Blower casing design

[Tony]

Bruce is making giant strides towards designing and fabricating a home made rotor for a suitably large, high pressure, but relatively low rpm flow bench blower.

I have already carried out some experiments here trying to discover a really simple way to construct a blower outer housing that combines ultra low cost and extreme simplicity of construction.

Commercial blowers all use the familiar scroll shaped outer casing, which is really a bit of a nuisance to fabricate. Yet vacuum cleaner motors seem to work reasonably well with a simple really crude round outer drum shaped casing.

Apart from the shape, there are two other fundamental requirements. It must be strong enough to withstand the considerable air pressures involved, and it absolutely must have a readily removable front cover to install and access the rotor.

What I decided to do, was first carry out some tests on a commercial belt driven high pressure blower, and then remove the original outer scroll housing, and replace it with a cut down 55 gallon drum. The original scroll was 24 inches diameter across the widest section, the new drum 23 inches diameter.

Everything else would remain exactly the same, same rotor, same drive motor, but I could do some careful back to back tests against a dumb drum housing and directly compare the results. I honestly did not expect it to work as well as it did.

Cutting straight through to the final results, my conclusion is that a round drum works perfectly well, and the flow and pressure figures that I measured were identical to that of a really sexy sculptured volute shape. It need be made no larger either.

I will go into more precise details of this later, but here are pictures of the original commercial Aerotech blower, and after it had been fitted with my ugly drum housing.

[bruce]

I was hoping you'd jump in here with your design!

Thanks for posting the pics and letting everyone in on what we have been talking about "behind the scenes"

[Tony]

Making this housing was unbelievably simple, but the first thing required will be a suitable drum.

Try and find a drum without bungs, that has a bolt on lid. These have a wonderful sturdy rubber sealing gasket, and a very efficient outside clamping ring. Drums like this are used to import dry bulk foods and chemicals. The companies that have them usually have to pay to get rid of them. Once you locate a source, you will have an unlimited supply to experiment with.



Cut the drum four and a half inches down from the top, shown here on the right hand side.
Cut again one inch from the bottom, and make saw cuts every couple of inches as seen on the left hand side.



Tap all around the bottom flange with a hammer, to bend it slightly inwards. Then force the top section of the drum over the now inward tapered bottom flange. Once the whole thing is tightly pressed together, go around inside and bend the bottom section back flush with the inside walls. It could then be welded, but I used pop rivets, and that seems to work perfectly well. The two drum halves will be an extremely tight fit together, so the whole thing becomes very rigid.

Your new blower casing should now look like this.



The extra round hole in the front of the casing was one of several experiments, and was later blocked off. A conventional tangental outlet is definitely best.

I used a four inch tangental outlet pipe which was first tack welded onto the outside of the drum, before the drum wall was cut away to suit. The whole job took a single afternoon and cost virtually nothing.

A flange mount motor would be a lot neater than the pulley system I have here. The ideal motor would have both a flange and foot mounting provided. This was just made as a test prototype with what I had on hand.

Here are the fan curves for this particular blower. The measured performance with either scroll or drum were absolutely identical.



These fan curves are not terribly exciting in themselves, but above 4,000 rpm it becomes very interesting indeed. flow increases almost directly with rpm increase, and pressure goes up almost at the rate of rpm squared. So performance at 5,000 rpm would be almost entirely off the top and right hand side of the sheet.

The rotor size to obtain the above curves was only 18" in diameter and half an inch wide. A larger diameter rotor could easily be fitted within a 23" drum. More diameter means that higher pressures can be reached with less rpm. It would not be unreasonable to expect a 21" rotor running at 3,400 rpm to have a very similar performance to an 18" rotor running at 4,000 rpm

A VFD can easily run a motor faster than it's 50/60 Hz rated speed. And a VFD is also an excellent way to control bench airflow.

If anyone has any reservations that a round drum will work as well as a scroll, Garrett made the TEO6 range of turbochargers with round compressor housings. These, although now a very old design, can still be found found on large diesels and aircraft engines.





Note the third drawing down, on the left does not use a progressively increasing scroll area, but has a simple round collection drum with a parallel wall diffuser. At the comparatively low "boost" pressures we are interested in, just a fully shrouded rotor working inside an empty drum shape works perfectly well.



Apart from making an absolutely splendid low cost flow bench blower, this could also have application for an exhaust extraction blower for a dyno, or it would make an fantastic low cost high capacity shop ducted dust and chip extraction system.

The drum casing and rotor can be made as wide as you have horsepower available to drive it. So there is really no practical limit to upper flow capacity. Just remember when cutting your drum, both top and bottom are dished, and you lose perhaps about roughly an inch. That is, if you cut at 4.5 inches, you will likely end up with about 3.5 inches internally. So measure up very carefully first.

[sneukam]

There is a book that might be of interest called how to design and build centrifugal fans by David Gingery. It is available from Lindsay Publications Inc.
A lot of the information in the book is already covered in this forum. Square edge orifices U tube manometers, Pitot tubes and calculations for air speed and volume,
Here is guide that is given for high-pressure fans on wheel width to diameter ratio. Inlet dial 35% to 65% of wheel diameter. Wheel width 10% to 25% of wheel diameter.
Discharge opening"blast area" is calculated by multiplying the wheel diameter by its width and dividing that by a factor from 2.5 to 3. The larger area allows error and inefficiency.
A = (DxW)/3
A = blast area in square inches
D = wheel diameter in inches
W = wheel width in inches
There is also a section on balancing your homemade wheel using a fixture that uses 2 steel rulers mounted on edge on which the fan shaft rolls along.
Scroll housing design is covered in great detail. You can make it simple or as fancy as like. A simple thing as moving the axle of fan towards the cutoff point and closing up side clearances increases efficiency.
Interesting book only $9.95. I bought mine about ten years ago trying to gather information on a diy flow bench. I believe that a person took his time he could build a large diameter fan but must be aware of the tensile strength of the material used. I gave up on building one after coming across the old flow bench site. I now have 8 16-1193-c 230-volt motors and 3 old vacuum cleaner motors waiting to go in a one direction two opening pitot style bench when time and space allows me to build it.

How to Design & Build Centrifugal Fans



Lindsay books