Skip to main content

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.


Popular posts from this blog

WTF is a Franzinator?

Sometimes the most useful things have the oddest names. Take the Franzinator, for instance. Named after Franz©, its curmudgeonly inventor, the Franzinator is a device used to separate moisture from compressed air. Having moisture in compressed air is not a a good thing, especially in painting where it contaminates the painted surface, or in media blasting where it causes the media to clump and not work as well. As well as causing rust in the air tanks and air tools, moisture is best removed. A number of methods have been developed from expensive refrigerated driers used to pre-condition air before it gets to the compressor, to simple mechanical separators that sit in the air line between the tanks and the air tool. Here is a tank that uses a chemical desiccant to dry the air. There are also ways to install the air lines that are intended to either cause moisture to condense or collect before it is sent to the air tool. A non-mechanical separator causes the moisture in the hotte

Keeping Your Compressor Quiet

Air compressors are noisy machines. The two most commonly used ways to quiet them down are to locate the air intake outside (not good for the neighbors) or to build some type of enclosure with sound deadening material inside; that usually causes overheating problems due to poor circulation. The best solution is to construct a silencer that uses the same principles as a gun silencer. But isn't that illegal? Only if you use it on a gun. We're using it on an air compressor. Still, this topic seems to be controversial, mostly by people who have a bury-your-head-in-the-sand approach to security and safety. Here are the materials: Pipe the same size the air compressor port will use, 16"-18" long. Some exhaust pipe, the same length as the other pipe. Some washers you have to make with a hole saw, enough to put one on each end and one every 1-1/2" that will fit around the air compressor intake port pipe and fit inside the exhaust pipe. Steel wool to fit betwe

Retrofit Hydroboost Brakes

Traditional power brakes have used engine vacuum to reduce the physical effort required to stop a car. Some high performance engines, because of the design of their camshafts, do not produce enough vacuum to used vacuum-powered brake power boosters. One alternative is to add a vacuum pump to the accessories driven by the engine. Another is to use the hydraulic pressure supplied by the power steering pump to power the brake booster. This device is called Hydroboost and it replaces the vacuum-operated brake booster. It provides more than twice the pressure than a vacuum diaphragm booster. There are commercial kits to use a hydraulic booster, like Vanco . They offer kits to match the car you are modifying. The most complete kits provide the Hydroboost unit, a new master brake cylinder, a power steering pump and the appropriate hoses with fittings and they are priced at around $900. You can see the parts of the kit by visiting this link . A nice step-by-step installation of the Vanco h