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  1. 1. To order additional copies ofthis sourcebook, please call:Offfice of Industrial TechnologiesInformation Clearinghouse800-862-2086Co-sponsored by:Council of Industrial Boiler OwnersCouncil of Industrial Boiler Operators6035 Burke Centre Parkway, Suite 360 ImprovingBurke, VA 22015 Steam SystemNational Insulation Association99 Canal Center Plaza, Suite 222 PerformanceAlexandria, VA 22314 a sourcebook for industryNorth American Insulation ManufacturersAssociation44 Canal Center Plaza, Suite 310Alexandria, VA 22314Office of Industrial TechnologiesEnergy Efficiency and Renewable EnergyU.S. Department of EnergyWashington, D.C. 20585 One of a series of industrial energy efficiency sourcebooks NT OF ME EN RT Office of Industrial Technologies A ER DEP GY Energy Efficiency and Renewable Energy ICA U N ITDOE/GO-102002-1557 ER U.S. Department of Energy ED M ST A AT E S OFJune 2002
  2. 2. Acknowledgements Improving Steam System Performance: A Sourcebook for Industry is a development of the BestPractices Program under the U. S. Department of Energy’s Office of Industrial Technologies (OIT). BestPractices undertook this project as a series of sourcebook publications. Other topics in this series include: compressed air systems, pumping systems, fan systems, and motor and drive systems. For more information about DOE’s BestPractices, see OIT and BestPractices in the the Programs, Contacts, and Resources section in this publication. OIT, Lawrence Berkeley National Laboratory, and Resource Dynamics Corporation wish to thank the staff at the many organizations that so generously assisted in the collection of data for this Sourcebook. The Alliance to Save Energy, the Council of Industrial Boiler Operators, the National Insulation Association, and the North American Insulation Manufacturers Association provided valuable assistance in developing, compiling, and reviewing this publication. The BestPractices Steam program appreciates the participation of the Steam Technical Subcommittee. Special thanks are extended to its co-chairs, Dr. Anthony Wright, Oak Ridge National Laboratory, and Glenn Hahn, Spirax Sarco, an Allied Partner, for providing extensive technical guidance and review throughout the preparation of this publication. The efforts of these program and committee participants are greatly appreciated. Additionally, the contributions of the following participants are appreciated for their review of and suggestions for this Sourcebook: Deborah Bloom, ONDEO-Nalco Sean Casten, Turbosteam Corporation Bruce Gorelick, Enercheck Systems Robert Griffin, Enbridge Consumers Gas, Canada Dr. Greg Harrell, Energy, Environment and Resources Center, University of Tennessee-Knoxville Thomas Henry, Armstrong Service Carroll Hooper, Steam Solutions, Inc. James Kumana, Kumana and Associates Andrew W. Larkin, Trigen Energy Corporation Lloyd Mason, Condensate Return Specialists Gil McCoy, Office of Industrial Technologies Clearinghouse Kelly Paffel, Plant Support and Evaluations, Inc. W. Randall Rawson, American Boiler Manufacturers Association Douglas Riley, Millennium Chemical Thomas Scheetz, BASF John Todd, Yarway Corporation Prepared for: The United States Department of Energy Office of Industrial Technologies Prepared by: Lawrence Berkeley National Laboratory Washington, DC Resource Dynamics Corporation Vienna, VA Cover photo credit: NREL/PIX 05559. The Leathers geothermal power plant located in the Salton Sea, California. Photo by Warren Gretz.i Improving Steam System Performance
  3. 3. Contents Acknowledgements i Table of Contents ii List of Figures and Tables iii Quick Start Guide 1 Section 1: Steam System Basics 3 Why Steam? 3 Steam System Operation 3 Generation 5 Distribution 11 End Use 15 Recovery 21 Section 2: Performance Improvement Opportunities 25 Overview 25 Systems Approach 25 Common Performance Improvement Opportunities 25 BestPractices Steam System Improvement Tools 26 Overview of Financing Steam System Improvements 28 Section 3: Programs, Contacts, and Resources 33 OIT and BestPractices 33 Directory of Contacts 35 Resources and Tools 36 Appendices 55 Appendix A: Glossary of Terms 57 Appendix B: Steam Tip Sheets 61 Appendix C: Guidelines for Comment 101A Sourcebook for Industry ii
  4. 4. List of Figures Figure 1. Steam System Schematic 4 Figure 2. Firetube Boiler 5 Figure 3. Watertube Boiler 6 Figure 4. Thermostatic Steam Trap with a Bellows Element 13 Figure 5. Thermostatic Steam Trap with a Bimetallic Element 13 Figure 6. Inverted Bucket Steam Trap 14 Figure 7. Float and Thermostatic Steam Trap 14 Figure 8. Thermodynamic Disc Steam Trap 14 Figure 9. Shell and Tube Heat Exchanger 18 Figure 10. Components of a Plate and Frame Heat Exchanger 18 Figure 11. Configuration of a Jacketed Kettle Heat Exchanger 18 Figure 12. Thermocompressor Operation 20 Figure 13. Condensate Receiver Tank and Pump Combination 22 Figure 14. Flash Steam Recovery Vessel 23 List of Tables Table 1. Key IOF Steam End-Use Equipment 16 Table 2. Common Performance Improvement Opportunities for the Generation, Distribution, and Recovery Parts of Industrial Steam Systems 26iii Improving Steam System Performance
  5. 5. Quick Start Guide Quick Start GuideThis Sourcebook is designed to provide steam system overview of the finance considerations related tousers with a reference that describes the basic steam system improvements. Additionally, thissteam system components, outlines opportunities section discusses several resources and toolsfor energy and performance improvements, and developed by the U. S. Department of Energy’sdiscusses the benefits of a systems approach in (DOE) BestPractices Steam Program to identify andidentifying and implementing these improvement assess steam system improvement opportunities.opportunities. The Sourcebook is divided into threemain sections as outlined below. ◆ Section 3: Programs, Contacts, and Resources This section provides a directory of associationsThis Sourcebook is not intended to be a and other organizations involved in the steamcomprehensive technical guide on improving system marketplace. This section also provides asteam systems, but rather a document that makes description of the BestPractices Steam Program, ausers aware of potential performance improvements, directory of contacts, and a listing of availableprovides some practical guidelines, and directs resources and tools, such as publications, software,the user to helpful resources. A systems approach training courses, and videos.analyzes the supply and the demand sides of thesystem and how they interact, essentially shifting ◆ Appendicesthe focus from individual components to total The Sourcebook includes three appendices.system performance. The cost-effective operation Appendix A is a glossary defining terms used inand maintenance of a steam system require steam systems. Appendix B contains a series ofattention not only to the needs of individual pieces steam system tip sheets. Developed by DOE’sof equipment, but also to the system as a whole. BestPractices Steam Program, these tip sheetsOften, operators are so focused on the immediate discuss common opportunities that industrialdemands of the equipment, they overlook the facilities can use to improve performance andbroader question of how system parameters affect reduce fuel use. Appendix C provides guidelinesthe equipment. for submitting suggested changes and improve- ments to the Sourcebook.◆ Section 1: Steam System BasicsFor users unfamiliar with the basics of steamsystems, or for users seeking a refresher, a briefdiscussion of the terms, relationships, and importantsystem design considerations is provided. Usersalready familiar with industrial steam systemoperation may want to skip this section. This sectiondescribes steam systems using four basic parts:generation, distribution, end use, and recovery.◆ Section 2: Performance Improvement OpportunitiesThis section discusses important factors that shouldbe considered when industrial facilities seek toimprove steam system performance and to loweroperating costs. This section also provides an A Sourcebook for Industry 1
  6. 6. 2 Improving Steam System Performance
  7. 7. Steam System Basics Section 1: Steam System Basics Why Steam? The many advantages that are available from steam are reflected in the significant amount of energy that industry uses to generate it. For example,There are three principal forms of energy used in in 1994, industry used about 5,676 trillion Btus ofindustrial processes: electricity, direct-fired heat, steam energy, which represents about 34 percentand steam. Electricity is used in many different of the total energy used in industrial applicationsways, including mechanical drive, heating, and for product output1.electrochemical reactions. Direct-fired energydirectly transfers the heat of fuel combustion to a Steam use in the Industries of the Future2 isprocess. Steam provides process heating, pressure especially significant. For example, in 1994, thecontrol, mechanical drive, component separation, pulp and paper industry used approximatelyand is a source of water for many process reactions. 2,197 trillion Btu of energy to generate steam, accounting for about 83 percent of the total energySteam has many performance advantages that make used by this industry. The chemicals industry usedit an indispensable means of delivering energy. approximately 1,855 trillion Btu of energy toThese advantages include low toxicity, ease of generate steam, which represents about 57 percenttransportability, high efficiency, high heat capacity, of the total energy used in this industry. Theand low cost with respect to the other alternatives. petroleum refining industry used about 1,373 trillionSteam holds a significant amount of energy on a Btus of energy to generate steam, which accounts forunit mass basis (between 1,000 and 1,250 Btu/lb) about 42 percent of this industry’s total energy use3.that can be extracted as mechanical work througha turbine or as heat for process use. Since most ofthe heat content of steam is stored as latent heat, Steam System Operationlarge quantities of heat can be transferred efficientlyat a constant temperature, which is a useful This Sourcebook uses four categories to discussattribute in many process heating applications. steam system components and ways to enhance steam system performance: generation, distribution,Steam is also used in many direct contact end use, and recovery. These four areas follow theapplications. For example, steam is used as a path of steam as it leaves the boiler and returns viasource of hydrogen in steam methane reforming, the condensate return system.which is an important process for many chemicaland petroleum refining applications. Steam is also ◆ Generationused to control the pressures and temperatures of Steam is generated in a boiler or a heat recoverymany chemical processes. Other significant steam generator by transferring the heat ofapplications of steam are to strip contaminants from combustion gases to water. When water absorbsa process fluid, to facilitate the fractionation of enough heat, it changes phase from liquid to steam.hydrocarbon components, and to dry all types of In some boilers, a superheater further increases thepaper products. energy content of the steam. Under pressure, the steam then flows from the boiler or steam generator and into the distribution system.1 Arthur D. Little, Overview of Energy Flow for Industries in Standard Industrial Classifications 20–39, December, 2000.2 DOE’s Industries of the Future (IOF) include: agriculture, aluminum, chemicals, forest products, glass, metal casting, mining, petroleum refining, and steel.3 Resource Dynamics Corporation estimates. A Sourcebook for Industry 3
  8. 8. Steam System Basics ◆ Distribution at which point the trap passes the condensate into The distribution system carries steam from the the condensate return system. In a turbine, the boiler or generator to the points of end use. Many steam transforms its energy to mechanical work to distribution systems have several take-off lines that drive rotating machinery such as pumps, compressors, operate at different pressures. These distribution or electric generators. In fractionating towers, steam lines are separated by various types of isolation facilitates the separation of various components of valves, pressure regulating valves, and, sometimes, a process fluid. In stripping applications, the steam backpressure turbines. A properly performing pulls contaminants out of a process fluid. Steam is distribution system delivers sufficient quantities of also used as a source of water for certain chemical high quality steam at the right pressures and reactions. In steam methane reforming, steam is a temperatures to the end uses. Effective distribution source of hydrogen. system performance requires proper steam pressure balance, good condensate drainage, adequate ◆ Recovery insulation, and effective pressure regulation. The condensate return system sends the condensate back to the boiler. The condensate is returned to a collection tank. Sometimes the makeup water and ◆ End Use chemicals are added here while other times this is There are many different end uses of steam. Examples done in the deaerator. From the collection tank the of steam’s diverse uses include process heating, condensate is pumped to the deaerator, which strips mechanical drive, moderation of chemical reactions, oxygen and non-condensable gases. The boiler and fractionation of hydrocarbon components. feed pumps increase the feedwater pressure to Common steam system end-use equipment includes above boiler pressure and inject it into the boiler heat exchangers, turbines, fractionating towers, to complete the cycle. strippers, and chemical reaction vessels. Figure 1 provides a general schematic description In a heat exchanger, the steam transfers its latent of the four principal areas of a steam system. The heat to a process fluid. The steam is held in the following sections discuss the components in these heat exchanger by a steam trap until it condenses, areas in greater detail. Pressure Distribution Reducing Valve Combustion Gases Isolation Valve Combustion Air Forced Draft End Use Preheater Fan Process Heater Shell and Tube Economizer Heat Exchanger Steam Boiler Trap Steam Process Heater Trap Steam Trap Condensate Fuel Feed Receiver Combustion Air Pump Condensate Pump Tank Deaerator Recovery Figure 1. Steam System Schematic4 Improving Steam System Performance
  9. 9. Steam System Basics: Generation Generation Firetube Boilers. In firetube boilers, the combustion gases pass inside boiler tubes, and heat is transferred to water on the shell side. A representativeThe generation part of a steam system uses a boiler firetube boiler is shown in Figure 2. Scotch marineto add energy to a feedwater supply to generate boilers are the most common type of industrialsteam. The energy is released from the combustion firetube boiler. The Scotch marine boiler is anof fossil fuels or from process waste heat. The boiler industry workhorse due to low initial cost, andprovides a heat transfer surface (generally a set of advantages in efficiency and durability. Scotchtubes) between the combustion products and the marine boilers are typically cylindrical shells withwater. The most important parts of the generating horizontal tubes configured such that the exhaustsystem include the boiler, the fuel supply, gases pass through these tubes, transferring energycombustion air system, feedwater system, and to boiler water on the shell side.exhaust gases venting system. These systems arerelated, since problems or changes in one generally Scotch marine boilers contain relatively largeaffect the performance of the others. amounts of water, which enables them to respond to load changes with relatively little change in◆ Boilers pressure. However, since the boiler typically holdsThere are two basic types of boilers: firetube and a large water mass, it requires more time to initiatewatertube. The fundamental difference between steaming and more time to accommodate changesthese boiler types is which side of the boiler tubes in steam pressure. Also, Scotch marine boilerscontains the combustion gases or the boiler generate steam on the shell side, which has a largewater/steam. surface area, limiting the amount of pressure they Figure 2. Firetube Boiler44 Guideline for Gas and Oil Emission Factors for Industrial, Commercial, and Institutional (ICI) Boilers, American Boiler Manufacturer’s Association, Arlington, Virginia, 1997. A Sourcebook for Industry 5
  10. 10. Steam System Basics: Generation can generate. In general, Scotch marine boilers are Watertube Boilers. In watertube boilers, boiler water not used where pressures above 300 psig are passes through the tubes while the exhaust gases required. Today, the biggest firetube boilers are over remain in the shell side, passing over the tube 1,500 boiler horsepower (about 50,000 lbs/hr)5. surfaces. A representative watertube boiler is shown in Figure 3. Since tubes can typically withstand Firetube boilers are often characterized by their higher internal pressure than the large chamber number of passes, referring to the number of times shell in a firetube, watertube boilers are used where the combustion (or flue) gases flow the length of high steam pressures (3,000 psi, sometimes higher) the pressure vessel as they transfer heat to the water. are required. Watertube boilers are also capable of Each pass sends the flue gases through the tubes in high efficiencies and can generate saturated or the opposite direction. To make another pass, the superheated steam. In fact, the ability of watertube gases turn 180 degrees and pass back through the boilers to generate superheated steam makes these shell. The turnaround zones can be either dryback boilers particularly attractive in applications that or water-back. In dryback designs, the turnaround require dry, high-pressure, high-energy steam, area is refractory-lined. In water-back designs, this including steam turbine power generation. turnaround zone is water-cooled, eliminating the need for the refractory lining. The performance characteristics of watertube boilers make them highly favorable in process industries, including chemical manufacturing, pulp and paper manufacturing, and refining. Although Figure 3. Watertube Boiler6 5 1 boiler horsepower = 33,475 Btu/hr 6 Guideline for Gas and Oil Emission Factors for Industrial, Commercial, and Institutional (ICI) Boilers, American Boiler Manufacturer’s Association, Arlington, Virginia, 1997.6 Improving Steam System Performance
  11. 11. Steam System Basics: Generationfiretube boilers account for the majority of boiler designed to handle flue gas condensation, non-sales in terms of units, watertube boilers account condensing economizers must be operated atfor the majority of boiler capacity7. temperatures that are reasonably above the dew points of the flue gas components. The dew pointWaste Heat Recovery Boiler (WHRB). These boilers of the flue gases depends largely on the amount ofmay be either firetube or watertube design and use water in the gas, which, in turn, is related to theheat that would otherwise be discarded to generate amount of hydrogen in the fuel. For example, tosteam. Typical sources of heat for WHRBs include avoid condensation in the exhaust gases producedexhaust gases or high temperature products from by burning natural gas, the exhaust gas temperaturean external manufacturing process in refineries and should typically be kept above 250°F. Condensingchemical manufacturing facilities or combustion of economizers are designed to allow condensationa waste fuel in the boiler furnace. of the exhaust gas components. Due to latent heat recovery, these economizers typically extract moreHeat Recovery Steam Generators (HRSGs). HRSGs energy than do non-condensing economizers.transfer energy from the exhaust of a gas turbine to Often, special materials are unfired or supplementary fired heat-recoverysteam generator to produce steam. Exhaust gases For more information on economizers see theleave the gas turbine at temperatures of 1000°F Steam Tip Sheet Number 3 titled Use Feedwater(538°C) or higher and can represent more than Economizers for Waste Heat Recovery in Appendix B.75 percent of the total fuel energy input. This energycan be recovered by passing the gases through a Combustion Air Preheaters. Combustion airheat exchanger (steam generator) to produce hot preheaters are similar to economizers in that theywater or steam for process needs. If the amount of transfer energy from the flue gases back into thesteam needed by the process exceeds the amount system. In these devices, however, the energy isproduced by simple heat recovery, then transferred to the incoming combustion air. Thesupplementary fuel can be burned in the ducting efficiency benefit is roughly 1 percent for everybetween the gas turbine and the HRSG. 40°F increase in the combustion air temperature8.Superheaters. Superheaters add energy to steam, ◆ Boiler Insulationresulting in a steam temperature that exceeds the The walls and combustion regions of boilers aresaturation temperature at a specific pressure. typically lined with insulating materials to reduceSuperheaters can be convective or radiant. energy loss and to prevent leakage. There areRadiative superheaters rely on the energy several types of boiler insulating materials, includingtransferred directly from the combustion flame to brick, refractory, insulation and lagging. Theincrease the energy level of the steam while selection and design of boiler insulating materialsconvective superheaters rely on the transfer of depend largely on the age and design of the boiler.additional energy from the flue gases to the steam. Since the insulating lining is exposed to high temperatures and is subject to degradation, it shouldEconomizers. In many boilers, the flue gases still be periodically inspected and repaired whenhave useful amounts of energy even after they necessary.have passed through the boiler. In many of theseapplications, economizers provide effective ◆ Boiler Control Systemmethods of increasing boiler efficiency by Boiler control systems are designed to protect thetransferring the heat of the flue gases to incoming boiler and to ensure proper boiler operation.feedwater. There are two principal types of These systems include the combustion controleconomizers: non-condensing and condensing. system, flame safeguard, water level control, andNon-condensing economizers are usually air-to-water fuel control.heat exchangers. Since these economizers are not7 GRI, Analysis of the Industrial Boiler Population, Final Report No.-96/0200, 1996.8 Boiler Efficiency Institute, Boiler Efficiency Improvement, 1991. A Sourcebook for Industry 7
  12. 12. Steam System Basics: Generation Combustion Control System. The combustion control Safety Valve. The safety valve is the most important system regulates the fuel air mixture to achieve valve on the boiler and keeps the boiler from safe and efficient combustion and maintains steam exceeding its maximum allowable working system pressure. Control systems have varying pressure (MAWP). levels of sophistication. Simple systems use a fixed linkage between the fuel-regulating valve and the Steam Pressure Control. Steam pressure controls combustion air damper. This is called single point regulate the combustion equipment to maintain a positioning. A change in steam pressure makes a constant pressure in the steam header. As the proportional change in the combustion air and pressure rises above or falls below the pressure fuel. Advanced systems rely on signals from setting, the control adjusts the burner firing rate to transmitters to determine independent fuel valve bring the pressure back to the setpoint. and air damper positions. This is called a full mon- itoring system. Nonreturn Valve. The nonreturn valve is a combination shutoff and check valve that allows For more information see the Steam Tip Sheet steam out of the boiler, but prevents backflow from Number 4 titled Improve Your Boiler’s Combustion the steam header in the event the boiler pressure Efficiency in Appendix B. drops below that of the header. The valve is opened only when the pressure inside the boiler rises Burner Flame Safeguard System. A flame safeguard slightly above the steam header pressure. system is an arrangement of flame detection systems, interlocks, and relays which will sense Steam Flow Meter. Steam flow meters are helpful the presence of a proper flame in a furnace and in evaluating the performance of the system and cause fuel to be shut off if a hazardous condition can provide useful data in assessing boiler develops. Modern combustion systems are closely performance, calculating boiler efficiency, and interlocked with flame safeguard systems and also tracking the amount of steam required by the system. pressure-limit switches, low-water level cutoffs, In some systems, steam flow meters provide a and other safety controls that will stop the energy measurement signal for the boiler control system. input to a boiler when an unsafe condition develops. Additionally, steam flow meters can be useful in The flame safeguard system senses the presence of a benchmarking efforts. good flame or proper combustion and programs the operation of a burner system so that motors, blowers, There are three basic types of steam flowmeters: ignition, and fuel valves are energized only when differential pressure (DP), vortex, and Coriolis. they are needed and then in proper sequence. Differential pressure flowmeters rely on the change in pressure as steam flows by an element such as a Safety Shutoff Valve. Safety shutoff valves isolate nozzle, orifice, or venturi. This pressure difference the fuel supply to the boiler in response to certain provides an indication of flow velocity, which, conditions such as low or high gas pressure or in turn, can be used to determine the flow rate. satisfied load demand. The type of safety shutoff Vortex flowmeters rely on the principal that flow valves and the settings are often determined by past an element creates vortices that have frequencies code or insurance requirements. that correspond to the flow velocity. Coriolis flowmeters rely on tubes placed in the steam flow Water Level Control. The boiler water level control path that twist according to the velocity of the flow. system ensures a safe water level in the boiler. Typically, the control system provides a signal to ◆ Boiler Feedwater System the feedwater control valve to regulate the feed The boiler feedwater system supplies water to the rate. Simple water level control systems that only boiler. Sources of feedwater include returning sense water level are single element systems. More condensate and makeup water. Feedwater is complex systems incorporate additional data such typically stored in a collecting tank to ensure that as steam flow rate (dual element system) and a steady supply of heated water is available to the feedwater flow (triple element system) and will boiler. provide better water level control during abrupt load changes.8 Improving Steam System Performance
  13. 13. Steam System Basics: GenerationFeedwater Flow Control Valve. A modulating removal as deaerators and deaerating heaters,feedwater flow control valve moves up or down typically providing water with oxygen levels ofin response to the water level transmitter(s). On 0.5 to 1 parts per million (ppm).smaller firetube boilers, it is not uncommon for thefeedwater valve to operate in a closed or open In applications that require lower oxygen levels thanposition, depending on the water level transmitter achievable with a deaerator, deaerating heater, orsignal. open feedwater heater, a chemical agent, known as an oxygen scavenger, can be used to remove moreSoftener. Softeners remove hardness minerals, such oxygen. In most systems, an oxygen scavenger isas calcium, magnesium, and iron, from a water part of the system’s water treatment The presence of hardness in boiler waterleads to many problems, including scale buildup For more information on these devices see the Steamand foaming, which reduce boiler efficiency and Tip Sheet Number 18 titled Deaerators in Industrialcan cause tube failure. Softeners reduce this prob- Steam Systems, provided in Appendix B.lem through an ion exchange process. As the hardwater passes through a chamber filled with resin, Feedwater Pump. Feedwater pumps transfer wateran exchange occurs that removes hardness miner- from the deaerator to the boiler. Feedwater pumpsals from the water. The sodium that replaces the are driven by electric motors or by steam turbines.hardness minerals has a higher solubility in water In a modulating feedwater system, the feedwaterand generally will not form scale. pumps run constantly as opposed to an on-off operation in relatively small boilers.Pretreatment Equipment. Pretreatment equipmentimproves the quality of the incoming water so that Collecting/Storage Tank. The return of condensate isit may be used in the boiler without excessive often erratic due to changing steam requirementsscaling or foaming, which can reduce boiler by the end uses. The condensate is usually returnedefficiency and cause tube failure. Pretreatment to a condensate receiver or directly to the deaeratorequipment includes, but is not limited to, clarifiers, if the system does not have a receiver. Pretreatedfilters, softeners, dealkalizers, decarbonators, reverse water may also be stored in a tank prior to use.osmosis (RO) units, and demineralizers. This provides the boiler system with additional water capacity in case the pretreatment equipmentDeaerator, Deaerating Heater, and Atmospheric malfunctions. The condensate and pretreated water,Deaerator. The presence of oxygen in the boiler or makeup, are transferred from the storage tankssystem can be a significant problem due to its to the deaerator prior to being sent to the boiler.corrosivity at high temperatures. Deaerators anddeaerating heaters use heat, typically steam, to ◆ Boiler Combustion Air Systemreduce the oxygen content in water. Deaerators The combustion air system supplies the oxygenand deaerating heaters are typically pressurized necessary for the combustion reaction. To providetanks that raise the water temperature to the point enough air for the amount of fuel used in industrialof saturation. They also break the incoming water boilers, fans are typically required. Dampers, inletinto either fine droplets or thin sheets to facilitate valves, or variable speed drives typically controlthe removal of oxygen and other noncondensible the amount of air allowed into the boiler.gases. Depending on the design, the feedwateroxygen content can be reduced to levels ranging Forced Draft Fan. A forced draft fan is located at thefrom 7 to 40 parts per billion (ppb). inlet of a boiler and pushes ambient air into the burner region, ensuring that adequate air is deliv-Atmospheric deaerators are typically found in ered to the combustion process. These fans eithersmaller, lower-pressure boiler systems. They pull air directly from the boiler room or connect tooperate at atmospheric pressure, so the maximum a duct system that allows outside air to be drawnoperating temperature is 212°F. Most will operate into the temperatures lower than this. Atmosphericdeaerators can not achieve the same level of oxygen A Sourcebook for Industry 9
  14. 14. Steam System Basics: Generation Induced Draft Fan. Induced draft fans are located fuel, it is important to know the energy content of on the outlet gas side of the boiler and pull flue the fuel when determining boiler efficiency. gases out. The induced draft fan creates a slightly negative furnace pressure that is controlled by For more information see the Steam Tip Sheet outlet dampers on the boiler. In some systems where Number 15 titled Benchmark the Fuel Cost of a bag house, mechanical collector, or precipitator Steam Generation in Appendix B. is involved, special considerations should be given in sizing and selection of this fan. Burner. Burners combine the fuel and air to initiate combustion. There are many different types of Damper. Dampers control the amount of air burners due to the many different types of fuels. allowed into and out of a combustion chamber. Additionally, burners have different performance Dampers, in combination with fuel regulating characteristics and control requirements. Some devices, are positioned by the combustion control burners are on/off while others allow precise system to achieve certain fuel:air ratios. Dampers setting of the fuel:air mixture over a range of on the boiler outlet are used to regulate the conditions. Some burners can fire different types of negative furnace draft. fuel, allowing boiler operation to continue despite the loss of one fuel supply. ◆ Boiler Fuel System There are many different types of fuels used in ◆ Boiler Blowdown System boilers, requiring several different types of fuel The boiler blowdown system includes the valves handling systems. Fossil fuels such as coal, oil, and and the controls for the continuous blowdown and gas are most commonly used. Waste fuels are used bottom blowdown services. Continuous blowdown in many industries, particularly the forest products, removes a specific amount of boiler water (often petroleum refining, and chemical manufacturing measured in terms of percentage of feedwater flow) industries where there is an available supply of in order to maintain a desired level of total waste products such as bark, wood chips, black dissolved solids in the boiler. Setting the flow for liquor, and refinery gas. the continuous blowdown is typically done in conjunction with the water treatment program. Fuel Regulating Valve. In gaseous and liquid fuels, Some continuous blowdown systems rely on the regulating valves control the fuel delivered to the input of sensors that detect the level of dissolved boiler. In many systems these valves can be quickly solids in the boiler water. shut in response to an operating problem. The bottom blowdown is performed to remove Fuel. The fuel types that are commonly used in particulates and sludge from the bottom of the boilers include natural gas, coal, propane, fuel boiler. Bottom blowdowns are periodic and are oils, and waste fuels (for example, black liquor, typically performed a certain number of times per bark, and refinery gas). Fuel type significantly shift or according to a set schedule. In some affects boiler operation, including efficiency, systems, bottom blowdowns are controlled by an emissions, and operating cost. Natural gas accounts automatic timer. Bottom blowdown should never for about 36 percent of the total U.S. industry boiler be permitted unless it is recommended by the capacity. Coal accounts for about 14 percent of boiler manufacturer. This is because in higher the boiler capacity. Fuel oils account for about pressure boilers, especially those above 700 psig, 21 percent. Other fuels, which include waste fuels, bottom blowdown may cause water starvation in account for about 29 percent of the boiler capacity9. some portions of the boiler circuit. Fuel Flow Meter. Fuel meters measure the amount Boiler Blowdown Heat Exchangers and Flash Tank. of fuel delivered to a boiler. Fuel meters provide The continuous blowdown water has the same essential data in determining boiler efficiency. temperature and pressure as the boiler water. Before Since fuel flow meters measure volume or mass of this high energy water is discharged into the environment, it is often sent to a heat exchanger 9 Derived from GRI, Analysis of the Industrial Boiler Population, Final Report No.-96/0200, 1996.10 Improving Steam System Performance
  15. 15. Steam System Basics: Distributionand flash tank. Flash tanks permit the recovery of Important configuration issues are flexibility andlow-pressure flash steam, which can be used in drainage. With respect to flexibility, piping,deaeration or process heating. They also permit the especially at equipment connections, needs touse of a smaller heat exchanger than would be accommodate thermal reactions during systemrequired without the flash tank. Blowdown heat startups and shutdowns. Additionally, pipingexchangers are most often used to preheat boiler should be equipped with a sufficient number ofmakeup water. appropriately sized drip legs to promote effective condensate drainage. Additionally, the piping shouldFor more information on boiler blowdowns, see the be pitched properly to promote the drainage ofSteam Tip Sheets Numbers 9 and 10 titled Minimize condensate to these drip lines. Typically theseBoiler Blowdown, and Recover Heat from Boiler drainage points experience two very differentBlowdown in Appendix B. operating conditions: normal operation and start- up; both load conditions should be considered in Distribution the initial design. ◆ InsulationThe distribution system transports steam from theboiler to the various end uses. Although distribution Thermal insulation provides important safety,systems may appear to be passive, in reality, these energy savings, and performance benefits. In termssystems regulate the delivery of steam and respond of safety, insulation reduces the outer surfaceto changing temperature and pressure requirements. temperature of the steam piping, which lessens theConsequently, proper performance of the distribution risk of burns. A well-insulated system also reducessystem requires careful design practices and effective heat loss to ambient workspaces, which can makemaintenance. The piping should be properly sized, the work environment more comfortable.supported, insulated, and configured with adequate Consequently, the energy saving benefits includeflexibility. Pressure regulating devices such as reduced energy losses from the steam system andpressure reducing valves and backpressure turbines reduced burden on the cooling systems that removeshould be configured to provide proper steam heat from workspaces. In addition to its safety andbalance among the different steam headers. energy benefits, insulation increases the amount ofAdditionally, the distribution system should be steam energy available for end uses by decreasingconfigured to allow adequate condensate drainage, the amount of heat lost from the distribution system.which requires adequate drip leg capacity andproper steam trap selection. Steam distribution Important insulation properties include thermalsystems can be broken down into three different conductivity, strength, abrasion resistance,categories: buried pipe, above-ground, and building workability, and resistance to water absorption.sections, and selection of distribution components Thermal conductivity is the measure of heat transfer(piping, insulation, etc.) can vary depending on the per unit thickness. Thermal conductivity ofcategory. insulation varies with temperature; consequently, it is important to know the right temperature range when selecting insulation. Strength is the measure◆ Piping of the insulation’s ability to maintain its integritySteam piping transports steam from the boiler to under mechanical loads. Abrasion resistance is thethe end-use services. Important characteristics of ability to withstand shearing forces. Workability iswell-designed steam system piping are that it is a measure of the ease with which the insulation isadequately sized, configured, and supported. installed. Water absorption refers to the tendencyInstallation of larger pipe diameters may be more of the insulation to hold moisture. Insulationexpensive, but can create less pressure drop for a blankets (fiberglass and fabric) are commonlygiven flow rate. Additionally, larger pipe diameters used on steam distribution components (valves,help to reduce the noise associated with steam expansion joints, turbines, etc.) to enable easyflow. As such, consideration should be given to the removal and replacement for maintenance tasks.type of environment in which the steam pipingwill be located when selecting the pipe diameter. A Sourcebook for Industry 11
  16. 16. Steam System Basics: Distribution Some common insulating materials used in steam exchange components as well as result in water systems include calcium silicate, mineral fiber, hammer. Removing water droplets before they fiberglass, perlite, and cellular glass. The American reach end-use equipment is necessary. Society for Testing and Materials (ASTM) provides standards for the required properties of these and Steam separators remove water droplets, generally other insulation materials. relying on controlled centrifugal flow. This action forces the entrained moisture to the outer wall where Additionally, the North American Insulation it is removed from the separator. The means of mois- Manufacturers Association (NAIMA) has developed ture removal could be a steam trap or a drain. Some a software program titled 3E Plus that allows users manufacturers include the trap as an integral part to determine the energy losses associated with of the unit. Additional accessories include water various types and thicknesses of insulation. The gage connections, thermometer connections, and 3E Plus program facilitates the assessment of various vent connections. insulation systems to determine the most cost-effective solution for a given installation. See Section 2, page 27 Steam separators can be installed in either a for more about 3E Plus Insulation software, which horizontal or vertical line. They are capable of can help steam users assess insulation opportunities. removing 99% of particulate entrainment 10 microns and larger over a wide range of flows. Separators For more information on insulation, refer to Steam Tip are often designed in accordance with ASME Code, Sheets Numbers 2 and 17 titled Insulate Steam Section VIII, Division 1 with pressures to 300 psig. Distribution and Condensate Return Lines and Install Removable Insulation on Uninsulated Valves ◆ Steam Accumulators and Fittings. Both can be found in Appendix B. A steam accumulator is a large insulated pressure vessel, partially filled with hot water (saturated liquid). ◆ Valves When steam supply exceeds demand, the excess In steam systems, the principal functions of valves are high-pressure steam is charged into the accumulator to isolate equipment or system branches, to regulate through special charging nozzles. The steam is steam flow, and to prevent overpressurization. The condensed, giving up its latent heat, to raise the principal types of valves used in steam systems pressure, temperature, and heat content of the water include gate, globe, swing check, pressure reducing, body. When the steam demand exceeds the supply, and pressure relief valves. Gate, globe, and swing the pressure in the accumulator drops and the check valves typically isolate steam from a system additional required steam flashes from the water, branch or a component. Pressure reducing valves taking back the heat previously stored. A simple (PRV) typically maintain certain downstream steam system of control valves and check valves regulates pressure conditions by controlling the amount of the charging and discharging. The excess steam is steam that is passed. These reducing valves are charged quietly and smoothly, and when steam is often controlled by transmitters that monitor down- needed, it is available with the speed of a control stream conditions. Pressure relief valves release valve operation. There is also an accumulator design steam to prevent overpressurization of a system that stores hot water for use as boiler feedwater. header or equipment. ◆ Steam Traps ◆ Steam Separators Steam traps are essential for proper distribution In some steam systems, wet steam is generated. This system performance. During system startups, traps wet steam contains water droplets that can reduce allow air and large quantities of condensate to the effectiveness of the steam system. Water droplets escape. During system operation, the traps allow erode turbine blades and passages reducing efficiency collected condensate to pass into the condensate and life. Water droplets also tend to erode pressure return system, while minimizing the accompanying reducing valves. Furthermore, liquid water can loss of steam. There are three primary types of traps: significantly reduce heat transfer rates in heat thermostatic, mechanical, and thermodynamic10. 10 The following discussion of steam traps is based extensively on C. B. Oland, Review of Orifice Plate Steam Traps, Oak Ridge National Laboratory, January 2001.12 Improving Steam System Performance
  17. 17. Steam System Basics: Distribution◆ Thermostatic TrapsThermostatic traps use temperature differential to Hot or Subcooled Liquid Bimetallic Elementsdistinguish between condensate and live steam. CondensateThis differential is used to open or close a valve.Under normal operating conditions, the conden-sate must cool below the steam temperature before Steam or Liquidthe valve will open. Common types of thermostatic Condensate Condensatetraps include bellows and bimetallic traps. In & Flash Out Valve SeatBellows Traps. Bellows traps include a valveelement that expands and contracts in response to Figure 5. Thermostatic Steam Trap with a Bimetallictemperature changes. Often a volatile chemical Elementsuch as alcohol or water is inside the element. movement causes a valve to open or close. ThereEvaporation provides the necessary force to change are a number of mechanical trap designs that arethe position of the valve. At start up, the bellows based on this principle. They include ball float,trap is open due to the relative cold condition. This float and lever, inverted bucket, open bucket, andoperating condition allows air to escape and float and thermostatic traps.provides maximum condensate removal when theload is the highest. Bellows traps can fail either Ball Float Traps. Ball float traps rely on the move-open or closed. The configuration of a bellows ment of a spherical ball to open and close thesteam trap is shown in Figure 4. outlet opening in the trap body. When no condensate is present, the ball covers the outlet Steam and/or opening, thereby keeping air and steam from Hot condensate escaping. As condensate accumulates inside the Depending on Bellows Element Trap trap, the ball floats and uncovers the outlet Valve opening. This movement allows the condensate to Steam & Liquid Condensate Condensate flow continuously from the trap. Unless they are In & Flash Out equipped with a separate air vent, ball float traps cannot vent air on start up. Seat Figure 4. Thermostatic Steam Trap with a Bellows Float and Lever Traps. Float and lever traps are Element similar in operation to ball float traps except the ball is connected to a lever. When the ball floatsBimetallic Traps. Bimetallic traps rely on the bend- upward due to accumulation of condensate insideing of a composite strip of two dissimilar metals to the trap body, the attached lever moves and causesopen and close a valve. Air and condensate pass a valve to open. This action allows condensate tofreely through the valve until the temperature of the continuously flow from the trap. If the condensatebimetallic strip approaches the steam temperature. load decreases and steam reaches the trap, down-After steam or relatively hot condensate heats the ward ball movement causes the valve to closebimetallic strip and causes it to close the valve, thereby keeping steam from escaping. Unless theythe trap remains shut until the temperature of the are equipped with a separate air vent, float andcondensate cools sufficiently to allow the bimetallic lever traps cannot vent air on start up. See thestrip to return to its original shape and thereby discussion on float and thermostatic the valve. Bimetallic traps can fail in eitherthe open or closed position. The configuration of a Inverted Bucket Traps. Inverted bucket traps arebimetallic steam trap is shown in Figure 5. somewhat more complicated than float and lever traps. At start up, the inverted bucket inside the◆ Mechanical Traps trap is resting on the bottom of the trap body andMechanical traps use the difference in density the valve to which the bucket is linked is widebetween condensate and live steam to produce a open. The trap is initially filled with condensate.change in the position of a float or bucket. This A Sourcebook for Industry 13
  18. 18. Steam System Basics: Distribution As steam enters the trap and is captured inside the Steam & bucket, it causes the bucket to move upward. Condensate In Air Vent This upward movement closes the valve and keeps Steam steam from escaping. When the condensate Space collects and cools the steam, the bucket moves Condensate Level downward. This movement causes the valve to open thereby allowing the condensate to escape. Valve Unlike closed float traps, inverted bucket traps have intermittent discharge. These traps can be Float Lever Liquid Condensate depleted of their “condensate seal” when applied Seat & Flash Out in superheated steam service. If this occurs, the trap will continuously discharge “live steam.” This trap Figure 7. Float and Thermostatic Steam Trap type is not recommended for superheated steam service, unless special installation conditions are ◆ Thermodynamic Traps met. The configuration of an inverted bucket steam Thermodynamic traps use the difference in kinetic trap is shown in Figure 6. energy (velocity) between condensate and live steam to operate a valve. The disc trap is the most Seat Steam Spaces common type of thermodynamic trap, but piston Liquid Condensate Level or impulse traps are sometimes used. Condensate Steam Bubbles & Flash Out Vent Hole Valve Disc Traps. Disc traps use the position of a flat disc Inverted Bucket to control steam and condensate flow. When Lever condensate flows through the trap, the disc is raised thereby causing the trap to open. As steam and air Steam & Condensate pass through the trap the disc moves downward. In The force that causes the disc to move downward is generated by the difference in pressure between Figure 6. Inverted Bucket Steam Trap the low-velocity steam above the disc and the high-velocity steam that flows through the narrow Open Bucket Traps. Open bucket traps consist of an gap beneath the disc. Disc traps commonly have upright bucket that is attached to a valve. At start up, an intermittent discharge and, when they fail, they the bucket rests on the bottom of the trap body. In normally fail open. The configuration of a disc this position, the valve is wide open. As condensate steam trap is shown in Figure 8. Generally, the air accumulates in the trap body on the outside of the bucket, the bucket floats upward causing the valve Outlet Port Flash Vapor Closes to close. When sufficient condensate accumulates Seating Surface Inlet Port Valve Disc outside the bucket, it spills over the top and fills the Bonnet Chamber inside of the bucket. At this time, the bucket sinks Valve Disc causing the valve to open. This trap is also prone to failure when applied in superheated steam service Steam & Liquid because of the loss of the condensate seal. Like Condensate Condensate inverted bucket traps, open bucket traps have In & Flash Out intermittent discharge. Figure 8. Thermodynamic Disc Steam Trap Float and Thermostatic (F&T) Traps. Float and thermostatic (F&T) traps are similar to float and lever removal capability of this trap type is poor unless traps except they include a thermostatic element that equipped with additional components (like the allows air to be discharged at start up and during float and thermostatic trap). operation. The thermostatic elements used in these traps are the same as those used in thermostatic Piston Traps. Piston or impulse traps utilize the traps. The configuration of a float and thermostatic heat energy in hot condensate, and the kinetic steam trap is shown in Figure 7. energy in steam, to open and close a valve. Like14 Improving Steam System Performance
  19. 19. Steam System Basics: End Usedisc traps, piston traps are phase detectors that end-use equipment includes heat exchange devicessense the difference between a liquid and gas or to transfer thermal energy and turbines to recovervapor. They continuously discharge any air and mechanical energy. In manufacturing industries,condensate. Their primary failure mode is open. steam end uses often directly support production, making their performance and reliability essentialLever Traps. Lever traps are a variation of the to plant productivity. Improvements in end-usethermodynamic piston trap. They operate on the efficiency and effectiveness also tend to result insame principal as a piston trap but with a lever better performance and increased reliability. Thereaction to pass large amounts of condensate and air is a wide range of end-use equipment, largely dueon a continuous basis. Their primary failure mode to the advantages of steam that are discussed inis open. the Introduction. Some of the major end-use components are discussed in this section.Orifice Traps. Orifice traps are of two basic types:orifice plate and short tube. Both trap types For the purposes of this discussion, steam end-useoperate under the exact same principles. A simple equipment is grouped into three basic categories:orifice plate steam trap consists of a thin metalplate with a small-diameter hole (orifice) drilled ■ Industries of the Future11 (IOF) key end-usethrough the plate. When installed, condensate that equipment;accumulates is continuously removed as the steam ■ Conditioning and control equipment; andpressure forces the condensate through the orifice. ■ Additional equipment.During conditions when no condensate is present,a limited amount of steam flows through the orifice. The key IOF equipment category includes theThe report Review of Orifice Plate Steam Traps in largest uses of steam in those industries. AlthoughResources: Reports and Technical Papers on page IOF facilities use steam for other services as well,51 of the Programs, Contacts, and Resources the key end uses account for the largest amount ofSection, provides information for making informed steam use. The conditioning equipment categorydecisions about when orifice plate steam traps includes equipment that facilitates the effective useshould be considered for use in new or existing of steam. The additional equipment categorysteam systems. includes equipment that is used in other industries and, though significant, does not account for mostAdditional information regarding steam traps is of the steam use in IOF industries.available in the Steam Tip Sheet Number 1 titledInspect and Repair Steam Traps, found in ◆ Industries of the Future Key End-UseAppendix B. Equipment In the three IOF industries of forest products,◆ Steam Meters petroleum refining, and chemicals, steam accountsThe use of flowmeters within the distribution sys- for the largest amount of end-use energy. In anothertem can provide important data for monitoring the IOF industry, steel production, steam represents aefficiency of a process or an end use. Tracking the significant amount of end-use energy and is usedamount of steam required can be particularly to generate most of that industry’s on-site electricuseful in benchmarking efforts. The types of steam power. Table 1 provides a list of key steam-suppliedflowmeters are discussed in the Generation Section. end-use equipment for IOF industries. End Use ◆ Condensers In steam applications, condensers are associatedSteam system end-use equipment transfers steam with condensing steam turbines and with multipleenergy into other forms of useful energy. Common stage ejector systems. In steam turbine applications, condensers typically operate under a vacuum.11 Industries of the Future (IOF) include: agriculture, aluminum, chemicals, forest products, glass, metal casting, mining, petroleum refining, and steel. A Sourcebook for Industry 15
  20. 20. Steam System Basics: End Use Table 1. Key IOF Steam End-Use Equipment Equipment Process Application Industry Condenser Steam turbine operation Aluminum, Chemical Manufacturing, Forest Products, Glass, Metal Casting, Petroleum Refining, and Steel Distillation tower Distillation, fractionation Chemical Manufacturing, Petroleum Refining Dryer Drying Forest Products Evaporator Evaporation/concentration Chemical Manufacturing, Forest Products Petroleum Refining Process heat Alkylation, Process air heating, Process water Aluminum, Chemical Manufacturing, Forest exchanger heating, Gas recovery/Light ends distillation, Products, Glass, Metal Casting, Petroleum Isomerization, Storage tank heating Refining, and Steel Visbreaking/Coking Reboiler Fractionation Petroleum Refining Reformer Hydrogen generation Chemical Manufacturing, Petroleum Refining Separator Component separation Chemical Manufacturing, Forest Products, Petroleum Refining Steam ejector Condenser operation, Vacuum distillation Aluminum, Chemical Manufacturing, Forest Products, Glass, Metal Casting, Petroleum Refining, and Steel Steam injector Agitation/blending, Heating Chemical Manufacturing, Forest Products, Petroleum Refining Steam turbine Power generation, Compressor mechanical Aluminum, Chemical Manufacturing, Forest drive, Hydrocracking, Naphtha reforming, Products, Glass, Metal Casting, Petroleum Pump mechanical drive, Feed pump Refining, and Steel mechanical drive Stripper Distillation (crude and vacuum units), Chemical Manufacturing, Petroleum Refining Catalytic cracking, Asphalt processing, Catalytic reforming, Component removal, Component separation, Fractionation, Hydrogen treatment, Lube oil processing Thermocompressor Drying, Steam pressure amplification Forest Products They remove energy from the exhaust steam allowing applications, condensers are also used to condense it to be recovered as condensate. In steam ejector components from gaseous mixtures. In these applications, condensers increase the effectiveness applications, the condensers use a cooling medium of the ejectors by condensing both the motive to extract energy from the gases and collect the steam and condensables pulled from the process, condensed components. reducing the amount of motive steam required. ◆ Distillation Towers Condensers can be surface type or barometric. The petroleum refining and chemical manufacturing Surface condensers are supplied with cooling water industries use large amounts of steam to facilitate that circulates through condenser tubes providing the separation of crude oil or chemical feedstocks a cool surface area that causes steam condensation. into various components. This separation process The condensate is typically collected in a condensate relies on differences in the boiling points of these well, and pumped into the condensate return hydrocarbon components. Fractionating towers use system. Barometric condensers rely on direct a furnace to heat crude oil above 700°F. As the contact between the cooling water and the steam. volatile components boil off and rise up the tower, In petroleum refining and chemical manufacturing they cool and condense on trays. Steam is injected16 Improving Steam System Performance
  21. 21. Steam System Basics: End Useinto the bottom of these towers to reduce the ◆ Heat Exchangerspartial pressures of the hydrocarbons, which Heat exchangers transfer thermal energy from onefacilitates their separation, and to reduce coke fluid to another. In manufacturing facilities, steamformation on tray and tower surfaces. is a common source of heat for many reasons, some of which are discussed in the Introduction. There is◆ Dryers a wide range of heat exchanger designs that useDryers reduce the water content of a solid. Dryers steam, largely due to the wide range of products thataccount for the largest end use of steam in the pulp are heated with steam. Many process and productand paper industry12. The chemical manufacturing, considerations must be incorporated into the selectiontextiles, and food processing industries also use of a heat exchanger. Some basic heat exchangerlarge amounts of steam for drying. Dryers can be types are discussed below, including:indirect or direct. Indirect dryers remove moisturethermally as energy is transferred from condensing ■ Tubular;steam, flue gases, or high temperature process ■ Plate and frame;fluid to the product being dried. Common indirect ■ Jacketed; anddryer types are coil and rotating drum. Direct dryers ■ Coil.use hot gases that have been heated with steam orflue gases to directly contact and dry a product. Tubular Heat Exchanger. Tubular heat exchangers are tube bundles that are surrounded by the heatedDryers, like evaporators, can be arranged in multiple- or heating medium. This type of heat exchangerstage configurations. Multiple-stage steam dryers includes finned tube and shell and tube designs asuse a cascading set of steam pressures, allowing shown in Figure 9. Finned tube heat exchangers aresteam released from an upstream stage to supply often used to heat air for drying and space heatingsteam to the next stage. In many multiple-stage applications. Shell and tube heat exchangers aredryers, thermocompressors are used to increase the often used for liquid heating and evaporation.steam pressure of downstream-effect stages. Since the tube side of shell and tube heat exchangers can be designed to withstand high pressures,◆ Evaporators sometimes exceeding 1,500 psig, heat exchangersEvaporators reduce the water content of a liquid, of this type are often used in high temperature andgenerally by heating it with steam in order to high-pressure applications.concentrate the product. Evaporators are usedextensively in industries such as food processing, Plate and Frame Heat Exchanger. In plate and framechemical manufacturing, steel, forest products, and heat exchangers, the two heat exchange fluids aretextiles. separated by plates. The plates are corrugated, or ridged, as shown in Figure 10, to increase theIn most cases, evaporators are shell and tube heat surface area available for heat transfer. Plate andexchangers with the steam on the shell side and frame heat exchangers are often used in low-the product being concentrated in the tubes. viscosity applications, where the risk of clogging isEvaporators can be single effect or multiple effect. less severe. The plate ends are typically sealed byA single effect evaporator uses steam at one set of gasketed covers that can be removed to allowpressure and temperature conditions to boil off the disassembly and cleaning. This heat exchangervapor from a product. Multiple-effect evaporators type is used when temperatures and pressures aretake the vapor produced from one evaporator and use moderately low, typically below 300°F and 370 to heat the product in a lower-pressure evaporator. Plate and frame heat exchangers also have aMultiple-effect evaporators are generally more common design variation that has the plates weldedefficient at concentrating a fluid than single-effect or brazed together. This allows higher temperatureevaporators. service but eliminates the possibility of mechanical cleaning.12 Giese & Associates, Scoping Study of the Pulp and Paper Industry, EPRI, 1988. A Sourcebook for Industry 17
  22. 22. Steam System Basics: End Use drawn from a fractionating Tubesheet tower. These volatile compo- Tube Bundle nents are sent downstream for further processing. The residual components are sent back into Tube Side Fluid the fractionating tower or sent on to a vacuum distillation process. There are several types of reboilers, including jacketed Baffles kettle, kettle, internal reboiler, and thermosyphon reboiler. Shell Side Fluid These designs differ from one another in the way the product Figure 9. Shell and Tube Heat Exchanger is heated with steam. Jacketed Heat Exchangers. Jacketed heat exchangers ◆ Reformers use an enclosure to surround the vessel that Steam reformers are used to generate hydrogen, contains the heated product. A common example typically from a hydrocarbon feedstock such as of a jacketed heat exchanger is the jacketed kettle. methane (the largest component of natural gas). In A representation of a jacketed heat exchanger is turn, hydrogen is used in many petroleum refining shown in Figure 11. Jacketed heat exchangers are and chemical manufacturing processes. Reformers practical for batch processes and for product types use steam for both energy and as a source of that tend to foul or clog tube bundles or coils. hydrogen. Steam is injected with the hydrocarbon feedstock to initiate the following reaction: Coil Heat Exchangers. Coil heat exchangers characteristically use a set of coils immersed in the CH4 + H2O CO + 3H2 medium that is being heated. Coil heat exchangers Methane Steam Carbon Hydrogen are generally compact, offering a large heat transfer monoxide area for the size of the heat exchanger. Reformers often have secondary stages that are ◆ Reboilers used to convert the carbon monoxide to carbon Reboilers are typically used in distilling processes dioxide and additional hydrogen. Although large to increase component separation. Reboilers use amounts of steam are used throughout the heat, often provided by steam, to evaporate the reforming processes, steam is also generated by the volatile components of a product that has been reformers and is sometimes exported for other uses. Frame Plates Steam Kettle Steam Jacket Compression Fasteners Condensate Figure 10. Components of a Plate and Frame Figure 11. Configuration of a Jacketed Kettle Heat Heat Exchanger Exchanger18 Improving Steam System Performance