How is RH Controlled in AHU

To explain in layperson terms, to get the most moisture out of the air, you cool the air as much as possible. (The moisture thus condenses or even freezes on the cooling coil.) If this makes the air colder than you want, you then reheat it with a steam or hot water coil, an electric heater, or possibly a combustion process.

If instead you want high relative humidity, you cool the air as little as possible. This is done with large cooling coils with chilled water or refrigerant that is only a small amount cooler than the desired air temperature.

The chilled water removes heat from the incoming air stream. There are two types of heat removed from the air. There is the physical manifestation of temeprature in the air molecule - this is referred to as the sensible heat. There is also the "latent heat of vaporization" usually referred to as latent heat. This is the reverse removal of the heat that was required to vaporize the water molecule in the first place.

Once you cool the air to the point where the relative humidity is 100% (at this point only sensible heat is removed), you must now begin removing latent heat from the water vapor in the air.

The chilled water coil removes mositure by absorbing a combination of sensible and latent heat from the incoming air stream. Assuming your coil is desigend correctly, you should, in general, be able to cool your air stream down to about 5 degrees higher than your chilled water temperature. At the point the air leaves the chilled water coil, it is at 100% RH, but it is cooled. Depending upon your situation, you typically add heat back to the system to raise its temperature back up. Doing this will also lower the RH.

The important thing to concentrate on is dew point. You have to wring enough water out of your air stream to acheive the desired RH once you recombine your conditioned air with the rest of the air in your room that you are trying to condition.

I highly recommend that you familiarize yourself with psychrometrics if you want to truely understand how a chilled water coil controls RH.

http://en.wikipedia.org/wiki/Psychrometrics

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