Compressed Air Best Practices - June 2008 - (Page 26)

| 06–07/08 Focus Industry PHARMACEUTICALS | E VA L U AT I O N G U I D E L I N E S F O R T W O - S TA G E , L U B R I C A N T- F R E E R O TA R Y S C R E W A N D M U LT I S TA G E ( 3 ) C E N T R I F U G A L A I R C O M P R E S S O R S The following write-up compares operating parameters, design criteria and basic operating power efficiency of these two very popular “oil-free” compressors. When it is all said and done, they are both excellent products, and when well applied and maintained serve the user very well. There are areas of size (flow) where one and then the other are the most energy efficient and a wide operating area where the energy performance is very similar. To develop this baseline data we used the basic performance manufacturer’s data from 200 to 900 horsepower (hp) covering nine manufacturers, some of which offer both types. The summation of this data, along with the basic operating differences between a Positive Displacement and Dynamic compressor (Mass Flow), create some very interesting situations. Particularly with regard to the effect of inlet air conditions on both, and the basic turndown range and capabilities of both. The basic conclusion is that one technology is not “better” than the other, but certainly one may fit a given set of conditions better than the other. I. Basic Operating Characteristics of the Oil-Free, Two-Stage Rotary Screw Compressor in the 100-Psig Industrial Compressed Air Markets General Design — Whether lobe or screw, non-lubricated rotaries have some common design characteristics. These are positive displacement type. p They are a positive displacement clearance type design and do not require lubrication in the compression chamber The lobes or screws do not drive one another while rotating, but are driven by some type of “bull gear” arrangement. This drive system also acts as a “timing gear” to maintain the accurate rotor or lobe profile relationship Depending on the performance and pressures desired, they can be packaged into single or two-stage configurations. In the 100-psig class market, most units are two-stage and can deliver very acceptable efficiencies Cooling is accomplished through the cylinder walls through water jackets or air-cooled cylinders. Injected oil is not used to cool or seal These units are referred to as “clearance type compressors” which means that the basic design has a constant leakage rate for any fixed set of conditions, clearance, pressure and temperature. Inlet is taken in, sealed and the volume reduced to raise the pressure — positive displacement The general critical internal clearances are: • • • Between end covers and rotor Between rotor lobes Between the rotor “OD” and the cylinder “ID” p p p p p 26 www.airbestpractices.com http://www.airbestpractices.com

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

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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