1、32.1CHAPTER 32BOILERSClassifications 32.1Selection Parameters 32.5Efficiency: Input and Output Ratings. 32.5Performance Codes and Standards 32.6Sizing 32.6Burner Types 32.7BOILER CONTROLS . 32.7Flame Safeguard Controls . 32.8OILERS are pressure vessels designed to transfer heat (pro-B duced by combu
2、stion) to a fluid. The definition has beenexpanded to include transfer of heat from electrical resistance ele-ments to the fluid or by direct action of electrodes on the fluid. Inmost boilers, the fluid is usually water in the form of liquid or steam.If the fluid being heated is air, the heat exchan
3、ge device is called afurnace, not a boiler. The firebox, or combustion chamber, of someboilers is also called a furnace.Excluding special and unusual fluids, materials, and methods, aboiler is a cast-iron, carbon or stainless steel, aluminum, or copperpressure vessel heat exchanger designed to (1) b
4、urn fossil fuels (oruse electric current) and (2) transfer the released heat to water (inwater boilers) or to water and steam (in steam boilers). Boiler heat-ing surface is the area of fluid-backed surface exposed to the prod-ucts of combustion, or the fire-side surface. Various manufacturersdefine
5、allowable heat transfer rates in terms of heating surface basedon their specific boiler design and material limitations. Boilerdesigns provide for connections to a piping system, which deliversheated fluid to the point of use and returns the cooled fluid to theboiler.Chapters 6, 11, 12, 13, and 15 c
6、over applications of heating boil-ers. Chapter 7 discusses cogeneration, which may require boilers.CLASSIFICATIONSBoilers may be grouped into classes based on working pressureand temperature, fuel used, material of construction, type of draft(natural or mechanical), and whether they are condensing o
7、r non-condensing. They may also be classified according to shape andsize, application (e.g., heating, process), and state of output medium(steam or water). Boiler classifications are important to the specify-ing engineer because they affect performance, first cost, and spacerequirements. Excluding d
8、esigned-to-order boilers, significant classdescriptions are given in boiler catalogs or are available from theboiler manufacturer. The following basic classifications may behelpful.Working Pressure and TemperatureWith few exceptions, boilers are constructed to meet ASME Boilerand Pressure Vessel Cod
9、e, Section IV (SCIV), Rules for Constructionof Heating Boilers (low-pressure boilers), or Section I (SCI), Rulesfor Construction of Power Boilers (high-pressure boilers).Low-pressure boilers are constructed for maximum workingpressures of 103 kPa (gage) steam and up to 1100 kPa (gage) hotwater. Hot-
10、water boilers are limited to 120C operating tempera-ture. Operating and safety controls and relief valves, which limittemperature and pressure, are ancillary devices required to protectthe boiler and prevent operation beyond design limits.High-pressure boilers are designed to operate above 103 kPa(g
11、age) steam, or above 1100 kPa (gage) and/or 120C for waterboilers. Similarly, operating and safety controls and relief valves arerequired.Steam boilers are generally available in standard sizes from17 kW to 30 MW, many of which are used for space heating appli-cations in both new and existing system
12、s. On larger installations,they may also provide steam for auxiliary uses, such as hot waterheat exchangers, absorption cooling, laundry, and sterilizers. Inaddition, many steam boilers provide steam at various temperaturesand pressures for a wide variety of industrial processes.Water boilers are ge
13、nerally available in standard sizes from10 kW to over 30 MW, many of which are in the low-pressure classand are used primarily for space heating applications in both newand existing systems. Some water boilers may be equipped witheither internal or external heat exchangers for domestic water ser-vic
14、e.Every steam or water boiler is rated for a maximum workingpressure that is determined by the applicable boiler code underwhich it is constructed and tested. When installed, it also must beequipped at a minimum with operation and safety controls and pres-sure/temperature-relief devices mandated by
15、such codes.Fuel UsedBoilers may be designed to burn coal, wood, various grades offuel oil, waste oil, or various types of fuel gas, or to operate as electricboilers. A boiler designed for one specific fuel type may not be con-vertible to another type of fuel. Some boilers can be adapted to burncoal,
16、 oil, or gas. Several designs accommodate firing oil or gas, andother designs allow firing dual-fuel-burning equipment. Accommo-dating various fuel-burning equipment is a fundamental concern ofboiler manufacturers, who can furnish details to a specifying engi-neer. The manufacturer is responsible fo
17、r performance and rating ac-cording to the code or standard for the fuel used (see section onPerformance Codes and Standards). Construction MaterialsMost noncondensing boilers are made with cast iron sectionsor steel. Some small boilers are made of copper or copper-cladsteel. Condensing boilers are
18、typically made of stainless steel oraluminum because copper, cast iron, and carbon steel will corrodebecause of acidic condensate.Cast-iron sectional boilers generally are designed according toASME SCIV requirements and range in size from 10 kW to4.1 MW gross output. They are constructed of individu
19、ally castsections, assembled into blocks (assemblies) of sections. Push orscrew nipples, gaskets, and/or an external header join the sectionspressure-tight and provide passages for the water, steam, and prod-ucts of combustion. The number of sections assembled determinesthe boiler size and energy ra
20、ting. Sections may be vertical orhorizontal, the vertical design being more common (Figures 1A and1C).The boiler may be dry-base (the combustion chamber is beneaththe fluid-backed sections), as in Figure 1B; wet-base (the combus-tion chamber is surrounded by fluid-backed sections, except fornecessar
21、y openings), as in Figure 2A; or wet-leg (the combustionThe preparation of this chapter is assigned to TC 6.1, Hydronic and SteamEquipment and Systems.32.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)chamber top and sides are enclosed by fluid-backed sections), as inFigure 2B.The three types
22、of boilers can be designed to be equally efficient.Testing and rating standards apply equally to all three types. Thewet-base design is easiest to adapt for combustible floor installa-tions. Applicable codes usually demand a floor temperature underthe boiler no higher than 50 K above room temperatur
23、e. A steamboiler at 102C or a water boiler at 116C may not meet this require-ment without appropriate floor insulation. Large cast-iron boilersare also made as water-tube units with external headers (Figure 2C).Steel boilers generally range in size from 15 kW to the largestboilers made. Designs are
24、constructed to either ASME SCI orSCIV (or other applicable code) requirements. They are fabricatedinto one assembly of a given size and rating, usually by welding.The heat exchange surface past the combustion chamber is usuallyan assembly of vertical, horizontal, or slanted tubes. Boilers of thefire
25、-tube design contain flue gases in tubes completely submergedin fluid (Figures 1D and 1E show residential units, and Figures 3Ato 3D and Figure 4A show commercial units). Water-tube boilerscontain fluid inside tubes with tube pattern arrangement providingfor the combustion chamber (Figures 4C and 4D
26、). The internalconfiguration may accommodate one or more flue gas passes. Aswith cast-iron sectional boilers, dry-base, wet-leg, or wet-basedesigns may be used. Most small steel boilers are of the dry-base,vertical fire-tube type (Figure 1D).Larger boilers usually incorporate horizontal or slanted t
27、ubes;both fire-tube and water-tube designs are used. A popular horizontalfire-tube design for medium and large steel boilers is the scotchmarine, which is characterized by a central fluid-backed cylindricalcombustion chamber, surrounded by fire-tubes accommodating twoor more flue gas passes, all wit
28、hin an outer shell (Figures 3A to 3D).In another horizontal fire-tube design, the combustion chamber hasFig. 1 Residential BoilersFig. 2 Cast-Iron Commercial BoilersBoilers 32.3a similar central fluid-backed combustion chamber surrounded byfire tubes accommodating two or more flue gas passes, all wi
29、thin anouter shell. However, this design uses a dry base and wet leg (or mudleg) (Figure 4A).Copper boilers are usually some variation of the water-tubeboiler. Parallel finned copper tube coils with headers, and serpentinecopper tube units are most common (Figures 1F and 1G). Some areoffered as wall
30、-hung residential boilers. The commercial bentwater-tube design is shown in Figure 4B. Natural gas is the usualfuel for copper boilers.Stainless steel boilers usually are designed to operate with con-densing flue gases. Most are single-pass, fire-tube design and aregenerally resistant to thermal sho
31、ck. ASME limits operating tem-peratures to 99C and 1103 kPa (gage) working pressure.Aluminum boilers are also usually designed to operate withcondensing flue gases. Typical designs incorporate either cast alu-minum boiler sections or integrally finned aluminum tubing. ASMElimits operating temperatur
32、es to 93C and working pressure to345 kPa (gage).Type of DraftDraft is the pressure difference that causes air and/or fuel to flowthrough a boiler or chimney. A natural draft boiler is designed tooperate with a negative pressure in the combustion chamber and inthe flue connection. The pressure differ
33、ence is created by thetendency of hot gases to rise up a chimney or by the height of theboiler up to the draft control device. In a mechanical draft boiler,a fan or blower or other machinery creates the required pressure dif-ference. These boilers may be either forced draft or induced draft. Ina for
34、ced-draft boiler, air is forced into the combustion chamber tomaintain a positive pressure in the combustion chamber and/or thespace between the tubing and the jacket (breaching). In an induced-draft boiler, air is drawn into the combustion chamber to maintaina negative pressure in the combustion ch
35、amber.Condensing or NoncondensingTraditionally designed boilers must operate without condensingthe flue gas in the boiler. This precaution was necessary to preventcorrosion of cast-iron, steel, or copper parts. Hot-water units wereoperated at 60C minimum water temperature to prevent this corro-sion
36、and to reduce the likelihood of thermal shock.Because a higher boiler efficiency can be achieved with a lowerwater temperature, the condensing boiler allows the flue gas watervapor to condense and drain. Full condensing boilers are nowavailable from a large number of manufacturers. These boilers are
37、specifically designed for operation with the low return watertemperatures found in hot-water reset, water-source heat pump,two-pipe fan-coil, and reheat systems. Two types of commercialcondensing boilers are shown in Figure 5. Figure 6 shows a typicalrelationship of overall condensing boiler efficie
38、ncy to return watertemperature. The dew point of 55C shown in the figure varies withthe percentage of hydrogen in the fuel and oxygen/carbon dioxideratio, or excess air, in the flue gases. A condensing boiler is shownin Figure 1H. Condensing boilers can be of the fire-tube, water-tube,cast-iron, and
39、 cast-aluminum sectional design.Condensing boilers are generally provided with high-turndownmodulating burners and are more efficient than noncondensing boil-ers at any return water temperature (RWT), including noncondensing-temperature applications. Efficiencies of noncondensing boilers mustbe limi
40、ted to avoid potential condensing and corrosion. Furtherefficiencies can be gained by using lower RWT or higher t asrecommended by ASHRAE. For example, a natural gas condensingboiler operating with 15C RWT in a water-source heat pump appli-cation has potential boiler efficiency in excess of 98% (Fig
41、ure 6).For maximum reliability and durability over extended productlife, condensing boilers should be constructed for corrosion resis-tance throughout the fireside combustion chamber and heatexchanger.Fig. 3 Scotch Marine Commercial BoilersFig. 4 Commercial Fire-Tube and Water-Tube Boilers32.4 2012
42、ASHRAE HandbookHVAC Systems and Equipment (SI)Noncondensing heat plant efficiency may in some cases beimproved with the use of external flue gas-to-water economizers.The condensing medium may include domestic hot-water (DHW)preheat, steam condensate or hot-water return, fresh-water makeup,or other f
43、luid sources in the 20 to 55C range. The medium can alsobe used as a source of heat recovery in the HVAC system. Caremust be taken to protect the noncondensing boiler from the low-temperature water return in the event of economizer service or con-trol failure.Figure 7 shows how dew point varies with
44、 a change in the per-centages of oxygen/carbon dioxide for natural gas. Boilers thatoperate with a combustion efficiency and oxygen and carbon diox-ide concentrations in the flue gas such that the flue gas temperaturefalls between the dew point and the dew point plus 80 K should beavoided, unless th
45、e venting is designed for condensation. This tem-perature typically occurs with boilers operating between 83 and87% efficiency, and the flue gas has an oxygen concentration of 7 to10% and the carbon dioxide is 6 to 8%. Chapter 35 gives furtherdetails on chimneys.The condensing portion of these boile
46、rs may require specialmaterial or operating techniques to resist the corrosive effects of thecondensing flue gases. In the past, typical cast iron, carbon steel,and copper were not suitable materials for the condensing section ofa boiler. Certain stainless steels and aluminum alloys were suitable.Ho
47、wever, advances in design, controls, and manufacturing haveallowed materials such as cast iron to be used where they previouslycould not be; as with all products, consult the manufacturer forproper application. Commercial boiler installations can be adaptedto condensing operation by adding a condens
48、ing heat exchanger inthe flue gas vent.Heat exchangers in the flue gas venting require a condensingmedium such as (1) low pressure steam condensate or hot waterreturn, (2) domestic water service, (3) fresh water makeup, or(4) other fluid sources in the 20 to 55C range. The medium can alsobe a source
49、 of heat recovery in HVAC systems.Wall-Hung BoilersWall-hung boilers are a type of small residential gas-fired boilerdeveloped to conserve space in buildings such as apartments andcondominiums. These boilers are popular in Europe. The most com-mon designs are mounted on outer walls. Combustion air entersthrough a pipe from the outdoors, and flue products are venteddirectly through another pipe to the outdoors. In some cases, the airintake pipe and vent pipe are concentric. Other designs mount adja-cent to a chimney for venting and use indoor air for combustion.These units may be
