Compressed Air Best Practices - June 2008 - (Page 42)

| 06–07/08 Focus Industry PHARMACEUTICALS | REAL WORLD BEST PRACTICES EVALUATION GUIDELINES FOR TWO-STAGE, LUBRICANT-FREE ROTARY SCREW AND MULTI-STAGE (3) CENTRIFUGAL AIR COMPRESSORS (Continued from pg. 31) Centrifugals Within the compressor area there are no touching parts in a centrifugal. As long as the entering air is kept clean and the intercooler drains clear, there is no reason for a scheduled “Air End” replacement or overhaul. Machine performance can be easily monitored to assess current performance versus original performance and thus evaluate wear. It is not unusual for the centrifugal to run 10 years or more without any work in the “air end” Many . full-service warranty programs recommend a 5-year scheduled cleaning of the impellers, diffusers and intercooler. Wear will occur with inlet debris and “dirt” buildup will affect the aerodynamic performance. Basic Packaging Oil-free two-stage rotary screws are sold as basic standard packages. They can be selected based on volume and pressure desired. The capacity control system, when correctly installed with appropriate effective storage, is very versatile and will perform well under almost any operating requirement from base load to “trim.” Dynamic centrifugal compressors are sold as more of an engineered product. To correctly apply these compressors, more solid information is needed on installation data, such as: p Summary When selecting rotating air compressors for future “oil-free” applications, the following best practices guidelines based primarily on energy efficiency should apply. All selections should be investigated thoroughly as to operating costs, projected or guaranteed maintenance/overhaul costs, load profile and installed first cost, etc. p Below 1,500 cfm/90–135 psig First Choice: two-stage oil-free rotary screw, exceptions: Base loaded applications may fit centrifugal 1,500 cfm to 2,500 cfm/90–135 psig Somewhere between 1,500/2,500 cfm flow range, the specific power advantage shifts to a well designed three-stage centrifugal over the oil-free two-stage rotary screw. This is the gray area of selection and other selection criteria will be needed to make the optimum selection. 2,500 cfm to 3,000 cfm/90–135 psig This is the 500 to 600 hp class range and the specific power advantage is to the three-stage centrifugal, which would make it the first machine of choice unless it was to be a dedicated trim unit and the load demand could not be fitted into the turndown the majority of the operating hours. 3,000 cfm and up/90–135 psig In these sizes, the specific power advantage is to the three-stage centrifugal and improves as the size increases. This full-load, energy-efficiency advantage should be evaluated with all the other critical considerations. p p Normal ambient pressure, temperatures, relative humidity Expected flow demand over all 3 shifts and weekends Ambient air make-up and/or condition Cooling water condition temperature etc. Lowest effective pressure required p p p p p Other Key Considerations: Plant personnel training and capabilities — Centrifugals are a little more sensitive and require proper maintenance and knowledgeable operators. A plant can supply this either locally or remotely through outside contractors. Plants that have less intense maintenance, suspect water, hostile operating environment, etc. would probably be better off with an oil-free rotary screw. Part Load Performance Capacity Controls The standard capacity control on the two-stage oil free rotary screw is the two-step or full-load and no-load control. With this control, the compressor load and unload set points are used to activate a pressure that either opens or closes the inlet valve, which allows the unit to run fully loaded or fully unloaded (some bypass air Similar data is also required to correctly apply and install on oil-free two-stage rotary screw but usually it just has to fit within predetermined maximum and minimum limitations. Proper selection and impeller design to the operating conditions has direct impact on performance and operation. Data acquisition must be accurate and analyzed by knowledgeable personnel to optimize the installation opportunities and more important avoid operational problems. For example: p Be sure the highest operating temperatures are considered. Being too low on this could preclude “making full pressure” in hot weather. Be sure the coldest operating temperatures are considered. Being too high on this could cause a motor overload or cut back too much airflow during colder weather. p 42 www.airbestpractices.com http://www.airbestpractices.com

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Compressed Air Best Practices - June 2008 Contents From the Editor Utility-Air News A Pharmaceutical Compressed Air System Audit Talking Dew Point Puerto Rico, Pharmaceuticals and Airequipo Inc. Oil-Free Rotary and Oil-Free Centrifugal Compressor Comparison SMC: Managing Energy and Dew Point in Pneumatic Systems Utility Grade Compressed Air Systems Training Calendar & Product Picks Wall Street Watch Advertiser Index Job Market

Compressed Air Best Practices - June 2008

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