Fouling, shell-side pressure drop

The effect of the fouling on the shell-side flow is to increase the cross-flow and increase the overall heat transfer coefficient for a fixed pressure drop (assuming the same fouling coefficients in both cases). 

Should the liquid level in the bottom of the tower rise to the reboiler vapor return nozzle, the tower will certainly flood, but the reboiler heat duty will continue. Unfortunately, reboiler shell-side fouling may also lead to tray flooding. This happens because the fouling can cause a pressure-drop buildup on the shell side of the reboiler. 

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. 

If measured pressure drops are significantly lower than calculated drops, this might indicate fluid bypassing, which could occur either on tube side or shell side. On the other hand, if measured pressure drop is too high, this is often caused by severe fouling, freezing, or slug flow for two-phase flows. 

Equation 8.16 is a simplified equation which is based upon equipment with a certain amount of fouling present on the shell side. Consequently, it may predict values of pressure drop higher than actually present for certain applications. A more rigorous method for calculating pressure drop across banks of tubes is presented here. The pressure drop for fluids flowing across the tube banks may be determined by calculating the following components ... 

The concept is to ensure effective bundle penetration and avoid dead zones so as to leave no stagnant areas where shell-side fouling could accumulate. Also, helical baffle plates can be spaced to reduce unsupported tube spans (without affecting pressure drop or heat-transfer characteristics) to mitigate flow-induced vibration. The designers also claim that the helicoidal baffle arrangement permits increased heat transfer. 

Another example from oil refineries is in crude preheat service with vacuum resid again on the shell side (with the helical baffles). Once again, similar results are seen as described above, with less fouling, reduced rate of increase in pressure drop, and better maintenance of heat-transfer coefficient as compared to the conventional shell-and-tube exchanger design. Our more direct experience of this comes from current practice in the United States, but we have also seen evidence of similar applications in Australia, as discussed in a recent article on the subject of crude preheat exchanger train redesign. ... 

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