Compressed-Air Discharge-Pressure Setpoint Savings
Prices the cheapest compressed-air conservation measure there is: turning the discharge pressure down.
Example
You enter
- Current discharge setpoint (psig) 120
- Proposed lower setpoint (psig) 105
- Inlet pressure (psia) 14.7
- Current compressor input (kW) 50
- Run hours/yr 6000
- Energy rate ($/kWh) 0.1
You get
- Percent energy saved 7.07%
- Power saved 3.54 kW
- Annual energy saved 21,219 kWh/yr
- Annual savings $2,122/yr
Checked against published worked examples from: Oregon State University Energy Efficiency Center (DOE Industrial Assessment Center).
Details, formula, and sources
ScopeBecause compression power rises with the pressure ratio, the isentropic power ratio between two setpoints gives the exact percent saved - which reproduces the DOE rule of about half a percent of compressor energy per psi. Dropping 120 to 105 psig on a 50 kW compressor saves ~7 percent, roughly $2,000 a year, for turning a knob. The reduced setpoint must still hold the minimum tool pressure after system drop, which is why fixing leaks and piping is what unlocks it. An energy-savings estimate, not a metered result.
Formula or tablework_current = ((current_psig + inlet_psia) / inlet_psia)^((k-1)/k) - 1; work_reduced = ((reduced_psig + inlet_psia) / inlet_psia)^((k-1)/k) - 1; saved_fraction = 1 - work_reduced / work_current; pct_saved = 100 x saved_fraction; kw_saved = input_kw x saved_fraction; annual_kwh = kw_saved x run_hours; annual_savings = annual_kwh x rate_kwh. (k = 1.4)
Source and editionThe isentropic compression-power ratio (percent saved = 1 - [(P_reduced/P1)^((k-1)/k) - 1] / [(P_current/P1)^((k-1)/k) - 1]), which reproduces the DOE rule of roughly 0.5% of compressor energy per psi of reduction, by name; k = 1.4 for air is a named constant.
Free accessThe isentropic pressure-ratio relation and the DOE per-psi rule are public; the DOE compressed-air tip sheets are free at energy.gov.
What governsEstimate. AHJ and licensed professional govern.
API field namescurrent_psig, reduced_psig, inlet_psia, input_kw, run_hours, rate_kwh, pct_saved, kw_saved, annual_kwh, annual_savings
- Power ratio the saving is the isentropic work ratio between the two discharge pressures (absolute)isentropic compression
- Per-psi rule reproduces the DOE rule of about 0.5% of compressor energy per psi of reductionUS DOE
- Minimum pressure the reduced setpoint must still serve the tools after system pressure dropcompressed-air best practice