Calculating compressed air efficiency

In brief:

• The data sheets made available by the Compressed Air and Gas Institute (CAGI), in addition to resources from Compressed Air Challenge, can make a real difference in helping you choose a compressor.
• There is now a wealth of information about compressor and air dryer energy consumption available within the CAGI Compressor Data Sheets published by participating manufacturers.

It’s probably been years since you’ve even thought about buying a new air compressor, but now your old unit is broken, the boss is howling, and you need to make a decision. But there are so many choices. How do you decide the best one? The data sheets made available by the Compressed Air and Gas Institute (CAGI), in addition to resources from Compressed Air Challenge, can make a real difference in helping you choose.

Too often, the purchasing decision comes down to price. The difference between various compressor brands and models may be only a few hundred dollars, making the cheapest machine the obvious choice to the purchasing department. But what about operating costs?

At $0.10/kWh with a five-day, two-shift operation of about 4,200 hrs/year, a typical 100 hp compressor would require about $37,000 in annual electrical costs. Over a 10-year period, these costs can add up to about 76% of the total lifecycle cost of the air compressor with the purchase price representing only 12% (Figure 1). Even a 5% change in operating efficiency can equal an $18,500 difference over 10 years.

Figure 1. Over 10 years, the operating costs of an air compressor is mostly energy.

Data sheets can help

Figure 2. Typical CAGI Data sheet for a fixed speed, lubricated screw compressor.

When making decisions about energy efficiency, we need some accurate information to plug into power cost formulas. In past years, there has been a jumble of information based on a variety of test conditions. Fortunately, there is now a wealth of information about compressor and air dryer energy consumption available within the CAGI Compressor Data Sheets published by participating manufacturers.

Figure 2 shows an example data sheet for a selected 100-hp, 125-psi, air-cooled, fixed-speed, lubricated rotary screw air compressor. Using it and others you can make and compare cost projections on the electrical needs of fully loaded operation between different makes and models of compressors Table 1 shows a summary of comparisons of other compressors, including more efficient compressors from the same companies (shown as Opt 2).You can see the numbers vary, but the common ground is the specific power number. Specific power is like a gas mileage rating for compressors and shows the ratio of the total package power input kW of a complete compressor package, including cooling fans, pumps, and other electrical loads for every 100 cfm of output, not just the break horsepower on the screw inlet shaft.

Compare carefully

It should be noted that this table is not an apples-to-apples comparison. Two of the manufacturers have chosen to test their compressors at 115 psi, and this lower pressure results in lower specific power numbers by about 5%. To compare fairly, the numbers would have to be adjusted for a common pressure by about 1% for every 2 psi increase in pressure. The manufacturer should be consulted to provide assistance finding out the actual package kW consumption at the pressure you choose to run your system. Besides lower kW consumption for fixed-speed compressors, this lower pressure allows most VSD-controlled machines to produce more cfm output through an automatically programmed increase in maximum speed.

Table 1. Specific Power can range for different choices (various selected CAGI members).

An additional item has been added for interest in Table 1 as Item E, a 100-psi-rated compressor. This shows the potential savings in specific power gained if you operated your air compressor at 100 psi, rather than 125 psi. The resulting 9.5% reduction would reduce the lifecycle power costs significantly. Unregulated compressed air loads reduce flow at these lower pressures resulting in even more savings.

Part loads

If the compressor in question is operating at part load in load/unload trim duty, a different kind of calculation is required that takes the effective storage receiver capacity of the system into account. Figure 3 shows the relative power consumed by a lubricated screw compressor at various part loads and system storage sizes. These are the characteristic of lubricated screw compressors with a 10-psi wide pressure band and 40-sec blow down time and can vary with different conditions.

Figure 3. Part Load Performance of typical lubricated screw compressors, load/unload control.