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Posted:
Thu Dec 21, 2006 3:17 pmby Brett W
I have seen some larger flowbenches use big commercial blowers to drive the bench. Could use use a standard electric motor to drive the impeller from a VW aircooled engine? Those fans will flow something like 800+cfm when flowing through an engine. The aircooled engine has a pretty high pressure drop as well. SO in theory you could build a shroud around one an use it to supply the bench.
Could you use a belt driven supercharger from some of the bigger construction equipment to handle flow duties? I know some of the huge earth moving equipment uses a belt driven supercharger as big as a house.
Posted:
Thu Dec 21, 2006 7:58 pmby Tony
Not personally familiar with the VW cooling fan, but to develop useful pressure it would need to run a much higher Rpm. It may be safe, or it may not, I have no idea. The main issues are balance, and mechanical strength at the speeds required.
Centrifugal superchargers make superb flow bench blowers, only problem is the high cost. But if you can find one, it would be ideal.
Posted:
Thu Dec 21, 2006 8:11 pmby evilgeniuscycleworks
I was think of using the blower from a little wonder leaf blower coupled to a 5 horse electric motor. the blower is tall and narrow and had a high output with a 6 horse brigs.
what do you think?
John
Posted:
Fri Dec 22, 2006 1:55 amby Brett W
The VW impellers develope maximum flow in the 4800 rpm range. I don't know about the pressure but I would assume that in the case of an aircooled engine as flow goes up so would pressure.
Posted:
Fri Dec 22, 2006 10:15 pmby Tony
A fairly good guide to estimating pressure capability for any centrifugal blower can be judged just from the rotor tip speed.
As a starting point, an 18" rotor often develops pretty close to 17" pressure (or vacuum) at 2,900 Rpm. That will be at about 230 feet per second tip speed.
Doubling the tip speed raises the pressure four times, half the tip speed, you only get one quarter the pressure.
So around 400 feet per second might be reasonable to generate perhaps 50" for a flow bench.
So, very approximately for fifty inches of pressure:
24 inch rotor 3,800 Rpm
12 inch rotor 7,600 Rpm
6 inch rotor 15,200 Rpm
3 inch rotor 30,400 Rpm
Sometimes very poorly designed blowers cannot reach this, simply because the internal flow paths are restrictive at the 400 FPS internal air velocities. Because pressure is proportional to Rpm squared, a small Rpm increase can do wonders.
Big and slow is probably the best way to go for a home built bench blower. Far fewer balance and bearing problems, and much less noise from the motor and drive system. Only real problem is, it will be huge ! But does that really matter ?