Data for a heat pipe designed to preheat air for a boiler are given below. 

(a) Check the heat balance for the unit and calculate the average heat transfer rate in kW for a single pipe. 

(b) Estimate the temperature of the working fluid at the cold end and the hot end of the exchanger, assuming that the air film and the flue gas film have about the same resistance. 

(c) What is the value of U based on the outside area of the pipes, not counting the area of the fins? 

Flue gas flow rate: 36,700 kg/h. Air flow rate: 34,500 kg/h.

Flue gas temperatures: in, 260°C; out, 177°C.

Air temperatures: in, 27°C; out 120°C. 

Heat pipes: 144 pipes, 5.1 cm in diameter, 4.57 m long.