Getting Compressed Air under Control
Energy savings as part of a lean manufacturing strategy
J. Ned Dempsey, Pneu-Logic Corp. and Barry Hendrix, HBG Consulting
Lean manufacturing focuses on identifying and eliminating waste of all types. Waste is defined as any non-value-added activities or expenses that occur in any manufacturing or service process. Drawing its initial inspiration from processes instituted by Toyota that were intended to preserve customer value while eliminating waste from the automotive manufacturing process, Lean has evolved into a set of principles by which businesses of all types can learn to do more with less, while reducing their impact on the environment. According to the Hollings Manufacturing Extension Partnership (MEP) program (part of the US National Institute of Standards and Technology), there have been eight areas of primary focus: eliminating overproduction, excess inventory, optimizing transportation, reducing excess motion, eliminating defects, avoiding non-value-added processing, making more effective use of underutilized people and reducing the amount of waiting that occurs in plant operations. A ninth focus area, which is at least as important as these factors, is reducing energy usage. The purpose of educating US industry on the principles of lean manufacturing is to create a business culture of sensitivity to waste, to inform business leaders as to the tools and best practices that can be used to reduce waste, and in the process, to make business leaders aware of how waste reduction can improve their bottom line.
Where’s the energy waste?
Eliminating the waste often starts with an audit or Value Stream Map. Waste auditors map all aspects of the manufacturing process to identify inefficiencies. Some factors are obvious, easy to observe and measure, and therefore easier to improve by monitoring the results of changes over time. Others may require digging deeper. For example, to analyze energy usage, it is helpful to put monitoring devices on individual machines or work-cells and look at trends continuously. For example, month-end electric bills for the facility won’t provide the granularity that’s needed.
A particular area of electrical consumption that hasn’t received enough attention is compressed air. According to a government report, some 10% of the electrical power used in factories in this country goes to operate air compressors, but often a plant’s compressed air production is viewed as part of the facilities infrastructure instead of a variable cost of manufacturing. It turns out that plants waste as much as 40% of the energy used to power compressors, and reclaiming this waste is not as difficult as some may think. The secret is putting in place an automated monitoring and control system.
Eliminating wasted electricity can have a major impact on industries such as metals processing (foundries, forges, shot peening operations, fabrication and machining facilities), food, beverage and pharmaceutical processing (bottling, mixing and packaging facilities), minerals processing (quarries, bagging facilities and cement plants), automotive manufacturing (painting) and forest products processing (wood byproducts transport).
Mechanical controls or sequencers aren’t enough
The compressor controls that are used in typical manufacturing environments such as those mentioned above have traditionally been not very sophisticated. In the old days (when energy was cheap), manufacturing plants would leave compressors on all the time during a production shift. Typically, the target air pressure was set independently at each compressor using mechanical valving. When controlled in this manner, it is not unusual to have multiple compressors running at partial load, and with the overall pressure of the system maintained at a level that is higher than would be necessary if more sophisticated controls were employed. This situation results in unnecessary component wear and compressors fighting each other in an attempt to maintain pressure and airflow at different levels. In addition to wasting energy, running the system at a higher than necessary pressure may result in the factory purchasing and maintaining more compressors than it needs. Maintenance costs can also increase needlessly due to air leaks and compressor wear caused by pressures set too high.
An improvement over this philosophy is to take a system-level approach and bring the compressor resources under centralized control. Some 20 years ago, rudimentary sequencing control systems were introduced that turned compressors on or off, one at a time, in a predetermined order as the plant’s need for air fluctuated. This scheme, which is called cascade sequencing, is prone to producing a large pressure band, i.e., the pressure in the system and the way that the air pressure responds to loads can vary significantly depending on the number of compressors that are online at any given time. Not only is this an inefficient way to operate the factory’s compressed air resource, but energy can be wasted unnecessarily in the rigid sequential way that the compressor bank is controlled.
Getting compressed air under control
A much more efficient way of operating banks of air compressors is by enforcing a control strategy that operates each individual compressor in a performance range where it is most efficient. All other things being equal, the two modes of operation where compressor motors are most efficient are full on and full off. In varying load conditions, experience has told us that it makes sense to have at most one single compressor out of the group operating at partial load.
The controller shouldn’t base the decision as to which compressor to run on efficiency alone, however. If that were the case, the system would tend to wear out the most efficient compressors first. When it’s time to turn on another compressor, the control system should employ a program of readiness scoring that selects which compressor to run next according to various factors including compressed air demand, production loads, and maintenance schedules in addition to the efficiency of the individual compressors.
An example of a central control system is the PL4000, manufactured by Pneu-Logic Corp. (Portland, OR). The PL4000 selects compressors to run in a two-step process. First, it identifies which groupings of compressors to call upon to provide air to meet a particular need. Second, it selects which compressors to run within a particular group according to their scores. Figure 2 shows a typical staging table with compressor groupings identified and associated with system pressures to maintain. Figure 3 is a drill-down screen that focuses on the current flow requirement and shows which individual compressor(s) would be activated if the flow requirement changes (or if some other factor calls for shutting an operating compressor down). Pneu-Logic Corp. uses the term “airgonomics” to describe the process by which its controllers balance air supply with demand in an efficient manner.
With some intelligence built into the compressor controls comes the additional ability to inform users of system status. Operators would like to know at a glance how the system is functioning. For this, the PL4000 provides an operator screen that displays the status of all of the compressors in the network and also provides the ability to “drill down” to view the operational status of any individual compressor. Because the displays also provide information on the operating efficiency of the system, it’s easier for plants to sustain cost-savings initiatives due to this ability to review real-time results. The PL4000 can also export operational data to the plant’s distributed control system (DCS) for viewing and analyzing remotely.
Wasted energy is eliminated
A Pneu-Logic controller was installed at Pepsico’s Gatorade plant in Tolleson, AZ, where the use of compressed air is integral to the beverage bottling process. Before the upgrade, an independent compressed air analysis identified several issues with the plant’s compressed air system. The plant had seven compressors in three locations, but there was no monitoring in place to observe system-wide pressures. To ensure adequate air supply under all conditions, the compressors were operated at higher pressures than necessary, regardless of production requirements. By upgrading the plant’s compressed air system and installing the PL4000 compressed air controller, fewer compressors need to operate and the total amount of energy required to run the compressed air system has been decreased by 21%, resulting in an annual savings of over 1 million kWh of electricity and a 1.5 year payback on the project.
As this example shows, by paying attention to the principles of Lean manufacturing and installing systems that can eliminate waste from key areas of the plant infrastructure, companies can make significant progress toward meeting the productivity and profitability challenges of the 21st century.
For more information, visit www.pneulogic.corp.