Tube-side fouling

One of the mistakes 1 have made in the past is due to air accumulation in the channel head (see Fig. 32.1). That is. I ve confused the effect of trapped air with fouling. Especially on start-up, air may be trapped both above and below the pass partition baffles. The air can fill some of 

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Tube-side fouling coefficient. The heat transfer through the resistance created by the inside (tube-side) fouling is given by ... 

ATTf = temperature difference across the tube-side fouling resistance (K)... 

If one of the air coolers begins to experience tube-side fouling, the fluid flow will be reduced. But the tube-side pressure drop will remain the same. The pressure drop across all five air-cooler bundles, shown in Fig. 14.6, is 10 psig. 

Allowable tube-side fouling factor RDT The basic data required for the air side are ... 

C READ THE TUBE SIDE FOULING FACTOR FOLT... 

For fired heaters subject to creep problems, make sure that the tube metal temperature was considered in materials selection, hi the absence of better information, assume the fireside temperature is 100°F (38°C) higher than the process temperature. (If tube-side fouling is anticipated [e.g., coke formation], assume the tube metal temperature is 150°F [85°C] higher than the process temperature.) If necessary, make a note on the template to ensure that creep is accommodated during design of heater tubes, in accordance with API 530 [23]. 

Heat transfer in crude exchangers often declines because of tube-side fouling. The difference in crude preheat may be 50 "F for a dirty vs a clean exchanger train. 

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. 

At one Midwest refinery, on a 180,000 BSD crude unit, two parallel, 6000 square ft, titanium twisted tube bundles have been in service for a number of years. Crude tower overhead vapors plus steam are condensing on the shell side. Crude is on the tube side. Fouling on both the shell side and the tube side appears to be quite minimal. However, the heat-transfer coefficient is bad only about 20-25 Btu/hr/ ft /°F, even when the exchanger is clean. 

One excellent way of suppressing shell-side fouling (and tube-side fouling as well) is to maintain tube smoothness by retarding corrosion. Basically, use alloy tubes (316 s.s. if chlorides and caustic are not present), rather than carbon steel (c.s.). 

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. 

What is far more important on an air cooler is the tube-side fouling factor. 

The external finned area of an air cooler is typically 20 times larger than the tube internal area. Thus, if a tube-side fouling factor of 0.002 is used, it must be multiplied by 20 to obtain the effective fouling factor, based on the air cooler finned area. 

If the overall heat-transfer coefficient for the air cooler is 10 Btu/hr/ ftV°F, then the 0.002 tube-side fouling factor will represent 24 percent of the total heat-transfer resistance ... 

For ordinary tubes, I never bother about the small effect of the ratio of outside area + inside area. But for finned, studded, or serrated tubes (i.e., low fins), this ratio of area factor is really important, as it magnifies the effect on the tube-side fouling factor that is selected. 

Design a shell and tube heat exchanger to heat raw water at 75°F to 80°F using 150,000 Ib/h of demineralized water enters the exchanger at 95°F and cooled to 85°F. Assume that the tube side fouling resistance is 0.001 (ft h °F)/Btu. 

Design a shell and tube heat exchanger for 100,000 Ib/h of ethylene glycol (EG) at 250°F cooled to 130°F using cooling water heated from 90°F to 120°F. The shell side fouling resistance is 0.004 (h ft °F)/Btu and tube side fouling resistance is also 0.004 (h ft °F)/Btu. Compare results with Example 4.2. 

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