• Is your air compressor working at 100% load all the time?
  • Does your air pressure keep dropping?
  • Have you added or plan to add new equipment which uses air-but have not increased the size of your compressor? Is your air compressor maxed out and in need of replacing – but you just aren’t sure how to figure how much more horsepower you need?

When your business counts on air – you need to know the valuable formulas and steps to accurately determine how many CFM you actually use.  You also need to know how to accurately figure the additional CFM and horsepower you need when considering new equipment.

Do we add another compressor or do we replace?  That’s a BIG and costly question that deserves a careful and correct answer. 

Here are three specific formulas to help you determine how many CFM you presently use and how many more CFM you need to meet your desired PSIG.  We will show you how to take this information and use it to determine the size of the air compressor horsepower you actually need.

FORMULA ONE:  Find out how many CFM your air compressor delivers now.

1. STOP the compressor unit

2. CLOSE the outlet valve on the tank/air receiver

3. DRAIN the condensate from air receiver until there is 0 PSIG -then close the drain valve

4. NOTE THE TIME- in minutes & seconds (Best to write it down.) Then START THE UNIT.
When the compressor unit stops and unloads – then NOTE THE TIME again – in minutes & seconds. Convert the minutes into seconds and then total the number of seconds it takes between START and STOP/UNLOAD.

5. NOTE the GUAGE PSIG reading

6. NOTE the Air Receiver/Tank GALLON SIZE

TANK GALLONS x .538* x PSIG divided by SECONDS

You have an 80 gallon tank and 175 PSIG. Your total start to stop/unload time is 3 minutes and 9 seconds. Change the minutes to seconds timed (60 x 3= 180 seconds plus 9 seconds totals 189).  You will use the total number of seconds (189) and 175 PSIG within the formula as shown in the example below:
80 (tank size) multiplied by .538* = 42.88
42.88 multiplied by 175 (example PSIG) = 7504.00
7504.00 divided by 189 (total seconds)= 39.71 CFM delivered

You now know that your air compressor is delivering 39.71 CFM (example only)
Your Response to this evaluation should be to compare this number with what your air compressor manufacturer says your CFM should be and evaluate how efficiently your compressor is running.
*.538 is a set number that is part of this formula and not a variable as the other numbers would be.

Find out how many more CFM you need to raise your PSIG to your desired pressure.

1.    What is your desired pressure________ (Example: 100 psig)

2.    What is your present pressure________ (Example: 70 psig)

3.    Divide your desired pressure by your present pressure
(Example: 100 psig ÷ 70 psig =1.43)
This gives us the X-factor needed for our formula (1.43)

4.    Multiply the present air compressor CFM delivery from the first formula  times the x-factor to get the TOTAL CFM (39.71  X 1.43 = 56.79 total CFM)

5.    Subtract your PRESENT CFM from TOTAL CFM                                   
(56.79 total CFM minus 39.71 present CFM = 17.08)
This tells you how much additional CFM is needed to raise your PSIG to your desired level.  (Example: An additional 17.08 CFM needed)

Translate your answers into how much horsepower you actually need to operate.

Additional CFM’s needed divided by the CFM’s per horsepower provided by your existing compressor size and type (see chart below).  This answer will give you the horsepower you actually need to operate. (Example:  17.08 ÷ 3.5 = 4.88 hp.   Round this up to 5 horsepower.)

So using our example figures, we’ve determined we actually need an additional 5 horsepower to operate our equipment.

CFM per compressor horsepower chart:
3.5 CFM per hp for small piston compressors ½ -30 hp
4.0 CFM per hp for large piston 40 hp up & small screw compressors 2-30 hp
4.5 CFM per hp for 40-150 hp medium hp screws
5.0 CFM per hp for 200-2000 hp large screw & centrifugal compressors

Note:  Always buy a compressor that can supply the CFM’s of delivered air at the psig you need—do not purchase just by horsepower.
Note:  Always invest in at least 20% more CFM than your equipment needs.  This will cover extra air usage for such things as air leaks.


Compressor Terms you should know:
Cubic Feet Per Minute (cfm) - Volumetric air flow rate.

"psig" means pounds per square inch, GAGE pressure. Gage pressure is the absolute pressure of something, with the atmospheric pressure subtracted. In practice, when someone gives a pressure in just "psi" they probably mean gage pressure. If they mean absolute, they should be using "psia."

Gauge Pressure - The pressure determined by most instruments and gauges, usually expressed in psig.
Barometric pressure must be considered to obtain true or absolute pressure

Load Time - Time period from when a compressor loads until it unloads.

Unload - (No load) Compressor operation in which no air is delivered due to the intake being closed or modified not to allow inlet air to be trapped.

Receiver - A vessel or tank used for storage of gas under pressure. In a large compressed air system there may be primary and secondary receivers.

Demand - Flow of air at specific conditions required at a point or by the overall facility.

Tommy McGuire
McGuire Air Compressors, Inc.
"Real People with Real Compressor Experience"

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