Compressed Air

[jaybee]

How large do they need to be to run a car alternator?

How long would they run for if powered with as much compressed air as I can get in an old Calor gas butane bottle. (A foot in diameter and about two feet high for those elsewhere)

jaybee.

[wdyasq]

Using industrial tools as a guidline, I'll hazzard to guess not very long.  I shall guess we are speaking of 5 gallon 'propane' bottle to those on the US - it is ~5 gallons or 2/3 of of a cubic foot and will handle about 130 - 150 psi - 10 atmosphers.

A small hand held sander that has similar performance to 200W electric units willl consume that amount of air in a little over a minute.  My fairly efficient 3PH 5HP compressor will not power three of them continually.

Possibly a multiple expansion air motor would be more efficient.  I think I would look at old air-powered mine cars and see what they used early in my search if I were looking to try airpower as storage.  With some research, I think one wouod be looking somewhere else.

Ron

Ron

[BeenzMeenzWind]

I'd imagine you'd need a bigger turbine than would be practical for use with a calor bottle, even a big one.

I've a fair bit of familiarity with small turbine designs, as we intermittently mess about with a gas turbine powered model aircraft. While it's possible to build a pretty powerful one in a reasonably compact frame, doing so involves more complexity than I'd like to take on without a LOT of spare time to machine the 'fiddly bits'

Compressor and turbine efficiency goes up radically with size, to the point where modern 'lean burn' airliner engines are hugely more efficient than a car engine in terms of fuel used for the work done. Only HUGE diesels are better, in fact. Most modern diesel container ships have a fuel efficiency of around 1000 miles per gallon per ton of payload.

With a small turbine, efficiency is chronically bad. I read that in car terms it's about 100 yards to the gallon. I know ours eats through 2 litres of kerosene in 2 minutes at cruising power. It manages to make a whole 12 pounds of thrust. A 0.75 cubic inch 2 stroke engine with a properly sized propeller can easily beat that with 5-10 times the fuel economy. (Jets are faster than most propellor designs and sound way cool, but suck in every other way. No pun intended.)

If you run one of these little buggers in reverse, as an air driven device instead of pressure generating, the same problems with poor isentropic efficiency still appear. Also, getting a free power turbine to rotate at a sensible speed for power takeoff without elaborate gearing is pretty much a factor of blade diameter, just as a big mill will generally run more slowly than a small one for a given windspeed.

Opinions may vary however. There might be specialised designs that may be adaptable for what you want, but it's unlikely to be very efficient even with a turbine designed specifically for the job. Small turbines are good for making speed, really, like in rotary airtools. Power output is fairly miniscule.

It should be possible to use a piston based unit to do the job though. I'm sure plenty of the guys and gals on here will know far more about that than I.

[jaybee]

Thanks for clarifying my thoughts. I can cross compressed air off.

[nothing to lose]

Well my question is how were you planning to fill the tank? Run it down  to the service station for free air? Hmm, now that would be an idea for free power

Course lots of places now use those crappy pay machines, some are still large compressors with free air.

Anyway I had a thought on this for aircompressors in a workshop moving volumes of air.

Use about 4 large storage tanks, a motor between each one and vavle to controll the flow so that there is a large difference in presure between full and when it alough more air to enter.

First the main compresser tank builds to 120psi shuts off, turn back on at 80 PSi like normal. Using about a 1/2" pipe connect to air motor then out to next tank. do this about 4 times. Now when you start up the system the first tank pumps to about 90-100psi, valve pops on line and dumps that pressure and volume through the first motor to fill the second tank, when the second tank hits about 100psi it pops the valve to dump through the next motor and into the next tank. By the time the system is full you have dumped high volume at high pressure through about 4 motors filling 4 tanks. You should lose nothing if you have no leaks. You end up with all the compresses air filling 4 tanks at 120psi each. Now run tools!

When the first tank drops to 80psi a valve in the inlet line pops open, this aloughs the pressure to flow from the previous tank to the last tank. Being the supply tank is at 120psi it's oulet valve is already opened, you now have 40psi rushing though the motor to get to the lower pressure tank till it begins increasing in pressure. This will work backwards all the way to the main compressors tank. You should eventually have all motors running at one time. When the main tank drops to 80psi the compressor starts as normal and all motors should be running till the tanks are all full again at 120psi or so.

It's still going to take more power to pump all that air than you will get back out of it, but as it travels tank to tank like that through the motors you are getting someting back for basically nothing, if it works! Your pumping that air to run sanders and saws and drills and grinders in the first place and that is where the air pressure is doing in the end, the work it was actually compressed for to begin with. Your just storing 400gal in a bunch of tanks instead of 60gal in one tank

As for the torques of the motor, some of those air drills and die grinders seem to have pretty good power to them, how well or long they would work like this I don't know. Problem is most such tool type motors just blow out the exhaust air everywhere and not actaully a place to attache a line for that output.

A person could build an airpowered engine from a steam engine design, like for the old models, the ones that did work with lower pressure steam, maybe 45-60psi. As lsong as the first tank has more pressure than the second tanks the motor between them should run as air will flow though it while the tanks try to even out the difference or the tool is keeping air moving out the last tank as it's filling.

I think that little tank mentioned is far to small to do much, but there are some small steam engines it could run, double acting piston type with flywheel comes to mind. Air enters one end and pushes piston to other end air in that end flows out the  port, flywheel slides valve plate, that end now gets air and pushes piston back to first side letting the air out the port there. This type piston engine uses the same port for both intake and exhaust, controlled by a slide plate and box covering and un covering ports to direct the flow in or out each side at the same time. Pretty simple really, I am building one (very slowly).

 Should work at very low pressure but not sure the work it could do but the air would last longer. It would do more work at higher pressure but you lose your air faster. So it would be a trade off finding the sweet spot to do the most work and the longest run time on limited air supply. If nothing else it would look cool