I saw an infomercial for a palm sized air compressor that ran off of C size batteries. The host claimed extraordinary power of this unit to produce 100 PSI. Being in the fluid power industry I could only laugh at the uselessness of this product. It may be able to achieve those pressures, but the volume of air would be miniscule. I guess if you were blowing up party balloons or topping off air in your basketball it would have some use.
While the limitations of this compressor are obvious to pneumatic professionals, it does exemplify a lack of understanding in the industrial world as to the relationship of compressed air pressure and volume as they apply to the ability to do work. All too many process engineers focus on pressure and overlook volume when considering their compressed air capacity and delivery requirements. I see this verified by undersized air lines, valves, restrictive quick connections, and undersized air compressors.
Let’s start at the source of a compressed air system, the compressor, which in simple terms is a mechanical device that converts energy from a motor (electric or gas) into stored energy in the form of a compressed gas. So it stands to reason that the more volume of compressed air generated, the more energy, and hence the greater ability to do work. Work is measured in horsepower so the more energy you need the more compressor horsepower you will need.
Whenever you install a pneumatic component, check out its air consumption to see if you have the compressed air capacity needed. An industry rule of thumb is that you get 4 standard cubic feet of air per horsepower. So if you are trying to drive an impact wrench that uses 16 SCFM, doing the math, you will need at least a 4 horsepower air compressor. On a 115V circuit, a 4 horsepower compressor would require 30 amps. Obviously then this is not a home/contractor style air compressor. It would need to be run off of a 220V circuit.
From the air compressor you will need to pipe the air to the point of use. Here is where a lot of folks get into trouble with undersized plumbing. Even though the impact wrench may have a 1/4″ pipe port, depending on the distance, you may or may not be able to use 1/4″ pipe. As the air passes through the pipe, there are frictional losses. The longer the pipe, the more resistance there is to flow. To counteract the resistance, the size of the pipe needs to be enlarged.
Here is a table that you can use as a reference guide. Using the impact wrench example you see that you can use 1/4″ pipe for 25’ of length. Over that you must use 3/8″ pipe.
|100 feet||50 feet||25 feet|
|1/8 NPT||4 SCFM||6 SCFM||8 SCFM|
|1/4 NPT||10 SCFM||15 SCFM||20 SCFM|
|3/8 NPT||20 SCFM||30 SCFM||40 SCFM|
|1/2 NPT||40 SCFM||55 SCFM||80 SCFM|
|3/4 NPT||80 SCFM||120 SCFM||160 SCFM|
|1” NPT||150 SCFM||225 SCFM||300 SCFM|
Quick disconnects and push to connect fittings are another source of flow restriction. Pictured here is an example where a client was not able to get the performance from his air product. The unit required a fair amount of compressed air thus had a 1/2 NPT air inlet.
The client installed a reducer from 1/2 NPT to 1/4 NPT. That in itself should have been a clue as to why he was not getting enough air to the unit but he went on to add a quick disconnect. This next picture shows the extreme reduction in trough hole diameters. It is apparent as to why this client could not get the performance from his air product.
Compressed air is an area of study all its own. The few paragraphs here are only intended to get you thinking of some of the associated variables. If you have questions I would invite you to call EXAIR’s application engineering department 1-800-903-9247.