Mejores practicas uso vapor

Heat Transfer

Steam
System Management
David Jaber, Alliance to Save Energy Gilbert A. McCoy, Washington State Univ. Fred L. Hart, U.S. Dept. of Energy chieving operational excellence is an ongoing task for manufacturers seeking to reduce costs and keep their plants profitable. Fortunately, many opportunities exist for industrial plants to cut energy costs without jeopardizing jobs or theenvironment. The chemical process industries (CPI) in particular can benefit because they are very steam intensive, with boiler fuel accounting for over 50% of their total energy use (1). Common areas for energy savings opportunities are steam generation, distribution, end use and recovery. This article outlines some typical energy efficiency measures. The suggestions and information presented herecome from the U.S. Dept. of Energy’s (DOE) Energy Tips series, which can be consulted for further information; tip sheets are available online at www.oit.doe.gov/bestpractices/.

Follow These Best Practices in

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These tips on how to improve steam generation, distribution and recovery can help a plant to increase its steam system efficiency by as much as 20–30%.
Nalco), a process engineerdetermined that a steam header pressure of 125 psig was no longer necessary due to changes in some of the plant’s processes. A team of personnel from the maintenance, utilities and production departments evaluated the feasibility of reducing the header pressure and decided to incrementally decrease header pressure, while monitoring the effects of this change on system performance. The pressure wasreduced twice, first to 115 psig, and then to 100 psig. The plant determined that there were no detrimental impacts on system operation, and it now operates the system at 100 psig. This reduced energy consumption by 8%, far exceeding initial expectations, and has saved the plant $142,000 annually. Carbon emissions have also been reduced (2). Another approach is to reduce a specific end-usepressure. As part of its Operational Excellence Program, Vulcan Chemicals (Birmingham, AL) implemented a process optimization project involving two chloromethane production units. This four-month project required no capital investment. The steam system pressure in the first distillation column was lowered from 35 to 26 psig, which resulted in a lower condensing temperature that required less refluxduring component separation. Average reboiler steam demand per unit of product decreased by almost 6%, with yearly cost savings of $42,000 (3).

Demand reduction A plant’s boilers may be producing more steam or steam at a higher pressure than is necessary for the end uses. Understanding process requirements is especially important when downstream improvements, such as insulation and increasedcondensate return, are implemented. Lower losses translate into changes in steam production. At Nalco Chemical Co.’s Clearing Plant in Bedford Park, IL (Nalco is now known as ONDEO

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December 2001

CEP

Photos courtesy of Spirax Sarco, Inc., Energy Services

Boiler tune-up The major areas of opportunity in boiler tune-up encompass minimizing excess combustion air andcontinuous boiler blowdown optimization. Providing an optimum amount of excess combustion air minimizes stack heat losses, while ensuring complete fuel combustion. Stack temperature and flue gas oxygen (or carbon dioxide) content are the primary indicators of excess air. Optimum excess air is a function of fuel type and boiler and burner design; many well-designed systems use less than 10% excessair. Flue gas composition should be monitored regularly with gas-absorbing test kits or combustion efficiency analyzers. Highly variable steam flowrates or the use of fuels with variable composition or moisture content may warrant the installation of an online oxygen analyzer with oxygen trim control. Minimizing boiler blowdown helps maintain a high steam quality for effective production, while...