Positive displacement pump designs, efficiencies, small

[fastline12]

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?

 

[fastline12]

I should further add that this design will need to operate at a wide variety of of speeds. Typical might be 1k-11k rpm. I know typical roots and screw styles do not like to operate at these speeds but possibly with the down size of components, we can find acceptable limits in this operating range.

 

[MikeHalloran]

Consider renting an actual engineer to help you.


Mike Halloran
Pembroke Pines, FL, USA

 

[1gibson]

Look up some info on Eaton M24, it is fairly small. 11K is well within acceptable range for that blower, up to 16K is normal, many larger eaton blowers are regularly overdriven to 20k+ range.

 

[fastline12]

thanks, I am in aerospace engineering and this is somewhat out of my norm but I can certainly get my head around it.

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.

 

[gruntguru]

An essential piece of information is pressure. Inlet, outlet pressure ratio? Horses for courses, roots is fine (perhaps best choice) at low PR - up to about 1.6 - 2.0, screw and recip take over above that. There are a few other more exotic options - scroll etc.

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.

 

[fastline12]

Thanks for the reply. Outlet pressure is estimated at 5-10psi at approx 100cuft/min.

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.

 

[tbuelna]

fastline12,

For good efficiency at low PR, try a rotary vane compressor.

http://www.blackmer.com/tech-compress.jsp

Hope that helps.
Terry

 

[fastline12]

Thanks! I have barely looked at that design but thought they were fairly limited in speed due to the friction type vanes. Do the vanes create a significant amount of friction to reduce eff much? I would be targeting something that was around 95% eff if that will be possible. Shooting from the cuff there.

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.

 

[HRHtex]

What about a piston pump - as in a normal air compressor?

 

[MikeHalloran]

Rotary vane pumps' primary advantage is that they are relatively simple to build. ... or at least they appear to be simple to build.

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

 

[fastline12]

I am pretty familiar with a few roots designs and have several in the shop but my concerns with roots again is the shear weight and size of them. I will admit that I am rather interested in the vane style pumps. What would the life cycle look like for one? I seem to recall a company making industrial air compressors with them which I thought odd if they are a high wear item. that being said, they are not exactly flooding the market like roots styles are.

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.

 

[patprimmer]

http://www.eaton.com/EatonCom/Produ...Products/Products/Superchargers/TVS/index.htm

These are rather compact and rather efficient.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[fastline12]

Rotary vane has really sparked my interests but there is not much data out there about them. I would not see them as a good solution for constant, long term duty, but for our application requirements of compact, light, PD design, and occasional use, they seem promising.

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.

 

[MikeHalloran]

The traditional materials for rotary vane air pumps include cast iron for the rotor and housing, and some kind of carbon or graphite for the vanes.

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

 

[thruthefence]

Modern Aircraft instrument air pumps are the sliding carbon vane type. If you get 600 hrs out of them, you are lucky. The older "wet" pumps seen to last forever, (I've never seen a failure, in 35 years) but they require air/oil separators to clean up the exhausted, or pressure side air. Someone marketed a "life time guaranteed" aluminum pump a few years ago; quickly pulled of the market. Now being sold again, but with (guess what) no life time guarantee.

 

[fastline12]

In something like this, 600 hrs to swap a set of carbons would be a non issue here if that is the only real failure mode.

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...

 

[thruthefence]

Yeah, the fact is many applications use the pneumatic pumps to draw a negative pressure, generally 4-5 " hg, pulling through the turbine of a gyro instrument. The vacuum drives the gyro. The dust is simply exhausted overboard. (I wonder if this counts against the 'carbon debt'?)

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.

 

[gruntguru]

Thanks for the reply. Outlet pressure is estimated at 5-10psi at approx 100cuft/min. 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.
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.

 

[fastline12]

Gruntguru, I was unable to find any other mfgr online that made a scroll style SC. Are there others? I will admit they interest me enough to want to build one just for the heck of it. They seem to be as reliable as dirt in HVAC. I just wonder if the design could be simplified to reduce moving components. Just thinking out loud ther.

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?