The selection of fouling resistances to design plate-and-frame heat exchangers is usually based on experience or guess work. The primary aim of this study, was to compare fouling resistances for plate heat exchangers with values obtained in a parallel double-pipe heat exchanger under comparable operating conditions. The double-pipe heat exchanger it-as selected because it represents a shell-and-tube heat exchanger in a simplified form. Correction factors for plate heat exchangers have been obtained by comparing the performance of the two heat exchanger types during calcium sulfate crystallization fouling. Fouling was investigated in both heat exchangers for two conditions: (1) particles present in the process solution were allowed to enhance the crystallization process and (2) particles present in the process solution were removed by an in-line 1-mum filter. For similar flow velocities, the plate heat exchanger was found to foul significantly less than the double-pipe heat exchanger, i.e., 20-25 times less in the presence of particles and around 15 times less when the particles were filtered out. When data for similar overall shear forces are compared, results for both heat exchanger types are much closer, even though the plate heat exchanger still seems to perform somewhat better. Fouling in plate-and-frame heat exchangers was also found to depend significantly on plate design. This will add a further complication in direct comparison with shell-and-tube heat exchangers.