Thread: Air Tool Efficiency

I have always run my air tools off bigger shop compressor and never thought much about it. This week I have been using them off a small portable and the poor thing runs constantly and can't keep up. Got me to thinking what is the advantage of converting electricity to air and then air to mechanical, specially now when good cordless tools are available.
Bob

I don't know the figures, but I do know my 800w die grinder outpunches the air-fed equivelent powered by a 2kw compressor....

I saw an air powered skillsaw at a thrift store. It had a one inch inlet! airtools are lighter and run cooler. they can be used where sparks might be an issue and they can be used underwater.

Air tools are prety inefficient. Besides two adiabatic losses you have major friction losses in the vane motors. Compress air to 120 PIS and lose abou 25" of the energy to heat of compression. Expand it through an air tool and you lose another 25% to adiabatic cooling. Then the tool is only about 50% efficient mechanically. Work the math and you recover a little over 1/4 the energy yield of the compressos motor. Add the motors efficiency and you have less but if you are comparing air to electric the electrical efficiency and more or less off-setting.

You can with some justification assert that a rotary air tool yields 25 -30% of the mechanical power use to compress the air that drives it. People will quibble details one way or anther but that figure is a fair characterisation admittable in most any technical discussion as a place to start. A single stage compressor equipped with an honestly rated 2 HP motor wiill run almost continuously when driving a typical 4" disk sander. The same disk when mounted on an electric grinder will demand maybe 500 watts at full load.

I don't use air tools. I think they are expensive, noisy, and inefficient. Their two benefits are they are rugged as hell and no-on has ever been electrocuted with compressed air. The second benefit has disappeard bucause of plastic housings and double insulated design - which leads to pretty rugged construction too.

Anyway, I see a lot of electric powered hand maniulated production tools in factory videos these days.

A big operation like a vehicle assembly plant would have 2000 - 3000 HP in distributed air system and metering its output was a fair indicator of worker activity. Used to be "upgrading comressed air system" was a frequent entry in the line item list of plant improvement projects for factories, assembly plants, etc. I wonder if it still is.

Used to use a lot of air tools. I could never get the efficiency out of them that the manufacturer claimed I should. Turns out "Average use" is 25% - 50% according to the manufacturer. I was running them about 90% of the time.

All of what Forrest said plus the cost of the compressed air itself. We've metered our compressors just to find some costs. With a dryer, it's now close to $500/yr/hp@100 PSIG. That's all at 480 VAC 3 phase. You don't want to know what it costs to run our 400 PSIG system. It pays for us to do an "air audit" every other year. All maintenance does is look for leaks.
JR

Originally Posted by JRIowa

All of what Forrest said plus the cost of the compressed air itself. We've metered our compressors just to find some costs. With a dryer, it's now close to $500/yr/hp@100 PSIG. That's all at 480 VAC 3 phase. You don't want to know what it costs to run our 400 PSIG system. It pays for us to do an "air audit" every other year. All maintenance does is look for leaks.
JR

One summer, I walked around my assigned building's, and just with everyone gone, I could find a bunch
of air, oxygen, shield gas, and other leaks with my ears.

I made up a spreadsheet, and it was a couple of pages long.

As said before, $1.00 per C.F.M. is conservative.

When it comes to air wrenches they are lighter and can fit in tighter spaces. A must have for sandblasting. Electric tool technology has come a long way though. Good point on analyzing efficiency, never considered it.

I ran a compressor plant at a gold mine a few years ago. A total of 5 compressors put out 11,500 cfm @ 100 psi. Had to run 3 compressors all weekend when nobody was underground just to keep the pressure up. Lots of leaks in 100 miles of piping.

I realize no piping system is perfect, but what would be an acceptable, normal figure for PSI (CFM more proper here?) loss per hour? I'm wondering if there is a threshold figure where shops will pull a maintenance session and go over any leaks like JR mentioned. I wouldn't be surprised if my question is too individualized to answer, but I'm curious what others consider unacceptable?

One of the most expensive things we make is compressed air! Our finishing dept. used to use air vibrators because they only cost about $100 ea. and the electric ones cost up to $1,000 ea.However the electric versions use a 1/16 HP motor but the air ones use 2hp or more of air.We have two 250hp GD's with a 5k gal receiver as our main supply and haven't ran out of air yet.At our old plant we had a mix of 40 & 50 hp compressors and many times the printing presses would shut down because of low air pressure and I would have to go to the finishing dept.and start turning off air blast nozzles and air vibrators to build up pressure.We used to come in on a weekend when it was nice and quiet to find all the air leaks.I sure miss that old place......like a case of clap.

Originally Posted by byawor

Got me to thinking what is the advantage of converting electricity to air and then air to mechanical, specially now when good cordless tools are available.
Bob

The proverbial Light Bulb above your head turned on at that very moment.

Originally Posted by Arthur.Marks

I realize no piping system is perfect, but what would be an acceptable, normal figure for PSI (CFM more proper here?) loss per hour? I'm wondering if there is a threshold figure where shops will pull a maintenance session and go over any leaks like JR mentioned. I wouldn't be surprised if my question is too individualized to answer, but I'm curious what others consider unacceptable?

Many times you'll see auction flyer's for closed plants, and inside is a list of the compressors.
Usually a couple small ones and 2 or 3 large ones.

I always figure that one large one supplied the plant air requirements, the second one supplied the air for the leaks, and if there's a third, it was a backup in case one of the other 2 goes down....

The only air tools I use are the sandblaster, impact and die grinder... bare minimum for what I would consider to be the tool electric hasn't replaced yet.

As to leak allowance why? If it was a flammable gas or water there would be no allowable amount of leakage would there? Why lower the expectations just because its air?

I agree with a lot the folks that have extolled the virtues of air op. tools, i.e. cooler, sparkless operation etc. However I do remeber reading an article not that long ago that stated compressed air is one of the most inefficient and expensive means of powering a tool. Now this of course is subject to the cost eletricity in your area and the efficiency I would postulate will increase dependent upon the number of tools being run. But without running some real numbers it is hard to tell and then factor in the type of compressor piston, screw etc.. But energy loss has to be high. they produce alot of heat which is just wasted, more energy goes down the tubes when friction comes into play (motor, belts, pistons).
Are all these down sides to comp. air canceled out when you compare the cost of say a battery operated ratchet where your paying for a motor, charger, battery pack (finite life not to mention re-charging every so often requiring a second battery.). Plus I have never worked inplace that properly maintained it's compressor(s), infact the compressor is one of the first places I look to evealuate a plants preventative maint program and I almost always find clogged intake filters, heat exchangers that look like a layer cake made of dirt, wrong belt tension and blank stares when I ask how often the oil is changed, So the topic is interesting and it would be very cool from an academic point to run the numbers and see. The other detractor is that if your running pretty much any type of pneumatic device you need a dryer ( more power ). Been inplaces where there are a couple of CNCs running straight off the compressor no dryer and piped wrong. When I mention how bad that is someone usally points to the filter, oiler, water trap that have benn maintaned with same due dilligence of the air comp. itself. That water separator removes water already condensed NOT the vapor that is still in the air and will condense in the tool changer and everyother bad place you can imagine. Some folks get away with it for a while but eventually it come up behind you and tear your wallet out.

Originally Posted by dkmc

Many times you'll see auction flyer's for closed plants, and inside is a list of the compressors.
Usually a couple small ones and 2 or 3 large ones.

I always figure that one large one supplied the plant air requirements, the second one supplied the air for the leaks, and if there's a third, it was a backup in case one of the other 2 goes down....

Yup,
a friend of mine is a maintanance man at a local, old foundry.

Says he comes in the morning, punches on the 50 h.p. to take care of the leaks,
punches on the other 50 h.p. to supply the foundry.

Originally Posted by digger doug

Yup, a friend of mine is a maintanance man at a local, old foundry. Says he comes in the morning, punches on the 50 h.p. to take care of the leaks, punches on the other 50 h.p. to supply the foundry.

Well, we 'had' a local aluminum foundry and pattern shop here for 100+ years up until about 4 years ago. They had (2) 25HP screws. The leaks got so bad the clueless owner assumed that since the 2 screws got little to no maintnenece, that they were 'worn out' and he went out about 8 years ago and bought a 40HP IR screw. In the process of the place closing I went there and bought the 2 35HP screw compressors for scrap. A little maintenance and common sense, and they had a lot of life left in them, and I resold them. The owner pissed and moaned about how expensive it got to keep the doors open........ DUH ?


"and blank stares when I ask how often the oil is changed"

You don't CHANGE oil in a compressor!
It's not like it's a CAR engine or anything!

about 30 years ago I computed the cost of drilling a hole with a hand held air drill vs electric, and I think it cost 5 times as much. I did not include depreciation in there. Selling machinery for many years I was in lots of shops and always saw air tools being used where an electric would be just as good or better, and I sold lots of compressors to guys who needed them to basically make up for waste.

An air audit at my previous workplace found a 3/4" piece of pipe had been disconnected up near the ceiling, and was jsut whistling away.....
Joe

Ok so even assuming that electric is more expensive were you are, crunch the losses on a diesel engine running a compressor running a air tool compared to the same engine running a alternator driving a electric motor. It is not the air option that is going to win!