LMTD question 1

[semsagro]

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

 

[25362]

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

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

(Q₁+Q₂)[÷][(Q₁/LMTD₁)+(Q₂/LMTD₂)]​

 

[ione]

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-...ge&q=heat exchanger tema weighted MTD&f=false

 

[srfish]

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.

 

[ione]

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.

 

[srfish]

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.

 

[ione]

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.