To make the best use of the temperature difference between the evaporation media and the secondary fluid, the evaporator should operate in counter-current mode, as discussed in chapter 1.6. Furthermore, the more uniform temperature difference in a counter-current evaporator results in a less vigorous evaporation process than in a co-current evaporator. In a cocurrent evaporator, the large temperature difference in the beginning (see Figure 6.13) will result in a very vigorous boiling process, which creates a large volume of vapor in an early stage of the evaporator. Because the large vapor volume accelerates the flow inside the BPHE channels, the induced pressure drop will increase substantially. Pressure drops in co-current evaporators are much larger than in counter-current flow arrangements.
The temperature glide of non-azeotropic refrigerants has a positive effect on the mean temperature difference for counter-current evaporators, which is reversed for co-current flow. For a counter-current evaporator with a glide refrigerant, the increasing saturation temperature is balanced by the decreasing water temperature, giving a favorable temperature profile. In contrast, a co-current flow arrangement has a negative influence on the temperature profile, with the temperature glide of refrigerant and water temperature converging. At the end of the evaporator, where superheating takes place, the temperatures of the secondary medium and the refrigerant run the risk of crossing each other. Figure 6.14 shows these effects.
Reversible chillers, often called heat pumps, are designed to provide both cooling and heating depending on the requirements. This arrangement imposes special demands on the design of the evaporator and condenser in the refrigeration cycle. The refrigerant flow is reversed by means of a four-way valve, which makes the evaporator operate as a condenser and vice versa.
When mounting an evaporator in a reversible system, it has to be decided whether it will operate in counter-current or co-current mode. When reversing the system, the refrigerant flows in the opposite direction during operation as a condenser. The mounting depends on whether the demand is greater for heating or for cooling. If it is for cooling, the evaporator should work in counter-current mode. If it is for heating, the condenser should work in counter-current mode. There are some special applications where the most beneficial temperature profile is achieved if the evaporator is connected in co-current mode. For example, when using refrigerants that have a high pressure-drop over the evaporator, causing the saturated temperature to drop greatly as it passes the heat transfer surface (see Figure 6.15). Typical applications where co-current flow should be considered are low-temperature or cryogenic applications.