Vacuum units, roots or vane

[viper]

It has been a few days since I was here. I still have my bench at partial build. I bough like 20 little vacuum motors but decided not get rid of them and go with something easier to adjust. I want to use a 3ph motor mounted to a vac pump and vary rpm with a VFD.

I have attached a photo of a vane unit. I am curious if these are known to work well? Last time I was here, I had settled on a roots blower type and bought an 8-71 blower for this application. It will take some work to adapt so I thought this might be easier. Can anyone comment on performance on the two types of pumps? I am obviously looking to get serious with the bench and want to make sure I always have enough flow on hand.

[viper]

Here is my partially build unit. I planned to add an Al top plate. All the build is MDF. I hope the thing does not implode. I planned to gusset everything and seal the #### out of it. Obviously, I never designed it to hold the weight of a 3ph motor and vac unit so I will likely need to revise this thing a bit.

[Tony]

Don't know Viper.
You will need to do some flow tests with that vane pump.
See how much air it can draw through a few different sized orifices, they don't need to be extremely accurate, just something to get a bit of an idea.

Cut out some thin sheet metal plates, and make various sized holes, maybe 1.0", 1.5", 2", 2.5", 3.0", measure the pressure drops your blower produces, and then calculate the flows at each pressure drop.

That will give you five points on a flow versus pressure curve.
Then you will know exactly what you have there, and you can draw your own conclusions about it's suitability.

[viper]

I think I would track down the OEM and get a performance sweep before buying one but for some reason, I thought there was a good reason I bought this 8-71 blower when I did. Maybe not. The vane type sure would be easier. I do not remember the standards for test pressures but I want to say 18" and 36" of H2O. I also will have to get my book out to determine how much power it takes to say move 1200cfm at 36" and if that is even a realistic approach. That might be some massive carb or something but most of my testing would be 100-400cfm max. I was thinking around 5HP is what I needed at the time.

CIIR but a performance sweep of a pump that compares pressure to flow should be pretty accurate to what we would need on the bench?



EDIT: upon studying a few of these, it looks like most of them have too much vacuum and not enough flow. Some will pull to 126" of water or so but flow is not there, even no load.

[Tony]

1200 CFM at 36 inches.

That is 1,200 x 1.29 (psi) divided by 229 = 6.76 air horsepower.

A 50% efficient blower will require around a 13.5 Hp electric motor.
No problem, if you have sufficient electric power available to do it.

[viper]

So most blowers are about 50% efficient or our you referring more to the motor? Is this target number to high? Could ample testing be done at lower pressures to reduce the power side? We really want to stay in the 5-8HP area because of cost, weight, and space.

[Tony]

Fifty percent is a pretty good ball park starting figure for many different types and sizes of fairly well designed air blowers.

Something like a turbocharger may peak out at 75%, a leaf blower is more likely to be closer to 10%.

If you wish to stay in the 5-8Hp range, then you need to reduce either flow or pressure to get there. f you cut down your flow from 1,200 CFM to 600 CFM, but stay with 36 inches, that will halve your power requirement.

[viper]

Would you possibly have any idea what my detroit 8071 blower will do pressure vs flow wise? I am having a #### of a time finding any specs. My plan (if I were to use it) was to add sealed bearings and extra seals and run the ends open and build a pulley device to couple power from a motor. I would have no idea how many rpms I would need or if that blower would be a good choice or if the regen vane pump would be best. I would sure think the 8-71 could do the flow just because at 14.7psi, it should need to supply a 350Ci motor at around 800cfm. I am hoping my math is right there. I guess I could push one harder to achieve my target 1200 cfm.

I would prefer to back off the pressure and increase the flow here. This way I can at least do some testing at the higher volumes.

[Tony]

I would not bother going frantic over rotor sealing. The rotor leakage is proportional to "boost" pressure. As we are only going to be working in the 1 psi range anyway, even a very leaky blower will lose very little air.

My figures for the 8/71 are, rotor diameter, 5.778", rotor length 27.45 inches, blower displacement 436 inches per revolution.

That comes pretty close to 0.25 cubic feet displacement per revolution.

So just divide blower rpm by four to get a very rough estimate of CFM.

[viper]

Tony, I assume this is an open air situation? Would you know what relationship we might apply to flow and pressure in this? Also, would you know how fast an 8-71 can spin safely? I am looking at around 6K rpm or more. They have gears on the back of them to drive the two rotors so I am a little concerned about heat on the gears at those speeds.


Do you think you would lean towards the roots blower here or something else? The rotors in this one are the helical type which I understand might work better for a bench.

[Tony]

A roots blower basically pumps a fixed volume of air every revolution. So the pressure created only depends on whatever is restricting the flow through the blower.

If you are drive power limited to 5-8 Hp as you suggest, then something around 600 CFM at 2,400 rpm and 1.3 psi are all you are going to be able to get before you overload your electric motor. Actually you may get slightly more than that, I am being a bit conservative, but you will not get much more.

They work a lot harder than that even in the original diesel application. Something more like 4 psi and 4,500 blower rpm in the truck engine it originally came off.

So there is no need to think about modifying the blower in any way. On your flow bench, that blower will be doing the easiest job it has ever had in it's whole life.

[viper]

I was really thinking I could get by with using a lower test pressure for large flow numbers. I want to say the big super flow unit only flows 1200 cfm at 20in. Would testing be fairly accurate at testing say 18in H20 which should nearly double my cfm if I double the speed and just keep a close eye on the current?

7.5HP is the closest HP to what I am looking at. I guess I could try to find a 10HP so if I needed to go bigger, I would just need to find a bigger VFD I guess. The power requirements seem rather low though when compared to the SF unit at a rated 80 amps. That is 25HP!! Am i missing the efficiency side of this?

[Tony]

As you say, a VFD allows you to turn that blower over a very wide speed range and trade off pressure for flow any way you want. I would expect the efficiency to stay quite high too, over a wide speed and pressure range.
Your VFD should be able to be programmed to limit motor current to a safe maximum.

And a whole bunch of vacuum cleaner motors can sure move a lot of air, but not necessarily always do it efficiently.

[viper]

Is that what the super flow uses? Vacuum cleaner motors??? How do they throttle the flow and pressure?


Sounds like I might want to try and use the roots blower I have and try it. At least there will be some adjustable means to it...

[Tony]

Yup, Super Flow use vacuum cleaner motors, and the flow/pressure is controlled by an electronic speed control module.

The large roots blower with a VFD sounds like your best bet.