General Help On Subcooling Needed

[adamp716]

Perhaps someone can help me. My mentor recently passed away (unexpectedly) and I am now doing designs that I am unfamiliar with.

I have to design a steam condenser with subcooling. The steam (47500 #/hr) simply enters at 65 psia and 323F. Cools to 298F, condenses, and then condensate cools to 190F.

I know that there are precautions I need to take when designing the shell and tube exchanger but I simply do not know what they are.

Can anyone give me some tips?

 

[rmw]

Are you saying that you WANT sub cooling, or that you want to PREVENT sub cooling. Most condensers try to prevent subcooling in a variety of ways.

In those that need the subcooling, the condensate must be passed across the tubes at the cold inlet of the Hx in such a way that you maintain that tube section flooded and prevent vapor entry into the subcooling zone.

If you get vapor into the subcooling zone, you will have 2 phase flow and that will lead to tube cutting.

As far as the exit, I have seen side outlet and bottom outlets in sub cooling zones work equally well.

Boiler Feed Water Heaters are essentially high pressure condensers with subcoolers (and some with desuperheating zones). Google Yuba, TEI, and Struthers as well as the words "Feedwater heaters" and look at the way their Hx's are built. They will all have subcooling zones whether or not they have desuperheating sections.

rmw

PS: boy, Google spell checker didn't like a lot of words in this post.

 

[adamp716]

rmw...I'll give you all of the details that I have.

I am being asked to design a vertical condenser with steam in the tubes. 45700 #/hr at 65 psia and 323F. The condensing temp is 298F. They are asking for an outlet temp of 190F.

Water in tubes, minimum of 2 passes.

Now to me, it appears as though they want subcooling. However, they include a note that the condensate outlet temp can't be lower than the saturation temp corresponding to the shell pressure. So I'd think this means they want 298F outlet.

I'll study your answer for a little bit and see if I can follow what you are saying.

I'll also do the google search you suggested.

 

[adamp716]

oops...not sure how to edit my post.....steam is in the shell

 

[25362]

Are you sure the request doesn't imply condensing at vacuum conditions (9.34 psia) ?

 

[adamp716]

i have received some clarification of the process. apparently all they want is to cool the steam and then condense it. NO SUBCOOLING REQUIRED.

so i guess my question now is if there are any tips anyone can provide?

i apologize if these seem like dumb questions. again, although i am not new at designing shell and tubes i am by no means an expert and i do not want to make a colossal mistake.

 

[davefitz]

Well, I now feel completely reassured that the condenser I buy is going to be reliably designed and engineered.

I wonder if oil rig blow out valves are designed with the same attention to details.

 

[adamp716]

thanks for the input davefitz. very helpful.

 

[speco]

Adamp716,

It looks like you may have already learned one of the most valuable lessons. That is, to simply ask the customer what he really has in mind. You would be amazed at the number of times that specifications have conflicting requirements, or are just plain dumb.

Speco

 

[vpl]

adamp716

Tips on how to design a condensing heat exchangers are not especially easy to render on a forum like this. I recommend that you go take a look at your heat transfer books, any TEMA specifications that you have, and any design information that your company has available. See if your company has purchased any computer programs to assist you in the design. You might also peruse other threads on here about condensing heat exchangers. You might need to buy yourself (or have your company purchase) some additional texts or references.

You're in a hard situation where you've been handed a problem to solve without anyone to look over your shoulder and guide you. I'd like to say that you should be able to go to your boss and say "I'm not ready for this" ... but that might not be an option for you. Although it might not solve the immediate problem, there are classes out there on designing condensing exchangers -- maybe your employer will spring the $$ for you to take one.



Patricia Lougheed

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