If you work with air compressors in your industry, you must be familiar with the term SCFM, or standard cubic feet per minute. This is a critical measurement when it comes to air compressors. SCFM, along with CFM and PSI, are acronyms that you should know when you shop for an air compressor because understanding these measurements will help you make the right choice.
What does SCFM mean? If you’re new to the world of air compressors or are taking a greater role in your company’s air compressor management, here is what you need to know about SCFM and why it is important for air compressors.
Table of Contents
- What Is SCFM?
- Why Is SCFM Important for Air Compressors?
- What Is the Difference Between SCFM and CFM?
- How to Calculate SCFM and CFM
- What Does the Volumetric Flow Rate for Air Compressors Mean and Why Is It Important for Air Compressor Systems?
- What Does Free Air Delivery Mean?
- What Are the Most Important Ratings to Look at When Determining Air Compressor Capacity?
What Is SCFM?
Standard cubic feet per minute measures the flow rate of gas under standard pressure and temperature conditions. The accepted standard values for temperature and pressure are 68 degrees Fahrenheit and 36 percent humidity at sea level.
Pressure varies inversely with SCFM, so if your PSI is 120, your SCFM will be lower than if your PSI is 80.
SCFM vs. ACFM
The alternative measurement to SCFM is ACFM, or actual cubic feet per minute. This is the pure flow rate of gas independent of standard pressure or temperature conditions.
Why Is SCFM Important for Air Compressors?
Understanding the rate of free air flow with respect to your compressor is vitally important when you are purchasing or deciding where to install air compressors. The SCFM is probably your most useful measure for this information.
Knowledge of your air compressor’s SCFM will tell you if you have the right gas compression for the size of the job your air compressor has to do. An air compressor that does not push enough air will mean your applications will not work optimally, while one that pushes more air than you need will waste your company’s money and energy.
By calculating how much pressure you need for your application, you can verify you are working with the right air compressor by finding out its SCFM. An air compressor with 10 horsepower or more should generate around 3 or 4 cubic feet of air per minute at 90 PSI.
What Is the Difference Between SCFM and CFM?
In the United States, air-flow volume is typically measured in cubic feet per minute (CFM). It does not matter where the air is coming from — a fan, a compressor or a blower — the rate of air, as measured in volumetric terms, accounts for the amount of air that passes within a measurement of time.
To determine the amount of power needed to warm up a current of air, you will need to take two factors into consideration:
- The mass flow rate, also referred to as the amount of air that needs to be heated.
- The temperature rise, also known as the intensity of heat in the air.
On air compressors, CFM and SCFM are the two most common ratings used to indicate the capabilities of a given make and model. So how are SCFM and CFM different? As a measurable value, a compressor’s CFM rating will always be a lower number than the SCFM rating because the former value is measured at the point where the air supply is pressurized, usually at 90 PSI.
A few additional factors need to be taken into consideration to determine a compressor’s SCFM rating, which is impacted by standard temperature, humidity and pressurization. Consequently, the SCFM rating of a given machine will be a higher number than its CFM counterpart.
Inside an air compressor, the quality of the pressurized air reaches its peak quality just as the air passes through the pressurization chambers. Each supply of newly pressurized air that comes through the airend will inevitably lose some of its pressure once ambient factors take hold, like atmospheric pressure. As such, a compressor must produce a higher level of standard cubic feet per minute to make up for this loss.
How to Calculate SCFM and CFM
SCFM is calculated by a method that expands the air to accommodate the intervening factors of humidity, pressure and actual temperature. For example, 14.2 PSIA (0 PSIA), 63 degrees Fahrenheit at 34 percent relative humidity. For the buyer of an air compressor, CFM is more pertinent to the internal capacity of the air compressor while SCFM is more indicative of how the pressurized air will perform at the end-point applications.
For the best possible results in a factory setting, select the tool from your arsenal with the highest SCFM rating and choose an air compressor that yields an output 150 percent of that rating. For example, if a sander or blower requires 5.3 SCFM, make sure that the compressor you choose has an ideal gas output of 8.0 SCFM or higher. If you intend to run two or more pneumatic air tools simultaneously from a single compressor, add up the total SCFM of all the tools and multiply that number by 1.5 to determine the necessary SCFM for your next air compressor.
What Does the Volumetric Flow Rate for Air Compressors Mean and Why Is It Important for Air Compressor Systems?
The volume flow rate is defined as the amount of fluid that passes through a link in an air compressor at any given moment. In other words, the volume of air that flows between the different components of an air compressor system in the seconds it takes for newly pressurized air to travel from the tank to the endpoint application.
Volume flow rate is written in mathematical characters that represent the amount of pressurized contents that can pass through an area during a given stretch of time. In flow-rate terminology, the term “cross-sectional area” is used to specify the space through which the volume of pressurized contents travels to get from point A to B in the system.
For air compressors, volume flow rate is important because it indicates the amount of pressurized that air can pass per second through the compressor and its attached pneumatic tools. If you need to have a certain volume of air delivered to an end-point application in a split second, the volume flow rate of your system will indicate whether that would be possible.
Certain factors can affect the volume flow rate, such as the length and circumference of the hoses and the number of attachments present between the compressor and the tip of a given tool. Volume flow rate can also be impacted by errors in the setup, such as leaks that cause air to escape the system before reaching the endpoint.
What Does Free Air Delivery Mean?
In a compressed-air system, free air delivery (FAD) is the enlarged volume of air that the compressor releases into the network within a given measure of time. To determine FAD, measurements must be made of the ambient pressure, humidity and temperature present at the air inlet of the machine. The setting of an air compressor will impact the results of this measurement, as factors like ambient heat and moisture will impact the quality of air that travels through the inlet and, consequently, the pressurized air that travels to the system’s end-point applications.
Once the incoming air pressure, temperature and humidity are measured, the second round of measurements are made, this time of the maximum pressure, volume and temperature at the discharge. The results of this measurement could be affected by the quality of pressurization that occurs within the compressor and whether the machine is sound or flawed.
For example, a compressor with condensation and air leaks will yield different measurements at the discharge than a compressor with no such issues. In this case, you may need to invest in a water separator or at least a saturation pressure dew point monitor in order to get the best FAD.
After the measurements have been taken of the pressure, temperature and volume of the pressurized air that comes out of the compressor, the resulting figure must be calculated with the inlet measurement using the FAD mathematical structure. The result is the free air delivery of the compressor in question. FAD is not the same thing as the airend delivery of an air compressor.
What Are the Most Important Ratings to Look at When Determining Air Compressor Capacity?
When you decide to buy an air compressor, it is important to know how much air it will pressurize per minute. It is also crucial to note the amount of pressurized air that the machine can move on an inch-by-inch basis, as this will largely determine compressibility factors like the amount of air power that a compressor will produce. Other specs to note are the size of the tank, as this will dictate the total amount of air that the compressor can hold at a given time. Make note of the following ratings when you buy an air compressor.
CFM stands for cubic feet per minute, which refers to the amount of air being passed through an air compressor each minute. Each compressor has a CFM rating that indicates the amount of air that the machine will be able to pass through a pneumatic tool on a minute-by-minute basis. The higher the CFM rating, the more air the compressor will pressurize per minute. For applications that demand high volumes of pressurized air, you will need a compressor with a high CFM rating.
In a factory where furnishings are assembled along a conveyor belt, an air compressor will need to pressurize sufficient volumes of air per minute to drive the process along at a timely pace. For applications that need a more steady flow of air, the CFM will need to be at an adequate level to produce this constant supply. In general, high-powered pneumatic tools attached to large, stationary air compressors will require a CFM rating of 10 or higher, whereas tools attached to portable compressors may only require five-or-under CFM.
PSI stands for pounds per square inch, which refers to the amount of air pressure being delivered through an air compressor per inch. The vast majority of pneumatic tools in today’s factories require PSI in the range of 40 to 90 to pass sufficient pressure to an end-point application. Tools that cut through metal, such as pneumatic chisels, will require an air compressor with maximum PSI. Applications such as grinding, scaling, sanding, painting, nailing, cutting, nibbling and other tools used for parts shaping, surface preparation and fastening in an industrial setting should all be powered with a 90 PSI air compressor.
Examples of processes at the higher end would include the assembly tools used along the production lines at auto-assembly plants, where air-powered robot arms piece together the heavy components of automobiles. A high PSI rating will also be required at factories where aircraft and military vehicles are assembled, in full or in part. During the stages where the pieces of a vehicle are readied, high levels of air pressure will be drawn upon to mold and carve the parts into shape. Further rounds of high-pressure air will be needed as the parts are fastened together and given finishing coats.
3. Duty Cycle
During a given usage session, an air compressor will function in an on/off cycle. The amount of time that the compressor is active versus idle is referred to as the duty cycle. So what is the duty cycle for air compressors? If a compressor has a 50/50 duty cycle, that means the compressor will be active 50 percent of the time that it is powered on. Likewise, if a compressor has a 70/30 duty cycle, it will be active for 70 percent of the time that the machine is powered on for an application.
Off-time in a duty cycle does not mean that the machine turns off completely when it is not active. It just means that the compressor does not pressurize air during those moments. Rather, it experiences a momentary drop in pressure so it can recharge and continue working at its best. The on/off cycle can work at a second-by-second pace. Most applications require momentary air, so it is generally accepted for an air compressor to function in an on/off manner. There are exceptions, however, such as applications like spray painting that require a steady flow of air nonstop for several minutes at a time.
The internal capacity of an air compressor’s tank is indicated in gallons. The larger this number, the more air a tank can pressurize at a given time. An air compressor with a large tank volume is better for applications that require continuous airflow. For example, when vehicle parts undergo processes like buffering and painting, the air needs to flow continuously, without pause, for minutes at a time.
An air compressor’s capacity is indicated by its horsepower, which correlates to the CFM. Since the term “horsepower” is understood by most drivers, it is the value most readily identified on the specs of an air compressor when evaluated by a prospective buyer. However, the horsepower listed in the specs of a compressor is not necessarily the most telling value as far as performance is concerned.
The numbers listed in the specs of an air compressor are important to understand because they serve as indicators of how a compressor will perform for a given set of tools or applications. When you know and understand the CFM, SCFM, PSI, duty cycle and tank size of an air compressor, it will be easier to determine whether a prospective machine will have sufficient capacity to power the functions at your facility.
For more information on how to parse air compressor ratings, read the air compressor guide.
Contact Fluid-Aire Dynamics for More Information About Air Compressors for Your Business
At Fluid-Aire Dynamics, we have a wide range of air compressors, air dryers and complete compressed air systems to suit the requirements of numerous businesses and their applications. We've provided quality service and custom solutions to businesses in Pennsylvania, Delaware, New Jersey, New York, Virginia and D.C. since 1972, giving us the expertise to guide you in buying the right air compressor and keeping it meticulously maintained.
For help finding the right air compressors to buy for your business or a free estimate on any of our products, contact us today. We'll help you gain a better understanding of air compressors, oil-free compressed air systems and measurements like SCFM vs. CFM and ACFM.