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LMTD question

LMTD question

LMTD question

(OP)
I have a TEMA type BXU

shell side (glycerol)
Tin: 338 °F
Tout: 122 °F
Dew point: 241.1 °F

tube side (cooling water)
Tin: 95°F
Tout: 115°F

Correction factor : 0.99
T1 = 338-115 = 223
T2 = 122-95 = 27
this means LMTD corrected = 0.99 * [223 - 27]/LN [223/27] = 91.9 °F

According to the heat exchanger spec sheet they arrive at 97.73 °F .

Can someone help me out on this one?

Thanks  

RE: LMTD question


It is, apparently, a weighted MTD. Follow these steps:

1. Estimate the heat load Q1 from 338 to 241.1oF, and the heat load Q2 from 241.1 to 122oF.
2. Now estimate the water temperatures for these two heat loads to get the intermediate water temperature (against 241.1oF).
3. Calculate the LMTD's for both sections separately.
4. The weighted LMTD would be:
(Q1+Q2)÷[(Q1/LMTD1)+(Q2/LMTD2)]

RE: LMTD question

Semsagro,

There's nothing wrong in your calculations, except the fact that for your application you probably don't have to use LMTD.

I'd suggest to follow 25362's advise and use weighted MTD.

Take a look to the link below to get a more precise idea about special cases which ask for the weighted MTD approach.


http://books.google.it/books?id=RU-DnSXh1RAC&pg=PA73&lpg=PA73&dq=heat+exchanger+tema+weighted+MTD&source=bl&ots=5LprfTcI-L&sig=1T3RppdgnZJs7R7qyEPyZUPj_iE&;hl=it&;ei=DLYvS7nHHdT8sQal_JzsDw&;sa=X&oi=book_result&ct=result&resnum=8&ved=0CDEQ6AEwBw#v=onepage&q=heat%20exchanger%20tema%20weighted%20MTD&f=false
 

RE: LMTD question

What am I missing?

The heat exchanger specification sheet has a higher MTD than the straight line MTD. Normally in a case like this where the dew point is significantly lower than the inlet temperature, it is the other way around. Is there a bubble point? If this is pure Glycerol, there should be.         

RE: LMTD question

LMTD concept does not apply to multi-phase heat exchangers. In this specific case glycerol should suffer a change of phase during the heat transfer process, and being LMTD applicable when only sensible heats are involved in the heat transfer process, is simply meaningless to talk about LMTD here.

RE: LMTD question

I wouldn't throw out the LMTD concept entirely. In a counter-curent flow heat exchanger the weighted MTD can be calculated as shown by 25362. Then the appropiate heat transfer coefficient used with each temperature zone. If the heat exchanger is not counter-current flow, then it gets much more complicated. In this case, the weighted MTD is only a reporting mechanism to use on the specification sheet.

RE: LMTD question

srfish,

I agree with you. In my previous post dated 21st December, I suggested the OP to follow 25362's approach. In a broader view the classical method to calculate the LMTD (i.e. that which leads to LMTD = 91.8 °F) when dealing with complex HX doesn't work.

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