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 15 psig steam and up to 160 psig hot water. Hot-waterboilers
10、are limited to 250F operating temperature. Operating andsafety controls and relief valves, which limit temperature and pres-sure, are ancillary devices required to protect the boiler and preventoperation beyond design limits.High-pressure boilers are designed to operate above 15 psigsteam, or above
11、160 psig and/or 250F for water boilers. Similarly,operating and safety controls and relief valves are required.Steam boilers are generally available in standard sizes up to andabove 100,000 lb steam/h (60,000 to over 100,000,000 Btu/h), manyof which are used for space heating applications in both ne
12、w andexisting systems. On larger installations, they may also providesteam for auxiliary uses, such as hot water heat exchangers, absorp-tion cooling, laundry, and sterilizers. In addition, many steam boil-ers provide steam at various temperatures and pressures for a widevariety of industrial proces
13、ses.Water boilers are generally available in standard sizes from35,000 to over 100,000,000 Btu/h, many of which are in the low-pressure class and are used primarily for space heating applicationsin both new and existing systems. Some water boilers may beequipped with either internal or external heat
14、 exchangers for domes-tic water service.Traditionally, boilers were rated by boiler horsepower, a unitof measurement with one boiler horsepower being equal to33,475 Btu/h or the evaporation of 34.5 lb of water per hour at stan-dard atmospheric pressure (14.7 psia) and 212F.Every steam or water boile
15、r 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 such codes.Fuel UsedBoilers
16、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, oil, or gas. Several design
17、s 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 for performance and rating ac-
18、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 typically made of stainless
19、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 35,000 to13,975,000 Btu/h gross output. They are constructed of individu-ally cast sectio
20、ns, assembled into blocks (assemblies) of sections.Push or screw nipples, gaskets, and/or an external header join thesections pressure-tight and provide passages for the water, steam,and products of combustion. The number of sections assembleddetermines the boiler size and energy rating. Sections ma
21、y be verti-cal or horizontal, the vertical design being more common (Figures1A and 1C).The preparation of this chapter is assigned to TC 6.1, Hydronic and SteamEquipment and Systems.32.2 2012 ASHRAE HandbookHVAC Systems and Equipment The boiler may be dry-base (the combustion chamber is beneaththe f
22、luid-backed sections), as in Figure 1B; wet-base (the combus-tion chamber is surrounded by fluid-backed sections, except fornecessary openings), as in Figure 2A; or wet-leg (the combustionchamber top and sides are enclosed by fluid-backed sections), as inFigure 2B.The three types of boilers can be d
23、esigned 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 90F above room temperature. A steamboiler at
24、215F or a water boiler at 240F 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 50,000 Btu/h to thelargest boilers made. Designs are constructed t
25、o either ASME SCIor SCIV (or other applicable code) requirements. They are fabri-cated into one assembly of a given size and rating, usually bywelding. The heat exchange surface past the combustion chamberis usually an assembly of vertical, horizontal, or slanted tubes.Boilers of the fire-tube desig
26、n contain flue gases in tubes com-pletely submerged in fluid (Figures 1D and 1E show residentialunits, and Figures 3A to 3D and Figure 4A show commercialunits). Water-tube boilers contain fluid inside tubes with tube pat-tern arrangement providing for the combustion chamber (Figures4C and 4D). The i
27、nternal configuration may accommodate one ormore flue gas passes. As with cast-iron sectional boilers, dry-base,wet-leg, or wet-base designs may be used. Most small steel boilersare of the dry-base, vertical fire-tube type (Figure 1D).Larger boilers usually incorporate horizontal or slanted tubes;bo
28、th fire-tube and water-tube designs are used. A popular horizontalfire-tube design for medium and large steel boilers is the scotchFig. 1 Residential BoilersFig. 2 Cast-Iron Commercial BoilersBoilers 32.3marine, which is characterized by a central fluid-backed cylindricalcombustion chamber, surround
29、ed by fire-tubes accommodating twoor more flue gas passes, all within an outer shell (Figures 3A to 3D).In another horizontal fire-tube design, the combustion chamber hasa similar central fluid-backed combustion chamber surrounded byfire tubes accommodating two or more flue gas passes, all within an
30、outer 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-hung r
31、esidential 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 shock. ASM
32、E limits operating tem-peratures to 210F and 160 psig 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 temperatures to 200F an
33、d working pressure to50 psig.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 difference is created by
34、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 forced-draft boiler, a
35、ir 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 chamber.Condensing or
36、 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 140F minimum water temperature to prevent this cor-rosion and to reduce the
37、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 arespecifically desig
38、ned 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 efficiency to return wate
39、rtemperature. The dew point of 130F 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 cast-aluminum se
40、ctional 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 limited to avoid pote
41、ntial 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 60F RWT in a water-source heat pump appli-cation has potential boiler efficiency in excess of 98% (Figure 6).Fig. 3 Sco
42、tch Marine Commercial BoilersFig. 4 Commercial Fire-Tube and Water-Tube Boilers32.4 2012 ASHRAE HandbookHVAC Systems and Equipment For maximum reliability and durability over extended productlife, condensing boilers should be constructed for corrosion resis-tance throughout the fireside combustion c
43、hamber and heatexchanger.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 fluid sources in the 7
44、0 to 130F range. The medium canalso be 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 a change in the per
45、-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 140F should beavoided, unless the venting is designe
46、d 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 boilers may require speci
47、almaterial 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.However, advances in d
48、esign, 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 condensing heat exchanger i
49、nthe 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 70 to 130F range. The medium canalso be a source 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 en
