Question

1) A commercial refrigerator with refrigerant-134a as the working fluid is used to keep the refrigerated space at -30°C by rejecting its waste heat to cooling water that enters the condenser at 16 °C at a rate of 0.25 kg/s and leaves at 26°C. The refrigerant enters the condenser at 1.2 MPa and 65°C and leaves at 42°C. The inlet state of the compressor is 60 kPa and -34°C and the compressor is estimated to gain a net heat of 550 W from the surroundings Determine (a) the quality of the refrigerant at the evaporator inlet, (b) the refrigeration load, (c) the CoP of the refrigerator, and (d) the theoretical maximum refrigeration load for the same power input to the compressor. 2) A heat pump with refrigerant-134a as the working fluid is used to keep a space at 25°C by absorbing heat from geothermal water that enters the evaporator at 50°C at a rate of 0.075 kg/s and leaves at 40°C. The refrigerant enters the evaporator at 20°C with a quality of 23 percent and leaves at the inlet pressure as saturated vapor. The refrigerant loses 400 W of heat to the surroundings as it flows through the compressor and the refrigerant leaves the compressor at 1.4 MPa at the same entropy as the inlet. Determine (a) the degrees of subcooling of the refrigerant in the condenser, (b) the mass flow rate of the refrigerant, (c) the heating load and the COP of the heat pump, and (d) the theoretical minimum power input to the compressor for the same heating load 3) A gas refrigeration cycle uses helium as the working fluid. The pressure ratio between the inlet and out of the compressor is 3, and pressure drops in heat exchangers are negligible. The temperature of the helium is -15°C at the compressor inlet and 50°C at the turbine inlet. Assuming isentropic efficiencies of 83% for both the turbine and the compressor, determine (a) the minimum temperature in the cycle, (b) the coefficient of performance, and (c) the mass flow rate of the helium for a refrigeration rate of 18 kW. You may use the constant specific heats

Answer 1

Solution to question (1):

Block diagram o yabouy re iven 65"с. T134' ondensesLt voporation ( Ornpressor c va 5,2 з 111,2 kJlk Duin ovpansion in valve enfal 111+2 = 3.84 +"ху 223.95 evaporator talet

(b) Lon sidey en balante Quwatt leam ri p (24s. 2-111.2) 0,25 ( 109-68.2 (5 R rgeat Load : gu 6:59 kw 6-59 he(h2-hi) _ φι~ o.oss,(aaç.2ay

= 5-2826 Ye v 50+213 kール (16+223)-С 30 223 : Co Prev><@dhrM-유) ニ 5.28 2CYlo、05%(29S-2-239-о.ss 16.2 kW