Optimal Distribution of Condenser Area for Retrofits

A. 1 ohansson and P. Lundqvist Dept. of Energy Technology Div. of Applied Thennodynamics and Refrigeration The Royal Institute of Technology, KTH Stockholm, Sweden Test results from field retrofit situations, where R22 has been changed to Isceon 59 1 and R407C in two identical chillers, shows that both substitutes require more subcooling than R22 to give the system a satisfying performance. It was also noted that to achieve the same subcooling, one had to charge the system with a larger amount of refrigerant. To achieve the same efficiency, COP2, with Isceon 59 as with R22, one had to add approximately 25% more refrigerant, than the system had with R22. A computer model of a system consisting of coaxial heat exchangers, with refrigerant in the inner tubes, has been made. In the model five R22 substitutes (R134a, Propane, R404A, R407C and lsceon 59) and R22 were compared. The simulations showed a similar behaviour as was experienced in the field tests: To make the system perform its best with lsceon 59 the area used for subcooling had to be larger than when R22 were used. With other substitutes, like R134a, a smaller subcooling area gave the maximum performance of the system. Rl34a can be a very interesting R22 alternative if it is energy efficiency that is important, and not primarily capacity. To fmd the correct amount of refrigerant charge for a specific system, it is necessary to look at the shape of the wet region when plotted in a property plot, but it is also important to look at the relative size of the heat exchanger area in both condenser and evaporator, for the specific refrigerant used. Introduction Legislation in many countries makes the study of replacing R22 in old heat pump and refrigeration facilities interesting. Usually the substitutes are chosen from refrigerants with look alike volumetric refrigeration capacity and vapour pressure. These two parameters has been dealt with extensively in the literature. This paper addresses another factor: System charge. To achieve the same, or at least satisfYing performance with a new working media, it is often necessary to charge the system differently than was the case with R22. The question is just how much refrigerant should one charge into the system when retrofitting, to get the maximum performance with the chosen R22 replacement? It is not possible to make a general study of the appropriate/optimum system charge from volumetric refrigeration capacity and vapour pressure alone. Parameters such as transport properties and other more fundamental thermodynamic properties has to be taken into account. Comparing refrigerants in a system When the performance of refrigerants in a certain facility are compared, it is common to use the carnot efficiency as a "benchmarking" tool. The carnot efficiency definition that is often used is the definition describing the efficiency for the refrigerantthe refrigerant cycle cam~t efficiency, llc.cycie : llc,cyde o= COP 2,C,Cycle Where ... COP2 = ~2 ... and COP2,C,Cycle = T ~ T I 2 1 Isceon 59 is a zeotropic refrigerant mixture consisting ofR134a (50 mass-%), Rl25 (46 mass-%) and R600a (4 mass-%). 2 It is usually, more or less, impossible to achieve the same refrigeration capacity with the R22 substitutes presently available on the market, without altering the system design in a more thorough way than just changing the refrigerant, and possibly lubricant