Simple Vapour Compression Refrigeration Systems

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 (NH3), carbon dioxide (CO2) and sulphur dioxide (SO2).

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.

The vapour compression refrigeration system is now-a-days used for all purpose refrigeration. It is generally used for all industrial purposes from a small domestic refrigerator to a big air conditioning plant.

Following are the advantages and disadvantages of the vapour compression refrigeration system over air refrigeration system:

Advantages

1. It has smaller size for the given capacity of refrigeration.
2. It has less running cost.
3. It can be employed over a large range of temperatures.
4. The coefficient of performance is quite high.

Disadvantages

1. The initial cost is high.
2. The prevention of leakage of the refrigerant is the major problem in vapour compression system.

Mechanism of a Simple Vapour Compression Refrigeration System

A simple vapour compression refrigeration system consists of the following five essential parts:

• 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 valve B.

• 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.

• 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.

• 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.

• 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.

Note: In any compression, refrigeration system, there are two different pressure conditions. One is called the high pressure side and other is known as low pressure side. The high pressure side includes the discharge line ( i.e. piping from delivery valve B to the condenser), condenser, receiver and expansion valve. The low pressure side includes the evaporator, piping from the expansion valve to the evaporator and the suction line (i.e. piping from the evaporator to the suction valve A).

Pressure-Enthalpy (p-h) Chart

The most convenient chart for studying the behaviour of a refrigerant is the p-h chart, in which the vertical ordinates represent pressure and horizontal ordinates represent enthalpy (i.e. total heat). A typical chart is shown above. in which a few important lines of the complete chart are drawn. The saturated liquid line and the saturated vapour line merge into one another at the critical point. A saturated liquid is one which has a temperature equal to the saturation temperature corresponding to its pressure. The space to the left of the saturated liquid line will, therefore, be sub-cooled liquid region. The space between the liquid and the vapour lines is called wet vapour region and to the right of the saturated vapour line is a superheated vapour region.

Consider the chart below which is typical of the refrigerant R22, a common refrigerant in small refrigeration systems.

A closer analysis of the chart shows that there are distinct regions separated by three “boundary lines”. The region on the left is subcooled liquid. This is the refrigerant liquid at a temperature lower than the equivalent boiling point for the pressure noted.

The region inside the “dome” is a liquid-vapor mixture. If the liquid is at the boiling point, but just hasn't begun to boil, it is defined as saturated liquid. Adding any heat to this liquid will vaporize a portion of it. Adding more heat to the liquid-vapor mixture eventually evaporates all of the liquid. At some precise point (G), the vapor is fully saturated. Adding any more heat to the vapor will cause it to rise in temperature further; this is referred to as superheated vapor.

It is a general tendency to believe that a superheated vapor is “hot”. This is not always the case. Superheated vapours can be cold. By the term superheated, we simply mean that they are above the corresponding saturated vapor point. Similarly, subcooled liquid can be generally warm. It just means that the liquid is cooler than the saturation line at that pressure. We now present a detailed study of the pressure-enthalpy diagram.

We consider the refrigerant to be initially at point A. To reach this point after leaving the evaporator at G, the refrigerant is heated slightly and crosses the compressor suction valve to point A. The compressor elevates the refrigerant's pressure to a point at which it can push the discharge valve open and flow into the condenser. The refrigerant vapor leaves the compressor at point B, de-superheats to point C, and then begins to condense. After the vapor is completely condensed at point D, it is subcooled a bit further (E), at which time it is still at a much higher pressure than the evaporator.

Controlling the flow to the evaporator and throttling to the pressure of the evaporator is the job performed by the expansion device, a capillary tube or a throttling valve in small refrigeration systems. This pressure reduction step vaporizes a portion of the liquid which cools (called flash gas) the remaining liquid going to point F. The "average" mixture of vapor and liquid crossing the valve doesn't change in energy content. It simply separates into liquid and vapor at the reduced temperature and pressure according to its precise thermodynamic properties. The liquid at point F is then ready to pick up heat in the evaporator and form vapor at point G where the cycle repeats itself.