Hybrid control, flow maldistribution
The hybrid control, as proposed by Kim et al. (2008), employs a primary expansion valve that provides most of the pressure drop, while small secondary balancing valves in the distributor lines to the circuits of the evaporator adjust the refrigerant flow to provide equal circuit exit superheats. This paper shows the experimental results for the application of the hybrid control of expansion valves for a 3-ton R404A large room cooling system. Data with the inbuilt expansion device, a pressure compensated TXV, was taken for a limited number of tests. Baseline data with an electronic expansion valve was taken to determine the best possible performance without using individual circuit flow control. After that, secondary balancing valves were inserted into the distributor lines to complete the hybrid control scheme. The same tests as done with the EXV were repeated to determine the achievable performance improvement. Ice-up tests at high indoor room humidity were conducted with all control schemes to determine the influence of the control scheme on frost build up and system performance. For repeatable results, additional tests with partially blocked evaporator coil were conducted with the hybrid and EXV control scheme. It was found that for TXV and EXV, even with a clean coil, substantial maldistribution occurs. This maldistribution lead to different usage of the individual circuits in terms of area fraction used for evaporation of, and area fraction used for superheating of refrigerant. With the EXV control scheme, the evaporation temperature had to be decreased to obtain sufficient superheat on the circuits that did not feed liquid into the suction header in order to evaporate the liquid of the circuits that fed liquid into the suction header. In addition, this resulted in uneven ice build-up and very poor controllability, which was especially noticeable as severe hunting when using the TXV in the ice-up test. With the hybrid control scheme, the surface usage for evaporation on all circuits was larger than 75% for most of the time, which resulted in a higher evaporation temperature and by that in a greater COP and larger capacity.