Lesson 1: What Compressed Air Really Costs
Most customers think air is free. It comes out of a pipe on the wall, so nobody gets a bill for it. They do get a bill. It just shows up on the power bill, not the purchase order.
Why air is expensive
A compressor turns electricity into pressure, and it is not good at it. The U.S. Department of Energy says the overall efficiency of a typical compressed air system can be as low as 10 to 15 percent. The Compressed Air Challenge says as much as 80 to 93 percent of the electricity an industrial compressor uses turns into heat.
So most of what the customer pays for warms up the compressor room. Only a small slice ends up as useful work at the tool.
The cost formula
The DOE sourcebook gives a simple way to estimate what a compressor costs to run each year:
Annual cost = (hp x 0.746 x hours per year x $ per kWh x % time at that load x % of full-load hp) / motor efficiency
The 0.746 converts horsepower to kilowatts.
Shop example. A 50 hp compressor runs fully loaded across two shifts, about 4,000 hours a year. Power costs $0.12 per kWh. Motor efficiency is 93 percent.
- 50 x 0.746 = 37.3 kW
- 37.3 / 0.93 = 40.1 kW drawn from the line
- 40.1 x 4,000 hours = 160,400 kWh
- 160,400 x $0.12 = about $19,250 a year
That is one compressor, two shifts, on electricity alone. Over the life of the machine, the power bill is far bigger than the purchase price. That is why a cheaper compressor with poor efficiency is rarely the cheaper choice.
Note: this is a quick estimate. Real compressors run partly loaded, and some run above nameplate horsepower into the motor’s service factor. Use real hours and real load when you have them.
Where the money leaks out
Two numbers worth remembering:
- Leaks. The DOE sourcebook says a poorly maintained plant typically leaks about 20 percent of its compressor output, and as high as 20 to 30 percent. That is air paid for and thrown away.
- Pressure. Rule of thumb from the same source: in systems around 100 psig, every 2 psi of extra discharge pressure adds about 1 percent to energy use.
A plant running 10 psi higher than it needs pays roughly 5 percent more for power, every hour, for no reason.
What this means at the counter
When a customer says “we need a bigger compressor,” the cheaper fix is often fixing leaks, cutting pressure drop, or adding storage. Ask before you quote iron. You will save the customer money, and they will remember who did.
Sources
- DOE/Compressed Air Challenge, Improving Compressed Air System Performance, 3rd ed. (annual cost formula; leak rate typically about 20 percent, as high as 20 to 30 percent; 2 psi per 1 percent rule of thumb).
- U.S. Department of Energy, compressed air tip sheet (overall system efficiency as low as 10 to 15 percent).
- Compressed Air Challenge, Fact Sheet 10, Heat Recovery with Compressed Air Systems (as much as 80 to 93 percent of electrical energy becomes heat).
- Worked example arithmetic by the author using the sourcebook formula; inputs (50 hp, 4,000 hours, $0.12/kWh, 93 percent efficiency) are illustrative assumptions, not measured data.