Odor control duct design

[NADOR123]

Gary,

This is my first time designing an odor control system to replace an old odor control system in a weaste treatment plant; I’m hoping you can educate me. The ductwork on the original odor control system ran below the ground, and was constantly filled with water and therefore did not work. This sounds to me that there was not enough air velocity to carry the moisture out of the duct and it accumulated. Because of the problems in the past we want to run the ductwork above ground, however this becomes quite cumbersome, due to the duct size and clearance needed for trucks to drive under. In addition this will increase the external static pressure that the fan will have to overcome.

In your experience how is the ductwork usually run, above or below the ground?

Is the water accumulation due to a lack is air velocity?

How do we avoid moisture build-up in the ductwork?



Your help is greatly appreciated.



Thank you,

 

[JedClampett]

In your experience how is the ductwork usually run, above or below the ground? Either one is done. below ground needs to be designed for overburden, so the pipe wall gets pretty thick.

Is the water accumulation due to a lack is air velocity? I'm a structural pud, so this is a little out of my area, but if the water is affecting flow, I doubt it's dropping out of the gas. It's either siphoning into the system or leaking in through infiltration or stormwater.

How do we avoid moisture build-up in the ductwork? Add low spots with drains.

 

[cvg]

water will condense given the right conditions. Metal pipes underground often provide an ideal condition for condensation. Warm moist air drawn from the treatment process will readily condense on a cool metal surface. Provide a positive slope and/or drains are necessary to remove accumulated condensation. Leakage through pipe joints needs to be prevented by using leak resistant joints. Groundwater infiltration near the underground ducts can be handled by placement of impermeable backfill around the ducts and proper surface drainage.

 

[bimr]

Don't see how that you can run the ductwork underground unless you install the ductwork within a pipe chase.

The moisture laden air in the duct will condense at the ground temperature of around 55 Deg F. The ductwork must have installed drains to remove the liquid that condenses. The drains usually have traps to prevent the duct pressure from blowing through the drains.

Velocity has little bearing on the condensation. You will never have enough velocity to force the water out. The ductwork is sized on the basis of economical velocity/pressure drop as well as noise. High velocity air streams in ductwork can generate noise pollution. The typical velocity of 1500 CFM for this application is much less than the velocity to force the water along.

http://ateam.lbl.gov/Design-Guide/DGHtm/existingductsizingmethods.htm

You may have a larger problem with condensation if the ductwork is installed out of doors, depending on the climate.

To retain heat in the ductwork and lessen the amount of condensation, it is common to insulate the ductwork to retain heat.