The state of compressed air systems at many plants frustrates consultants and vendors. “Compressed air is one of the most inefficient utilities within any industrial plant, and often the most ignored, until trouble starts,” notes Ron Marshall, chief auditor, Marshall Compressed Air Consulting, Winnipeg, Man. Yet, most sites have ample opportunities to achieve better energy efficiency, specialists say.
The crucial first step for improving energy efficiency is to measure the key performance parameters of a compressed air system and its associated equipment with an air audit (Figure 1). “Plant personnel are often surprised at how bad their system really is and very happy that something can be done about it,” Marshall stresses.
Following the audit, operating companies frequently require guidance as to the best technology for their particular process. This usually involves answering questions such as whether to use oil-lubricated or oil-free compressors, how best to recover and re-use heat generated during compression, and whether a variable speed drive (VSD) is appropriate?
For example, pharmaceutical and other plants demanding very high air quality generally opt for oil-free systems. However, oil-free compressors are much costlier to purchase and maintain, cautions Marshall. “The oil-free compressors themselves are usually slightly less efficient at full load than oil-lubricated, but switching allows the heat of compression to be used in the desiccant drying of the air, which saves energy, where applicable,” he explains.
Because most of the energy input to an air compressor of any type comes out as heat, there’s a big drive to recover it and so improve system efficiency. “A common heat recovery strategy is to use the heat of compression for building heat but its best use is for a continuous process such as boiler makeup or producing hot water,” counsels Marshall. The compressor heat can help dry the air more efficiently, too, where very low dew points are required.
VSDs usually make sense because a compressor in the unloaded state can waste a lot of energy. “Unloaded screw compressors can consume between 20–40% of their full-load power while producing zero air. VSD technology helps reduce or eliminate unloaded run time while better stabilizing the system pressure. Use of VSD technology, where appropriate, can both improve the efficiency of the production of compressed air and help save more if leakage and compressed air waste is reduced at the same time,” he notes.
Modern onboard control systems also can contribute to better efficiency. However, Marshall feels more work is needed on standardizing the communication protocols for air compressors: “At this point, unlike the HVAC [heating, ventilation, air-conditioning] industry, it is rare to find any compressor from one manufacturer with the ability to easily integrate with another manufacturer’s communication system.”
Chemical makers are showing greater awareness of the total lifecycle costs of their compressed air systems, he says, particularly as a result of compressor and dryer verification initiatives and rating sheets offered by organizations such the Compressed Air & Gas Institute (CAGI).
More Efficient Options
The need to improve energy efficiency is one of the main drivers for the chemical industry’s move to more-efficient air compressor technology, emphasizes Richard Hinkle, global product director for Gardner Denver, Quincy, Ill. “There’s no question about that. Being efficient and using utility compressed air properly saves dollars and people do know that now.”