The evaporator is the component of the refrigeration system in which the heat is removed from the air, water, or any other body by the evaporating refrigerant.
It is a device in which the refrigerant is evaporated by absorbing the heat from a surrounding medium.
CLASSIFICATION OF EVAPORATORS
Based on the circulation
- Natural circulation evaporators
- Forced circulation evaporators
Based on the method of liquid (refrigerant) feed
- Dry expansion (DX) evaporators
- DX cooling coils- used for cooling air.
- DX chilers- used for cooling water or other liquids.
2. Flooded evaporators
A. Full-flooded evaporators
B. Liquid overfeed evaporators
NATURAL CIRCULATION- FLOODED EVAPORATORS
- A full- flooded evaporator is operated completely filled with liquid refrigerant, which covers the entrie tube surface and provides the highest possible heat transfer rate.
- A liquid over feed evaporator is one in which the amount of liquid refrigerant circulated through the evaporator is considerably in excess of that which can be vapourised.
- The flow of the liquid refrigerant in flooded evaporator is outside the tubes whereas in the dry expansion type the flow is inside the tubes.
- The evaporator coil is kept inside the shell through which the fluid to be chilled is recirculated again and again to get the required cooling.
- Natural convection current is set up between the refrigerant and the fluid to be chilled, due to the temperature difference between them.
- The liquid refrigerant enters the shell through a float valve and due to the heat transfer the liquid refrigerant gets vapourised which cools the fluid to be chilled.
- The vapour refrigerant in the top of the shell is drawn by the compressor through the suction line and liquid refrigerant remains in the evaporator.
FORCED CIRCULATION- DRY EXPANSION (DX) EVAPORATOR
- In the dry expansion type, the refrigerant flows through tubing, and there is no liquid storage of refrigerant in the evaporator.
- The amount of liquid refrigerant fed into the evaporator is limited in such a way that at the end of the evaporator, the refrigerant is completely vapourised and only vapour refrigerant enters the suction line.
- The refrigerant flow control employed with method is usually either a thermostatic expansion valve or a capilary tube.
- A dry type direct- expansion (DX) evaporator with thermostatic expansion valve and fins on the air side which is commonly used in air- conditioing systems.
- The name direct- expansion is derived from the fact that the medium is cooled directly by the refrigerant evaporation.
- DX- evaporators are used in small capacity refrigeration and air conditioning plants.
- This type of evaporator is used when it is to be installed near the compressor to avoid leakage of refrigerant and to avoid pressure drop.