Compressed Air Best Practices - January 2008 - (Page 8)

| 01/08 Focus Industry ENERGY MANAGEMENT | S E L E C T I N G A V S D R O TA R Y C O M P R E S S O R The variable speed drive, particularly in lubricant cooled rotary screws and metal vane compressors, is an excellent tool for optimizing the electrical energy cost to drive air compressors. However, it is only one of many available tools. Like other tools, it must be implemented correctly and at the right time to avoid misapplications and missed opportunities. Its capabilities and draw backs must be clearly understood. The electric motor variable speed driven compressors are not new and have been employed many times over the last 30 to 40 years. These drivers were historically almost all VFD (Variable Frequency Drive) and the units were usually built as “specials” by the manufacture or packaged by specially trained and experienced organizations. What Has Changed? New types of variable speed drives have become available in addition to the variable frequency drive. Variable frequency drives have also become more efficient and responsive. Today, almost all manufacturers offer some form of variable speed drive as a “standard” product in their rotary screw and vane compressor packaged lineups. Air compressor manufacturers now offer excellent “factory packages” and VSD is “new” and very desirable. It is not a cure-all for every application and might use more electrical energy than other options, depending on the facility’s unique requirements. Examine all the data carefully. There will always be an electrical energy loss going through the Variable Speed Drive equipment, which will show up dramatically at higher load conditions. VSD’s save energy at 80% demand Ascertain demand profiles Does It Really Work? Absolutely. Variable speed drives in positive displacement compressors is by far the most overall power efficient type of part load capacity control when applied correctly. As the compressor slows down to reduce capacity to meet demand it is operating at or near full load efficiency from 40 to 50% flow up to 100% flow. What keeps it from maintaining the “perfect efficiency” under all conditions? The electric power has to go through some kind of variable speed controller and there will be some energy loss — probably in the range of 4 to 6% at full load compared to the same unit in constant speed. Therefore it is not designed to supply compressed air at full load on a continuing basis; it is designed as a very effective trim unit from 75 to 80% flow and lower with the capability of going to full load as required. It is a misapplication to spend the majority of the operating hours at a high load. However, the capability of reaching full load “once in a while” may actually keep the plant from turning on another air compressor, further increasing the savings under certain conditions. The performance curves in Figure 1 show the relative efficiency of various constant speed capacity controls and typical variable speed drive. 8 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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