Chemical Processing - April 2008 - (Page 36)

hemical plants vary widely in size and complexity — and so do the types of compressors used in them, and the compressors’ capacity control systems. The units often differ significantly from those at most industrial manufacturing plants using compressed air, where the most common compressor now is the oil-injected rotary screw. While chemical plants generally pose less variation in flow requirements from shift to shift than other manufacturing facilities, there can be differences in pressure requirements for the various processes, plus instrument air. Air quality requirements, including dryness, also may be stricter for some processes, requiring different drying and filtration methods. Each of these aspects may require segregation of some systems for more efficient operation. In addition, the cost of loss of production, due to an interruption of air supply, must be considered. We’ll look at these considerations. Demand proﬁle It’s essential to know how, where and in what quantities compressed air is being used. List each process or application, together with the minimum, average and peak 36 • April 2008 C rates of ow and any cycle times on and off. Staggering the timing of peak demands can reduce the total maximum demand. One common problem is that distribution piping is sized for the average demand, with pressure drop based on that. When peak demand is signi cantly higher, the pressure drop also will be signi cantly higher, resulting in a lower pressure at the point of use or requiring a higher discharge pressure from the compressor(s). So, size distribution piping for the peak ow rate with an air velocity in the pipe not to exceed 30 ft/sec. Also, establish the required operating pressure for each process or application. Significant differences in required operating pressure may identify potential segregation of processes or applications for overall system efficiency. Operating the total compressed air supply at the highest required pressure results in increased power consumption (an approximately 1% increase for every 2 psi). Having a separate air supply at the required pressure for a specific process or application can improve overall system efficiency. In each case, ensure that sufficient backup air supply is available. While adequate primary compressed air storage (near the compressors) is essential for ef cient capacity control of www.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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