6 replies to this topic

[Dnyandeoingale]

Hello,
I would like to know, whether the temperature cross occurs in shell and tube heat exchanger is desirable or not? if it is how?

Thanks in advance.

Dnyandeo

[Shivshankar]

Hi,

This has been discussed in the past, find below links

http://www.cheresour...eat-exchangers/

http://www.cheresour...perature-cross/


Regards
Shivshankar

Edited by Shivshankar, 24 January 2013 - 03:11 AM.

[Dnyandeoingale]

Thank you Shivshankar,

I have seen those topics but it is not clear whether temp cross is desirable in shell and tube heat echanger? As per my understanding most are saying it is undesirable so avoid it.

But as per my understanding if I am getting outlet temp of cold fluid more than the hot fluid outlet.... why should i try to avoid it..if i am getting better heat transfer coefficient..

Thank You!!

[fallah]

I have seen those topics but it is not clear whether temp cross is desirable in shell and tube heat echanger? As per my understanding most are saying it is undesirable so avoid it. Yes it is undesirable..
But as per my understanding if I am getting outlet temp of cold fluid more than the hot fluid outlet.... why should i try to avoid it..because indicates a negative driving force for heat transfer between the hot and cold fluids...

if i am getting better heat transfer coefficient..It requires a large heat transfer area or high velocity to increase OHTC, otherwise to be avoided by, let say, using more exchanger shells in series...

[sheiko]

Dny,
You can't design a 1-2 heat exchanger with a temperature cross. But if the cross is not avoidable, then it is preferable to use 2 or more 1-2 heat exchangers in series. Moreover, the more shell you add, the less surface area you need (because the corrected mean temperature difference increases).
On the the other hand, temperature cross can and do occur during operation (after the HX is designed, built, procured and commissioned). I believe this is mainly due to excess surface. In this case the outlet temperature of the cold product can deviate and exceed the outlet temperature of the hot product. The result could be cold product degradation or exceeded design temperature on the cold side.
So I would say temperature cross is better avoided in both situations.

Edited by sheiko, 07 September 2013 - 12:07 PM.

[nickle]

I believe for true counter current flow, like 1-1 shell and tube HX. It is possible to get external temperture cross. But in some limitation.

[srfish]

A slight temperature cross is alright in a one shell pass multi-tube pass shell-and-tube. I wouldn't go below a LMTD correction factor of 0.75. One time in an existing heat exchanger of this type I backed out by calculation a LMTD correction factor of 0.65. But don't ever design for correction factors that low.