Compressed Air Best Practices - October 2008 - (Page 12) | 10/08 Focus Industry AUTOMOTIVE TRANSIT | COMPRESSED AIR AUDIT OF THE MONTH Transit Bus Manufacturing Grit Blasting 150 HP 115 psi 650 cfm To Grit Blast Particulate Filter Refrig Dryer 650 cfm To Paint Booths 400 USG 400 USG Main 1 200 HP 115 psi 750 cfm To Plant Particulate Filter Coalescing Filter Dessic Dryer 850 cfm Particulate Filter To Paint Booths Coalescing Filter 400 USG Refrig Dryer 425 cfm Particulate Filter Breathing Air 100 HP 125 psi 410 cfm Main 2 150 HP 115 psi 625 cfm Particulate Filter Refrig Dryer 650 cfm New Flyer Prev ious System Diagram Monitoring and testing of the supply equipment in this system showed the following issues: p System 2 — Blasting System This system consisted of a 150 hp compressor and refrigerated dryer set up an independent supply for a grit blasting operation used to clean bus frames. This compressor was installed with no storage and was operating using inlet modulation control with pressure switch unloading. The unit was equipped with the auto-dual option which is designed to unload and even turn off the compressor during light loads, however, because the system had no storage capacity, the compressor ran constantly, rapid cycling between loaded and unloaded operation even though blasting air was required only 15% of the time. Data loggers showed that this compressor was lightly loaded during times when the main system was starved for air, making it a good potential source of extra capacity during peak flows. The 200 hp compressor controls were set up incorrectly. The unit used spiral valve capacity control and inlet modulation to control the output of the compressor, however, the capacity control was not operating correctly, greatly decreasing the efficiency of the unit. The unit was flow tested and was found to be producing 83% of its full load capacity. The main system was experiencing low pressure events caused by peak air flows that exceeded the capacity of the two compressors. Plant pressures were dropping below 85 psi which shut down plant equipment and machines. The 900 cfm desiccant air dryer was malfunctioning and consuming 220 cfm of air or 25% of its nameplate rating. The dryer filters were causing a high pressure differential which prevented the compressors from fully loading during times of high air demand. Compressor coordination issues caused the 200 hp unit (the most expensive unit to run) to run constantly, even during lightly loaded evening and weekend shifts. This unit had no automatic blow down control. The refrigerated dryer was operating in non-cycling mode causing it to run even when the associated compressor was turned off. p p p p p Capacity Controls like this are controlled by a simple screwdriver adjustment. Incorrect adjustment can greatly affect the efficiency of the compressor. 12 www.airbestpractices.com http://www.airbestpractices.com
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