Compressed Air Best Practices - March 2009 - (Page 34)

® | 03/09 Focus Industry ENERGY MANAGEMENT | DEMAND-SIDE SYSTEM OPTIMIZATION: POINT-OF-USE A I R C Y L I N D E R S , VA LV E S , D I S T R I B U T I O N A N D S T O R A G E (Continued from pg. 27) This size pipe does not exist in any branch lines that feed the end use machines. Some do however have 2-inch rubber hose as a connection but when you factor in the end hose barbs and pipe fittings, there still is a reduction in flow and thus an increase in pressure drop. Pictured right is a classic example of using rubber hose to feed a machine that requires in excess of 600 scfm in peak flows. Rubber hose has a greater pressure drop than equivalent sized pipe. End hose barbs further restrict the flow. All of our calculations shown above are based on the velocity calculation shown below. Remember it is desirable to keep the velocity in distribution piping down to 30 fps. This will provide minimal flow restrictions. STORAGE SIZING FOR A HIGH VOLUME INTERMITTENT USE Note: point-of-use storage works if there is a time delay between events or actuations. When the repetitive speed of the event is very fast, one must use the correct diameter piping to allow the flow unrestricted into the end use. The following is an example of how storage can be utilized in lieu of modifying distribution, branch lines size or undersized filters, regulators or lubricators. Let’s say our 363 scfm cylinder actuates one way in three seconds. The pressure starts at 80 psig and we do not want the pressure to vary by 5 psig. The receiver volume required to support this event is calculated as: V fps = cfm (14.5) 60 a ( P2 +14.5) πd2 a= 4 x 144 a= 144 QPa V 60( Pd + Pa ) Where: V = Velocity in ft/sec cfm = Air flow, free air, in ft3/min a = Cross sectional area of the pipe bore in ft2 d = Pipe bore diameter in inches P2 = Gauge pressure in header or pipe With a known flow, pressure and pipe cross sectional area (a), you can calculate V in feet per second. Pictured right is a machine with a very large air cylinder. The 8-inch diameter by 8-foot stroke cylinder used as an example previously is not unique at most plants. We came up with 363 scfm flow if the cylinder actuated in three seconds. That’s just the push stroke. What about the retract portion of the stroke? What happens if one or two more cylinders are actuating at the same time as this big one? The answer is very clear. As shown by the high speed data, the pressure drops can be upwards of 20 psig or more, simply because we cannot pass the large flows required through the existing branch piping. In a previous example, the restrictions were so severe that the 12-second cycles weren’t allowing enough time for the pressure to even recover back to the 81 psig of the header. So how do we supply these large volumes of air required by most cylinders during their actuation? The answer is point-of-use storage. Vcf = Vcf = Tmin Q(14.5) ΔP 3 60 (363)(14.5) x 7.48 Gal/CuFt 5 Gallons = 393 Where: Tmin = 3/60 minutes (event time) Q = 363 cfm Pa = 14.5 psia (atmospheric pressure) P1 = 80 psig header pressure into receiver P1-P2 = ΔP P2 = 75 psig (lowest allowable pressure) This is a simplified calculation and does not take into account the flow into the tank during the event. This gives conservative sizing. 34 34 www.ai b estpractices.com www.airbestpractices.com http://www.airbestpractices.com

Table of Contents Feed for the Digital Edition of Compressed Air Best Practices - March 2009

Compressed Air Best Practices - March 2009 Contents From the Editor Utility-Air News Compressed Air Audit of the Month Air Standards Assessment Improves Electroplater Production and Saves Energy Demand-Side System Optimization Seven Sustainability Projects for Industrial Energy Savings Personal Productivity Resources for Energy Engineers Wall Street Watch Advertiser Index Classifieds

Compressed Air Best Practices - March 2009

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