Evaporators, maldistribution, propane, performance
Brazed plate heat exchangers (BPHE) are used widely in heating and cooling systems, mainly for liquid-to-liquid heat transfer but also for two-phase-applications like evaporation and condensation. Using them as evaporator or condenser in a heat pump the proper performance of them strongly influences the energy efficiency of the cycle and the required amount of refrigerant. One of the most common factors affecting the performance of BPHE is the not uniform distribution of the refrigerant. This problem is especially important in the evaporator as the refrigerant enters in it in two phase flow and usually the distribution of liquid and vapor is not uniform over all the channels. This problem is very well known in the literature, and manufacturers have implemented some technical solutions like introducing distributors in the inlet port in order to minimize this effect. Nevertheless, the applied solutions usually are developed only for nominal conditions. The deviations in performance at part load conditions of the BPHE can be strong, but they are normally not known. In this contribution, an experimental analysis of the refrigerant distribution in an evaporator BPHE as a function of the inlet and outlet conditions will be presented. The BPHE has been tested and evaluated working at several inlet conditions (quality) and outlet conditions (superheat) for different temperature difference in the secondary fluid. As refrigerant propane was used, the refrigerant distribution inside the BPHE has been registered using thermography. It has been seen that: i) the flow maldistribution is present in most of the tests, ii) the quality of the refrigerant at the inlet influences the flow distribution, iii) the required superheat can have some influence, iv) the influence of the refrigerant maldistribution on evaporating temperature depends on the testing conditions resulting on a very significant degradation of the COP in some cases.