Blower design

[Tony]

It all comes down to cost and convenience. A proper tong tester is the safest and most convenient, but you may not like paying perhaps $100.00 or a great deal more than that, for something you will rarely ever use,
With one of these, you press the yellow trigger on the side, and the two yellow jaws (tongs) open up. You just close the jaws around the outside of a single wire carrying the ac current, and take a reading. It is the safest way, as the instrument works off the magnetic field around the wire, there being no need to connect anything to exposed high voltages.



A shunt is the ultra low cost way, assuming you already have a digital multimeter that measures millivolts.



Yet another way would be to buy an ac panel meter, either cheap and nasty like this one.



Or a proper professional quality analog ac panel meter:



There are some vintage ac amp meters that turn up occasionally, often at very reasonable cost. These are precision instruments, despite their age and appearance, they are usually extremely accurate and reliable, and should not be dispised. I have several ancient instruments like this myself, some going back to the 1930's and they still work perfectly. In their day they were fantastically expensive, and built to the very highest quality.



Plenty of different ways to go about measuring ac motor current, and it need not cost a fortune.

[SWR]

Thanks,Tony & John for educating me. I am starting to think that actually wiring up a meter to continually show me the amps going would be benefical in the long run,especially as I'm about to move on to a big blower instead of multiple motors and I'd like to not make a blackout in the whole district when I pull as much as I can on it. :laugh:

Another question,related to the blower design: To pull more inches,would you need a bigger diameter blower? Or just rev it more? What would you think is the best way?

(I guess making it as strong as possible is a must anyway,as the forces generated are high and just as dangerous as the tons of force working against the panels of your bench).

[Tony]

Either will do the job. Tip speed is what determines the developed pressure, or more precisely tip speed squared.

An 18 inch rotor running at 3,600 rpm should just about reach 28" .

A three inch rotor running 21,600 rpm would be expected to do pretty much the same.

Doubling either the speed or rotor diameter would develop four times the pressure 112" !!

So a little bit more of either speed or diameter can produce a DRAMATIC pressure increase. And a little bit less speed or diameter means you are going to fall way short.

You can either start off with a motor, and design your rotor diameter to suit the motor rpm, or start off with an existing commercial blower, and work out the required drive speed.

[SWR]

Thanks for that,you probably didn't mean to but you just told me how big my rotor has to be... About 40" and do 3600 rpm... If I now could fathom how "deep" it has to be to move enough air,then I'm ready to prototype one...

[Tony]

First of all figure out your required maximum flow, then try to estimate what would be reasonable flow areas throughout the whole bench. A bit of imagination and some wild guessing is required here. But let's say you decide to run a four inch diameter test hole, and no flow area except maybe a flow measurement orifice is to be smaller than four inches diameter.

This is just an example, my own bench is 3.5 inches. Yours could be more or less than that.

Anyhow, a four inch minimum flow area might require a blower casing with a four inch entry, and a four inch discharge diameter. Your rotor will need a four inch minimum entry eye diameter. The rotor width will need to be at least quarter of that, or one inch wide at the air entry point.

Now four inches diameter is a flow area of roughly 12.5 square inches.

Your rotor ideally should have an almost constant flow area from inlet to periphery. If your diameter is going to be forty inches, which is pretty enormous ! the circumference will be 126 inches. To achieve a flow area of 12.5 square inches, the wheel minimum width only needs to be 0.1 inches wide at the outside!!! If the rotor was to be only twenty inches diameter, the rotor would need to be around 0.2" wide minimum.

These are all minimums because we are talking about minimum flow areas. A bit wider, even double is not going to hurt.

Like most things, it is the minimum flow restriction point that mainly determines flow. Making one section vastly larger than somewhere else is not going to increase flow, unless the part you are unblocking is the most restrictive point in the whole system that is causing the limiting.

Our blowers run at a low pressure ratio, perhaps 13.7 psi absolute inlet, and 14.7psi absolute outlet pressure. That is only a pressure ratio of 7%
So the inlet does not need to be made significantly larger than the outlet as it would normally be for a much higher pressure ratio.

Just forget for a moment that the blower is rotating, and figure out what sort of flow areas the air has to negotiate from inlet to outlet. Try to keep everything in rough proportion.

Forty inches diameter sounds very large, at 3,600 rpm that will generate something like 130+ inches, almost 5psi. Do you really need such a high pressure ?

[SWR]

I'm seeing things... that is,with higher pressures in the "MotoTune"-style 4-valve and 5-valve ports I now run I need it to. Lose flow,gain "impossible" port speeds and still make power... I think the 4-valvers need a new set of "rules of thumb"... Especially those like these...with a choke with a proper venturi,and not just a valve seat acting like one.

[ThomasVaught]

Quote:

"Tom,

You need to find a better electrician, basic testing 101 - is it connected right, are motor currents less than rated max, .... "

The best intentions are worthless when you are dealing with a drunken electrician

Tom Vaught

[WPH]

This is about overall blower design:

I'm a huge fan of aircooled VWs and I found some interesting numbers of their cooling "fans". It is capable of 1400cfm @ 4000rpm (engine) with a roughly 1.5:1 pulley ratio. Any idea to dig into? They are quite easily available and CHEAP, you just need to build some housing and get sufficient electric motor to run?
Blower has a approximately following measures: blade tip dia 9.5", entrance dia 5.8" (streamlined nicely to blades) and 28 1.8" curved radial blades.
CFM seems fine but how about pressure? Couldn't find any numbers.

[106-1194218389]

hmmm... I used to race VW aircooled. You could just gut the generator that drives the fan and have it stick through the box where the vacuum motors normally would be to the outside and drive the pulley with an electric motor. Now if we can just figure a way to test the pressure.

[Tony]

[gadget]

For what it is worth I have seen belt drive turbochargers. It got me thinking of getting a large truck turbo to get the compressor side and impeller and making a bearing suport and shaft to mount it to. This company has done a impressive job of putting this togeather. I will link it for example only http://www.vortron.com/pdf/Z40e_data_sheet.pdf
This gives a good description of their build I think a machinest could build the necessary parts. Also alot has been said about hp requirements this chart might help http://www.vortron.com/pdf/selection_guide.pdf

[thomasvaught-1]

The Vortron People are also the same owners of the Vortech Racing Superchargers and the Paxton Superchargers used on submarines for purging bad air on surfacing of the sub.

Tom V.

[gadget]

That is good info thanks. I had been looking at paxton superchargers as a pump for a flow bench but the ones that I have seen used are less than 300 cfm at 20" wc. I liked vortron's use of a automotive belt tightener. I love off the shelf engeneering when possible. I think that their layout could be done by some one with access to basic machine tools.

[Tony]

Mother nature has decreed that developed pressure is pretty much determined by rotor tip speed, which is really diameter times rpm. So you might get to choose between :

1.5" diameter rotor, 50,000 rpm maybe 35 inches of developed pressure.

6" diameter rotor , 12,500 rpm maybe 35 inches of developed pressure.

24" diameter rotor, 3,125 rpm maybe 35 inches of developed pressure.

Which is most convenient to drive, balance, and lubricate, is entirely up to you.

Unless it really needs to be made very small, perhaps for a vehicle, or a vacuum cleaner, bigger will always be quieter and potentially have a lot fewer mechanical dramas.

[115-1172523331]

Just looking at some older posts and wondering what happened to this one? Did Bruce get seduced by the "Dark Side" (electronics)? :p -- Doug