Experimental investigation of upstream individual superheat control on two-pass water-cooled refrigeration system

Abstract

The impact of individual upstream superheat control on a two-pass water-cooled refrigeration system has been studied. Previous research has verified that the loss of cooling capacity and coefficient of performance (COP) of the system due to non-uniform superheat can be recovered by applying upstream individual superheat control. This thesis presents the analysis of upstream individual superheat control. The experiment apparatus consisted of a two-pass water-cooled refrigeration system composed of a 2040 watts scroll type compressor, a water-cooled coaxial type condenser and two water-cooled coaxial type evaporators. R410A was selected as refrigerant. The design phase was based on refrigeration cycle of thermodynamics. AUTOCAD and Pro/Engineering 4.0 were used in order to do the simulation. Agilent 34980A was used as data acquisition hardware and Agilent BenchLink Data Logger Pro was used as data acquisition software. Engineering Equation Solver (EES) was used to do all the calculations, including the superheat, subcooling, enthalpy, cooling capacity and coefficient of performance (COP). Three different conditions were considered in this thesis. Condition I: without control. Condition II: with control. Condition III : minimum stable superheat (MSS) phenomenon. In condition I, no control was applied and non-uniform superheat was observed. In condition II, upstream individual superheat control was applied, and the superheats in two evaporating circuits were observered almost evenly distributied. In condition III, close the control valve on the corresponding circuit of 98%, and observed the sudden change of superheat. Results showed that there exist significant benefits of system cooling capacity and coefficient of performance (COP) by using upstream individual superheat control method. In Condition I, the cooling capacity was found to be 7.671kw and the COP was 3.715. In Condition II, the cooling capacity was found to be 8.138kw and the COP was 3.955. By applied the upstream individual superheat control method, the cooling capacity increased about 5.739% and the COP increased about 6.068%. Furthermore, the minimum stable superheat (MSS) phenomenon was examined. Close one of the control valves on the corresponding circuit of 98%. Instead of getting superheat increased, it was found that the superheat of this circuit suddenly decreased. This phenomenon is referred as minimum stable superheat (MSS). The exact reasons that cause MSS haven`t been found yet. But some reasonable factors that may affect MSS were presented. First, the suddenly change of heat transfer coefficient inside the evaporator. Second, different types of refrigerant may affect MSS.