Plant Services - September 2007 - (Page 49) EfficiEncy HVAC 3. Variable-frequency drives for evaporator fans represent to a person a problem difficult to solve or a situation difficult to resolve. Approach variable-speed drives for refrigeration compressors with caution. Figure 1 illustrates the potential in a freezer application by showing the combined compressor and evaporator operating efficiency serving a -20°F space with an evaporator temperature difference of 8.4°F. Note the increasing advantage of the VFD fan option over the fixed-speed fan option as the evaporator’s part-load ratio decreases. Low-capacity operation with fixed speed fans is costly. 4. Variable-speed compressors Approach variable–speed refrigeration compressors with caution. Compressors in these systems tend to be driven by a large-frame motor, which makes the VFD option expensive. At most, consider having only one variable-speed compressor per suction pressure level in the plant. Use the VFD compressor as a trim machine for capacity modulation. 5. Variable-speed condenser fans September 2007 Puz-zle / p z- l \ vb puz-zled; puz-zling: To offer or In many cases, VFD condenser fans can yield operating costs savings of 2% to 3%, depending on a number of factors including: relationship of www.PLANTSERVICES.com e e Because most evaporators don’t operate at their design load 100% of the time, their capacity needs to be varied to meet instantaneous thermal loads. Evaporator efficiency at part-load conditions can be improved in most systems by using variable-frequency drives (VFD) on evaporator fans. The savings attributable to this technology depends on a number of factors including system suction pressure, evaporator part-load ratio, evaporator fan type, and face velocity of air over the evaporator coil. heat rejection capacity available to liquid management problems on the that required, minimum head-pressystem’s high side. sure set point, condenser fan type, and others. If you pursue a VFD 6. Heat recovery at oil coolers project for condenser fans, install It’s possible to recover heat from the VFDs on every condenser fan and discharge gas on high-stage compresmodulate their capacity equally to sors. However, a more effective option Plant energy savings and avoid Page 2 maximize Services ad for 07.qxp 4/26/2007 10:53 AMfrom oil-cooling is to recover heat Puz-zle / p z- l \ vb puz-zled; puz-zling: To offer or represent to a person a problem difficult to solve or a situation difficult to resolve. e e 49 http://www.PLANTSERVICES.com
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