Thanks for your input! Interesting point re. pressure vs. vacuum. The friend who gave it to me insists that it sucks really well but a short test run seems to confirm your statement. A strong airflow at the outlet and barely noticeable flowing in. I'm hoping that containing it in a box will create more suction but I am totally inexperienced in this. If all else fails, I'll just sell the fan and buy a ShopVac :)

I tried this same design using a "Dust Deputy" cyclone collector and a fairly large squirrel cage blower.  The problem was that the blower is very good at creating pressure, but not so good at creating a vacuum.  Regular vacs and shop vacs use a flatter squirrel cage and a high rotational speed to get a good vacuum, the downside is the noise. A real shop vac and the Dust Deputy work fantastically well together, but I hate the noise. I hope that you can find a way around this problem.