|A big drawback to using compressed air|
|Air drills. Impact wrenches. Air sanders. Air grinders. Air motors. Air cylinders. Air this. Air that.
If you ever saw the inner workings of an auto body shop, you've probably seen at least 3 of the above air tools at work. One reason is that air tools don't create any electrical sparks which could cause a fire in the presence of the paint and othe flammable materials found in a body shop. Another reason is that air-powered tools tend to be lighter in weight than electrically powered tools.
A few times when I visited body shops, I noticed that if one guy was using a small air sander, the big 5 horse air compressor seemed to be running pretty much non-stop to supply the air for this tool. At the time, I brushed it off as unimportant. Now that the price of energy is an arm and a leg, I did a little investigating to see what the deal is.
And here's the deal. Compressing air is a TERRIBLY INEFFICIENT PROCESS! Due to the laws of physics, a lower-quality air compressor like a homeowner might use is LESS THAN 10% EFFICIENT. More like 5 or 6%. Even the large industrial compressors are LESS THAN 20% EFFICIENT. So for every dollar's worth of electricity used to run the compressor, only 5 to 20 cents worth of useable air energy is produced by the compressor. The rest of the power is wasted as heat. Just what we need more of!
And worse yet, the process of turning the compressed air back into useful power to run a sander or a drill appears to be equally, or even MORE INEFFICIENT! So for the dollar you spent to run that air compressor, in the end you wind up with a few pennies worth of useful work being done.
The moral of the story, you can do drill a hole with an electric drill that uses 200 watts of power, or you can drill the same hole with an air drill, and use 4000 watts of power!
Of course, there are ways to make air compressors more efficient. One way is to slow down the compression process, which results in a big flywheel on the compressor pump, driven by belts from the motor. But this isn't "kewl" or lightweight, and costs too much to ship from China. A two or more "stage" air compressor uses several cylinders, each one increasing the pressure by a fraction of the total, allowing the air to be cooled somewhat between the stages. There again, we wind up with a machine that is "big and slow", but more efficient. Combining both of the above puts us near the 20% efficiency point. Even bigger and slower yet, we could build near-isothermal compressors with much higher efficiency. But in America, we're taught that "small and fast" is the way to go. That's why people buy the cheapo "direct drive" toys that attempt to compress air at 1800 or even 3600 strokes of the piston per minute. This results in about 5% efficiency, about as much noise as a chainsaw, and a very short life of the unit. But we can fit hundreds of them on the ship from China, and sell them for $199.00 apeice! What a deal! Even the U.S. Patent Office endorses such ideas.
Of course there's one more thing you need to know about air compressors. Most of the "consumer" units sold at home and auto stores really lie about their horsepower. Be sure to read this: <aircomp>
|As long as we're discussing compressors, let's not forget about the little toys you buy that run off of your car's lighter socket. So many of these brag that they can produce 175 or even 200 PSI of pressure.
It's important to remember that compressing air involves two different variables. The pressure, or PSI, is the measure of how hard the air is pushing against the inside of whatever it is contained in, trying to escape. You could put 30 PSI in a car tire, and you could put 30 PSI inside a pill bottle. The size of the container is unimportant, it's the strength that matters for PSI. The other variable is the volume of the container. This determines how much air will fit inside of a container. In our example above, we could fit a lot more compressed air inside of the car tire, than the pill bottle, even though we have 30 PSI in both.
On an air compressor, both Volume and Pressure (PSI) require power from the motor driving the compressor. If you have X amount of power available from the motor, the compressor pump can be designed to use most of the motor's power to create pressure (high PSI) but at a low volume, or it could be designed to move a lot of air (high volume) but at a low pressure (PSI).
How often have you used your little compressor to pump something up to 200 PSI ? But you did notice that it takes a long time to pump your car tire up to 30 PSI.
That's because people don't know the difference between volume and PSI. They associate "PSI" with air compressors, and assume that more PSI is better, so they buy the compressor with the bigger PSI rating plastered on it. But the tiny motor on these little toys has limited power, and the manufacturer chooses to devote most of the power to higher PSI, because that's what sells compressors. But if you knew that a compressor that could only produce 50 PSI, but at 4 times the volume, would pump up your tire four times faster, you might buy it! Someone should market one like this. But you'd have to come up with a way to convince the consumer that ".7 CFM" (cubic feet per minute) is more important than "200 PSI". Good luck!
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