Reversible refrigerant systems are designed to provide cooling and heating from the same heat exchangers, depending on the requirements of the recipient. The same BPHE thus acts both as an evaporator and as a condenser. The condenser-evaporator duty is switched by a four-way reverse valve, as shown in Figure 7.20.
Because the refrigerant flow is reversed when changing from evaporator evaporator to condenser, while the water direction remains constant, the BPHE cannot operate in counter-current flow for both duties. When deciding whether the condenser or the evaporator should work with a co-current flow pattern, some design parameters must be taken into account:
- The main operating mode: If the reversible system is to be installed in a warm climate, it is more important that the evaporator operates in the most efficient way, and it should thus be designed for counter-current flow. For a cold climate, the operating efficiency of the condenser as a heat pump becomes more important, and the condenser should be designed for counter-current flow.
- If one of the heat exchangers is of the air coil type, the selection of flow pattern through the BPHE is affected. Air coils operate with cross-flow, so they are less sensitive to reversed flow even if their efficiency is reduced when operating in reversed mode. The most efficient operation for the BPHE should therefore determine the normal flow pattern.
In Figure 7.20, the evaporator works in co-current mode and the condenser in counter-current mode. Normal performance can thus be expected for the condenser. Operating the evaporator with co-current flow would usually give a lower level of performance than with counter-current flow, due to a lower LMTD. In this specific case, however, the evaporator performs almost as in a normal counter-current application, for the following reason: part of the superheating is created by the relatively hot reverse valve, which acts as a small heat exchanger in the suction line. Because the expansion valve bulb must always be connected after the reverse valve, for protection against high gas temperatures in condenser duty, the evaporator works with a very low level of superheating. This gives nearly the same performance as a counter-current case with "normal" superheating. A disadvantage of controlling the refrigerant flow this way is that the expansion valve reacts very slowly to changes.
Evaporator models as condensers
BPHEs mounted in reversible systems must operate both as evaporators and as condensers, as mentioned above. The BPHE performance in evaporator duty is often vital for the system economy. It is therefore common to install a dedicated BPHE evaporator with a distribution device in the lower left refrigerant inlet port (F3). When the system is reversed, the evaporator operates as a condenser instead, with the hot gas entering in the top left port (F1) and leaving as liquid through the lower left port (F3), where the distribution device is situated.
One may wrongly suspect that the distribution device, which induces a high pressure-drop when operating as an evaporator, disrupts the operation of the condenser. However, because the leaving refrigerant flow is 100% liquid, which has a much smaller specific volume than gas, the pressure drop through the distribution device is negligible when the heat exchanger operates as a condenser.