Although it is often viewed as an essentially free resource, compressed air (CA) is anything but free. In fact, in many industrial plants, air compressors consume more energy than any other single end use. And once the air is compressed to the desired pressure, it often has to be dried and cooled before it is sent through the distribution system to the end use, requiring even more energy (Figure 1). According to the U.S. Department of Energy (DOE), CA accounts for 10 percent of industrial electricity consumption.

Obviously, CA can be quite energy-intensive and costly. Fortunately, in most industrial facilities there are many opportunities to improve CA system efficiency with rapid paybacks. A DOE study found that about 15 percent of CA system usage can be saved with simple paybacks of less than 2 years in small to midsize industrial facilities, and that, in larger facilities, these savings could range from 30 to 60 percent of current system usage. Although a comprehensive optimization of your CA system usually requires the services of a highly trained professional, there are a number of very cost-effective steps you can take today, without the aid of an expert, to make your CA system more efficient. These steps include reducing system pressure to the minimum necessary to get the job done, identifying and fixing leaks, eliminating inappropriate uses of CA, and improving system control.

The concepts presented briefly here (and many more) are discussed in far greater detail in Improving Compressed Air System Performance: A Sourcebook for Industry, published by the DOE. See the section on Resources for Further Information.

It is not unusual to find CA systems operating at higher pressures than necessary. This can happen for a variety of reasons, such as inadequate information about end-use requirements, changes to production demands over time, or suboptimal air storage capacity. Operating your CA system above the minimum necessary pressure is wasteful for three reasons. First, it takes more compressor energy to pump air to higher pressure. One rule of thumb is that for systems operating at about 100 pounds per square inch (psi), every increase of 2 psi raises input power to the compressor by 1 percent at full flow. Second, for unregulated CA end uses, the volume of air consumed is highly dependent upon the air pressure—the higher the pressure, the more air consumed, so CA requirements and energy costs can often be reduced significantly without affecting performance simply by reducing system pressure to the minimum necessary. Finally, leaks can be considered another unregulated end use, and the higher the system pressure, the more air is driven through the leaks. For example, at 80 psi, about 21.4 cubic feet per minute (cfm) will flow through a leak with a diameter of 1/8 inch. At 100 psi, that flow would increase by over 20 percent to 26 cfm, wasting thousands of dollars annually.

Therefore, reducing system pressure to the minimum that is absolutely necessary is frequently the most cost-effective, quickest payback opportunity for energy savings in a CA system, and it should be your first step in system optimization. Reducing pressure without affecting production processes requires that you be aware of the minimum pressure at which each CA end use can operate. If you find that none of the CA end uses in your plant require the pressure being delivered, you can save energy at almost no cost by dialing back compressor discharge pressure in small increments to the minimum that maintains satisfactory equipment performance. Note that some of the other suggestions provided here, such as eliminating leaks, can result in higher air pressures at the end use, so it's important to measure and record air pressure at each end use both before and after you make any improvements to the system, because you may be able to gain even greater energy savings by further reducing system pressure.

Often though, it's not just a matter of turning down the discharge pressure setpoint. Sometimes, elevated pressures are maintained to compensate for unacceptable pressure drops that would otherwise occur due to large, intermittent CA consumers on the same distribution system. In such cases, adding secondary storage capacity at or near the point of use is an inexpensive solution to smooth out systemwide pressure fluctuations. Section 5.F of the DOE Sourcebook describes how to calculate the volume of secondary storage needed for a particular application.

Another common reason CA systems operate at unnecessarily high pressure is that one or more end uses require this pressure. In such cases, it can often be profitable to install either a booster compressor with local storage or a separate compressor and air distribution system dedicated to these high-pressure end uses. Doing so allows the rest of the plant to operate at lower pressure and can result in dramatic energy savings.

Finally, excessive pressure drop through the components of the air treatment and distribution system can necessitate higher compressor discharge pressure to ensure that the pressure will be adequate by the time it gets to the end use. In a well-designed and -maintained CA system, the pressure at the end use should be at least 90 percent of compressor discharge pressure. Virtually every component of the CA system downstream of the compressor can be a source of pressure drop, such as dryers and filters on the supply side and undersized distribution piping, equipment hoses, disconnect couplings, filters, regulators, or lubricators on the demand side. If you find pressure at the end use significantly below 90 percent of compressor discharge, work upstream one component at a time to identify where the major pressure drops are occurring. When specifying or replacing this equipment, always ask manufacturers to provide information on pressure drop at the maximum anticipated flow rate and select equipment that minimizes pressure drop. And be sure to clean or replace filter elements regularly.

Air leaks can waste a considerable fraction of a compressor's output. According to the DOE Sourcebook, it's not unusual for leaks to consume 20 to 30 percent of compressor output, which can add up to thousands of dollars per year in unnecessary electricity costs. Moreover, leaks reduce system pressure, which can cause air tools to operate inefficiently, which in turn could affect production. Finally, all the air that leaks out must be replaced by the compressor, causing compressors to run longer and reducing their lifetime.

As you can see, there are many reasons to find and eliminate leaks. The first step in doing so is to estimate the amount of air leakage. There are two straightforward ways to do this, but both methods must be done while production is shut down. For systems that have on/off or load/unload controls, allow the compressor to bring the system up to the pressure setpoint. Then allow the CA system to run through several cycles (more cycles will give you greater accuracy) as the pressure drops due to leakage and the compressor kicks on or loads up to bring pressure back to the setpoint. On each cycle, record the amount of time that the compressor is on/loaded. The ratio of the on/loaded time to the total time of the test is the leakage fraction.

For systems that have other types of capacity control, leakage can be estimated by noting the time it takes for system pressure to drop from its setpoint to one-half of setpoint pressure with the compressor shut off and no production activity. The leakage rate (L) in cfm is then determined by

L = PS ÷ (2T) x V x 14.7 x 1.25

where PS is setpoint pressure; T is the time in minutes it takes for the system to drop to PS ÷ 2; and V is the total system volume, including the volume of all storage and the distribution piping. By comparing this leakage rate to the total volume of CA delivered, you can estimate the fraction of CA costs that are wasted by leaks.

The next step is to find and eliminate the leaks. Your goal should be to bring the leakage rate down well below 10 percent. The easiest and fastest way to do this is with a tool called an ultrasonic leak detector (available from a number of manufacturers), but many leaks are audible and easily located, especially during nonproduction periods. Once you've found a leak, eliminating it is often just a matter of tightening the connection, but sometimes it will be necessary to open a joint, clean the threads, and apply proper thread sealant. In some cases you may find that you need to remove and replace faulty equipment. Finally, ensure that the air supply to all equipment is shut off when that equipment is not in use. Solenoid valves are available to automate that process.

Once you've completed your leak hunt and eliminated as many leaks as possible, reevaluate the leakage rate to determine the impact you've had on the system and to estimate the resulting savings. Also, be sure to remeasure the system pressure during normal plant operation—you may find that you are now able to further reduce the compressor discharge setpoint and gain additional savings.

Compressed air is used in a wide variety of applications that could be performed more efficiently (and in (many cases more effectively) in other ways. Table 1 lists several common misapplications of compressed air, along with better alternatives. You'll find a more comprehensive discussion of this issue in the DOE Sourcebook.

Although upfront capital investment will be necessary to eliminate some inappropriate applications, performing them with compressed air is so inefficient that the required investment will usually be repaid quickly. Eliminating inappropriate uses will reduce CA consumption and may allow you to shut down one or more compressors entirely. This can save capital in the future as well—should expanded production require additional compressor capacity, you'll have it ready and waiting.

Optimizing CA system control normally requires careful analysis by a trained professional. However, there is one symptom of a poorly controlled CA system that even the untrained eye can identify. All air compressors operate most efficiently when running at full load. Unfortunately, it is a rare application indeed for which CA demand precisely matches the full load output of its air compressor. Much more frequently, some type of control system operates one or more compressors to match CA supply with demand. There are numerous types of control technologies and strategies, some of which are specific to the type of compressor in use. But regardless of compressor type, system efficiency will be optimized by operating as many compressors as necessary at full load and operating only one "trim" compressor with good part-load efficiency to follow the load. If you find that you've got two or more compressors feeding into the same CA system that are simultaneously running under partial load, it's probably time to revamp your control strategy.

As noted above, comprehensive optimization of CA systems usually requires the services of a CA expert. Because most CA systems have opportunities for cost-effective savings, hiring a trained professional to perform an audit of your CA system is a good idea. But whether you're looking to hire an expert or planning to work on your CA system yourself, you'll find the following resources quite helpful.

• U.S. Department of Energy Industrial Technologies Program web site: www1.eere.energy.gov/industry/bestpractices/compressed_air.html. This web site provides a variety of resources, including tip sheets, guidelines for selecting a CA service provider, free software for assessing your existing CA system and estimating potential savings from efficiency measures, and Improving Compressed Air System Performance: A Sourcebook for Industry.
• Compressed Air Challenge (CAC) web site: www.compressedairchallenge.org. The CAC is a voluntary collaboration of industrial users; manufacturers, distributors, and their associations; consultants; state research and development agencies; energy-efficiency organizations; and utilities. On its web site, you can learn about upcoming CAC training opportunities and download documents covering a wide variety of CA concepts as well as case studies that demonstrate these concepts. Of particular note, the CAC-produced Best Practices for Compressed Air Systems manual can be ordered here. This comprehensive and easily understandable book compiles information on virtually all aspects of CA system maintenance.