

They are used where small capacity (2 to 10 TR) liquid cooling is required. It may

be noted that the shell and coil evaporator is restricted to operation above 5°C in order

to prevent the freezing problems.

2. Tube – In – Tube or Double Tube Evaporator:

The tube-in-tube evaporator (or double tube evaporator) as shown in Fig. 7,

consists of one tube inside another tube. The liquid to be cooled flows through the inner

tube while the primary refrigerant or secondary refrigerant (i.e. water, air or brine)

circulates in the space .between the two tubes. The tube- in-tube evaporator provides

high heat transfer rates. However, they require more space than shell and tube

evaporators of the same capacity. These evaporators are used for wine cooling and in

petroleum industry for chilling of oil.

Shell and coil evaporator – Tube in tube – Flooded evaporator – Dry expansion evaporator

38

Fig. 7. Tube-In-tube or double tube evaporator.

3. Flooded Evaporator:

In a flooded evaporator, as shown in Fig. 8, a constant liquid refrigerant level is

always maintained.

Fig. 8. Flooded evaporator.

A float control valve is used as an expansion device which maintains constant

liquid level in the evaporator. The liquid refrigerant from the receiver passes through a

low side float control valve and accumulator before entering the evaporator coil. The

accumulator (also called a surge drum or surge tank) serves as a storage tank for the

39

liquid refrigerant. It maintains a constant liquid level in the evaporator and helps to

separate the liquid refrigerant from the vapour returning to the compressor. Due to the

heat supplied by the substance to be cooled, the liquid refrigerant in the evaporator coil

vaporises and thus the liquid level falls down. The accumulator supplies more liquid to

the evaporator in order to keep the liquid refrigerant in the evaporator at proper level. In

this way, the level of liquid refrigerant in the accumulator also falls down. Since the float

within the float chamber rests on liquid refrigerant at the same level as that in the

accumulator, the float also falls down and opens the float valve. Now the liquid

refrigerant from the receiver is admitted into the accumulator. As the liquid level in the

accumulator rises and reaches to the constant level, the float also rises with it until the

float control valve closes.

Since the evaporator is almost completely filled with liquid refrigerant, therefore

the vapour refrigerant from the evaporator is not superheated but it is in a saturated

condition. In order to prevent liquid refrigerant to enter into the compressor, an

accumulator is generally used with the flooded evaporators. The liquid refrigerant

trapped in the accumulator is re-circulated through the evaporator. The evaporator coil is

connected to the accumulator and the liquid flow from the accumulator to the evaporator

coil is generally by gravity. The vapour formed by vaporising the liquid in the coil being

lighter, rises up and passes on to the top of the accumulator from where it is supplied to

the suction side of the compressor. The baffle plate arrests any liquid present in the

vapour.

The advantage of the flooded evaporator is that the whole surface of the

evaporator coil is in contact with the liquid refrigerant under all the load conditions. Thus,

it gives high heat transfer rates (i.e. more efficient cooling) than a dry expansion

evaporator of the same size. However, the flooded evaporator is more expensive to

operate because it requires more refrigerant charge.

The flooded evaporators have many industrial applications, especially in the

chemical and food processing industries.

4. Dry Expansion Evaporator:

The dry expansion evaporators are not really dry at all. They simply use relatively

little refrigerant as compared to flooded evaporators having the same coil volume. They

40

are usually only one-fourth to one-third filled with liquid refrigerant. A simple bare-tube

dry expansion evaporator is shown in Fig. 9.

Fig. 9. Dry expansion evaporator.

In dry expansion evaporators, the liquid refrigerant from the receiver is fed by the

expansion valve to the evaporator coil. The expansion valve controls the rate of flow of

liquid refrigerant in a way that all the liquid refrigerant is vaporised by the time it reaches

at the end of the evaporator coils or the suction line to the compressor. The vapour is

also superheated to a limited extent. In a dry expansion system, the refrigerant does not

recirculate within the evaporator as in flooded type evaporator.

41

Lecture No. 14

1. Natural Convection Evaporator:

The natural convection evaporators are used where low air velocity and minimum

hydration of the product is desired. The domestic refrigerators, water coolers and small

freezers have natural convection evaporators. The circulation of air in a domestic

refrigerator by natural convection is shown in Fig. 10. The evaporator coil should be

placed as high as possible in the refrigerator because the cold air falls down as it leaves

the evaporator.

Fig. 10. Air circulation with correct shape of coil.

In Natural Convection, the velocity of air depends upon the temperature

difference between the evaporator and the space to be cooled. When the temperature

difference, using a natural convection evaporator, is low (less than 8°C), the velocity of

air is too low for satisfactory circulation. A low temperature difference than 8°C may even

cause slime on certain products such as meat or poultry. On the other hand, too great a

temperature difference causes excessive dehydration. The coil should occupy at least

2/3rd of the width of the path of the air circulation and 3/4th the length of box. The

natural convection can be improved by the use of baffles. These are simply Sheet metal

plates which guide the ascending and descending air currents in their proper channels.

2. Forced Convection Evaporators:

In this the air is forced over the refrigerant cooled coils and fins. ·s is done by a

fan driven by an electric motor. The fins are provided to increase the heat transfer rate.

The forced convection evaporators are more efficient than natural convection

evaporators. They are suited for cooling units as well as for refrigerator cabinets used to

store bottled beverages or foods in sealed containers.

Natural convection evaporator – Forced convection evaporator – Frosting evaporator – Non Frosting evaporators – Defrosting evaporators

42

They may be divided into three groups according to the velocity of air leaving the

unit.

a. Low velocity cooling units: These units have a discharge air rate from 60

m/min to 90 m/min. They are used in comfort air conditioning where low noise and low

air velocity rates are needed. Both centrifugal and propeller type fans are used with low

velocity cooling units.

b. Medium velocity cooling units: They have an exit velocity of air from 150

m/min to 240 m/min. They are frequently used in refrigerators and freezers where drafts

and noise are not problem. The propeller fans are usually the source of air circulation in

these units.

c. High velocity cooling units: They have a discharge air rate above 240

m/min. They re used principally in blast freezers in special product refrigerators requiring

quick pull-down of temperature. The high velocity cooling units usually use centrifugal

fans as the source of air circulation.

3. Frosting Evaporators:

They always operate at temperatures below 0°C. This means that the coil frosts

continually when in use and it must be removed at regular intervals for most efficient

operation. The frost which forms on the evaporator comes from the moisture in the air.

as the frost grows in thickness, the coil or cooling efficiency decreases until the ice and

frost is removed. The evaporators in household refrigerators, bare pipe coils in storage

boxes and low temperature evaporators fall under the frosting evaporators.

4. Non-frosting Evaporators:

The non-frosting evaporators operate at a temperature above 0°C at all times.

So, the frost does not form on the evaporator. Occasionally, the evaporator may build up

a light coat of frost just before the compressor shuts off. This frost immediately melts on

the off cycle.

The big advantage of a non-frosting evaporator is its operation at a temperature

close to freezing (0.6°C to 1°C). At this temperature, it does not draw the moisture out of

the air rapidly.

43

This permits to maintain a relative humidity from 75 to 85% in the cabinet. This

helps to keep the food fresh and stops shrinking in weight. They are used to high

temperature work such as air conditioning, bakery refrigeration and candy storage.

5. Defrosting Evaporators:-

A defrosting evaporator is one in which frost accumulates on the coils when the

compressor is running and melts after the compressor shuts off. During the running of

the compressor shut off. During the running of the compressor, the evaporator remains

at a temperature of about -7°C to - 6°C. At these temperatures, the frost forms on the

coil surface. But after the compressor stops, the coil warms upto a temperature slightly

above 0°C and the frost melts before the compressor starts again.

A defrosting evaporator is usually a finned coil because of the necessity of rapid

heat transfer during the off cycle. It also keeps a high relative humidity of about

90 to 95 %.

44

Lecture No. 15

Introduction:

A refrigerant compressor, as the name indicates, is a machine used to compress

vapour refrigerant from the evaporator and raise its pressure so that the corresponding

saturation temperature is higher than that of cooling medium. It also continually

circulates the refrigerant through the refrigerating system Since the compression of

refrigerant requires some work to be done on it, therefore, compressor must be driven by

some prime over.

Note: Since the compressor virtually takes the heat at a low temperature from the

evaporator and pumps it at the high temperature to the condenser, it is often referred to

as a heat pump.

Classification of Compressors:

1. According to the method of compressors

(a) Reciprocating compressors

(b) Rotary compressors

(c) Centrifugal compressors

2. According to the number of working strokes

(a) Single acting compressors

(b) Double acting compressors

3. According to the number of stages

(a) Single stage (or single cylinder) compressors

(b) Multi-stage (or multi-cylinder) compressors

4. According to the method of drive employed

(a) Direct drive compressors

Compressor – Classification – Suction pressure, Discharge pressure, Compression ratio, Suction volume, Stroke volume, Clearance factor, Compressor capacity, Volumetric efficiency

45

(b) Belt drives compressors

5. According to the location of the prime mover

(a) Semi-hermetic compressors (direct drive, motor and compressor in

separate housings),

(b) Hermetic compressors (direct drive, motor and compressor in same

housings).

Important Terms:

The important terms used in reciprocating compressor (Fig. 11) are given below.

Fig. 11. Reciprocating compressor

1. Suction pressure: It is the absolute pressure of refrigerant at the inlet of a

compressor.

2. Discharge pressure: It is the absolute pressure of refrigerant at the outlet of a

compressor.

3. Compression ratio: It is the ratio of absolute discharge pressure to the

absolute suction pressure. Since the absolute discharge pressure is always more than

the absolute suction pressure, the value of compression ratio is more than unity.

The compression ratio may also be defined as the ratio of total cylinder volume to

the clearance volume.

4. Suction volume: It is the volume of refrigerant sucked by the compressor

during its suction stroke. It is the volume swept by the piston when it moves from its

inner dead position to outer dead centre.

46

Mathematically,

LDV 2

4

πρ =

Where

D = Diameter of cylinder,

L = Length of piston stroke.

6. Clearance factor: It is the ratio of clearance volume (Vc) to the piston

displacement volume (Vp). Mathematically, clearance factor,

p

c

V

VC =

7. Compressor capacity: It is the volume of the actual amount of refrigerant

passing through the compressor in a unit time. It is equal to the suction volume (Vs). It is

expressed in m3/s.

8. Volumetric efficiency: It is the ratio of the compressor capacity or the suction

volume (Vs) to the piston displacement volume (Vp)

Mathematically,

p

sv v

v=η

A good compressor has a volumetric efficiency of 70 to 80 %.

47

Lecture No. 16

Reciprocating Compressors:

The compressors in which the vapour refrigerant is compressed by the

reciprocating (i.e. back and forth) motion of the piston, are called reciprocating

compressors. These compressors used for refrigerants which have comparatively low

volume per kg and a large differential pressure, such as ammonia (R-717), R-12, R-22,

and methyl chloride (R-40). The reciprocating compressors are available in sizes from a

fraction of a ton which are used in domestic refrigerators and upto about 100 tons for

large capacity installations.

The two types of reciprocating compressors in general use are single acting

vertical compressors and double acting horizontal compressors. The single acting

compressors (Fig. 12) usual have their cylinders arranged vertically, radially or in a V or

W form. The double acting compressors usually have their cylinders arranged

horizontally. The compressor may be operated with an electric motor or an internal

combnstion engine.

Fig. 12. Parts of reciprocating compressor.

An important parameter that influences the performance of a compressor is the

compressor capacity. The compressor capacity is affected by several factors that are

inherent to the design of equipment include

Reciprocating compressor - Parts of a reciprocating compressor - Cycle of a reciprocating compressor

48

a) Piston displacement

b) Clearance between the piston head and the end of cylinder when the piston is

at the top of its stroke and

c) Size of the suction and discharge valves.

Other factors that influence the compressor capacity are associated with

the operating conditions – a) Revolutions per minute b) Type of refrigerant c) Suction

pressure d) Discharge pressure.

Fig. 13. A complete cycle of a reciprocating compressor.

The piston displacement can be examined by considering the above

Fig. 13 Point A refers to the cylinder volume of 100% with the vapour at the suction

pressure. The vapours are compressed to about 15% of cylinder volume shown by point

B. during the compression cycle, both the suction valve and discharge value remain

closed. During the discharge cycle B C, the discharge valve opens and the gas is

released. This value represents the volume is decreased to 5%. This value represents

the volume between the head of the piston and the end of the cylinder.

49

As the piston starts travelling in the opposite direction, during the expansion

cycle CD, the high pressure gas remaining in the clearance expands. Attempts are

always made to keep the clearance as small as practically possible, since the clearance

space represents a loss in capacity process DA represents suction; the suction value

remains open during this process and the vapour are admitted to the cylinder. The piston

retracts to the location indicated by point A the cycle is then repeated.

In Actual practice the suction and discharge pressure lines (BC and DA) are not

straight lines due to the pressure drop across the values. The discharge lines above and

the suction line lies below the theoretical straight lines indicated in figure. This difference

can be explained by the fact that at the beginning of discharge, the cylinder pressure

must be higher than the discharge pressure to force the value open against the spring

pressure. Similarly, at the beginning of the suction process, the pressure must be lower

than the suction pressure to open the suction value.

Compressor capacity control:

The speed can be controlled by the use of a variable speed electric motor.

Bypassing gas is the most commonly used method of capacity control. In one

bypass system, the discharge side of the compressor is connected to the suction side.

A solenoid value is used to bypass the discharge gas directly to the suction side of the

compressor. Thus the refrigerant delivered from a bypassed cylinder of the compressor

is stopped, and the compressor capacity is reduced. The bypass system does not result

in any significant savings in power requirement.

A more desirable bypass system involves keeping the suction value open,

allowing the gas to simply surge back and forth in the cylinder. Thus, the bypassed

cylinder delivers no refrigerant. Hermetically sealed solenoids are used to control the

opening of the suction values.

In mulitcylinder compressors, it is possible to inactivate desired cylinders when

the load is low. For a four – cylinder compressor with three cylinders inoperative, the

compressor capacity can be reduced by as much as 75%.

In a double acting reciprocating compressor, the suction and compression takes

place on both sides of the piston. It is thus obvions, that such a compressor will supply

double the volume of refrigerant than a single acting reciprocating compressor.

50

The low capacity compressors are air cooled. The cylinders of these

compressors usually have fins to provide better air cooling. The high capacity

compressors are cooled by providing water jackets around the cylinder.

Hermetic Sealed Compressors :

When the compressor and motor operate on the same shaft and are enclosed in

a common casing, they are known as hermetic sealed compressors. These types of

compressors eliminate the use of crank shaft seal which is necessary in ordinary

compressors in order to prevent leakage of refrigerant. These compressors may operate

on either reciprocating or rotary principle and may be mounted with the shaft in either the

vertical or horizontal position. The hermetic units are widely used for small capacity

refrigerating systems such as in domestic refrigerators, home freezers and window air

conditioners.

The hermetic sealed compressors have the following advantages and

disadvantages

Advantages:-

1. The leakage of refrigerant is completely prevented.

2. It is less noisy.

3. It requires small space because of compactness.

4. The lubrication is simple as the motor and compressor operate in a sealed

space with the lubricating oil.

Disadvantages:

1. The maintenance is not easy because the moving parts are inaccessible.

2. A separate pump is required for evacuation and charging of refrigerant.

51

Lecture No. 17

Rotary Compressors:-

In rotary compressors, the vapour refrigerant from the evaporator is compressed

due to the movement of blades. The rotary compressors are positive displacement type

compressors. Since the clearance in rotary compressors is negligible, they have high

volumetric efficiency. These compressors may be used with refrigerants R-12, R-22,

R-114 and ammonia.

There are two basic types of rotary compressors:

1. Single stationary blade type rotary compressor

2. Rotating blade type rotary compressor

1. Single stationary blade type rotary compressor:

It consists of a single stationary cylinder, a roller (or impeller) and a shaft

(Fig. 14). The shaft has an eccentric on which the roller is mounted. A blade is set into

the slot of a cylinder in such a manner that it always maintains contacts with the roller by

means of a spring. The blade moves in and out of the slot to follow the rotor when it

rotates. Since the blade separates the suction and discharge ports, it is often called a

sealing blade. When the shaft rotates, the roller also rotates so that it always touches the

cylinder wall.

Fig.a) shows the completion of intake stroke (i.e. the cylinder is full of low

pressure and temperature vapour refrigerant) and the beginning of compression stroke.

When the roller rotates, the vapour refrigerant ahead of the roller is being compressed

and the new intake from the evaporator is drawn into the cylinder, as shown in Fig. b).

As the roller turns towards mid position as shown in Fig. b) more vapour refrigerant is

drawn into the cylinder while the compressed refrigerant is discharged to the condenser.

At the end of compression stroke, as shown in Fig. d), most of the compressed vapour

refrigerant is passed through the discharge port to the condenser. A new charge of

refrigerant is passed through the discharged port to the condenser. A new charge of

refrigerant is drawn into the cylinder. This in turn, is compressed and discharged to the

Rotary compressor – Centrifugal compressor – Advantages and disadvantages of a centrifugal compressor

52

condenser. In this way, the low pressure and temperature vapour refrigerant is

compressed gradually to a high pressure and temperature.

Fig. 14. Stationary single blade rotary compressor.

2. Rotating blade type rotary compressor:

It consists of a cylinder and a slotted rotor containing a number of blades

(Fig. 15). The centre of the rotor is eccentric with the centre of the cylinder. The blades

are forced against the rotation of the motor.

53

Fig. 15. Rotating blade type rotary compressor.

The low pressure and temperature vapour refrigerant from the evaporator is

drawn through suction port. As the rotor turns, the suction vapour refrigerant entrapped

between the two adjacent blades is compressed. The compressed refrigerant at high

pressure and temperature is discharged through the discharge port to the condenser.

3. Centrifugal compressor:-

The centrifugal compressor (Fig. 16) for refrigeration systems was designed and

developed by Dr. Willis H. Carrier in 1922.

This compressor increases the pressure of vapour refrigerant to a high pressure

by centrifugal force. The centrifugal compressor is generally used for refrigerants that

require large displacement and low condensing pressure, such as R -11 and It 113.

However, the refrigerant R-12 is also employed for large capacity applications an low-

temperature applications. It consists of an impeller to which a number of curved vanes

are fitted symmetrically, the impeller rotates in an air tight volute casing with inlet and

outlet points.

The impeller draws in low pressure vapour refrigerant from the evaporator. When

the impeller rotates, it pushes the vapour refrigerant from the centre of the impeller to its

periphery by centrifugal force. The high speed of the impeller leaves the vapour

refrigerant at a high velocity at the vane tips of the impeller. The kinetic energy thus

attained at the impeller outlet is converted into pressure energy when the high velocity

vapour refrigerant passes over the diffuser. The diffuser is normally a vaneless type as it

permits more efficient part load operation which is quite usual in any air- conditioning

plant. The volute casing collects the refrigerant from the diffuser and it further converts

54

the kinetic energy into pressure energy before it leaves the refrigerant to the condenser.

The centrifugal compressors have no valves, pistons and cylinders. The only wearing

parts are the main bearings.

Fig. 16. Centrifugal compressor.

Advantages and Disadvantages of Centrifugal Compressors over Reciprocating

Compressors

Advantages:

1. Since the centrifugal compressors have no valves, pistons, cylinders,

connecting rod etc., the working life of these compressors is more as

compared to reciprocating compressors.

2. These compressors operate with little or no vibration as there are no

unbalanced masses.

3. The operation of centrifugal compressors is quiet and calm.

4. The centrifugal compressors run at high speeds (3000 r.p.m. and above),

these can be directly connected to electric motors or steam turbines.

5. Because of the high speed, these compressors can handle large volume of

vapour refrigerant, as compared to reciprocating compressors.

55

6. The centrifugal compressors are especially adapted for systems ranging from

50 to 5000 tonnes. They are also used for temperature ranges between -

90°C and + l0°C.

7. The efficiency of these compressors is considerably high.

8. The large sizes centrifugal compressors require less floor area as compared to

reciprocating compressors.

Disadvantages:

1. The main disadvantage in centrifugal compressors is * surging. It occurs when

the refrigeration load decreases to below 35 percent of the rated capacity and

causes severe stress conditions in the compressor.

2. The centrifugal compressors are not practical below 50 tonnes capacity load.

3. The refrigerants used with these compressors should have high specific

volume.

56

Lecture No. 18

Introduction:

The condenser is an important device used in the high pressure side of a

refrigeration system. Its function is to remove heat of the vapour refrigerant discharged

from compressor. The hot vapour refrigerant consists of the heat absorbed by the

evaporator and heat of compression added by the mechanical energy of the compressor

motor. The heat from the hot vapour refrigerant in a condenser is removed first by

transferring it to the walls of the condenser tubes and then from the tubes to the

condensing or cooling medium. The cooling medium may be air or water or a

combination of the two.

The selection of a condenser depends upon the capacity of the refrigerating

system, the type. refrigerant used and the type of cooling medium available.

Working of a Condenser:

The working of a condenser may be best understood by considering a simple

refrigerating system as shown in Fig 17. The corresponding p-h diagram showing three

stages of a refrigerant cooling is shown in Fig 18.

Fig. 17. Simple refrigerating system.

Condensers – Working of a condenser – Factors affecting the condenser capacity – Heat rejection factor

57

Fig. 18. p-h diagram of a simple refrigerating system.

The compressor draws in the superheated vapour that the refrigerant contains

the heat it absorbed in the evaporator. The compressor adds more heat (i.e the heat of

compression) to the superheated vapour. This highly superheated vapour from the

compressor is pumped to the condenser through the discharge line. The condenser

cools the refrigerant in the following three stages.

1. First of all, the superheated vapour is cooled to saturation temperature

(desuperheating) corresponding to the pressure of the refrigerant. This is shown by the

line 2-3 in Fig 18.The de superheating occurs in the discharge line and in the first few

coils of the condenser.

2. The saturated vapour refrigerant gives up its latent heat and is condensed to a

saturated liquid refrigerant. This process, called condensation, is shown by the line 3-4.

3. The temperature of the liquid refrigerant is reduced below its saturation

temperature (i.e. sub cooled) in order to increase the refrigeration effect. This process is

shown by the line 4-5.

58

Factors Affecting the Condenser Capacity:

The condenser capacity is the ability of the condenser to transfer heat from the

hot vapour refrigerant to the condensing medium. The heat transfer capacity of a

condenser depends upon following factors.

1. Material: Since the different materials have different abilities of heat transfer,

th ' the size of a condenser of a given capacity can be varied by selecting the right

material. It may be noted that higher the ability of a material to transfer heat, the smaller

will be the size of condenser.

2. Amount of contact: The condenser capacity may be varied by controlling the

amount of contact between the condenser surface and the condensing medium. This

can be done by varying the surface area of the condenser and the rate of flow of the

condensing medium over condenser surface. The amount of liquid refrigerant level in the

condenser also affects the amount of contact between the vapour refrigerant and the

condensing medium. The portion of the condenser used for liquid sub-cooling can not

condense any vapour refrigerant. I

3. Temperature difference: The heat transfer capacity of a condenser greatly

depends upon the temperature difference between the condensing medium and the

vapour refrigerant. As temperature difference increases, the heat transfers rate

increases and therefore the condenser capacity increases. Generally, this temperature

difference cannot be controlled. But when temperature difference becomes so great that

it becomes a problem, devices are available that will change the amount of condensing

surface and the air flow rate to control condenser capacity.

Most air-cooled condensers are designed to operate with a temperature

difference of 14° C.

Heat Rejection Factor:

The load on the condenser per unit of refrigeration capacity is known heat

rejection factor. The load on the condenser (Qc) is given by

Qc = Refrigeration capacity + Work done by the compressor

59

=RE+W

Heat rejection factor,

E

E

E

c

R

WR

R

QHRF

+==

COPR

W

E

111 +=+= (

W

RCOP E= )

Heat rejection factor depends upon the coefficient performance (COP) which in

turn depends upon the evaporator and condenser temperatures.

In actual air-conditioning applications for R-12 and R-22, and operating at a

condenser temperature of 40°C and an evaporator temperature of 5°C, the heat

rejection factor is about 1.25.

60

Lecture No. 19

Classification of Condensers:

According to the condensing medium used, the condensers are classified into the

following three groups:

1. Air cooled condensers,

2. Water cooled condensers, and

3. Evaporative condensers.

1. Air Cooled Condensers:

An air-cooled condenser (Fig. 19) is one in which the removal of heat is done

by air. It consists of steel or copper tubing through which the refrigerant flows. The size

of tube usually ranges from 6 to 18 mm outside diameter, depending upon the size of

condenser.

Fig. 19. Air cooled condenser.

Generally copper tubes used because of its excellent heat transfer ability. The

condensers with steel tubes are used in , ammonia refrigerating systems. The tubes are

usually provided with plates or fins to increase the surface area for heat transfer, The

fins are usually made from aluminium because of its light weight. The fin spacing is quite

wide to reduce dust clogging. The plate condensers have no fins, so they require

Classification of condensers –Air cooled condensers – Water cooled condensers – Tube in condenser – Shell and coil condenser

61

considerably large surface areas. However, they are cheaper to construct and require

little maintenance. Both these types of condensers can be founds in house hold

refrigerators.

The condensers with single row of tubing provide the most efficient heat transfer.

This is because the air temperature rises at it passes through each row of tubing. The

temperature difference between the air and the vapour . refrigerant decreases in each

row becomes effective. However, single row condensers require more space than multi-

row condensers.

The air cooled condensers may have two or more rows of tubing, but the

condensers with up to six rows of tubing are common. Some condensers have seven or

eight rows. However more than eight rows of tubing are usually not efficient. This is

because the air temperature will be too close to the condenser temperature to absorb

any more heat after passing through eight rows of tubing.

There are two types of air-cooled condensers:

1. Natural convection air-cooled condensers: In natural convection air-cooled

condenser the heat transfer from the condenser coils to the air is by natural convection.

As the air comes in contact with the warm condenser tubes, it absorbs heat from the

refrigerant and thus the temperature of air increases. The warm air being lighter, rises up

and the cold air from below rises to take away the heat from the condenser. As the rate

of heat transfer in natural connection condenser, they require a larger surface area as

compared to forced convection condensers. These are used only in small capacity

applications such as domestic refrigerant freezers, water coolers and air-conditioners.

2. Forced convection air-cooled condensers: In the fan (either propeller or

centrifugal) is used to force the air over the condenser coils to increase its heat transfer

capacity.

2. Water Cooled Condensers:

A water cooled condenser is one in which water is used as the condensing

medium. They, are always preferred where an adequate supply of clear inexpensive

water and means of water disposal are available. These condensers are commonly used

in commercial and industrial refrigerating units.

62

The water cooled condensers operate at a lower condensing temperature then

an air cooled condenser this is because the supply water temperature is normally lower

than the ambient air temperature, but the difference between the condensing and

cooling medium temperatures is normally the same (i.e. 14°C) Thus, the compressor for

a water cooled condenser requires less power for the same capacity.

The water cooled condensers are classified into three groups.

1. Tube – in – Tube or double tube condensers

2. Shell and coil condensers

3. Shell and tube condensers

1.Tube-in-tube or double tube condensers: It consists of a water tube inside a

large refrigerant tube (Fig. 20). The hot vapour refrigerant enters at the top of the

condenser. The water absorbs the heat from the refrigerant and the condensed liquid

refrigerant flows at the bottom. Since the refrigerant tubes are exposed to ambient air,

therefore some of the heat is also absorbed by ambient air natural convection.

Fig. 20. Tube-in-tube condenser.

The cold water in the inner tubes may flow in either direction. When the water

enters at bottom and flows in the direction opposite to the refrigerant, it is said to be a

63

counter flow system. On the other hand, when the water enters at the top and flows in

the same direction as refrigerant, it is said to be a parallel flow system.

The counter-flow system is preferred in all types of water cool condensers

because it gives high rate of heat transfer. Since the coldest water is used for final

cooling of the liquid refrigerant and the warmest water absorbs heat from the hottest

vapour refrigerant, the temperature difference between the water and refrigerant remains

fairly constant throughout the condenser. In case of parallel flow system, as the water

and refrigerant flow in the same direction, the temperature difference between them

increases. Thus the ability of water to absorb heat decreases at it passes through the

condenser.

2. Shell and coil condensers: A shell and coil condenser consists of one or

more water coils enclosed in a welded steel shell (Fig. 21). Both the finned and bare coil

types are available. It may be either vertical or horizontal. In vertical type of condenser,

the hot vapour refrigerant enters at the top of the shell and surrounds the water coils. As

the vapour condenses, it drops to the bottom of the shell which often serves as a

receiver. Most vertical type shell and coil condensers use counter flow water system as it

is more efficient than parallel flow water system.

Fig. 21. Shell-coil condenser.

In the shell and coil condensers, coiled tubing is free to expand and contract with

temperature changes because of its spring action and can withstand any strain caused

by temperature changes. Since the water coils are enclosed in a welded steel shell, the

mechanical cleaning of these coils is not possible. The coils are cleaned with chemicals.

They are used for units upto 50 tonnes capacity.

64

3. Shell and tube condensers: It consists a cylindrical steel shell containing a

number of straight water tubes (Fig. 22). The tubes are expanded into grooves in the

tube sheet holes to form a vapour- tight fit. The tube sheets are welded to the shell at

both the ends. The removable water boxes are bolted to the tube sheet at each end to

facilitate cleaning of the condenser.

Fig. 22. Shell-tube condenser.

The condenser tubes are made either from steel or copper, with or without fins.

The steel tubes without fins are usually used in ammonia refrigerating systems because

ammonia corrodes copper tubing.

In this type of condenser, the hot vapour refrigerant enters at the top of the shell

and condenses as it comes in contact with water tubes. The condensed liquid refrigerant

drops to the bottom of the shell which often serves as a receives

65

Lecture No. 20

Fouling Factor:

The water used in water cooled condensers always contain a certain amount of

minerals and other foreign materials, depending upon its source. These materials form

deposits inside the condenser water tubes. This is called water fouling. The deposits

insulate the tubes, reduce their heat transfer rate and restricts the water flow. The fouling

factor is the reciprocal of heat transfer coefficient for the material of scale.

1. For copper tubes used for R-12 and R-22 condensers, the fouling factor is

0.000 095 m2s K/ J.

2. For steel tubes used in ammonia condensers, the fouling factor is

0.000 18 m2s K/J.

Table 6. Comparison of air cooled and water cooled condensers

Air cooled condenser Water cooled condenser1. As the construction of air cooled

Condenser is very simple, the initial

cost is less. The maintenance cost is

also low.

2. There is no handling problem with

air cooled condensers.

3. The air cooled condensers do not

require piping arrangement for carrying

the air.

4. There is no problem in disposing of

used air.

5. Since there is no corrosion, The fouling

effect is low.

6. They have low heat transfer capacity

due to low thermal conductivity of air.

1. As the construction of water cooled

condenser is complicated, the initial cost

is high. The maintenance cost is also

high.

2. The water cooled condensers are

difficult to handle.

3. The pipes are required to take water to

take water to and from the condenser.

4. There is a problem of disposing the

used water unless a recirculation

system is provided.

5. Since corrosion occurs inside the tubes

carrying the water, the fouling effects

are high.

6. They have high heat transfer capacity

due to high thermal conductivity of

Fouling factor – Difference between air cooled and water cooled condensers – Evaporative condenser

66

7. They are used for low capacity plants

(< 5 TR )

8. The fan noise becomes objectionable.

9. The distribution of air on condenser

surface is not uniform.

10. They have high flexibility.

water.

7. They are used for large capacity plants.

8. There is no fan noise.

9. There is even distribution of water on

the condensing surface.

10. They have low flexibility.

Evaporating condensers: It uses both air and water as condensing mediums to

condense the hot vapour refrigerant to liquid refrigerant. These condensers (Fig. 23)

perform the combined functions of a water cooled condenser and a cooling tower.

Fig. 23. Evaporative condenser.

During operation, the water is pumped from the sump to a spray header and

sprayed through nozzles over the condenser coils through which the hot vapour

refrigerant from the compressor is passing. The heat transfers from the refrigerant

through the condensing tube walls and into the water that is wetting the outside surface

of the tubes. At the same time, a fan draws air from the bottom side of the condenser

and discharged out at the top of the condenser.

The air causes the water from the surface of the condenser coils to evaporate

and absorb the latent heat of evaporation from the remaining water to cool it. Though

most of the cooling takes place by evaporation, the air can also absorb some sensible

67

heat from water. Since the heat for vaporising the water is taken from the refrigerant the

vapour refrigerant condenses into a liquid refrigerant.

The cold water that drops down into a sump is recirculated. In order to make up

the deficiency caused by the evaporated water, additional water is supplied to the sump.

A float valve in the sump controls the make-up supply. The eliminator is provided above

the spray header to stop particles of water escaping along with the discharge air.

68

Lecture No. 21

Introduction:

The expansion device (also known as metering device or throttling device) is an

important device that divides the high pressure side and the low pressure side of a

refrigerating system. It is connected between the receiver (containing liquid refrigerant at

high pressure) and the evaporator (containing liquid refrigerant at low pressure). The

expansion device performs the following functions.

1. It reduces the high pressure liquid refrigerant to low pressure liquid refrigerant

before being fed to the evaporator.

2. It maintains the desired pressure difference between the high and low pressure

sides of the system, so that the liquid refrigerant vaporises at the designed

pressure in the evaporator.

3. It controls the flow of refrigerant according to the load on the evaporator.

Note: The expansion devices used with dry expansion evaporators are usually called

expansion valves where as the expansion devices used with flooded evaporators are

known as float valves.

Types of Expansion Devices:

Following are the main types of expansion devices used in industrial and

commercial refrigeration and air conditioning system.

1. Capillary tube

2. Hand-operated expansion valve

3. Automatic or constant pressure expansion valve

4. Thermostatic expansion valve

5. Low side float valve and

6. High side float valve

Expansion Devices – Types of expansion devices – Capillary tube

69

Capillary Tube:

The capillary tube (Fig. 24) is used as an expansion device in small capacity

hermetic sealed refrigeration units such as in domestic refrigerators, water coolers, room

air conditioners and freezers. It is a copper tube of small internal diameter and of varying

length depending upon the application. The inside diameter of the tube used in

refrigeration work is generally about 0.5 mm to 2.25 mm and the length varies from 0.5

m to 5 m. It is installed in the liquid line between the condenser and the evaporator. A

fine mesh screen is provided at the inlet of the tube in order to protect it from

contaminants. In its operation, the liquid refrigerant from the condenser enters the

capillary tube. Due to frictional resistance offered by a small diameter tube, the pressure

drops. Since the frictional resistance is directly proportional to the length and inversely

proportional to the diameter, more pressure drop will be created for the long capillary

tube once selected for a given set of conditions and load can not operate efficiently at

other conditions.

Fig. 24. Capillary tube.

The refrigeration system using capillary tube has the following advantages.

1. The cost of capillary tube is less than all other forms of expansion devices.

2. When the compressor stops, the refrigerant continues to flow into the

evaporator and equalizes the pressure between the high side and low side of the

system. This considerably decreases the starting load .on the compressor. Thus a low

starting torque motor (low cost motor) can be used to drive the compressor.

3. Since the refrigerant charge in a capillary tube system is critical, no receiver is

necessary.

70

Lecture No. 22

Hand-operated Expansion Valve:

The hand-operated expansion valve (Fig. 25) is the most simple type of

expansion valve but it requires an operator to regulate the flow of refrigerant to the

evaporator manually. The conical-shaped needle valve extends down into the valve port

and restricts the flow area through the port. When closed, the valve rests on its conical

seat. This valve is limited to systems operating under nearly constant loads for long

periods of time, such as in ice making plants and cold storages.

Fig. 25. Hand operated expansion valve.

Low-side Float Valve:

Low side float valve (Fig. 26) is located in the low pressure side (i.e. between the

evaporator and compressor suction line) of the refrigeration system. It maintains a

constant level of liquid refrigerant in the evaporator and float chamber by opening and

closing a needle valve.

Hand operated expansion valve – Low side float valve – High side float valve

71

Fig. 26. Low-side float Valve.

The float valve is a hollow ball attached to one end of a float arm is connected to

a needle valve. The movement of the float valve is transmitted to the needle valve by the

float arm which closes or opens the flow of liquid refrigerant.

When the liquid refrigerant in the evaporator vaporises, its level falls down. This

causes the float to drop and thus opens the needle valve, thereby allowing liquid

refrigerant from the liquid line to the float chamber and then to the evaporator to make

up for the amount of vaporisation. When the desired liquid level is reached, the float

rises and closes the needle valve. The major advantage of the low-side float valve is that

it maintains a constant liquid level in the evaporator under all loading conditions

regardless of the evaporator pressure and temperature.

High-side Float Valve:

The high side float valve (Fig. 27) located on the high pressure side (i.e. between

the condenser and evaporator) of the refrigeration system. It controls the flow of liquid

refrigerant to the evaporator according to the load and maintains a constant liquid level

in the evaporator and float chamber by opening or closing of a needle valve.

The liquid refrigerant from the condenser flows to the float chamber. As the level

of liquid refrigerant in the float chamber rises, the float ball also rises, thereby opening

the needle valve. This allows the liquid refrigerant to flow into the evaporator. when the

liquid level in the float chamber falls down, the float ball also drops, thereby closing the

needle valve.

72

Fig. 27. High-side float Valve.

The condenser supplies the liquid refrigerant at the same rate as it evaporates in

the evaporator. Since the rate of vaporisation depends upon the load on the evaporator,

the high side floats valve functions according to the load. The high side float valve may

be used with dry expansion evaporators.

73

Lecture No. 23

Ice Manufacture:

The commercial ice is produced by freezing potable water in standard cans

placed in rectangular tanks. The tanks are filled with chilled brine. For increasing the

heat transfer from the water in the can to the chilled brine, the brine solution is kept in

constant motion by agitators. The agitators can be either horizontal or vertical and are

operated by means of electric motors. The brine temperature is maintained by the

refrigeration plant at –l0°C to -11°C. The ammonia gas is used as the refrigerant

because of its excellent thermal properties. It also produces very high refrigerating effect

per kg of refrigerant and low specific volume of the refrigerant in vapour state.

The high temperature, high pressure ammonia vapours are condensed in a

condenser which may be of shell and tube type or evaporative type. The condensed

liquid ammonia is collected in the receiver and then expanded through the expansion

valve. Due to the expansion, the pressure of the liquid ammonia is considerably

reduced. It then passes through the evaporator coils surrounding a brine tank in which

brine solution is filled. The low pressure liquid ammonia absorbs heat from the brine

solution, equivalent to its latent heat of vaporisation, gets converted to vapour state and

is once again fed to compressor to complete the cycle. The layout of an ice

manufacturing system is shown in Fig. 28.

The brine tanks are usually fabricated of 6 mm thick mild steel plates with tie rods

welded end to end. The depth of tank is such that the brine level is around 25 mm higher

than the water level in the cans. The tank is insulated on all the four sides and from the

bottom. The insulated wooden lids are provided to cover the top insegments, to facilitate

the removal or replacing of ice-cans. The ice-cans are fabricated from galvanized steel

sheets and are given chromium treatment to prevent corrosion due to chemical

reactions. It also prevents corrosion from reaction with ammonia (NH3) in case it leaks

from the system.

Ice manufacturing – principle of ice production

74

Fig. 28. Ice plant.

In order to get clear transparent ice, water in the can is agitated by the use of low

pressure air, through the tubes suspended from the top. Due to agitation, the dissolved

impurities such as salts, even colours get collected in the unfrozen water core. It is

desirable that it should be taken out and replaced with fresh water.

It may be noted that the ice of potable water (treated or untreated) frozen at a

temperature lower than -12°C can crack. Therefore, brine temperature is kept at a higher

level, say _11°C to _10°C. Water in the ice cans placed in the brine cools rapidly up to a

temperature of about 3°C to 4°C. Thereafter, it takes more time for water to touch 0°C.

The rate of freezing decreases substantially as the thickness of the ice layer increases.

This is because of the reason that the ice layer offers more thermal resistance to the

heat flow from the water to the brine.

Notes: 1. Generally sodium chloride (NaCl) or calcium chloride (CaCl2) is used as brine.

Due to its low cost and being less injurious, sodium chloride brine is preferably

used.

2. The freezing time is dependent upon the brine temperature and the extent of

brine and water agitation.

75

Cooling of Milk (Milk Processing):

The milk is known to be one of the most perishable foods and if it is not

maintained at sufficient low temperature, it gets spoiled due to the growth of bacteria and

other organisms. Some kind of bacteria rapidly multiplies in number at temperatures of

21°C to 38°C (294 K to 311 K) and the milk gets spoilt and sour. As the temperature of

the milk is reduced, the bacterial growth decreases and practically ceases at 0°C to 5°C

(273 K to 278 K), though the bacilli are not killed even at very low temperatures. The

bacterial content can be eliminated to a great extent by heating the milk to 62°C (335 K)

and holding it at that temperature for about 30 minutes. Thereafter to minimize the

bacterial growth and preservation, the milk is cooled to 4°C to 5°C (277 K to 278 K). This

process of heating and immediately cooling the milk for controlling the bacterial growth is

known as pasteurization.

The pasteurization is generally done in a batch type process. In this process, the

raw milk is heated by hot water or steam to 62°C. In case the heating is done by hot

water, it is sprayed around the outside lining of the vat by a distributor, which gets

collected in a sump at the bottom of the vat, reheated and once again sprayed. The

steam heating is carried out by flowing steam in the space provided between lining and

the casing of the vat. The heated milk is then cooled, first by the cooling tower water and

then by the chilled water or brine to 4°C to 5°C, which is the desired temperature for

filling the milk in bottles. The heating and cooling is done by passing the milk (by milk

pumps) through the heat exchange plates. The milk flows between the two plates and

the hot water, cooling tower water or chilled water (brine) is circulated through alternate

pairs of plates. The direction of flow of heating or cooling fluids is opposite to that of milk

to obtain better heat transfer.

In order to control the fat content of milk, it is desired to churn the milk. Such milk

is retailed as 'toned milk'. The fat thus removed is processed as butter and stored at 4°C

to 5°C. The cheese is another product from milk and is stored at about 4°C.

The ice-cream is another product, which is manufactured by using milk fat along

with sugar and other ingredients of ice-cream. The mixture is pasteurized to a

temperature of 70°C to 80°C (343 K to 353 K) and homogenized. The homogenization is

the process of breaking down the fat and solid content of the mixture to a very finely

divided state to avoid any lump formation while freezing the mixture. The plate type heat

76

exchangers are used for heating and cooling the liquid mix to 5°C. Then the mixture is

frozen in ice-cream freezers. During freezing, the ice-cream is whipped to mix certain

amount of air. Generally the ice-cream is frozen to about - 2.5°C to - 5°C (270.5 K to 268

K), depending upon the type of mixture used. Thereafter, it is hardened in the hardening

rooms to a temperature of -18°C (255 K). The hardening rooms are maintained at about

-30°C to -35°C (243 K to 238 K) for rapid freezing of the balance water content in the

ice-cream. The rapid freezing ensures small size of the ice crystals.

77

Lecture No. 24

Applications of refrigeration in different food products:

The most significant application of refrigeration is in food preservation, whether it

is by way of processing or for storage. Processing is done by heating, heat drying, etc.,

and by refrigeration such as in chilling, freezing or freeze-drying. Storage may be of

either chilled or frozen product. Some of the important products involved in processing

are candy, beverages, meat, poultry, fish, bakery and dairy products, fruits and

vegetables, fruit-juice concentrates, precooked foods, etc.

The common products preserved by storage after chilling are fruits such as

apples, pears, grapes, citrus fruits etc. vegetables such as onions, potatoes, tomatoes,

etc. and dry fruits, candies, milk, eggs and their products. Storage under frozen

conditions is resorted to for preserving the food value as well as to store perishable

products over a long period. The common items of frozen food are fish, meat, poultry,

and some vegetables such as peas, beans, carrots, cauliflower etc. It may be noted that

all products are not amenable to freezing.

An interesting feature of the chilled and frozen food industry is the cold chain that

must be maintained from the form to the consumer. An important link in this chain is that

of transport refrigeration. It must be noted that processing by simple heat drying is a

more economical way of food preservation. However, with many products, it may involve

loss of quality in which case it is not considered suitable.

Application of refrigeration in different food products

78

Lecture No. 25

Examples of Food processing by refrigeration and storage:

Fruits and vegetables:

The rapid pre – cooling after harvesting increases the storage life considerably.

Futher, controlled atmosphear (CA) storage greatly reduces the respiration rate and

retards the deterioration of the product, thus increasing its storage temperature. A

controlled atmosphere aims at maintaining the oxygen level at a low value of 3-5

percent.

A few vegetables such as peas, beans, carrots and cauliflower are also found

suitable for freezing. They are partially cooked before freezing. Quick freezing is not so

critical in the case of vegetables. Never less, air blast freezers are commonly used for

individual freezing is also being resorted to on a small scale packaged products.

De hydro – freezing is another process that has been found successful in the

preservation of some products such as apples, apricots, peas etc. this involves partial

dehydration, and then freezing. The process retains the advantages of both, without the

disadvantages of either. One clear advantage is the reduced refrigeration load during

freezing. Another is the reduced weight of the product for transport. In simple de

hydration, there is a marked deterioration in quality during the last stages of drying. This

is eliminated in de hydro – freezing.

Examples of Food processing by refrigeration and storage –Fruits and vegetables

79

Lecture No. 26

Examples of Food processing by refrigeration and storage:

Meat Products:

Ammonia-refrigeration systems are commonly used for the chilling of carcass

and holding them in rooms. The cooling is by chilled-brine spray or sprayed coil.

In the case of brine, a 20 percent solution of NaCI is chilled to a temperature -7

to -9°C. The brine absorbs heat and moisture from the air. The diluted brine leaving the

room must be strengthened regularly. The majority of installations these days use a

sprayed-coil system with air being cooled with brine sprayed over a direct expansion

coil.

The average specific heat of carcass is 3 kJ/kg.K. The carcass should be

reduced to a temperature of 1.5°C in a short time after slaughter. The room temperature

could be as low as - 3.5°C. Refrigeration is also required for chilling water for washing

the product.

More and more meat products are, however, being preserved by freezing. The

processing steps before freezing involve cutting, chipping, grinding, forming, pressing

and slicing. The recommended storage temperature is -29°C for a storage life of 12

months or more for beef, lamb, pork, etc. At -23°C, a reasonable length of storage

between 8 to 18 months is possible. The product quality is seriously impaired if the

temperature goes up to - 9°C at any stage.

Fishery Products:

Refrigeration is required in many ways in the processing, preservation and

transport of fresh and frozen fish and their products. Care starts from the stage of

catching in fishing boats and trawlers.

Icing of fish is first done to chill it to a temperature of 0°C. The melt-water also

helps to wash off the slime and bacteria. Often the fish are stored in tanks using

refrigerated sea water at -1°C. It is found that shrimp when stored with refrigerated sea

Examples of Food processing by refrigeration and storage - Meat products – fish – poultry

products – dairy products

80

water is superior to that stored with ice. The storage life of chilled fish is normally 10-15

days.

The valued varieties of fish that are frozen include salmon, shrimp, shell fish, etc.

Individual Quick Freezing (IQF) techniques are preferred for these products. Quick

freezing has many advantages. It prevents bacterial spoilage and ensures rapid

handling, packaging and good appearance of the product, and also makes optimum

utilization of the freezer space. Quick-freezing is achieved by using low refrigeration

temperatures.

Contact plate freezers are suitable with large size products. Blocks of fish are

frozen in multilayered plates in contact with primary or secondary refrigerant. For

obtaining individual freezing of small-sized fish, such as shrimp, two methods are

employed. Air-blast freezers use air at -29 to -40°C flowing at 2.5 m/s or more.

Immersion freezers make use of liquid Freon or even liquid nitrogen at atmospheric

pressure. The freezing time depends on the water content and thermo physical

properties of fish. It also depends on the refrigeration temperature. Frozen fish is

normally stored at a temperature of -23°C or lower.

Poultry Products:

Poultry meat is preserved both by chilling and freezing. Chilling requires a lower

capital investment where as freezing offers quality and flexibility of operation.

A major fraction of poultry meat is processed and transported by liquid chilling by

the use of flake ice. Air chilling results in a considerable amount of dehydration at the

surface. The approximate time required for chilling to 7°C is about 115 minutes for

dressed birds, 50 minutes for eviscerated birds and 25 minutes for cut-up ones.

As for freezing, the use of air-blast tunnels operating at air temperatures of -29 to

-40°C and air velocities of 2.5 m/s or more is recommended. This ensures rapid freezing

which is essential to obtain good quality and appearance.

Cooling is also required in poultry farms. A temperature higher than 29°C results

in reduced egg production and weight and thinner shells. A temperature of 24°C is

recommended for poultry-keeping.

81

Dairy Products:

Most dairy plants pre-cool milk to 2.5°C before processing for the separation of

cream and blending for producing standardized quality. There are hot as well as cold,

milk separators. In the latter, the separation process takes place at a temperature of

4°C. After separation and blending, the milk is sent for pasteurization which is

accomplished by heating followed by the chilling of milk. Heating is done either by hot

water or steam and holding the milk at 62°C for about 30 minutes. The milk is then

cooled first by water from a cooling tower and then by chilled water. The rate of, flow of

chilled water should be such that its rise in temperature is not more than 5.5°C. When a

direct-expansion system is used for cooling, the refrigerant flow is controlled by an

automatic expansion valve so that the temperature does not drop below -2°C lest the

milk may freeze. Water-chilling equipment normally consists of an ice-bank type water

chiller consisting of an insulated tank with water, direct expansion coil and pump. This

system ensures a constant temperature of chilled water of 0-1°C. Also, it takes care of

load fluctuations by allowing excess refrigeration to be stored in the form of ice on

immersed coils.

Refrigeration is also required for cooling the cream before churning and the

mechanical separation of butter. For the storage of butter, a temperature of 0 to 4°C is

satisfactory. But for storage for 6 months or more, a temperature of -23 to -18°C must be

used. This is the same as for some frozen products. Thus butter can be kept in freezer

compartment of refrigerators for storing beyond a period of six months or more.

Ice-cream manufacture is another industry that makes great use of refrigeration.

The ice-cream freezer has to whip in a certain amount of air in the mix, simultaneously

with freezing. The product is 0 to 100% frozen between the temperature ranges of

-2.5 to - 55°C approximately.

82

Lecture No. 27

Food Freezing:-

Preservation of a food by freezing occurs by several mechanisms. At

temperatures below 0°C there is a significant reduction in growth rates for micro

organisms and in the corresponding deterioration of the product due to microbial activity.

The same temperature influence applies to most other reactions that might normally

occur in the product, such as enzymatic and oxidative reactions. In addition, the

formation of ice crystals within the product changes the availability of water to participate

in reactions. As the temperature is reduced and more water is converted to a solid state,

less water is available to support deteriorative reactions.

Although freezing as a preservation process generally results in a high-quality

product for consumption, the quality is influenced by the freezing process and frozen-

storage conditions. The freezing rate or time allowed for the product temperature to be

reduced from above to below the initial freezing temperature will influence product

quality, but in a variable manner depending on the food commodity.

For some products, rapid freezing (short freezing time) is required to ensure

formation of small ice crystals within the product structure and minimal damage to the

product texture. Other products are not influenced by structural changes and do not

justify the added costs associated with rapid freezing. Still other products have

geometric configurations and sizes that do not allow rapid freezing. The storage-

temperature conditions influence frozen food quality in a significant manner. Any

elevation in storage temperature tends to reduce the preservation quality of the process,

and fluctuations in storage temperature tend to be even more detrimental to product

quality.

It is obvious that the optimum freezing process depends on the product

characteristics. The importance of residence time in the freezing system must be

emphasized, as well as the need for accurate freezing time prediction.

Food Freezing – Freezing systems – Indirect contact systems

83

Freezing system:

To achieve freezing of a food product, the product must be exposed to a low-

temperature medium for sufficient time to remove sensible heat and latent heat of fusion

from the product. Removal of the sensible heat and latent heat results in a reduction in

the product temperature as well as a conversion of the water from liquid to solid state

(ice). In most cases, approximately 10% of the water remains in the liquid state at the

storage temperature of the frozen food.

To accomplish the freezing process in desired short times, the low-temperature

medium is at much lower temperature than the desired final temperature of the product,

and large convective heat-transfer coefficients are created.

The freezing process can be accomplished by using either indirect or direct

contact systems. Most often, the type of system used will depend on the product

characteristics, both before and after freezing is completed. There are a variety of

circumstances where direct contact between the product and refrigerant is not possible.

Indirect Contact Systems:

In numerous food-product freezing systems, the product and refrigerant are

separated by a barrier throughout the freezing process. This type of system is illustrated

schematically in Fig. 29. Although many systems use a nonpermeable barrier between

product and refrigerant, indirect freezing systems include any system without direct

contact, including those where the package material becomes the barrier.

Fig. 29. Indirect contact freezing system.

84

Lecture No. 28

Plate Freezers:

The product is frozen while held between two refrigerated plates. In most cases,

the barrier between product and refrigerant will include both the plate and package

material. The heat transfer through the barrier (plate and package) can be enhanced by

using pressure to reduce resistance to heat transfer across the barrier (Fig. 30). In some

cases, plate systems may use single plates in contact with the product and accomplish

freezing with heat transfer across a single package surface. As would be expected,

these systems are less efficient, and they are costly to acquire and operate.

Fig. 30. Plate freezing system.

Plate-freezing systems can be operated as a batch system with the product

placed on the plates for a specified residence time before being removed. In this

situation, the freezing time is the residence time and represents the total time required to

reduce the product from the initial temperature to some desired final temperature. In

general, the batch plate-freezing system has significant flexibility in terms of handling

diverse product types and product sizes.

The plate-freezing system operates in a continuous mode by moving the plates

holding the product through an enclosure in some prescribed manner. the product is

held between two refrigerated plates throughout the freezing process. The movement of

Plate Freezers – air blast Freezers – Freezers for liquid foods

85

plates (and product) occurs as the plates index upward or across within the

compartment. At the entrance and exit to the freezing system, the plates are opened to

allow the product to be conveyed to or from the system. In a continuous plate-freezing

system, the freezing time is the total time required for the product to move from entrance

to exit. During the residence time, the desired amounts of sensible and latent heat are

removed to achieve the desired frozen product temperature.

Air-Blast Freezers:

In many situations, the product size and/or shape may not accommodate plate

freezing. For these situations, air-blast freezing systems become the best alternative. In

some cases, the package film is the barrier for the indirect freezing, with cold air being

the source of refrigeration.

Air-blast freezers can be a simple design, as in the case of a refrigerated room.

In this situation, the product is placed in the room, and the low-temperature air is allowed

to circulate around the product for the desire residence of freezing time. This approach

represents a batch mode, and the refrigerated room may act as a storage space in

addition to the freezing compartment. In most cases, freezing times will be long because

of lower air speeds over the product, inability to achieve intimate contact between

product and cold air, and the smaller temperature gradients between product and air.

Most air-blast freezers are continuous, In these systems, the product is carried

on a conveyor that moves through a stream of high-velocity air. The length and speed

of , the conveyor establish the residence of freezing time. These times can be relatively

small based on the use of very low-temperature air, high air velocities, and good contact

between individual product packages and the cold air.

Continuous air-blast freezing systems use a variety of different conveying

arrangements such as tray conveyors, spiral conveyors, and roller conveyors. Most

often, the system used will depend on product characteristics.

Freezers for Liquid Foods:

The third general type of indirect freezing systems includes those designed

primarily for liquid foods. In many situations, the most efficient removal of thermal energy

from a liquid food can be accepted, the most common type is a scraped-surface system

(Fig. 31).

86

Fig. 31.Scrapped-surface heat exchanger

The food contact areas of a scraped – surface cylinder are fabricated from

stainless steel, pure nickel, hard chromium plated nickel, or other corrosion resistant

material. The inside rotor contains blades that are covered with plastic laminate or

molded plastic. The rotor speed varies between 150 and 500 rpm. Although higher

rotation speed allows better heat transfer, it may affect the quality of the process product

by possible maceration, thus we must carefully select the rotor speed and the and the

annular space between the rotor and the cylinder for the product being processed.

The cylinder containing the product and the rotor is enclosed in an outside jacket.

The cooling medium is supplied to the out side jacket. Commonly used media include

brine or a refrigerant such as Freon. The constant blending action accomplished the

scraped surface heat exchanger is often desirable to enhance the uniformity of product

flavour colour, aroma and textual characteristics.

The heat exchangers for freezing liquid foods are designed specifically for

freezing, with the heat-exchange shell surrounding the product compartment becoming

an evaporator for a vapour compression refrigeration system.

For freezing liquid foods, the residence time in the freezing compartment is

sufficient to decrease the product temperature by several degrees below the

temperature of initial ice-crystal formation. At these temperatures, between 60 and 80%

of the latent heat has been removed from the product, and the product is in the form of a

87

frozen slurry. In this condition, the product flows quite readily and can be placed in a

package for final freezing in a low-temperature refrigerated space. The scraped-surface

heat exchanger ensures efficient heat exchange between the slurry and the cold

surface.

Freezing systems for liquid foods can be batch or continuous. The batch system

places a given amount of unfrozen liquid in the compartment and allows the freezing

process to continue until the desired final temperature is reached. The product

compartment is a scraped-surface heat exchanger but is operated as a batch system. In

the case of ice-cream freezing, the system is designed with facility for injection of air into

the frozen slurry to achieve the desired product consistency.

In a continuous freezing system for liquid foods, the basic system is a scraped

-surface heat exchanger using refrigerant during phase change as the cooling medium.

The rotor acts as a mixing device, and the scraper blades enhance heat transfer at the

heat-exchange surface.

88

Lecture No. 29

Direct-Contact Systems:

Several freezing systems for food operate with direct contact between the

refrigerant and the product as shown in Fig. 32. In most situations, these systems will

operate more efficiently since there are no barriers to heat transfer between the

refrigerant and the product. the refrigerants used in these systems may be low-

temperature air at high speeds or liquid refrigerants with phase change while in contact

with the product surface. In all cases, the systems are designed to achieve rapid

freezing, and the term individual quick freezing (IQF) will apply.

Fig. 32. Direct contact freezing system.

Air Blast:

The use of low-temperature air at high speeds in direct contact with small product

objects is a form of IQF. The combination of low temperature air, high convective heat-

transfer coefficient (high air speed), and small product shape leads to short freezing time

or rapid freezing. In these systems, the product is moved through the high-speed-air

region on a conveyor in a manner that controls the residence time. The types of product

that can be frozen in these systems are limited to those that have the appropriate

geometries and that require rapid freezing for maximum quality.

Direct contact systems – Air blast - Immersion

89

Immersion:

By immersion of the food product in liquid refrigerant, the product surface is

reduced to a very low temperature. Assuming the product objects are relatively small,

the freezing process is accomplished rapidly or under IQF conditions. For typical

products, the freezing time is shorter than for the air-blast or fluidized-bed systems.

Fig. 33. Immersion freezing system.

As illustrated in Fig. 33, the product is carried into a bath of liquid refrigerant and

is conveyed through the liquid while the refrigerant changes from liquid to vapour and

absorbs heat from the product. The most common refrigerants for this purpose are

nitrogen and carbon dioxide.

One of the major disadvantages of immersion-type freezing systems is the cost

of the refrigerant. Since the refrigerant changes from liquid to vapour while the product

freezes it becomes difficult to recover the vapours leaving the freezing compartment.

These refrigerants are expensive, and the overall efficiency of the freezing system is a

function of the ability to recover and reuse the vapours produced in the freezing

compartments.

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Lecture No. 30

Frozen food properties:

The freezing process has a dramatic influence on the thermal properties of the

food product. As the water within the product changes from liquid to solid, the density,

thermal conductivity, heat content (enthalpy), and apparent specific heat of the product

change gradually as the temperature decreases below the initial freezing point for water

in the food.

Density:

The density of solid water (ice) is less than the density of liquid water. Similarly,

the density of a frozen food will be less than the unfrozen product. The gradual change

in density is due to the gradual change in the proportion of water frozen as a function of

temperature as shown in Fig. 34. The magnitude of change in density is proportional to

the moisture content of the product.

Fig. 34. Influence of freezing on predicted density of straw-berries.

Frozen food properties – Density – Thermal conductivity – Enthalpy – Apparent specific heat – Apparent thermal diffusivity

91

Thermal Conductivity:

The thermal conductivity of ice is approximately a factor of four larger than the

thermal conductivity of liquid water. This relationship has a similar influence on the

thermal conductivity of a frozen food. Since the change of phase for water in the product

is gradual as temperature decreases, the thermal conductivity of the product changes in

a manner illustrated in Fig. 35. The majority of the thermal conductivity increase occurs

within 10°C below the initial freezing temperature for the product. If the product happens

to contain a fibrous structure, the thermal conductivity will be less when measured in a

direction perpendicular to the fibers.

Fig. 35. Thermal conductivity of frozen lean beef as a function of temperature

Enthalpy:

The heat content or enthalpy of a frozen food is an important property in

computations of refrigeration requirements for freezing the product. The heat content is

normally zero at -40°C and increases with increasing temperature in a manner illustrated

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in Fig. 36 Significant changes in enthalpy occur at 10°C just below the initial freezing

temperature, when most of the phase change in product water occurs.

Fig. 36. Enthalpy of sweet cherries as a function of temperature.

Apparent Specific Heat:

Based on the thermodynamic definition of specific heat, the profile of apparent

specific heat for a food product as a function of temperature would appear as in the

Fig. 37. This illustration reveals that the specific heat of a frozen food at a temperature

greater than 20°C below the initial freezing point is not significantly different from the

specific heat of the unfrozen product. The apparent specific heat profile clearly illustrates

the range of temperature where most of the phase changes for water in the product

occur.

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Fig. 37. Predicted apparent specific heat of frozen sweet cherries as a

function of temperature.

Apparent Thermal Diffusivity:

When the density, thermal conductivity, and apparent specific heat of a frozen

food are combined to compute apparent thermal diffusivity, the profile in Fig. 38 is

obtained. The relationship illustrates that apparent thermal diffusivity increases gradually

as the temperature decreases below the initial freezing point. The magnitudes of the

property for the frozen product are significantly larger than for the unfrozen food.

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Fig. 38. Relationship between thermal diffusivity and temperature during

freezing of food products.

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Lecture No. 31

Freezing time:

A key calculation in the design of a freezing process is the determination of

freezing time. Three distinct periods are noticeable at any location within a food

undergoing freezing: prefreezing, phase change, and post freezing.

Consider a simple experiment that illustrates these three periods. First, we

measure temperature change in freezing pure water into ice by placing an ice-cube tray

filled with water in the freezer section of a home freezer, with a thermocouple located

inside the tray.

In the second part of the experiment, we measure temperature in a small stick of

potato (ex. a French fry) placed in a freezer with a thermocouple embedded inside the

potato stick. The temperature-time plots for water and potato obtained in these

experiments will be similar to those shown in Fig. 39.

Fig. 39. Freezing diagram of water and a food material.

During the precooling period, the temperature of water decreases to the freezing

point as sensible heat is removed. The temperature plot shows a small amount of super

cooling (below 0°C); once nucleation occurs and ice crystals begin to form, the freezing

point increases to 0°C. The temperature remains at the freezing point until a complete

Freezing time – Factors influencing freezing time – Freezing rate – Thawing time

96

phase change occurs as latent heat of fusion is removed from liquid water to convert it

into solid ice. When all the liquid water has changed into solid ice, the temperature of the

ice decreases rapidly as sensible heat is removed during the post freezing period.

For the potato, the temperature plot obtained during freezing is similar to that of

water but with important differences. Like water, the temperature decreases during

precooling as sensible heat is removed. However, the temperature at which initial

nucleation occurs and ice crystals begin to form is lower than that of water due to the

presence of solutes in the food. After a brief period of super cooling , latent heat is

gradually removed with decreasing temperature. This deviation in temperature profile

from that of pure water is the result of the concentration effect during freezing of foods.

As water in the food converts into ice, the remaining water becomes more concentrated

with solutes and depresses the freezing point. This gradual change in temperature with

the additional removal of latent heat continues until the food is largely a mix of the initial

solid food components and ice. After this time, mostly sensible heat is removed until

some final, preselected, endpoint temperature is reached.

Typically, fruits and vegetable are frozen to lower temperatures of around -25°C.

The Conclusion of the experiment is:

1) Freezing involves removal of both sensible and latent heat.

2) Freezing of pure water exhibits sharp transitions between the different freezing

periods, where as with foods, the transitions are more gradual.

3) At the end point temperature for freezing foods, the frozen food may still have

some water may be in liquid state for foods fact, up to 10 percent water may

be in liquid state for foods frozen to - 18°C. This highly concentrated unfrozen

water may play an important role in determining the storage of frozen foods.

Freezing time is the most critical factor associated with the selection of a freezing

system to ensure optimum product quality. Freezing – time requirements Help establish

the system capacity.

Freezing time for foods can be predicted by two methods.

1) By planks equation

2) Pham’s method

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Factors Influencing Freezing Time:

One of the first factors is the freezing-medium temperature, where lower

magnitudes will decrease freezing time in a significant manner. The product size

influences freezing time, and the effect can be quite significant.

The factor with the most significant influence on freezing time is the convective

heat-transfer coefficient. This parameter can be used to influence freezing time through

equipment design and should be analyzed carefully. At low magnitudes of the convective

heat-transfer coefficient, small changes will influence the freezing time in a significant

manner. Product properties (limited freezing point of the material, density of the frozen

material, thermal conductivity of the frozen material) will influence freezing–time

predictions.

Freezing Rate:

The freezing rate (°C/h) for a product or a package is defined as the difference

between the initial and the final temperature divided by the freezing time. Since the

temperature at different locations of a product may vary during freezing, a local freezing

rate is defined for a given location in a product as the difference between the initial

temperature and the desired temperature, divided by the time elapsed until the moment

at which the desired temperature is achieved at that location.

Thawing Time:

Industrial processes, frozen foods may be thawed for additional processing.

Although freezing and thawing processes have some similarities in that both involve

phase change processes, there are also number of differences. For example, surface

boundary conditions during thawing become complicated due to the formation and later

melting of frost on the surface.

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Lecture No. 32

Quality Changes in Foods during Frozen Storage:

A commonly used descriptor for the storage life of frozen foods is practical

storage life (PSL). The practical storage life of a product is the period of frozen storage

after freezing during which the product retains its characteristic properties and remains

suitable for consumption or other intended process (International Institute of

Refrigeration (IIR), 1986).

The shelf life of frozen fish is considerably less than that of most other

commodities. The typical temperature used for storing frozen foods in the commercial

food chain is -18°C. However, for seafood’s, lower storage temperatures may be more

desirable for retaining quality.

Another term used to describe the storage life of frozen foods is the high-quality

life (HQL). As defined by IIR (1986), the high-quality life is the time elapsed from freezing

of an initially high-quality product and the moment when, by sensory assessment, a

statistically significant difference (p < 0.01) from the initial high quality (immediately after

freezing) can be established. The observed difference is termed the just noticeable

difference (JND). When a triangular test is used for sensory assessment of the quality, a

just noticeable difference can be postulated when 70% of experienced panelists

successfully distinguish the product from the control sample stored under such

conditions as have been proven to produce no detectable, degradation during the time

period under consideration (IIR, 1986). A typical temperature used for control

experiments is - 35°C.

The quality loss in frozen foods can be predicted using the experimentally

obtained data on acceptable time for storage.

The following Fig. 40 presents data on acceptable times for storage of frozen

strawberries. The acceptable shelf life in this figure is based on experimental data.

Quality Changes in Foods during Frozen Storage

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Fig. 40. Semilogarithmic plot of shelf life versus temperature for frozen

strawberries.

Table. 7 : Conditions of storage and quality loss in strawberries in different stage

in a frozen food chain

Stage Time

(days)

Temperature

(°C)

Acceptability

(days)

Loss per day

( % 1 day)

Quality loss

percentProducer 250 - 22 660 0.15 37.8Transport 2 - 14 220 0.45 0.9Wholesale 50 - 23 710 0.14 7.0Transport 1 - 12 140 0.71 0.7Retail 21 - 11 110 0.90 19Transport 0.1 - 3 18 5.55 0.5Home -freezer

20 - 13 180 0.56 11.1

Looking at Table. 7, column 1 describes frozen chain, starting with the producer's

warehouse and finishing at the consumer's freezer. Columns 2 and 3 denote the

expected time and temperature at various locations in the chain. Column 4 is the

acceptable days corresponding to the temperature as obtained from figure. Values in

column 5 are the inverse of those in column 4, multiplied by 100 to obtain percent loss

per day. Column 6 gives the calculated values of loss obtained by multiplying values in

column 2 with those in column 5. Thus, in the example shown in Table, the frozen

strawberries have lost 77.3% of their acceptable quality after 344.1 days frozen food

chain. The major losses in quality occur in the producers and retailers warehouse. A

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reduction of either storage temperature or time in the identified components can help

reduce quality loss for a given food.

101