Another Metafilter-inspired post, but at least this time I've got some comments in the thread.
Here in Australia, we've got a TV show called The New Inventors, which does not always do as much due diligence on the inventions they feature as they ought.
So, every now and then, something turns up on the show that sounds absolutely fantastic, and is therefore picked as the best invention of the three featured in that episode, and gets significant publicity as a result - but which is actually a scam.
I mention a couple of previous examples in this comment; any readers who watch the show more often than I do (those two examples turned me off it...) may be able to suggest others.
The Exhausted Air Recycling System has done very well for itself. It was named Invention of the Year for 2006.
The trouble is, it just doesn't make sense.
It's meant to be able to make air tools (which are notoriously inefficient) consume up to 80% less power, by routing exhaust air back to the compressor. But I, and others, can't see how this is possible without reducing tool power by exactly the amount you're apparently increasing efficiency.
My bullshit detector didn't trip the first time I saw the EARS - or the second, third or fourth, for that matter - because on the face of it, you'd think that it would be possible to take the above-atmospheric-pressure air coming out of an air tool and do something useful with it.
But now that I've thought about it some more, it seems quite clear that whatever you put on the outlet of the tool will restrict outgoing air flow, which will inescapably reduce the tool's power.
Other commenters on the Metafilter thread and elsewhere have gone on to express severe reservations about other aspects of the system, like for instance the fact that the return hose is the same narrow diameter tubing as the feed hose, despite the fact that the return has to handle a much larger volume of air, now that the pressure is lower.
[blinks innocently] Comments, anyone?
I doubt this'll end up being as much fun as the Firepower saga, but there still ought to be some entertainment to be had.
(The latest update on the Firepower story, by the way, is a good summary of the whole sordid story.)
24 April 2007 at 8:41 pm
This feels more like journalists doing their usual shtick of using the wrong terminology and giving misleading explanations whenever they report something technical.
Dan's quite right in thinking back pressure on the return line will reduce the power output of the tool, all other things being equal. The article doesn't state whether the pressures are relative or absolute, but assuming a fairly high feed pressure of ten bar the power loss could easily be 1/3 or more.
An intake pressure of "up to" four bar absolutely will not double the power of a given compressor as the article states. Useful energy output will, of course, still be exactly equal to the output of the motor minus the usual losses.
However, increased feed pressure will increase the mass flow possible with a given displacement and shaft speed. That ties up with the claim of needing a smaller compressor.
The pressure ratio is the same, so assuming nothing ruptures you can also deliver a higher feed pressure. This could compensate for the loss of power from back pressure.
Reduced noise from air tools is no bad thing as they are quite capable of damaging your hearing. Having to drain condensation out of the tank less often is another (minor) bonus. On the other hand, you need twice as much plumbing to make it work.
24 April 2007 at 9:55 pm
About Firepower... I am confused after reading this article.... does this pill work or not. Of course I always assumed it doesnt. But then why does this Belfast professor make comments to the effect that it does work?
24 April 2007 at 10:07 pm
Actually, I just realised what doesn't make sense with EARS system. Besides the tiny plenum on the compressor intake, there doesn't appear to be a storage tank for the return air. They imply the compressor will run less often for a given demand, whereas I don't see how the return air can be used for anything if it isn't been stored or continuously pumped back into the receiver.
24 April 2007 at 10:32 pm
Well said there. The New Inventors is right up there with A Current Affair, Today Tonight, Big Brother, et al., on the leaderboard for Most Loathsome Show on Australian Television (a hotly contested prize).
This may be partly coloured by my experience with brutal and demoralising process of inventing, developing and attempting to commercialise a new product in Australia, but I've never come so close to actually throwing heavy objects at my TV as when watching on TNI some insipid 'expert' in a totally irrelevant field pass over a well conceived, well executed, elegantly engineered - if unglamourous - solution to a real problem in favour of a gee-whiz sub-Chindogu gimmick.
They'd might as well just award the prize each week to the most attractive inventor, or the product with the cleverest name. Of course they're going to wind up with scam artists - it's all they deserve.
24 April 2007 at 11:46 pm
With a little extra thought, I can actually see how this could actually work. Take a nail gun, for example. It's got a pneumatic piston that extends under high pressure to shoot the nail. The pressure it works against is the friction of the nail against the wood, rather than its own exhaust pressure. Once the piston is fully extended, it is still fairly highly pressurized.
In other words, the nail gun hasn't used all the available energy of the compressed air. If it had, the air pressure at the end of the stroke would be equal to the atmospheric, which means that the force of the piston would decrease along the length of the stroke, ending with zero force at the end. That wouldn't work in a practical sense for two reasons. First, in order to get the necessary force to push a nail into wood at something near atmospheric pressure, you'd have to have a big cylinder. Second, because the wire nails are so small (18 gauge), they must go into the wood very quickly in order to not bend. So the piston must accelerate very quickly.
So nail guns, at least, only use the energy of the air when the air is most highly compressed, and waste the rest. Yes, it's very inefficient, but that's how they work. Recycling this now-useless-to-the-nailgun-but-still-pressurized air *would* save on the compressor's duty cycle.
I can't vouch for other air-powered tools, but for this specific application, the idea works. My father-in-law is a wood shop teacher, and they also use the compressed-air system to blow all the sawdust off their projects before finishing, and for clean up at the end of the class. The high-velocity air makes the sweeping job much easier.
25 April 2007 at 1:23 am
Surely for every psi you "retain" to feed back into the system, is one less psi of pressure drop you get across the tool... and thus less power. I mean, the logical limit to the idea is having the same pressure on either side of the tool... no work needed to be done by the compressor at all! Just no work done by the tool either.
As jrowland said, I can't see how this invention leaves you with anything other than a quieter airgun that needs less bleeding of water, but that has twice as much plumbing. Any efficiency you gain on the compression side you lose on the power side. And (whilst I'm just guessing here), I wouldn't be surprised if that's not a linear relationship either... I wonder if an airgun running at only 50% operating pressure produces 50% power, or even less...
As for the nail-gun option... I can see that making a bit more sense, though I wouldn't be surprised if at least some of that left-over high pressure air is then used to re-cock the mechanism and load the next nail.
25 April 2007 at 4:59 am
I'm guessing from that near-insignificant plenum that EARS operates much like a normal system unless both tools and compressor are running. Why? I'll explain:
When tools are running but the compressor isn't that little plenum would reach 4 bar in no time. The system must be dumping the surplus air to the atmosphere, otherwise the tools would stop. Note that you're getting the back pressure disadvantage and no advantage in this situation.
When the compressor starts but the tools are not running, there's no return air but that which is stored in the return line and plenum. Once that's gone, the device must be letting air back in. Once again, it's working no better than a normal system.
If I'm right, you'd have to install this somewhere with more-or-less constant air demand close to the performance limits of your compressor for it to work. Most air systems aren't like this; there're bursts of demand, and the compressor also runs in bursts as the pressure fluctuates between the limits of the pressure switch.
25 April 2007 at 12:19 pm
Ah, but Does It Come In Pink?
25 April 2007 at 12:34 pm
Seriously though, look at the extra engineering required to make a triple-expansion steam engine work. I don't see the low-pressure storage tank that a rule-of-thumb says should be sized to the cube of the volume of the small tank, probably larger.
Yes, you could "recycle" the air, but only in a tool that requires a lot less pressure. Again, look up triple expansion engines to see how this is done. The main reason they were used on ships was that ships had to recycle their boiler water, and massive condensers cooled by seawater would slow the ship down. When you actually have an air compressor, and a nearly (it pays to be precise!) inexhaustable supply of air, there would be little reason to recycle the air. They only way you would be truly recycling the air would be to drive a small dynamo and feed power back to the grid. The problem with that would be the resources and energy required to create the dynamo and step-up transformer required to feed back to the grid would far exceed the energy you could ever save.
Thats the principle that bothers me about things like the Totyota Prius - is the energy (or fossil fuels) saved buy the use of the car outweighed by the extra manufacturing required? Those things are pretty fancy inside, with not a lot of timber products used. Vegetable and animal products are the only truly renewable resources; every seems to forget that steel and aluminium are 'fossil' products - granted, there is a lot of it in the ground, but it takes metric sh1tloads of energy to refine both of them, orders of magnitude more than felling and shaping timber.
Whoa, time to turn down the gain on my rant output!
28 April 2007 at 9:25 am
@jaws_au - In this case, "retaining" the excess pressure doesn't deprive the air gun of any power. It's a bit like using a capacitor to light an (ideal) LED. Once the voltage of the cap gets below the forward voltage of the LED, the LED stops using that electricity, even though there's still energy left in the capacity. The LED only uses the portion of the capacitor's charge that has the greatest energy. Similarly, the nail gun uses the highest portion of the energy contained by the compressed air, then throws the rest away by exhausting the still-pressurized air after driving the nail in.
It kinda reminds me of the old joke about the billionnaire who ordered a new private jet because the ashtrays were full on the old one.