Shell-side fouling

Figure 10-39. Chart for determining U-dirty from values of U-clean and the sum of tube-side and shell-side fouling resistances. Note Factors refer to outside surface. Fouling resistance is sum of (r + rj, as hr-ft -°F/Btu. (Used by permission Standards of Tubular Exchanger Manufacturers Association 1959 and 1968. Tubular Exchanger Manufacturers Association, Inc. All rights reserved.)...

If the ho appears to have possibilities of satisfying the design, continue to a conclusion by assuming the tube-side and shell-side fouling (Tables 10-12 and 10-13 Figures 10-39, 10-40A, 10-41, 10-42, and 10-43). 

Now, the hb coefficient can be used with the overall U equation, including shell-side fouling, to calculate a final overall coefficient for boiling. 

The heat transfer through the resistance created by the outside (shell-side) fouling is quantified by a fouling coefficient given by ... 

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. 

Actually, retrofitting a tube bundle with low fin tubes often reduces heat-transfer capacity. This happens when the controlling resistance to heat transfer is shell-side fouling. The fouling deposits get trapped between the tiny fins. This acts as an insulator between the shell-side fluid and the surface of the tubes. In severe shell-side fouling services, I have replaced fin tubes with bare tubes, and doubled the heat-transfer duty on the exchanger. 

C READ THE SHELL SIDE FOULING FACTOR POLS... 

I have seen towers equipped with kettle reboilers flood due to high liquid levels a dozen times in my career. The story is always the same. The elevation difference between the reboiler vapor return nozzle and the overflow baffle inside the kettle is only 2 or 3 ft. The designer has forgotten about shell-side fouling. 

A reasonable allowance for shell-side fouling could be an extra 1 psi of AP. If we are dealing with a reboiler feed with a specific gravity... 

In one debutanizer, I had to revamp a new tower by removing the bottom two trays and elevating the reboiler return nozzle by 5 ft. The designer had made no allowance for shell-side fouling. The shutdown of the new debutanizer tower was the price my client paid for the designer s error. 

Whereas shell-side fouling in a surface condenser is uncommon, loss of heat transfer coefficient due to air binding is as common as tube-side fouhng due to poor-quality cooling water. 

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. 

We have also heard of a case where a heat exchanger was retrofitted with a helical baffled bundle and the shell-side fouling actually increased. The client theorized that being as how the angle used to slope the baffles can apparently be varied by the designer according to service conditions, the angle for the baffle had not been correctly selected. 

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.). 

I see this frequently when tube bundles are extracted from the shell during a unit turnaround. The distorted tubes interfere with the proper fluid flow through the shell side of the exchanger and likely promote both shell-side fouling and shell-side bypassing. Also, as the tubes plug off, tube-side AP increases. If half the tubes plug, then the differential pressure across the channel head pass partition baffle will increase by a factor of four and may result in the failure of the channel head pass partition baffle. 

Design a shell and tube heat exchanger to cool 50,000 Ib/h of diethanolamine (DEA) solution (0.2 mass fractions DEA/0.8 water) from 144°E to 113°E by using water at 77°E heated to 100°E. Assume tube inside fouling resistance, = 0.004 ft h °E/ Btu, ignoring shell side fouling resistance. 

Film coefficients Overall (/-value estimate is 100 Btu/h ft °F, tubeside fouling resistance is 0.004 h ft °F/Btu, shell side fouling resistance is 0 and scale factor is 1. 

The corrected overall heat transfer coefficient is within the design range (140-260 Btu/ft h °F). The assumed value should match U-value estimated from the heat exchanger design specifications that depends on the film heat transfer coefficient of tube side and shell side, fouling factor, and metal resistance. 

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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