Evaporators

It is the only component of refrigeration system, which is fitted inside the refrigerated space. You must have seen the evaporator coil wrapped around the freezer box/icebox in a domestic refrigerator or fitted inside a cold room. All other components i.e. compressor, condenser and capillary etc. are fitted outside the refrigerator cabinet on the backside. An evaporator or evaporator coil is a bundle of copper/steel tubes fitted in the refrigerated space. One end of the coil is connected to expansion valve and the other is connected to suction side of compressor. The refrigerant liquid after expanding in the expansion valve/control valve situated just outside the refrigerated space enters the evaporator coil. After expansion the temperature of refrigerant liquid is very low. As it flows through the evaporator coil,it absorbs heat from the surrounded refrigerated space outside the coil through the wall of tubes and get evaporated i.e. convert into vapours. Absorbing more heat in the evaporator may further superheat these vapours. These vapours are at very low pressure and temperature and are continuously sucked/pulled by the compressor on the other end. With continuous flow of refrigerant through evaporator the air or water, whatever is surrounding the evaporator tubes gets cooled and it further cools all the items placed there. In this way the working of an evaporator is just opposite to that of the condenser i.e. here the refrigerant vapours absorb heat from the air/water surrounding the evaporator coil in place of releasing heat as in the condenser.

Therefore, being a heat exchanger, the construction of evaporators is also same as that of condensers. The evaporators may be of natural convection type or forced convection type depending on the airflow naturally over evaporating coil or flows forcefully by a blower. However, depending on the manner the refrigerant liquid is fed to the evaporator coil, evaporators are of two types (i) Dry Expansion Evaporator(ii) Flooded Evaporator.

i. Dry Expansion Evaporator
Actually these types of evaporators are not completely dry. But the dry word is said because these are usually partially filled with liquid refrigerant and other part of evaporator coil is exposed to vapour only. It is because the amount of refrigerant liquid fed to coil is less as compared to the space available in the coil. Due to less amount of refrigerant, the temperature of outgoing refrigerant vapours is more than the saturation temperature at evaporator pressure i.e. vapours gets superheated.

Dry expansion evaporators generally used in small units with less cooling requirements. The advantage of these evaporators is that the quantity of refrigerant filled is less and so these are cheap. But the heat transfer is poor because only vapours are in touch with most of the inside surface of evaporator tubes.

ii. Flooded Evaporators

Flooded evaporators are those in which evaporator coil is fully filled with liquid refrigerant. There is an accumulator to which the evaporation coil is joined.The accumulator is joined with the low side float valve chamber. A fixed level of liquid refrigerant is automatically maintained in the accumulator and flood chamber by the low side control valve. At this level of liquid refrigerant in the accumulator, the evaporator coil is almost filled with liquid refrigerant.

As only liquid is in touch with almost all the inside surface of evaporator tubes, heat transfer from the surrounding of evaporator to the refrigerant liquid through wall of tubes is more. So, the cooling produced by flooded evaporator is more as compared to dry expansion evaporator but the quantity of refrigerant used is more so these are costly. On absorbing heat some liquid convert to vapours. These vapours lift up in the form of bubbles and accumulate at the top of accumulator above the level of liquid. Simultaneously more liquid comes in the evaporator coil from accumulator. As level of liquid falls down in accumulator more liquid comes from low side float control valve. The top of accumulator is connected to suction line of compressor through which vapours are sucked by compressor.