Compressed Air Best Practices - June 2008 - (Page 28)

| 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 speed drive, but offer better efficiency performance throughout the specific “sweet zone” VSD . drives are less storage dependent to deliver proper operation, but to achieve optimum cycling, proper storage should be calculated and installed. The VSD drive delivers a constant, smooth, continuous flow and generally avoids cycling. Two staged, lubricant-free rotary screw compressors are limited to a maximum compression ratio of 11:1 or 12:1. This may have a significant effect on the ability to deliver pressure at altitudes and under other varying conditions. For example: The nominal maximum pressure will be 150 psig at 1,500 feet elevation, while at 5,000 feet elevation, the nominal maximum pressure will be 125 psig. backpressure and normal flow is restored through another reversal. If operation in the surge region continues, these flow reversals also continue, resulting in surge. Surges create a temperature and pressure spike at the impeller discharge resulting in automatic shutdown to protect the unit. Units are normally applied with surge protection. A modern electronic control can very effectively monitor inlet conditions and keep the unit clear of operating within the surge range. Some important points regarding “rise to surge”: p II. Basic Operating Characteristics of Multi-Stage Centrifugal Compressors in the 100-Psig Class Industrial Compressed Air Market The centrifugal compressor used in industry is a dynamic compressor with rapidly rotating impellers accelerating the airflow. The air then passes through a diffuser section, which converts the velocity head into pressure through flow resistance. In a dynamic or mass flow compressor like the centrifugal, the power to compress the air basically is a function of the weight of the air, the flow, volume and temperature and the head or pressure. Once the impeller is designed and a speed set, the energy that a pound of air will absorb in passing through the impeller is established. This is true despite variation in inlet temperature, pressure level, throttling, etc. A pound of air will vary in cubic feet by temperature and pressure. A centrifugal compressor, therefore, will deliver a pound of air with a constant expenditure of energy, winter or summer. The actual volume of inlet air to be compressed will vary for a period of time with the inlet condition of pressure and temperature. A centrifugal compressor therefore, will deliver a positive displacement compressors. pound of air with a constant expenditure of energy, winter or summer. At this point, you must note that your “design conditions” for a centrifugal must be at your warmest temperatures to be sure you have adequate flow during those conditions. As the pressure falls or rises in the positive displacement rotary screw, the flow stays relatively constant; the power to run the compressor is directly proportional to the discharge pressure. The dynamic type centrifugal compressor reacts to a change in discharge pressure; indirectly as the pressure rises, the flow is reduced. As the pressure falls, the flow increases, all at similar input power. Depending on design, the “surge point” limits the actual amount of turndown in a given unit. This unstable operating region is entered when system pressure exceeds that being developed within the compressor. Then flow reverses and air attempts to flow toward the compressor. This reduces system Performance characteristics of centrifugal versus The surge point will vary with inlet conditions At colder temperatures and higher inlet pressure conditions, the density of the air increases, resulting in lower volume of inlet air (icfm or acfm) to reach the maximum design point of pounds of airflow. This means to hold the constant target discharge pressure, the inlet air flow will have to be reduced to avoid “running out on the curve” too far and reaching the stonewall area of potentially very unstable operation The opposite occurs at higher temperatures and lower pressure inlet air p p If the centrifugal compressor has to respond to a varying demand efficiently, it can usually only do this effectively throughout its “turndown range” . After full turndown is reached, the unit either: p Blow offs — all excess air produced beyond turndown. This results in less air delivered to production with no reduction in power (kW) or electrical energy (kWh) — OR — 28 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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