Compressed air is a form of stored energy that is used to operate machinery, equipment, or processes. Compressed air is used in most manufacturing and some service industries, often where it is impractical or hazardous to use electrical energy directly to supply power to tools and equipment.
Powered by electricity, a typical air compressor takes approximately 7 volumes of air at atmospheric conditions, and squeezes it into 1 volume at elevated pressure (about 100 psig, [7 bar]). The resulting high pressure air is distributed to equipment or tools where it releases useful energy to the operating tool or equipment as it is expanded back to atmospheric pressure.
In the compression process, and the subsequent cooling of the air to ambient temperatures, heat and moisture, are released as illustrated in Figure 1.
Recovered heat from the air compressor can potentially be used as an energy efficiency measure for other processes, such as space and water heating.
Depending on the application, excessive moisture in compressed air needs to be managed as it can cause problems with piping (corrosion) and end use equipment.
a. Compressed Air Costs
This section will help you to understand how much it costs to produce and use compressed air.
Over the first ten years of life of a typical air cooled compressor (see Figure 2), with two shift operation, the operating cost (electricity and maintenance) will equal about 88% of the total lifetime cost. The cost of the original equipment and installation will account for the remaining 12%.
As energy accounts for about 76% of the overall lifetime operating cost, it is very important to design and purchase the most efficient components for your compressed air system. It is recommended to make purchase decisions on the overall expected lifetime operating costs, and NOT just on the initial cost of the equipment.
Figure 3 illustrates the typical losses associated with producing and distributing compressed air. Assuming 100 HP energy input, approximately 91 HP ends up as losses, and only 9 HP as useful work. In other words, about 90% of the energy to produce and distribute compressed air is typically lost.
Always question if compressed air is the most appropriate power source for an end use application. In many cases, you would be better off to use a direct drive electric tool instead of a compressed air driven one.
Some industrial compressors are cooled with water. In such cases, the additional sewer and water charges, chilled water system operating costs, pumping costs and chemical treatment need to be evaluated. Figure 4 is a simplified table to provide you with an indication of the electricity costs associated with one, two and three shift operations at a typical industrial facility. The table also shows the approximate electricity cost for 10, 15, 25, 50 and 100 HP-sized air compressors. The table assumes the compressor average loading is 65% of full load.
The table in Figure 4 uses a blended (energy and demand) electricity rate of $0.10 per kWh. Depending on your local electricity tariff rates, you will need to multiply the values in Figure 4 by your inpidual electricity rate (dollars per kWh) and then multiply by 10 to derive your annual electricity costs. Contact your local utility or compressed air service provider for help to determine your own blended electricity rate.
Add approximately 25% to the annual compressed air electricity cost to account for maintenance and life cycle capital cost (purchase price) of compressed air system. For example, a 50 HP compressor operating for one shift would cost approximately $8,600 for electricity alone, and approximately an additional $2,150 for maintenance and capital costs, for a total of $10,750.
Most facilities can easily save 10-20% of their compressed air energy costs through routine maintenance such as the fixing of air leaks, lowering air pressure, and replacing clogged filters. Even higher savings numbers can be gained by choosing better compressor control, adding storage receiver capacity, and upgrading air dryers and filters.
Using the information in Figure 4, what is the potential to save money and energy at your facility?