Specific Power

Specific power is the standard metric used to measure air compressor efficiency. It is essentially the “gas mileage” rating for a compressed air system, measuring the total electrical power required to produce a certain volume of compressed air.

The modern standard measurement is kilowatts per 100 cubic feet per minute (kW/100 CFM). In the past, it was calculated as brake horsepower per cubic foot per minute (BHP per CFM). A lower specific power rating indicates a more efficient machine that requires less electricity to run.

Since electricity is the largest lifetime cost of operating a compressor, this metric is critical for decision-makers. Industries like agriculture, automotive and food and beverage rely on tracking specific power to minimize energy waste and maximize cost savings.

FAQs

To calculate specific power, use this formula:

Specific Power = Total Package Input Power ÷ (Free Air Delivery in CFM / 100)

For a more detailed way to determine specific power and get an accurate representation of system efficiency, follow these steps:

Measure the total input power: Include all electrical loads of the compressor package, including cooling fans, motors and internal pumps.
Identify the free air delivery: Measure the actual compressed air output in cubic feet per minute.
Calculate the specific power: Divide the input power by the free air delivery.

As the primary indicator of energy efficiency, determining the specific power between compressors allows facility managers to accurately forecast energy costs. A system running at a higher specific power rating than necessary is wasting energy.

Knowing the specific power can turn compressed air from a “hidden cost” into a measurable and controllable utility expense while delivering advantages, such as:

Better operating conditions
Improved overall efficiency
Lower capital expenditures
Reduced operating costs
Increased revenue potential

Specific power does not stay static. Here are some key factors that contribute to the decline of specific power, causing the compressor to work harder and consume more power:

Increased system resistance: Clogged intake filters, contaminant buildup and dirty heat exchangers or coolers affect the performance of the air compressor. These changes lead to pressure drops that worsen specific power.
Internal mechanical wear and tear: Wear in airends, bearings and belts damages the internal components over time. Efficiency is affected when this happens.
Leakage and operational waste: Undetected leaks in pipes, hoses, couplings and joints force the compressor to run longer and increase total energy consumption. Excessive pressure setting is another major cause of inefficiency.
Poor maintenance practices: From lubricant degradation to missed service intervals, lack of maintenance can lead to compound failures and increased power draw.

Improving the compressor’s specific power is critical to energy consumption and equipment longevity. Schedule regular system evaluations and perform routine maintenance as the first step.

Upgrading to modern technology is another strategy to improve the specific power rating. Quincy Compressor’s variable speed drive (VSD) rotary screw compressors automatically adjust motor speed to match fluctuating air demand.