air jet ejector

I have spent an intense week attempting to develop an air jet ejector that would evacuate about 300 cubic feet/min (cfm) of air for my lab hood (not yet built).

I'm attempting to use my salvaged home heating/AC blower as the motive air source. My piping is 4" ID PVC sewer pipe with a WYE (45 deg tee) connector to form the suction point. Internally there is a 4"x3" reducer leading to a 2.85" orifice located at the suction point. I have no diffuser per se. I don't see the point of one at such low motive air speed.

By increasing the blower speed from the original 1125 rpm to 2050 rpm I am able to develop a 1.25"H2O suction at no flow. When I open the suction the static pressure drops to about 0.06"H2O, and my calculated flow is about 25 cfm.

I develop 2.5"H2O static pressure in the blower plenum. I already know I will have to increase my motor size from 1/4 HP to 1/2 HP. And I don't know if my blower wheel can stand up to these more demanding conditions for very long.

If any of you have any experience with low pressure air ejector design and/or suggestions to increase my suction flow significantly (other than abandoning the ejector) I would like to hear them.

[Edited on 23-3-2005 by Magpie]