How to size an air compressor
Undersize a compressor and line pressure sags so tools lose torque; oversize it and the machine spends its life unloaded, burning electricity for nothing. Enter the combined demand of your tools along with a use factor, leakage allowance and discharge pressure to see the required delivered capacity, the power needed, and a standard motor size.
Required capacity is the sum of tool demand multiplied by the simultaneous use factor, then increased by the leakage allowance and any spare capacity you want. Power comes from the adiabatic compression relation using a specific heat ratio of 1.4 for air, referenced to standard atmospheric pressure, and brake power is that theoretical value divided by the overall compressor efficiency.
Leakage grows as a system ages. New piping may lose about 10 percent, while older plants often measure 20 to 30 percent, which an ultrasonic leak survey can confirm. Discharge pressure matters just as much: every extra 10 psi of pressure adds roughly 5 percent to the power bill, so set the setpoint from the actual pressure needed at the tool.
This estimate assumes standard inlet conditions near sea level. High altitude sites, hot compressor rooms, multi-stage machines and variable speed drives all shift the result, so confirm against manufacturer performance curves before committing to a final specification.
Frequently asked questions
Tools rarely all run at once, so 60 to 80 percent is common. If the plant has automated lines whose cylinders cycle together, push the factor to 90 percent or higher.
When demand swings widely, several units with one on a variable speed drive usually run more efficiently. Splitting the load also keeps production going during maintenance.