Fouling factors - Reasonable intervals? 2

[BaAn]

Fouling factors are used to provide added heat transfer surface area. This means that the exchanger is oversized for clean operation and barely adequate for conditions just before it should be cleaned. TEMA and other sources provide values for fouling resistances "so that the exchanger will meet performance requirements with reasonable intervals between shutdowns and cleaning".

Unfortunately TEMA does not state what "reasonable intervals" are. Does anyone know on what intervals those values are roughly based?

Thanks for any comments!
Andre

 

[rmw]

Those intervals are variable depending upon what is causing the fouling. The determining factor for cleaning is performance. When the X-changer performance begins to degrade to a point that it can no longer do the duty, it is time to clean.

Only your process can determing that, and process variables can make it such that it is not even readily able to be set in stone in a given process.

rmw

 

[unclesyd]

Given the good advice posted byrmwthere a little caveat in cleaning heat exchangers.
Do let the fouling let exchanger get to close to the duty required for the specific process. Some fouling mechanisms are not very linear as the velocity slows down.

You cannot chemically clean a fully plugged heat exchanger. Cleaning benefits from velocity of the cleaning media.

Once you determine the fouling mechanism develop a plan for cleaning, like chemical, water blasting, or something simple like back flushing.

In short it is much more economical to clean 75% fouled exchanger than a 95% fouled one.

 

[BaAn]

Thanks for the operational advices. I agree that those values are not set in stone and strongly process dependent. There is big influence of fluid velocity, temperature, heat flux, fluid composition and construction of the exchanger. Sometimes fouling is quite linear or asymptotic and sometimes rather saw teeth like.

However, when designing a heat exchanger for an unknown fouling behaviour TEMA values are used for estimating the fouling rates as a starting point to provide an adequate heat transfer area. Those values have been reviewed in 1990 by TEMA and HTRI. Of course, the factors are only rough rule of thumb values, but based on what? From what do those data originate? Do they represent mean experience values for an average refinery, say after three to five years? Or two years? Or do they originate from lab testing at specific conditions (like those mentioned above) and if yes, after what time interval?
I couldn't figure out any sources. Thanks again!

 

[25362]

FWIW, you may visit

thread391-22720
thread391-88954
thread391-69689

 

[25362]

For a review of the various types of fouling:

http://www.wlv.com/products/databook/ch1_6.pdf

 

[sshep]

I would hazard a guess that "reasonable intervals" at the time that these guidelines were first suggested was 2 years. This seemed a pretty common TAR frequency in the petrochemical industries 20+ years ago. Now days this could be considered high frequency for many processes, as there are big economic incentives to prolonging the intervals between shutdowns.

It almost sounds like you are trying to scale up fouling factors based on a time interval. My observation is that tubewall temperature related fouling does not start out linear- the first six months usually has a rapid decline in performance followed by a more linear decline after that. In otherwards if the "reasonable frequency" on which the tables were based really was 2 years, you generally wouldn't double the resistance specification just because you plan to run for 4 years.

I have specified many exchangers, and fouling resistance specification usually comes down to a qualitative assessment of whether the fouling is very low to very high based on exchangers in similar service. The fouling factors for each side are then specified in accordingly round numbers (for example 0.0005 to 0.004 hr-sqft-F/btu). I think this is the way most experienced engineers probably do it.

sshep

 

[BaAn]

Sshep,
I was indeed thinking about scaling up fouling factors based on a time interval. Or rather if the given fouling factors are appropriate for TAR intervals of 5-6 years.
Your statement gives me a better feeling of what is appropriate and what not.
Thanks to all of you for your valuable comments and links!
Andre

 

[srfish]

That was a good response sshep.

Many years ago I worked for a shell-and-tube manufacturer. Although there was nothing published, they believed at the time that the fouling time interval was 18 months.

Being a pack rat, I dug my 1952 TEMA out of storage. There has been little change in the 1952 fouling factors but there has been additions. Back then there were more river water fouling factors. No longer do you see that whopper, the .008 fouling factor for the Chicago Sanitary Canal flowing at a velocity of less than 3 ft/sec.