Combustion air in a manufacturing facility is to be preheated before entering a furnace by hot water at 90 C flowing through the tubes of a tube bank located in a duct. Air enters the duct at 15 C and 1 atm with a mean velocity of 4 m/s, and flows over the tubes in normal direction. The outer diameter of the tubes is 2 cm, and the tubes are arranged staggered with longitudinal and transverse pitches of SL=ST = 6 cm. There are eight rows in the flow direction with eight tubes in each row.