Plant Services - July 2008 - (Page 36)

efficiency Compressors Percent of rated discharge pressure formance and turndown limits at various times of year. The scfm rating reflects the relative air density and several other factors to establish limitations: Volute • Effects of ambient temperature and pressure collector • Mass flow rate that avoids overloading the motor Discharge Diﬀuser • Cooling water temperatures • Limited maximum volumetric flow rate With these thoughts in mind, it’s obvious that an effective Impeller capacity-control system needs accurate data regarding inlet conditions, power drawInducer and operating temperatures to optimize turndown. This, in conjunction with inlet guide vanes, allows the widest range of operation at or near full-load efInlet Plenum Rotation ficiency. Also, the air-management system should keep the units within their turndown ranges, except one that might be in blow-off. There are some effective centrifugal compressor central air-management systems that do this well. Watch for surge 140 Surge line 130 140 Inlet air temperature Higher temperature: lower scfm/lower hp – 130 increase turndown lower temperature Percent of rated horsepower Lower inlet temperature B 120 120 110 * A* 110 110 Horsepower 90 Design temperature 110 90 Other efficiency improvements Select an electric motor that’s large enough to handle the maximum expected flow at the cold conditions. A system designed for the southeastern United States used four 500-hpclass, three-stage units with 700-hp motors and an effective central air-management system. During cooler weather, we ran only three units. During the hottest months, we needed four most of the time. As a matter of interest, we used heat of compression desiccant regenerative dryers and held the velocity in the interconnecting piping to less than 20 fps, both of which resulted in using significantly less electrical energy for desiccant-dried air. Run the centrifugal compressor at the lowest pressure while avoiding surge. This can increase the flow at, or nearly at, the same horsepower draw. The magnitude of the increase depends on individual unit performance curves. One unit we audited delivered 1,884 acfm at 453 bhp at 135 psig. 80 A-Base load set point B-Acceptable surge point 70 70 80 Higher inlet 80 90 100 110 Percent of inlet capacity 70 120 Figure 2. The three curves compare operating characteristics with pressure and flow, including changes in surge point and horsepower requirements without a controlling device in the inlet and discharge piping. The impeller’s design and speed establishes the energy imparted to a pound of air as it passes through the impeller. That energy is independent of inlet temperature, pressure, throttling and other variables. At 90 psig, it delivered 2,074 acfm at 453 bhp – an increase of 190 acfm using the same power. This unit has a relatively steep operating curve; another unit might exhibit a much greater flow differential. You might need to modify the pipHeadline ancillary air-treatment equipment. ing and Upgrade your existing units. Many original equipment manufacturers and some aftermarket organizations offer performance-upgrade packages for older units. In one case, a completely new high-performance compressor package 36 can be set on the old drive train, in effect delivering a new unit at lower cost. Add new technology. The centrifugal compressor lends itself to significant advances from technology, particularly with electronic controls. The electronic controls continue to improve performance with electronic devices that can cycle in milliseconds. A new variable-speed drive uses a high-speed drive combined with uses asynchronous induction motors with magnetic bearings that can operate as fast as 60,000 rpm. This drive system eliminates the need for a speed-increasing gear box. The magnetic bearing provides the motor and directdriven compressor assembly with stable control throughout the operating range. The shaft is supported by a magnetic field resulting in no contact and no wear. This technology offers up-front advantages: • No oil anywhere in the drive train • No startup air required to hold oil back from the compression chamber when a double labyrinth seal is used • Normal drive train mechanical losses are reduced from between 12% and 17% to between 6% and 9% • The current lineup is two-stage units that are about 4% to 6% less power-efficient than a comparable threestage unit. This is more than offset by the savings in mechanical losses. July 2008 www.PLANTSERVICES.com http://www.PLANTSERVICES.com

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Plant Services - July 2008

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