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



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
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301-975-5810 - office
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david.brown@nist.gov
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