When a flow is disturbed, a pressure drop (ΔP) is created, i.e. the flow pressure at the beginning of a passage is higher than at its end. Pressure drop is a phenomenon with both positive and negative consequences for the heat transfer process.

## Pressure drop utilization

Excessive pressure drop is of course negative, because the flow must then be pushed through the BPHE using a lot of pump power. High pump power can be achieved only with large pumps, which demand a large amount of electricity and thus make the operation expensive.

The positive result of pressure drop is the greater extent of turbulence obtained. Turbulence is desirable in heat exchangers, because it improves heat transfer (as discussed in chapter 1.4). There are some proportional(~) relations that are useful to keep in mind when a BPHE is to be designed:

Relation [1] tells that decreasing the area, A, increases the flow velocity by the same factor. The interpretation of relation [2] is that if the velocity, for example, is doubled, the pressure drop is increased four times. The interpretation of relation [3] is that if the pressure drop, for example, is increased four times, the heat transfer coefficient becomes 4^1/3 = 1.59 times higher than the original value, i.e. a 59% increase. Note that these proportionalities only apply to fully turbulent flow.

## Pressure drop relations

The pressure drop in a BPHE channel mainly depends on different variables, shown in table below.

The plate pattern is one of the tools the designer can use to increase or decrease the pressure drops through the BPHE. A high θ pattern leads to bigger pressure drop than a low θ pattern. In practice, the high θ pattern is much more common due to the greater turbulence it creates. The difference between high θ and low θ patterns is visualized in **Figure 1.13**.