By Ron Marshall
A wood products manufacturer decided to save money by hiring a small service company to maintain its compressors. The company had minimal experience in repairing compressors, but agreed to do the job. The company didn’t have parts available to replace the faulty controls on the two screw compressors — so they did the best with what they had, even when the parts didn’t fit. Never a good start to a story!
Fig. 1 shows the subtractive pilot valve for the compressor inlet valve control resting on the floor rather than being mounted on the side of the compressor. This probably would have been okay, had the repair worked. However, the style of valves was all wrong for the compressors, leaving them to regulate the pressure very poorly. The system pressure varied from a high of 120 psi at light load to a low of 85 psi at higher loads. This caused problems with the compressors, as they were only rated at 100 psi. The modulating control caused the system specific power to peak as high as 100 kW per 100 cfm — much higher than the typical 22 kW/ 100 cfm of a normal system, making the system extremely inefficient.
The poor pressure during peak flows left the plant wanting more. In fact, there was a critical machine at the end of the line that was malfunctioning and slowing down the plant production.
A compressed air system auditor assessed the system and found that the compressors could be running in a more efficient load/unload mode, all the proper controls existed for this to happen. The system storage capacity totals about 3,000 gallons, which would allow very good compressor control if set up properly. The pressure problem turned out to be undersized piping in the compressor room and dirty filters, a somewhat easy fix. A remote storage tank with a check valve was installed at the critical machine to help stabilize the pressure.
The plant is purchasing a variable speed drive compressor, which will be even more efficient than the load/unload operation. Because the system was running so poorly, the simple installation of a properly sized VSD compressor will save about 50% in power costs, worth roughly $70,000 per year. That’s monster savings!