A seed cleaning company was looking to expand its operations by purchasing new compressed air using equipment. Plant people spoke with the sales engineer — who informed them that each proposed cleaning machine required a minimum of 50 cfm of compressed air. Doing quick addition in his head, the engineer calculated they would need a least 150 cfm for the three new machines, and an additional 50 cfm for a future unit.
The seed company’s maintenance manager was not comfortable installing the bare minimum size, so he added a safety factor of 30% to the 200 cfm size. They added in the existing load and found the compressor company they used did not have a compressor that exact size, so they went the next size bigger. Ultimately, the company settled on a 75-hp variable speed drive compressor with an output of 344 cfm.
The compressor was installed and started up, and before long the maintenance manager noticed then compressor was not ever running in its variable range. It was constantly starting and stopping — sometimes 20 times per hour! And, at the next scheduled service, the compressor technician noticed water in the compressor lubricant. They called in a compressed air auditor to investigate.
The auditor placed data loggers on their system and installed a flow meter. The readings determined that the compressor was greatly oversized for the plant flow, which averaged only 20% of the capacity of the compressor (35 cfm). The auditor dug further and found that the 50 cfm flow quoted for the new cleaning machines was actually a peak flow value with safety factors applied … only true if the machines were operating in worst case conditions, a very rare occurrence. In normal conditions, the actual flow was much lower. The actual need was a much smaller compressor.
The water in the compressed air was typical of a lightly loaded variable speed drive machine: not enough heat is generated by a machine running under minimum speed ( the lowest rpm a VSD compressor can go) to boil off the water that condenses during the compression process. Manufacturers recommend the machines not run under minimum speed for any length of time.
The auditor recommended a system with multiple smaller compressors sized to ensure the VSD would never run below minimum speed, but large enough to cover worst case conditions. After the installation, the maintenance issues disappeared, and the compressors ran at their best efficiency points.
Certainly this example shows that data must be understood prior to making decisions, and that guesses based on estimations are not likely to be correct.
Decisions should always be made on adequate understanding of the correct data.