Industry represents about three-quarters of annual U.S. energy consumption. One of the most common energy needs is for compressed air, accounting for some $1.5 billion in energy costs and some 0.5 percent in emissions annually. In fact, almost all industrial facilities operate at least one compressor and, according to the DOE's Sourcebook for Industry, a medium-sized industrial plant may have hundreds of different uses for compressed air.
Clearly, different industries will have different uses. Air-powered tools offer a number of advantages in many industrial operations, including the delivery of smooth power, infinitely variable speed and torque control, and low heat build-up. Some manufacturing operations also use compressed air and gas for combustion or process operations such as oxidation, fractionation, cryogenics, refrigeration, filtration, dehydration and aeration. Automotive plants and metal fabricators are likely to use compressed air for assembly station powering, tool powering, stamping, injection molding, forming and conveying. Food manufacturers, on the other hand, may use compressed air for dehydration, bottling, vacuum packing, coatings applications, controls and actuators and conveying.
Yet for all of the appropriate uses of compressed air, there are an equal number of potentially inappropriate uses. According to the DOE, some examples include open blowing, sparging, aspirating, dilute-phase transport, vacuum generation, secondary air conditioning and cabinet cooling.
Compressed air is one of the more expensive utilities in any industrial plant. When used properly, it represents a safe and justifiable cost; when used inappropriately – in any application that can be done more effectively or more efficiently by an alternative method – it is a costly waste for the business. Compressed air systems account for some $1.5 billion in annual energy costs to U.S. industry. One of the important aspects of that $1.5 billion figure is, according to the DOE's Industrial Technologies Program, that "optimization of compressed air systems can provide energy efficiency improvements of 20 to 50 percent."
This is a frequent situation in which financial improvements dovetail with environmental improvements. Reducing energy use inevitably represents an environmental improvement and, given the past year's nose-bleed upward spiral in fuel costs, that 20 to 50 percent can translate into some real money.
Compressed air improvements
The DOE's Office of Industrial Technologies (OIT) focuses on new technologies that can improve operations and reduce energy use. "BestPractices" is one of the OIT's programs, in which the office works with industry to identify plant-wide opportunities for energy savings and process efficiency. Not surprisingly, many of those opportunities center on compressed air.
BestPractices maintains a decent software collection that can be accessed through the OIT Clearinghouse [(800) 862-2086] or via its website (www.oit.doe.gov/bestpractices/software_tools.shtml). Even though they are available for downloading, implementation by someone trained in its use will benefit the user. In fact, the OIT notes for many of the software systems that, "DOE and the Compressed Air Challenge® recognize[s] qualified … specialists for their ability to use the … software effectively with industrial end users."
Beyond the software tools, the OIT offers BestPractices case studies highlighting the experiences of individual companies that were able to reduce energy consumption, save money and minimize emissions by improving their compressed air systems. Here's a look at a few of those examples:
Case 1
Lehigh Southwest Cement Company saves $90,000 in annual energy costs, and reduced annual energy consumption by 900,000 kWh, through the implementation of a system-level project that improved the compressed air system at its Tehachapi, Calif. cement plant. Compressed air to served dust collectors, cylinders, air knives and pneumatic clutches. Before the project, the plant's arsenal consisted of four rotary-screw compressors. The three largest served the main plant system and the smallest unit was dedicated to air knives. Before the project came on line, the smallest compressor failed and was replaced with a rental unit of 300 hp. The system produced pressure fluctuations from 85 to 120 psig; low pressure periodically forced production shutdowns.
An initial review of the system showed several problems. The pressure level was unstable, with air demand fluctuating between fewer than 2,200 to over 3,300 scfm, which required frequent loading and unloading of compressors. Set points for loading and unloading were set lower than the design set points, causing the compressors to operate at 15 to 20 percent below maximum efficiency. Despite frequent air filter changes, cement dust clogged the filters, preventing the compressors from generating their rated volume of air. The after-coolers for the large compressors and some condensate traps were not operational, forcing the dryer to overcompensate and subsequently increase the pressure gradient. Finally, the distribution piping system had leaks, was complex and restrictive compounding the system's pressure drop.
The company's solution involved the installation of a pressure/flow controller (P/FC) and a 5,000 gallon storage receiver to stabilize system pressure. Compressor discharge pressures were set at 110 psig and the pressure downstream of the P/FC was set at 85 psig. Two new 350 hp rotary-screw units replaced the existing 220 hp compressor. Other improvements included construction of a filter wall with several ventilation fans to reduce dust in the intake air. Doors to the compressor room were sealed, while the room's piping was reconfigured to lead more directly to the new storage tanks. To reduce compressed air waste, six nonfunctional condensate traps were replaced with six high-efficiency drain traps, and broken solenoids on the dust collectors were repaired, as were the largest leaks in sub-headers, drop piping and hoses. The total project achieved simple payback in less than 20 months and improved production.
Case 2
American Water Heater Company saved $160,000 annually (2,345,000 kWh) while increasing production by 12 percent following a system-level project on the compressed air system at its Johnson City, Tenn. water heater plant. The project enabled the compressed air system to support the plant's production processes more effectively, which allowed for greater production, reduced the compressor capacity required for the plant to operate, and improved the quality of the final product. Before the upgrade, the compressed air system included one 250 hp and four 350 hp lubricated rotary screw compressors, which were used for various industrial applications in four separate production centers. Each center required different pressure or air quality levels. The plant operated all five compressors during normal production to maintain a pressure level of 90 psig, but the main header pressure fluctuated between 83 and 111 psig. Poor air quality negatively impacted production.
Initial assessments showed inadequate storage and shifting air demand patterns, producing large swings in pressure; a physical layout that effectively created two compressed air systems that operated independently of one another; a poor control system; a convoluted distribution piping network; leaks; and excess heat. A particular problem involved the plant's air quality. Contamination was caused by undersized and deliquescent air dryers that were incapable of providing the pressure-dew-point (PDP) for the plant's applications. Excess condensate passed into the air stream and the condensate drains in the dryers had unreliable ball valves allowing the waste to pass into the air. The greatest problem caused by the poor air quality occurred in the powder coating area, which required a PDP of –40°F for reliable production. Even if the dryers had been properly sized, they would not have been able to achieve the required PDP level.
The solution was implemented in two phases to mitigate the production impact. In Phase 1, pressure/flow controllers were installed at each of the four production areas to partition the system into four zones. System pressure was stabilized to the lowest level that satisfied production requirements of each zone. In Phase 2, the deliquescent dryers were replaced with a combination of refrigerated and desiccant dryers. System piping was reconfigured, the cooling system was changed from an air-cooled to a closed-loop system, the condensate drains were replaced and a compressor sequencer with a data acquisition system was installed to upgrade the controls.
Each production zone now has stable pressure, with the compressors delivering air to the storage receivers at 96 psig. Stable, lower pressure and reduced air demand produced by the leak repair efforts meant the system requires less compressor capacity. The plant now operates only two 350 hp compressors and a 250 hp unit in trim mode. Plant personnel can rotate the 350 hp units to avoid excess wear of either compressor. The more optimal size and dryer type improved air quality and eliminated moisture contamination, resulting in air that would be dry enough for the compressed air applications to function well. Reduced compressor use result in an annual energy savings of $129,000, but the plant also saves about $31,000 annually in maintenance and cooling process costs. Production has increased by 12 percent and warranty claims dropped by 22 percent. Simple payback was achieved in 17 months.
