The purpose of the evaporator is to absorb heat from its surroundings. To do this it must be colder than the object or product being cooled. Remember that heat always flows from the warmer temperature object to the colder one. Evaporators are typically used to cool air or a fluid. Two types of evaporators are:
1. Flooded Evaporator
The term "flooded" generally refers to a coil system with an accumulator mounted above and the liquid refrigerant flowing by gravity into the evaporator. The sketch shows air being blown across a finned coil, but this could have also been a liquid to liquid evaporator. The warmer air can be likened to a flame under a kettle of water causing the refrigerant to evaporate. The action in this coil is similar to a coffee percolator with the evaporated gas pushing unevaporated liquid back to the accumulator where the two can be separated. The liquid falls down to the pool while the gas rises and is drawn off by the compressor. With proper designs, one can usually figure that three to five times the amount of liquid will be circulated through the coil during a given pass than will actually evaporate.
2. Pumped Liquid Overfeed
The term pumped liquid overfeed is also a type of flooded system, but the accumulator is often lower than the coils. Most designs call for 1-1/2 to 5 times the amount of liquid refrigerant pumped to the coils than will actually evaporate. Like the above flooded evaporator the gas and liquid are separated in the accumulator with the vapor pulled off by the compressor.