From Figure 1, it is apparent that the fouling process is time-dependant with zero fouling initially. The fouling then builds up quite rapidly and in most cases and levels off at a certain time to an asymptomatic value as represented by curve A. At this point, the rate of deposition is equal to that of removal. Not all fouling levels off, however, and curve B shows that at a certain time the exchanger would have to be taken off line for cleaning. It should be noted that a Paraflow is a particularly useful exchanger for this type of duty because of the ease of access to the plates and the simplicity of cleaning.
In the case of crystallization and suspended solid fouling, the process usually is of the type A. However, when the fouling is of the crystallization type with a pure compound crystallizing out, the fouling approaches type B and
Figure 1. Buildup of fouling resistance.
the equipment must be cleaned at frequent intervals. In one particularly severe fouling application, three Series HMB Paraflows are on a 4-1/2 hour cycle and the units are cleaned in place for 1-1/2 h in each cycle.
Biological growth can present a potentially hazardous fouling since it can provide a more sticky surface with which to bond other foulants. In many cases, however, treatment of the fluid can reduce the amount of biological growth. The use of germicides or poisons to kill bacteria can help.
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