Tube-side fouling, rate

The concept is that the tube-side fouling rate will be reduced by continuous scrubbing of the tube wall and also have a favorable impact on heat transfer. The associated increase in tube-side pressure drop with this device is about 1.5 psi per pass at 3 ft/s tube-side velocity, about half that of Spirelf or Fixotal. 

When a tube in fouling service (where the colder fluid is on the tube side) begins to foul due to low tube velocities, the flow through the tube is reduced. The tube gets hotter, which further reduces the flow as it fouls quicker. The resulting suppressed velocity then accelerates the rate of tube-side fouling until the tube plugs off entirely. 

But suppose we are operating a heat exchanger subject to rapid rates of initial fouling. The start-of-run heat-transfer coefficient U is 120 Btu/[(h)(ft2(°F)]. Four months later, the U value has lined out at 38. The calculated clean tube-side velocity is lV2 ft/s. This is too low, but what can be done ... 

The resulting four-pass tube bundle will have a tube-side velocity twice as high as it did when it was a two-pass exchanger 3 ft/s. Experience has shown that in many services, doubling this velocity may reduce fouling rates by an order of magnitude. That is fine. But what about pressure drop ... 

Acetone (s = 0.79) at 250°F is to be sent to storage at 100°F and at a rate of 60,000 Ib/hr. The heat will be received by 185,000 Ib/hr of 100 percent acetic acid (s = 1.07) coming from storage at 90°F and heated to 150°F. Pressure drops of 10.0 psi are available for both fluids. Assuming that the fouling factor on the tube side is 0.001 and that on the shell side is 0.003, calculate the heat transfer coefficients for the tube and shell sides, the overall heat transfer coefficient for the exchanger, outside area of unit, and the heat transferred. 

Heuristic 55 The tube side is for corrosive, fouling, scaling, hazardous, high-temperature, high-pressure, and more expensive fluids. The shell side is for more viscous, cleaner, lower-flow rate, evaporating, and condensing fluids. 

The pH value has a considerable role in the corrosion fouling rate and the corrosion rate is at minimum for a pH of 11 to 12 for steel surfaces. Low oxygen concentrations in the flue gases promote the fire-side corrosion of mild steel tubes in coal-fired boilers. 

The concept is to increase turbulence in the tube-side flow, especially at the tube wall by constantly mixing slow moving fluid near the tube wall back into the bulk, thus increasing tube-side heat-transfer rate and reducing fouling. The additional tube-side pressure drop... 

To determine the heat transfer coefficient ki in Eq. (7-14), see, for example [0.1, 12A-126] covering the field of heat transfer problems in evaporation processes. During solution evaporation and crystallization, fouling and incrustation of the heat exchange areas has to be considered, which may lead to substantially lower evaporation rates in the evaporator. With incrustation on the product side of the evaporator tube wall, the heat transfer resistance is... 

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