lecture 1. basic concepts in refrigeration

7/29/2019 Lecture 1. Basic Concepts in Refrigeration

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Lecture 1

Basic concepts inRefrigeration

By: Addisu Dagne1

Engineering and Technology college

Mechanical Engineering Department


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Introduction to Refrigeration

Definition: Refrigeration may be defined as the process of

achieving and maintaining a temperature below that of thesurroundings, the aim being to cool some product or space to

the required temperature.

One of the most important applications of refrigeration has

been the preservation of perishable food products by storingthem at low temperatures.

The purpose of refrigerator is to transfer heat from a cold

chamber which is at a temperature lower than that of its

surroundings. The natural flow of heat from the surroundings back to the

cold chamber can be resisted by insulating the chamber from

the surroundings.

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The refrigerationsystem (R)

transfers heat

from a cooler

low-energyreservoir to a

warmer high-

energy reservoir

(see figure 1).


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Q: Which of the following can be called as a refrigerationprocess?

a) Cooling of hot ingot from 1000oC to room temperature

b) Cooling of a pot of water by mixing it with a large

block of icec) Cooling of human beings using a ceiling fan

d) Cooling of a hot cup of coffee by leaving it on a table

e) Cooling of hot water by mixing it with tap water

f) Cooling of water by creating vacuum over it

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Applications of Refrigeration

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Modem refrigeration has many applications. Thefirst, and probably, still the most important is thepreservation of food.

Most foods kept at room temperature spoil

rapidly. This is due to the rapid growth of bacteria.At common refrigeration temperatures of about

40oF (4oC). Bacteria grow quite slowly. Food atthis temperature will keep much longer.

Refrigeration preserves food by keeping it cold. Other important uses of refrigeration include air

conditioning, beverage cooling, and humiditycontrol.

Many manufacturing processes also userefri eration.


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Refrigerators and Heat pumps

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Naturally heat flows in the direction of decreasing

temperature, that is, from high-temperatureregions to low-temperature ones. This heat-

transfer process occurs in nature without

requiring any devices.

The reverse process, however, cannot occur by

itself. The transfer of heat from a low-temperature

region to a high-temperature one requires special

devices called refrigerators.

Refrigerators are cyclic devices, and the working

fluids used in the refrigeration cycles are called

refrigerants


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A refrigerator is shown

schematically in Fig.

QLis the magnitude of the heat

removed from the refrigerated

space at temperature TL ,

QHis the magnitude of the heatrejected to the warm space at

temperature TH , and

Wnet,in is the net work input to

the refrigerator. QLand QHrepresent magnitudes

and thus are positive quantities.


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Another device that transfers heat from a low-temperaturemedium to a high-temperature one is the heat pump.

Refrigerators and heat pumps are essentially the samedevices; they differ in their objectives only.

The objective of a refrigerator is to maintain the

refrigerated space at a low temperature by removing heatfrom it.

Discharging this heat to a higher-temperature medium ismerely a necessary part of the operation, not the purpose.

The objective of a heat pump, however, is to maintain aheated space at a high temperature. This is accomplished byabsorbing heat from a low-temperature source, such as wellwater or cold outside air in winter, and supplying this heatto a warmer medium such as a house


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Refrigerator and Heat Pump

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There is no difference in

operation between arefrigerator and a heatpump.

With the refrigerator theimportant quantity is the

energy removed from coldchamber called therefrigerating effect, and

With the heat pump it isthe energy to be rejectedby the refrigerant forheating purposes.

The machine can be usedfor both purposes


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Refrigerator and Heat Pump

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The performance of refrigerators and heat pumpsis expressed in terms of the coefficient ofperformance (COP), defined as

Notice that both COPR and COPHP can be

greater than 1. COPHP = COPR + 1 for fixed values ofQLand QH.

This relation implies that COPHP > 1 since COPRis a positive quantity.


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NEED FOR THERMAL INSULATION

Heat will always migrate from a region of high temperatureto a region of lower temperature, there is always a

continuous flow of heat into the refrigerated region from the

warmer surrounding.

To limit the flow of heat into the refrigerated region to somepractical minimum, it is usually necessary to isolate the

region from its surroundings with a good heat-insulating

material.

THE REFRIGERATION LOAD The rate at which heat must be removed from the

refrigerated space or material in order to produce and

maintain the desired temperature conditions is called the

refrigeration load, the cooling load, or the heat load.


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Sources of Cooling Load:a) The heat transmitted by conduction through the

insulated walls.

b) The heat that must be removed from the warm

air that enters the space through opening and

closing doors.

c) The heat that must be removed from the

refrigerated product to reduce the temperature

of the product to the storage temperature;

d) The heat given off by people working in the

space and by motors, lights and other heat-

producing equipment operating in the space.


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The cooling capacity of a refrigeration system

The cooling capacity of a refrigeration system

that is, therate of heat removal from the refrigerated spaceis often

expressed in terms oftons of refrigeration.

One ton of refrigeration is defined as the quantity of heat

removed to freeze one ton of water into ice at 0

o

C in 24hours.

One ton of refrigeration is equivalent to 211 kJ/min (3.5 KW)

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Refrigerant

It is substance, which is used as a working fluid inrefrigerators.

The refrigerant has low boiling point, whichmeans that it vaporizes at low temperature and

takes away the heat from a substance.Properties of good refrigerant:

Have low freezing and boiling point

Have high COP

Be non toxic and non corrosive to metal Be non explosive

Easily be liquefied

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Types of Refrigeration systems

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The most widely used types of refrigerationsystems are

o Vapour compression Refrigeration system and

o Vapor absorption Refrigeration system

The vapor compression refrigeration system is

the mainstay of the refrigeration and air

conditioning industry.

Vapour Absorption refrigeration provides analternative to the vapor compression approach,

particularly in applications where a heat source is

economical and readily available.


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Vapor Compression Refrigeration system:

This type of refrigeration system is the most commonlyused system in domestic refrigerators.

In VCRS the vapor alternatively undergoes a change of

phase from vapor to liquid and vice versa during a cycle.

Vapor compression refrigeration system has the followingcomponents at its basic parts.

1. Compressor: The function of the compressor is to

compress the input refrigerant of low pressure and low

temperature. As a result the pressure and the temperatureof the refrigerant increases. Generally reciprocating

compressors are used in a refrigeration system. An

external motor is used to drive the compressor.

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2. Condenser: The condenser

is a coil of tubes, which aremade of copper. This is used

to condense the refrigerant

which is in the form of

vapor and convert into

liquid.

3. Expansion Valve: this is

otherwise called throttle

valve. This valve is used to

control the flow rate of

refrigerant and also to

reduce the pressure of the

refrigerant.


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4. Evaporator: This is

the part in which the

cooling takes place.

This is kept in the

space where cooling is

required. It is a coil oftubes made up of

copper.


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Working Principle:

The refrigerant, which is at low pressure and lowtemperature flows into the compressor. In thecompressor the refrigerant is compressed andconverted into a high pressure and high temperaturerefrigerant.

This high pressure and high temperature refrigerant invapor form then passes through the condenser whereit is condensed into high pressure liquid refrigerant.

The high pressure liquid refrigerant thus producedpasses through the expansion valve. In the expansion

valve the pressure and temperature of the refrigerantdrops and it partly evaporates. It is the allowed to flowinto the evaporator at a controlled rate.

In the evaporator, the partly liquid and vaporrefrigerant is mostly evaporated and converted into a

low pressure vapor. During this process, therefrigerant absorbs its latent19


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Vapor Absorption Refrigeration system:

As the name implies, absorption refrigerationsystems involve the absorption of a refrigerantbya transport medium.

The most widely used absorption refrigeration

system is the ammonia

water system, whereammonia (NH3) serves as the refrigerant and

water (H2O) as the transport medium.

Other absorption refrigeration systems include

water

lithium bromide and water

lithium chloridesystems, where water serves as the refrigerant.

The latter two systems are limited to applications

such as air-conditioning where the minimum

temperature is above the freezing point of water.21


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The vapour absorption refrigeration system looksvery much like the vapor-compression system,

except that the compressor has been replaced by

a complex absorption mechanism consisting of

an absorber, a pump, a generator, a and arectifier.

Once the pressure of NH3 is raised by the

components in the box (this is the only thing they

are set up to do), it is cooled and condensed inthe condenser by rejecting heat to the

surroundings, is throttled to the evaporator

pressure, and absorbs heat from the refrigerated

space as it flows through the evaporator. So,


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Ammonia vapor leaves the evaporator and enters theabsorber, where it dissolves and reacts with water to formNH3 H2O. This is an exothermic reaction; thus heat isreleased during this process.

The amount of NH3 that can be dissolved in H2O isinversely proportional to the temperature. Therefore, it isnecessary to cool the absorber to maintain its temperatureas low as possible, hence to maximize the amount of NH3dissolved in water.

The liquid NH3 + H2O solution, which is rich in NH3, is thenpumped to the generator. Heat is transferred to thesolution from a source to vaporize some of the solution.The vapor, which is rich in NH3, passes through a rectifier,which separates the water and returns it to the generator.

The high-pressure pure NH3 vapor then continues itsjourney through the rest of the cycle. The hot NH3 + H2Osolution, which is weak in NH3 , then passes through a

regenerator, where it transfers some heat to the rich


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Generator: the generator receives the strong solution ofaqua-ammonia from the absorber and heats it. Because ofthis heating, the aqua-ammonia solution gets separatedinto ammonia vapor at high pressure and hot weakammonia solution which contains mostly water.

Condenser: The condenser converts the high pressureammonia vapor received from the generator into high

pressure ammonia liquid. This condensation is done bymeans of circulating cool water.

Expansion valve: This valve is otherwise called thethrottling valve since the expansion, which takes placehere, is throttling. While passing through this valve, theliquid ammonia gets expanded and gets converted into

low pressure and low temperature ammonia. Evaporator: The evaporator is otherwise known as cold

chamber. Here the refrigerant absorbs the heat from thematerial which is to be cooled and gets evaporated. It hasmany coils made of copper.

Absorber: The absorber receives the low pressureammonia vapor from the evaporator and the weak


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VAR Working Principle

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The working fluid in vapor absorption refrigeration

system is normally ammonia. The ammonia vaporand water are mixed to form a strong solution of

aqua-ammonia in the absorber This aqua-ammonia

solution is then pumped into the generator.

In the generator, this solution is heated. Because of

heating, ammonia gets evaporated at high pressure

and leaves behind the weak ammonia solution, which

mostly contains water.

The high pressure ammonia vapor produced by the

generator is condensed in the condenser and itbecomes ammonia liquid, which is at high pressure.

This high pressure liquid ammonia is allowed to pass

through the expansion valve or throttling valve where

it expands and becomes a low pressure and low


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VAR Working Principle

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Ammonia at low pressure and low temperaturethen passes through the evaporator where it

absorbs the heat from the material which is to be

cooled and gets evaporated.

The evaporator is where the real cooling takesplace. Because of the heat absorbed by

ammonia, it gets evaporated and becomes low

pressure ammonia vapor. The low pressure

ammonia vapor is then sent into the absorber andthe cycle is repeated.

lecture 1. basic concepts in refrigeration

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