Compressed Air Best Practices - January 2008 - (Page 12)

| 01/08 Focus Industry ENERGY MANAGEMENT | REAL WORLD BEST PRACTICES Selecting a VSD Rotary Compressor Example: Lubricant Free Two Stage Rotary Screw (350 HP VSD and 200 HP Constant Speed Trim) Annual Electrical Energy Cost for Compressed Air Production Existing system: 1,526 cfm base load (350 hp) 1,026 cfm (200 hp) trim unit Measure Average system ﬂow Average compressor discharge pressure Input Electric Demand Operating hour of air system Speciﬁc power Annual electric cost for compressed air Total annual electric cost Three production shifts 2300 cfm 100 psig 393 kW 6,240 5.85 cfm/kW $122,616 / yr Holidays and weekends 900 cfm 100 psig 148.3 kW 2,720 6.07 cfm/kW $20,168 / yr Proposed system: Existing (350 hp) 1,526 cfm base load / new (350 hp) 1,520 cfm variable speed drive trim unit Three production shifts 2300 cfm 100 psig 405 kW 6,240 5.68 cfm/kW $126,360 / yr Holidays and weekends 900 cfm 100 psig 171 kW 2,720 5.26 cfm/kW $23,256 / yr How Can I Tell What the Power Draw Is — Can I Use Amperage? We do not use amperage to measure power; power is kW and kWh is energy power over time. Amperage can be very misleading with regard to power because it is not proportional to power throughout the motor operating cycle. For example: The example left shows typical HP data — BHP (at compressor shaft) output motor HP and input HP/kW. Basic specification: CFM PSIG FL Amps Power Factor VSD 490 100 124 .944 Constant Speed 490 100 133 .84 $142,784 / yr $149,616 / yr Existing Operating System Data Proﬁle Unit # Compressor: Manufacturer/Model Full Load Demand kW Air Flow (acfm) Actual Elec Demand % of Full kW Actual kW Actual Air Flow Actual acfm Production Shift: Operating at 100 psig discharge pressure 2300 cfm 1 2 1,526 / 350 hp constant-speed drive 1,026 / 200 hp two step full load / no load 100 78 100 87 254.7 x 1 157.8 x .874 254.7 138.3 393 kW 1,526 800 2,300 acfm At first glance, the VSD at 124 amps would appear to be more efficient than the constant speed at 133 amps at the same flow and pressure. However, in this case, the power factor of the VSD driven motor is .944 and of the constant speed motor is .84. When you put the data in the formulas shown to calculate kW you generate: VSD drive Constant speed 93.26 kW 89 kW TOTAL (Actual): 1,526 / 350 hp constant-speed drive 1,026 / 200 hp two step full load / no load 88 94 Limited Production Shift: (weekends, holidays) Operating at 100 psig discharge pressure and 900 cfm 1 2 OFF 157.8 x .94 148.3 148.3 kW 900 900 acfm TOTAL (Actual): This particular VSD unit appears to be about 4 to 5% less power efficient than a comparable constant speed at full load — as one would expect. Proposed Operating System Unit # Compressor: Manufacturer/Model Full Load Demand kW Air Flow (acfm) Actual Elec Demand % of Full kW Actual kW Actual Air Flow Actual acfm Variable Speed Drive and Oil-Free Rotary Screw Compressors Variable speed drives with lubricant free rotary screw air ends, when properly applied, can bring energy savings. In addition, they provide excellent trim capability when required. The lubricant free two-stage rotary screw has a much more narrow efficient operating speed range than the lubricant cooled models. The overall energy reduction will probably be of a lower magnitude than the lubricant cooled. The following case study is an example of misapplication of a VSD caused by attempting to select the proper unit on “rules of thumb” rather than “attention to details.” Production Shift: Operating at 100 psig discharge pressure 2300 cfm 1 2 1,526 / 350 hp constant-speed drive 1,520 / 350 hp variable-speed drive 100 50.7 100 53 254.7 x 1 288.9 x .52 254.7 150.3 405 kW 1,526 774 2,300 acfm TOTAL (Actual): 1,526 / 350 hp constant-speed drive 1,520 / 350 hp variable-speed drive 59 59 Limited Production Shift: (weekends, holidays) Operating at 100 psig discharge pressure and 900 cfm 1 2 OFF 288.9 x .59 171 171 kW 900 900 acfm TOTAL (Actual): 12 www.airbestpractices.com http://www.airbestpractices.com

Table of Contents Feed for the Digital Edition of Compressed Air Best Practices - January 2008

Compressed Air Best Practices - January 2008 Contents Focus Industry Features Compressed Air Industry Articles From the Editor Real World Best Practices: Selecting A VSD Rotary Compressor Audit of the Month: Energy Savings in Peru The Low-Hanging Fruit in Pneumatic Systems Mikropor America, Inc. Wireless Sensor Networks: Improve Efficiency in Compressed Air Systems Special Report: The Chicago Pneumatic Compressor Launch in North America The Importance of Measuring Vacuum The Comp-Air Service Co. Special Report: The 2007 NAACD Wall Street Watch Advertiser Index Job Market

Compressed Air Best Practices - January 2008

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