Reliability of the supply of compressed air to your process(es) is what really matters.Variable speed drives add additional potential reliability risks with no (yes NO!) efficiency advantage for powering  reciprocating compressors.  To accommodate varying flow requirements, recips can be operated at idle for reasonable periods of time with astoundingly low power consumption requirements.  If your process has significant periods of time with relatively small flow requirements, you may want to consider one quite small compressor to cover these periods and then an assortment of larger compressors to cover other periods.  The process needs will control a rational compressor selection process.Don't forget that a variable speed drive imposes parasitic losses of its own on the order of 5% or more in addition to significantly increasing motor and control costs.  As I have often stated, variable speed drives, when properly applied to suitable applications, can provide dramatic energy savings (I have seen as much as 90% savings at low loads), but they are anything but a universal answer to any or every problem.  An industiral air compression system is almost certainly not a suitable application for a variable speed drive.Compressors and Suber-water pump compressed air systems are expensive, but false savings on these can impose staggering costs on your processes if they are not sufficiently reliable and durable.Be careful when considering so-called non-lubricated air compressors.  Any compressor will concentrate ambient oil vapors or fumes, so if any portion of your process has any specific sensitivity to oil, filtration and separation equipment will still be required.  Non-lubricated compressors can perform quite well, but they are not likely to compare favorably with lubricated compressors for very long-term durability, reliability, and maintenance requirements.  I've seen and heard many claims favoring non-lubricated reciprocating compressors, but I have never seen a case where a non-lubricated compressor performed as well as claimed in terms of durability and maintenance requirements.  I have seen low speed lubricated multi-stage reciprocating air compressors provide durability, reliability, and performance greatly exceeding all claims, guarantees, and expectations.For really serious energy savings, you will be wise to pay very close attention to compressed air system leakage, and more importantly, to air usages that can be better served by non-compressed air based items.  Compressed air should only be used where it is the only reasonable choice.  For example, an air motor may be the prudent choice where a safety consideration controls, but otherwise, an electric motor would require dramatically less total energy.  If significant amounts of compressed air are required at different pressure levels, serious consideration should be given to multiple compressed air systems.  It may be reasonable to have a lower pressure system serve as the main one with booster compressors drawing air from that system to serve higher pressure systems.  Valuable advice from a professor many years ago:  First, design for graceful failure.  Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs.  Only then can practicality and economics be properly considered.

2011-08-28

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