Heat Exchanger Design

[sandy2023]

I need 200 litre/hr liquid condensate at 170 Deg.F. steam available at 320 Deg.F and on other side, water (13 gpm) is available at 55 Deg.F.

Please help me how to design heat exchanger ?

 

[TD2K]

Well, since you know the condensate flow rate you need to have, and you have the steam (saturated I assume?) and condensate temperatures, you can get your heat duty from that information.

You also have a cooling water flow rate and an inlet temperature. Since the heat given up by the condensing steam is picked up by the water, a thermal balance will give you the outlet water temperature.

Now, you have the duty and all the temperatures. Do you have a type of exchanger in mind? Shell and tube, plate, double pipe, etc or is this all part of your question?

 

[sandy2023]

Thanks for your reply. Please correct me, if I am wrong, to calculate the heat duty ,I need is Latent heat of vaporization at 320 Deg.F * flow rate + (temp.diff)* Specific heat * flow rate.

I would like to go for plate and frame heat exchanger.

Sandy

 

[TD2K]

Correct, if the steam is saturated at inlet conditions. The heat duty is the sum of the latent heat plus the sensible heat from subcooling.

One thing to check, you likely don't want the cooling water to be leaving at more than about 120F or you can have problems with calcium carbonate deposition and fouling. You can handle higher temperatures by playing around with your cooling water treating program but for one small exchanger, I doubt you will want to do that.

We just had Alfa-Laval in to do a presentation on plate and frame exchangers and I asked the question (wrt another process they were showing) whether it was possible to condense AND subcool in a single unit. They said it wasn't possible. No big problme, it just means you have one unit to condense the steam and another unit to subcool it. You can supply the cold cooling water to the subcooling unit and then use the warm cooling water from it to the condenser.

Coming up with a heat transfer area is a bit more tricky for plate and frame as the correlations aren't nearly as widely available (there have been some links and information posted on Eng-Tips, you might want to do some browsing). In any event, I'm not sure I would be actually determining how many plates I needed, I'd leave that up to the vendor though I might do some back checking to seee if the overall heat transfer coefficient looked reasonable (they'll be much higher than for a shell and tube obviously).

Have you looked on Alfa-Laval's or Trantor's web site to see what information might be available? I'd likely call up one of their technical people, explain what you are looking for and ask if they can give you some general information.

 

[tickle]

Shell and tube exchangers are able to condense and sub cool (and desuperheat as well) in one unit. It requires a section where the baffles are closer together to give higher velocities for the liquid. if doing tube side condensing (shell side is generally preferred) a multipass exchanger can be used with the sub cooling pass(es) with fewer tubes in it.

However, S&T HE's are more expensive than plate exchangers, so it may be cheaper to install multiple plate exchangers.

 

[cymro]

sandy2023,
I suggest you ask Alfa Laval again . . about their TS6M and TS20M PHEs. These are designed specifically for steam primary use, and will allow the steam space to be flooded with sub-cooled condensate. Alfa laval should be able to predict your needs with their software.