I have read sites like this, but I am just not getting it yet.

I'm looking to understand the variables involved, which I think are:

  • air moisture content
  • air flow (cfm?)
  • condenser surface area?
  • temperature of the heat exchange medium (fluid)?

Is there more? Are there terms used besides these?

Background: The problem I'm trying to solve

I have a water foundation and I want to regulate the humidity of the air that flow past / out of it.

I think I would have an area about 14" diameter and 6-10" tall to put the dehumidifier system. I'm not presuming that I'll find one off-the-shelf, but better if I can.

I'm looking forward to understanding about the theory of how they work.

  • $\begingroup$ Dear NewAlexandria (is your username a metaphor for the internet stored knowledge BTW) I don't think you'll get an answer here, but unfortunately there is no mechanical or civil engineering stack exchange yet. The kind of knowledge you are seeking is that found in engineering specifications and standards, which gather together a great deal of experimental and practical experience in coming up with the procedures for designing things like this. Physicists and mathematicians in general are not going to be any wiser than you on this one. $\endgroup$ Commented Dec 1, 2013 at 23:49
  • $\begingroup$ I got rid of the exchange interaction tag and put thermodynamics instead. The former is unrelated to your problem. I hope this helps - there may be someone here who has come across this kind of thin in their experience. $\endgroup$ Commented Dec 1, 2013 at 23:51
  • $\begingroup$ @WetSavanna The absence of an Engineering site simply means all the engineers hang out here. :) $\endgroup$ Commented Dec 2, 2013 at 4:24
  • 1
    $\begingroup$ @Asad I'm glad to see it! My point was partly that many people see physicists as all-knowing, and able to work anything out from first principles. But some problems are simply messy, do not lend themselves well to theoretical study and are wrongly trivialised: we physicists, in my experience, grossly underestimate the experience and work that goes into solving a problem like the OP's. Also, such problems can take on a subjective human element: what level of humidity feels comfortable and under what conditions. I was warning the OP not to expect too much; if the right people are here, grand! $\endgroup$ Commented Dec 2, 2013 at 4:31

2 Answers 2


Dehumidifiers work by first cooling the air, then heating it back again. The cooler air can hold less moisture, so the cool air dumps much of its moisture load. This separates the incoming moist air into two streams, dry air and liquid water.

The heat output of the heat engine that makes the cold temperature is used to warm the cooled air again. Actually this will make it a bit warmer than it was originally, with the extra representing the power required to operate the heat engine.


From the article you cited:

(source: dantherm.com)

A fan draws in humid air and carries it through a refrigerated evaporator.

Evaporator is just a heat exchanger in which the working fluid (refrigerant, inside the tubes) evaporates. It accomplishes this by sucking out the heat from the air flowing through. This is the rectangular portion on the right side of the apparatus in the image.

The air is cooled well below its dew point.

The dew point is the temperature of the air at which it can't carry any more water vapour than it already does. If you cool this air, the excess water vapour

The water condenses on the cold surface of the evaporator and drips into a water container or is led directly to a drain.

This is pretty self-explanatory. You now have cold air, minus the water vapour.

Then the cold dry air continues through a hot condenser which heats it up and returns it to the room to pick up new humidity.

The cold, dry air from the previous step can't be introduced into the room as-is (this would mean the apparatus is a cooler). Remember the working fluid that was in the pipes of the evaporator, which sucked out heat from the incoming air? That fluid is pumped to another heat exchanger called the "condenser". Here, this fluid (which is a gas currently), is condensed into liquid. In doing so, it throws out its excess heat, which is taken by the "cool, dry air" flowing through the condenser, rising to slightly above the original temperature. This is for two reasons, because some work is involved in running the system and because the water gives off some latent heat in order to change phase. This air is then sent to the room.

This procedure is continued until the desired condition is achieved.

The end result is (slightly-more-than-)room-temperature air, minus the moisture.

  • $\begingroup$ "This procedure is continued until the desired condition is achieved" does this mean that most dehumidifying units must include a switched shunt that recirculates air back into the system until the output is the desired humidity, and then the shunt switches to emit the air into the 'room'? or that the 'room' air is recirculated? $\endgroup$ Commented Dec 2, 2013 at 22:39
  • $\begingroup$ A portion of the room air is recirculated, along with fresh air. You're now moving into the territory of a refrigeration / air conditioning engineer. $\endgroup$
    – pho
    Commented Dec 3, 2013 at 0:57

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