Positive displacement pump designs, efficiencies, small
Positive displacement pump designs, efficiencies, small
(OP)
We are helping a friend with an air pump requirement and thought you guys might have some info from the automotive super charger area. Basically making a pump to test some proprietary components. pump MUST be compact, light weight, and positive displacement.
So far, all I am familiar with is twin screw and roots. Centrifugal is out because it is not pos disp. I think most roots type pumps are pretty heavy. We only need to move about 20-30ci/rev of air. As I understand it, a 6-71 blower is designed to provide 71ci of air to 6 cyl per rev??? Is that right?
This will have to be a custom design for fitment reasons so not really shopping for a ready made solution, but rather just the design types so we can hug a direction. I am not quite sure what efficiencies have been achieved but I seem to remember the twin screw has been able to hold decent eff over a decent range in the past. is that accurate?
So far, all I am familiar with is twin screw and roots. Centrifugal is out because it is not pos disp. I think most roots type pumps are pretty heavy. We only need to move about 20-30ci/rev of air. As I understand it, a 6-71 blower is designed to provide 71ci of air to 6 cyl per rev??? Is that right?
This will have to be a custom design for fitment reasons so not really shopping for a ready made solution, but rather just the design types so we can hug a direction. I am not quite sure what efficiencies have been achieved but I seem to remember the twin screw has been able to hold decent eff over a decent range in the past. is that accurate?





RE: Positive displacement pump designs, efficiencies, small
RE: Positive displacement pump designs, efficiencies, small
Mike Halloran
Pembroke Pines, FL, USA
RE: Positive displacement pump designs, efficiencies, small
RE: Positive displacement pump designs, efficiencies, small
Regarding the Eaton pumps, they appear to use a helical lobe design? I am not sure where the line is drawn in the sand to be called a screw style. Not real sure what kind of pumping losses these pumps typically have.
Are there any radial type pos disp pumps being used in the automotive ind? The length of a screw/roots style might be troublesome for fitment here.
RE: Positive displacement pump designs, efficiencies, small
If it has internal compression its a screw type, no internal compression - its a roots.
All of the above require a massive budget to design and build from scratch, although there is such a huge range available I doubt you need to do that. Worst case you might need to make some custom housings and utilise off-the-shelf rotors.
Engineering is the art of creating things you need, from things you can get.
RE: Positive displacement pump designs, efficiencies, small
Being rather green regarding air pumps, my only experience with scroll design has been in HVAC compressors but I understand them to be rather efficient and a true PD pump. Why are scroll pumps not used for for automotive apps? Do they need a super clean environment? Do they have practical limitations or efficiency issues? The whole design seems rather simple but I still do not fully understand how you would keep one balanced.
RE: Positive displacement pump designs, efficiencies, small
For good efficiency at low PR, try a rotary vane compressor.
http://www.blackmer.com/tech-compress.jsp
Hope that helps.
Terry
RE: Positive displacement pump designs, efficiencies, small
What is the typical application for the rotary vane? I am most intrigued and will admit, I am learning some things here. I certainly appreciate those who have thrown some ideas to chew on.
RE: Positive displacement pump designs, efficiencies, small
RE: Positive displacement pump designs, efficiencies, small
If you need to make one that's durable, you will learn a lot more about tribology than you ever dreamed could be learned, and you will produce a mountain of worn-out or fragged scrap. They do have a problem with friction, even with fluids that are self-lubricating like hydraulic oil, and the problem is much, much worse with gases. The state of the art is probably the low-pressure carbon vane pumps used as smog pumps in vehicles when smog controls were new. The newer smog pumps of which I am aware use regenerative non-contacting vanes, and are not positive displacement.
In short, forget vane pumps.
Roots pumps are harder to make, but the rotors don't touch until the phasing gears wear out, and the technology is mature enough that you'll probably find one in the size you need without having to actually make it yourself.
Topologically, I'd include the Eaton screw pumps in the same general class as Roots pumps. I think they have phasing gears so the rotors don't rub or touch each other when things are running right. Do check on that; I haven't disassembled one myself.
Mike Halloran
Pembroke Pines, FL, USA
RE: Positive displacement pump designs, efficiencies, small
I certainly want to stay with a PD design. Pretty interested to learn more about them. To date, I have only seen them in pneumatic air tools and they seem to last forever in that application. Also, it looks like centrifugal force alone provides the sealing force for the vanes. Does this mean it takes a substantial amount of speed to seal them? A certainly level of slip or regeneration at slow speed would not be a bad thing for this application but just curious.
RE: Positive displacement pump designs, efficiencies, small
These are rather compact and rather efficient.
Regards
Pat
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RE: Positive displacement pump designs, efficiencies, small
Problem is the calculations for that critter seem daunting. Hell, I cannot really even look at the mechanical efficiency unless I know the materials used in the stator and vanes. I am also wondering about carbon dust or wear swarf needing filtering on the exit tube. Some companies claim very long life from them but I have yet to find ANY efficiency claims on any of them, thermal or mechanical.
Seems like a really fun R&D project IMO. Since we are just doing this work as a favor for a piddly project, it might be an opportunity to learn a few things here. I know we would target Al for the case and rotor so that really just leaves the vane material selection. Might have to take another look though. The wear surface of the stator might need a hard coating or even plated to provide acceptable wear characteristics.
RE: Positive displacement pump designs, efficiencies, small
Aluminum should be very effective at burning up your R&D budget, not least in searching for a magical coating.
Do include an exit filter to limit collateral damage to whatever is downstream, and to assist in the many post mortems you will conduct.
Mike Halloran
Pembroke Pines, FL, USA
RE: Positive displacement pump designs, efficiencies, small
RE: Positive displacement pump designs, efficiencies, small
Is carbon dust a real issue on the output side? Has no one come up with a better solutions for this? I could imagine that anything of substantial density could create a centrifugal balancing nightmare but bronze type bushings seem to run a long time dry, especially leaded. Well, maybe that could create an air quality hazard...
RE: Positive displacement pump designs, efficiencies, small
Other systems use the outlet, or pressure side, of the identical pump to blow through the gyro instrument's turbine. In these applications, there is an inline canister filter in series with the pump & instrument.
RE: Positive displacement pump designs, efficiencies, small
At 5-10 psi a roots is probably the best choice. A roots to fit your specification would be very small. Something with internal compression will be a little more efficient but will need to have the right compression ratio for your application.
Scroll compressors have been used extensively as automotive superchargers - by VW in particular. Google search "G-Lader".
Engineering is the art of creating things you need, from things you can get.
RE: Positive displacement pump designs, efficiencies, small
Regarding the twin screw or root style, I have yet to find anything that is even close in size to what we require. Most use large, finned, casted designs. Are there any compact units out there?
RE: Positive displacement pump designs, efficiencies, small
Regards
Pat
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RE: Positive displacement pump designs, efficiencies, small
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Engineering is the art of creating things you need, from things you can get.
RE: Positive displacement pump designs, efficiencies, small
Also, on the rotary vane stuff, I wanted to ask about the elliptical housing designs with dual inlet/outlet. It would seem these might be a great design for high volume, low pressure to reduce pump speed requirements and reduce wear? Looking at some ballpark numbers, it looks like PR 2 could be easily obtained as well as nearly double the flow and reduced radial load on the rotor. Plumbing complexity would be substantially higher though.
RE: Positive displacement pump designs, efficiencies, small
Seems the movement of the vanes would LARGELY limit the rev ability of the pump so they could probably be made huge, swallow a large gulp real slow but doing it fast might really wear it out. hmmm.
RE: Positive displacement pump designs, efficiencies, small
Engineering is the art of creating things you need, from things you can get.