Tractorsport Flowbench Forum Archive

Posted: **Sun Jan 29, 2006 4:26 pm**

Hello everyone i've been reading here for a while and finally decided to join in, this is a great forum with lots of knolegable people. Here is my problem, I managed to find a paxton vr-70 centrifugal blower and according to the seller it is capable of 600cfm nominal at 3000rpm input 9000rpm impeller and maximum 950 cfm and 6-7 psi at a input rpm of 10650 at 31950rpm impeller which is maximum allowable. I dont know how accurate his claims are but thats what ive been basing my calculations on. For the motor i used tonys required blower drive hp formula and for 500cfm at 60" i need 9.46 hp assuming 50% blower eficiency. I found a 10hp 1phase motor but what i'm woried about is that it only turns 1725rpm so to get a input rpm of 10650 i need a overdrive ratio of .16:1 I dont know if my motor will be capable of that because i dont know if it will have enouph torque i figure the torque is 30lbft at full load so that will be 4.8lbft at the blower input. I know i will probably not have to drive it that fast but i would like to know if it is capabe, I have know idea how fast i will have to drive the blower for my actual flow bench testing but im sure it will be slower.

Blake

Posted: **Sun Jan 29, 2006 6:17 pm**

Hi Blake,

I have been looking for a flow map for the vr-70 on the internet, but without success. But you can do some simple tests and make some measurements yourself fairly easily.

First of all just hook up the electric motor to the blower with any suitable drive pulleys that you happen to have there, get it going reasonably fast, say three or four times motor Rpm, block off the flow, and measure the developed pressure with a water manometer.

From that, you can easily figure out what sort of final drive ratio is actually required for your bench. Final pressure will be the blower Rpm squared, so doubling the Rpm will get you four times the pressure.

The other thing is motor power. Measure the motor current with an amp meter, and compare that to the amps on the motor rating plate. That will give you a pretty good idea of how hard the motor is working, and where you are with motor capacity.

A little bit of experimentation will soon give you a pretty good idea of what you have, and how best to set it all up. Belt slip will be a huge problem with V belts, but they are cheap and convenient to try initially. You will probably require at least four, and even then it will most likely still slip. A six rib flat automotive belt, or toothed belt would be far better for the final installation.

As a rough starting guide, available flow is proportional to Rpm, pressure increases with Rpm squared, and drive horsepower (and amps) go up with Rpm cubed.

The other interesting thing is that as you block off flow, and pressure rises, motor amps fall, which may be the opposite of what is expected. Running the blower with full open flow and no back pressure will almost guarantee belt slip or motor overload. It is better to run it with the intake blanked off when starting up for the very first time. The noise will still be quite impressive !

Posted: **Sun Jan 29, 2006 6:37 pm**

I do boosting for a living for the Ford Motor Company.

The VR-Paxton blowers used to have a "ball drive" inside of them that would "step-up" the speed of the impeller shaft much like a planetary gear set does in a automatic transmission.

Say the actual step up ratio: Pulley ratio times ball drive ratio times engine speed is:

3 to 1 ball drive ratio and 2 to 1 pulley ratio.
Your actual step up ration is 6 to 1. If you had
a electric motor spinning 1750 rpm, the impeller would be turning 10,500 rpm and moving 600 cfm easily at most flow bench test pressures.

Forget about trying to run boosted engine pressures on a flow bench.

I have personally owned two paxton supercharge rs designed for flow bench work and both were driven by a 10 hp 3 phase electric motor. They worked just fine.

Tom V.

Posted: **Mon Jan 30, 2006 12:23 am**

Thanks for the replies, I will have to do a lot of testing , my goal is 500cfm at 60". But how will i know im getting that much flow? you say to block off the blower and measure the pressure but should i also build a orifice to figure out the flow then change gear ratios until i get maximum?

Blake

Posted: **Mon Jan 30, 2006 6:41 pm**

Yes indeed, test with a series of different orifice sizes to determine the pressure/flow relationship at a number of different operating points.

Basically you need enough wheel tip speed to reach the desired pressure ratio. Whatever flow the blower has at that Rpm, is what you get. Tom and I have both reached the sort of flows and pressures you are seeking with a 10 Hp motor, so there will certainly be enough drive power.

Monitor the motor current with an amp meter, and just experiment with a few different drive ratios. I had forgotten that the Paxton had the variable ratio ball drive system, but that shouldn't really make much difference.

The only other thing that I recall with a Paxton, is that they are designed for blow through carby applications. The blower intake normally operates at atmospheric pressure, with positive boost pressure at the discharge. In a flow bench, the blower intake will be operating at a significant negative pressure, and the oil seals may not be happy. I read somewhere that the trick is to vent the oil reservoir to the blower intake, so the shaft seals see a zero pressure difference.

I have never owned a Paxton myself, only know what I have read and been told by others. Tom may be able to comment on the oil seal design.

Posted: **Mon Jan 30, 2006 7:17 pm**

Tony and Blake,

If you set up your paxton like the flow benches a famous head porter (C.J. Batten) had in his shop you will be fine. He had three test stands.

The flow schematic is as follows:

From the head to the measurement device:

From the head, the flow went through the cylinder head bore fixture and then into the top of a 55 gallon drum. They took the air flow out the side of the drum and then went to the measurement tubes. All plumbing was 4" pvc pipe.

Test Tubes:

In his shop they used commercially available sharp edged orifices in a pvc tube. One tube had a orifice with a range from 50 to 100 cfm with the right Beta ratio for that orifice. The second test pipe was a 100 to 200 cfm range. Same deal on the Beta ratio. Third pipe had a 200 to 400 cfm range, Beta ratio, etc. So with three changable test pipes they could read from 50 cfm to 400 cfm. The smaller diameter pipes used an adaptor on each end to get back to the 4" pvc pipe diameter.

From the test pipes to the supercharger:

The supercharger was mounted on the mezzanine which was above the flow bench room. A 4" piece of pipe went from the test pipe upstairs to the supercharger.

From the supercharger to outside the shop:

From the supercharger, the piping went to a throttle valve. Paxton's like to be throttled downstream of the blower. The throttle valve set the test pressure in the 55 gallon drum by regulating the suction on the drum. From the throttle valve the pipes went out of the building. This eliminated most of the air noise in the system. There was still air noise on the suction side of the head.

The benches used Merrium manometers.

The benches could pull 60 inches of water.

Very basic but very effective.

They built cabinets around the parts on the main floor to keep the "smoke and mirrors" deal of a fancy cabinet for "the tourists". The most expensive parts of the deal were the professional orifices and the supercharger.

Hope this helps.

Tom V.

Posted: **Mon Jan 30, 2006 8:05 pm**

Excellent Tom,

Throttling the blower outlet is one way, a variable frequency drive would be more efficient and reduce noise significantly. Outside venting of exhaust air is also an excellent way to reduce noise, I blow all my air down a storm water drain, and the flow bench is no noisier than a small window airconditioner, even when run flat out.

If you do vent the air to outside, just make damned sure you leave a door or window open, otherwise the developed air pressure could easily burst the building.

Posted: **Mon Jan 30, 2006 8:28 pm**

Thank you Tony and Tom, I will start testing as soon as i get some pulleys and belts to try. Hopfully i'll stop losing so much sleep over this, i just cant stop thinking about it.

Posted: **Mon Jan 30, 2006 9:40 pm**

The other deal I found out some years ago, Tony, is to not send the air outside with too small of an inlet opening or you suck all of the water out of the plumbing traps and the wife gets really irritated with the smell!!! LOL!

Go for it Blake.

Tom V.

ps Tony, a friend of mine built a home built 2000 hp dyno (water brake) and the exhaust from the dyno he sent through a couple of passages he dug into his hillside (like a muffler) and you could hardly hear the engines run, but the ground vibrated pretty good sometimes!

Posted: **Tue Jan 31, 2006 6:32 pm**

Blake, yes it is certainly fun stuff to play with.

I still believe it is well worth spending some time testing and optimizing suitable blower, and getting that part exactly right before starting construction of the main bench.

I just bought myself another blower from e-bay last week , but will keep it for a future upgrade. This one has an 18" diameter rotor, four inch inlet, and 3.5" outlet. As it is, it develops 19" and 450 CFM at 3,250 Rpm with a 1.5 Hp motor.

If I double the Rpm I should be able to get around 900 CFM, at 75" pressure with about 12-15 Hp needed to drive it. I cannot see myself ever needing any more air than that.

Posted: **Wed Feb 01, 2006 1:41 am**

What about powering that bad boy with a lawn mower engine? Put the blower and motor out side with an electric start and vroommm. Go baby go. If I was going that route I would try it.

Nick

Posted: **Wed Feb 01, 2006 9:23 am**

Good Idea Nick.

When I set up my dyno I used a little twin cylinder Lister diesel engine for the water brake water pump. I paid 125 for that Lister, it was from a light plant, the generator was shot. It is different to have a variable speed drive on that water pump. A IC engine is an easy cheap way to do it when using an aircraft vernier throttle cable.

Posted: **Thu Feb 02, 2006 3:27 am**

Tony, I completely agree it's best to get the blower setup optimized before designing and building the rest of the bench. That blower you bought sounds like a pretty good find, 900 cfm at 75" is a massive amount of flow. Are you using a vfd or a valve to adjust depression ?

Posted: **Thu Feb 02, 2006 4:24 pm**

Blake, this is my third flow bench, and it uses a VFD.

Fortunately, I have three phase power available at home. The VFD is a wonderful sensitive way to control airflow, and at lower running speeds noise is minimal.

Posted: **Fri Feb 17, 2006 9:47 pm**

I'm pretty familiar now with the variac's, but what is a variable frequency drive? Is it similar? I found some on e-bay that seem pretty resonable in price by comparison. If i wanted one for my 10hp 3 phase motor to run the centrifugal blower for my bench, would this be the prefered method?
thanks,
shawn

Tractorsport Flowbench Forum Archive

Paxton blower, speed, hp, tq required - Will i have enough with 10 hp