Compressed-Air Leak Load and Annual Cost (Load/Unload Test)
Turns a stopwatch reading into a repair budget.
Example
You enter
- Compressor delivered capacity (cfm) 500
- Loaded time over test cycles (min) 3
- Unloaded time over test cycles (min) 12
- Specific power (kW/100cfm) 22
- Run hours/yr (energized) 8760
- Energy rate ($/kWh) 0.1
You get
- Leak fraction 0.2
- Leak (CFM) 100
- Leak (kW) 22
- Annual leak energy 192,720 kWh/yr
- Annual leak cost $19,272/yr
Checked against published worked examples from: US DOE Industrial Technologies Program.
Details, formula, and sources
ScopeWith production off, the fraction of a cycle the compressor runs loaded (t_load / (t_load + t_unload), the DOE Compressed Air Challenge method) times the compressor's actual delivered capacity (not the nameplate) is the leakage flow, converted to kilowatts at the system specific power (about 16-22 kW per 100 cfm by compressor type) and to dollars at the run hours and rate. A neglected system loses 20-30 percent of its air to leaks, a well-run one under 10, and the difference is thousands a year on the plant's most expensive utility. Needs no flow meter. An estimate from a stopwatch test, not a metered audit.
Formula or tableleak_fraction = load_min / (load_min + unload_min); leak_cfm = compressor_cfm x leak_fraction; leak_kw = leak_cfm x specific_power / 100; annual_kwh = leak_kw x run_hours; annual_cost = annual_kwh x rate_kwh.
Source and editionThe US DOE Compressed Air Challenge load/unload leak test (leak fraction = t_load / (t_load + t_unload), leak flow = fraction x compressor cfm) and the compressed-air specific-power convention (about 16-22 kW per 100 cfm at 100 psig (Compressed Air Challenge Sourcebook: 16-19 single-stage lubricant-injected screw, 15-17 two-stage, 17-22 lubricant-free; DOE Tip Sheet 3 uses 18)), by name; the relations are public.
Free accessThe DOE Compressed Air Challenge load/unload method and the specific-power convention are public; the DOE compressed-air tip sheets and sourcebook are free at energy.gov.
What governsEstimate. AHJ and licensed professional govern.
API field namescompressor_cfm, load_min, unload_min, specific_power, run_hours, rate_kwh, leak_fraction, leak_cfm, leak_kw, annual_kwh, annual_cost
- Leak fraction the loaded fraction of the load/unload cycle with production off, t_load / (t_load + t_unload)US DOE Compressed Air Challenge
- Leak flow the leak fraction times the compressor's delivered capacity at the operating pressureUS DOE Compressed Air Challenge
- Specific power the whole system's wire-to-air kW per 100 cfm (16-19 lubricant-injected screw, 17-22 lubricant-free, at 100 psig; DOE Tip Sheet 3 uses 18)compressed-air specific-power convention