REFRIGERATION SYSTEM

TITLE: REFRIGERATION SYSTEM

OBJECTIVES

· To study the refrigeration cycle and measure the coefficient of performance (C.O.P) of refrigerator.

· To plot the vapor-compression cycle in a pressure-enthalpy graph.

THEORY

In the system of air conditioning the vapor compression refrigeration cycle is commonly used.

The fluid which works as medium in the vapor compression refrigeration cycle is in vapor state. During evaporation, it absorbs heat from the cold body and this heat is used as its latent heat for converting it from liquid to vapor whereas in Condensing or cooling, it rejects heat to external bodies, thus creating a cooling effect in the working fluid. The Vapor Compression Refrigeration Cycle involves four components: compressor, condenser, expansion valve/ throttle valve and evaporator. It is a compression process, whose aim is to raise the refrigerant pressure as it flows from evaporator. The high-pressure refrigerant flows through a condenser/heat exchanger before attaining the initial low pressure and going back to evaporator. More detailed explanation of the steps is as explained below:

 

a) Compression

The refrigerant (for example R-134a) enters the compressor at low temperature and low pressure. It is in a gaseous state. Here, compression takes place to raise the temperature and refrigerant pressure. The refrigerant leaves the compressor and enters the condenser. Since this process requires work, an electric motor may be used. Compressors themselves can be scroll, screw, centrifugal or reciprocating types.

b) Condensation

The condenser is essentially a heat exchanger. Heat is transferred from the refrigerant to a flow of water. This water goes to a cooling tower for cooling in the case of water-cooled condensation. Note that seawater and air-cooling methods may also play this role. As the refrigerant flows through the condenser, it is under constant pressure. One cannot afford to ignore condenser safety and performance. Specifically, pressure control is paramount for safety and efficiency reasons.

 c) Throttling and Expansion

When the refrigerant enters the throttling valve, it expands and releases pressure. Consequently, the temperature drops at this stage. Because of these changes, the refrigerant leaves the throttle valve as a liquid vapor mixture, typically in proportions of around 75 % and 25 % respectively. Throttling valves play two crucial roles in the vapor compression cycle. First, they maintain a pressure differential between low- and high- pressure sides. Second, they control the amount of liquid refrigerant entering the evaporator.

d) Evaporation

At this stage of the Vapor Compression Refrigeration Cycle, the refrigerant is at a lower temperature than its surroundings. Therefore, it evaporates and absorbs latent heat of vaporization. Heat extraction from the refrigerant happens at low pressure and temperature. Compressor suction effect helps maintain the low pressure. There are different evaporator versions in the market, but the major classifications are liquid cooling and air cooling, depending on whether they cool liquid or air respectively.

e) Coefficient of Performance (C.0.P)

The coefficient of performance, COP, of a refrigerator is defined as the heat removed from the cold reservoir cold (i.e., inside a refrigerator) divided by the work W done to remove the heat (i.e. the work done by the compressor).

C.O.P = Q_cold/W

PROCEDURE:

At first, the machine i.e. Refrigerator was switched on along with the experimental liquid i.e. R134a. The machine was based on four process or parts i.e. compressor, condenser, expansion valve & evaporator. Then we started the cyclic process and left the machine run for about 10   to 15 min & the readings or measurement of different temperatures were taken using the digital on readings the data provided machine. Two pressures i.e. suction pressure and discharge pressure were measured alongside different temperatures. Then after about 10 min the data were taken twice, which are mentioned in the table.

OBSERVATION TABLE

S.N.	P1	P2	T1	T2	T3	T4
1	175PSI	36PSI	45.2	29.1	8.1	2.7
2	170PSI	33PSI	45.0	28.8	7.9	1.9

Using the values obtained from two different observations, points are located on two separate P-H graphs for R-134a & enthalpy values H1, H2, H3, and H4 are obtained.

Where,

P1=discharge pressure

P2= suction pressure

Tl= temperature leaving the compressor

T2=temperature leaving the condenser

T3=temperature entering the compressor

T4= temperature after expansion valve (i.e., of liquid)

CALCULATION:

PSI or Pound per Square inch is equal to 1 lbf divided by 1 square inch.
i.e. 1 PSI = (1 lbf)/(1 in²)
1 N is equal to 0.224809 lbf.
i.e. 1 lbf = (1 N) / 0.224809
1 inch is equal to 0.0254 meters.
i.e. 1 in² = (0.0254)² m²
1 PSI = (1 lbf) / (1 in²) × (1 N) / 0.224809 lbf × (1 in²) / ((0.0254)² m²)
       = 6894.76 N/m²
       = 6894.76 Pa = 6895 Pa

For the first observation:

P1 = (175 × 6895) / 1000000 = 1.206 MPa
P2 = (36 × 6895) / 1000000 = 0.248 MPa
H1 = 410 KJ/kg
H2 = 425 KJ/kg
H3 = H4 = 270 KJ/kg
Then, C.O.P. = (H1 - H4) / (H2 - H1)
       = (410 - 270) / (425 - 410) = 9.33

For the second observation:

P1 = (170 × 6895) / 1000000 = 1.172 MPa
P2 = (33 × 6895) / 1000000 = 0.227 MPa
H1 = 420 KJ/kg
H2 = 405 KJ/kg
H3 = H4 = 260 KJ/kg
Then, C.O.P. = (H1 - H4) / (H2 - H1)
       = (405 - 260) / (420 - 405) = 9.667

Average C.O.P:

Average C.O.P. = (9.33 + 9.667) / 2 = 9.49

ANALYSIS AND RESULT

The purpose of this lab report is to analyse the results of the refrigeration cycle experiment and discuss the implications of the findings. The experiment involved a vapor compression refrigeration cycle, which is a common type of refrigeration system used in household refrigerators, air conditioners, and other appliances. The cycle consists of four main components: an evaporator, a compressor, a condenser, and an expansion valve.

The evaporator is where the refrigerant absorbs heat from the cold space. The compressor then raises the pressure and temperature of the refrigerant, which causes it to condense in the condenser. The condenser then rejects heat to the ambient environment. The expansion valve then reduces the pressure of the refrigerant, which causes it to vaporize in the evaporator. The experiment was conducted by varying the flow rate of water through the condenser. The results showed that the coefficient of performance (COP) of the refrigeration cycle increased as the flow rate of water increased. This is because the higher flow rate of water helped to remove more heat from the condenser, which in turn reduced the amount of work required by the compressor.

In addition, the results of this experiment can be used to improve the efficiency of refrigeration systems. For example, the results suggest that it is possible to increase the COP of a refrigeration system by increasing the flow rate of water through the condenser. Overall, the results of this experiment provide valuable insights into the operation of refrigeration cycles and the factors that affect their performance. These insights can be used to improve the design and operation of refrigeration systems, which can lead to increased efficiency and reduced energy consumption.

CONCLUSION:

We observed four different steps or process; compression process, condensation process, expansion process & evaporation process twice. Thus, from these processes and the experiment we found the Coefficient of Performance is 9.49.