If you're new to construction work, all of the tools and equipment can be overwhelming. Whether you're working on a large-scale construction job or a small remodeling project, you may find that you need to buy specialized equipment from a company like Bourget Bros Building Materials to get the job done. One of the things that is often required to power those tools is an air compressor. The output ratings published on the tank itself are often the optimal levels for the tank, not what you'll get on a consistent basis. Here's a look at what you need to now about the cubic feet per minute rating if you're going to be using a compressor and air-powered tools on your site.
Defining Cubic Feet Per Minute
The cubic feet per minute rating defines how much air the compressor is able to put out every minute. It's often listed as the CFM rating on the tank, and it's expressed as an amount at a specific pressure level. As an example, some large compressors may be rated for something like 80 cubic feet per minute at 110 pounds per square inch (PSI).
A cubic feet per minute rating is usually an average that's calculated from several tests over a predetermined period. In most situations, the testing is done in a few spurts over the course of a couple of seconds. For example, if the test runs for fifteen seconds, the amount of air measured will then be multiplied by four to arrive at a per-minute figure. This calculation process is why the rating on the tank may not represent the actual consistent production of the tank.
The Importance of Cubic Feet Per Minute
With so many different air-powered tools in demand on a construction site, each one has its own air pressure requirement for safe operation. For example, a small tool like a framing nailer may not need a lot of air pressure, but an impact wrench or a large grinding wheel might need much more. You'll have to know what your air compressor's actual CFM output is to know for sure what tools are safe to use with that compressor. After all, if you put a light-duty tool on a compressor with an output much greater than the tool requires, you could damage the tool and risk injuring yourself or those working around you.
Confirming Your Compressor's Cubic Feet Per Minute
The good news is that you can verify the cubic feet per minute of your compressor with your own brief testing. The first thing you need to know is what the compressor's tank volume is. The tank should have a label from the manufacturer with a label that includes the tank's capacity in gallons.
There are 7.48 gallons per cubic foot, so divide the gallon capacity by 7.48 to arrive at the cubic feet of volume in the tank. Empty all of the air from the tank, then let it start refilling. You'll want to time the refill cycle with a stopwatch so you know how long it takes the tank to completely fill. Make note of the pressure rating (listed in pounds per square inch) on the gauge when the compressor turns on, then again when the compressor turns off.
Calculate the difference between those two pressure ratings. For example, if the compressor turned on at 50 pounds per square inch and then turned off at 200 pounds per square inch, the difference would be 150 pounds per square inch. To determine how much pressure is added to the tank when it fills, divide the difference calculated here by 14.7. This determines how much pressure is added in terms of atmospheric pressure and tells you how many cubic feet of air the compressor can produce in the time it takes it to refill.
Finally, you can convert this figure into cubic feet per minute by dividing the cubic feet you just calculated by the number of seconds it took the tank to fill. Multiply that figure by 60 to create a cubic feet per minute rate. If you're using the example here, you'd have 10.2 cubic feet, which is calculated by dividing 150 by 14.7. If the tank took 20 seconds to fill, that equals .51 cubic feet per second, which is 30 cubic feet per minute.