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Friday, March 18, 2011

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 expensive sources of energy in a plant. The over-all 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 inchgauge (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:

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

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 = .90). The aggregate electric rate is $0.05/kWh.

Cost when fully loaded =

Cost when fully unloaded =

Annual energy cost = $48,792 + $2,272 = $51,064

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, 5 days a week at an aggregate electric rate of $0.05/kWh over 10 years of equipment life.


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