Compressed Air Best Practices - February 2008 - (Page 14)

| 02/08 Compressed Air Industry BULK HANDLING | COMPRESSED AIR AUDIT OF THE MONTH Dense-Phase Transport Wreaks Havoc on Compressed Air System kW Control Valves Flow Based Analysis (FBA) FBA applies this formula for every row of data: ΔP = ((T)(C)(PSIA)) ÷ V, and then updates pressure in supply side storage. If pressure reaches a set point, the flow from another compressor is either added or removed from the calculation for “C”. Where: CO2 O V= ΔP = PSIA = T= C= volume of the tank in cubic feet (for a 30,000 gallon tank: = 30,000/7.48 = 4,010.7 ft3) the change in pressure in the tank atmospheric pressure (14.5 PSIA) Elapsed time in minutes (for 5 second intervals: = 5 ÷ 60 = 0.08333 min) flow rate (difference between supply and demand, which may have a negative value) Pressure in the tank must be able to change for the tank to be useful. Meanwhile a stable pressure is desired in the plant. In this case, even with the addition of a large tank, we still needed a delta of nearly one Bar (14.5 psi), more than the change we wanted in the plant. Higher pressure increases artificial demand: and in this facility, that was having a significantly detrimental effect. Additionally, lowering overall system pressure is more difficult when it is not stable. A flow control valve was designed and site-built to meet the situation where the flow rate would change rapidly and by a large magnitude. The valves (redundant arrangement) were selected for good control with a low-pressure loss (< 0.5 psi) at peak flow rates. We had already installed an excellent pressure transmitter as part of the permanent flow meter, and it was wired to the plant control system. We decided to use the plant’s control system to control the valves as well. Once the valves were operational, we began to lower pressure in the plant and create the ΔP necessary to store the volume of air needed to meet the largest demand events. For example: If T= 5 seconds then T = 5 ÷ 60 = 0.08333 minutes an if demand exceeds supply by 1500 SCFM then C = -1500 Then: ΔP = ((T)(C)(PSIA)) ÷ V = ((0.08333)(-1500)(14.5)) ÷ 4010.7 = -0.451 PSI change in 5 seconds Secondary Issues Surface As soon as we began to lower pressure, the Low-Pressure Alarm on the Instrument Air System sounded, forcing us to raise it back up to 110 psig. Lowering pressure is essential to reducing both power costs and artificial demand, so we kept investigating. The Instrument Air System was supplied by the main system where we had lowered pressure about 10 psi. But the alarm was set another 15 psi below this point. In spite of its name, there actually weren’t any instruments on this system. So we asked both why a desiccant dryer was needed to further suppress the dew point and what was causing the additional 15 psi drop that tripped the alarm. I installed a pressure transmitter and data logger. The data revealed that pressure was dropping sharply by 15 psi or more at regular intervals. The interval matched precisely the tower switching interval for this type of dryer, so we called a service technician and had the dryer repaired. We also walked the system. In doing so we found that the dryer served dust collectors and valve actuators located outside the heated spaces, and that the low dew point was to prevent frosting or freezing in the winter. Since this plant is in the deep South, I wondered how often the low dew-point was really needed. But at the very least, the dryer could be by-passed in the warmer months, eliminating the purge air requirement (pressure swing dryer). 14 www.airbestpractices.com http://www.airbestpractices.com

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Compressed Air Best Practices - February 2008 Contents From the Editor Utility-Air News February Audit of the Month: Dense-Phase Transport System Wreaks Havoc on Compressed Air System Real World Best Practices: Material Conveying with Pneumatic and Vacuum Systems Industrial Vacuum Cleaners Use Compressed Air in Portland Cement Manufacturing Processes The Numatics Air Preparation Group The Ins and Outs of Vacuum Generators Dekker Vacuum Technologies Do Your Meetings Sabotage Your Profits? Resources for Energy Engineers Wall Street Watch Advertiser Index Job Market

Compressed Air Best Practices - February 2008

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