Calculator
Tip: If you enter average demand already, set load fraction to 1.00.
Example data table
This example demonstrates a common plant scenario with demand charges and leakage. Your local tariff and compressor performance may differ.
| Example input | Value | Example output | Value |
|---|---|---|---|
| Useful demand | 600 cfm | Total annual cost | 26,354.00 |
| Pressure (gauge) | 100 psi | Annual energy | 138,133 kWh |
| Operating hours | 5,000 h/year | Cost per 1,000 m³ (useful) | 7.95 |
| Load fraction | 0.65 | Extra cost from leaks | 5,086.60 |
| Leakage | 20% | Peak electrical power | 37.104 kW |
Formulas used
- Leak-adjusted produced flow: Qprod = Quseful / (1 − L), where L is leakage fraction.
- Isentropic compression power (estimate): Pis = (k/(k−1)) · p1 · \dot{V} · \big[(p2/p1)^((k−1)/k) − 1\big].
- Electrical full-load power: Pfull = Pis / (ηcomp·ηmotor·ηdrive).
- Specific power option: Pfull = SP · Qprod, where SP is kW per (m³/min).
- Average power: VSD: Pavg=Pfull·f, Load/Unload: Pavg=Pfull·(f+(1−f)·u), where f is load fraction and u is unload power fraction.
- Annual energy: E = (Pavg + Paux) · H, in kWh, where H is operating hours.
- Annual costs: Cenergy=E·r, Cdemand=Ppeak·d·12, Cmaint=E·m + Cfixed.
- Annualized capital (optional): Equivalent annual cost with a capital recovery factor and salvage value.
How to use this calculator
- Enter your useful air demand and system pressure.
- Estimate leakage and your average load fraction.
- Select a power method: manufacturer specific power, or estimate.
- Fill in energy rate, demand charge, and maintenance assumptions.
- Optional: add auxiliary loads and capital costs for full ownership.
- Press Calculate to view results above the form.
- Use the CSV/PDF buttons to export the latest report.
FAQs
1) What does “useful air demand” mean?
It is the air your processes actually need at standard conditions. The calculator then increases produced air to cover leakage, so you can see the true energy and cost impact.
2) How should I choose load fraction?
Use the average fraction of full output during operating hours. If you already entered an average demand rather than a peak value, set the load fraction to 1.00 for consistent results.
3) When should I use specific power?
Use it when you have a datasheet or measured kW per delivered flow for your compressor at your operating pressure. It usually reflects real performance better than generic efficiency assumptions.
4) What is unload power fraction?
In load/unload control, the compressor may consume power while producing little or no air. The unload fraction models that idle consumption, which can significantly raise annual energy at low load.
5) How do demand charges affect results?
If your utility bills a monthly fee based on peak kW, the calculator adds demand charges using peak electrical power and your demand rate. Set the demand rate to 0 if you are not billed this way.
6) Why include auxiliary loads?
Dryers, cooling fans, condensate drains, and pumps often run whenever the system is energized. Adding auxiliary kW captures these loads so your annual energy estimate matches what the meter sees.
7) How is leakage extra cost calculated?
The calculator runs a second case with leakage set to 0% while keeping all other inputs the same. The difference in total annual cost is reported as the extra cost attributed to leakage.
8) Is this suitable for audits and investment cases?
It is a strong screening tool for comparing scenarios, leak repairs, and control upgrades. For final decisions, validate with logged kW, flow measurements, and a tariff review for your facility.