Calculating humidity increase

[Takket2]

I’ve got a heck of a situation and I’m not sure how to calculate it.

I have a computer room that is set at 70 degrees. The computer room is 19838 square feet. On the north and south end of the mechanical room there are mechanical rooms, each with 4 AHUs that move about 30,000 CFM each (N+1), possibly 3 running at most in each mechanical room). One mechanical room is 2767 square feet and the other is 2439.

The current humidity in the computer room is 61%. The humidifiers are set to maintain 40%.

The AHU discharge temp is 55 and the return temp is 72. There is NO outside air to the AHUs (well, there is a SMALL amount, but not relevant.)

A contractor is going to apply spray on fireproofing foam to the roof deck in each mechanical room. There is concern that as the foam dries/curies, the water released will cause an unsafe humidity level in the computer room. I have figured out by talking with the contractors that they will use 360 gallons per day of water to prepare their spray on foam. It will take them 12 days of spraying to cover the mechanical rooms. The foam takes 24 hours to cure/dry (meaning that 360 gallons will, more or less, all be vaporized and pass through the AHUs, and then into the data room). Now, I am aware that SOME of the water will be removed in the form of condensation over the cooling coil. What I do not know how to calculate is, after that 61% RH, 72 degree air enters the mechanical room, what can I expect for a humidity increase from the process of the fireproofing drying? If I can figure that out, I should be able to figure out from my psychometrics how much water will be removed by the AHUs as condensate, and how much will be transferred to the computer room.

Any suggestions?

 

[SAK9]

Will moisture release rate remain constant throghout the drying time? unlikely I would imagine.The rate may be high initially and gradually going down with time.I would treat this as a constant enthalpy process along the wet bulb line similar to a water spray ahead of the coil.Being a computer room unit,the coil is most likely a dry one and may not condense a lot of moisture.

 

[EnergyProfessional]

I'm not sure I see the problem here. any excess moisture will be taken out by the cooling coil. CC always discharges air at 100% RH at the given temperature. so the more water you introduce, the more moisture they remove.It may happen that this goes above the capacity of the chiller and the air will be slightly warmer. On the other hand the water evaporating in the same room has the same cooling effect.

I also don't think higher humidity, especially for such short period of time, hurts the servers. Newer ASHRAE guidelines have a very wide allowable range of humidity and Dell etc. have similar wide specs. Many large datacenters (incl. Microsoft, Google etc.) cool just with OA in suitable climates. It really is not such a big deal.

The old humidity values came from when "computers" still had paper cards with he "code" and the paper would get stuck when too humid. But old habits just carried over without need for pure electronic computers. I assume the computers you talk about are post 1970

Unless the computer manufacturer specifically demands or you use air-economizer, I wouldn't even bother humidity control. the cooling coils take care of it all. considering how inaccurate humidity sensor s are (and never get calibrated or replaced as they should), you probably do more harm attempting to control humidity than by just doing nothing.

 

[Drazen]

yep, if it could be assumed that water spraying will function as adiabatic cooling of mechanical room, chiller load would not change at all...

but that will not help if chiller is not designed to work as wet-cooling dehumidifier, as dry or "uncontrolled" wet chillers will not command humidity and it will raise, no matter the fact that the same amount of enthalpy will be extracted by chiller.

 

[KiwiMace]

I don't think this will be a problem. If you are worried, run the fourth AHU and open the access door/panel at the discharge so it circulates air in the mech room. Or if VFD, four slowly for better DH performance.

I would be more concerned at why your humidity is at 60% when you don't have OA. Sounds unsustainable. Are your coils clean?

 

[marcoh]

if you RH is set to 40% and you only achieving 60% this would indicate an issue of some sort with the controls, heaters, cooling coil or chw temperature.

Depending on your ambient conditions you could try to go to 100% outside air?

 

[tys90]

I think he meant that the humidifiers keep the humidity ABOVE 40%. He doesn't say what the limit (if there is one) on the high end is.

 

[317069]

Please correct me if I understood in wrong way.
- you have 19838 sq.ft of a space used as a computer room
- this space is served by 8 AHU's with small or neglegted fresh air
- there are two mechanical rooms with total area of about 25% of the computer room area, these rooms will have a high level of humidity ratio for a short time because of a work has to be done.

- computer room's current hummidity is 61%(before sparying), is this a design value, I mean does the system work fine for now or has some problems?
- computers give most (if not all) of the heat as sensible, there is no fresh air for this system, how come the humidity is 61%.?

- I guess the room return is connected directly to each AHU, how come the mechanical room's air will find his way to computer room ducts?
- mechanical room air has no way to go into conditioned space, it should be phisically separated from conditioned space.

- in your case why don't you use an exhaust fan to exhaust mechanical rooms during the work time?

 

[cluelessinohio]

This work is probably already complete so it would be interesting how it turned out.

One of the other responses already mentioned why is it so humid in the space now. If the room conditions is measured at 72F/61%, and you are supplying 55F/54F, you are gaining 13 grains/lbair across the space. This is significant even at one AHU running let alone if 6 is running at 30000 cfm.

Either you have large leaks between the room and surronding spaces/outside or the supply temperature is not really 55F.

Check the outside air conditions when the condition were measured and see if there is a relationship between this the absolute humidity gain and the outside humidity.

 

[ChasBean1]

Just had a deja vu with this post. Have you asked this same question before?

 

[willard3]

Well, takket 2 has only posted once unless he's changed his account.