Essay Example: A Vapor Compression Refrigeration System

Abstract.

A vapor compression refrigeration system is one of the most commonly used methods in refrigeration and air conditioning where the refrigerant is used as a medium for absorbing heat from an enclosed space and undergoes varies phase changes resulting to cooling. It therefore necessary to have a concise and clear understanding of how the cycle operates and the relationship between the two main parameters (temperature and pressure).this report serves as an overview of the vapor compression refrigeration by highlighting how it operates and also showing the relationship between the saturation temperatures and pressure in the evaporator and condenser.

Introduction.

Refrigeration is the process of transferring heat from a cold chamber and releasing it to its surrounding. A basic refrigeration system, therefore, consists of a number of processes with the same refrigerant in a continuous refrigeration (Rao, 2010).

A refrigerator is as a machine whose core function is to remove heat from a low-temperature region. Since energy cannot be destroyed according to the first law of thermodynamics, the heat taken in at a low temperature must be dissipated to the surroundings.

Theory

Refrigerators are cyclic devices, and the working fluids used in the refrigerator cycles are called refrigerants. A refrigerator requires an external energy for it to operate. This energy input may be in the form of work or a heat transfer at a high temperature. The most common type of refrigerator uses a work input and operates on the vapor compression cycle.

The Vapor Compression Cycle

The vapor compression cycle in the most widely used cycle for refrigerant, air-conditioning system, and heat pumps. Schematic diagram of a vapor compression cycle is shown in Figure 1 below.

Figure 1 Schematic of the vapor compression refrigeration cycle

The vapor compression refrigeration cycle has four main components:

An evaporator where heat is taken in at a low temperature as a liquid evaporates at a low pressure.

A compressor which uses a work input to reduce the pressure in the evaporator and increase the pressure of the vapor being transferred to the condenser.

A condenser where the high-pressure vapor condenses, rejecting heat to its surroundings.

A flow control device which controls the flow of liquid back to the evaporator and which brings about the pressure reduction.

Vapor Compression Refrigeration Principles.The vapor compression refrigeration cycle process can be represented on a T-s diagram as shown in Figure 1 above. 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 valve

4-1 Constant-pressure heat absorption in an evaporator.

The refrigerant enters the compressor at state 1 as saturated vapor and is compressed isentropically to the condenser pressure. The temperature of the refrigerant increases during this process to well above the surrounding temperature.

The refrigerant then enters the condenser as a superheated vapor at state 2 and leaves as saturated liquid at state 3 as a result of heat rejection to the surroundings (in this case the surrounding is the water circulate through the coil in the condenser). The temperature of the refrigerant at this state is still above the surrounding temperature.

The saturated liquid refrigerant at state 3 is throttled to the evaporator pressure by passing through an expansion valve. The temperature of the refrigerant drops below the temperature of the refrigerated space during this process.

The refrigerant enters the evaporator at state 4 as a low-quality saturated mixture, and it completely evaporates by absorbing heat from the refrigerated space (in this case the surrounding is the water circulate through the coil in the evaporator). The refrigerant leaves the evaporator as a saturated vapor and reenters the compressor, completing the cycle.

Methods.

General Operating Procedures.General Start-up Procedure.Check the unit and all instruments are in proper condition.

Check that the cooling water is supplied to the cooling coil and then allow sufficient water to flow through the cooling coil.

Close all valves except for the valve at the base of the condenser, V1, and the valves at the water inlet, V6, and V7.

Adjust the control valves on the evaporator water flow meter, V7 and condenser water flow meter valve, V6 to give approximately 90 LPH (1.5 LPM).

Ensure the power supply is connected and then turn on the main switch and the compressor switch at the main panel.

Ensure that the vapor is drawn by the compressor to the condenser and the condensed liquid is returned to the evaporator through the expansion valve without any problem.

If air needs to be vented out from the unit, wait until the condenser pressure is high and then slowly open the venting valve at the top of the condenser, V5 until the pressure gauge is about to reach zero and then quickly close the vent valve. Repeat venting the air several times to ensure all the air is drawn out. (Note: do not fully open the vent valve as the refrigerant will be vented out as well)

Let the unit run for a while to stabilize all the parameters.

The unit is ready to proceed with the experimental procedures.

Experiment 1: Relationship between Saturation Pressure and Temperature in the Condenser

Objective:

The objective of this experiment is to investigate the relationship between saturation pressure and temperature in the condenser.

Procedure:

Perform the general start-up procedure

Set the evaporator and condenser flowrate to 1.5 LPM.

Adjust the condenser cooling water flowrate to the maximum.

Allow the system to run for 15 minutes for the temperature and pressure reading to stabilize.

Record the condenser pressure, PT01, evaporator pressure PT02, the condensing temperature TT1, and the evaporating temperature TT4 into the experimental data sheet.

Reduce the condenser cooling flowrate by adjusting valve V6 by a small increment so that the condenser pressure, PT01 is increased.

Allow the unit to stabilize for a few minutes and again record the above parameters.

Repeat the procedure up to the maximum condenser pressure required (dont exceed PT01 of 2.0 bar)

Plot the results of the saturation pressure against saturation temperature and discuss the result.

Results.Cooling water flowrate [LPM] TT1, [oC] PT01, [bar] TT4, [oC] PT01, [bar]

1.6 31.8 2.43 24.5 0.71

1.4 34.1 2.49 24.6 0.72

1.2 35.1 2.53 24.9 0.73

1.0 35.8 2.56 25.1 0.74

0.8 36.5 2.59 25.4 0.75

Observation.

Evaporation and condensation take place when the refrigerant is receiving or rejecting latent heat and this is a constant temperature and pressure process and is clearly demonstrated in the graph. From the chart, as the temperature increases, the pressure in the condenser also increases. This indicates that the two components (pressure and temperature are directly proportional to each other.)Therefore an increase in pressure will increase the cooling/vaporization of the refrigerant.

Discussion.Saturation is synonymous with temperature. Saturation temperature refers to the temperature at which a fluid changes phase from liquid to vapor or vice versa. Its a combination of latent heat of vaporization ( the liquid absorbs heat changed from liquid to gas ) or latent heat of condensation (gases give off heat when changed from gas to liquid).the liquid at saturation temperature is known as a saturated liquid while vapor is known as saturated vapor. ( Rao, 2010)

Conclusion.

Saturation takes place in the condenser and evaporator where it involves phase changes for the vapor and liquid take place.at saturation temperature, the vapor experiences minimum temperature for that given pressure while the liquid experiences its maximum temperature for a given pressure. However, the liquid and vapor remain at a constant pressure when saturation is taking place.

References.

Rao, Y. V. (2010). An introduction to thermodynamics. Hyderabad: Universities Press.