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Calculating CFM due to pressure difference

by Jeb Kerman
Tags: difference, pressure
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Jeb Kerman
#1
Apr5-14, 03:55 PM
P: 4
I am working on plans for a portable air motor system, to power a scooter discretely. The motor I have picked out is a Gast 1UP-NRV-15, which is a vane-style oil-less motor, with the following specifications:
  • 6000RPM (628 radians/second)
  • Max torque 0.68 Newton-meters @ 1000RPM (6 in.-lbs)
  • 373 watts (0.5 Hp)
  • 5.5*10^5 Pa max pressure (80 PSI)
  • 12.74 liters/second air consumption (I think, taken from 27CFM, so 127 milliliters per revolution)

I was thinking to myself that 27 CFM isn't that much, but it turns out to be a lot. The biggest tank I can really use is a 1.2 liter pony tank that stores air at 2*10^7 Pa (3000PSI), which using the combined gas law at constant temperature, I calculated to be about 204 liters of air at 1 atm. This would mean that I would blow through the whole tank in a little less than 15-16 seconds due to the residual pressure left in the tank and the regulator valve closing and whatnot (The regulator valve feeds air at 80PSI from the source 3000). The motor is only 4.6 inches in diameter, so this seems extreme. I don't see how anyone has made compressed-air vehicles with this horrible efficiency, so I must be doing something wrong. According to this, if I wanted to drive around for an hour on full throttle, I would need a 4.6 cubic meter tank, which is huge. Using a more conventional radial-piston motor of the same size would yield 1/5th the power and also consume 1/6th the gas, which isn't a big increase in efficiency. Plus, I don't think a motor that weak could even accelerate the scooter.

I think I am making some mistakes. For one, I know that gas changes temperature as it expands and contracts, so the tank would cool down as I empty it, but I am not sure of the equation for that, or the effect it would have as the air changes temperature further down the lines. Second, I don't know if the 27CFM rating actually describes atmospheric pressure air moving through the motor. I also don't know how much % of the max power I need to keep the scooter moving, or if I have enough torque for it to be useful at all (It would be geared down on about a 10:1 input-output ratio). The lines I plan on using are 1/8" NPT for the whole setup, if that helps at all.

I know there are prototype air-powered cars which store air at 4500PSI and can run for at least a hundred miles, and I don't see them towing a giant trailer of air behind them. They have a lot more displacement, too. I hope that you can prove my theoretical calculations wrong!
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Travis_King
#2
Apr5-14, 05:37 PM
P: 841
You want to be able to drive around for an hour on a 1.2 liter tank?
Jeb Kerman
#3
Apr5-14, 05:53 PM
P: 4
Quote Quote by Travis_King View Post
You want to be able to drive around for an hour on a 1.2 liter tank?
No, I was hoping for 10 minutes or so. Enough to justify filling the tank for a dollar at the local scuba shop...

etudiant
#4
Apr5-14, 06:56 PM
PF Gold
P: 866
Calculating CFM due to pressure difference

You are getting a better understanding of why compressed air motors have not swept the field.
I've not seen anyone drive around a compressed air vehicle for an hour, or even ten minutes, for that matter.
It is a field rich in dreams and sparse in achievements.
Travis_King
#5
Apr5-14, 07:02 PM
P: 841
More reasonable, but considering the air requirement of the motor...

The 27 CFM likely refers to SCFM, basically, it's saying that it is moving the mass of 27 cubic feet of free air through the pump each minute at the given pressure. ACFM will depend on pressure, temperature, etc. You can find these calculations online pretty easily by just typing in "ACFM to SCFM" or vice versa.

A 1.2 L tank is only roughly .042 CF. At 3000 psi, that's an estimated 8.5 ACF of air assuming temperature has been allowed to cool to room temp and relative humidity of the air is a conservative 80%.

So, using 27 SCFM at 80 psi, you'll be draining your tank at a rate of 5.178 ACFM (that's assuming pressure stays the same, which it wont, but anyway), so you'd be lucky to get 1.5 minutes from that tank.



The .68 N-m torque is in the right range. A lot of scooters are in the .5-1 N-m range, with appropriate gearing.
Q_Goest
#6
Apr5-14, 07:09 PM
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P: 2,907
Hi Jeb. Welcome to the board. The vane type motors are not particularly efficient. I calculate the one you're referring to has an isentropic efficiency of around 21% which is not unusual from what I've seen.

The flow is in SCFM. I think that's what you were using. If you ran it at 27 SCFM from a supply that had a 1.2 L water volume and a pressure of 3000 psi, then assuming you could heat the gas fast enough to maintain it at 70 F, it would only last for about 18.7 seconds. That assumes it's putting out the 0.5 hp for that period of time of course. Unfortunately, you still need to heat the gas since it will be expanding rapidly inside the bottle and getting very cold.

One thing you notice right away is the poor isentropic efficiency. The second thing is that you are regulating the pressure down from 3000 psi to 80 psi before using it which also wastes energy.

In general, air powered motors are a poor way of powering a vehicle.
Jeb Kerman
#7
Apr5-14, 10:10 PM
P: 4
Quote Quote by Q_Goest View Post
One thing you notice right away is the poor isentropic efficiency. The second thing is that you are regulating the pressure down from 3000 psi to 80 psi before using it which also wastes energy.

In general, air powered motors are a poor way of powering a vehicle.
Hmm, I figured a 6000 RPM motor would consume a ton of air. I wish that I could get a high-torque, long stroke piston motor. I was thinking about converting a 2-stroke engine to air, but they usually have a ton of unnecessary heat sinks on them that I would be tempted to cut off to make it more discrete. Too bad THEY STILL HAVEN'T STARTED SELLING THIS THING: http://www.engineair.com.au/ AAHHH, anyway...

Does the regulator waste energy because it is stopping the air from moving, and getting rid of its kinetic energy? Or is it something else? I can't imagine that accounting for a whole lot, though.

So here's another idea: What if I scrap the scuba tank idea and instead put a big 100 PSI tank or whatever in it, like a compressor pancake. Then have some lever with a modified bike pump. The highest PSI bike pump I've seen is 160PSI. So then, it's not as dangerous, and I can pump it up when I need it. (Don't ask why, I need something to do with my free time!) I would want to get the least powerful motor that I could use to keep the scooter rolling, but not actually accelerate. How weak could I go? Also, what kind of motor would be best? Custom machined giant Airhogs motor?
P K Pillai
#8
Apr5-14, 10:39 PM
P: 14
I would like to power a cycle rickshaw with a pneumatic engine.A large diameter cylinder could be fitted with electric air heaters inside the cylinder, on either side of the piston.The heaters will increase pressure during each stroke of the piston.This will reduce pressure requirements.Speeds are low.
Jeb Kerman
#9
Apr5-14, 11:03 PM
P: 4
Quote Quote by P K Pillai View Post
I would like to power a cycle rickshaw with a pneumatic engine.A large diameter cylinder could be fitted with electric air heaters inside the cylinder, on either side of the piston.The heaters will increase pressure during each stroke of the piston.This will reduce pressure requirements.Speeds are low.
That's a cool idea. But, I feel like the super high wattage used by electric heaters would just turn it into an electric/air hybrid with an emphasis on electric. That isn't necessarily bad, I guess.
Barron
#10
Apr8-14, 06:24 AM
P: 7
engineair.com.au looks like it would get you to the other side of the workshop where you'd have to refill it before the next trip. Might be convenient for frequent trips within the same building or yard, but then why not use an electric vehicle and just recharge it overnight? Perhaps some weight saving compensates for the extra cost of inefficiently compressing air?

Still sounds like a fun project!


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