Chemical Processing - April 2008 - (Page 37)

>> Effect of inlet temperature 200 180 160 140 ◆ ◆ ◆ ◆ ◆ ◆ Pressure, psig 120 100 80 60 40 20 0 800 900 1,000 1,100 1,200 1,300 Capacity scfm 1,400 1,500 1,600 1,700 1,800 ◆ ◆ ◆ Inlet temp 95°F ◆ Inlet temp 55°F Control line ◆ Surge line Figure 1. Reduced capacities in centrifugal compressors can lead to a reversal of ﬂow. Centrifugal compressors have a characteristic pressure curve, where pressure increases as capacity decreases. This is an important consideration in capacity control, which is further complicated because pressurehead-making ability varies inversely with absolute inlet temperature and resultant mass. This means that, when designed for a given inlet capacity in actual cubic feet per minute (acfm), the mass flow increases inversely with absolute inlet temperature and the pressure gen◆ ◆◆ erated also increases because ◆ the denser inlet air. ◆ of ◆◆▲▲ So, for a given discharge pressure, the compressor will ◆◆▲▲ have◆ “grown.” See Figure 1, which presumes the same ◆ ▲ ▲ inlet temperature to each stage. Cooling water tem◆ ▲ ▲ ◆ peratures can further affect the curve. Figure 1 has a ▲ ◆ 60 ▲ control pressure of 125 psig. It also shows a “surge” Compressor types ◆ ▲ Three kinds of 50 compressors generally find use at chem- line, to the left of which, at reduced capacities, a flow ▲ ◆ reversal can occur and should be avoided. ical plants: ◆▲▲ 40 Generally, plants want to maintain a relatively constant 1. Centrifugal. Larger plants often use centrifugal com◆ 30 ◆ pressors, which are capable of substantial ows of air, pressure to the system; controls will reduce capacity if the 20 although they’re now available in as low as 100 hp. pressure tends to rise above the set point due to decreasing These are 10 dynamic compressors, where a continuous demand. With centrifugal compressors, a common means of ow of air has its velocity increased by impellers ro- capacity reduction is by progressively closing an inlet valve 0 This tating at high0 speeds. The velocity energy, or kinetic or guide vanes.60 causes a pressure drop at the compressor 10 20 30 40 50 70 80 90 100 inlet, capacity energy, is translated into pressure energy in diffuser or Percentreducing the mass ow in proportion to the absolute volute chambers. The number of impeller stages de- inlet pressure. Because inlet guide vanes impart rotation in a 1gal/cfm 2gal/cfm ▲ 5gal/cfm 10gal/cfm ◆ pends upon the desired nal pressure and ef ciency. similar direction to the impeller, this provides energy savings Cooling occurs between each stage. An important compared with a simple butter y valve. However, as stated, ow reduction must be limited to avoid surge and generally advantage of centrifugal compressors is that they ow shouldn’t go below 80% or 70% of capacity. are inherently oil-free, with no lubricant coming into The biggest potential energy problem is when furcontact with the air passing through the compression ther capacity reduction requires opening a blow-off process. They generally are water-cooled. www.chemicalprocessing.com Percent kW input the units, adequate secondary storage (near each point of use) can prevent wide swings of pressure at each point of use. Compressed air leakage in the distribution system also contributes to the cost of operation. Leak detection equipment is readily available and an ongoing leak detection and repair program should be standard practice. In addition, idle production machines or processes should 110 be tted with shut-off valves that close upon shut down. Adequate pressure sensing points must be available 100 from compressor discharge, before and after dryers and 90 filters and throughout the distribution system to each 80 point of use. This can allow monitoring the dynamic ◆ variations in the entire system. 70 April 2008 • 37 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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