The exchanger you oversized is fouling because you slowed the water
A fouling resistance buys area. The area you buy cuts the velocity. And below about one and a half metres a second, the cut velocity is what lays the deposit down. Every number on this page is worked, and the slider in section two is the whole argument.
Out now
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Reflux Student drives your own Aspen Plus V14 from plain English. You type what you want, it opens the case, makes the change in Aspen, runs it, reads the result back and tells you what it verified. Aspen does the math. You keep the judgment. Windows, your own licence, and three free runs to start: it is a trial with a limit on it, not a free product.
Enter your email on the next page and the download link lands in your inbox, so you can open it on the Windows machine Aspen lives on. Windows will say it does not recognise the app the first time: click More info, then Run anyway. The certificate is new and Windows trusts it by reputation, which takes downloads to build. Nothing is wrong with the file.
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The forty percent you already bought
Overall heat transfer is four resistances in series. The designer adds a fifth before the exchanger is ever built, so that the duty is still met when it is dirty:
At fixed duty and fixed driving force, Q = U A dT means the area has to rise by
exactly the ratio of the coefficients. With the cooling-water allowance of
0.0004 m2K/W:
| U clean | U dirty | Area you buy |
|---|---|---|
| 1000 W/m2K, a good one | 714 | +40% |
| 500 W/m2K, an ordinary one | 417 | +20% |
That allowance has a thickness. R_f = x / k, so 0.0004 m2K/W is
0.88 mm of hard calcium carbonate scale at k = 2.2 W/m·K, 0.36 mm of coke,
or 0.24 mm of biofilm. Under a millimetre, against a 1.65 mm tube wall, is the whole forty percent.
The tube in four layers, and the velocity you chose
Cooling water in the tubes, hot process in the shell. Heat crosses a hot film, the steel wall, whatever is growing on the water side, and a cold film. The bands below are drawn to scale against a 3/4 inch 16 BWG tube: wall 1.65 mm, bore 15.75 mm.
Above the threshold. The deposit does not grow.
Drag it left. More tubes at the same flow is a slower fluid, and under about 1.5 m/s the balance tips: the deposit starts, the deposit insulates, the wall runs hotter, and the threshold velocity you needed climbs away from the one you have.
The 1941 number still in your spreadsheet
The allowance above is not a measurement of your exchanger. It is a tabulated value, and the table it comes from is an industry table published in 1941 whose numbers have barely changed since.
| Stream | Fouling resistance, m2K/W |
|---|---|
| cooling tower water | 0.00018 to 0.00035 |
| river water | 0.00035 to 0.00053 |
| crude oil | 0.00035 to 0.00123 |
| steam, oil free | 0.00009 |
Two things follow. A single number cannot describe something that changes every week the unit runs. And designing for high fouling is what produces the conditions that cause high fouling, which is the rest of this page.
It is a rate, not a constant
Deposition and removal run at the same time. Deposition climbs with wall temperature through an Arrhenius term. Removal climbs with wall shear, which is to say with velocity. What you see on a pulled bundle is the running balance of the two:
Set that to zero and there is a locus in velocity and wall temperature where the deposit stops growing altogether. Not slower: stops. That is the line in the next section, and it is why the slider above has a cliff in it rather than a slope.
The line to design above
Velocity across, wall temperature up. The curve rises to the right because a faster fluid tolerates a hotter wall. Below it, removal wins. The curve here is pinned so that a 65 C wall needs 1.5 m/s, which is the cooling-water number people quote.
The blue dot is a design at 2.0 m/s: below the line, clean. Buy the forty percent margin at the same flow and the tube count goes 200 to 280, the velocity goes 2.0 to 1.43, and the dot slides left across the line without the duty changing at all. That is the whole trap. Design on the right of the line and the extra forty percent was never needed.
The model calls fouling one number. The plant runs it as a rate. Bridging those two is the calculation, and it is the layer we are building at Reflux.
Run it on your own exchanger
Reflux Student drives your Aspen Plus V14 from plain English: open the case, change the fouling resistance, re-run, and read back what moved. Three free runs, Windows, your own licence.
Sources: tabulated fouling resistances from engineeringpage.com; the area penalty and the 1.5 m/s guideline from Awad, Fouling of heat transfer surfaces; the deposition minus removal threshold from Ebert and Panchal, as reviewed by Wilson. The threshold curve is illustrative of the trade, not a fitted correlation for your service. No cleaning interval is quoted because none was sourced.
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