Exterior Dust Collection Ducts

[flyrfan]

What are your thoughts regarding insulating dust collection ducts that are located outdoors in the Northeast U.S.(14*F design DB)? Specifically, I have an application where ~ 200 ft of duct will be installed outdoors. Some people in my office suggest that the ducts be insulated to prevent condensation on the interior as the exhaust air will be humidified to 70*F DB / 30% RH in the winter time. The dewpoint at those conditions is approx. 36*F, so theoretically water vapor could form on the cold duct surface. I've designed many DC systems, with no problems w/ condensation on outdoor duct work (or collector housings for thath matter!) Could it be that since the air velocity is so high in DC ducts and the surface film is highly turbulent thus preventing condensation?

Thanks in advance for your thoughts!

 

[CH5OH]

the high velocity will somewhat decrease the dewpoint, however even in high velocity ducts, there are places the air doesn't move at all (where it touches the sides, corners,...

 

[tys90]

I would use double wall duct with insulation sandwiched between. Find out what r-value you need to prevent the inside wall from reaching 36F at design conditions and use the appropriate amount of insulation to reach that r-value.

You are correct that the air velocity is probably the reason why DC systems don't condense but I would guess that the reason has to do with heat transfer not turbulence, if I understand what you are saying correctly. There will be a much larger convective coefficient with high velocity air.

I think of it like this: the more surface area/the slower the velocity, the more chance of condensation if everything else is the same.

 

[RJonTilt]

i am not sure what the DC means but You are most sure that the condensation is occurring if you use the following equation to determine at what temperture condensation will occur:

q/A= (Tduct-Toutside)/(Rduct+R(metal+insulation) +Routside)=(Tduct-Tdp)/(Rduct)
convection Resistance values are as follows:

Rduct surface = 1/hduct so use h inside duct of 1.0 btu/hr ft^2 F ( you can vary this based upon velocity in duct but will the aire always be moving?)

R metal surface/insulation = L/K L is thickness .. it is duct gauge if not insulation Assume steel thermal conductivity (K) factor of 0.27 Btu/hour/square foot/°F/inch thickness

R outside surface = 1/h outside so use h outside of 0.25 btu/hr ft^2 F ( you can vary this based upon velocity in wind)

So now you all the information Solve the equation for T outside.Once you know Toutside the amount of insulation can be iterated to determined breakpoiint to prevent the inside of the duct from reaching this temperature.