The values of r^ and rfc (the film resistances for the hot and cold fluids, respectively) can be calculated from the Dittus—Boelter equations described previously and the wall metal resistance rw can be calculated from the average metal thickness and thermal conductivity. The fouling resistances of the hot and cold fluids rdh and rdc often are based on experience.

The value taken for A is the developed area after pressing. That is the total area available for heat transfer and, because of the corrugations, will be greater than the projected area of the plate, ie, 1.81 ft2 versus 1.45 ft2 for an HX plate.

The value of AT is calculated from the logarithmic mean temperature difference multiplied by a correction factor. With single pass operation, this factor is about 1 except for plate packs of less than 20 when the end effect has a significant bearing on the calculation. This is due to the fact that the passage at either end of the plate pack only transfers heat from one side and therefore the heat load is reduced.

When the plate unit is arranged for multiple-pass use, a further correction factor must be applied. Even when two passes are countercurrent to two other passes, at least one of them must experience cocurrent flow. This correction factor is shown in Figure 17 against a number of heat

Figure 17. LMTD correction factor.

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