Gas Mixing in Ducts 1

[baglan]

Does anyone have any suggestions as to how to effectively mix low pressure gases in a large duct (3 to 4 metre diamter) without a large pressure drop? Hot products of combustion from a vertical up-fired burner i.e. CO2, water vapour, nitrogen and some excess air are surrounded by ambient air.

I would like to ensure that the hot gases (1200 to 1400 °C) moving at about 100 m/s are thoroughly mixed with the air before they enter a heat exchanger at about 500 °C, in the shortest distance possible.

 

[TD2K]

Koch makes static mixers for mixing gas streams (as well as liquids). Ask them about your application.

 

[zdas04]

Remember that static mixers are notorious for having a very high pressure drop. I've seen improved mixing from a vortex tool (see

http://www.vortexflowllc.com/

), but I've never done anything with the temperature drop you're anticipating.



David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[TD2K]

I've seen pictures of static mixers for ducts, they were basically tabs attached to the wall and not really restricting the duct area that much, I wouldn't exect huge pressure drops but I guess huge is relative on gas systems.

I'd be interested hearing what sort of pressure drops you hear from Koch or some other vendors.

 

[unclesyd]

If water injection is viable option it would keep from raising the duct pressure or enlarging the duct.
I think you need a desupheater type arrangement where you inject water to cool the gas stream. We use such an arrangement on a much larger scale on a large thermal oxidizer. We depend on 2 rings of atomizing nozzles to inject water into the gas stream. The first one blows in more air than water and the second ring injects more water than air. The gas temperature is dropped from 2800°F to 2200°F then to 1700°F.
The gas heats a boiler and then an SCR and exits still above the dew point.

You might want to give these people a call if water injection is a possibility.

http://www.komax.com

 

[baglan]

Thanks for the comments, guys.

I don't think a static mixer with tabs around the circumference will be enough in this case because of the large diameter of the duct. If they don't protrude too much they probably don't have much mixing effect?

We thought about cooling the gas with water but unfortunately water is just not available for this application. (Water would have been good because the increase in mass flow is not so much.)

 

[rmw]

BD Heat Recovery has vortex type static mixers, but nothing appears on their static mixer page at

http://www.bdheat.com.

You can contact them for the information you need.

rmw