Ventilation Ducting - Psychometric Chart

[ajoven]

I am ventilating a second floor area in a recip engine power plant with just outside air. I will distribute the air through a duct with registers. The duct will be insulated inside with 1" insulation and outside with 2" insulation. The area gets very warm (120+ with engines running during the summer and 90+ during the winter with not engines running). I don't think I will have a problem with condensation during the summer however I am concerned with condensation during the winter. I have done some heat tranfer - temperature calulcations. I calculated the ducting sandwiched between the outside and inside insulation will have a temperature of 60 deg. F. I started out with 110 deg outside and 32 deg inside temperatures (outside and inside of ducting). If I were to use the psychometric chart and with 30% RH, the dew point temperature is above the 60 deg duct temperature. 1. Can I assume that there will be condensation on the duct even if sandwiched between insulation? 2. Or if the duct is properly insulated and sealed condensation will not be an issue? 3. Does this application warrant the use of the psychometric chart? Thanks in advance for any comments.

AJ

 

[ChrisConley]

The reason we insulate cold ducts (well one of the reasons) is to prevent condensation.

If you calculate the surface temperature of you duct through the insulation, and that temp is below the dewpoint of the space you will have condensation.

If the duct is insulated with a continuous vapour barrier you shouldn't have probelms.

 

[ChasBean1]

Chris is right. It would depend heavily on vater vapor generation in the space and air exchange rates. If you have relatively low humidity generation in the space (which it sounds like you might if it's an engine plant and not a steam plant) and a decent air exchange rate, the dew point inside and outside might be nearly the same. This would mean that you might not even need to insulate the duct.

3 inches of duct insulation sounds almost overkill for this application, unless there's a substantial moisture source in the room.

CB

 

[trashcanman]

If you are just ventilating with outside air to remove engine heat, why are you insulating the ducts?

As for moisture, where is it coming from, people? Moisture will migrate towards the cold side and condense only if the surface is at or below the dew point. If you are supplying cold outside air during the winter, the inside temperature of the building will be warmer so how can it condense?

 

[willard3]

As I read the original question, the generator is NOT running during the winter

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The area gets very warm (120+ with engines running during the summer and 90+ during the winter with not engines running).
snip

Why would you be ventilating in winter if the engines are not running? It is possible to condense on the duct in the summer, but OA would have to be pretty low and inside have very high wet-bulb.

Is the inside temp heat gain mostly sensible as it would be with an engine? If not, what is the moisture source?