Plant Services - July 2008 - (Page 35)

efficiency Compressors pability, yielded a $600,000-per-year energy cost savings ($0.05 kWh, 8,760 hrs per year). The new units cost $600,000 and the installed cost was $1.2 million, which meant there was a two-year simple payback. Utility incentives further reduced the payback period. This was a project spawned from a comprehensive, full-plant compressed air audit. Turndown and electrical energy The industrial centrifugal compressor is a dynamic unit with a rapidly rotating impeller to accelerate airflow (Figure 1), which then passes through a diffuser that converts velocity head into pressure head. In the dynamic, mass-flow compressor, the power needed is a function of the air’s weight, flow, volume, temperature and head. The impeller’s design and speed establishes the energy imparted to a pound of air passing through the impeller. That energy is independent of inlet variables. A centrifugal compressor, therefore, delivers a pound of air with a constant expenditure of energy, winter or summer. The volume of air compressed varies with the inlet pressure and temperature. If more air is produced than is needed, the centrifugal compressor must unload (deliver less air) to avoid overpressure. A centrifugal compressor has a maximum pressure it can achieve under specific inlet conditions before the air flow reverses and surges, which triggers a protective compressor shutdown. This description of surge is oversimplified; however, each unit has a rise-to-surge limit or maximum pressure. Turndown is the fraction of full-load flow the compressor can handle without surging. For example, 15% turndown means the unit must run at no less than 85% capacity flow to avoid surging. The surge point varies with inlet conditions (Figure 2). Air density increases at colder temperatures and higher inlet pressure, reducing the volume of inlet air that reaches the maximum mass flow rate. To hold a constant discharge pressure, the inlet air flow must decrease to avoid “running out on the curve” too far and reaching the area of potentially unstable operation. The opposite occurs at higher temperature and lower inlet air pressure. The centrifugal compressor can respond to a varying demand efficiently only within its turndown range. Beyond full turndown, the unit either Seasonal swings Average Winter Max. icfm Max. scfm Min. icfm Max. scfm Max. BHP (at full load scfm) 2,810 2,871 1,695 1,729/TD 41% 638 delivers less air with no reduction in power consumption, or the inlet valve closes partially and operates with blow-off or recirculation at a 25% to 35% power draw with no flow to the plant. Such capacity control is storage-dependent. Turndown is another area where modern units are superior to older ones. The greater the turndown, the more flexible the unit is in meeting demand profiles efficiently. Achieving a turndown of 25% or more in an older unit usually requires impellers designed for higher pressure and sacrificing lowerpressure operating efficiency. Although true, in reality, the efficiency difference is much lower. We now have as much as 37% turndown at 5.86 scfm/kW – a solid situation. Modern inlet guide vanes are much improved over older versions, and are relatively easy and economical to retrofit to most existing units. Guide vanes don’t increase turndown, but they allow the unit to operate throughout the turndown range at or near full-load efficiency. The data in Table 2 are drawn from a 2,500 cfm-class, 125-psig, three-stage compressor operating in the Midwestern part of the United States and show calculated per- Dynamic in action Volute collector Discharge Diffuser Impeller Inducer Inlet Plenum Rotation Figure 1. The impeller is the heart of the centrifugal compressor. Extreme Winter 2,830 3,224 1,504 1,711/TD 47% 680 Average Summer 2,805 2,612 1,844 1,715/TD 34.3% 600 Extreme Summer 2,810 2,422 2,004 1,726/TD 29% 573 Table 2. These are the effects of inlet temperature and ambient condition on performance and turndown for a specific three-stage centrifugal 2,500 scfm-class compressor at 125 psig. July 2008 www.PLANTSERVICES.com 35 http://www.PLANTSERVICES.com

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Plant Services - July 2008

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