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

| 01/08 Focus Industry ENERGY MANAGEMENT | REAL WORLD BEST PRACTICES Selecting a VSD Rotary Compressor Capacity vs. Power Estimating Chart 2-Step Control Oil Free Rotary Screw Fixed Speed and Variable Speed Drive 1500 Class 100 psig 300 270 240 210 180 kW 171 150.3 148.3 157.8 350 HP Oil-Free 2-Stage Rotary Screw, VSD 350 HP Oil-Free 2-Stage Rotary Screw, 2-Step Control 200 HP Oil-Free 2-Stage Rotary Screw, 2-Step Constant Speed 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 222 230 288.9 254.7 150 120 90 60 30 0 Flow SCFM 69.4 78.9 92.83 94.42 107.3 138 Translate this into actual “input kW” throughout the “operating load profile.” Apply it to the number of hours of operation to determine the “total kWh per year.” In your calculations, compare some typical capacity controls at various loads. The manufacturer is capable of giving you actual input kW. The conventional method of using BHP x .746 or .7457 and dividing by the motor efficiency is misleading. It will miss the VSD equipment loss and/or belt losses in belt driven equipment. Keep in mind the VSD starts at a higher full load kW than a comparable constant speed model. Accurate performance charts will show you where the “break even” point is and where you start saving energy in your operation. Properly selected and applied the VSD controls on positive displacement rotary compressors are a great help to improve your overall compressed air system efficiency and to help maintain it. Utilizing all its strengths in the operation plan can: p Figure 1. Typical performance curves of percentage of power to percentage of flow for lubricant cooled rotary screw compressors of various capacity controls. Based on 40-second blow down from no flow to full idle kW. Depending on the brand, size and age of the unit in question this time can vary from 10 seconds to 3 minutes or more. Note that the variable displacement control is actually more efficient from about 75% to 99%. Lubricant Cooled Rotary Screw Compressor Capacity Control Comparison 40 Second Blow Down 110 100 90 80 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 2-Step Control 1 gal/cfm storage IV Mod No BD IV Mod 60% BD Lower overall average and peak kW — lowering the base electric cost and reducing the effect of demand changes. Increase savings with its capability of almost unlimited starts and stops per hour — “nothing is less costly to run than a stopped electric motor.” p % Full Load Aver age B H P VSD Compared to Constant Speed Control Var. Displacement 2-Step Control 5 gal/cfm storage Coventional VSD Spend the time and effort to do it right — if improperly selected and applied it not only may achieve no savings, you could spend more dollars to actually increase the operating cost. All data taken from the Compressed Air Challenge 1999, sponsored by the U.S. DOE. % of Compressor Capacity Abbreviations: IV Inlet Valve Mod Modulation BD Blow Down VSD Variable Speed Drive NOTE: All percentages are stated as a percent of full load horsepower at a specific discharge pressure (i.e., 100 psig) on a specific machine. NOTE: Curves are based on a lubricant cooled screw with a 40 second blow down, other parameters see miscellaneous section. 14 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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