vapour compression refrigeration system|vapour compression refrigeration|refrigeration cycle diagram

Introduction

A vapour compression refrigeration system is an improved type of air refrigeration system in which a suitable working substance, termed as refrigerant, is used. It condenses and evaporates at temperatures and pressures close to the atmospheric conditions. The refrigerants, usually, used for this purpose are ammonia (NH 3), carbon dioxide (C0 2) and sulphur dioxide (SOz). The refrigerant used, does not leave the system, but is circulated throughout the system alternately condensing and evaporating. In evaporating, the refrigerant absorbs its latent heat from the brine (salt water) which is used for circulating it around the cold chamber. While condensing, it gives out its latent heat to the circulating water of the cooler. The vapour compression refrigeration system is, therefore a latent heat pump, as it pumps its latent heat from the brine and delivers it to the cooler.

Components of a Simple Vapour Compression Refrigeration System

1. Compressor - The low pressure and temperature vapour refrigerant from evaporator is drawn into the compressor through the inlet or suction valve A, where it is compressed to a high pressure and temperature. This high pressure and temperature vapour refrigerant is discharged into the condenser through the delivery or discharge valve B.

2. Condenser -The condenser or cooler consists of coils of pipe in which the high pressure and temperature vapour refrigerant is cooled and condensed. The refrigerant, while passing through the condenser, gives up its latent heat to the surrounding condensing medium which is normally air or water.

Simple vapour compresion diagram

3. Receiver - The condensed liquid refrigerant from the condenser is stored in a vessel known as receiver from where it is supplied to the evaporator through the expansion valve or refrigerant control valve.

4. Expansion Valve - It is also called throttle valve or refrigerant control valve. The function of the expansion valve is to allow the liquid refrigerant under high pressure and temperature to pass at a controlled rate after reducing its pressure and temperature. Some of the liquid refrigerant evaporates as it passes through the expansion valve. but the greater portion is vaporised in the evaporator at the low pressure and temperature.

5. Evaporator - An evaporator consists of coils of pipe in which the liquid-vapour refrigerant at low pressure and temperature is evaporated and changed into vapour refrigerant at low pressure and temperature. In evaporating, the liquid vapour refrigerant absorbs its latent heat of vaporisation from the medium (air, water or brine) which is to be cooled .

Theoretical Vapour Compression Cycle with Dry Saturated Vapour after Compression

A vapour compression cycle with dry saturated vapour after compression is shown on T-s and p-h diagrams in Fig. At point 1, let Tl' p 1 and S p be the temperature, pressure and entropy of the vapour refrigerant respectively.The four processes of the cycle are as follows:

1. Compression process. The vapour refrigerant at low pressure P1 and temperature T1 is compressed isentropically to dry saturated vapour as shown by the vertical line 1-2 on T-S diagram and by the curve 1-2 on P-H diagram. The pressure and temperature rises from P1 to P2  and T 1 to T2  respectively.

The work done during isentropic compression per kg of refrigerant is given by

w = h 2 - h1

h1 = Enthalpy of vapour refrigerant at temperature T1 , at sunction of the compressor, and

h2 Enthalpy of the vapour refrigerant at temperature T 2,  at discharge of the compressor.

P-V diagram of Vapour Compression Refrigeration System

2. Condensing process. The high pressure and temperature vapour refrigerant from the compressor is passed through the condenser where it is completely condensed at constant pressure p 2 and temperature T 2, as shown by the horizontal line 2-3 on T-s and p-h diagrams. The vapour refrigerant is changed into liquid refrigerant. The refrigerant, while passing through the condenser, lives its latent heat to the surrounding condensing medium.

T-S diagram

3. Expansion process. The liquid refrigerant at pressure p 3 = p 2 and temperature T3 T 2  isexpanded by *throttling process through the expansion valve to a low pressure p 4 = p1 and temperature T4 = T 1• as shown by the curve 3 - 4 on T-s diagram and by the vertical line 3-4 on p-h diagram. We have already discussed that some of the liquid refrigerant evaporates as it passes through the expansion valve, but the greater portion is vaporised in the evaporator. We know that during the throttling process, no heat is absorbed or rejected by the liquid refrigerant.

4. Vaporising process. The liquid-vapour mixture of the refrigerant at pressure p 4 = p 1 and temperature T4 = T1 is evaporated and changed into vapour refrigerant at constant pressure and temperature, as shown by the horizontal line 4-1 on T-s and p-h diagrams. During evaporation. the liquid-vapour refrigerant absorbs its latent heat of vaporisation from the medium (air, watec urbrine) which is to be cooled. This heat which is absorbed by the refrigerant is called refrigeration effect and it is briefly written as RE. The process of vaporisation continues upto point 1 which is the starting point and thus the cycle is completed.

Advantages

• It has smaller size for the given capacity of refrigeration.

• It has less running cost.

• It can be employed over a large range of temperatures.

• The coefficient of perfonnance is quite high.

Disadvantages

• The initial cost is high.

• The prevention of leakage of the refrigerant is the major problem in vapour compression system.

Comparison between gas cycles and vapor cycles

Thermodynamic cycles can be categorized into gas cycles and vapour cycles.In a typical gas cycle, the working fluid (a gas) does not undergo phase change, consequently the operating cycle will be away from the vapour dome. In gas cycles, heat rejection and refrigeration take place as the gas undergoes sensible cooling and heating. In a vapour cycle the working fluid undergoes phase change and refrigeration effect is due to the vaporization of refrigerant liquid. If the refrigerant is a pure substance then its temperature remains constant during the phase change processes. However, if a zeotropic mixture is used as a refrigerant, then there will be a temperature glide during vaporization and condensation. Since the refrigeration effect is produced during phase change, large amount of heat (latent heat) can be transferred per kilogram of refrigerant at a near constant temperature. Hence, the required mass flow rates for a given refrigeration capacity will be much smaller compared to a gas cycle. Vapour cycles can be subdivided into vapour compression systems, vapour absorption systems, vapour jet systems etc. Among these the vapour compression refrigeration systems are predominant.