Fouling Factors for Heat Exchanger Design

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In this post I’ll list a few resources to find fouling factors for your heat exchanger designs. But first, let’s take a step back and review the definition and purpose of fouling factors in heat exchanger design.

And I thought they smelled bad...on the outside!

And I thought they smelled bad...on the outside!

Recall that in the heat exchanger sizing equation Q = U * A * LTMD, the “U” factor was a representation of all the resistances of heat exchange between the two sides. “U” was influenced by the types of fluids in the exchanger and also to a lesser degree by the material of construction of the heat exchanger. The fouling factor helps us add some additional detail, by representing the extra resistances that appear on the inside and outside of tubes after an exchanger has been operating awhile: the caked on products of fouling. There are different types of fouling, from crystalline scale to literal pieces of gunk. Fouling introduces a wrinkle into our simple equation, because now the resistance to heat exchange varies over time. But we still have to pick one exchanger design and one “A” value for the entire cycle of operation.

A properly chosen fouling factor will inform the detailed design of heat exchangers by vendors and/or software. As always, the best values to use are real-world values based on experience or tests in your own plant. Nevertheless, it’s helpful to have literature values for a starting point.

Some Literature Sources for Fouling Factors

Note that some of these links are PDF files:

Using whatever sources you can find, look up the value that most closely matches your source. These factors can then be placed into your equipment datasheet, your equations for “U”, or into your design software.

These fouling factors are all for shell and tube exchangers. For plate-and-frame heat exchangers, the higher velocities imply much less fouling should occur. My software manual for Aspen Exchanger Design and Rating says as a rule of thumb, fouling factors in plate exchangers are often taken as one fifth the shell and tube values. On the other hand, the nature of plate-and-frames with their small flow paths means they are wholly unsuited to most fouling fluids, which can gum up the works. So fouling factors shouldn’t be a major issue in plate heat exchangers anyway.

You’re not always fouled

To operations, fouling is a creeping buildup that gradually reduces heat exchanger performance. Heat exchangers can be cleaned, and you have to judge when cleaning is necessary. Too little cleaning, and your heat exchanger performance degrades unacceptably. You may end up wasting utilities or other resources compensating for the poor exchanger performance. Too much cleaning, and you have a lot of downtime and wasted man-hours.

Both operators and designers need to consider the exchanger in at least two states: fully cleaned and therefore transferring heat very well; and fully fouled just prior to a cleanup and transferring heat poorly. Your exchanger and your plant’s control scheme need to work for both states: you need the process to work with a clean exchanger, yet adjust as the exchanger gets more and more fouled. For example, adjusting an upstream temperature, or flow through an exchanger bypass, or other factors may let your process adapt to the life-cycle of a fouling exchanger.

The ironic challenges of fouling

You also need to be careful in your designs that you don’t overestimate or overcompensate for fouling. Why? Because ironically, you can end up causing fouling! Consider this dialogue between two engineers working in front of their design program:

“This exchanger uses really heavy, dirty oil! What fouling factor should we put into the program?”

“Eh, I don’t know. Let’s put in a really high fouling factor, just to be safe.”

“Yeah, better safe than sorry.” *runs program* “The program’s predicting a huge area now!”

“Well, I guess that makes sense. When there’s a lot of fouling resistance, we’ll need the extra area to compensate. Right?...”

The problem is, the really high surface area means your inlet flow is dispersed over a huge area, and fluid velocities are low. Low velocities encourage fouling! By making a huge exchanger, you created a self-fulfilling prophecy: when clean and new, the exchanger has low velocities, fouls fast, and you quickly end up needing your extra area. With a properly chosen fouling factor a smaller area could have been chosen: you’d have a tighter design, higher velocities, and a slower buildup of fouling. You wouldn’t be wasting money buying excess area that actually ends up contributing to your problems.

Like the threat of chattering in an oversized relief valve, this is a case where playing it too safe actually makes you worse off! You want to use the right fouling factors and then you want to target the right velocity: not too high (which promotes erosion and high pressure losses), but not too flow (which promotes fouling).

For reasons like this, heat exchanger design remains something of an art.

Further Articles

Free:
Reduce fouling in shell-and-tube heat exchangers (No Foul Design Method) by J. NESTA and C. A. BENNETT HYDROCARBON PROCESSING JULY 2004.

Subscription Required:
To Foul or Not to Foul that is the question by T. Reg Bott, Chemical Engineering Progress (CEP) Magazine, Nov. 2001

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