Air User Tips

Determine the Cost of Compressed Air for Your Plant

Most industrial facilities need some form of compressed air, whether for running a simple air tool or for more complicated tasks such as the operation of pneumatic controls. A recent survey by the U.S. Department of Energy showed that for a typical industrial facility, approximately; 10% of the electricity consumed is for generating compressed air. For some facilities, compressed air generation may account for 30% or more of the electricity consumed. Compressed air is an on-site generated utility. Very often, the cost of generation is not known; however, some companies use a value of 18-30 cents per 1,000 cubic feet of air. Compressed air is one of the most extensive sources of energy in a plant. The overall efficiency of a typical compressed air system can be as low as 10%-15%. For example, to operate a 1-horsepower (hp) air motor at 100 pounds per square inch gauge (psig), approximately 7-8 hp of electrical power is supplied to the air compressor. To calculate the cost of compressed air in your facility, use the formula shown below:

Cost ($) =
(bhp) x (0.746) x (no. of operating hours) x ($/kWh) x (% time) x (% full-load bhp)
Motor Efficiency

Where:
bhp - Motor full-load horsepower (frequently higher than the motor nameplate horsepower- check equipment specification)
0.746 - conversion between hp and kW
Percent time - percentage of time running at this operating level
Percent full-load bhp  - bhp as percentage of full-load bhp at this operating level
Motor efficiency - motor efficiency at this operating level

Example
A typical manufacturing facility has a 200-hp compressor (which requires 215 bhp) that operates for 6800 hours annually. It is fully loaded 85% of the time (motor efficiency = .95) and unloaded the rest of the time (25% full-load bhp and motor efficiency = .95). The aggregate electric rate is $0.075kWh.

Cost when fully loaded =
(215 bhp) x (0.746) x (6800 hrs) x ($0.075/kWh) x (0.85) x (1.0) = $73,188
      .95
Cost when unloaded =
(215 bhp) x (0.746) x (6800 hrs) x ($0.075/kWh) x (0.15) x (0.25) = $3,229
  .95

Annual energy cost = $73,188 + $3,229 = $76,417

Typical Lifetime Compressed Air Costs in Perspective (Costs Over 10 Years)

Assumptions in this example include a 75-hp compressor operated two shifts a day, 5days a week at an aggregate electric rate of $0.075/kWh over 10 years of equipment life.

Air User Tip courtesy of:
U.S. Dept. of Energy
Energy Efficiency and Renewable Energy
www.eere.energy.gov/industry/bestpractices