Air System Basics
Air tools are a great compliment to any shop. They can perform a variety of tasks easier and quicker than by many other means. If not properly matched to the air system, however, tool performance can suffer and result in lost productivity. To get the most out of the air tool, air system design is critical. A properly designed system will provide optimum tool performance and productivity.
Air Tools are designed to operate at certain air pressures. Air pressure is the force per unit area acting on a surface. Pressure is measured in Pounds per Square Inch (PSI). When an air tool is turned on, energy is released as air expands from the starting pressure (usually 90 PSI(G)) to atmospheric pressure (0 PSI(G)). Most air tools operate at 90 PSI. Some tools, such as spray guns and caulk guns, operate at lower pressures. Be sure to check the air tool manual for recommended air pressure.
Air Flow Ratings
Air tools consume a certain flow or volume of air. Air consumption varies by tool depending on the type and size of the motor, mechanisms, and overall design. Airflow is typically measured in Cubic Feet per Minute (CFM). The air compressor industry rates compressors in Standard Cubic Feet per Minute (SCFM). SCFM means that the airflow has been corrected for air temperature and altitude differences. SCFM also indicates that the volume of air being measured is not pressurized. CFM and SCFM usually vary by a factor of 7.1 when the air pressure is 90 PSI. The tool industry rates tools in CFM meaning the air is pressurized when it is measured.
Two methods are used to rate SCFM, average and continuous. Continuous SCFM is the amount of air the tool consumes under normal, no load operation. Most air tool manufacturers advertise average SCFM. Average SCFM is determined by multiplying the continuous SCFM by a use factor. The use factor is determined by the industry as the amount of time that the tool is run. Tools, such as impact wrenches, have a shorter run time and thus lower use factors and lower average SCFM requirements. Tools that run for long periods of time, such as sanders, have a higher use factor and higher average SCFM requirements. Average SCFM is only used in matching the tools to the air compressor. To size the air lines and fittings, use continuous SCFM.
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Know Your Pressure
Always regulate air pressure. Air tools require a certain amount of pressure (PSI) to operate properly. An air tool that does not have enough pressure may not perform as expected (even if it “sounds” right). An air tool that has too much pressure can dramatically shorten the tool’s life.
Typical compressed air systems contain several restrictions. These restrictions can hinder airflow to the tool and reduce pressure. Each time there is a reduction in air pressure in the system it is called a pressure drop. Insufficient pressure to the tool will result in poor tool performance. Several major causes of pressure drop are:
- Too small diameter of piping and air hose.
- Piping and air hose are too long.
- Number of filters, regulators, lubricators, and quick connects in the system.
- Performance of filters, regulators, lubricators, and quick connects in the system.
No matter how good the system, there will always be pressure drops, however you can compensate for them. Two mistakes are often made when adjusting air pressure that can lead to poor tool performance. The first mistake is regulating the pressure far from the tool. A significant pressure drop can occur between the regulator and the tool. To ensure accurate pressure at the tool, always check and measure the pressure at the tool inlet while adjusting the regulator. Furthermore, always locate the regulator as close to the tool as possible. The second mistake is adjusting pressure with the tool in the static or off position. The regulator must be adjusted with the air tools running. Pressure drops only occur when air is flowing through the line. Make sure you remove all accessories, such as sockets, when testing your tools.
When air flows through an object, friction is induced. This friction results in a pressure drop. The greater an airflow through any given object, the greater the friction and hence the greater the pressure drop. As stated earlier, the primary causes of pressure drop are the length and diameter of piping and hose and the number and performance of quick connects, filters, regulators, and lubricators. Every component of the system has certain flow characteristics. While it is virtually impossible to eliminate all pressure drops, it is possible to minimize them by sizing components of the air system to the air consumption of the tool being used.
A common mistake is sizing air lines and fittings to the air delivery of the air compressor. Regardless of the size of the compressor pump, air tools will consume a given amount of air. To properly size the system you must look at the tools’ continuous air consumption in SCFM. This is the actual amount of air the tool consumes when it is switched on. Average SCFM numbers should only be used when sizing the compressor pump.
To start planning the air system, it is important to figure out what tools you will be using now and in the future. It is best to make a work sheet, step one: list the tools you will be using. Step two: enter the continuous SCFM for each tool. If multiple tools will be running at the same time, enter the SCFM of all tools that will be operating on each line. Step three: enter the largest SCFM of each air line. Step four: total system SCFM (sum of all air lines in step three). After you have determined the tools, determine the total SCFM requirements for the tools that will be used. Now that you know how much SCFM the system needs to flow you are ready to begin designing an efficient air system.
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