3 replies to this topic

[sheiko]

Hi,

Does somebody know why hot streams almost always enter the top and leave the bottom, and just the opposite with cold streams?

I'm asking because none of the people I have questionned in my refinery seems to know the reason(s)...

[Art Montemayor]

Sheiko:

If you are talking in generalities - and this question is a general one; it doesn't identify the phase of the streams, whether there is a phase change, etc. - then one possible answer is:

Hot streams can be condensed within a heat exchanger if the other stream is a colder stream. If condensation takes place, the resulting condensate has to be drained off and gravity is used for that. That explains why some hot stream are better entering at the top.

Cold streams tend to enter the bottom portion of a heat exchanger because cold streams might have dissolved gases that strip out in the course of being heated up. You want to get rid of these gases because they are nearly 100% fouling due to their notoriously bad heat transfer coefficient that acts as an insulator. Having upward flow is condusive to sweeping out any gases or inerts.

There may be other reasons - but the above two are generally applicable.

I always design my heat exchangers to have cooling water enter at the bottom and exit at the top - without any dead spots in the flow direction. This avoids any gas binding in the CWS flow direction.

[sheiko]

Thank you Art. Yes my question was indeed general.

This question arose from the following quote from Mr Lieberman's Working Guide to Process Equipment:
"The cold fluid may be on the shell side or the tube side of an exchanger. The convention is to put the cold fluid nozzle on the bottom of the exchanger. Sometimes this is necessary. Sometimes it does not matter, but it is still the convention."

I wanted to have more clue on it.

Cheers.

[Jiten_process]

@sheiko

I dont think i could have anything above what Mr. Art has explained. However there is a possibility of taking an advantage of density difference. If we talk about no phase change heat transfer (sensible heat transfer) then as temperature of hot fluid decreases density increases and you may take advantage of gravity by placing inlet (hotter) from top and outlet (colder) from bottom to let the flow passes with less energy requirement and similary the otherway round for cold fluid. I guess this benifit is not so substantial but in absence of any other technical merit this can be justified.