existing HEX evaluation
existing HEX evaluation
(OP)
What is the best approach in evaluating an existing heat exchanger for a New Service??
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RE: existing HEX evaluation
RE: existing HEX evaluation
If there are phase changes on the process side, then a model based rating is advised.
best wishes,
sshep
RE: existing HEX evaluation
RE: existing HEX evaluation
http://en.wikipedia.org/wiki/NTU_method
http://www.cheresources.com/hteffzz.shtml
or call these guys:
http://reratellc.com/
-MJC
RE: existing HEX evaluation
The affinity relationships include: viscosity, thermal conductivity, heat capacity, mass flowrate, density, and various geometry changes that could be considered.
best wishes,
sshep
RE: existing HEX evaluation
GPSA refers to the U calculated based on the affinity law the Available U, and the U based on the Q=UA(LMTD) the Required U. So Available U should be less than the Required U, Correct?? Wording kind of misleading...TEMA calls the Available U, Dirty U.
Thanks
RE: existing HEX evaluation
If you run a full check rating of the existing exchanger you should get a new U value and a new LMTD value. Your new heat load (Q) may be higher or lower than the original.
So the product of the new U, LMTD, and existing A (surface) needs to be equal to or greater than your new Q.
Of course, that assumes that there are no pressure drop or other problems on either side of the exchanger.
Regards,
Speco
RE: existing HEX evaluation
The affinity laws work on resistances. As U is calculated from resistances (1/sum(r)), it can be confusing, however: it only makes sense that U available should be equal to or greater than U required in order to be adequate.
I think that the GPSA example has a mistake in their text on this point, and I have written them for clarification.
With respect to clean vs service U, this is a different issue altogether. Fouling factor is a totally user defined input to the overal heat transfer coefficient (U), it cannot be calculated, rather it is estimated from experience.
If I hear back from GPSA, then I will forward their response. With luck, they may post a clarification for us.
best wishes always,
sshep
RE: existing HEX evaluation
If you have a service that you don't realistically expect to foul, (clean both sides) then the use of a generous fouling factor will produce an oversized Hx which may overperform.
rmw