The thermal length demand is a measure of how "difficult" a certain operational case is to solve for the heat exchanger. The thermal length can be expressed as the Number of Heat Transfer Units (NTU or θ). As shown in Figure 1.10, it is possible to calculate the NTU for each side of the heat exchanger.
A BPHE with a long thermal length can solve cases that are thermodynamically more problematic than a BPHE with short thermal length. Different cases are shown in Figure 1.11, and the possible solutions for these cases are discussed below.
Operations with close temperature programs (Figure 1.11 a) demand long BPHE plates or a multi-pass BPHE. The purpose of those solutions is to enhance heat transfer through the BPHE. Another solution is to design the BPHE with a high-theta pattern (cf. Figure 1.13 ), which will increase the turbulence of the fluid and thus increase the heat transfer efficiency.
An easier case, i.e. with small temperature changes on each side (Figure 1.11 b), will most probably be solved with a short single-pass BPHE. The plate pattern for easy jobs may very well be a low-theta pattern (cf. Figure 1.13), which leads to a low pressure-drop through the heat exchanger.
Asymmetric operation (Figure 1.11 c ) occurs when the temperature change on one side of the BPHE is much larger than on the other. The solution for this is a two-pass over one-pass BPHE, as shown in Figure 1.12, or an asymmetric BPHE. An asymmetric BPHE uses a design with a mixture of high θ and low θ plates.