Hot Pinch and Cold Pinch

[thiru4vino]

Dear All!

Can you please help me to understand Hot Pinch and Cold Pinch temperatures. What is the relation between them and the minimum hot-stream outlet temperature possible?

This doubt came to me in light of doing an exercise in Hysys, where I encountered temperature cross problem, and it was suggested that we should go for more shells in case of temperature cross? I am not able to understand this relation too..
Can any one help?

Advance thanks to all!

Cheers
atm

 

[gedomin]

Heat transfer is limited by either: T1-t2 (hot end) or T2-t1 (cold end). Say hot pinch and cold pinch, or temperature approach.
T1 = Hot inlet temp
T2 = Hot outlet temp
t1 = cold inlet temp
t2 = cold outlet temp
Assume you are dealing with single phase on both sides

If you try to design for a very close "temperature approach", a conventional one-shell-pass may not work, as this is half co-current and half counter-current. Cross temperature limitation may occur. Watch for the MTD correction factor, this should be > 0.8.
In your case, if maximum heat recovery is of importance, you might try a fully countercurrent arrangement. This has an MTD correction factor = 1.
MTD: mean temperature difference.

 

[EmmanuelTop]

In such case, choosing TEMA F shell may be of help.
You can try to locate R. Mukherjee's article "Does your application call for F-shell heat exchanger", where it has been explained in details about temperature crosses and achieving maximum HEx duty.

Try this link:

http://engineeredwriting.com/samples/040440.pdf