Insulation on 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!

 

[RJonTilt]

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+Rmetal+Routside)=(Tduct-Tdp)/(Rduct)
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)

R metal surface = L/K L is thickness .. it is duct gauge
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 determined to prevent the inside of the duct from reaching this temperature.
Condensation and possibly freezing will occur unless you can find a solution for this equation that does not allow it. .