Compressed Air Best Practices - June 2008 - (Page 43)

Focus Industry PHARMACEUTICALS | 06–07/08 | goes through the unit) over the full operating band which is usually 10 psig. This is very energy efficient throughout the total operating range. Full idle is usually 15–20% of normal input full load power. This creates some application characteristics that should be addressed in all installations: p Regardless of the type of butterfly valve and bypass or unloader valve, the inlet butterfly valve is used in several distinct types of controls: Constant pressure or base load: This is designed to control the discharge at a constant point and matches the compressor output to the demand. As the pressure rises, the bypass valve opens, venting excess air to atmosphere. There is no reduction in power. This is a base load control only and basically obsolete today in industrial applications if they are to experience any varying load. Inlet throttle or modulation control: The inlet butterfly is controlled to permit the compressor to operate on its characteristic curve when the demand for air is less than the compressor’s rated capacity. As the inlet valve closes and the rising pressure approaches surge point, the bypass valve begins to open slowly, bleeding off excess air to match demand. Once the bypass valve is fully open there is no further reduction in power. Full unload or auto dual control: At a predetermined discharge pressure (before the surge point), the compressor is unloaded by: 1. 2. Closing the inlet valve Opening the bypass or unloading valve Backpressure from small piping, high turbulent fittings and connecting small filters, etc., must be minimized to allow enough effective storage to avoid “short cycling” When installed in the same system with modulating controls, such as a centrifugal compressor or variable speed drive rotary screw, the load in point of the two-step control must be above the set point of either type unit because the modulating and VSD controls react immediately to any pressure above their set point and back down into part load Variable speed drive is available on two-stage oil-free rotary screws and in addition to enhancing efficiency compared to fixed speed over part of its load band, it also creates a smooth modulating flow rather than full load/no load p p Centrifugal Compressors — Inlet Butterfly Valve (IBV) (and Bypass/Unloading Valve): When the IBV closes with constant pressure regulation in reaction to a sensed rise in system pressure over the full-load set point (indicating an oversupply), the pressure on the downstream side of the throttle valve falls. This is the actual pressure entering the impeller/diffuser. As the pressure drop across the IBV increases, the density of the entering air decreases, resulting in a lower “mass flow” in relation to icfm (inlet/ambient cubic feet per minute). There is a resultant decrease in power as the mass flow falls — but not in the same proportion. The specific power — scfm of compressed air produced per kW (input kW) — deteriorates. Additionally, as the butterfly valve reaches the end of its closure, it will cause some increased turbulence further reducing the effective flow into the impeller. At “full idle” all manufacturers close or partially close, the inlet butterfly , valve and open the “inlet bypass valve” or “unloader valve” The compressor . is now theoretically flowing just enough air through the stages for cooling and to avoid vacuum and minimize the power draw on the unit. At full unload; the power draw will usually be from 25–35% for a “nominal” 20%. INLET GUIDE VANES (IGV): Inlet guide vanes are usually mounted on the inlet to the first stage in industrial air compressors but may be installed on each stage in larger process units. The volume flow can be increased or decreased within specific limits at a constant pressure. Inlet guide vanes vary the flow and produce a swirl in the inlet airflow usually in the direction of the impeller rotation. At signals from the controller, the vanes rotate to any position from parallel to the air stream to fully closed as these vanes produce the swirl, which induces rotation of the air in the same direction as the impeller. The effect is to reduce the work required to produce the same air discharge condition. The net result is lowering input power requirements and improving specific power (cfm/kW) at lower flow conditions. IGV’s can also increase the flow up to 20% when in the “over throttle” position (flow against rotation) at the same pressure compared to an inlet butterfly valve. This increased flow will require a commensurate amount of additional horsepower. www.airbestpractices.com 43 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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