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1 energydesignresources On the supply side, typical opportunities include installation of a variable speed drive (VSD) compressor, a more efficient dryer matched to the quality and quantity of compressed air demand, the addition of compressed air storage, and modifications to or implementation of more effective compressor and system controls. The optimization goal for the supply side is to operate the compressors at their highest efficiency point. This goal may be achieved by operating the minimum number of compressors at full load and at the lowest possible pressure range, and using a VFD-controlled compressor for trimming. On the demand side, opportunities include reducing compressed air usage through selecting appropriate equipment and practices, impledesign brief Compressed buildingair design Summary Compressed air commonly called the fourth utility is in high demand in most industrial facilities. Despite its widespread application, however, up to two-thirds of the compressed air systems in operation have either an obvious problem that affects production or a hidden problem that drives compressed air production costs higher [3]. In some cases, according to the U.S. Department of Energy, compressed air generation may account for as much as 30% of the total electricity consumed by the facility [2]. Compressed air is one of the most expensive uses of power in an industrial facility. Opportunities for increasing the overall efficiency of compressed air systems occur on both the supply side and the demand side of the systems. conte nts Summary 1 Introduction 2 Supply Side System Components 6 Demand Side System Components 20 Optimization of Existing Systems 22 Best Practices Design Methods for a New System 31 Notes 33 For More Information 34

2 menting effective leak repair programs, managing pressure swings without additional compressor capacity, and lowering system pressure when appropriate. Optimizing a compressed air system has the potential to generate energy efficiency improvements in the range of 20% to 50% [1]. By incorporating Best Practices design methods into the front-end development of new construction or major retrofit projects, end users of compressed air can achieve dramatically lower electricity consumption, along with improved reliability and stability of the compressed air system, monetary savings from reduced electricity consumption and reduced wear and tear on the equipment, and reduced emissions of greenhouse gases associated with electricity generation. Introduction in Industrial Facilities Compressed air systems are an integral part of many industrial processes up to 90% of industrial facilities utilize a compressed air system. Compressed air is a valuable source of power for hand-held tools and for equipment used for pressurizing, atomizing, agitating, and mixing applications. The convenience and relative safety of using compressed air for motive force have made it as commonplace as electricity, gas, and water systems in industrial facilities, which is why industrial facility engineers and operating staff often refer to compressed air as the fourth utility in the plant. Compressed air provides end users with a practical method to convert electrical power into mechanical power, enabling end users to store, transmit, and ultimately deliver mechanical power to the required point of use. Examples of applications include actuating air cylinders, operating hand tools (e.g., hammers, rotary guns), blowing molten plastics into molds, mixing, and powering automatic conveying, sorting, and packaging equipment. Pneumatic hand tools are typically smaller, lighter, and more maneuverable than hand tools driven by electric motors; they deliver power smoothly and are not easily damaged by overloading. The industrial sector uses compressed air in a wide range of applications, and plants frequently have multiple compressor systems with extensive distribution systems to provide compressed air throughout the facility. In addition to the examples of applications listed in Table 1 [1], some manupage 2

3 facturers utilize compressed air for combustion in boilers firing liquid fuels and for process operations, such as painting, oxidation, fractionation, cryogenics, refrigeration, filtration, dehydration, and aeration. Table 1: Typical Uses of in Industrial/Manufacturing Sectors Apparel Industry Automotive Chemicals Food Furniture General Manufacturing Lumber & Wood Metals and Fabrication Petroleum Primary Metals Pulp & Paper Examples of Uses Conveying, clamping, tool powering, controls and actuators, automated equipment Tool powering, stamping, controls and actuators, conveying Conveying, controls and actuators Dehydration, bottling, controls and actuators, conveying, spraying, coatings, cleaning, vacuum packing Air piston powering, tool powering, clamping, spraying, controls and actuators Clamping, stamping, tool powering and cleaning, controls and actuators Sawing, hoisting, clamping, pressure treatment, controls and actuators Assembly station powering, tool powering, controls and actuators, injection molding, spraying Process gas compressing, controls and actuators Vacuum melting, controls and actuators, hoisting Conveying, controls and actuators Rubber & Plastics Tool powering, clamping, controls and actuators, forming, mold press powering, injection molding Stone, Clay and Glass Textiles Conveying, blending, mixing, controls and actuators, glass blowing and molding, cooling Agitating liquids, clamping, conveying, automated equipment, controls and actuators, loom jet weaving, spinning, texturizing Compressed air is also important in non-manufacturing sectors, including transportation, construction, mining, agriculture, recreation, and service industries. Examples of applications in these sectors are shown in Table 2 [1]. Current Efficiency and Cost The use of compressed air for performing mechanical or process-related work is inherently inefficient; the overall efficiency of a typical compressed air system can be as low as 10% to 15% [2]. For example, to operate a 1-horsepower (hp) air motor at 100 pounds per square inch gauge (psig), approximately 7 to 8 hp of electrical power must be supplied to the air compressor. page 3

4 Sector Agriculture Table 2: Typical Uses of in Non-Manufacturing Sectors Examples of Uses Farm equipment, materials handling, crop spraying, dairy machines Mining Power Generation Recreation Service Industries Transportation Pneumatic tools, hoists, pumps, controls and actuators Starting gas turbines, automatic control, emissions controls Amusement parks air brakes Golf courses seeding, fertilizing, sprinkler systems Hotels elevators, sewage disposal Ski resorts snow making Theaters projector cleaning Underwater exploration air tanks Pneumatic tools, hoists, air brake systems, garment pressing machines, hospital respirator systems, climate control Pneumatic tools, hoists, air brake systems Waste Water Treatment Vacuum filters, conveying Generating compressed air is expensive. A survey conducted by the U.S. Department of Energy (DOE) indicated that for a typical industrial facility, approximately 10% of all of the electricity used was attributable to generating compressed air, and that in some cases compressed air generation may have accounted for as much as 30% of the total electricity consumed by a facility. As illustrated in Figure 1, the cost of the electricity is approximately 76% of the total cost of owning and operating a compressed air system [2]. The total cost of producing compressed air at 100 psig has been estimated to be in the range of 18 to 32 cents per 1,000 cubic feet [4]. Compressed air systems consume an estimated 90 billion kwh per year in the United States, accounting for approximately $1.5 billion in energy costs, and electricity production for compressed air is responsible for 0.5% of total greenhouse gas generation in the United States [3]. However, the operation of these systems is often overlooked as a contribution to the overall production cost. Once installed, a typical compressed air system is largely neglected as long as it continues to satisfy production needs; occasional failures in meeting production needs may result in additional inefficiencies as piecemeal expansion without appropriate analysis and optimization of the overall system performance takes place. page 4

5 Figure 1: Typical Lifetime Costs Source: Energy Tips Tip Sheet #1, August [2] Energy Savings Potential and Benefits Despite their widespread application, up to two-thirds of the compressed air systems in operation have some sort of problems, from an obvious air leak to an improper sequencing control that keeps all compressors running continuously without consideration of demand. These problems may stem from a wide range of causes, including: Improper compressor types and/or sizes Inappropriate controls Improper cleanup equipment (such as filters, dryers, regulators) Unsound installation and operation practices Poor maintenance practices Poorly optimized systems waste energy, increase operating costs, and reduce reliability. According to the DOE s Industrial Technologies Program, optimizing compressed air systems could improve their energy efficiency by 20% to 50% [3]. In California, the recently completed California Energy Efficiency Potential Study estimates that implementing high-efficiency compressed air systems at industrial facilities in the state could generate annual savings of 1,000 MWh and 115 MW [5]. In addition to the energy savings potentials described above, optimizing compressed air systems can lead to reduced greenhouse gas emissions from electricity generation and improved reliability. page 5

6 Key Elements of Systems A compressed air system consists of a supply side and a demand side. The supply side comprises compressors, air treatment equipment, and primary compressed air storage. It is responsible for providing an adequate amount of clean, dry air at the lowest operating pressure to meet the end-use requirements. The demand side consists of the distribution system and the end-use equipment. The objective of the demand side is to distribute and utilize the compressed air as efficiently as possible while eliminating, or at least minimizing, wasted air. Given the continuous interaction of supply-side and demand-side elements, it is essential that owners of compressed air systems adopt an integrated (whole system) approach to design, operation, and maintenance. Figure 2 illustrates the components of a typical industrial compressed air system. Supply Side System Components The key supply-side components are the compressors and compressor controls, air treatment equipment, and primary compressed air storage. Compressors Compressors fall under two general types: positive displacement compressors, and dynamic compressors. Positive displacement machines trap a volume of air in a compression chamber and mechanically compress the air by reducing the volume of the chamber. Dynamic machines change the velocity of a continuous flow of gas into pressure by means of impellers rotating at high speed, and downstream diffusers. The pressure and flow relationship in a dynamic machine is influenced by the shape of the rotor and the diffuser volutes. Figure 3 shows the family tree of positive displacement and dynamic compressor types. Compressor efficiency is expressed as the ratio of the input power to the production rate of compressed air at a specific pressure. The industry norm for comparing compressor efficiency is expressed in terms of brake horsepower per actual cubic feet per minute (bhp/100 ACFM) at a compressor discharge pressure of 100 psig. Rated capacities, pressures, and package power are generally available in the manufacturer data sheets for page 6

7 Figure 2: Components of a Typical System Source: Improving System Performance, a Sourcebook for Industry, November [1] new compressors; for older compressors, generic ratings may be available in the AIRMaster+ program [7] and on utility websites [8]. Compressors typically found at industrial facilities include rotary and reciprocating compressors (both are positive displacement machines), and centrifugal compressors (dynamic machines). A brief discussion of each of these three common compressor types follows. Rotary Screw Compressors These are by far the most common compressors found at or specified for industrial facilities. Figure 4 is a drawing of a direct drive rotary screw compage 7

8 Figure 3: Components of a Typical System Source: Improving System Performance, a Sourcebook for Industry, November [1] pressor showing the two intermeshing rotors: a male rotor with helical cut lobes and a female rotor with helical cut grooves mounted in a stator case. As the two rotors turn and mesh together, a fixed volume of air is drawn from the inlet port on the left side of the compressor shown. As the lobe and groove begin to mesh, the trapped gas is compressed in the decreasing volume between the two rotors; the compressed charge of air is forced along the two rotors until it is discharged at a higher pressure at the right side port on the drawing. A fixed volume of air is drawn into the two rotors for a constant rotor speed, so the discharge pressure rises or falls as compressed air demand decreases or increases. Rotary screw compressors are available in lubricant-injected and lubricantfree designs. Lubricant-injected compressors use a specialized fluid that is injected into the inlet air stream to remove some of the heat of compression, form a seal between the rotors, and lubricate the rotors as they mesh along their length. The oil is removed from the compressed air after the discharge port, first by gravity as the air flow slows, and later by coalescing filters that remove all but a few parts per million of oil that remains page 8

9 Figure 4: Cutaway View of a Direct Drive Rotary Screw Compressor Source: Quincy Compressor aerosolized. Typical sizes for these workhorse compressors range from 3 to 900 hp, although the more common sizes are less than 200 hp. Capacities for these compressors are 8 to 5,000 CFM at pressures from 50 psig up to 250 psig, with higher pressures possible for multi-stage compressors. Specific power ratings for lubricant-injected rotary screw compressors cover a wide range of values depending on both the operating pressure of the system and the size of the compressor. Smaller compressors with load/unload type capacity controls require as much as 37 kw per 100 ACFM for a 5 hp compressor discharging at 175 psig and 21 kw per 100 ACFM for a 20 hp compressor discharging at 100 psig. Larger industrial compressors of 100 to 200 hp sizes have a lower specific power, and are commonly rated at between 18 to 25 kw per 100 ACFM for similar discharge conditions. Compressors larger than 200 hp typically have similar specific power ratings. Some industrial applications require oil-free air. Lubricant-free compressors do not use lubricants in the compressed air; bearings and timing gears are lubricated, but they are kept isolated from the compression chamber. In a lubricant-free rotary compressor, external timing gears prevent the rotors from touching. However, because the heat of compression is not removed by a lubricant and the sealing between rotors is less efficient, these compressors are often operating at higher rotor speeds, and often have two-stage designs with an intercooler between the two stages to remove the heat of compression. The resulting compressed air is lubricant free and safe for use in critical applications. These compressor types are page 9

10 available in a wide range of sizes, from 25 hp up to approximately 4,000 hp for capacities of 90 to 20,000 CFM at pressures of 50 to 150 psig. Reciprocating Compressors Reciprocating compressors are similar to bicycle pumps, in that a piston attached to a crankshaft moves up and down in a cylinder, compressing the air as the volume of the compression chamber is reduced. Single-acting reciprocating compressors provide compression of the gas charge in one direction only, whereas double-acting compressors provide compression in both directions of the piston travel. Large multi-stage, water-cooled, double-acting industrial compressors have the highest efficiency, but are loud, expensive to maintain and operate, and are becoming less common in industrial facilities. Increasingly they are being replaced with rotary screw or centrifugal compressors. Reciprocating compressors are available in a wide range of sizes, from fractional and single digit horsepower, aircooled single-acting hobby compressors up to 25 hp, and double acting, typically water-cooled compressors in sizes up to approximately 500 hp. With compressor efficiency ranges as high as 13.2 to15.8 kw per 100 ACFM for 400 hp double-acting, multi-stage compressors at discharge pressures of 80 to 125 psig respectively, they are 5% to 10% more efficient than rotary screw compressors at similar pressure ratings, but with significant trade-offs for maintenance and installation costs [8]. Centrifugal Compressors Centrifugal compressors are typically large capacity, water-cooled machines designed to satisfy high flow demands at relatively stable flow rates. Pressure in a centrifugal compressor is achieved partly by the rapidly rotating impeller, spinning to over 50,000 RPM (approximately 50% of pressure is generated in the impeller), and partly by converting the velocity of the air flow to pressure after the impeller by means of a diffuser and volute. These machines are usually multi-stage, with two to four stages being the most common configuration for compressors at 100 to 150 psig ratings. Between stages, the compressed air is cooled to ambient temperatures in water-cooled intercoolers prior to recompression in the next stage. Centrifugal compressors are available in a wide range of sizes, with capacities of 300 to more than 100,000 ACFM at pressures up to 125 psig. page 10

11 With specific power ratings of between 15 and 20 kw per 100 ACFM, they compare favorably with rotary screw and reciprocating compressor efficiencies for applications requiring high flow at relatively stable pressures. An inherent characteristic of a centrifugal compressor is that as pressure in the system decreases, the flow capacity of the compressor increases. The slope of the pressure/flow curve depends on the impeller design, with a steeper curve for higher degrees of backward lean of the impeller blades from the radial position. As rapidly rotating machines, centrifugal compressors are sensitive to imbalances in the impeller and shaft and are usually equipped with monitoring equipment to prevent damage from vibration. Centrifugal compressors are also limited to a minimum flow rate for a condition known as surge; as flow decreases, the compressor discharge pressure increases. Eventually the discharge pressure cannot overcome system pressure and the flow in the machine reverses from discharge to inlet, causing a condition known as surge within the compressor. If the discharge pressure increases past a preset point below the critical pressure that leads to surge, the compressor controls go into blow-off mode and vent the excess compressed air to the atmosphere, or unload to avoid possible equipment damage. Compressor Controls A compressed air system rarely operates at full load continuously. Part load operation is more typical, and is largely influenced by the type of compressor(s) and the control strategy used to vary the capacity of the individual compressors. A single compressor with a steady demand might be equipped with simple on/off controls. With variable compressed air demand, any one of the compressor control strategies shown below might be employed. However, for multi-compressor systems with large swings in flow, careful consideration of the size and type of each compressor and the integration of the individual compressor controls can be the key to minimizing the cost of operating the system. The primary objective of a control system is to shut off any un-needed compressors, and/or delay starting up another compressor until needed. In multi-compressor systems, all operating compressors should be run at full load, except one that will provide trim capacity to the system. page 11

12 Capacity control of individual compressors is accomplished through one of several control strategies. All of the strategies below depend on monitoring the discharge pressure of the compressor to respond to changes in the system demand: Start/Stop Load/Unload Inlet Modulation Variable Displacement Variable Speed Drive Each of these strategies is described in more detail below, along with compressor performance graphs for all but the Start/Stop control strategy, which essentially operates at full load whenever it is active. The performance curves show the compressor control s effect on an individual compressor motor load as the compressor capacity changes; the performance curves are all based on lubricant-injected rotary screw compressors. Start/Stop This simple approach turns on the compressor when pressure drops below a set point, and then turns it off after reaching the higher limit. Demand is met by storage during periods when the compressor is off. This strategy is typically used on smaller motors and compressors, as rapid start/stop cycles on large motors can lead to overheating and failure. Load/Unload The motor runs continuously for a load/unload compressor, or until a pre-set time interval of unloaded operation is completed in order to prevent motor damage in this strategy. Sensing a drop in system pressure, the load cycle starts by fully opening the inlet valve, and closing the sump pressure relief valve; compressed air continues to be produced until the upper pressure set point is reached, at which point the compressor inlet valve is closed, and pressure in the sump is slowly bled off for the unloaded phase of operations. During unloaded periods, the compressor will continue to consume 15% to 35% of full power while delivering no compressed air. The four curves in Figure 5 correspond to system performance for a load/unload compressor equipped with varying amounts of compressed air storage. A load/unload compressor essentially has two operating points, either loaded, or unloaded. A compressor with a large volume of compressed air storage capacity operates closer to the virtual linear line between page 12

13 the load and unload points, while compressors associated with lower levels of storage are represented by the curves that are increasingly bowed upward. The upward bowing represents increased average power demand for a given average flow of compressed air. Figure 5: Performance Curves for Load/unload Control with Varying Levels of Storage Capacity Source: Improving System Performance, a Sourcebook for Industry, November [1] Inlet Modulation Throttling the inlet air flow reduces the output of a compressor to closely match the demand. This strategy cannot be used on lubricant-free rotary screw or reciprocating compressors. For lubricant-injected rotary screw compressors, the modulating range is limited to about 40% of rated capacity; below this threshold the compressor is unloaded. Modulating controls provide good capacity control in the control range, but at a penalty in efficiency at part load operations. Often, rotary screw compressors with modulation control are also equipped with blowdown to relieve pressure in the sump. By relieving the sump pressure, the input power is dramatically reduced. However, similar to a load/unload compressor operation in unload mode, the compressor delivers no compressed air to the system after the sump pressure is relieved while continuing to run at 15% to 35% of full load (see Figure 6). page 13

14 Figure 6: Performance Curves for Inlet Modulation Control With and Without Sump Blowdown Figure 7: Variable Volume Control Performance Curve Source: Improving System Performance, a Sourcebook for Industry, November [1] Source: Improving System Performance, a Sourcebook for Industry, November [1] Variable Displacement Reciprocating compressors can unload cylinders to modulate capacity (see Figure 7). Control strategies include twostep (start/stop or load/unload), three-step (0, 50, 100 percent), or fivestep (0, 25, 50, 75, 100 percent). Variable displacement controls for rotary screw compressors include varying the compression chamber volpage 14

15 ume using sliding or turn valves; the strategy is used in concert with inlet modulation to provide accurate pressure control and improved part load efficiency. Variable Speed Drive Especially suitable for lubricant-injected rotary screw compressors, a VSD slows the rotor speed and air flow through a compressor while retaining the compression ratio of a constant speed compressor when demand drops. Similar in closely matching demand, but superior in performance to inlet modulation control for a constant speed compressor, a VSD extends the range of efficient part load operations to low capacity levels with a near linear relationship between power and capacity, with accurate control of pressure (+/- 1 psig) and matching of capacity to system demand (see Figure 8). Figure 8: VSD control performance curves for compressor with and without unload control Source: Improving System Performance, a Sourcebook for Industry, November [1] System Controls Multiple compressor systems benefit from a control system to orchestrate the sequencing and operations of the compressors. As previously stated, the primary goal of a multi-compressor control system is to operate all but one compressor at full load, with one compressor designated to provide page 15

16 trim capacity. However, system controls can range from relatively simple to very complex, with compressor control based on real-time flow and pressure measurements of the system. A single compressor control system typically consists of a simple pressure transducer that produces a signal based on the discharge pressure to the compressor. As the discharge pressure decays with increased compressed air flow, a signal is sent to the compressor capacity control to increase the inlet flow, begin to load, or start up, depending on compressor control type. For a single compressor with modulation control, load/unload, variable volume, VSD or start/stop compressor control, this type of system may be perfectly adequate. But in a large industrial multi-compressor system with combinations of dynamic and positive displacement machines, and varying compressor control types, managing system capacity to meet demand while optimizing for energy efficiency can be challenging for the best master control systems. Many older multi-compressor systems are equipped with electro-mechanical controls that start up compressors based on a series of cascading pressure set points. An individual compressor is started up whenever the discharge pressure drops below the set point; the next compressor in line starts up when its discharge pressure sags below a pressure set below that of the previous compressor. Imprecise control and slow response times, characteristic of these system controls, can often result in a wide swing of system pressure, and multiple compressors are often forced to run at inefficient part load conditions. For many systems with two, three, or more screw compressors, the addition or conversion of a constant speed compressor to an appropriately sized VSD trim compressor may be a relatively low-cost option that meets the system demand for variable capacity while maintaining high overall system efficiency. The VSD compressor is able to modulate over a wide range of flows, allowing the other compressors to either remain off or operate at high load conditions. In other instances, however, a more sophisticated master control system may be a more appropriate solution. New microprocessor-based control systems now available are capable of monitoring conditions throughout a complex compressed air system. These control systems manage comprespage 16

17 sor operations based on measured flow and pressure in a real-time fashion; they are capable of communicating with practically all compressor brands and communications protocols, and they work with centralized engine rooms and/or remote satellite compressors. A sophisticated master control system harmonizes the operations of all of the compressors for the highest possible system efficiency. The limitations of such a master control system are typically governed by initial cost, practicality, and the cost of additional control points and functionality. Air Treatment Equipment An important part of a compressed air system is the air treatment equipment that protects the tools and process equipment served by the system. This air treatment equipment includes dryers and filters to remove moisture and contaminants from the compressed air. Ambient air supplied to the air compressors may contain moisture, oil, and dirt, and can dramatically affect equipment life or operation: Moisture - Ambient air contains varying amounts of water vapor, which can condense in pipes. Sludge, rust, freezing, and tool damage can result from poor moisture control. Oil - Lubricant-injected rotary screw compressors and reciprocating compressors always have some carryover of oil, but excessive amounts can lead to problems with tools or processes exposed to compressed air. In some processes, food processing for example, use of oil-free compressors and elimination of any other oil contamination is an important function for cleanup equipment. Dirt - Dust and grit that enter the system through the air intake are concentrated by the compression process by a factor of seven times for 100 psig air. Combined with oil and/or moisture, dirt can lead to equipment failure or poor operation of tools. The international standard that specifies the quality of compressed air is ISO This standard specifies limits for three categories of air quality: Maximum allowable dew point temperature Maximum particle size for any remaining particles Maximum remaining oil content page 17

18 Each category is given a rating number between 1 and 6, as shown in Table 3. In many cases a Class 4 rating is adequate for compressed air needs for hand tools and other pneumatic equipment. Cleanup of compressed air beyond the needs of the process equipment can lead to additional pressure drops, excessive use of compressed air for purge air type dryers, and additional costs to install, operate, and maintain the cleanup equipment. Oil Carry- Dust Carry- Moisture Over Over Carry-Over Class mg/m3 µg mg/m3 PDP* (degf) mg/m *PDP - Pressure Dew Point, ºF Dryers Table 3: ISO 8573 Air Quality Classifications Dryers are designed to remove water vapor carried into the system from ambient inlet air. Three types of dryers are briefly discussed below: Refrigerated dryers are the most common means of removing water, with cycling and non-cycling refrigerated dryers capable of achieving dry air at dew points between 35 F and 39 F. Desiccant dryers adsorb moisture from the air in desiccant beds. The desiccant beds typically require regeneration when the desiccant has reached its capacity for holding moisture. This can be achieved by using purge air previously dried and heated, or by using compressor waste heat. Purge air requirements in the regenerating tower can be a relatively large efficiency loss to the system. In a deliquescent dryer, the medium absorbs water as it dries the air. These dryers are non-regenerative, and the medium is used up as it changes from a solid to a liquid. The most efficient dryer for a compressed air system is one that minimizes pressure losses, minimizes power consumption, and meets the design flow page 18

19 and dew-point requirements for supply of dry air to the demand side. Refrigerated dryers typically have the lowest capital cost and operating cost; however, desiccant or deliquescent dryers may be necessary for meeting lower dew point requirements. Filters Filters serve two primary purposes cleaning grit and dust from inlet air, and removing oil or other contaminants from the discharged compressed air. As with dryers, optimization of filtration systems involves specifying filters capable of removing the contaminants while minimizing any pressure drops, either on the inlet side or the discharge side. Maintenance and replacement of filters are key elements in maintaining the system efficiency and reliability of the system. Storage Compressed air storage receivers are simple but effective additions to compressed air systems to address demand surges (high flow but short duration events) and reduce pressure swings in a system. By storing a volume of compressed air, the receiver can meet sudden compressed air demands much more quickly than a compressor, and thus can prevent or delay startup of another compressor. Tank capacity ranges from a few gallons to several thousand gallons. The volume of storage should be sized to meet a flow event by supplying compressed air over a sufficient length of time that the pressure drop from the demand event will not cause another compressor to start up to meet the intermittent demand. In large distribution systems, remote compressed air storage or remote metered storage may be a good solution to intermittent flow events. Large intermittent flow events from equipment located far from the central storage or engine room can lead to localized equipment operation problems, cause compressors to come on line for short load cycles and then continue to run afterward at no-load, or affect other equipment on the system (e.g., poor operations from flow event pressure sags). With long distribution lines, the delay between the start of a large remote flow event, with its localized pressure sag, and the response from either centralized compressed air storage or compressor capacity response, may be too long to meet the equipment demand in a timely manner. A metered storage syspage 19

20 tem equipped with a check-valve allows localized storage of compressed air near equipment that creates the demand; the storage tank builds up a sufficient volume of compressed air between event cycles, with the entire storage capacity devoted to meeting the intermittent demand. Other equipment and the compressors do not see the event, and the rest of the system is unaffected by the now isolated pressure swings. Although compressed air receivers are passive pieces of equipment, they can be a critical element in the supply side of a compressed air system; they help the system operate at higher efficiency by minimizing the number of compressor startups necessary to meet fluctuating compressed air demands. Careful consideration of total storage needs, location of storage, choice of wet or dry storage, and integration of storage with the control system will provide significant benefits in most settings. While not quite a cure-all, additional storage can be one of the single best modifications for existing system operational and efficiency improvement. Demand Side System Components The key demand-side components include the distribution piping, valves, connectors, filter/regulator/lubricators (FRL), remote storage receivers and metering devices, and end-use equipment. The distribution piping transports compressed air to the point of use. The compressed air may be treated by the FRL to achieve the proper pressure and air quality before being used. Successful system operation is dependent on maintaining an adequate flow of compressed air at a pressure sufficient to meet the requirements of the end-use equipment. Failure to meet flow or pressure requirements can result in production problems or equipment damage. Preventing unnecessary pressure losses is thus a key consideration in configuring a distribution system. Any devices in the distribution system that increase friction also reduce the pressure and flow of compressed air delivered to the end use potentially causing improper operation of the end-use equipment. Figure 9 is a pressure profile diagram that illustrates how pressure losses add up in a system between the compressor discharge and the final point of use. page 20

21 Ideally, system capacity should match demand at all operating conditions. Thus distribution systems should be designed with maximum expected flows and velocities in mind. As shown in Figure 9, large pressure drops between the cleanup equipment (e.g. dryer and filter pressure drop) and the end use are possible; often the control range of a system is set higher than necessary to prevent or mitigate problems associated with system pressure losses. Specifying low (pressure) loss filters and FRLs, and adequately sizing the disconnects and drop hoses can help minimize these point-of-use pressure drops and lead to substantial savings opportunities by allowing lower discharge pressure for the compressors. Careful consideration in choosing and setting up end-use equipment can also make a significant difference in overall system performance. For example, remote end-use equipment that imposes large but intermittent loads on a system may benefit from additional local metered storage tanks to isolate the load from the rest of the system; another common situation Figure 9: Pressure Profile for a System at a Point in Time Source: Improving System Performance, a Sourcebook for Industry, November [1] page 21

22 is use of engineered nozzles instead of open blowing. Understanding the system pressure profile is very useful in specifying end-use equipment that can operate on actual available pressure within an existing system. Optimization of Existing Systems The energy required to operate a compressed air system can be reduced in many ways, some of which are low-to-no cost, while others require capital expenditures. Because a compressed air system operates as an integrated set of components, changes to one component may cascade through the system, affecting the operations of other parts. Sometimes, however, operating improvements in one area will only have a beneficial impact if other actions are taken, such as reducing system pressure before implementing a leak repair program. Implementing the basic principles of a systems approach to optimizing compressed air system performance involve taking the following steps [3]: 1. Develop a basic block diagram of the system. 2. Measure the baseline performance of the system (kilowatts, system pressure profiles, compressed air flows, and leak loads). 3. Work with a specialist to implement appropriate compressor control strategies, which may include installing a VFD compressor, compressor sequencer, or compressed air storage. 4. After controls adjustment or improvements, re-measure system performance through measurements of compressor power, system pressures, and system flows; evaluate whether system pressure can be reduced; implement the pressure reduction; and re-evaluate the system and end-use performance. 5. Walk through to identify obvious preventive maintenance items, and other opportunities for cost reduction (changing inappropriate air uses, for example). 6. Implement a leak repair program, correct inappropriate air uses, and then re-measure system performance and re-adjust controls as noted in Steps 3 and 4 above. page 22

23 7. Evaluate the results from the previous steps and implement an awareness and continuous improvement program. Although not specifically outlined above, capital cost measures may be appropriate to satisfy the broad principles of a systems approach. Brief discussions of these basic principles, which underlie the steps above, are presented below. Audit the System and Establish Baseline Performance (Steps 1 and 2) Auditing the existing performance of a compressed air system should be one of the first steps taken to optimize system performance. The audit should be focused on identifying problems in production and end uses of compressed air, previous work-arounds for persistent issues, and understanding of the operations of the existing system. The process begins with Step 1 above, drawing a block diagram of all system components. Next is carrying out Step 2, by measuring compressor electrical demand, measuring pressure at various points in the system, and if possible, determining compressed air flow to various parts of the system. Useful work is only accomplished with compressed air flow, and measuring only pressure and power may not provide the right answers for complex system problems. For example, increasing flows through a distribution system that is too small can rapidly cause pressure drops through the line; evaluation of flows allows the auditor to calculate compressed air velocities, the primary determinant of distribution line-size problems. Bringing in an experienced compressed air specialist with the specialized training, knowledge, and the correct tools to make an accurate assessment can be very cost-effective for facilities that lack in-house expertise. The Challenge [9] has guidelines on selecting a compressed air service professional for Level 1 Walk-Through Assessments through more sophisticated Level 3 System Audits, which can include monitoring and detailed measurement of system pressures, flows, and compressor power. Reports of audit findings can guide facility owners to next steps in correcting deficiencies and improving system performance. A list of qualified Best Practices AIRMaster+ specialists is also available on DOE s Industrial Technology Program website [10]. Energy auditing or engineering and design assistance may also be available from the electric utility serving the facility. page 23

24 Implement Compressor and System Control Strategies (Step 3) Adjusting, modifying, or replacing controls and control strategies are among the more effective changes that can be made to a compressed air system. Individual Compressor Controls System Controls From a system perspective, proper sequencing of individual compressors can have a dramatic effect on the overall performance of the system at different load conditions. By ensuring that the control system keeps any base load compressors operating at high load levels while the trim compressor only serves the trim load (or the whole load if demand is low enough), a facility can improve overall system performance. With an accurate pressystem failures With respect to individual compressor controls, older inlet modulation compressors not equipped with a blowdown capability (i.e., the sump is de-pressurized during unload periods) deliver the poorest efficiency of any of the rotary screw compressors. Up to 60% of the rated power is required to operate at no flow or unloaded conditions with un-relieved sump pressures. Retrofitting these compressors with relatively simple blowdown controls and valves allows an inlet modulation compressor to reduce its unloaded energy consumption significantly. The modulation capability is retained at higher part loads above about 60% of full flow, but energy consumption for unloaded operation drops to 15% to 35% of rated full load power. Other types of individual compressor controls may not be as easily modified, and when this is the case, replacement or reassignment of a compressor in the overall control sequence may be the best approach. A VSDcontrolled compressor is a fairly common retrofit strategy for controlling system capacity and meeting trim loads. In this case, other compressors are designated as base-load compressors and are kept operating at high part load ratios. A single VFD compressor, sized to meet variations in the compressed air demands, serves to meet the trim load. The performance curve of a VSD-controlled rotary screw compressor makes it ideal to run at part loads while retaining high efficiency. page 24

25 sure profile of the system in hand, along with an evaluation of the actual demands from the distribution system and end uses, re-sequencing of compressors may present opportunities to lower the overall system pressure as large pressure swings are mitigated. To meet widely varying system demands with a large multi-compressor system, especially systems that have dynamic machines, a more sophisticated System Master Controller may be an appropriate solution. Older systems with slow and/or imprecise mechanical cascading pressure set point control systems can benefit from retrofits that deliver more precise control of all components in the system. A system master control can: (1) narrow the pressure control band range for all compressors, (2) ensure that compressors are only brought on line when necessary, and (3) ensure that all operating compressors are kept at full load with the exception of a single trim compressor. As previously noted, currently available master controllers can communicate with most brands of compressors and over most communications protocols. They can monitor compressor status in real time (including pressure, flow, and end-use status), are capable of sequencing the appropriate compressors to match system conditions, and can trend monitored data. These capabilities allow operators to track performance and implement continuous improvement programs for compressed air production and use. As master controller capabilities increase, the cost of the installation also increases; however, improved capabilities to manage system performance, add demand response options, enhance protection of large centrifugal compressors from surge and physical damage, and improve system reliability may make these capabilities cost-effective in many industrial facilities. Storage Having sufficient storage to ride out short duration, high-flow demand events is an essential adjunct to a control strategy. As a general rule, adding storage will help correct many operating problems in compressed air systems. Technically, additional storage capacity is not a control system component, but adding appropriate volumes in the right places allows the compressor controls and pressure/flow controllers to keep the system operating in a more stable fashion without large pressure swings. page 25

26 system failures Short duration and high flow demands can create pressure swings in a system. A common scenario is that a rapid drop in system pressure, caused by a big compressed air demand out in the plant, eventually arrives at a compressor s discharge pressure sensor and signals the controls to start up an additional compressor. The compressor quickly starts, and satisfies the short duration load, but is then forced to continue operating for a long period afterward in an unloaded condition. With limitations on the number of start/stop cycles per hour for most large compressors, once the compressor starts, it may only be loaded for a few seconds or minutes to meet a transient pressure drop, followed by running at 15% to 35% of full power until it can finally stop; or it repeats the cycle of loading up again in response to the next pressure drop from a flow event. Operating this way can turn even the most efficient compressor into a poorly performing system component. By measuring a few parameters during the flow event, the amount of additional compressed air storage necessary to meet the intermittent demand without turning on an additional compressor can be calculated using Equation 1 below: Eq 1. Where V = Necessary receiver volume, cubic feet T = Time for pressure drop to occur, min C = Compressed air flow rate during event, cfm Pa = Absolute pressure, psi P = Change in pressure from beginning to end of the event, psig Installation of additional primary storage equal to the calculated volume from Equation 1 should prevent the system pressure from dropping below the point at which another compressor must start to satisfy the system demand. Similarly, installation of remote storage near a point of use can stabilize the distribution system pressure and prevent startup of an additional compressor, or help keep a process operating properly and without interruption. Metered storage is a more specialized application for some end uses in which a one-way valve allows a receiver to fill and then rapidly flow to the end-use point when needed to satisfy a large demand. The metered page 26

27 storage isolates the equipment and its intermittent demand from the distribution system such that the event has essentially no impact on the rest of the system. Provided that the metered storage has sufficient capacity to ride out the event, the system pressure responds only minimally as the receiver refills before the next event. The compressor performance curves for a load/unload compressor previously illustrated in Figure 5 showed the effect of differing amounts of storage on the performance of a load/unload compressor. With greater system storage, the compressor follows a more linear relationship for percent power vs. percent flow. Conversely a load/unload compressor will tend to consume additional energy with an insufficient amount of storage as the compressor wastes additional energy cycling between load and unload conditions. Evaluate and Make System Adjustments (Step 4) Once system improvements are implemented, evaluation of the changes to system performance is a critical step in the process. The pressure profile, compressor power, and flows should be measured again, to assess the adjusted system performance. With lower pressure swings from additional storage, re-sequencing of compressors, or addition of VSD compressor, the operator should review whether the system is now operating at the right pressure. Minor changes to the system pressure of just a few psig can result in fairly significant energy savings, both for compressor power and for reduced leakage in the system. Estimates are that for every 2 psig drop in pressure, the compressors use approximately 1% less power [1]. Careful assessment of potential changes to operating the system, combined with evaluation of the results of those changes can lead to persistent gains in system performance and energy savings. Implement Preventive Maintenance, Eliminate Inappropriate Uses, and Identify Other Opportunities (Step 5) Good opportunities for system improvement are often related to timely, regularly scheduled maintenance of system components: a prime example is to change filter elements at regular intervals both at the compressor end and at end-use points to minimize pressure losses from clogged filters. page 27

28 system failures Eliminating inappropriate uses of compressed air, substituting electricpowered tools and equipment, or correcting behavioral practices in compressed air use are the among the most direct and cost-effective methods of reducing compressed air usage and saving energy. With poor energy conversion efficiency, compressed air should generally be utilized only when: (1) safety considerations require doing so, (2) productivity gains justify the expense, and (3) the associated reduction in labor costs justifies the expense. In general, compressed air use should be continuously monitored and evaluated to ensure that it is prudent and judicious. Examples of potentially inappropriate uses of compressed air are: Open blowing Aspirating, atomizing, dilute-phase transport, padding, sparging Vacuum generation Ventilation Improperly sized and installed diaphragm pumps Cabinet or personal cooling Among the continuous improvement opportunities that might be considered, relocation of the intake for inlet air is worthy of consideration. Since less energy is required to compress cooler, denser air than warmer, lighter air, compressors operate more efficiently when the intake air to the compressor is drawn from outdoors, and not from within a hot engine room. If an existing compressor arrangement draws its inlet air from inside a warm building (such as a boiler room), there are energy savings available just from installing ducts to bring in cooler air from the outside. Implement a Leak Repair Program (Step 6) A leak repair program should be undertaken after other system improvements have been completed. Leak loads in compressed air systems are typically quite large, and can be at 20% of compressor production, with some systems observed with over 50% losses [1]. Every system has leaks, and no leak repair program is likely to correct all of the problems. Additionally, the pressure stresses on the system will likely generate new page 28

29 sources of leakage over time. Thus, leak repair should be an ongoing program that is repeated at regular intervals. The first step in any leak repair program is identifying the severity of the problem, which can be done in several ways. For simple systems equipped with a single load/unload compressor, the percentage of leak load can be calculated by timing a load/unload cycle with no end-use equipment operating. The proportion of loaded time to the total time in a load/unload cycle is the percentage of air leakage in the system. Another approach to estimating leak loads requires monitoring a pressure gauge and the time required for a system with no end-use equipment operating to drop to another pressure. The time the system takes to drop to the lower pressure and the beginning and end pressures can be used in Equation 2 to estimate the normal leak load. Eq. 2 Where Leakage is in cfm V = Storage volume, cfm P1= Beginning pressure, psig P2=Ending pressure, psig T = Time, minutes If the end-use equipment operates continuously, or leak loads cannot be isolated, specialized ultrasonic equipment can locate the source and determine the general magnitude of leaks. Once the leaks are identified and tagged, corrective actions can be taken to repair distribution piping leaks or replace defective valves, leaking disconnects, and fittings for hose drops. Reducing overall leakage in a system to 10% of total free air delivery is a reasonable goal that will typically be cost-effective, depending on the initial state of the system. It bears repeating that leak repair programs should not be undertaken prior to other system improvements. Reductions in artificial demand (excess air required by a system s unregulated uses from high system pressure) and measures to minimize pressure losses in the system will tend to page 29

30 increase the rate at which leaks waste compressed air. Completing a leak repair program first will likely result in a false total for leak repair savings as the increased pressure in the system from other measures will only accentuate the remaining leaks, and perhaps initiate new ones. Figure 10 illustrates the ongoing nature of leak repair programs. Figure 10: Best Practices for Eliminating Leakage Implement an Awareness and Continuous Improvement Program (Step 7) Some of the activities described above are iterative and focus on evaluating the results of system improvements between major steps to ensure continued improvement. Performance evaluation and adjustments to system controls, for example, should be repeated regularly to ensure that any performance gains are maintained and possibly improved on especially if production changes occur in a plant. Benchmarking of the baseline system performance and regular evaluation of ongoing system performance can help to prevent degradation of system performance, and simultaneously provide opportunities for improving production and use of compressed air. Unlike electricity and gas, compressed air has a relatively poor energy conversion ratio into mechanical work to start with. Thus compressed air system performance can easily degrade through poor maintenance practices, changes in production, or even simple changes to system controls by well meaning operators trying to solve production problems by turning up the pressure just a bit. Regular evaluation and implementasystem failures Source: Adapted from U.S. DOE Best Practices Manual. page 30

31 tion of improvements to a system can lead to dividends in both compressed air costs and an improved production environment. Best Practices Design Methods for a New System The U.S. Department of Energy has an established set of best practices for compressed air systems. These practices provide end users with an effective roadmap for designing new systems that provide an efficient, reliable source of power for operating pneumatic machines and tools. Designing the System The design methods recommended include five key design steps, which are similar to the basic principles outlined above for optimizing an existing compressed air system: Step 1: Create a Demand Profile Chart. Step 2: Review the end user s stated air requirement. Step 3: Determine the expected average flow rates during each shift and expected maximum demands. Step 4: Consider the need for future increases in compressed air use. Step 5: Determine the minimum operating pressure requirements. DOE s Improving System Performance, a Sourcebook for Industry [1], provides an in-depth analysis of these design principles, and detailed step-by-step procedures. Assessing Total System Costs As with all energy efficiency projects, estimating capital costs when designing a new compressed air system is an exercise in balancing competing demands for scarce capital with the need to minimize future operating costs. As indicated earlier, production of compressed air is expensive, with lifetime electricity costs estimated at six times the initial capital investment in the equipment [2]. Clearly, minimizing the life cycle electricity costs should be a priority in the design phase and should be highlighted in presenting the business case for making marginal improvements in the specification of high efficiency system components and integrated design. page 31

32 In addition to first-cost capital requirements, recurring costs associated with compressed air should also be considered, including: Filter maintenance and replacements Compressor maintenance and rebuilds Rental equipment in case of breakdowns Spare parts system failures Maintenance and leak repair programs by outside contractors or inhouse staff Disposal costs for lubricants and condensate Calculating compressed air costs on a life cycle basis including all capital expenditures, recurring costs for maintenance and replacement of filters and other consumables, and especially electricity costs to produce compressed air is key to understanding the true costs of owning and operating these systems, and the importance of operating them at the highest possible efficiency level. Recognition of the true cost of producing compressed air is likely to help decision makers approve proposals for best practices design of new compressed air systems. Some costs can be offset by utility-sponsored incentive programs. The compressed air system measures covered in a number of these programs in California include a wide range of custom energy efficiency measures, a sample of which includes: Direct replacement of one or more compressors with compressors of higher efficiency, including those equipped with variable speed drives Installation of new compressors to service increased production capacity Sequence optimization of multiple compressors Installation or upgrade of system storage Installation of intermediate pressure/flow control valves Up-to-date information about incentives and other assistance can be found on the utilities websites or through utility representatives. page 32

33 Notes 1. Improving System Performance, a Sourcebook for Industry, prepared for: U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE), Best Practices. November Prepared by Lawrence Berkley National Laboratory and Resource Dynamics Corporation. urcebook.pdf 2. U.S. Department of Energy, Energy Efficiency and Renewable Energy, Industrial Technologies Program, Energy Tips Tip Sheet #1, Determine the Cost of for Your Plant, August U.S. Department of Energy, Energy Efficiency and Renewable Energy, Industrial Technologies Program, s Role in Productivity, Energy Matters, Fall 2002, 4. Best Practices for Systems, The Challenge, Inc., 2003, page California Energy Efficiency Potential Study, Volume 1. Itron. May Best Practices, Smith Onandia Communications LLC, Fairhope, AL, September 2006, page AIRMaster+, V1.2.0, U.S. Department of Energy, Energy Efficiency and Renewable Energy, Industrial Technologies Program, DOE Industry Tools. Available for download at 8. Table C.4 Air Compressor Equipment, Minimum efficiency ratings for rotary screw and reciprocating air compressors. %20Efficiency.pdf 9. Guidelines for Selecting a System Provider, 2002, Challenge, Inc.: Qualified Best Practices AIRMaster+ Specialists: Full List, DOE Industrial Technologies Program, ialists.cfm?software_id=1&display=full_list page 33

34 For More Information and Gas Institute (CAGI): Challenge : Improving System Performance, a Sourcebook for Industry: compressed_air_sourcebook.pdf U.S. Department of Energy Best Practices: page 34

35 Energy Design Resources provides information and design tools to architects, engineers, lighting designers, and building owners and developers. Our goal is to make it easier for designers to create energy efficient new nonresidential buildings in California. Energy Design Resources is funded by California utility customers and administered by Pacific Gas and Electric Company, the Sacramento Municipal Utility District, San Diego Gas and Electric, Southern California Edison, and Southern California Gas Company, under the auspices of the California Public Utilities Commission. To learn more about Energy Design Resources, please visit our website at: This design brief was prepared for Energy Design Resources by Nexant, Inc., San Francisco, CA. page 35