STEAM CONDENSER 1

[dkm0038]

I need help in sizing an air cooled heat exchanger for a steam condenser. There will be de-superheating of the steam, condensing of the steam to water, and finally subcooling the water.

Usually when you size a heat exchanger you use dQ=dm*cp*(Tin-Tout)=U*A*LMTD. Yet for the above situation there is three different U terms (heat transfer coefficeint) and it is not clear what the LMTD would be. (since the classical equation for it doesnt seem applicable.)

What is the standard way of calculating dQ for a heat exchanger: would you calculate U, A, and LMTD for each independent regime and add them upp, or take a weighted average of the terms, or something entirely else?

Please help

 

[speco]

dkm0038,

Your approach is basically correct. Each heat transfer zone has its own heat transfer coefficient and LMTD. The overall LMTD and coefficent are normally weighted. The question of HOW they are weighted really depends on the way the exchanger is built. If it's a single-pass condenser, then it's pretty simple: Just treat each portion of it as a defined length. The air inlet temperature is the same for each zone, so it's pretty easy to come up with an LMTD for each portion, then weight the whole thing together.

However, if your design is for a multipass tube bundle, the weighting of the LMTD is a bit trickier, since you have to look at the effective air temperatures for each portion.

If your desuperheating range is small, it may be possible to simplify your problem by ignoring the desuperheating zone, and simply incorporating the desuperheating heat load into the condensing zone. There is a good chance that the inside tube wall will be below the saturation temperature at the inlet, but you should check it to be sure.

Regards,

speco (

http://www.stoneprocess.com)

 

[danberry]

Even if your desuperheating range is high, you can eliminate the exchanger's desuperheating zone by spraying part of the condensate in the superheated steam. Doesn't change the exchanger load but it makes for a better average heat transfer coefficient.

 

[dkm0038]

thanks for the help speco and danberry.

Does anyone know any good books or references that go into detail on air cooled steam condensers I am having trouble finding things relevant to my problems.

 

[25362]

Lieberman & Lieberman in their A Working Guide to Process Equipment, Mc Graw-Hill, when referring to air cooled condensers of "clean" vapors, mention values of 10-12 Btu/(h.^oF.ft² of finned surface) for the overall heat transfer coefficient.

I wonder whether these values could be referred to as "preliminary" assessment figures.

 

[dkm0038]

I have no problem sizing a steam condenser. I am just lost when there is desuperheating or subcooling along with the condensing, specifically how to handle the different overal all heat transfer rates and different lmtds.

 

[rmw]

Most condensers are designed to minimize the subcooling because that is just heat load to reheat the condensate to boiling point again. I would assume that air cooled condensers are designed similarly. Google GEA and rummage around the hits until you find their division that makes air condensers. You may pick up something there.

rmw