Lesson 6: Sizing Basics
Size a compressor to the work, not to the motor the customer had before. This is the counter-level method. For a large or critical system, have the compressor maker or a system engineer check the numbers before you quote.
Step 1: List every air user
For each tool or machine, get:
- Air use in cfm, from the tool’s spec sheet
- Pressure needed at the tool
- How much of the time it actually runs
A grinder rated 20 cfm that runs 30 percent of the shift averages about 6 cfm. Add the averages, but also note which users run at the same time. Peaks matter.
Step 2: Cylinders, if there are any
Air per cycle for a double-acting cylinder, ignoring the rod (a little high, which is the safe side):
Free air per cycle (ft³) = bore area (in²) x stroke (in) x 2 x [(psig + 14.7) / 14.7] / 1,728
Example: 2 in bore, 6 in stroke, 80 psig, 10 cycles a minute.
- Area = 3.14 in²
- 3.14 x 6 x 2 = 37.7 in³
- Compression ratio = (80 + 14.7) / 14.7 = 6.44
- 37.7 x 6.44 = 243 in³, divided by 1,728 = 0.14 ft³ per cycle
- x 10 cycles = about 1.4 scfm
Step 3: Add leaks and growth
Add the leak estimate from Lesson 5. Ask about new machines or a second shift and leave room for them.
Step 4: Set the pressure
Take the highest pressure any user needs, plus the expected drop through dryer, filters, and piping. Nothing extra “just in case.” Every extra 2 psi costs about 1 percent in energy (Lesson 1).
Step 5: Match the compressor to the duty
Compare total cfm at your pressure against rated capacity on the CAGI data sheet. Check the duty rating too. Many small piston compressors are not built to run continuously.
Step 6: Size the storage
The DOE sourcebook gives a formula for receiver volume to cover a demand spike:
V (ft³) = [t x (C - S) x Pa] / (P1 - P2)
- t = length of the spike, minutes
- C - S = cfm needed beyond what the compressor supplies
- Pa = atmospheric pressure, 14.7 psia
- P1 - P2 = allowed pressure drop in the tank, psi
Example: a 1 minute spike, 50 cfm short, tank allowed to fall from 110 to 90 psig. V = 1 x 50 x 14.7 / 20 = 36.75 ft³. Multiply by 7.48 for gallons: about 275 gallons.
Rule of thumb when there is no spike data: current Compressed Air Challenge guidance is 3 to 5 gallons of receiver per cfm, and 5 to 10 gallons per cfm for load/unload compressors. The older 1 to 3 gallons per cfm rule is no longer considered good practice. Follow the compressor maker’s recommendation when they give one.
The formula sizes for a known spike. The rule of thumb sizes general storage. Use the larger answer.
Shop example
A shop has 4 grinders averaging 6 cfm each, 2 blow guns averaging 3 cfm each, and leaks estimated at 10 cfm. That is 40 cfm. Add 25 percent for growth and you are near 50 cfm at about 100 psig. Storage by rule of thumb: 150 to 250 gallons, or 250 to 500 gallons with a load/unload compressor. Now you can compare real machines on their data sheets.
Sources
- DOE/Compressed Air Challenge, Improving Compressed Air System Performance, 3rd ed. (receiver volume formula; 2 psi per 1 percent rule of thumb; demand analysis approach).
- Compressed Air and Gas Institute (CAGI), Performance Verification Program data sheets (rated capacity at full-load operating pressure).
- Cylinder air consumption formula is standard gas-law arithmetic (free air = compressed volume x absolute pressure ratio; 1,728 in³ per ft³; 7.48 gal per ft³).
- Receiver gallons-per-cfm rules of thumb: Compressed Air Challenge guidance as reported in Plant Services, “Optimize compressed air storage” (2015), and Air Best Practices receiver guide (2020): 3 to 5 gal/cfm, 5 to 10 gal/cfm for load/unload. ORNL Better Plants compressed air training (2025): the older 1 to 3 gal/cfm rule is no longer regarded as good practice. Rules of thumb, not a standard. Confirm with the selected compressor maker.
- Shop example inputs (tool cfm, use factors, leak estimate, 25 percent growth allowance) are illustrative assumptions.