Humidity Control

Hi Friends,

In Datacenters with low thermal load we can observe humidity control issues. The relative humidity at the room have a high variation (humidity swings) on a short time.
I have observed that when the precision air condition equipment are on cooling or dehumidifying mode (compressors on) the RH stays Ok, but when the compressors turn off by setpoints the RH rises quickly.
I know that with this scenario (units that are oversized for the heat load) will cause this situation, but I would like to understand better by psychrometry why this happen and what can be done to reduce the humidity swings.

Thank you.

Hi,

I think the biggest problem is that with stopped compressor and still running evap fans you will bring back huminity from evap to the datacenter.
You will need something like a dripping time after compressor stop to avoid this.

Quote:

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Originally Posted by RODRIGO BR

Hi Friends,

In Datacenters with low thermal load we can observe humidity control issues. The relative humidity at the room have a high variation (humidity swings) on a short time.
I have observed that when the precision air condition equipment are on cooling or dehumidifying mode (compressors on) the RH stays Ok, but when the compressors turn off by setpoints the RH rises quickly.
I know that with this scenario (units that are oversized for the heat load) will cause this situation, but I would like to understand better by psychrometry why this happen and what can be done to reduce the humidity swings.

Thank you.

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The keyword to understand this scenario is "psychrometric chart". If the doors are closed the RH will go down, but if the door is opened, moisture will enter the room and RH will go up.

When is compressor is tripped, there exists condesate water on the cooling coils. Since the air inside the room is already dry, this air will pick up the condensate adding to the increase in RH of the room. The solution is to keep the data center isolated so no outside air enters the room. But practically this is not possible. The only solutions are, to have a good positive pressure air lock installed, or use a VRF unit, as the compressor is always ON (at low speeds) maintaining the temperature.

Also, how many units are there in the room concerned?

If more than one then try offsetting the temperature set point of the various units so that one unit is running at all times which will help to stabilise the humidity. The other units can then stage up or down holding the temperature.

when you say high RH swings, what is RH setpoint and what is the swing differential?
I work in a Data centre cooled by outside air and depending on outside conditions we are happy for the RH to drift between 25% - 75%. So i wouldnt be concerned of RH value drifting out of a tight band.
If so just adjust your high RH alarm setpoint

It's all about the temperature of the air flowing through the evap coil, as inferred by the air off temp. The air off temp should be 20F/11K lower than the room air temp. If a volume of humid air is cooled to say 55F/12.5C it will lose just enough moisture so that if it is then warmed to 75F/23.5C it's relative humidity will be about 50%.

Brian, Friends, Tks for the replies,

Unfortunately in the room there is only one unit.

Hi ,

The RH setpoint setup is 50% with 15% of proportional band (differential)
The RH swings are between 46% to 80 % in a short time.
The problem is that the room needs to keep the RH constant without swings.Only change the alarm setpoints doesn't fix the problem.

Thanks

Hi Gary, Thanks for your reply,

The inlet temperature of the air on the unit is about 21.0 C and the outlet temperature of the air is ~12.0 C.
The problem is that there low thermal load on the room (~10KW) and the AC unit have 35 KW of cooling capacity.There are no enough thermal load to warm up these air volume.
To put the electrical reheats (already instaled inside of the unit)to work all time, could it help to avoid the RH swings?

Thanks to all.

Quote:

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Originally Posted by RODRIGO BR

Hi Gary, Thanks for your reply,

The inlet temperature of the air on the unit is about 21.0 C and the outlet temperature of the air is ~12.0 C.
The problem is that there low thermal load on the room (~10KW) and the AC unit have 35 KW of cooling capacity.There are no enough thermal load to warm up these air volume.
To put the electrical reheats (already instaled inside of the unit)to work all time, could it help to avoid the RH swings?

Thanks to all.

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If we assume that the inlet air temperature is the same as the room temperature, then the outlet temperature should be 11K lower than the inlet temperature. In this case, the outlet temperature should be 10C.

Lower the outlet temperature by slowing the evaporator fan speed.

Reheat does not remove moisture, it simply extends the run time.

Fresh air input is possibly effecting RH % with cooling stopped, either limit fresh air make up or cycle fresh air with cooling cycle

Quote:

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Originally Posted by Magoo

Fresh air input is possibly effecting RH % with cooling stopped, either limit fresh air make up or cycle fresh air with cooling cycle

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There has been no mention of fresh air input. I assume there is none.

"when the compressors turn off by setpoints the RH rises quickly."

That simply means there is water in this room. Find out where is it, in evaporator or from open the door or..
Otherwise when room temperature rise the RH drop.
And look at the sensor where you measure the RH, is that near water? You should measure RH at a point in the room and should not measure it near evaporator.

Quote:

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Originally Posted by Magoo

Fresh air input is possibly effecting RH % with cooling stopped, either limit fresh air make up or cycle fresh air with cooling cycle

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Hi,

There is no fresh air , the room is completely sealed.

Thanks.

Quote:

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Originally Posted by Gary

If we assume that the inlet air temperature is the same as the room temperature, then the outlet temperature should be 11K lower than the inlet temperature. In this case, the outlet temperature should be 10C.

Lower the outlet temperature by slowing the evaporator fan speed.

Reheat does not remove moisture, it simply extends the run time.

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The idea to keep the reheat on is just to try minimize the short cycles of the compressors(due the low thermal load), keeping the cooling run for a higher time , drying the air.
Previously I have already tried slowing the evaporator fan speed, but the suction pressures slowing down to values not so good.( ~53 PSIG). (R407C).

Thanks.

Slow the fan speed. Get the air off temp right.

The suction pressure is what it is. It is not important.

Quote:

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Originally Posted by Gary

Slow the fan speed. Get the air off temp right.

The suction pressure is what it is. It is not important.

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Couldn't low suction pressures causes coil freezing?

Quote:

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Originally Posted by RODRIGO BR

Couldn't low suction pressures causes coil freezing?

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The air off temp is 12C and needs to be reduced to 10C, neither of which is anywhere close to 0C which is where freezing occurs.

Here's a psychrometric chart:

http://www.uigi.com/UIGI_SI.PDF

Locate the intersection of 21C and 50%RH.

Draw a horizontal line left to where it intersects the 100%RH line. Note that this coincides with the vertical 10C line.

Humid air at 10C@100%RH drops out excess moisture. When this air is then warmed to 21C it will be @50%RH.

Hi Guys
just an idea :)
I had a similar problem several years ago with an Airdale A/C unit I could watch the the evap coil as it ran
the problem was the fan speed was to high the water could not form a droplet to run off the evaporator and effectively remove the humidity from the room as soon as the cooling stopped the coil warmed and the moisture was pushed back into the room
if you can reduce the fan speed on the evaporator a little if the unit is oversized by as much as you say then the air flow could be way to much for the space you are cooling

just an idea :)

Hi Gary,

Thank you so much for your attention. You help me a lot.

Regards

Quote:

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Originally Posted by simon@parker

Hi Guys
just an idea :)
I had a similar problem several years ago with an Airdale A/C unit I could watch the the evap coil as it ran
the problem was the fan speed was to high the water could not form a droplet to run off the evaporator and effectively remove the humidity from the room as soon as the cooling stopped the coil warmed and the moisture was pushed back into the room
if you can reduce the fan speed on the evaporator a little if the unit is oversized by as much as you say then the air flow could be way to much for the space you are cooling

just an idea :)

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Hi Simon@parker ,

Thank you to share with us your experience . Gary have mentioned on previously post to slow the fan speed too.
Its a good idea . I will post the results later.

Thanks to everyone .