Is Very small LMTD reasonable for the 2-2 type exchanger in the real w

[knapee]

Dear Sir/Madam

I got the temperature profile result (please see the attached file)from my designed 2(shell)-2(tube heat exchanger), but this result seems not reasonable.

I can only see one reason behind this in the following :

1. The hold side outlet temperature of heat exchanger(44 C) is smaller than the cold side outlet
temperature of heat exchanger(50.5C), but it seems different than Temperature cross phenomena
cause the hold&cold curve not cross each other.

Someone told me that the temperature difference at the end of top & bottom curve in this temperature profile becomes much greater, and this will cause the LMTD becomes unreasonable small value.

Could you please explain how to calculate the LMTD base on the graph attached just by quickly figuring this temperature difference changing on the graph?

And, is it possible to change this counter current to concurrent to maximize the LMTD ?


Please tell me how to tell the greater of the temperature difference of the s&t exchanger will cause the smaller LMTD.

Thank you very much for your concern and help, and really appreciate it.

 

[speco]

Knapee,

If you re-arrange the configuration such that the shell side nozzles are near the tube side nozzles, it is possible to achieve a more counter-flow design. Then your LMTD calculation would be totally straightforward.

Regards,

Speco (

http://www.stoneprocess.com)

 

[IRstuff]

Is your math or your equation correct here? Using the same values, I get 27, since the denominator that I get is more like 0.11, given that the two deltas are only differing by 3 degrees out of 30. ln(~1) is close to zero.

TTFN
faq731-376

 

[speco]

Knapee,

Let's try this LMTD calculation again. Here's what I get:

Hot Side 69 --> 44

Cold side 50.5 <-- 30

Diff 18.5 14

Therefor LMTD in COUNTERFLOW is 16.14

However, to achieve this, the exchanger has to be arranged in counterflow.

Regards,

Speco

 

[kjw1956]

Have a read of the attachment section on multipass and cross flow heat exchangers.