EXPERIMENT NO:-1

AIM: To Compare And Analyze Advance Refrigeration Cycle For Different

Refrigerant.

1. INTRODUCTION

In nature, heat transfer occurs from the region of higher temperature to lower temperature

without requiring any external devices. The reverse process cannot occur by itself. The transfer

of heat from lower temperature to higher temperature requires special devices called

refrigerators. Refrigerator works under the principle of reversed Carnot cycle. Many types of

refrigerants are available for getting cooling effect. It is very much important to use a suitable

refrigerant, which gives the maximum cooling effect by consuming minimum power.

In present situation, most of the domestic vapour compression refrigerators are equipped with

R134a due to its thermodynamic properties. However, it is known that it have a higher global

warming potential. So it is important to find alternative refrigerant to R134a. Here R134a is

compared with R290 and R600a of different mass fraction.

Mohanraj have done an experimental investigation with hydrocarbon refrigerant mixture

(composed of R290 and R600a in the ratio of 45.2:54.8 by weight) as an alternative to R134a in

a domestic refrigerator. They performed a continuous running test under different ambient

temperatures (24, 28, 32, 38 and 43˚C), and cycling test (ON/OFF) were carried out at 32˚C

ambient temperature. The results show that the hydrocarbon mixture has lower values of energy

consumption; pull down time and ON time ratio by about 11.1%, 11.6% and 13.2%, respectively,

with 3.25–3.6% higher Coefficient of performance (COP).

Maclaine-Cross shows that the usage of hydrocarbon refrigerants in the refrigeration systems

instead of halogenated refrigerants (such as R134a and R12) reduces energy consumption by

20%. Fatouh conducted an experiment in domestic refrigerators with hydrocarbon refrigerants

such as propane and isobutene and shows that the performance of hydrocarbon refrigerants

matches with that of R134a.

1

Baskaran performed an analysis on a vapour compression refrigeration system with various

refrigerant mixtures of R152a, R170, R600a and R290 at various mass fractions. From their

results, the alternative refrigerants except R431a (which is a combination of R152a, R290 at 29%

and 71% respectively) have a slightly higher performance than R134a at the condensation

temperature at 50˚C and evaporator temperature ranging between -30˚C and 10˚C.

Dongsoo Jung examined the performance of a propane/isobutane (R290/R600a) mixture for

domestic refrigerators. This thermodynamic cycle analysis signified that the propane/isobutene

mixture in the composition range of 0.2 to 0.6 mass fraction of propaneyields an increase in the

coefficient of performance (COP) of up to 2.3% as compared to R12. The experimental results

obtained indicated that the propane/isobutane mixture at 0.6 mass fraction of propane has 3±4%

higher energy efficiency and faster cooling rate than R12.

The above literature reviews show that the performance analysis on vapour compression

refrigeration system with the various possible alternatives. However the possibility of using

propane and isobutane in different combinations as alternative to halogenated refrigerants needs

further investigation.

2. REFRIGERANTS:

The thermodynamic efficiency of a refrigeration system depends mainly on its operating

temperatures. However, important practical issues such as the system design, size, initial and

operating costs, safety, reliability, and serviceability etc. depend very much on the type of

refrigerant selected for a given application. Due to several environmental issues such as ozone

layer depletion and global warming and their relation to the various refrigerants used, the

selection of suitable refrigerant has become one of the most important issues in recent times.

Replacement of an existing refrigerant by a completely new refrigerant, for whatever reason, is

an expensive proposition as it may call for several changes in the design and manufacturing of

refrigeration systems. Hence it is very important to understand the issues related to the selection

and use of refrigerants. In principle, any fluid can be used as a refrigerant. Air used in an air

cycle refrigeration system can also be considered as a refrigerant. However, in this lecture the

attention is mainly focused on those fluids that can be used as refrigerants in vapour compression

refrigeration systems only.

2

3. SELECTION OF REFRIGERANT BASED ON PROPERTIES

3.1. Selection of Substance:

Hereby Select R 404a from predefined mixture for the analysis purpose as shown.

Fig-1 Specify Saturation Table of R 407C at Constant Temperature

Fig-2 Saturation Table Information of R 404A at Constant Temperature

3

3.2. Specify Saturation Table of R 404a at Constant condenser pressure Pressure:

1.182MPa

By keeping condenser pressure as constant and varying the evaporator pressure, the

specific saturation table of R 404A is obtain as shown.

3.3. Saturation Table Information of R 404a at Constant Evaporator Pressure 0.164

MPa:

By keeping evaporator pressure as constant and varying the condenser pressure, the

specific saturation table of R 404a is obtain as shown.

Fig-3 Saturation Table Information of R 404A by varying Evaporator pressure

Fig-4 Calculation Table Information of R 404A at Constant condenser Pressure

4

3.4. Calculation Table of R 404a at Constant condenser Pressure:

Figure-5 Calculation Table of R 404A at Constant condenser Pressure

3.5. Calculation Table of R 404a at Constant Evaporator Pressure :

Fig-6 Calculation Table of R 404A at Constant Evaporator Pressure

5

Fig-7 Calculation of C.O.P of R404A at different sub cooling temperatures

3.6. Calculation Table of R 404a at Different Sub- cooling Temperatures:

By keeping Condenser pressure and Evaporator pressure as constant and varying the sub

cooling temperature, the calculation of C.O.P of R404A is as shown in fig.8..

3.7. Comparison graphs of R22 vs. R404a, R507A & R410A at constant Condenser

Pressure:

The Figure 8 shows the variation of COP with evaporator pressure for a given constant

condensing pressure of 1182KPa for different alternative refrigerants.

Fig-8 Comparison of C.O.P of R22 vs. R404A, R507A & R410A

3.8. Comparison graphs of R22 vs. R404a, R507a & R410a at constant Evaporator

Pressure:

The Figure 10 shows the variation of COP with condenser pressure for a given constant

evaporator pressure of 164 KPa for different alternative refrigerants.

6

Fig-9 Comparison of C.O.P of R22 vs. R404a, R507a &R410Aa

3.9. Comparison graphs of R22 vs R404a, R507a & R410a at sub-cooling

Temperatures:

The Figure 11 shows the variation of COP with sub-cooling temperature for different

alternative refrigerants. The sub-cooling temperature varied from 200C to 300C.

Fig-10 Comparison of C.O.P of R22 vs. R404a, R507Aa&R410a

4. CONCLUSIONS:

The theoretical investigations were carried out to study performance characteristics of three

alternative refrigerants namelyR-404a, R-507a & R-410a to replace R-22 in the present day air

conditioners that employed the same heat exchangers and compressors of the same-efficiency.

7

The effects of evaporator pressure, condenser pressure and degree of sub-cooling on coefficients

of performance (COPs) were investigated. The analysis is performed at condenser pressures

ranging from 982 to1182 KPa and evaporator pressures ranging from 164 to 364KPa. The sub-

cooling temperatures varied between 20 and 30°C. The following conclusions were drawn from

the analysis:

The COP of R410a is found to be better than that of R-22 at all evaporator pressures,

condenser pressures and at various degrees of sub-cooling.

R410a has less harmful influence on the environment than R22 as its ODP is zero.

The thermo-physical properties of R410a are similar to R22, so when changing from R22

to R410a, minimal alterations in the refrigeration system are required.

The performance coefficient (COP) of the system increases with increasing evaporating

pressure for constant condensing pressure in the analysis.

The overall evaluation of the performance of the three alternative refrigerants in the

Standard vapour compression air-conditioning system favored the use of R410a.

8

EXPERIMENT NO:-2

Aim: Performance analysis of VCR system using capillary tube as a throttling

device.

Refrigeration: - Refrigeration may be defined as an art of producing and maintaining a

temperature in a space below the surrounding temperature. It also includes the process of

removing heat from the substance under controlled conditions. It is used for the manufacturing of

ice and similar products. This is widely used for cooling of storage chambers in which perishable

foods, drinks and medicines are stored. The refrigeration has also wide application in submarine

ships, aircrafts.

Standard Vapour Compression Cycle: - (SVCC)

This cycle consists of following four processes:

(1) Reversible adiabatic compression from the saturated vapour to a superheated

condition.

(2) Reversible heat rejection at constant pressure (de-superheating and condensation of

the refrigerant)

(3) Irreversible is enthalpy expansion from saturated liquid to a low pressure vapour.

(4) Reversible heat addition at constant pressure.

Fig-1 Vapour Compression Refrigeration cycle

9

System components and its function: -

(1) Compressor: - Compressor is most important part of the system. The compressor raises the

measure of incoming vapour from the evaporator to a high pressure. Different type of

compressor:

- Reciprocating compressor

- Rotary compressor

- Screw compressor

- Centrifugal compressor

The selection of the above mentioned compressor depends upon the usage. Usually

domestic refrigerator are installed with reciprocating compressor.

A hermetic type compressor is one in which the compressor and motor integral

on one shaft and they are both contained in a pressure shield housing. It is compact in

size, quite, low in cost and no problem of refrigerant gas leakage. The motor in a

hermetic type compressor is cooled by refrigerant suction gas.

(2) Condenser: - The function of a condenser is to remove heat from the superheated high

pressure refrigerant vapour & to condense the vapour into a sub cooled high pressure

liquid. This is accomplished a cooling medium either air or water. The air cooled

condenser may be of static cooled type where natural convective motion of air

surrounding heat is enough to cool the condenser or they may be of fan cooled type. The

static cooled condenser is used in domestic refrigerators. In commercial appliances and

windows air-conditioners usually fan cooled condensers are used. The water condenser is

normally used in large system firms 5 tons and above.

(3) Expansion Devices: -As the high pressure sub-cooled refrigerant liquid from the condenser

passes through the expansion device its pressure and temperature is reduced, and outlet

refrigerant, mostly in liquid stage.

Expansion device are of many types and are selected as per the requirement. For

example, for constant evaporator pressure requirement automatic expansion valve are

used. If variation in cooling load is high the thermostatic valves are best suited.

10

Hermetic compressor are almost invariably are used with capillary tube, through a few of

them might also be used with thermostatic expansion valves.

(a) Thermostatic expansion valve: -

Thermostatic expansion valve controls the mass flow rate of refriogerent by

sensing evaporator outlet temperature. Thus the valve is sensitive to the cooling load. If

the load is more than the degree of super hit of refrigerant coming out of the evaporator

will increases. To maintain the degree of super heat to preset level, more liquid is fed to

evaporator. When the load is low, valve closes and less liquid quantity is fed.

Selection of thermostatic expansion is done on the basis of refrigerant used in capacity.

(b) Capillary tube: -

It is the simplest and cheapest form of expansion device. It does not include any moving

parts hence no maintenance is required. Capillary tube is supposed to be a single point

operation device in the sense that the best control is achieved only at a given set of

operating parameters. Under varying loads the capillary tube does not function

satisfactorily. For example at lower loads than designed value capillary tube may

overfeed the evaporator causing liquid to return to compressor. Under higher loads than

designed the capillary tube starves the evaporator, causing excessive return gas superheat.

Hence for variation in cooling loads, it is not suitable.

(4) Evaporator: -The function of evaporator is to remove heat from the product or the area to be

cooled and maintain it at any desired temperature. The liquid refrigerant inside the

evaporator evaporates by absorbing heat and converts into vapour refrigerant and than it

returns back into the compressor. Various types of constructions of the evaporator used in

refrigerant system:

(a) Finned tube evaporator coil

(b) Bare tube soldered/clamped to the tank

(c) Bare tube dipped in the liquid to be cooled

(d) Shell & coil type evaporator

11

The choice of particular construction depends on the type of appliance. However in every

case the heat capacity depends on these factors viz. temperature difference between the

load and the refrigerant, heat transfer co-efficient and areas of the heat transfer.

(5) Drier: - If by chance refrigerant is containing any water particle then in low temperature

region (i.e. at and after the expansion valve) it forms ice and chokes the valve or bends

thereby preventing the smooth passage of refrigerant through it. To prevent this drier is

used to remove any water particles carried into the refrigerant. It is used in between the

evaporator and compressor.

(6) Accumulator: -It is fitted in between the evaporator and compressor. It prevents the liquid

refrigerant from entering the compressor.

Description: -

The unit is equipped with Kirlosker make compressor model HAG operate on 220V AC

supply and works on R-134 a. The unit is fitted on sun mica base with compressor, air cooled

condenser, condenser fan motor drier, thermostatic expansion valve, solenoid valve, capillary

tube, expansion valve refrigerant flow motor water calorimeter (cooling coil), suction gauge,

discharge gauge, digital temperature indicator with probe to measure different temperatures,

heater inside the calorimeter, cooling thermostat, one number charging valve provided to

charge the liquid refrigerant. The voltmeter and ampere meter for compressor have been

provided which are duly interlocked type writing for safety point of view.

Experimental procedure: -

1. Switch in main board, check voltage. It should not be less than 180 V.

2. Start the condenser fan motor.

3. See that all the respective indication lights are on.

4. Put the water in the evaporative tank/calorimeter.

5. Switch on the compressor.

6. At the time of the start of the unit, note down the reading of voltage, ampere, suction and

discharge gauge- pressures reading for compressor.

7. Check and note down the reading of various temperature through digital temp. Indicator.

12

8. Always close the door of the evaporator.

9. Note down the reading of the pressure gauge. Absence of any reading will indicate the

blockage or leakage of gas.

10. After the gap of 15 minutes, start the agitator motor for two minute for equalization of

water temperature and note down all the readings.

11. Now if you want to provide or test load on our compressor, switch on the heater.

12. Check and note don the readings of temperature, pressure and energy.

13. Now let the unit run for at least 10-20 minutes.

14. Switch on the agitator motor for 2 minutes for equalization of water temperature.

15. Check the water temperature through digital temp. indicator. It must not go ahead of

350C.

16. After taking the required readings, switch off the heating process if the unit runs with

compressor or cooling process.

17. While closing the unit, first switch-off the compressor, condenser, fan motor, and all the

other valves and switches on the unit.

18. Always check the indication lights provided on the board for each component.

Specifications: -

Compressor : Hermetically sealed Kirloskar make

Agitator : Compatible capacity

Condenser : Air cooled compatible to compressor

Condenser cooling fan : Compatible capacity with permanent lubricating motor

Evaporator : Made of Stainless steel insulated with ceramic wool/P.U.F

Rotameter : Make Eureka

Expansion device : Capillary tube solenoid & thermostatic valve

Pressure gauges : 2 Nos.

13

Safety control : Time delay circuit

Temperature sensor : RTD PT-100 type

Control panel : Digital voltmeter (0-300V) and ammeter (0-10A)

Temperature measurement : Digital temperature indicator with multi channel switch

Observation table: -

ρ – Density of water = 1000 kg/m3

Cpw – specific heat of water = 4.186 KJ/kg 0C

Sr.

No

P1 P2 T1 T2 T3 T4 T5 T6 Volume of

water (W)

V I

1

2

3

Calculation: -

1. C.O.P.Theo.:

COPth=

REth

CW th

COPth=H1−H 3

H2−H1 ( H3 = H4 )

[Note: H1 at P1-T1 , H2 at P2-T2 and H3 at P2-T3 from P-H diagram of R-134a]

14

2. C.O.P act :- Heat given away by water = Actual refrigerating effect

CWact. = V * I * cosΦ Watt (cosΦ = 0.7)

REact. = m * Cp* (T5 – T7) Watt

COPact=REact

CW act

COPRe lative=COPact

COP theo

15

Nomenclature: -

T1 – Temperature of compressor inlet (suction) (0C)

T2 – Temperature of compressor outlet (discharge) (0C)

T3 – Temperature of condenser outlet (0C)

T4 – Temperature of evaporator inlet (0C)

T5 – Temperature of water inlet (0C)

T6 – Temperature of water outlet (0C)

H1 – Enthalpy at compressor inlet (KJ/kg)

H2 – Enthalpy at compressor outlet (KJ/kg)

H3 – Enthalpy of sub-cooling at condenser outlet (KJ/kg)

H4 - Enthalpy of refrigeration at evaporator inlet (KJ/kg)

P1 – Pressure at compressor suction (kg/cm2)

P2 – Pressure at compressor discharge (kg/cm2)

V – Voltmeter reading

I – ammeter reading

Conclusions

16

EXPERIMENT NO:-3

Aim: Performance analysis of VCR system using thermostatic expansion valve as

a throttling device.

Refrigeration: - Refrigeration may be defined as an art of producing and maintaining a

temperature in a space below the surrounding temperature. It also includes the process of

removing heat from the substance under controlled conditions. It is used for the manufacturing of

ice and similar products. This is widely used for cooling of storage chambers in which perishable

foods, drinks and medicines are stored. The refrigeration has also wide application in submarine

ships, aircrafts.

Standard Vapour Compression Cycle: - (SVCC)

This cycle consists of following four processes:

(5) Reversible adiabatic compression from the saturated vapour to a superheated

condition.

(6) Reversible heat rejection at constant pressure (de-superheating and condensation of

the refrigerant)

(7) Irreversible is enthalpy expansion from saturated liquid to a low pressure vapour.

(8) Reversible heat addition at constant pressure.

Fig-1 Vapour Compression Refrigeration cycle

17

System components and its function: -

(1) Compressor: - Compressor is most important part of the system. The compressor raises the

measure of incoming vapour from the evaporator to a high pressure. Different type of

compressor:

- Reciprocating compressor

- Rotary compressor

- Screw compressor

- Centrifugal compressor

The selection of the above mentioned compressor depends upon the usage. Usually

domestic refrigerator are installed with reciprocating compressor.

A hermetic type compressor is one in which the compressor and motor integral

on one shaft and they are both contained in a pressure shield housing. It is compact in

size, quite, low in cost and no problem of refrigerant gas leakage. The motor in a

hermetic type compressor is cooled by refrigerant suction gas.

(2) Condenser: - The function of a condenser is to remove heat from the superheated high

pressure refrigerant vapour & to condense the vapour into a sub cooled high pressure

liquid. This is accomplished a cooling medium either air or water. The air cooled

condenser may be of static cooled type where natural convective motion of air

surrounding heat is enough to cool the condenser or they may be of fan cooled type. The

static cooled condenser is used in domestic refrigerators. In commercial appliances and

windows air-conditioners usually fan cooled condensers are used. The water condenser is

normally used in large system firms 5 tons and above.

(3) Expansion Devices: -As the high pressure sub-cooled refrigerant liquid from the condenser

passes through the expansion device its pressure and temperature is reduced, and outlet

refrigerant, mostly in liquid stage.

Expansion device are of many types and are selected as per the requirement. For

example, for constant evaporator pressure requirement automatic expansion valve are

used. If variation in cooling load is high the thermostatic valves are best suited.

18

Hermetic compressor are almost invariably are used with capillary tube, through a few of

them might also be used with thermostatic expansion valves.

(a) Thermostatic expansion valve: -

Thermostatic expansion valve controls the mass flow rate of refriogerent by sensing

evaporator outlet temperature. Thus the valve is sensitive to the cooling load. If the load

is more than the degree of super hit of refrigerant coming out of the evaporator will

increases. To maintain the degree of super heat to preset level, more liquid is fed to

evaporator. When the load is low, valve closes and less liquid quantity is fed.

Selection of thermostatic expansion is done on the basis of refrigerant used in capacity.

(b) Capillary tube: -

It is the simplest and cheapest form of expansion device. It does not include any moving

parts hence no maintenance is required. Capillary tube is supposed to be a single point

operation device in the sense that the best control is achieved only at a given set of

operating parameters. Under varying loads the capillary tube does not function

satisfactorily. For example at lower loads than designed value capillary tube may

overfeed the evaporator causing liquid to return to compressor. Under higher loads than

designed the capillary tube starves the evaporator, causing excessive return gas superheat.

Hence for variation in cooling loads, it is not suitable.

(4) Evaporator: -The function of evaporator is to remove heat from the product or the area to be

cooled and maintain it at any desired temperature. The liquid refrigerant inside the

evaporator evaporates by absorbing heat and converts into vapour refrigerant and than it

returns back into the compressor. Various types of constructions of the evaporator used in

refrigerant system:

(a) Finned tube evaporator coil

(b) Bare tube soldered/clamped to the tank

(c) Bare tube dipped in the liquid to be cooled

(d) Shell & coil type evaporator

19

The choice of particular construction depends on the type of appliance. However in every

case the heat capacity depends on these factors viz. temperature difference between the

load and the refrigerant, heat transfer co-efficient and areas of the heat transfer.

(5) Drier: - If by chance refrigerant is containing any water particle then in low temperature

region (i.e. at and after the expansion valve) it forms ice and chokes the valve or bends

thereby preventing the smooth passage of refrigerant through it. To prevent this drier is

used to remove any water particles carried into the refrigerant. It is used in between the

evaporator and compressor.

(6) Accumulator: -It is fitted in between the evaporator and compressor. It prevents the liquid

refrigerant from entering the compressor.

Description: -

The unit is equipped with Kirlosker make compressor model HAG operate on 220V AC

supply and works on R-134 a. The unit is fitted on sun mica base with compressor, air cooled

condenser, condenser fan motor drier, thermostatic expansion valve, solenoid valve, capillary

tube, expansion valve refrigerant flow motor water calorimeter (cooling coil), suction gauge,

discharge gauge, digital temperature indicator with probe to measure different temperatures,

heater inside the calorimeter, cooling thermostat, one number charging valve provided to

charge the liquid refrigerant. The voltmeter and ampere meter for compressor have been

provided which are duly interlocked type writing for safety point of view.

Experimental procedure: -

1. Switch in main board, check voltage. It should not be less than 180 V.

2. Start the condenser fan motor.

3. See that all the respective indication lights are on.

4. Put the water in the evaporative tank/calorimeter.

5. Switch on the compressor.

6. At the time of the start of the unit, note down the reading of voltage, ampere, suction and

discharge gauge- pressures reading for compressor.

7. Check and note down the reading of various temperature through digital temp. Indicator.

20

8. Always close the door of the evaporator.

9. Note down the reading of the pressure gauge. Absence of any reading will indicate the

blockage or leakage of gas.

10. After the gap of 15 minutes, start the agitator motor for two minute for equalization of

water temperature and note down all the readings.

11. Now if you want to provide or test load on our compressor, switch on the heater.

12. Check and note don the readings of temperature, pressure and energy.

13. Now let the unit run for at least 10-20 minutes.

14. Switch on the agitator motor for 2 minutes for equalization of water temperature.

15. Check the water temperature through digital temp. indicator. It must not go ahead of

350C.

16. After taking the required readings, switch off the heating process if the unit runs with

compressor or cooling process.

17. While closing the unit, first switch-off the compressor, condenser, fan motor, and all the

other valves and switches on the unit.

18. Always check the indication lights provided on the board for each component.

Specifications: -

Compressor : Hermetically sealed Kirloskar make

Agitator : Compatible capacity

Condenser : Air cooled compatible to compressor

Condenser cooling fan : Compatible capacity with permanent lubricating motor

Evaporator : Made of Stainless steel insulated with ceramic wool/P.U.F

Rotameter : Make Eureka

Expansion device : Capillary tube solenoid & thermostatic valve

Pressure gauges : 2 Nos.

21

Safety control : Time delay circuit

Temperature sensor : RTD PT-100 type

Control panel : Digital voltmeter (0-300V) and ammeter (0-10A)

Temperature measurement : Digital temperature indicator with multi channel switch

Observation table: -

ρ – Density of water = 1000 kg/m3

Cpw – specific heat of water = 4.186 KJ/kg 0C

Sr.

No

P1 P2 T1 T2 T3 T4 T5 T6 Volume of

water (W)

V I

1

2

3

Calculation: -

1. C.O.P.Theo.:

COPth=

REth

CW th

COPth=H1−H 3

H2−H1 ( H3 = H4 )

[Note: H1 at P1-T1 , H2 at P2-T2 and H3 at P2-T3 from P-H diagram of R-134a]

22

2. C.O.P act :- Heat given away by water = Actual refrigerating effect

CWact. = V * I * cosΦ Watt (cosΦ = 0.7)

REact. = m * Cp* (T5 – T7) Watt

COPact=REact

CW act

COPRe lative=COPact

COP theo

23

Nomenclature: -

T1 – Temperature of compressor inlet (suction) (0C)

T2 – Temperature of compressor outlet (discharge) (0C)

T3 – Temperature of condenser outlet (0C)

T4 – Temperature of evaporator inlet (0C)

T5 – Temperature of water inlet (0C)

T6 – Temperature of water outlet (0C)

H1 – Enthalpy at compressor inlet (KJ/kg)

H2 – Enthalpy at compressor outlet (KJ/kg)

H3 – Enthalpy of sub-cooling at condenser outlet (KJ/kg)

H4 - Enthalpy of refrigeration at evaporator inlet (KJ/kg)

P1 – Pressure at compressor suction (kg/cm2)

P2 – Pressure at compressor discharge (kg/cm2)

V – Voltmeter reading

I – ammeter reading

CONCLUSION:

24

EXPERIMENT NO:-4

AIM- Design of Steam Jet Refrigeration System for particular application

INTRODUCTION:

Nowadays, the most widely used refrigeration system for air-conditioning in the household

sector and buildings are a vapour compression system. This system essentially requires electrical

energy to produce the useful refrigeration. This is because such systems must be driven by means

of a mechanical compressor. As a result, green house gas (GHG) emission that is generated

during electricity generation is released increasingly to the environment. This reflects the fact

that refrigeration and air-conditioning systems are one of the high emitters of GHG to the

environment.

To reduce the demand of electricity for refrigeration application, alternative refrigeration systems

that can be operated by using thermal energy, heat-powered refrigeration cycle, is introduced at

present time. This cycle can be powered by low-grade thermal energy as low as 90-130°C. This

kind of energy is usually available from renewable resources or waste-heat rejected from

industrial processes. Therefore, the electricity is not required for producing useful refrigeration.

A reduction in GHG emission is a consequence.

Steam jet refrigeration system, which is one of the heat-powered refrigeration cycles, is of

current interest. The distinctive point of such cycle is that it is relatively simple to design,

construct and operate compared to the other types of heat-powered refrigeration systems. In this

cycle, the important equipment known as an ejector is used as a main driving part for the system.

This is because an ejector is used to elevate refrigerant pressure similar to the use of a

mechanical compressor.

The components of an ejector are shown in Fig.1 and the schematic diagram of a jet refrigeration

system is shown in Fig.2. Referring to Fig.1, as high pressure steam from the boiler, known as a

primary fluid, is accelerated passing through the primary nozzle. A supersonic jet stream of the

primary fluid is produced within the mixing chamber. Very low pressure region at the mixing

chamber is obtained as a consequence. This low pressure region can draw secondary fluid from

25

the evaporator (where the refrigeration effect is produced) into the mixing chamber. The primary

fluid and the secondary fluid then mix together within the mixing chamber. Due to high

momentum of the primary fluid, the mixed stream is still in form of the supersonic flow. At the

end of the throat section, due to the large difference in pressure between mixed stream and back

pressure (condenser pressure), the series of oblique shocks are thought to be induced. The shock

causes a major compression effect to occur and flow form is suddenly changed from supersonic

to subsonic. A further compression of the flow is achieved as it is brought to stagnation through a

subsonic diffuser. The ejector is discharged at a pressure (back pressure) equal to the saturation

pressure in the condenser. A significant parameter used to indicate the performance of an ejector

is the entrainment ratio:

(1)

For Steam jet refrigeration system referring to Fig.2, an ejector entrains a low pressure saturated

vapour from the evaporator, where the refrigeration effect is produced, as the secondary fluid. It

uses a hot and high-pressure vapour from the boiler as the primary fluid. The ejector discharges

its exhaust to the condenser where the fluid is condensed to liquid by rejecting heat out to the

surroundings. Performance of a steam jet refrigeration system is defined in terms of the

Coefficient of performance (COP):

(2)

Where, hg@evap is the enthalpy of the saturated vapour at the evaporator (kJ/kg)

hg@boiler is the enthalpy of the saturated vapour at the boiler (kJ/kg)

hf@cond is the enthalpy of the saturated liquid at the condenser (kJ/kg)

Usually, the enthalpy change at the evaporator is approximately equal to the enthalpy change at

the boiler and therefore, the COP can be defined as:

(3)

26

According to the literature survey, it is found that most of the jet refrigeration cycle is tested

experimentally based on the Lab-scale. Very few literatures in which the jet refrigeration cycle is

tested at the actual application (used for actual air-conditionings) are found. In addition, it is also

found that the existing jet refrigerator is tested at low ambient temperature where the cooling

water produced by commonly used cooling tower is relatively low (17-20°C) compared to

ambient temperature of Thailand (28-33°C). This is a critical point for designing the jet

refrigerator to be used for the actual ambient condition in Thailand.

The aim of this paper is to develop the jet refrigerator and test it under the actual ambient

condition in Thailand. The prototype steam jet refrigerator with a cooling capacity of 3000W is

newly designed and constructed. It is driven by low grade thermal energy. In this case, the LPG

burner is used to simulate the heat source for driving the boiler. The geometries of the ejector are

fixed throughout the test. Two primary nozzles with its throat diameter of 3.3 and 3.8 mm are

used to test the system performance. They produce the nozzle’s exit Mach number of 4. The

refrigerator is used to produce the chilled water at various cooling load. In such case, a 3 kW

electric immersion heater is used to simulate the cooling load. A condenser is cooled by the

cooling water produced by a commonly used cooling tower. The results indicate that the

prototype jet refrigerator can completely be operated at the actual ambient condition in Thailand

where cooling water for a condenser is between 29 and 33°C. It also shows that when the

primary nozzle, 3.8M4, is used, it provides the maximum COP. In such case, the maximum COP

of 0.45 is achieved when the refrigerator is operated at boiler temperature of 110°C, evaporator

temperature of 17°C and cooling capacity of 3000W.

Fig-1 Main Component of Ejector

27

28

2. The prototype of the steam jet refrigerator

Fig. 3 shows the schematic diagram of the prototype steam ejector refrigeration cycle. It consists

mainly of the steam-boiler, evaporator, ejector, condenser and pumping system. All components

were fabricated from stainless steel, brass and polymer. The Argon-TIG welding technique was

used in order to assemble all stainless steel vessels. Mixing chamber, throat, subsonic diffuser

and primary nozzle were made up of brass. All valves and fittings used in the system were

manufactured by stainless-steel and fittings used were compression fitting type. Stainless-steel

tubes were used for piping work to link all components together. The steam-boiler consists of

two components which are steam boiler and steam separator. The steam-boiler shell was

fabricated from a 5-inch, 50 cm long, stainless steel pipe, with two flanges welded at the top and

the bottom. The mixed steam and liquid in the top part of boiler is separated at the steam

separator. It was fabricated from a 4-inch, 40 cm long, schedule 40s stainless steel tube. To

prevent the heat loss between boiler’s vessel and surroundings, the boiler was well-insulated by

glass-fiber wool covered by aluminum foil. The LPG burner was used to generate the heat for

producing the steam. The steam produced by the boiler is used as the primary fluid for the

primary nozzle.

29

The evaporator vessel was made up of 4-inch stainless steel pipe. In order to promote the

refrigeration effect, the 10 baffles were fitted along the evaporator column. The working fluid

within the evaporator was circulated using magnetic-coupling pump to promote the refrigeration

effect. A 3000W electric immersion heater used together with variable transformer was used to

produce the cooling load. It was installed within the insulated box which the water is contained

inside. A cooling load was transferred to the working fluid within the evaporator via a plate heat

exchanger. Therefore, the chilled water produced by this refrigerator is directly related to the

evaporator temperature. To prevent the heat loss and gain between evaporator’s vessel and

surroundings, it was well-insulated by neoprene foam.

The condenser was modified from three plate heat exchangers. They were connected together in

parallel. The cooling water produced by a commonly used cooling tower was used to absorb heat

from the working fluid and was then rejected to the surroundings. The working fluid condensed

from the condenser was accumulated within the receiver tank. It was then pumped back to the

boiler and evaporator by means of a pneumatic diaphragm pump.

The ejector and primary nozzle used were fabricated by brass. They were designed based on

compressible flow theory recommended by literature. The significant dimension of the ejector

and primary nozzle is shown in Fig 4.

Type-k thermocouple with the accuracy of 0.5°C was used to measure the temperature at the

point of interest. The pressure inside the boiler, evaporator, and condenser were detected using

the pressure transducer and pressure gauge. The level of the working fluid of all vessels can be

observed using attached sight glass. In case of protecting the system, the relief valve was

installed at the steam boiler to limit pressure inside the vessel.

3. Discussions

3.1. Variation of the evaporator temperature with the cooling capacity

Fig. 5 shows the variation of the evaporator temperature with an increase in the cooling capacity

after the steady state is reached. In this case, the primary nozzle, 3.8M4, is used to investigate.

The boiler temperature is varied between 110°C and 120°C. During the test, the chilled water

used for cooling the condenser is varied between 29 and 33°C.

30

According to the Fig. 5, it can be seen that with the fixed boiler temperature, the evaporator

temperature is increased with cooling capacity. The refrigerator can provide the lowest

evaporator temperature of 5°C at the cooling capacity of 500W and the maximum evaporator

temperature of 17°C at the cooling capacity of 3000W. This evaporator temperature range can be

used properly for air conditioning system. The reason of an increase in evaporator temperature

with cooling capacity is that at higher cooling capacity, the larger amount of refrigerant within

the evaporator (secondary fluid) is produced. This causes the pressure within the evaporator to

increase. Thus, the saturation evaporator temperature is also increased. When the boiler

temperature is increased from 110°C to 120°C, the similar result is obtained as can be seen in

Fig. 5. It is found obviously that at the same cooling capacity, it produces the same evaporator

temperature when the boiler temperature is changed. This is because a suction pressure of the

ejector is independent of the boiler temperature. A suction pressure of the ejector depends only

on the designed primary nozzle’s exit Mach number that agrees well with the compressible flow

theory. It reflects the fact that in order to achieve a better performance for one particular primary

nozzle, the refrigerator should be run at relatively low boiler temperature.

31

Fig. 6 shows the results that are tested by the primary nozzle, 3.3M4 and 3.8M4. In this case, the

boiler temperature is kept constant at 120°C. It can be seen from the Fig. 6 that with a fixed

cooling capacity, the evaporator temperature is independent of the primary nozzle used. This is

because both primary nozzles are designed to produce the same nozzle’s exit Mach number. As a

result, both primary nozzles can produce the same suction pressure. Therefore, the refrigerator

with the use of these two primary nozzles can provide the same evaporator temperature when the

cooling capacity is fixed. This implies that the primary nozzle with a fixed nozzle’s exit Mach

number which is used for the jet refrigerator does not affect the ability of producing the

refrigeration effect.

3.2. Variation of the coefficient of performance with the cooling capacity

In order to analyze the system performance of the jet refrigerator, the well known term, the

coefficient of performance (COP), is necessary to know because it indicates the overall

performance of the refrigerator. The coefficient of performance of the jet refrigerator can be

defined by equation (4).

(4)

32

Where, Qcooling is the cooling load produced at the evaporator

Qboiler is the heat supplied to the boiler

The heat supplied to the boiler can be calculated by equation (5).

(5)

Where, hg@boiler is the saturated vapour enthalpy produced by the boiler (kJ/kg)

hf @cond is the saturated liquid enthalpy at the condenser (kJ/kg)

mp is the primary fluid mass flow rate (kg/s)

In this case, the primary fluid mass flow rate produced by the boiler can be calculated by

equation (6).

(6)

Where, A is the cross section area of the primary nozzle’s throat (m2)

Po is the boiler pressure (kPa)

To is the boiler temperature (K)

k is the specific heat ratio

R is the ideal gas constant (kJ/kg.K)

Fig. 7 shows the variation of the COP with an increase in the cooling capacity when the primary

nozzle, 3.8M4, is used to test. The boiler temperature is varied between 110°C and 120°C. The

value of COP is obtained from equations (4), (5), and (6).

33

It can be seen from Fig. 7 that with a fixed boiler temperature the COP of the jet refrigerator is

increased linearly with an increase in the cooling capacity. The lowest COP for the test is of 0.03

for the cooling capacity of 500 W. Meanwhile, the highest COP of 0.45 is obtained at the cooling

capacity of 3000W. An increase of COP with cooling capacity is the result of an increase in

cooling capacity while the heat supplied to the boiler is kept constant due to a fixed boiler

temperature. Therefore, COP of the jet refrigerator is increased with cooling capacity. However,

it comes together with an undesired cooling temperature, due to an increase in the evaporator

temperature.

When the boiler temperature is increased from 110°C to 120°C as can be seen in Fig. 7, with a

fixed cooling capacity, a decrease in the COP is found. This is due to the fact that at the higher

boiler temperature, the higher heat load supplied to the boiler is required while the cooling

capacity is constant. This causes the COP of the refrigerator to drop when the boiler temperature

is increased. This implies that for one particular cooling capacity, the jet refrigerator should be

operated at relatively low boiler temperature. However, if the boiler temperature is too low, it

may cause the malfunction mode of the jet refrigerator due to an inadequate boiler pressure for

producing the nozzle’s exit Mach number of 4. As a result of this effect, the ejector is not able to

draw a secondary fluid from the evaporator.

34

Fig. 8 shows the variation of the COP with an increase in the cooling capacity when the primary

nozzle, 3.3M4 and 3.8M4, are used to test. The boiler temperature is fixed at 120°C throughout

the test.

At fixed cooling capacity, it can be seen from Fig.8 that when the primary nozzle used is

changed, the COP also varies significantly. In such case, if the larger primary nozzle’s throat is

used, it provides lower COP than that of the case of using smaller one. With the use of larger

primary nozzle’s throat, the larger amount of primary fluid mass flow rate produced by the boiler

is allowed to pass through the primary nozzle. This causes the heat supplied to the boiler to

increase. At the same time, when the cooling capacity is kept constant, a reduction in COP is a

consequence. Therefore, with a fixed boiler temperature, the use of a smaller primary nozzle is

preferred

However, the use of a smaller primary nozzle may cause the malfunction of the ejector’s

operation due to a decrease in total momentum of the mixed fluid within the ejector. In such

case, lower temperature of the cooling water is needed to cool down the condenser. Therefore,

the jet refrigerator may not be workable with the actual ambient condition in Thailand

35

4. Conclusions

The prototype steam jet refrigeration cycle driven by low-grade thermal energy is developed for

testing at actual ambient condition in Thailand where the cooling water produced by cooling

tower is between 29 and 33°C. It is found that the prototype steam jet refrigerator can completely

be operated with various range of the operating-condition and primary nozzle used which is

suitable for air-conditioning system (chilled water produced by refrigerator between 5°C and

17°C). The refrigerator can provide the maximum COP of 0.45 at the cooling capacity of

3000W, evaporator temperature of 17°C and boiler temperature of 110°C.

36

EXPERIMENT NO:-5

Aim: Design of cascade refrigeration system for particular application

Introduction

When the vapour compression system is to be used for the production of low temperature, the

common alternative to stage compression is called Cascade System.

The cascade refrigeration system consists of two or more vapour compression refrigeration

systems in series which use refrigerants with progressively lower boiling temperatures. The two-

stage cascade system using two refrigerant is shown in fig. and its corresponding p-h and T-s

diagrams are shown in fig.

Fig.1 Two stage Cascade System

In this system, a cascade condenser serves as an evaporator for the higher temperature cascade

system and a condenser for the low temperature cascade system. The only useful refrigerating

effect is produced in the evaporator of the low temperature cascade system.

The high temperature cascade system uses a refrigerant with high boiling temperature such as R-

12 or R-22.The low temperature cascade system uses a refrigerant with low boiling temperature

such as R-13 or R-13. These low boiling temperature refrigerants have extremely high pressure

which ensures a smaller compressor displacement in the low temperature cascade system and

high COP.

37

The difference in low temperature cascade condenser temperature and high temperature cascade

evaporator temperature is called Temperature overlap. Thisis necessary for heat transfer. If these

temperatures are equal, then it is known as Intermediate Temperature.

Analysis of Cascade refrigeration system

Fig.2 P-h Diagram Fig.3 T-s Diagram

If Q tonne of refrigeration is load on the low temperature cascade system, then the mass of

refrigerant flowing through the system is given by

m1=210Qh1−h2

kg /min (1)

The mass of refrigerant m2 required in the high temperature cascade system in order to liquefy

the refrigerant of low temperature cascade system in the cascade condenser may be obtained by

balancing the heat of both systems.

Heat absorbed in high temp. system = Heat rejected in low temp. system

m2 ( h5−h8 )=m1 ( h2−h f 3 ) (2)

m2=m1 (h2−hf 3 )

(h5−h8 ) =

m1 (h2−h4 )(h5−h8 )

kg /min (3)

Total work done by the system,

W =m1 (h2−h1 )+m2 ( h6−h5 ) kJ /min (4)

38

Refrigerating effect,

RE=210Q kJ /min

Coefficient of performance of the system,

C .O . P .=RE

W= 210Q

m1 (h2−h1 )+m2 ( h6−h5 )

(5)

Power required to drive the system,

P=m1 ( h2−h1 )+m2 (h6−h5 )

60kW

(6)

Design of Cascade Refrigeration System:

Consider suitable evaporator temperature (TE) and condenser temperature (TC) as -800c and 320c

respectively for cooling capacity 0.5 TR of cascade refrigeration system. The superheating

before compression and the sub-cooling after condensation at both high temperature and low

temperature stages are considered. Compressor efficiency of 80% for high temperature side

compressor and 85% for low temperature side compressor are considered. In evaluation of heat-

transfer coefficient for refrigerant neglecting fouling factor during flow of refrigerant inside the

tube as well as outside the tube and also neglecting wall resistance for tube wall.

Assuming the following data:-

High temperature side:- Low temperature side:

Tc= 32 0C TC= -38 0C

Te= -40 0C Te= -800C

Pc= 19.74 bar Pc= 7.627 bar

Pe= 1.759 bar Pe= 1.136 bar

Refrigerent used :- R-410A Refrigerent used :- R-23

39

Cooling capacity = 0.5 TR Refrigeration effect = 1.758 kW

Considering refrigeration enters in compressor with dry and saturated condition & also there is

no sub-cooling after condensation process for both stages.

First finding the refrigerant properties for both refrigerants.

For HTS side refrigerant

From the refrigerent properties chart of HTS and LTS side refrigerant, (using thumb rule)

finding thermodynamic properties at all possible points, and also finding at the remaining

points by using governing equations for the vapour compression refrigeration system.

The enthalpy and entropy of the refrigerant after compression process formulatted by given

equation,

Fig-4 Schematic diagram of cascade refrigeration

40

Fig-5 Temperature-entropy diagram of cascade refrigeration system

h=h'+C pv(T−T ' )(7)

s=s=s,+C pv ln TT .

(8)

Now finding the COP of the system,

COPcascade system = R . E . at LTS

m.

RH ( Δh)+m.

RL ( Δh )

(9)

Considering cooling capacity or refrigerating effect(capacity) = 1.758 kW

After finding values of Enthalpy and entropy at all points for both HTS & LTS,

R .E .=m.

R ( Δh )

Now finding also mass flow rate of Refrigerant

Applying energy balance to the cascsde condenser,

41

Heat absorb by HTS refrigerant = Heat rejected by LTS

m.

RH ( Δh )HTS, evaporator = m.

RL ( Δh )LTS ,condenser

Compressor design:-( LTS and HTS)

Considering compressor speed 1440 RPM & ratio of stroke length to bore diameter L/D=1.1

We know that governing equation for swept volume of compressor

V swept=π4×D2×L× N

60=m

.v1

(10)

from above relation diameter (D) and stroke length (L) can be evaluated.

Condenser design :-(HTS)

Here considering water-cooled condenser (concentric tube type) in which temperature rise

( Δt )water of water is about 3 to 6 K, so here we take ( Δt )water=6 K . Selecting tube material as

copper & ID=0.0060 m and OD= 0.0127 m. then after finding all necessary thermo-physical

properties of refrigerant and water found at bulk mean temperature of the fluid. also finding the

heat duty of the condenser.

Fig.-6 Line diagram of condenser

42

Qcondenser=m.

R ( Δh )

heat duty of condenser can be also formulated by given equation

Qcondenser=U A s ΔT m

So for finding heat transfer surface area overall heat-transfer coefficient and log-mean

temperature difference has to be found out.

LMTD =( th , in−tc , out )−(th ,out−t c ,in )

ln [ (th, in−t c ,out )(th, out−t c , in) ]

(11)

Now overall heat transfer coefficient is estimated by considering both of refrigerant and water

film heat transfer coefficient. Overall heat-transfer coefficient based on outside surface area,

But due to very small wall thickness of pipe so neglecting wall resistance and neglecting fouling

on both side,

1UA0

= 1h i A i

+ 1h0 A0

+ln (D0 / Di )

2 π kL+F i+F0

(12)

Now refrigerant flows through tube, the phenomenon of condensation inside horizontal tube is

quite complicated. At low vapor velocities (Ref<35000), then experimental results agree very

well with Chato’s correlation,

hi=0 . 555[ gρf (ρ f−ρg )k f3 hfg

'

μf ΔtD i ]1/4

(13)

43

Where modified latent heat,

h fg' =hfg+

38

C f ΔT i

Water side heat-transfer coefficient:-(h0)

Using Dittus-Boilter correlation

h0 De

kw=0.023×(Re )0. 8×( pr )0. 4

(14)

Where,D e=

4 A f

p

After finding heat transfer coefficient for both fluid refrigerant and water side ,then finding the

overall heat-transfer coefficient by neglecting the fouling factor and also considering A i=Aoand

finally found the heat transfer area and length of condenser (heat-exchanger) requirement.

Evaporator design: - (LTS)

Considering air inside the cabinet to be cooled by evaporator coil ( concentric tube type) which is

accommodate by cabinet and material of coil is copper having ID = 0.0952 m & OD = 0.00812

m. Refrigerating effect has been already considered as 0.5 TR. process occurs inside the the

evaporator is almost reverse to the condenser.

Qevaporater= R .E .= mR−508 B ( Δh)=m.

airC pair ΔT air(15)

44

Fig-7 Line diagram of evaporator

So for finding heat transfer surface area overall heat-transfer coefficient and log-mean

temperature difference has to be found out by using the eq.no.(12) and (11) Now overall heat

transfer coefficient is estimated by considering both of refrigerant and water film heat transfer

coefficient.

Refrigerant side heat-transfer coefficient:- (hi)

In commercial equipment, the boiling process occurs in two types of situation:-

(1) Pool boiling as in flooded evaporators with refrigerant boiling on the shell-side.

(2) Flow or forced convection boiling as in direct expansion evaporator with refrigerant on

the tube side.

Now, find physical properties of Refrigerant from charts or tables at the saturation or evaporation

temperature Te by using correlation:

Nud=0 . 027 Red0. 8 Pr0 .33 ( μ

μw )0 .14

(16)

Air-side heat transfer coefficient:-(ho)

If we considering forced convection, air flowing outside the tubes, then average heat-transfer

coefficients for the forced convection of air across a tube may be calculated from the “Grimon’s

correlation. Assume that air circulated with the rate of 120 m3/min and face velocity of air 6

m/sec. By using correlation,

ho D e

k=C×(Re )n×( pr )1/3

(17)

45

Overall heat transfer coefficient for evaporator also found by using equation (4.9) considering

same assumption as in previous case. Finally found out the heat-transfer area and require heat-

exchanger length (evaporator).

Cascade condenser design: - (concentric tube type)

Cascade condenser behaves as evaporator for HTS while as condenser for LTS. HTS Refrigerant

flowing through the one tube and LTS Refrigerant flowing through the another tube. Taking ID

= 0.0060 m & OD = 0.0127 m for both the tubes having material is copper. Thermal conductivity

of copper kcopper = 386 W/m K.

Fig-8 Line diagram of cascade condenser

Applying energy balance to the cascsde condenser, also if we considering the efficiency of the

cascade condenser is 0.75, then finding the mass flow rate of one of the refrigerant by using the

eq. (18).

ηcascadeHE=mRH ( Δh)HTS ,evaporator

mRL( Δh )LTS , condenser

(18)

Now, Heat duty of cascade condenser formulated by eq. (19).

Qcascadecondenser=mRH ( Δh )HTS, evaporator+mRL ( Δh )LTS, condenser(19)

46

The LMTD and required heat-transfer area of the cascade condenser is calculated likewise

condenser of HTS.

R410A/R23

Cascade refrigeration

system

TCH= 320C, TeL=-800C & DT=2

Heat-transfer

area in m2

Required length of heat

exchanger in m

Heat duty in H E in

kW

Condenser (HTS) 0.4287 10.74 1.361

Cascade condenser 0.3359 8.418 0.7276

Evaporator (LTS) 0.2171 7.26 0.5

Thermal design for R410A/R23 cascade refrigeration system

R404A/R508B

Cascade refrigeration

system

TCH= 320C, TeL=-800C & DT=2

Heat-transfer

area in m2

Required length of heat

exchanger in m

Heat duty in H E in

kW

Condenser (HTS) 0.6838 17.14 1.352

Cascade condenser 0.4682 11.73 0.7371

47

Evaporator (LTS) 0.2158 7.217 0.5

Design of expansion device: - (capillary tube)

For HTS & LTS :- The design procedure for capillary tube for HTS and LTS is similar

The state of the entering refrigerant is assumed saturated liquid.

The mass flow rate m.

RH is evaluated.

The condensing temperature TCH& pressure PCHwhereas evaporating temperature

&TeL&peLare known.

In actual practice, expansion takes place adiabatically thus, enthalpy does not remain

same,

Fig-9 Expansion process for HTS

Dividing expansion line into small divisions such as 7-a, a-b, b-c, c-d, d-e, & e-8.

Now considering isenthalpic expansion and assuming tube diameter (d i ) = 2.3mm or

0.0023 m.

Now cross-section Area of capillary tube evaluated and by using of it calculated the mass

velocity of the refrigerant flowing through the capillary,

48

mass velocity (G )=m.

RH

A= ρ uA

A=ρu=u

ν(20)

Now finding the all necessary thermodynamic properties of refrigerant like enthalpy, dryness

fraction (χ), specific volume (υ) and velocity (u) at all points.

For all sub-point find dryness fraction, specific volume, velocity, dynamic viscosity and friction

factor by using below equations.

h = hf + xh fg(21)

v=v f + x (v g−v f )(22)

Re= ρuDμ

(23)

μ= (1−x )μ f+xμg(24)

friction factor ( f )= 0. 324Re0 . 25

(25)

The pressure drop due to the friction for first decrement Δp F for single phase

ΔpF=ρ f sp ΔL u2

2D(26)

49

Δp F=Δp−Δp A(27)

The pressure drop due to the friction and acceleration for second decrement for two phases

ΔL=−2 DG2 [ Δp

f tp

−v+ G2 Δv

f tp

−v

]

(28)

The total length required for capillary tube to get the desire pressure drop between condenser and

evaporator

Ltotal=ΔL3−a+ΔLa−b+ΔLb−c+ ΔLc−4(29)

The required diameter of capillary tube to get desire pressure drop for given length is obtain

using equation,

2 DΔp−8 mπD

Δu=Ltotal ρ fu2

(30)

Application of Cascade system:

1) Liquefaction of oxygen using SO2 and CO2 as intermediate refrigerants.

2) Liquefaction of petroleum vapours

3) Liquefaction of industrial gases

4) Manufacturing of dry ice

5) Deep freezing

Comments:

1) Using a cascade system the power consumption could be reduced by about 9.5 %.

2) In actual systems, the compared to the single stage system, the compressors of cascade

systems will be operating at much smaller pressure ratios, yielding high volumetric and

50

isentropic efficiencies and lower discharge temperatures. Thus cascade systems are

obviously beneficial compared to single stage systems for large temperature lift

applications.

3) The performance of the cascade system can be improved by reducing the temperature

difference for heat transfer in the evaporator, condenser and cascade condenser,

compared to larger compressors.

51

52