Heat Exchanger Fouling Factors for Air Separation Plant 1

[Smith55]

Hi all,

Looking for guidance on recommended good practice for specifying fouling factors for the process air side of heat exchangers used in an air separation process.

We have air cooled heat exchangers, S&T heat exchangers and a brazed aluminium plate-fin heat exchangers in a cold box.

I note in the ALPEMA standard that "Fouling is generally not encountered for processes in which brazed aluminium plate-fin heat exchangers are traditionally used: air separation, hydrocarbon separation and liquefaction of gases." In the TEMA standard for S&T Heat Exchangers, there is a recommended fouling factor for "Compressed Air" which is the closest match I can find to our process air stream.

Our main and recycle air compressors are oil free units and the main air inlet is provided with a prefilter droplet separator, class F9 pulse filter and class H13 static filter.

Based on the statement in the ALPEMA standard, could I assume that a fouling factor need not be applied throughout for this process air stream, or would it be recommended to apply fouling factors to particular types of heat exchangers or to distinguish between heat exchangers upstream and downstream of the air purification unit.

Any advice or recommendations for further reading would be appreciated.

Many thanks.

 

[pierreick]

Hi,
Consider the brochure attached to support your work. On a regular basis fin should be cleaned to remove the dirt from the air (dust, pollen, dirt), no provision for fouling.
Good luck
Pierre

 

[pierreick]

Hi,
Another document.
Pierre

 

[EdStainless]

In an ASU I would consider the process side clean and with near zero fouling.
The other side is where it will be more complicated.
Air cooled units foul and you cooling water chemistry will control how much those units foul.

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P.E. Metallurgy, consulting work welcomed

 

[Smith55]

Thank you Pierre and EdStainless, that's very helpful.

 

[davefitz]

There are some differnces in the air separation processes used by the different ASU design outfits, due to patent issues. You may wish to contanct the designer of your ASU and ask them what values of fouling factor they use in their design. If you arew the ASU designer and have other existing units already in service, it should be easy to obtain the field data and then calculate the effective fouling factor yourself.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[sarahrad]

For Brazed Fin Aluminum exchangers, no fouling factors are typically applied. Overdesign or excess surface area margins are typically requested by licensors of about 10-15%.

For shell and tube exchangers and air cooled exchangers, TEMA has widely accepted fouling factors for all the process streams found in the Recommended Good Practice Section 10. Compressed Air has a fouling resistance of 0.001 (hr ft2 °F/Btu)which is relatively low and a standard fouling resistance for clean gases. For air cooled exchangers, if the exchanger is in an environment where there's potential for dust and debris to cover the high finned tubes, it is good practice to add an air side fouling resistance of 0.002.

I don't think modifying the fouling resistances upstream or downstream of the purifier is necessary since the fouling resistance will not add that much margin to the design where the gas heat transfer coefficient is going to govern the design, not so much the fouling.