Vapor under vacuum in a heat exchanger

[methylester]

I'm working with a manufacturer to size a heat exchanger that will be used to condense water vapor that is being extracted from a mixture under vacuum. The vapor is also being condensed under vacuum. For the sake of round numbers, it'll be under about 29.5" Hg of vacuum, at which point the saturation temp is about 60F. We'll be using an hxer with a cold side at about 40F.

His sizing software only works with a minimum pressure of 0.5 psig. At this point, the saturation temp is about 214F. So to get a rough analogy, I suggested that he just plug in an hxer cold side of 194F, to maintain the same 20 deg difference between the saturation temp and the hxer cold side.

What I'm not sure how to compensate for, though, is the difference in density of the vapor. The mass flow rate will be the same either way. But at 0.5 psig, you would have much denser vapor moving more slowly through the hxer, while at 29.5" Hg of vacuum, you would have a much less dense vapor moving more rapidly through the hxer. This has got to have some effect, but I'm not sure how to figure out what that effect is.

Anybody know of a method or rule of thumb to figure out how to approximately compensate for this difference?

Thanks

 

[ione]

Well, not only density would be different but also viscosity and thermal conductivity, and these parameters would all affect the heat transfer coefficient. Moreover your approach foresees to keep a constant temperature difference (20 F), but what usually matters here is the mean log temperature difference.

Is it possible to enter manually the fluid properties at your conditions (p,T) in the manufacturer’s software?

P.S. At 29.5" Hg vacuum (0.0133 bara) the saturation temeprature is 52.15 F

 

[srfish]

Hi,

A shell-and-tube condenser that operates at 29.5" Hg abs is a special situation. Even most surface condensers do not operate at this low of pressure. The slightest pressure drop will have a profound effect on the design. Because of this the tube pattern is usually descending.

I don't know how you can use a rule of thumb or other method to make an approximation. For more information on steam condensers that operate at this low of pressure, check out the Heat Exchange Institute (HEI) in Cleveland, Ohio. They specialize in standards for equipment of this type.

 

[TJOrlowski]

Work with a different manufacturer. One with the know-how (not just the software) to design a unit with your actual operating conditions.

As srfish pointed out, a small change in pressure (even within the heat exchanger itself) has a large effect on the way the unit performs.

-TJ Orlowski

 

[rmw]

I am with TJ Orlowski on this one. Not only do you want someone who has the tools to size it properly but you want someone who has the knowedge of how to design it.

Condensers are Hx's but Hx's are not necessarily condensers.

You will also need to know how much non-condensables are coming over with your vapor and have your condenser designed to handle and segregate them so that they can be extracted.

What type of air (non-condensable) extraction will you be using?

rmw

 

[800502]

From my understanding and experiences, there is no commercial software that can handle your case. Yes, the only way is approximation to an existing condenser like this.

 

[rmw]

I suggest going to other vendors who are in the business and have a successful track record. If what they propose is similar to what the vendor you are working with is proposing, then you are probably OK. If not, that should be a warning sign.

If this is not the one you are working with, the one I would go to is

http://www.graham-mfg.com

. There are others, but they pop into my mind right off, especially for smaller Hx's.

They do this type of fabrication with regularity.

rmw

 

[methylester]

Thanks for the suggestion - I'll give graham-mfg.com a try. In answer to your question about what kind of air extraction, we're simply using a rotary vane vacuum pump for this.

 

[rmw]

If you are going to actually reach 29.5" Hg vacuum, you are going to need some first class vacuum equipment to function way down there.

It occurred to me that Graham can help you there too as they are a world leader in the field.

rmw

 

[PISA]

you will find many vacuum condenser manufacturers:

http://www.vacuum-guide.com/vacuum-pump/vacuum-ejector/vacuum-condenser-europe.htm

(most of them manufacture vacuum ejectors)

 

[StoneCold]

Something you might want to consider is condensing the water after the vacuum pump. Assuming your pump can handle the volume of vapor. You will get better vacuum because the pressure drop of your condenser will be on the other side of the vacuum pump.
This is a common arangment for dry screw pumps.
Regards
StoneCold

 

[PISA]

answering to StoneCold:
I always recommend to condense under vacuum (with chilled water if necessary) and before the vacuum pump - reducing (dramatically) the size of the vacuum pump. An after-condenser (at atm. pressure) make sense in addition of a pre-condenser (under vacuum) for a solvent recovery (reducing emissions).
Regards. Pisa,

http://www.vacuum-guide.com

 

[StoneCold]

I think that whether you condense before the vacuum pump or after has a lot to do with what temperatures you are running and what utilities you have to work with. Size can be an issue. Most of the posts on this site neglect to tell us the size of the system. Which usually has an impact on the best solution.

Regards
StoneCold