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Calculating the Cost of Compressed Air

Key Points
  • About 80 percent of compressed air energy use is lost as heat, creating a cost-saving opportunity.
  • Calculate costs using a simple formula and information about system equipment and operations.
  • Complete a compressed air audit to determine the most effective energy-saving measures. 

Compressed air systems are widely used in commercial and industrial facilities, but they're very inefficient. About 80 percent of compressed air energy use is lost as heat. This type of waste puts a lot of unnecessary pressure on your operating budget. By pinpointing the costs involved, you can make a better case for energy-saving investments.

Estimating compressed air costs

Every facility is different. So how do you evaluate compressed air system costs without turning to the time-consuming process of tracking energy use with submetering or a data logger? Simply gather some information about system equipment and operations and use the following formula:

Compressed air gif

Where:

  • Bhp = compressor brake (shaft) horsepower
  • 0.746 = conversion factor between horsepower and kilowatt (kW)
  • Percent time = percentage of time running at this operating level
  • Percent full-load bhp = bhp as a percentage of full-load bhp at this operating level
  • Motor efficiency = motor efficiency at this operating level

Include the amount of time the compressor is operating at full load and partial load. Partially loaded compressors operate at a lower compressor shaft horsepower and are less efficient. As an example, consider a compressor with a shaft horsepower of 215 bhp that operates 6,800 hours per year. It is fully loaded 85 percent of the time with a motor efficiency of 95 percent and partially loaded the remaining 15 percent of the time. The partial load shaft horsepower is 25 percent of full-load shaft horsepower, and the motor efficiency is 80 percent. The electric rate is $0.08/kWh.

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So, the annual cost of running that compressor is more than $82,000. Remember though, the process of compressing air is very inefficient. When you take out the 80 percent lost as heat, you're spending more than $65,000 a year for no useful work. That's a number that should get the CFO's attention.

Putting the squeeze on energy waste

Numerous strategies are available to help you improve system efficiency. Start with a compressed air audit. Commonly used energy-saving measures include:

  • Inspect the system to locate and fix leaks and other issues that may affect energy efficiency.
  • Review compressed air end-uses and determine the required level of air pressure. Look for processes that can be reconfigured to use air more efficiently.
  • Install modulating inlet or multistep controls, which enables the compressor to operate at part load during periods of low demand.
  • Design a pressure control system to separate air supply and demand, allowing compressors to operate at optimum pressure to maximize efficiency. Higher pressure air is delivered to storage tanks and is available to meet fluctuating demand at a constant, lower-pressure level.

Develop a load profile of the system (compressed air requirements over a specified period) to determine which energy-saving measures will be most effective.

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