Plant Services - August 2008 - (Page 38) uses 10% Artificial demand 15% Normal production 50% EfficiEncy Compressors Leaks 25% Speed factor Compressors with variable-speed drives (VSD) change the storage variables in the balance equation because they have no consequential cost penalty from operating partially loaded. Horsepower is essentially matched to the demand load. Oversizing a VSD compressor to provide an energy reserve is an acceptable practice. For example, if a system needs 100 hp in reserve, a 200 hp VSD compressor could run at 50% load without introducing appreciable waste and inefficiency. In systems with VSD compressors, the energy required to maintain the system balance can rely more on a reserve of rotating motor horsepower and less on storage than a system configured with only fixed-speed machines (Figure 3). The arbitrary substitution of storage with the reserve energy of motor horsepower from a VSD compressor must be weighed carefully. There’s still a time lag between the demand event and the compressor response that you must take into account. Insufficient storage puts the compressor in a catch-up mode, in which it constantly chases system demand, never taking maximum advantage of its inherent highly efficient part-load performance. In general, you can realize an additional 7% to 10% savings by trimming the system using the VSD compressor through controlled A versatile approach 200 hp VSD with built-in dryer Supply side Demand side Air receiver 500 gallons 200 hp compressor Intermediate control Figure 3. This schematic illustrates a typical application of a VSD with intermediate control. (Block diagram courtesy of Tom Taranto and ConservAIR.) Headline storage produced by the pressure/flow control. The storage required to swing a trim compressor into the base position without interrupting production in the event of an unanticipated compressor failure still must be addressed. Also, VSD compressors frequently are networked with fixed-speed machines and centrifugal units. Proper application of storage ensures the VSD always can trim the system and not cause other networked compressors to load up or blow off. Applying storage to control the system balance is essential for optimizing the energy efficiency, regardless of the compressor configuration. Storage ensures that a stable, reliable source of compressed air always is available for production. Effective compressor sequencing can be automated in a balanced system. The reduced air consumption from leak management and the elimination of wasteful practices and inappropriate uses will fully translate into real energy savings back at the compressor motors. You can profile the system, design the storage response and then control the energy balance of the system at the optimum level. Bob Wilson is the products manager at Pneumatech LLP in Kenosha, Wis. Contact him at rwilson9@tampabay.rr.com and (727) 866-8118. More resources at www.PlantServices.com Optimum receiver location – “The compressed air receiver: the endless question” Efficiency through higher loading – “Load ‘em up” Baseline measurements – “Vital signs” Speed-regulated drives – “Drive down the cost of compressed air” Air system design – “Keep it simple” Proper line sizes – “The secret is in the pipe” Compressed air leaks – “Stop the bleeding” Efficiency gains – “Compress efficiently” For more, search www.PlantServices.com using the keywords demand, horsepower and pressure. 38 www.PLANTSERVICES.com August 2008 http://www.PlantServices.com http://www.coxreels.com http://www.PlantServices.com http://www.coxreels.com http://www.PLANTSERVICES.com
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