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To avoid the inconvenience of buying compressed "air" cleaners and scavenging quarters for refilling tires, I'd like to get a pump that can do 72 PSI (vapor pressure of difluoroethane minus atmosphere pressure) at ~2 liters/minute for dusting, and 35 PSI @ ~40 L/minute for filling tires (I calculated to pressurize a 33L tire @30 PSI to 35 PSI, 10.4L of uncompressed air is needed, and I'd like to do this in 15s). The dilemma is that I can't seem to find any portable pump that can operate at both these 2 points.

I have one of those 12V portable pumps that can go up to 250 PSI, but the time it takes to fill a single tire is painfully slow (and loud). The problem is that at a low pressure of like 30 PSI, the flow is pretty much the same (I measured about 6 L/minute with a balloon) as at a higher pressure like 80 PSI. In other words, the motor isn't delivering it's full power at low pressures. This is just like the limited power band on diesel and gasoline engines.

Are there pumps that have a wide power band that would meet my requirement?

Or do pumps in general have limited operating points? Here's the impression I got:

(top - high pressure, low flow)

  1. piston
  2. diaphragm
  3. centrifugal pumps
  4. axial pumps / propellers

(bottom - low pressure, high flow)

One idea is to make a piston pump with a large bore volume coupled to an electric motor with a constant power curve (torque = power / RPM). That way, it can operate at both high flow, low pressure (high RPM, low torque on the motor), and low flow, high pressure. But I don't think a constant power motor exist, so you'll end up needing a transmission, which will make it complicated.

Other ways?

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    $\begingroup$ Most pneumatic systems have a pressure tank kept stable through a pressure switch turning on and off a compressor. $\endgroup$
    – ratchet freak
    Jan 11, 2016 at 11:49
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    $\begingroup$ How are you going to set pressure and flow rate independent of the external "resistance" ? That's like making a battery which dumps X amps at Y volts into arbitrary circuits. $\endgroup$
    – Carl Witthoft
    Jan 11, 2016 at 13:40
  • $\begingroup$ Where have you found an electric motor that has a constant power curve? In any case, what you're asking for requires a pump with variable displacement, which is just as complicated as using a transmission. $\endgroup$
    – Dave Tweed
    Jan 11, 2016 at 15:00
  • $\begingroup$ Whoops, I don't need 87 PSI for dusting. After subtracting atmospheric pressure, the actual pressure is only 72 PSI. @ratchet freak. Right I can pair a tank with an under powered pump to achieve variable pressure/flow, but for my purposes, that would be prohibitively big. I tried refilling a compressed "air" can to 120 PSI and it lasted only 1/10 as long as liquefied difluoroethane. $\endgroup$
    – Yale Zhang
    Jan 11, 2016 at 21:27
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    $\begingroup$ Alright, I think I've found the most compact and reliable solution: mini refrigerator compressors! Look at the coke can sized Samsung UX0T011ZNAE5 amstechnologies-webshop.com/media/pdf/UX0T011ZNAE5SBMC1.pdf and a similar one youtube.com/watch?v=aN6IQDzOLfI . The rotary piston pump (similar to the more well known scroll pump) makes it very reliable and quiet due to not having any sliding surfaces. The flow is a bit lower than I like (2.4 cc * 6000 RPM = 14.4 L/min), but I probably can find a model with a bigger displacement. Plus, it can run on 12V. Very promising. $\endgroup$
    – Yale Zhang
    Jan 13, 2016 at 12:34

3 Answers 3

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There are many cases in engineering where one device does not economically cover two situations. A sports car can go 250mph and a pickup can tow a very large trailer, but a vehicle that can do both would not be economical to own or drive. You would be better off buying one of each; even though at first glance the situation seems wasteful.

Both of your applications will fall in the diaphragm or piston pump region. And sure, your could put a continuously variable transmission on and air compressor, but it would be complicated and very expensive. For your situation there are much easier/cheaper ways to increase portability and performance.

If you have access to a 120v outlet or a 1500W inverter, the cost to cover both situations is minimal. This low-cost oil-less DEWALT Model DWFP55126 compressor will easily handle both of your applications. It has a maximum delivery of 2.6 SCFM (74L/minute) at 90psi.

This smaller pancake HDX Model 0210284C compressor would only require a 400W inverter. It max flowrate is only 0.7 SCFM (20L/min), but it will continue filling the storage tank while you are getting ready for the next tire, so the effective delivery for an intermittent task like filling tires will be greater than the listed amount.

The noise level and longevity of the pump will depend on the quality of pump purchased and the rpm it operates at.

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    $\begingroup$ Those looks promising, but I don't consider it that portable since it won't fit in a drawer (sorry to be miserly, but I live in Silicon Valley, where space is a premium), unless I replace the tank with a smaller one. Anyways, I think I've found the ultimate solution - mini refrigerator pumps! (see comment under question) – Yale Zhang 41 mins ago $\endgroup$
    – Yale Zhang
    Jan 13, 2016 at 12:51
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It is possible to use a venturi nozzle to create a low pressure/high volume airflow from a high pressure/low volume one.

In essence these are very similar to the way that a venturi gas burner works from a pressurised gas tank. The way it works is that high pressure gas is expanded through a small diameter nozzle, accelerating it and causing a consequent pressure drop which sucks more air into the stream through holes in the back of a large diameter shroud.

This is one example

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What you are looking for is a Variable displacement pump. These often take the shape of axial piston pumps with a (somewhat) movable swashplate. I've seen us in use in hydraulic systems (low volume flow, high pressure).

Variable displacement compressors also exist:

enter image description here

This could be modified with a control scheme that controls displacement for a constant power at the shaft.

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