An automobile radiator is constructed of six rows of staggered tubes as illustrated in Fig. 7.36. There are twenty tubes in each row. Water at about 200°F is pumped through each tube at a high enough rate so that the resistance to heat transfer inside the tubes is negligible and the water temperature is nearly constant at 200°F. The resistance to heat transfer in the tube walls can also be neglected so that the outside surface of the tubes are maintained at 200°F. If the average air velocity in the tube bundle is 60 ft/sec, what is the total rate of heat transfer from the radiator to the air if the ambient air temperature is 76°F and the tubes are 18 in. long? If it is necessary to increase the total rate of heat transfer, while maintaining the longitudinal and transverse pitch constant and keeping the mass velocity constant, should the tube diameter be increased or decreased?
