The various symbols in these equations have their usual meanings. The new symbol F stands for a correction factor that must be used with the log mean temperature difference for a countercurrent heat exchanger to accommodate the fact that the flow of the two streams here is more complicated than simple countercurrent or cocurrent flow. Consider the simplest possible shell-and-tube heat exchanger, called 1-1, which means that there is a single shell “pass” and a single tube “pass.” The sketch schematically illustrates this concept in plan view. Note that the contact is not really countercurrent, because the shell fluid flows across the bank of tubes, and there are baffles on the shell side to assure that the fluid does not bypass the tube bank. The entire bundle of tubes (typically in the hundreds) is illustrated by a single line in the sketch. The baffle cuts are aligned vertically to permit dirt particles settling out of the shell side fluid to be washed away.