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where π2 and πΜ2 expressed in J/kg and W respectively are the heat absorbed, or evaporator load, h1 and h2 the enthalpies and the inlet and outlet of the evaporator respectively, in J/kg and πΜ the refrigerant mass flow, in kg/s. The required work, ideal and real, are given by equations (3.5) and (3.6). where π€ππ and π€ are expressed in J/kg. In a real vapour compression system, the enthalpy at the exit of the compressor differs from the isentropic one, as the losses in this component cannot be neglected. The isentropic efficiency of the compressor can thus be defined according to (3.7). The performance of a refrigerating cycle is evaluated through the coefficient of performance (COP). To represent the difference in the purpose of the heat pump and the refrigerating machine, two different COP can be expressed. In the case of a heat pump, the desired product is the rejected heat and thus the COP is given by (3.8); in the case of a refrigerating machine, the sought effect is the absorbed heat and therefore the COP is given by (3.9). where πΜ1 is the rejected heat during the condensation process, πΜ2 the absorbed heat during evaporation and W is the required net work input, in kW. The COP1 of the heat pump can also be thought of as a multiplier, representing the number of times the used work is gained as heat at the higher temperature level. The relationship between the two COPs, in the same machine β operating at the same conditions β can be expressed as seen in (3.10). 3.1.1. Basic assumptions A set of experimental data obtained from runs of the real heat pump were available and were used as starting points for the creation of the model. Specifically, the runs were classified by inlet temperature of brine in the evaporator (0, 5, -5 Β°C) and outlet temperature of water in the condenser (35, 45, 55 oC), creating a combination of nine runs. Superheat is set at 4.8 Β°C and subcooling at 4 Β°C. Overall, the input data chosen are as seen in Table 3.1: π€ππ = h3,ππ β h2 π€ = h3 β h2 (3.5) (3.6) Ξ·is = h3,is β h2 h3 β h2 (3.7) ΜΜ πΆππ = π /π 11 ΜΜ πΆππ = π /π 22 (3.8) (3.9) πΆππ =πΆππ +1 12 (3.10) -23-PDF Image | Next generation of refrigerants for residential heat pump systems
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