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Power Units

Power determines how much energy the compressor consumes and how efficient it is.

Power Conversions​

UnitEquivalent
1 HP (horsepower)0.746 kW
1 kW1.341 HP
1 HP745.7 W
PkW=PHP×0.746P_{kW} = P_{HP} \times 0.746 PHP=PkW×1.341P_{HP} = P_{kW} \times 1.341

Specific Consumption​

Specific consumption measures compressor efficiency:

Specific Consumption=kW consumedm3/min delivered\text{Specific Consumption} = \frac{\text{kW consumed}}{\text{m}^3/\text{min delivered}}

By compressor type​

Compressor typeTypical specific consumption
Lubricated screw5.5-7 kW/(m³/min)
Oil-free screw7-9 kW/(m³/min)
Piston6-8 kW/(m³/min)
Centrifugal (large)5-6 kW/(m³/min)
Rule of thumb

At 100 psig (7 bar), an efficient compressor consumes approximately 6 kW per m³/min of air.

Operating Cost​

Annual Cost=HP×0.746×$/kWh×hours/day×days/yearηmotor\text{Annual Cost} = \frac{HP \times 0.746 \times \$/\text{kWh} \times \text{hours/day} \times \text{days/year}}{\eta_{motor}}

Example: 50 HP Compressor

  • 24 hours/day, 365 days/year
  • $0.08/kWh
  • 91% motor efficiency
Cost=50×0.746×0.08×24×3650.91=$28,725 USD/year\text{Cost} = \frac{50 \times 0.746 \times 0.08 \times 24 \times 365}{0.91} = \$28{,}725 \text{ USD/year}

The Energy Reality​

8 electrical HP = 1 pneumatic HP

Approximately 80% of compression energy is lost as heat.

ηpneumatic=PusefulPinput≈12−20%\eta_{pneumatic} = \frac{P_{useful}}{P_{input}} \approx 12-20\%