Humidity Increase Due to Rainstorm

[russmech]

I'm designing HVAC systems with humidity control for a museum and wish to know if someone can tell me or lead me to information on the short term effects of a passing thunderstorm. Project is in Fairbanks, Alaska where where summer temps and rainstorms could dramatically raise humidity levels for a short time. But how much, and for how long? Can anyone help or suggest a source of help?

 

[ChasBean1]

Russ,

The standard HVAC system will "wring the air out" based on its discharge temperature. During the storm, your humidity will rise to 100% (outside air is saturated).

As long as the system you install cools the air to the design value (say 55°F), your dewpoint at the unit outlet will not exceed 55°F. This means that you will be providing a properly-dehumified volume of air to your space.

The subject here is consistency. Modern HVAC systems generally provide air at 55°F or slightly lower because that wrings the appropriate moisture from the air such that a tenable, non-mold growing environment can be attained inside the building. If your central unit discharge temperature is 55°F and constant and the intake air velocity is low (e.g., below 500 feet per minute) any storm can pass through without a concern for indoor conditions deviating much from 72°F and 50% humidity.

If your system doesn't cool the air to 55°F (or lower), be prepared to face problems later.

If you find that the 55°F discharge air makes the space too cold, use reheat to warm the air back up before entering the space. This final heat addition can be done at each location where the air enters the space, or it can be done as the final, conditioning step at the AHU discharge. Regardless the air should be consistently cooled (initially) to remove the right amount of moisture.

My best as always, -CB

 

[quark]

Russmech!

I will add one more point to CB. Moisture permeation from outer walls will be one more problem. For the rooms attached to outer walls you should install some vapor barriers. As suggested by Chasbean the reheating of supply air will further decrease Relative Humidity.

But as for 55 deg. F, I think it is better to check the apparatus dew point first. During rainy season if you decrease the fresh air quantity, you can decrease the intensity of the problem.

However I strongly feel that desiccant dehumidifier is a better option, for there will be lot of moisture load addition from respiration and clothes of visitors in the museum.

Regards,

 

[ChasBean1]

Russ, trying to keep things simple, my response above was with respect to a passing rainstorm, not internal people load, infiltration due to poor vapor barrier, or negative building air pressure drawing in moisture. And it is with the understanding that this is Alaska, not the Bahamas.

CONDITION 1, PRE-RAIN

Say for example you can have a nice, comfortable day that's 75°F outside and 50% humidity (55°F dewpoint). If the central AHUs are operating normally, they will discharge saturated (~100% RH), 55°F air to condition the space.

CONDITION 2, AFTER RAINFALL

Now, weather gets hot and sticky and starts to rain. Temperature outside increases to 90°F and 100% RH (90°F dewpoint - worst case; will never happen in Alaska). Rain falls. If the central AHUs are operating normally, they will discharge saturated (~100% RH), 55°F air to condition the space.

Notice any difference between the two discharge conditions? The only difference is that the chilled water control valve (or compression stages for the refrigerant) will be open wider or operate more, and more water will come out the condensate drain during hot and humid conditions.

Establish design criteria from ASHRAE (I believe Alaska design conditions are included). Cool the air centrally to allow the system to cool and dehumidify based on design conditions. As Quark says, ensure you have a good vapor barrier. And ensure your building is slightly positive (more air in than out).

I don't see an advantage of desiccant dehumidification over the standard method of cooling the air centrally, in this case. -CB

 

[quark]

Russmech!

As you said yours is a museum I thought it is better to have desiccant dehumidifier. People coming in during rain bring extra (than usual) moisture inside.

Secondly there is always coil bypass factor. When your AHUs are designed for normal load conditions this will create some problem.

It is easy to remove absorbed moisture from material lying in the museum if you use desiccant dehumidifiers. Ofcourse I may be thinking in only one line.

Regards,