Common flow arrangements of the fluids in a heat exchanger are classified in Fig. The choice of a particular flow arrangement is dependent on the required exchanger effectiveness, available pressure drops, minimum and maximum velocities allowed, fluid flow paths, packaging envelope, allowable thermal stresses, temperature levels, piping and plumbing considerations, and other design criteria. Let us first discuss the concept of multi-passing, followed by some of the basic ideal flow arrangements for a two fluid heat exchanger for single- and multipass heat exchangers.

Multipassing: The concept of multipassing applies separately to the fluid and heat exchanger. A fluid is considered to have made one pass if it flows through a section of the heat exchanger through its full length. After flowing through one full length, if the flow direction is reversed and fluid flows through an equal- or different-sized section, it is considered to have made a second pass of equal or different size.

Singlepass: A heat exchanger is considered as a single-pass unit if both fluids make one pass in the exchanger or if it represents any of the single-pass flow arrangements when the multipass fluid side is unfolded (note that the folding is used to control the envelope size).