Condensation and Freezing in a Duct

[teoman877]

I have a simple question that I couldn't figure out.

We have L=60 m, with dia 1600 mm Stainless Steel (thickness=1,6 mm)round duct at outside. Outside minimum temperature is -36 C, with RH = 90%. The two ends of duct are open. From right end, 23500 m3/h air enters at -36 C. At the center of the duct, 6500 m3/h air at 18 C, mix with that 23000 m3/h cold air. So, from the left end, totally 30000 m3/h air is being exhausted with a fan.

The question is that whether a condensation and/or a freezing occurs in that duct or not.

Thank you.

 

[AbbyNormal]

work out the dewpoint of the mixture and the wall temperature of the duct

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[ChasBean1]

You left out the critical component, which is dew point of the air mixture inside the duct. With your givens, condensation could happen inside the duct and not outside the duct. If the dew point of the mixed INTERNAL airstream is greater than -36°C, condensation will occur inside the duct… Don’t worry about heat transfer across the metal – it does exist but is negligible for this application.

 

[teoman877]

Thanks a lot for your attention. But, what about the freezing possibility of this condensated water inside the duct? Does this condensated water turn to ice easily with air flow 30000 m3/h, velocity 4 m/s, and mixed air temperature inside the duct approximately -14 / -15 C?

 

[AbbyNormal]

In sub-zero ambients, when air touches a surface below its dew point, they do not call it condensation they call it frost



The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[AbbyNormal]

maybe you get a blizzard

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[sterl]

Actually blizzard would be pretty close.

Unless that 18-deg Air Stream is at less than about 8% Rel Humidity, this mix is going to be saturated or supersaturated and the outbound temperature is going to be higher than the pure mixed enthalpy numbers would indicate due to the latent of the frost.

The frost in the immediate downstream portion of the junction is going to choke the duct before long and the assembly is going to weight many times more than the duct and support would represent. And there will be snow spitting out the fan.