A rail yard purchased and installed a 100-hp variable speed drive compressor for supply of maintenance activities in their repair shop — and to supply air to the yard to charge train braking systems. They did everything right, purchasing a large storage receiver, desiccant dryer with dew point control, oversized filters, and a pressure/flow controller to keep pressure down to as low as possible in the yard. The extra cost for this equipment was partially covered by a power utility incentive.
Everything went well until the electronic drive failed in the compressor. After finding out the cost of replacement and that the exchange would not be covered by warranty, the yard managers decided to convert the compressor to load/unload control, bypassing the drive. The compressor service company grudgingly complied with this request because the compressor design allowed this to happen.
Years later, a scoping study was performed on the system; the compressor monitoring that was done determined that the compressor was consuming about 71 kW while producing an average of 127 cfm. Calculations showed that the system specific power was 55.5 kW per 100 cfm, well above an optimized system with a level of about 20 kW/100 cfm.
Some sort of problem had occurred within the compressor during conversion, the unloaded power consumption was measured at 88% of full load, rather than about 35% as with a with a normal compressor. This problem was costing 240,000 kWh per year worth about $17,000 per year in wasted power costs.
Over the life of the compressor, the hour meters show that it has spent 39,000 hours running unloaded. Based on the power readings, this works to lifetime wasted power costs of $110,000! If the electronic drive had been replaced when it failed, at an estimated cost of $25,000, significant lifetime costs could likely have been avoided.
The power utility has offered to help this customer improve this system by granting an incentive to address the compressor problem, perhaps with a new more efficient compressor will be installed.