Direct expansion vs Flooded evaporator

For direct expansion (DX) evaporators, the refrigerant (in two-phase) enters an evaporator straight after the expansion process while for flooded evaporator, the low pressure refrigerant after the expansion enters a gas-liquid separator which is connected to an evaporator. A saturated liquid refrigerant is fed to the evaporator, and partially evaporated (two-phase mixture). This benefits the efficiency of the evaporator. The most heat transfer at the evaporator comes from the latent heat of evaporation of the refrigerant, and the coefficient of performance (COP) of the refrigeration cycle is ratio of the heat transferred to the energy consumed in terms of electricity (this varys). Therefore, it is important to maximise the surface area of the evaporator for phase change, not super heating the vapour phase. In DX system, the refrigerant exiting an evaporator should be superheated so that no liquid dropplets get sucked into the compressor. In order to do that, the expansion valve opening is adjusted based on the superheat at the outlet of the evaporator, which means a certain section of the evaporator is used to superheat the vapour phase. In flooded system, on the other hand, the refrigerant entering the evaporator is 100% saturated liquid, and the entire surface area is used for phase change. The vapour fraction of the return stream is about 0.5 to 0.8. The vapour phase in the separator is drawn by the suction pressure generated by a compressor, while the remaining liquid circulates the evaporator. The expansion valve is used to regulate the level of the separator, and there is no super heat requirment for the flooded system.

 

Flooded evaporators are divided into forced-flow evaporators and thermosiphon evaporators. Forced-flow evaporators use a pump or an ejector to drive the circulation between a gas-liquid separator and an evaporator, while the density difference between liquid and gaseous refrigerant drives thermosiphon evaporators. 

 

In summary, The flooded systems generally exhibit higher COP, better fluid distribution through evaporators, no superheating is required, and the system can regulate it self for certain degree of disturbances. However, the flooded systems are more expensive than DX systems, one needs to take great care about the pressure drop through the evaporator, and the systems are more difficult to design as they are sensitive to deviations from design refrigerant flows.