Published by EPRI Industrial Program - Environment & Energy Management No. 1, 1991

Capturing Wasted Air

Compressed air is an expensive utility. Al- though many people believe that compressed air systemlosses (e.g., airleaksandequipment inefficiencies) are economically insignificant and unavoidable, a properly designed and main- tained compressed air system could save a company thousands of dollars per year. For example, the adjacent case study identifies one instance where consultants found that a paper manufacturing plant could save over $230,000 in annual energy costs through a one-time investment of about $41,000. As shown in Table 1, even a single small air leak can amount to significant annual costs.

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Compressed air systems can be found in most industries, including automotive manufactur- ing, pulp and paper, chemical processing, com- munication, food and beverage, shipyards, coatings manufacturing, and textiles. Com- pressed air is typically used to provide air to a process application or to provide power to pneumatic (i.e., air-driven) equipment. Ex- amples of process applications include com-

Case Study: Paper Manufacturing Plant, North Carolina Auditor Finds Over $233,000 in Annual Energy Savings

In July 1990, the compressed air system of a large specialty paper manufacturing plant was audited by a consultant. The purpose of the audit was to determine the condition of the compressed air equipment and distribution system, and to locate system leaks.

The plantoperates7daysa week, 365daysayear, and hasl,700employees. The production and warehouse areas occupy approximately 1.6 million square feet. Eleven compressors generate over 7,600 scfm of compressed air to be used throughout the plant for instrumentation, atomizers, cleaning rolls, air motors, and filter bags.

The audit identified five action areas that could substantially reduce the plant’s overall energy cost, energy use, and peak load demand. Asshown below, the consultant determined that a one-time investment of about $41,000 in equipment and maintenance would reduce the compressed air system’s demand by about 530 kilowatts, resulting in an annual savings of over $230,000.

Estimated Demand Resulting Annual Correction Reduction Energy Savings

Action Area costs ($) (kW) (kWh) ($1

170,500 Fix air leaks 30,500 389.3 3,410,300

Correct equipment 3,400 77.5 678,900 33,900 misuse

Match equipment to plant needs 5,900 18.2 159,400 8,000

Shut down unneeded equipment 0 3.3 28,900 1,400

Reduce air pressure 1,000 44.0 385,400 19,300

TOTAL $40,800 532.3 kW 4,662,900 kWh $233,100

Note: Energy savings were calculated using an energy rate of $0.05 per kilowatt-hour and assuming 8,760 operating hours per year. These savings do not reflect the potential additional savings due to reduced peak demand charges as a result of improved equipment efficiency and control.

Source: Scales Air Compressor Corporation, Carle Place, New York

bustion, polymerization, quenching, instrumen- tation, and drying. As a power service, it is used for a variety of automated and manual machinery, such as pumps, jack hammers, air guns, drills, and impact wrenches.

Industry sources estimate that the total con- nected capacity of U.S. compressed air sys-

tems exceeds 17 million horsepower. Of this total, an estimated 20 to 35% of air compres- sion energy costs could be saved. In other words, assuming a rate of $0.05 per kilowatt-hour, U.S. industry could save from $1.1 billion to $1.9 billion annually on its energy costs by maintaining energy-ef€icient compressed air systems,

Techcommentary /No. 1 1

System Components As depicted in Figure 1, the typical compressed air system is comprised of six principal compq- nents. Each component helps to ensure that compressed air will arrive clean, dry, and free of pressure fluctuations at its point of use. If any component is working inefficiently, the system’s performance suffers and operating costs rise.

The air compressor creates pressurized air within a distribution system through either the positive displacement or dynamic method. In the positive displacement method, successive volumes of air enter a cylinder, where the vol- ume is reduced either by a piston or from the intermeshing of two mechanical lobes. The dynamic method uses a mechanical force, such as a fan or impeller, to increase air velocity and force it (under pressure) through the distribu- tion system.

The aftercooler and moisture collector re- move as much moisture as possible from the compressed air by reducing air temperature. As the temperature is reduced, water vapor is con- densed, collected, and drained from the system.

The air receiver has three functions. First, it should continue to cool the air so more moisture can be removed. Second, it acts as a “pulsation damper” to eliminate the pressure pulses that reciprocating compressors generate. Third, it provides storage capacity to allow a small com- pressor to meet occasional high demands for compressed air.

The air dryer helps to eliminate any remaining moisture in the compressed air by using either a a refrigerated condenser or a desiccant. Refrig- erated condensers cool the air to condense water vapors into a liquid that is then drained from the system. Desiccants are powders or gels that remove water by absorbing it.

The air distribution system links the compo- nents of the compressed air system. Although the specific configuration depends on the needs of the plant, the system frequently consists of an extended network of main lines, branch lines, valves, and air hoses.

The tool stations connect the dried compressed air to pneumatic tools. The station is comprised of a shut-off valve, filter and moisture separa- tor, pressure regulator, lubricator, and quick-change shut-off coupler body. If the tool uses a large volume of air suddenly and inter- mittently, a storage receiver is installed to pre- vent pressure fluctuations in the line.

In addition to system air leaks, each component can contribute to air and energy losses associ- ated with an industrial compressed air system. Table 2 lists common problems that can result in substantially increased energy costs.

Atmospheric Air

T I LEGEND

0 GAUGE 8 AUTOMATIC DRAIN TRAP

SAFETY VALVE

AIR COMPRESSOR Adapted From Scales Air Compressor Corporation, Carle P/ace, New York

Figure 1 : Compressed Air System Components

2 Techcommentary /No. 1

Steps to an Energy-Efficient Compressed Air System An appropriate compressed air system man- 0 agement program can help a plant minimize its energy losses. While the particular program needed depends upon the specific plant and its existing practices, seven key steps typically are required.

1 . Establish a regular maintenance pro- gram. Follow the manufacturer’s recom- mended inspection, maintenance, and service procedures for each system component. If leaks and system misuses are found in a timely fashion, the resulting energy losses can be minimized.

a. Educate system operators on the need to maintain energy-efficient compressed air systems. As the day-to-day caretakers of the system, system operators are frequently the first to know if a leak has developed. If they believe that leaks are unimportant, however, they will be less likely to fix the problem in a timely fashion. Plant personnel, particularly system operators, should be educated on the cost of compressed air, techniques to minimize air losses, and the importance of identifying and correcting leaks and system misuses early.

5 Plant air compressors operating at ltP0 psig should deliver a minimum of 4.5 scfm per input hp. For example, a 150 hp compressor should deliver 675 scfm of usable air at 100 psig. A W o w Energy LM., North Vancouver, British Columbia

8. Conduct a detailed audit of the com- pressed air system. Whether an audit is conducted in-house or by an independent audi- tor, a company should know the impact its compressed air system has on its overall pro- duction costs. An audit will show the system’s annual operating cost as well as.the additional costs contributed by leaks and system misuses. To compute the annual costs of an inefficient system, an auditor measures air flow leakage in standard cubic feet per minute (scfm) in order to estimate the lost horsepower (hp). Costs are then computed based on the lost hp. Besides computing costs, an independent au- ditor should also help detect system misuses and air leaks. Most leaks can be found audibly; however, for hard-to-reach places, such as underground air mains, ultrasonic devices are often used. Table 3 lists the general services an independent auditor usually offers.

4. Install control devices. Many systems contain several compressors, some of which may not be needed during periods of low demand. Control devices help ensure that unneeded compressors are tumed off. For instance, automatic start-stop controls allow the compressor to run on demand instead of idling. Constant speed controls tell the com- pressor when to “unload” (stop providing air but continue running) and when to compress air again. Dual control, which can be auto- mated, combines automatic start-stop and con- stant speed controls. Automatic sequencers are programmed to run only the compressors required to meet compressed air supply needs. Without them, compressors usually run con- tinuously because of personnel and time con- straints. Controls are not limited to compres- sors. Regenerative dryers, for example, can be installed with dew point sensors rather than time cycle controls.

5. Buy energy-efficient compressed air system equipment matched to plant needs. Some plants operate equipment incompatible with their needs. For instance, if a dryer is the wrong size or uses an inappropriate method @e., a desiccant instead of a refrigerated con- denser), pressure drop and energy consump- tion may be excessive and an acceptable dew point may not be met. Furthermore, some dryer and compressor designs are 10 to 15% more efficient than others.

8. Improve the system’s distribution lay- out. When designing a system layout, particu- lar emphasis should be placed on the location of the air compressors and related accessories. The best location is where the humidity and temperature are the lowest. Generally speak- ing, as long as the compressor’s inlet will not be over a moist area, the north side of a plant is often the best location for the compressor and receiver because that side offers shaded, cooler intake air. In addition, possible future de- mands and tie-ins to the existing distribution system need to be carefully considered. The

Installaton of a variable frequency speed control on a compressor, combined with the elimination of system air leaks, reduced total annual air compressor energy usage by 20% (2.159 million kilowatt-hours) at a Georgia-Pacijic Corporation integrated pulp and paper mill.

U.S. Dept. of Energy, Bonneville Power Administration, DOEJBP-92222-1, March 1988

distribution system should not be placed un- derground, where the leaks are harder to de- tect, and its length should be minimized to help prevent leaks from forming.

7. Reexamine the use of compressed air. For many industrial tasks, hydraulic or electric power might prove to be a more cost-effective power source than compressed air.

Some U.S. and Canadian govemment agen- cies actively encourage the use of energy-efficient compressed air systems. The State of Utah and the Energy Division of the State of North Carolina have funded energy management programs that include workshops that teach plant engineers, maintenance super- intendents, and mechanics the practical steps of compressed air system management. Some Canadian utilities, such as B.C. Hydro and Ontario Hydro, fund independent auditors to help improve the efficiency of a plant air compressor system. In addition, B.C. Hydro offers rebates to purchasers of more efficient compressors (those exceeding 4.5 scfm/hp) and provides free leak testing equipment if an audited plant agrees to test monthly and report the results to B.C. Hydro.

TechCommentary /No. 1 3

Advantages of an Energy=Eff icient Compressed Air System A plant that implements a new management program for its compressed air system can reduce its operating costs while improving the system’s performance. Table 4 indicates the impact that air leaks alone can have on plant operating costs for a variety of industries.

Lower energy costs. An energy-efficient compressed air system can minimize substan- tial energy losses occurring through air leaks and system misuses.

Increased output. A maintenance program helps ensure that compressors are delivering the air supply at their designed capacity level. System misuses force some plants to purchase extra compressors to compensate for losses.

Improved quality of air. An efficient system ensures that when the compressed air reaches its point of usage, it will arrive clean, dry, and free of pressure fluctuations. Contaminated compressed air performs poorly, increases equipment maintenance cost, and decreases service life.

Improved control and convenience. Control devices, such as automatic sequencers, elimi- nate the plant’s reliance on manual labor to manage compressed air.

Reduced capital and repair costs. Without an efficient system, premature repair and re- placement costs may be incurred because of increased wear and tear on equipment. Fur- thermore, a well managed system might elimi- nate the need to purchase extra compressors that would compensate for the existing equip- ment working inefficiently.

Recaptured lost heat. Plants can supplement their heating needs by utilizing compression heat generated by compressors.

Implementing a compressed air management program will also benefit the environment. Energy-efficient measures and programs, in- cluding maintenance programs for compressed air systems, help conserve fuel used to gener- ate electricity, such as coal and gas, and reduce emissions by using less energy.

Summary Many people mistakenly believe that com- pressed air is free or at least relatively inexpen- sive when compared with other plant utilities. In actuality, compressed air can be the most

expensive utility in a plant, particularly if the compressed air system is operating ineffi- ciently. Most air system losses can be avoided by implementing a compressed air system management program. By incorporating such a program, users of compressed air systems can obtain significant operating savings while simultaneously improving system efficiencv and service life. k

The information presented in this Techcommentary is intended to provide a ba- sic understanding of the need for a compressed air system management program. For more information, contact your electric utility mar- keting representative, a consultant specializ- ing in air system audits, or a compressed air equipment supplier.

Basic funding for this TechCommentury isprovidedby the Electric Power Research Institute (EPRI), a non-profit institute that conducts applications development on behalf of the United States electric utility industry. TechCommentury is one way the EPRI Industrial Program assists in communicating information concerning energy-efficient, electric-based technologies.

This issue of TechCommentury was written and produced by the National Environmental Technology Applications Corporation (NETAC) at the University of Pittsburgh. NETAC is a non-profit corporation with the mission of accelerating the commercialization and use of priority environmental technologies.

Tom Beck of AnditAir Inc.; Richard Okrasa, P.E., of Ontario Hydro; Lanny Pastemak of Airflow Energy Ltd.; William Scales, P.E., of Scales Air Compressor Corpora- tion;andCraigWohlgemuthofBonnevillePowerAdmin- istration made valuable contributions to this issue.

Copyright 01991 Electric Power Research Institute Palo Alto, California

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