How to Calculate CFM of The Air Compressor?

One of the most important factors determining how powerful an air compressor is CFM that stands for cubic feet per minute. Unfortunately, the CFM rate is often exaggerated by the manufacturers to make more sales. Don’t get fooled by overrated air flow given by dishonest companies.

Then how to know the actual CFM on an air compressor? You don’t need to do a laboratory test to determine the true CFM. What you can do, instead, is to test it by yourself. Here is how you can check CFM following a few simple steps with an example for better understanding.

Step 1: 

Step 1: Find out the tank capacity which is often mentioned by the manufacturer on the tank. Then covert it in cubic foot dividing by 7.48. ( 1 gallon = 7.48 cubic feet)

Step 2:

Make sure that the compressor is completely empty. You can do this by opening the drain valve.

Step 3:

Start refilling the tank and record how long it takes to refill the tank entirely. At the same time, record the PSI two moments, once when the compressor kicks in and another when kicks out.

Step 4:

Then, find out the difference by deducting the PSI shown in the tank’s gauge while kicking in from the PSI while kicking out. Now, all that you need to do is to divide the difference by 14.7 to obtain the amount of pressure consumed to fill in the tank in atmospheric pressure. ( 1 atm = 14.7 psi)

Step 5:

Now multiply the tank volume converted in cubic feet by the pressure obtained with regard to atmospheric pressure. Doing so will give the number of cubic feet that the compressor pumped in to refill the tank in the recorded time.

Step 6:

As you found the cubic feet already, you just need to convert it into the minute. To do so, first, divide the number of cubic feet by the time in the minute that it took to refill the tank. The result is CFM.

Let’s consider an example.​

Suppose you had gone to the market and found an air compressor bearing a sticker where it’s written that compressor is of 5 horsepower, tank capacity 60 gallons, and 12 CFM at 90 PSI. Let’s calculate the true CFM applying the technique as mentioned above.

Assume that it took 0.58 minutes to refill the tank, and the PSI difference between kick in and kick out was 30.
What we need to do first is convert the gallons into cubic feet. To do this, we divide 60 by 7.48 and get the cubic feet (60/7.48 = 8.02).

Now we need to convert the PSI found between two moments into atmospheric pressure. To do this, we divide 30 by 14.7 and obtain the PSI in terms of atmospheric pressure ( 30/14.7 = 2.04).

We have already all data, and what we are going to do now is to put this data into the CFM formula that we know.

CFM = Tank volume in cubic feet X ATM of pressure during cycle X Crank time in minute
= 8.02 X 2.O4 X 0.58
= 9.48

Here is the video guide for you to calculate CFM

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