Oversized AHU & High humidity 2

[Takket2]

We have an AHU that was sized to cool a UPS system. The UPS is only half loaded and is not producting enough heat so the AHU is essentially oversized. The AHU has the room cooled but since it hardly has to run, we aren't dehumidifying. Tried closing the outdoor air damper to zero and still only getting beading on the coil and no condensate at all. System has no VFD or reheat so either the humidity is high, or we subcool the room to dehumidify. Suggestion has been made to raise the discharge air temperature above 55. Does this make sense? What should we set it at? Setpoint for the room right now is 78. Coil is chilled water. Fan shuts off at 68 and turns back on at 70 but as said, the AHU just doesn't have enough heat load to run enough to dehumidify.

 

[DRWeig]

You won't dehumidify with warmer supply air either.

Make the unit "temporarily" smaller with an adjustable sheave for the fan belt. If the fan is direct drive, stick a VFD on it.

What size is this unit? CFM and horsepower, please.[tt][/tt]

Best to you,

Goober Dave

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[Takket2]

I didn't think so..... that's why I asked LOL. AHU is 2900 CFM, 5HP fan, designed for 3.04 brake horsepower.

 

[DRWeig]

If you can do the pulley thing, figure the actual load in the room and lower the speed to give you maybe 15 percent excess capacity, with the supply air temperature as cold as you can make it. Adjustable sheaves are cheap, common, and reliable in my experience.

If you have to put in a drive, they're just not that expensive anymore in small sizes like 5 HP. Maybe $500 if you shop around, even if it happens to be 200-230V. Maybe a bit cheaper at 460V. That's cheaper than the dehumidifier you'd have to buy, in all likelihood.

Good luck!




Best to you,

Goober Dave

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[DRWeig]

I forgot to ask -- what sort of humidities are you getting right now? What size is the UPS that only has half a load on it?

I also forgot to say, after you get your humidity under control (the purpose of my coldest-supply-air comment), you should be able to reset supply air upward to keep from overdoing it. If you have a BAS, a little programming is in order. Maybe even a reheat box or local heater. It wouldn't take too big of a heater.

Best to you,

Goober Dave

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[Compositepro]

Put a small portable dehumidifier in the room.

 

[SAK9]

Step 1: Reduce fan speed to meet the reduced load condition.Since the room is functional an accurate estimate of heat load is possible from current draw.
Step2: Blank off coil face area to maintain a healthy coil face velocity otherwise flow will go laminar reducing heat transfer and extent of dehumidifcation
Step3:Assuming this is a chilled water system,compressor capacity control may not be necessary.If not make provisions for compressor tu run loaded for long periods.

 

[EnergyProfessional]

what RH (at what temp) do you actually want to achieve? You shouldn't take RH to serious.... all the large Microsoft/Yahoo datacenters in non-humid climates all just cool with untreated (only filtered) outside air and don't give a damn about RH.

the old RH standards for IT were based on when computers sucked in their code on paper rolls and those would expand at high RH and get stuck. Assuming you now have fully electronic data storage, you likely are resolving a problem that doesn't exist.

 

[hoodsman]

Can you manually run the fan on the air handler? If so wouldn't that cut the humidity even if the thermostat wasn't calling for cooling? I remember something about this from school but maybe I'm wrong.

 

[DRWeig]

OP still hasn't told us the humidity level the room is experiencing. I agree with a dehumidifier, Compositpro, as long as it costs less than a small (5 HP) variable speed drive.

If he's running 75 percent and wants 50 percent, that won't be a small humidifier. Humidifiers other than direct steam injection are a pain to maintain. Say, that rhymes.

It is just about a moot point in modern data centers, as HerrKaLeun stated -- you're not supposed to store paper in there. You certainly don't have to worry about condensation on a server, either. They kinda have to be warmer than the dry bulb by a bit. To that point, most server rooms and other electrical process rooms I have encountered had sensible heat ratios of 85 percent to 95+ percent. The latent load is primarily a coil load, not a space load.

Best to you,

Goober Dave

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