Lower Resistance

It generally is considered that resistance due to fouling is lower with Paraflow plate heat exchangers than with tubular units. This is the result of five Paraflow advantages:

1. There is a high degree of turbulence, which increases the rate of foulant removal and results in a lower asymptotic value of fouling resistance.

2. The velocity profile across a plate is good. There are no zones of low velocity compared with certain areas on the shell side of tubular exchangers.

3. Corrosion is maintained at an absolute minimum.

4. A smooth heat-transfer surface can be obtained.

5. In certain cooling duties using water to cool organics, the very high water film coefficient maintains a moderately low metal surface temperature, which helps prevent crystallization growth of the inversely soluble compounds in the cooling water.

The most important of these is turbulence. HTRI (Heat Transfer Research Incorporated) has shown that for tubular heat exchangers, fouling is a function of flow velocity and friction factor. Although flow velocities are low with the plate heat exchanger, friction factors are very high and this results in lower fouling resistance. The effect of velocity and turbulence is plotted in Figure 2.

Marriot of Alfa Laval has produced a table showing values of fouling for a number of plate heat-exchanger duties.

Figure 2. Effect of velocity and turbulence.

Fouling resistance

Water

Fouling resistance

Water

Demineralized or distilled

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