The hot condensate liquid from the condenser can be utilized to superheat the cold vapor from the evaporator in a suction heat exchanger, with two positive effects. The higher level of sub-cooling increases the evaporator capacity, as discussed above. At the same time, the superheating in the evaporator can be minimized, because the suction heat exchanger ensures that no liquid enters the compressor. This results in a more efficiently utilized heat surface inside the evaporator, allowing a higher evaporation temperature or the use of a smaller BPHE. Because the refrigerant mass flow is the same on both sides of the suction heat exchanger, the enthalpy decrease of the condensate exactly corresponds to the enthalpy increase of the vapor. An excessively high level of vapor superheating may create problems with elevated discharge gas temperatures, which may limit the level of sub-cooling.
Systems with suction gas heat exchangers can become unstable if the system load fluctuates, so this system solution is favored for stable systems. Alternatively, an electronic expansion valve can be used to handle the fluctuations. A suction heat exchanger system is shown in Figure 10.14.