Humidity Control while Cooling the computer Room in Winter

[fcxdfm]

The computer room I am working on is in Houston. The winter outdoor design temperature 20 DB, and 60% RH are used. The space needs to be maintained at 75 +/- 5 Deg. F, and 50% +/- 5% RH throughout the year. The cooling load is about 6 ton and almost 100% sensible load. The outdoor air is about 20% of the total supply. The system is DX unit with 5 KW electric heating, and with hot gas reheat for dehumidity control. The psychrometric chart shows in the winter, the mixed air has 64 DB and 60% RH, about 49 deg. F dew point, before it enters the cooling coil (the room still needs cooling in the winter). However, 49 deg. F dew point is lower than the 55 Deg. F dew point to maintain the space at 75 Deg. and 50% RH. I am confused here - does it mean the space can not be mainted at 75 deg. and 50% RH, and maybe only at 70 Deg. and 49% RH, or 75% deg. and 42% RH? Do I need provide humidifier to keep RH no lower than 45% then?

Thank you very much for help!

 

[imok2]

All designed for computer room air conitioners come with humidifiers and it's almost a must for mfg warrenty.

 

[fcxdfm]

Imok2, thanks for the response. Could you further explain the types of humidifier that are typically used for the computer room? Thanks again for help!

 

[fcxdfm]

By the way, to be accruate, the sapce is not a computer room, and it is mainly electrical room housing UPS, VFD, and other electrical power cabinets. I feel the humidifier is not needed in my case since the winter weather is quite mild in Houston, and only about 10% outside air mixes with 90% return air. But I just cannot approve it through psychrometric chart to make sure the RH never drops below 45% RH, either in winter cooling mode or electric heat mode (in case all elect. equipment is off, and the space needs elect. heat).

Thanks for the advise from any of you !!!

 

[Yorkman]

Not, to nudge in but there are commonly two types of humidifiers that I see in Computer Rooom Air Conditioning units in our area (Colo). (CRAC units as they are sometime called). One type will be a stainless steel pan with radiant heat quartz lamps above it. The other type is a steam canister, which is a small high temp plastic barrel with electrodes immerssed in it to generate steam. Both require periodic maintanance some prefer the canister over the lamps, I think it's generally a toss up.
I have only seen it once, a Liebert unit that actually used building steam for humidification, and I suppose if that's available they can be ordered that way also it depends on the manufacterer.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[Yorkman]

If your trying to maintain a higher humidity level in the space than out side the space you may need to look at putting a vapor barrier around the room.
I've seen time and time again were a tenant will take a space, turn it into a data center or printer room, and try to maintain 55% Rh or better. Then find they can't do it because they never took the time and expense to insure that they had a room with a quality vapor barrier.
Just somethimg to be aware of if your stuggling with maintaining your humidity levels.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[imok2]

Yorkman nailed the humidity question.

 

[fcxdfm]

Thanks, Yorkman and Imok2. So, can we avoid the humidifier (In this case, with 90% return air, I just dont' feel I need one) if a vapor barrier is provided? Please correct me if I am wrong.

 

[KiwiMace]

You need both.

Your heated supply air will still dry out the room because when 20degF/60% OA is heated to 80degF the RH drops to about 15%. There are very few moisture-producing sources within the room itself, unless it is heavily occupied.

With an artificially raised RH inside, you will need the vapour barrier to prevent moisture migration outside, and to protect your insulation from condensation.

 

[MAragorn]

I am also in Houston.

I agree with what they have said, though you are right also, low humidity is not a "typical" problem in Houston. But as indicated, unless there is a moisture source in the space, the 10% OA will gradually dry out your space.

If you do install a vapor barrier, be sure and install it correctly for a hot and humid climate.

Vapor barriers installed where they are normally done in the great white north reportedly cause huge mold problems in the south, particularly Houston.

 

[fcxdfm]

Thank you all for the advises!