Theoretical Analysis And Experimental Study On The Influence Of Degrees Of Sub-cooling On The Performance Of Refrigeration Cycle

With the rapid urbanization process and the improvement of residents’ livingstandard, the ratio of energy consumption of air conditioning systems to the totalconsumption is increasing. Improving the energy efficiency of air conditioning system isimportant to achieve energy conservation and emission reduction and sustainabledevelopment. Sub-cooled liquid is benifit for the COP (coefficient of performance) ofair-conditioning system. Therefore, by using the method of theoretical analysis andexperimental study, this paper studies the characteristics of the working mediumsubstitution and frequent operation with partial load of air conditioning systems, andresearches that how to utilize sub-cooling technology to improve the cycle performanceof air conditioning system.Taking the single-stage vapor compression refrigeration cycle as an example,according to the standard GB/T18430.2-2008, use the EES program to calculate. Forworking medium R134a, the increase of COP caused by unit sub-cooling degreedecreases, with the increase of condensing temperature,high pressure and the reduce ofevaporating temperature. For common refrigerants, though sub-cooling always improvesthe COP, the contribution of unit sub-cooling degree to the improvement of differentrefrigerants’ COP is different, like larger increase for R218and R215, while rather smallfor R717and R407C.An experiment scheme is done on the experiment table of R134a single-stage vaporcompression refrigeration, validating the effect of the change of condensationtemperature and evaporation temperature on cycle performance, under the condition ofsub-cooling. Both the experiment and theoretical analysis show that the COP change ofunit sub-cooling degree caused by evaporation temperature change is larger than thatcaused by condensation temperature change.As for the single-stage vapor compression refrigeration cycle, retional design ofcondenser can obtain sub-cooling. According to model building and calculation analysis,for R134a, the heat transfer area of condenser needs to add6.5%with1℃increase ofsub-cooling degree. So considering the system performance and economy, sub-coolingdegree needs to be controlled at a reasonable range.Sub-cooling effect can also be achieved by utilizing independent mechanicalsub-cooling cycle, jet sub-cooling cycle, regenerative sub-cooling cycle and cool storagesub-cooling cycle. For independent mechanical sub-cooling cycle,5℃sub-coolingdegree can increase the COP with16%, and reduce the optimum intermediate temperature. Jet sub-cooling cycle also enhances the refrigeration effect. Nevertheless,when the sub-cooling degree reaches to a certain temperature value, there exists amaximum for COP. Regenerative sub-cooling is related with thermal physical propertiesof the refrigerants. For refrigerants which are good for the regenerative sub-cooling, thelarger regenerative sub-cooling degree is, the larger the added percentage of unit masscooling and COP will be. For cool storage sub-cooling cycle, both the increasing amountand ratio of cooling increase with the enhancement of sub-cooling degree.