Question

A refrigeration unit operates between pressure of 244.83 kPa and 1043.9 kPa, and uses refrigerant-22. The saturated vapour R-22 enters the compressor at -20 °C and leaves the condenser at 25 oC as a saturated liquid. Assume isentropic compression and neglect all losses. Use the property tables and/or figures in the text book. Calculate the coefficient of performance; and

Answer 1

As per the problem statement a refrigeration unit working on the VCRS cycle as shown in figure below. Given: - (1) Entry to the compressor is from saturated vapour. (2) Exit from the condenser is from saturated liquid. 1-2 implies isentropic compression 2-3 implies isobaric heat rejection 3-4 implies constant enthalpy expansion 4-1 implies isobaric head addition \r\n From saturated property table @ -20 degree C h_f = 177.0737 kJ/kg h_g = 397.0627 kJ/kg S_g = 1.7826 kJ/kg - k @ 25 degree C h_f = 230.2894 kJ/kg h_g = 413.0288 kJ/kg s_g = 1.7174 kJ/kg - k (C_p)_v = 0.8724 kJ/kg - k Calculating Enthalpy at every state h_1 = (h_g)_20 degree C = 397.0627 kJ/kg h_2 = h_2 + C_pv (T_2 - T_2) - eqn (1) for isentropic process 1 - 2 S_1 = S_2 implies (S_g)_-20 degree C = (S_g)_25 degree C + (C_p)_v ln T_2/T_2 implies 1.7826 = 1.7174 + 0.8724 ln T_2/298 T_2 = 321.124 k = 48.124 degree C \r\n h_2 = 413.0288 + 0.8724 (48.124 - 25) using eqn (1)