The flows inside a heat exchanger can be arranged in various ways to fulfil different purposes. The possibilities are counter-current flow, co-current flow and cross-flow, the first two of which are shown in Figure 1.8. There are different forms of so-called cross-flow heat exchangers in which the flows are more or less mutually perpendicular.
In BPHEs, counter-current flow is by far the most common arrangement. In this case, it is possible for the cooling liquid to leave at a higher temperature than the heating liquid outlet temperature. One of the great advantages of counter-current flow is the possibility of extracting a higher proportion of the heat content of the heating fluid. It is important to note that the LMTD value for counter-current flow is much larger than for cocurrent flow at the same terminal temperature (see Figure 1.9).
The motive for using counter-current flow becomes obvious by referring to equation 6. A high LMTD implies that a smaller heat transfer area is needed, i.e. the BPHE can be manufactured with fewer plates. However, co-current flow also occurs in BPHEs when the application so demands, e.g. in flooded evaporators. A consequence of a co-current arrangement is that the outlet temperature of the cooling medium can never exceed the outlet temperature of the warming medium. It is also worth noting that in a cocurrent arrangement there is a large temperature gradient at the beginning of the heat exchanger, which makes boiling start earlier.