Humidity Control for Tape Storage

[Pgee630]

Hi,

I've a case where it requires an indoor air conditioned environment be maintained at 16 to 25 degC and 20% to 50% RH, for tape storage equipment. Assume the load basically is sensible load (tape storage equipment only). If just using a normal chilled water system, e.g. 7 degC supply and 12 degC return, can the system maintains the humidity level? Or it requires dehumidifier and humidifier for more precise humidity control?

Thanks for advice.

 

[MintJulep]

No.
Yes.

 

[Pgee630]

@MintJulep

Thanks for your response. May I understand a bit more on the theory behind?

 

[EnergyProfessional]

Chiller temp is not relevant. If anything, worry about the AHU temps.

And you have zero infiltration and zero water from use (i.e. humans, cleaning)? I wouldn't design a system under such assumptions.

I'd look a psychrometric chart to see the relatively large allowable area.

 

[MintJulep]

If your load is only sensible there is no humidity to control.

You will not meet the 20% RH minimum.

 

[nuuvox000]

I don't know where you're located but I'm in the western United States where it's very dry most of the year. Whenever a customer asks me to maintain a certain humidity level I ask them what happens if the humidity leaves the range they're requiring. The price of a dehumidifier is VERY expensive and always catches people off guard. If it's not such a big deal to leave the humidity range then they usually tell us not to install the dehumidifier. If millions of dollars in product will be lost then they tell us to install the dehumidifier. Basically, it really depends on what area of the world you're in and what the risk is. If a customer MUST have this range of humidity at all times then install a humidifier and a dehumidifier. Most of my customers are okay being out of spec sometimes during the year when they see the equipment costs.

 

[Pgee630]

@nuuvox000

the humidity range I described must be maintained. I'm just wondering if the chilled water coil can already do the job without a dehumidifier. Though the load is basically near to 100% of sensible load, we may still have a bit of latent load due to infiltration, or if there are people in and out of the room for doing job (people will not stay for hours regularly but still will have a bit of time there to do some adhoc works), then the humidity may rise or go down (depends on the outside condition) for certain amount, and due to the AHU have very minimal latent capacity, then is it necessary to have a dehumidifier to dealt with this transient? Or say, if the system is initially started up, the room humidity may be high and it may take very long time to lower down the humidity to the required range?

 

[IRstuff]

https://bmcnoldy.rsmas.miami.edu/Humidity.html

I think your dew points are above the chiller temperatures.

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[MedicineEng]

A quick psychometric chart analysis would show that it would be very, very difficult for you to maintain your required temp/RH range with those chilled water temperatures.
Remember, depending on the distance between your chiller and the AHU and how effective is your thermal insulation, your 7C chilled water at chiller point might easily become 8C or more.
Also, please be aware that when the air is leaving the cooling coil is basically at 100%RH so you can go from there to see if the quantity of water vapour in your air stream matches your supply air requirements (assuming no more water vapout is introduced in the air stream).

Of course this is a cursory analysis just for you to see if you are in the ballpark or if you are already tight in your inputs to get the outputs that you require.
If you are confortable with the results, then a proper and deeper analysis must be performed.

2 years ago we did a art gallery project here at our property and despite our best efforts, we couldn't get away from desiccant dehumidifier and humidifier, in order to control humidity.
But again, there are quite a few million dollars worth of art in the gallery, so the equipment cost was kind of irrelevant when put into perspective of having a valuable painting full of mold or a wooden work cracked...

Good luck

 

[nuuvox000]

Let's take your upper limits, 25 deg C and 50% RH. Your discharge temperature from your air handler could never discharge air above 13.8 deg C (at least on my 5000 ft elevation chart). And that would be at the very limit of your constraints which obviously wouldn't work out well.

If you take the middle of the range they gave you which gives you some more room for error, that would be 20.5 deg C and 35% RH. Your discharge air temperature from the air handler should always be around 4.5 deg C. If you can't hit that temperature with your air handler then I wouldn't even consider doing it without a dehumidifier.

If it EVER gets dry in your area then you'll be screwed without a humidifier since the air handler can't add water to the air by itself.