1.4 Fluid Leakage and Contamination 2 Whereas in some applications. fluid leakage from one fluid side to the other fluid side is permissible within limits, in other applications fluid leakage is absolutely not allowed. Even in a good leak tight design, carryover and bypass leakages from the hot fluid to the cold fluid (or vice versa) occur in regenerators. Where these leakages and subsequent fluid contanination is not permissible, regenerators are not used The choices left are either a tubular, extended surface. or some plate type heat exchangers. Gasketed plate exchangers have more probability of flow leakage than do shell-and-tube exchangers. Plate-fin and tube-fin exchangers have potential leak- age problems at the joint between the corrugated fin passage and the header or at the tube-to-header joint. Where absolutely no fluid contamination is allowed (as in the processing of potable water), a double-wall tubular or shell-and-tube exchanger or a double-plate PHE is used