Compressed Air Best Practices-August 2008

| 08/08 Focus Industry COMPRESSED AIR AUDITS | KNOW YOUR SYSTEM Load/Unload with Blow Down Issues or Short Cycling Similar issues are experienced with load/ unload style compressors where varying cycle frequency and issues with blow down times can throw the real-world energy consumption off the theoretical curves. A screw compressor operating in load/unload mode is controlled by a pressure switch and operates within a set pressure band. The unit will alternately load and unload within this pressure band at a duty cycle that is determined by the percent compressor capacity. When the compressor unloads, it blows down its sump, and as it does so the compressor input power drops in relation to the sump pressure. The slower this blow down occurs the more power the compressor consumes during the transition from loaded to unloaded. Short cycling compressors or units with slow blow down will have a characteristic curve that has a more pronounced hump shape rather than the theoretical straight line between zero flow and full load. See Figure 3 for an example of this comparison. Referring to Figure 3, a given compressor might be assumed to be operating on curve 1, but issues with the blow down time might make it actually operate on curve 2. The resulting assumed flow, at 80% input power for this unit, would be 60% of full load, when in actual fact the flow might be 25%. Figure 3 — A compressor with slow blow down characteristics due to a malfunction might follow Curve A rather than its rated Curve B. At 80% power the compressor would be assumed to be 60% loaded when it is actually only running at 25% output. compressors is to construct a theoretical curve from the measured maximum and minimum amp readings, at zero and 100% flow, and then pick the flow at various actual operating points off the curve. This is one of the methods used to calculate flows using the US DOE’s AirMaster+ software, (http:// www1.eere.energy.gov/industry/bestpractices/ software.html#air) a tool that can be used by compressed air users to optimize their air systems. Once system flows are known, flow profiles can be constructed. These flow profiles are then used to calculate “what if” scenarios of different control strategies, alternate compressor types and/or lower flows due to leak reduction, or other efficiency efforts are implemented. Using the theoretical curves for these new scenarios and reversing the previous calculations, resulting projected new system power consumption can be estimated. The comparison of the existing energy consumption to the new consumption can be used to calculate potential savings if efficiency measures are implemented and to help justify project expenditures. 22 www.airbestpractices.com Results Can be Off Unfortunately real-world conditions can sometimes throw a wrench into theoretical calculations. Numerous conditions can affect the actual amp and power input to a compressor and/or the air output. The amp readings, for example, can be affected by changes in voltage. Many industrial sites have fluctuating voltage at the compressors depending on the capacity of the plant power distribution system. Part way into our study experience, Manitoba Hydro started seeing significant issues with estimates done for certain types of compressors, such as ones that have inlet valves with no visual indication of their position. If the customer is fortunate enough to have a compressor with an inlet valve with an external linkage, or a compressor with a capacity gauge, it is a relatively easy task to calibrate the amp or power readings to the theoretical curves. But some compressors have valves that are fully internal and the exact position cannot be determined during the calibration phase. This can lead to large errors in flow estimates. Centrifugals Major problems are also encountered when trying to estimate the output flows of centrifugal compressors using amp or power readings. Most centrifugals we have encountered in our area operate in inlet modulation mode with blow-off or bypass control to prevent the units from entering a damaging surge condition when compressor loading falls below stable flow. Centrifugals http://www1.eere.energy.gov/industry/bestpractices/software.html#air http://www1.eere.energy.gov/industry/bestpractices/software.html#air http://www1.eere.energy.gov/industry/bestpractices/software.html#air http://www.airbestpractices.com

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

Compressed Air Best Practices- August 2008 Contents From the Editor Utility-Air News The August Audit of the Month: A Vacuum System Audit Energized for Savings at Seattle City Light and Tacoma Power Know Your System CASE: Compressed Air Supply Efficiency Ecos Air: Managing Incentive Programs Kaeser Compressors: Loyalty, Energy and Growth Building Relationships at Cummins-Wagner McIntire is Moving Forward The 2008 A.I.C.D. Conference & Exhibition Tuthill Vacuum and Blower Systems Resources for Energy Engineers: Training Calendar & Product Picks Wall Street Watch Advertiser Index Job Market

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