Vapor under vacuum in a heat exchanger
Vapor under vacuum in a heat exchanger
(OP)
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
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





RE: Vapor under vacuum in a heat exchanger
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
RE: Vapor under vacuum in a heat exchanger
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.
RE: Vapor under vacuum in a heat exchanger
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
RE: Vapor under vacuum in a heat exchanger
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
RE: Vapor under vacuum in a heat exchanger
RE: Vapor under vacuum in a heat exchanger
If this is not the one you are working with, the one I would go to is 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
RE: Vapor under vacuum in a heat exchanger
RE: Vapor under vacuum in a heat exchanger
It occurred to me that Graham can help you there too as they are a world leader in the field.
rmw
RE: Vapor under vacuum in a heat exchanger
ht
(most of them manufacture vacuum ejectors)
RE: Vapor under vacuum in a heat exchanger
This is a common arangment for dry screw pumps.
Regards
StoneCold
RE: Vapor under vacuum in a heat exchanger
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, www.vacuum-guide.com
RE: Vapor under vacuum in a heat exchanger
Regards
StoneCold