Gas Stack Exit Velocity

[macmet]

I'd like people's opinions on what the minimum design velocity of flue gas should be from the gas stack.

In the past I have used numbers between 50 and 60ft/s. I did a search online and saw a minimum of 16ft/s. I checked B&W Steam and it said that the 'typical' was 30-35ft/s.

Also, are there any serious issues with the low gas velocity? My only concern now is the low velocity we may get will require us to do a new dispersion model, plus we're being charged for a stack that is larger than it needs to be. In this particular case the vendor is recommending between 10-15 ft/s.

 

[dvd]

High stack velocity can carry condensate up the stack and result in water droplets raining down in the area around the stack. If the stack is located adjacent to a parking or office area you may want the lower velocity.

 

[macmet]

Thanks for the response DVD, but I'm actually concerned that this exit velocity may be too low in this case. I have the vendor re-evaluating this velocity now.

 

[ash9144]

If you were to significantly change your velocity you would need to do a new dispersion model unless your new velocity was in the range covered by the original model.

Also most environmental permits (at least in US) include the stack height and exit velocity so you may need a permit modification for a new velocity.