atmospheric dispersion/dilution.......question

[Dave Brown <david.brown@nist.gov>]

I have a question related to an observation of some calculations I've

done recently regarding atmospheric diffusion of effluent releases. 



Consider a case where there is a fixed release rate, Ci/sec, at a fixed

stack flow rate, cfm.  There is also the option of a large dilution

fan that uses outside air to dilute the effluent concentration going out

the stack by say a factor of 500.  In this case, although the

effluent concentration has significantly decreased, the effective Q or

Ci/sec remains the same.  My observation is that the only thing this

dilution fan accomplishes in the calculations is to increase the

effective stack height thus changing the vertical dispersion coefficient

and the effective wind speed. This does not have a proportionally

significant affect on the resulting downwind concentrations. It would

seem that this dilution fan should have more impact on the downwind

dilution....but it doesn't. Anyone have a good explanation/justification

for this...? My initial thoughts are that in the scope of atmospheric

dilution , the dilution fan is small in comparison. This is just an

observation though, I'm looking for a more technical explanation

Regards,

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Dave Brown, CHP
National Institute of Standards and Technology
100 Bureau Drive, Stop 3543
Bldg 235 Rm B104
Gaithersburg, MD  20899-3543

301-975-5810 - office
301-921-9847 - fax
david.brown@nist.gov
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