An approach to fouling allowances in the design of compact heat exchangers

It is well known that the straight-forward application of fouling factors such as those recommended by TEMA is often inappropriate for compact exchangers. This is because in many applications they result in unrealistically high surface area increments, which in turn mean reduced throughflow velocities and increased fouling propensity, thereby counteracting their raison d'etre. It is argued in this paper that if a mean thermal conductivity of the deposit or fouling layer can be estimated for given process conditions, the application of a fouling factor R_f in m²K/W implies a calculable deposit thickness. It is proposed that the acceptable deposit thickness, and hence R_f, should be scaled in proportion to thehydraulic diameter to give the same proportional decrease in flow area, which in turn gives a calculable increase in pressure drop. Then provided that 1/R_f is in reasonable proportion to the clean overall heat transfer coefficient, the reduction in thermal performance and the increase in pressure drop will be acceptable. Values of reduced R_f calculated in this way are in broad agreement with those recommended for both shell and tube exchangers and plate exchangers (PHEs). The approach puts an increased emphasis on the need for more direct data on deposit conductances, rather than derived values from R_f values, although these can be used for first approximations. © 2002 Published by Elsevier Science Ltd.