Although the tests were performed on one water system only experience with tubular data indicates that the overall trends generally are valid; ie, plate exchangers should be designed to substantially lower values of fouling than would be used on tubular equipment for the same stream. However, the comparison is not always that straightforward, as flow velocity itself is not a valid criterion. The wall shear stress in a PHE operating at 2.8 ft/s is equivalent to that in a tube-side exchanger operating at 8.2 ft/s. On the basis of typical industrial velocities, it was found that the PHE at 1.5 ft/s fouls about one-half as much as a tube-side exchanger operating at 5.9 ft/s, Furthermore, because of the high-heat-transfer coefficients typical to the PHE, surface temperature may differ from those in tubular equipment and should be carefully watched.

The turbulence inducing corrugation pattern prevalent in heat-exchanger plates produces very high local velocities. This results in high friction factors and therefore high shearing. This high shear, in turn, results in less fouling in plate exchangers than in tubular units. For cooling-water duties, Heat Transfer Research Incorporated (HTRI) has shown that the plate exchanger fouls at a much lower rate than either the tube side or the shell side of tubular s. TEMA recommendations for fouling therefore should never be used for plate units since they probably are five times the value found in practice. In particular, the plate exchanger is far less susceptible to fouling with heavy sed-imented waters. This is contrary to popular belief. It must be noted, however, that all particular matter must be significantly smaller than the plate gap and generally particles over 0.1 in. in diameter cannot be handled in any plate heat exchanger.

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