Close control RH

Guys,

I need to design a facility which requires relative humidity to be controlled to 55 +/- 2%.Could anyone advise what precautions need to be taken?

 

[quark]

Sac22!

55+/-2% RH is relatively easy to maintain.

1. Proper sealing of return duct joint so that no ingression of moisture.

2. Hot water control will maintain RH presicely when compared to electric heaters.

3. Fins of cooling coil should be vertical.

4. Proper sealing of door bottoms should be done.

5. Finally use a good RH indicator.

Note: You can provide temperature check points at various places in the ducting system viz., before cooling coil, after cooling coil, mixing boxes and return ducting etc. so that you can check specific humidity. This will be helpful in knowing the humidity trend.

Regards,

 

[fredb]

Hello,

An important point to remember is that water vapor acts in the same way that heat does, it always tends to migrate from an area in a higher state to one in a lower state. You must make certain that your controlled area is vapor-tight so that whatever water you introduce can be controlled by your equipment and not by losses.

 

Thanks Quark and Fredb.I was advised to have a Dehumididfier afetr AHU outlet with a portion of air passing through it.The room humidistat will then have to control the bypass damper to dehumidifier to maintain RH.Do you think such a system is superior to reheat control?

WhatI have on site for reheat is steam at 100psi.Can a steam coil hold RH at +/_2% ?If I really need to use hot water for reheat,what kind of temperature range would you recommend for effective control?

Thanks a lot for your help guys!

 

[ChasBean1]

sac, NO, that system is not superior to a system that centrally cools/dehumidifies and reheats to provide comfort. By using such a system, you are bordering on a technology for which you will likely not be able to hire sufficient automatic temperature control (ATC) expertise.

My first question: who is driving the guideline of 55 ± 2% relative humidity? Obviously someone who is not familiar with mechanical engineering and HVAC. The control need is in the moisture content, not the relative humidity, as a room can be 80°F and 55% relative humidity and be a swamp, or 40°F and 55% relative humidity and be dry enough to make your skin flake and create static shocks.

If you need to maintain 55% relative humidity, you need absolutely precise room temperature control; fluctuations of greater than about 1°F will continuously put you either high or low out of a two percent humidity range. Then you must ask at what room temperature is the 55% relative humidity required.

I need to close this - it's late and I need to go to bed, but I'm seeing this more and more - projects where the need is driven by someone who doesn't have the understanding of the physics that provide the need, and who may not fully understand the result of their request. I don't mean to sound crass, but relative humidity control to me is sort of like trying to control the location of the moon with respect to your house... It's all relative and in humidity's application, based wholly on temperature. Relative humidity is more a result than a control parameter. Re-post if you have any further info on this requirement. Thanks, CB

 

Chasebean,

The temperature needs to be held at 72+/-1F.The application is for a production facility.

 

[gently]

Hi
Agree with most of what Chasbean has said, but would like to add that you can only achieve the required tolerance AT THE LOCATION OF THE SENSOR.

 

[quark]

Sac22!

If I am correct you may be working in a pharmaceutical facility (where conditions are stringent by ego not by statute)

Dehumidifier is obviously a good option. But the temperature of heating media depends upon what type of desiccant you are using. Typically for silicagel bed or honey comb type wheel regeneration temperature is 150 deg. C. For 55+/-2% desiccant dehumidifier is not economic. But you can use the bypass damper control using room humidistat.
Be sure that the humidistat is placed near return air grill for better control.

Relative humidity is controlled because at any dry bulb temperature the RH should be controlled to the specified limit so that product degradation doesn't occur. (stability I mean)

Regards,

 

[fredb]

Hello again,

I purchased what I would guess is a similar testing lab many years ago. As I recall the system had a small electric steam boiler running at 15 lbs with a multiport nozzle in the return air duct for moisture injection. The air had previously passed over a dx coil for temperature reduction and then over an electric reheat coil for final temperature adjustment.