A. Plate specification

The two most important parameters are the maximum flow capacity of the heat exchanger and the NTU range of the plate, where NTU, the number of transfer units, is defined as the temperature change of one fluid divided by the logarithmic temperature difference across the plates.

Maximum flow capacity determines the size of the port in the four corners of the plate and, in turn, decides the overall plate dimensions. The NTU range is related to the heat transfer and pressure drop characteristics of the plate and governs the type of trough or corrugation to be adopted. The actual choice of NTU range must be considered in relation to the types of duty for which the plate is intended. Achievement of the highest possible NTU value is not necessarily representative of the optimum design; a careful investigation of the proposed market is necessary, and the range of thermal performance eventually chosen must ensure that (1) the lowest NTU duties can be handled without the heat transfer area being limited by pressure drop so that surface is wasted, and (2) the highest NTU duties can be handled without an arrangement involving too many passes so that pressure drops are unduly high.

It is, however, the ability to combine multiple passes with true counter-current flow that gives the PHE one of its major operating advantages — the achievement of high NTU values. This enables high recuperative efficiencies to be obtained — 12 NTU, for instance, corresponding to 92% efficiency — for a unity heat capacity ratio, and this makes the PHE particularly suitable for heat recovery in the chemical, petroleum, food, dairy, and brewing industries.

B. Factors affecting plate design