Horizontal tubes are often used in waste heat boilers, refrigerant evaporators, and a number of other types of heat exchange equipment. For the sake of compactness, the horizontal sections are often relatively short in length and connected by return bends to form so-called serpentines; alternatively, the tube may be formed into a helical coil. It is therefore convenient to consider the influence of bends and coils on heat transfer rates in this section.

A common feature of flows in non-vertical rubes is that of departure from axial symmetry in both flow and heat transfer. However, the nature and magnitude of the effects arising from this departure from symmetry vary with flow rate and with the type of geometry. In what follows, we will deal in turn with horizontal tubes (Section A), inclined tubes (Section B), tubes with bends (Section C) and, finally, helical coils (Section D).

A. Horizontal tubes

Heat transfer rates in horizontal tubes differ from those in vertical tubes to the extent that the gravitational forces cause an asymmetric flow pattern. A useful review has been given by Butterworth and Robertson (1977). Because the differences in behavior from that in a vertical tube are associated directly with the occurrence of dry portions of the tube perimeter, we shall consider this aspect first.

(a) Flow patterns and heat transfer