A metal processor has a six-day per week production cycle with Sunday off. To save energy, the staff turns the compressed air equipment off during the down time.
Recently, a compressed air auditor installed a dew point probe on the system — and found that on start-up there was a dew point spike to as high as 18° C. Because the compressors feed very expensive laser cutters (that require compressed air with no water contamination), this presents a risk.
There are two things happening with the system on start-up. The first is the compression of the compressed air. Because the air pressure in an inactive system drops down to ambient pressure when the compressor starts up, the air in the system receiver and pipes have started to reach equilibrium with the ambient air by the time the system is started up again.
Water vapor leaks in through all the various leaks in the system and causes the dew point in the system to rise. When the system starts back up this air is compressed, and because compressed air cannot hold as much moisture, there is an increase in dew point when the pressure rises. This effect can be seen on the leading edge of the dew point spike, and the opposite effect observed when the system pressure drops; the second graphic shows that the compressed air dew point reaches about -3° C, lower than can be produced by the installed dew point, when the pressure falls, caused by the expansion of the compressed air in the system.
But the most significant effect is due to the start up of the refrigerated air dryer. This unit is a cycling style containing a large quantity of brine solution as a thermal mass. Normally, during normal operating hours, this thermal mass is cold and it reduces the temperature the compressed air so it can drop its moisture. However, the staff incorrectly turns off this dryer when they turn off the air compressor, which allows the brine to warm. Then, when the compressor is started up with the dryer, it takes about 15 minutes for the brine to cool back down, all the while poor quality compressed air flows into the plant.
The solution is to keep the air dryer energized during the downtime or start up the dryer well before turning on the air compressor.