Posted:
Thu Apr 01, 2004 10:47 pmby sauer_power
Im building my first bench and need help selecting motors. Will shop vac motors work if so how many do I need and what hp should they be. I will be flowing heads mostly but maybe a few bigger things like carbs or intakes.
Posted:
Sat Apr 03, 2004 9:01 amby Mouse
Yes, any vacuum motor will work. In fact, you can go to your local Good Will and buy a bunch of shop vacs and other houshold vacuum cleaners and mount all the motors on a single board.
It may be surprising to know that my 2.5HP shop vac pulls only slightly less air than my 4.5HP shop vac. And I have some friends whos 6.5HP shop vacs pull only a click more than my 4.5HP.
The Audie Tech QuikFlow web page has some vacuum motor recomendations ffom Grainger.
John
Posted:
Mon Apr 05, 2004 1:14 amby 84-1074663779
Finding a suitable air blower is a very important first step, as it is something that cannot easily be later changed once your bench is finally in one piece and running.
Your best bet is to test a bare blower, and then decide if it is going to suit your needs.
So what I suggest you do is make yourself a series of test orifices out of light gauge sheet metal, with hole sizes in steps from perhaps half inch, to maybe two and a half inches in diameter.
These will stick to the blower intake, held in position by air pressure alone, and you can measure how much vacuum the blower can pull across each orifice size.
Flow can be estimated from the basic orifice formula where CFM equals 13.55 multiplied by the square root of pressure drop (inches of water) multiplied by the orifice diameter in inches squared.
So if you can pull sixteen inches of water pressure across a 1.25 inch hole:
CFM = 13.55 x square root of 16" multiplied by 1.25" squared
CFM = 13.55 x 4 x 1.5625
CFM = 84.7
You can then plot a rough pressure flow curve for your bare blower. Another important thing to do is measure the motor current.
You will then be able to see how many similar blowers you will need to reach the design goal of your flowbench, always assuming that sufficient electrical power is available.
To the flow bench neophyte, this is usually a pretty terrifying experience. The idea of building an 800 CFM bench that will test at 28 inches suddenly looks like a fairly large piece of very high powered machinery, and it certainly would be.
Another useful formula is the supercharger drive power formula, where for a 100% efficient machine drive power equals psi multiplied by CFM divided by 229. One psi is almost exactly 28 inches of water.
So 500 CFM times one psi divided by 229 equals 2.18 horsepower. Now that is shaft power required to thermodynamically compress that air at 100% efficiency.
Your vacuum cleaner motor might be only 20% efficient, if you are lucky. So maybe ten horsepowers worth may be required, or even more. That is going to require a lot of amps to run it.