performance of the vapour compression cycle as a refrigerator and as a heat pump (2)

8/12/2019 Performance of the Vapour Compression Cycle as a Refrigerator and as a Heat Pump (2)

http://slidepdf.com/reader/full/performance-of-the-vapour-compression-cycle-as-a-refrigerator-and-as-a-heat 1/7

Objectiive :-To find out the performance of the vapour compression cycle as a refrigerator and as a heat pump

THEORY:-Vapour compression refrigeration cycle:-The vapor-compression refrigeration cycle is the most widely used cycle for refrigerators, air-conditioning systems, and heat pumps. It consists of four processes:

1-2 Isentropic compression in a compressor 2-3 Constant-pressure heat rejection in a condenser 3-4 Throttling in an expansion device

4-1 Constant-pressure heat absorption in an evaporator

Schematic and T - s diagram for the ideal vapor-compression refrigeration cycle



Schematic and T - s diagram for the actual vapor-compression refrigeration cycle.

THE P-h DIAGRAM :-



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

refrigerantsAnother device that transfers heat from a low-temperature medium to a high temperature one is

the heat pump The performance of refrigerators and heat pumps is expressed in terms of the coefficient of

performance (COP), defined as

For Refrigerator:-

E

R

heat absorbed at the lower temperatureCOP

compressor net work

Q

W = =

For a heat pump COPH:-

C

H

heat rejected at the higher temperatureCOP

compressor net work

Q

W = =

Here where E, C, R, H stand for Evaporator, Compressor, Refrigeration, and Heat

pump respectively.



OBSERVATIONS AND CALCULATIONS

h1=375 kJ/kg

h2=465 kJ/kg

h3=240 kJ/kg

h4=240 kJ/kg

h2’=420 kJ/kg

CALCULATIONS

1. C.O.P R

=1.5

2. C.O.P. (HP) =

=2.5

57 Evaporator Gauge Pressure(KPa) 079 Condenser Gauge Pressure(KPa)

385 Evaporator Outlet Temp., T1 (°C) 338 Evaporator Inlet Temp.,T4 (°C) 5580 Condenser Outlet Temp., T3 (°C) 8 Condenser Inlet Temp., T2 (°C) 5.8 Condenser Water Inlet Temp., T5 (°C) 98 Condenser Water Outlet Temp.,T6 (°C) 9 Condenser Water Flow Rate,m˙ (g/s)

Refrigerant flow rate,m˙(g/s)



3. coefficient of performance of the Heat pump is:-

C.O.PHP= C.O.PR +1

=1.5+1

=2.5

4. Heat gained by water=mcpΔt

=1.338E^6 kJ/kg

Heat effects

5. The rate of heat rejection from the refrigerant to the environment=m’(h2-h3)

=1.35 kW

6. Refrigeration effect=m’(h1-h4)

=.81 kW

7. W actual= m˙(h2-h1)

=.54kJ/sec

8.

W isentropic= m˙(h2’-h1)

=.27 kJ/sec

9. Degree of super cooling=(T3-Tsatliq)=

Tsatliq

=30°C(from P-h diagram)

=6°C

10. Degree of superheating=(T1-Tsatvap)

Tsatvap

=9°C (from P-h diagram)

S0 ans=9.20C

11. Efficiency η

=50%



Discussion

The reason behind using p-h diagram instead of t-s diagram was that On this diagram,

three of the four processes appear as straight lines, and the heat transfer in the condenser

and the evaporator is proportional to the lengths of the corresponding process curves.

In our calculations we neglected pressure drops in concentric tube heat exchange, this

assumption is not correct, because there are always some loses due to friction

There are also limitations associated with the refrigerant as we cannot get cooling n that

environment whose temperature is less than the freezing temperature of refrigerant. So

for better efficiency The temperature of refrigerant, higher than which its condensation

cannot be achieved should be high. Refrigerant should have better characteristics of heat

transfer, since they permit less difference in temperature of evaporator and condenser,

and low lift in temperatures. Viscosity of refrigerants should be low.

One ton cooling or heating means that how much heat is withdrawn or absorbed to freeze

or melt one ton of material.

Usually it is one ton water at zero0C and the process time is of 24 hours.

Mostly used refrigerant in use in industry are ammonia ,water, and most absorbers used

are water and lithium bromide

Co efficient of performance of heat pump is always 1 greater than that of cop of

refrigeration. this is proved by our calculations. this relationship is correct.

Water temperature used for cooling is increasing gradually.the reason behind is that we

are using same water.ie recycling without cooling.for better cooling purposes we shoulduse fresh water cooled water.

We can perform experiments related to heat exchanger on this unit.and can study heat

transfer phenomenon.

Both pumps and compressors increase the pressure of the working fluid. However, a

pump is used to increase the pressure of a liquid, while a compressor is used to increase

the pressure of a gas. A conventional vapor-compression refrigeration cycle uses R-134a

(or some variant), which exhibits a phase change during the cycle. The fluid exits the

evaporator in a gaseous phase at a relatively low pressure, so a compressor is used to

bring the fluid up to the proper pressure to return to a liquid phase in the condenser. Note

that any vapor-compression cycle utilizes a fluid that exhibits a phase change during the

cycle, and is therefore NOT an ideal gas. The ideal gas law does not apply for these fluids



Conclusion:-

Readings were taken from different instruments installed on the unit.With the properties of the refrigerant R134a at all the stages of theRefrigeration cycle known, the heat transfer at the condenser and evaporator were calculated.

Performance of the unit is also calculated ideally and actual. also efficiency was determined and

found it to b 50%

Applications of Vapor Compression Refrigeration

This system is generally used for the purpose of air conditioning, with the aim of providing a comfortableenvironment.It is also employed in refrigerators used in homes, for preservation of foods, vehicles in

which food items are carried, and other similar purposes

performance of the vapour compression cycle as a refrigerator and as a heat pump (2)

Documents