Chemical Processing - April 2008 - (Page 38)

>> Load/unload capacity control 110 100 90 80 Percent kW input 70 60 50 40 30 20 10 0 0 10 20 1 gal/cfm 30 40 2 gal/cfm 50 Percent capacity 60 70 80 90 100 ◆ ◆ ◆ ◆ ◆ ▲ ▲ ◆ ▲ ◆ ▲ ◆ ▲ ◆ ▲ ◆ ▲ ◆ ▲ ◆ ▲ ◆ ▲ ◆ ▲ ◆ ▲ ◆ ▲ ◆ ▲ ◆◆ ◆ ◆ ▲ 5 gal/cfm 10 gal/cfm Figure 2. Oil-injected rotary screw compressors require ample capacity to avoid cycling with this type of control. valve, discharging already compressed air to atmosphere. This should be avoided. Some centrifugal compressors can be unloaded at this point to keep from wasting compressed air. Some units also can be run in a load/unload mode. Unloading involves closing the inlet valve and discharging the substantially reduced minimum flow to atmosphere, resulting in an unloaded power of approximately 15%. The bearing designs must be capable of running in this markedly reduced mass-flow condition; any desired change in capacity control should be discussed with the manufacturer to avoid potential problems. Advantages of centrifugal compressors include: • oil-free air delivery; • generally well packaged and no need for special foundations; • relatively smooth air delivery; and • relative ﬁrst cost per cfm or hp improves with size. Disadvantages include: • limited constant discharge pressure capacity control range; and • need for specialized bearings for high rotational speeds and monitoring of vibrations and internal clearances. At full capacity and 100-psig discharge pressure, operating cost typically runs 16–20 kW/100 cfm. 2. Rotary screw. Many chemical processes require oil-free air, so oil-free compressors are popular. Oil-injected compressors sometimes are used with additional clean-up equipment including specialized filtration — but this adds pressure 38 • April 2008 drop, mandating a higher compressor discharge pressure than otherwise would be needed. The oil-free rotary screw compressor generally requires two stages of compression, with inter-cooling, for pressures in the 100–150 psig range. Capacity reduction on constant speed compressors by inlet valve throttling is limited to about 80% of capacity, due to increasing discharge temperature. Below this it’s necessary to unload the compressor. Ample storage capacity then is required. Compressors with variable speed drive (VSD) provide more flexibility in capacity reduction. Oil-injected rotary screw compressors can use various means of capacity control. The most common but least energy efficient is by inlet valve throttling. At about 40% of capacity, the power requirement is more than 80% of full load power. Below 40%, the compressor is unloaded, with no flow to the system, but will then require about 25% of full load power. Again, ample storage capacity is required. Load/unload (full capacity/zero capacity) control can cause excessive cycling unless ample storage capacity is provided (Figure 2). Some models are available with variable displacement, which provides capacity control by changing the effective length of the rotors. This offers some improvement over inlet valve throttling but also is limited in the capacity control range. VSD rotary screw compressors provide much more efficient capacity control, although they sacrifice a bit of efficiency at full capacity. Compare the percent powwww.chemicalprocessing.com http://www.chemicalprocessing.com

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Chemical Processing - April 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Is It a Tragedy or Comedy for Engineers? Better Understanding Boosts Mixer Scale-up Don't Err With Air Compressors Control Performance Supervision Enhances Revamp Process Puzzler Plant InSites Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - April 2008

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