Dry air at Ta, in = 30°C, and atmospheric pressure is blown at Va= 1.0 m3/s through a cross-flow heat exchanger in which refrigerant R134a is evaporating at a constant pressure of pR = 345 kPa. The air exits the heat exchanger at Ta, out = 13°C. The tubes and fins of the heat exchanger are both made of copper. The tubes have an outer diameter of Dout, t = 1.64 cm and tht = 1.5 mm tube wall thickness. The fins are circular
with a spacing that leads to 275 fins per meter, an outer diameter of Dout, f = 3.1 cm and a thickness of thf = 0.25 mm. The heat transfer coefficient between the R134a and the inner tube wall is estimated to be R h = 2,500 W/m2
-K. The heat transfer coefficient between the air and tubes and fins is estimated to be a h = 70 W/m2 -K. The total length of finned tubes is L = 110 m.
a) Determine the rate of heat transfer from the air.
b) Determine the value of the heat exchanger conductance for this heat exchanger.