Consider an ideal vapor-compression air conditioner with a cooling capacity of Q
ID: 2997991 • Letter: C
Question
Consider an ideal vapor-compression air conditioner with a cooling capacity of Q(dot)c=4 Tons whose evaporator is used to cool dry air in an air handler duct from Ta,in=75oF down to Ta,out=62oF. The refrigeration cycle uses refrigerant R-134a. The evaporator operates at a pressure of 60 psia, and refrigerant exits the evaporator as saturated vapor. The condenser operates at a pressure of 200 psia, and refrigerant exits the condenser as saturated liquid. Recall that 1 Ton of cooling capacity=12,000 Btu/hr. Find: (a) the compressor work per unit mass, wc, in Btu/lbm, (b) the heat transfer from the evaporator per unit mass, qc, in Btu/lbm, (c) the cycle COP, (d) the refrigerant mass flow rate, m(dot)r, in lbm/hr, (e) the mass flow rate of air through the air handler, m(dot)a, in lbm/hr, and (f) the electric power consumed by the compressor assuming a 100% efficient electric compressor motor. [(a) 10.7 Btu/lbm, (b) 55.8 Btu/lbm, (c) 5.21, (d) 884 lbm/hr (e) 15,400 lbm/hr, (f) 2.78 kW]
Explanation / Answer
From R-134a properties at P1 = 60 psia and quality = 1 (sat. vapor) we get v1 = 0.794 ft^3/lb, h1 = 110 Btu/lb, s1 = 0.221 Btu/lb-R
From R-134a properties at P2 = 200 psia and s2 = s1 = 0.221 Btu/lb-R we get, h2 = 121 Btu/lb
From R-134a properties at P3 = 200 psia and quality = 0 (sat. liquid) we get v3 = 0.0146 ft^3/lb, h3 = 54.3 Btu/lb
h4 = h3 = 54.3 Btu/lb
a)
wc = h2