CFM is a vital specification for the effectiveness of an air compressor as well as an air tool. CFM stands for ‘Cubic feet per minute,’ which indicates the flow rate of a compressor.
Essentially, this measure is the volume of air a unit can deliver in one minute at a given pressure (PSI) level.
Too much or too low airflow can lead to reduced system performance, even deteriorate the components. Therefore CFM plays a significant role while choosing a suitable compressor with the right capacity.
Now you must be wondering how much CFM you would need for a particular application. This article will help you answer that query and provide you a detailed insight into the topic.
The CFM you need for your air compressor will largely depend on the type and number of tools you use. Typically, portable air compressors connected to air tools for general use require a CFM rating of 0.1-5 at 70-90 psi.
Contrarily, a stationary compressor system for heavy-duty tools may need a CFM rating more than 10, at 100-120 psi.
If you are asking how much CFM do I need, as you can see, the answer to the CFM requirement is not so straight forward. Let’s discuss in detail how to choose your air compressor wisely and how the CFM can be determined.
How Much CFM You Need for Your Compressor
Before determining how much CFM you need, you should consider the required airflow, i.e., CFM of your specific tool/s. This information should be available in the tool manual or user guide, which is set by the manufacturers.
If you are using a single air tool, your required compressor CFM will be the highest CFM marked for that tool. Conversely, while using multiple instruments at a time, first, you should add the individual CFMs of all the tools.
The minimum CFM you need for your air compressor should be 1.5 times the total required CFMs of the tools.
For example, if you need to run a spray gun of CFM 4 at 40 PSI, the CFM of the air compressor should be at least 4 x 1.5= 6 CFM.
In this way, you can extend the lifespan of the air compressor as well as prevent it from overworking. However, this recommendation bases on the assumption that the average CFM rating usually considers a 25% duty cycle.
Here duty cycle means the percentage of time an air compressor runs before it needs time to cool down. If you are aiming for continuous operation of tools, you may need to multiply the total CFM by 4.
How to Calculate CFM of Air Compressor
The principal idea is to measure the time it takes to pump the air compressor tank of known volume. The pressure added in the process is found from the difference between the known final pressure and the starting pressure.
This added pressure, times the tank volume, divided by the pump-up time gives you the true CFM of the compressor.
Factors of CFM Calculation
As mentioned, here are three main factors that are taken into account while calculating the CFM of an air compressor:
Tank Volume: The size of the air compressor tank regulates the time between cycles. Hence it determines how long an air tool will run before the compressor has to cycle back on.
A larger tank is convenient for tools that need a high volume of air and is intended for continuous use. Typically tank volumes are indicated in gallons, which converts into cubit ft during CFM calculation.
Standard Pressure (atm): This is the amount of pressure generated to fill the empty tank with air. Ideally, this pressure is measured in psi (pound-force per square inch).
However, it is converted into a standard atmospheric pressure unit (atm) while calculating CFM.
Tank pump-up time: This is the time for the compressor tank to fill up. This is usually measured in seconds, which helps to determine the per minute value of the compressor airflow.
Steps for CFM Calculation
Following are the detailed steps to calculate CFM of an air compressor:
Determine the tank volume in gallons as marked by the manufacturer. Now divide the volume by 7.48 to convert the tank volume into cubit feet. Note that 1 cubit feet= 7.48 gallons.
For example, if the tank volume is 60 gallons, it will be 60/7.48= 8.02 in cubic feet.
Now release all the remaining air from the compressor tank. Then start to refill the air compressor while paying close attention to the tank gauge (PSIG-pounds per square inch-gauge).
Simultaneously, you need to precisely record the time it takes for the tank to reach its full capacity. Suppose, it takes 86 seconds.
- Take note of the PSIG reading at two times, once at the moment you turn the air compressor on and secondly as soon as the tank is full when you turn it off.
- Subtract the first PSIG reading from the second, this difference in pressure is the pressure added to the tank while being refilled. For instance, if the start pressure is 70 PSIG and the final pressure is 110 PSIG, the difference is 40 PSIG.
- Convert this PSIG reading to the atmospheric pressure unit, where 1 atm= 14.7 PSIG. Hence, for 30 PSIG, it will be 40/14.7= 2.72 atm.
- Now multiply the tank volume (cubic feet) with the pressure added to fill the tank (atm). This is the amount of air in cubic feet that the compressor pumps during the time it took for the tank to fill. As per our example, 8.02 x 2.72= 21.81 cubic feet.
- To obtain the CFM, you need to convert the calculated result into a ‘per minute’ value. Hence, divide the result by the filling time in seconds, and multiply by 60 (as 1 minute= 60 seconds). Therefore, the final result for our example case would be (21.81/86)*60 = 15.21 CFM.
Air Compressor CFM Vs. PSI
The CFM value varies inversely as the PSI (pound-force per square inch) changes. That means, if your PSI output is lowered, CFM value will increase and vice versa.The reason behind this is simple. At lower pressure, the air compressor has to overcome less resistance to deliver 1 cubic ft of airflow.
Generally, the PSI requirement of most air tools ranges from 70 to 90. However, it can be as low as 30 PSI or can be higher than 150 PSI, depending on the device.
At a given PSI, compressors with high CFM ratings can provide more air. Hence, they are well suited for continuous use or heavy applications.
For example, at 90 PSI level, a constantly spinning sanding machine may require a CFM rating of 8. On the other hand, a nail gun needing a single burst of air may have a low CFM of 2.8.
Check here a CFM Vs PSI chart, where average CFM at recommended psi level is compiled for various pneumatic equipment.
How to Increase CFM on Air Compressor
If you need to increase the CFM of an air compressor for heavier applications, there are several ways for that. However, each of these methods has some side-effects. Let’s elaborate:
Combining Another Compressor with Existing One
One method is to purchase another air compressor and combine it with the existing one. In this way, you would be able to enhance the maximum output.
There are few drawbacks in this method-
- An additional cost to purchase a supplementary compressor
- Longer time required to fill up both tanks
- Possibility of overheating and maintenance issue
- Experience required to execute the operation properly
Allowing the Compressor More Time
Another way to increase the CFM is to allow your compressor some more time to work on the compressed air. The process is pretty simple; all you need to do is dial down the regulator at a lower setting.
Be careful not to go all the way down in the first place. If the compressor adjusts well and works fine, gradually dial down and repeat till you reach the lowest setting.
Thus the pressure build-up will be less and the compressor will be able to run for a much longer time.
This method has more benefit relative to the previous one:
- The method is simple and inexpensive
- This is helpful to prolong the lifespan of the compressor
Tips To Get More CFM Out Of Air Compressor
In some circumstances, an air receiver or storage tank can help to get more CFM out of a low CFM compressor. As the receiver can store compressed air, it helps to reduce the loaded run time of the compressor.
For optimal results, the receiver should be at least 5 gallons and can go over 100 gallons as per requirement. This can be a nice strategy if you don’t want to invest in a high CFM air compressor.
Besides, you can consider purchasing a two-stage compressor, which is around 15% more efficient than single-stage compressors. Therefore, a two-stage compressor can deliver more CFM than the single-stage compressors, especially for bulky pneumatic tools.
I hope, the discussion above helped you to have a better understanding of CFM. Now you can make an educated decision while purchasing an air compressor according to your need.
Frequently Asked Questions
How Much CFM Do I Need To Paint A Car?
As painting a car requires the continual, consistent operation of the compressor, it requires a high CFM rating. A low CFM may lead to uneven paint or paint spattering. Typically, 14-18 CFM at 90 PSI is recommended for the spray gun to paint a car.
How Much CFM Do I Need For Air Tools?
CFM rating of most air tools ranges from 30-120 at a PSI level range of 70-120. Lower CFMs are usually for a small-scale operation, whereas higher CFM ratings are typically meant for industrial applications.
You can also refer to the chart linked to getting an idea of the CFM needed for specific air tools.
How Much CFM Do I Need For Dust Collection?
Dust collection requires a system with a high volume, low-pressure design. That is because the dust collector operates relatively slowly to accumulate a large number of airborne particulates.
In general, at least 1000 CFM is needed for fine-dust collection, whereas 350 CFM could be enough for chip collection.