Blower design

[bruce]

There has been so much discussion lately about some type of air handler besides the small screaming amp hogging vacuum blowers that I have been "secretly" working with a/some forum members on the design of a simple to build air handler.

I'm almost at the point of starting to cut the metal to make one to try out and I'd like to share a simple drawing I made of the blower wheel for discussion . . .

The blades are 5" in length



Edited By bruce on 1177261895

[Thomas Vaught]

Glad you created a new "board only" area.

Most centrifugals started out as straight blade units but over time went to a more curved vane approach. This is to get higher pressure ratios which you may not need for a flow bench.

Keep us informed.

Tom V.

[Tony]

Great work Bruce, I have not experimented with different blade profiles, but I did read somewhere that most high pressure blowers use short straight radial (non angled) blades. As that is probably the easiest type to fabricate anyway, it may be worth a try to begin with.

The rotor that I have inside my bench right now has twelve rather short blades only three inches long and arranged around the outside of an eighteen inch diameter fully shrouded rotor. This wheel is only half an inch wide around the outside edge.

The yellow Aerotech rotor is the same size, 18" x 1/2" but has ten almost straight radial blades, that extend quite a bit further into the centre eye of the rotor.




The back of the rotor is a dead flat disk onto which the blades are spot welded. Drive is via a bolted on central hub that has a taper lock fitting.

The flare shaped front rotor cover is then pop riveted onto the front. This front shroud is made of very light gauge pressed steel, but spun, or pressed aluminium or even fiberglass would probably be easier to fabricate for a homemade rotor.

[bruce]

I picked up some alum this past week to use for the side plates on my blower wheel and I am in the process of machining them this weekend. Right now they are cut into 22-1/8" squares and the diameter will be machined on the mill using my rotary table. With any luck that will happen in the next few days after machining up some adaptors to assist the rotab setup . . . I'll post some pics of my progress

[DaveMcLain]

Every so often we do some repair work on a blower for a local company that produces plastic bags. They use a blower to suck tiny plastic pellets from a rail car, across the road and into the factory. Every once in a while something happens and the blower will break off a blade and start shaking violently. They used to just weld the blade back on without balancing and that was terrible. What we would do is weld the blade on and then balance the rotor in the balancing machine, worked great.

For what it's worth, it was about 13 inches in diameter and it had about 8 blades that were slightly curved. It was flat on the back and the blades were exposed I would assume then that it just keyed to a shaft and spun in a housing with a bearing at each end they said it pumped through about an 8 inch pipe and really moved some air.

[Tony]

The two commercial blower rotors that I have here, both have had very light spot welded on balance weights added. There is no obvious vibration problem with either rotor.

The trick seems to be to build a very light wheel, but attach it to a fairly heavy hub. The mass of the hub will absorb any slight residual unbalance. Direct coupling a blower rotor to a motor seems to work very well, because the motor frame is heavy enough to absorb any slight vibration. Motor bearings are always designed to be fairly massive, to absorb side loads from very tightly stretched pulley belts. So a bit of blower rotor wobble is unlikely to destroy the motor bearings.

Balance might be more critical if the rotor is supported only by a very short low mass drive shaft with just a small diameter pulley, and then driven to very high speed. But fitting a blower rotor direct onto the motor shaft is probably a much better idea for many reasons.

My experience has been that vibration is a non issue, but a quick static balance on knife edges is pretty easy to arrange, and more than sufficient.

[bruce]

Here are a few pics of the start of my blower project. Blanking the side plates for the blower wheel on my milling machine/rotary table.

[DaveMcLain]

I think you're absolutely right about having the rotor connected directly to an electric motor as being the best about vibration and such. This blower that was used to move the plastic pellets was supported with a shaft and two pillow block style bearings which were probably pretty light and just bolted to some fabricated steel which would not be good at all when it comes to absorbing any vibrations. Either way, the fan was easy to balance by just welding a small washer to the back side where it was needed. Keeping the whole thing light is probably another key, this rotor was rather heavy since it was pumping both air and plastic through a pipe.

[bruce]

Side plates machined round and the holes pre-drilled for the blades to be installed. They don't look real pretty though, had to use some alum found in the scrap. Had to tear the rotary table apart twice while machining them. Kept getting tight, but did get real good with centering it back up! LOL

I'm trying to keep it in the realm of the average DIY'er if they care to build a blower. So this is a low-cost build as most of my projects are.



Edited By bruce on 1182029725

[gofaster]

For what it's worth, I've had occasion to do maintenance work on some large industrial fans, about 12 to 15 feet in diameter. Here are a few things that I noticed:

They had an inlet area that seemed large in relation to the length of the radial blades

The radial blades had about a 45 degree pitch (guessing)

There were always an odd number of blades

[bruce]

On this blower I only machined it for 6 blades, they are offset from the centerline 1".

I've had some lengthy emails with Tony over the past year on this subject and finally decieded that we could "talk it to death" but doing it is the best way to findout what works and what doesn't. He has given me some very good direction on this project and has done some testing using his blower to assist me.

If it doesn't work as machined I can always add in more blades, or even build another blower wheel. With "scrounging around" for parts I should have less than $100 into this blower (not counting my time). I setup the square sides on my rotary table to machine them round but those with access to a waterjet or laser could have them cut without that time consuming step. Accurate location of the blade mounting holes would be the most difficult thing to do.

Expected outcome? Rough figures are in the 500cfm @ 28" range with a 5hp motor without a VFD which would increase the cost and is always an add-on option.

Who wouldn't like that from a single blower, that could be built by most members of the forum?



Edited By bruce on 1182088371

[Tony]

I have no idea how important the number of blades will be. One of my blowers has ten blades, the other twelve.

Turbochargers usually have eleven turbine blades, and either ten or twelve compressor blades.

[gofaster]

Possibly to avoid having the pressure pulses go into sync, and set up a destructive harmonic?

[Tony]

That is my thinking as well.

[misc.motorsports]

the blower fan is looking good bruce. I finally started building my vaccum box and am going to order my other motors to make 8 motors total. well good luck with the blower.
Mark