PTS Flowbench Forum

Hey guys, I'm building a bench with 8 vacuum motors which should flow around 400 - 500 cfm @ 28".
I also have an industrial 3 phase blower that flows approximately 1600 cfm but only @16". I know its depending on bench design, but if I have both vacuum sources in the bench, what would be my max flow @ 28"?
I know it sounds extreme but I have some quite large parts to test.
I have got Bruce's plans and the bench will be the pts design just a bit longer.
Thanks Jim.

That industrial blower may be 1600 cfm at 16" but it may be 800 cfm at 40". Check the specs and I bet you will find this to be close to the case.

Thanks John, I don't have specs on the unit, 1600 @ 16" is an approximate figure that Tony worked out a few years ago. Tony used the motor rpm and fan diameter to come up with it.
Jim

Jim, If you hook it up to a VFD and speed it up the available pressure increase will be phenomenal.
If it is truly capable of 1,500 CFM open flow, with the airflow throttled (by what you are testing) motor load actually decreases as mass airflow decreases.

A blower that size would I expect have about a 7.5 Hp motor, an 18 inch rotor running at about 2850 rpm.
If you can speed it up by about 75% to roughly 87Hz (5,000 rpm) with a VFD it should be able to reach the required 44 inches easily enough.
Flow will have to be decreased (throttled) by about the same ratio as the pressure multiplication to prevent overloading the motor .
A rough guess at this might be pressure increase (16" up to 44") x 2.75
Flow decrease (1,500 CFM down to 545 CFM) 1 / 2.75

It won't work out exactly to that, but it should come close.
And you could always fit a much more powerful motor to it.

I did this with my own bench, the original blower had a 1Hp motor, 18 inch rotor, and very roughly 300 CFM at 17 inches.
By doubling the speed, it created almost twice the flow, four times the pressure, and required roughly eight times the drive horsepower.
Fitted with a 10 Hp motor and VFD run up to 100Hz, that wimpy little 1Hp blower really perked up...
I have seen 60 inches total pressure and can just reach 600 CFM at around 5,900 blower rpm.
I don't normally run it up to anywhere near that maximum speed, but it has proved it can do it.

Not brave enough to explore over twice the original design speed, but it has been totally reliable over the last ten years or so without any issues.

Thanks John and Tony. That all makes sense.
I did some testing on the blower, it is a dual housing unit like a bench grinder but fan and housing each end.
I tested at one end only with a straight hose in water.
5.5 orifice 3.5" wc ( open housing entry)
3.75 orifice 8" wc
2.75 orifice 11" wc
Does this equate to 767, 539 and 340 cfm?
Tony, rotor diameter approx 10.5" and 3450 rpm 3 hp motor
Tony you mentioned before about hooking up both sides in series, would this mean with 2.75 orifice I would have 22" and 680 cfm? Or not that simple?

With a .590" orifice it went down to 8" wc.
Blower not efficient? Housing leak?

Jim

RACEPUMPER wrote:Thanks John and Tony. That all makes sense.
I did some testing on the blower, it is a dual housing unit like a bench grinder but fan and housing each end.
I tested at one end only with a straight hose in water.
5.5 orifice 3.5" wc ( open housing entry)
3.75 orifice 8" wc
2.75 orifice 11" wc
Does this equate to 767, 539 and 340 cfm?
Tony, rotor diameter approx 10.5" and 3450 rpm 3 hp motor
Tony you mentioned before about hooking up both sides in series, would this mean with 2.75 orifice I would have 22" and 680 cfm? Or not that simple?

If you arranged one blower half to blow into the other, in series flow, you would still get only 340 CFM but at twice the pressure (maybe about 22 inches). That isn't going to be enough.
However you do it, the motor is rather small at only 3Hp, and the rotor diameter not really large enough to develop serious pressures.

You really need something with a much larger motor (7.5Hp +) and a rotor diameter of perhaps 24 inches in diameter. Air entry into the eye of the rotor might ideally be about the same flow area as the test hole in your bench. Rotor width at the periphery one half, to one inch maximum will be more than sufficient.

These are not rigid rules, but just give a bit of an indication of the size and physical proportions of a prospective flow bench blower to be looking out for.
It will be large diameter and really skinny (with the proportions of a cookie) and fitted with a pretty big motor.



These turn up occasionally at machinery junk yards and auctions for scrap value, typically $50 to $200, you just need to keep your eyes wide open, and be very patient.
There is not a big market for these large three phase pressure blowers, and they often sit around for years out in the rain and the weeds.
Just don't appear too enthusiastic, ask about a few other things he has there first, and make him a really low ball offer, the seller will probably be very glad to finally get rid of it.

Thanks again Tony, I will store this blower for now and keep looking and thinking
Did you read my new water brake idea? I need some feedback on it.
Jim