To turn the basic vapour compression cycle into a practical refrigerator, the evaporator should superheat the refrigerant after all the liquid has evaporated. It is not practical to control precisely at the point where evaporation is just finished. Unless it is complete, some liquid will leave the evaporator, which is useful cooling potential wasted. Moreover, compressors do not generally appreciate liquid arriving with the vapour. It can cause damage. So control is provided in such a way as to ensure that the vapour leaving the evaporator is superheated.
The upper diagram shows the refrigeration circuit, and the lower one is the corresponding P-h diagram The process starts with evaporation of the refrigerant in the evaporator. Point 2 is in in the vapour region, to the right of the saturated vapour curve. Compression raises the pressure of the refrigerant, point 3. The vapour is now hot, and is cooled before condensation starts.
It is not possible in practice to control exactly the completion of condensation. We want liquid to flow through the line from the condenser to the control or expansion valve. If some vapour is present here, it can cause excessive pressure drop and reduction in performance of the system. The pressure drop should occur in the valve itself. Some degree of subcooling is necessary to ensure 100% liquid flow. This subcooling can occur in the condenser, and further cooling of the liquid can take place between the condenser and the valve. Point 4 is now in the liquid region, to the left of the saturated liquid curve, the pressure is reduced in an expansion device, and the refrigerant is returned to its original condition 1.
Superheat and Subcooling occupy quite small sections of the diagram, but they are very important for the effective working of the system. When refrigerant flows from one process to the next it always moves through the pipes as either a superheated vapour or a subcooled liquid. The amount of superheat or subcool may be quite small.
Now we have sufficient information to build a real circuit. By taking the components shown below and connecting them with tubing, and adding refrigerant first remove all air and moisture) a real cooling system can be built.
Connect up, use fans to circulate air over the evaporator (the air will be cooled), and condenser (the air will be heated), switch on the compressor and we have a refrigeration machine. Sounds simple, but careful design and specification of components is needed. The control valve is a key component. Usually termed « Expansion Valve » this device regulates the superheat at the outlet of the evaporator. The temperature sensor at the outlet of the evaporator is connected to the valve to provide feedback on the adjustment of the valve. Most valves work automatically by means of a diaphram, and are termed Thermostatic Valves, whilst other types are electronic.
How can we tell if the system is working correctly? It is really quite simple in principle. The properties of the Refrigerant or Working Fluid are known to a high level of accuracy and by measuring the pressure and temperature at points 1, 2, 3, 4 the P-h diagram can be established. In practice only two pressure measurements 2 and 3 are required. Instrumentation and computer techniques are now available which allow fast diagnostics of almost any system.