If U-tubes are used the number of tubes will be slightly less than that given by
equation 12.3a, as the spacing between the two centre rows will be determined by the
minimum allowable radius for the U-bend. The minimum bend radius will depend on the tube diameter and wall thickness. It will range from 1.5 to 3.0 times the tube outside
diameter. The tighter bend radius will lead to some thinning of the tube wall.