What does compressed air cost per 1,000 cubic feet?

Most plants know what they pay for electricity. Few know what they pay for compressed air, even though it is often one of the largest single uses of electricity on site. Without that number, a new air tool, an open blow-off or a leak looks free.

The short answer

Compressed air is paid for in electricity. A typical plant uses about 20 kW of compressor power to make 100 scfm of air at around 100 psig. Making 1,000 cubic feet at that rate takes about 3.3 kWh. At $0.12 per kWh:

• About $0.40 per 1,000 cubic feet of air.

That is the energy alone, for a plant whose compressors run well loaded. With 18 to 22 kW per 100 scfm, and electricity from $0.08 to $0.16 per kWh, the figure runs from about $0.24 to $0.59 per 1,000 cubic feet.

A more useful yardstick: each cfm, per year

Cost per 1,000 cubic feet is hard to picture. What one more cubic foot a minute adds to the bill is easier. At 20 kW per 100 scfm and $0.12 per kWh, each scfm of steady demand costs:

• Over two shifts (4,160 hours a year): about $100 a year.
• Around the clock (8,760 hours): about $210 a year.

So an open blow-off using 20 scfm through two shifts costs about $2,000 a year. A cabinet cooler, an air motor or a vacuum generator that runs all day can cost more to feed than it cost to buy.

Try your own numbers: Cost of compressed air calculator

Why your number may be higher

The 20 kW per 100 scfm figure assumes compressors that are well loaded. Many plants aren't, and that changes the answer more than the electricity rate does.

1. Part-loaded compressors

A compressor that throttles its inlet to match demand (modulation) can still draw 80% or more of its full power while making half its air. In that case each 100 scfm takes about 32 kW, not 20, and the air costs about $0.64 per 1,000 cubic feet. The air is the same. The way the compressor makes it is not.

2. Running unloaded

A load/unload compressor makes no air while it is unloaded, but its motor keeps running. With too little storage it unloads and loads often, and those unloaded minutes are paid for with nothing to show.

3. More compressors running than the demand needs

Two compressors each running half loaded cost more than one running fully loaded. Without controls that run only the units the demand needs, this is common, especially on second shift and weekends.

4. Higher pressure than the plant needs

Each 2 psi of added pressure costs about 1% more compressor power. Higher pressure also pushes more air through every leak and every unregulated use, so the plant uses more air as well as paying more for each cubic foot.

What the usual figure leaves out

• Dryers, cooling water and fans that run with the compressors, and the purge air a desiccant dryer uses.
• Maintenance, repairs and the equipment itself. Over a compressor's life, energy is usually about three-quarters of the total. The rest is the purchase, installation and upkeep.
• Demand charges on the electricity bill, when the compressors add to the plant's peak.

So the energy figure is a floor. For a decision such as fixing leaks or replacing an air motor with an electric one, it is usually the right number to start with.

Getting your own number

• Start from what the compressors draw. A power meter, or logged amps, on each compressor gives the energy side.
• Measure the air. A flow meter, or a careful estimate, gives the other side. Divide one by the other and you have your plant's real kW per 100 scfm.
• Log a full week, including a weekend. The cost per 1,000 cubic feet is often worst at light load, nights and weekends, when the compressors are least loaded.
• Look unit by unit. One poorly loaded compressor can raise the plant's figure for every cubic foot.

Once you know what your air costs, every use of it can be judged the same way: is it worth what it costs to feed?

Source: U.S. Department of Energy, Improving Compressed Air System Performance: A Sourcebook for Industry, compressor part-load performance and life-cycle costs. The examples assume 100 psig, 20 kW per 100 scfm and $0.12 per kWh, as in our calculator.