Wednesday, April 29, 2009

The Science Behind Piping Your Air Compressor

How many of us have spent hours examining compressor horsepower and CFM ratings, reading shop forum posts and comparing prices to decide what the "perfect" compressor for our shop will be, but then connect a few Harbor Freight air hoses and wonder why the tools don't run well and spit water everywhere?

A well-written article at the website delves into the science behind piping your shop's air compressor. In brief, our problems arise because we do not have a properly-sized pipe system to deliver the air to the tool.

Compressed air will lose pressure because of the friction from the walls of the pipe. This is expressed in pressure drop per 100 feet of pipe at a particular pressure for a specific diameter of pipe. Of course, designing a proper system is not simple because there are many factors that affect the performance other than pipe size.

Another point made in the article is that the velocity of the air through the system is rarely considered. Why does it matter? When the velocity is less than 20 FPS, moisture and debris are not pushed past traps and can be easily drained away. When the velocity is greater than 30 FPS, all the moisture and debris is blown out of the tool you're using. So if you have a long run of small diameter pipe or hose,the velocity is high and stuff shoots out of the tool along with the air. Does that sound familiar?

To further complicate matters, the tables are designed under the assumption that un-compressed air is being pushed through the pipe. Compressing air increases the volume of air that flows through the pipe, so some more calculations are necessary to adjust the figures and you'll need to determine the ration of atmospheric pressure to the pressure of the compressed air. You'll need to know the average air pressure in PSI where you live; at sea level it's 14.7 PSI and that will drop as elevation increases. What's yours? Remember, Google is your friend.

Generally speaking, the larger the diameter of the pipe, the better off you'll be. A table of inner and outer pipe diameters can be found here. A table showing the pressure drop per 100 feet of Schedule 40 pipe is here. You can see the problem we face: using a small diameter pipe (or hose) to reduce pressure loss will increase the velocity of the air which explains why so much water gets past those filters and traps. But if we use a large diameter pipe, we need higher pressure at the receiver (the tank) to compensate for the pressure drop. The benefit is less water and debris blown out the tool, but a more powerful compressor is more costly.

The Quick Soultion
At some point, somebody sat down and calculated a rule-of-thumb guide to piping you shop, based upon quite a few assumptions that probably aren't correct for your shop. That "rule" is 3/4" for the mains and 1/2" for the drops. That usually works OK for a small garage shop, . . .but maybe not.

Read the article, work the math (you can get your kids to help), and you'll be able to see why you have so much trouble with compressed air at your shop. Having understood well enough to have done the math, you'll be able to understand how to go about improving your own system and improve it. Better yet, it can help you design a good system from scratch.

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