1、35.1CHAPTER 35CHIMNEY, VENT, AND FIREPLACE SYSTEMSTerminology . 35.1Draft Operating Principles 35.1Chimney Functions 35.2Steady-State Chimney Design Equations. 35.3Steady-State Chimney Design Graphical Solutions. 35.11Vent and Chimney Capacity Calculation Examples. 35.12Gas Appliance Venting. 35.18O
2、il-Fired Appliance Venting 35.19Fireplace Chimneys 35.20Air Supply to Fuel-Burning Appliances . 35.25Vent and Chimney Materials 35.26Vent and Chimney Accessories. 35.27Draft Fans 35.29Terminations: Caps and Wind Effects 35.30Codes and Standards 35.32Symbols 35.32PROPERLY designed chimney or vent sys
3、tem provides andA controls draft to convey flue gas from an appliance to the out-doors. This chapter describes the design of chimneys and vent sys-tems that discharge flue gas from appliances and fireplace systems.Sustainability. Good chimney and vent design is not only asafety issue, but also can e
4、nhance a buildings sustainability. Thischapter explains how to design vent systems to optimize and mini-mize the materials used to construct fuel-burning appliance ventsand chimneys for low cost and long reliability, reducing the need forvent or chimney replacement, thus saving natural resources. Al
5、so,systems designed to bring outdoor air directly into the appliancespace for combustion and vent gas dilution, instead of relying on airinfiltration into the building, reduce heat load and conserve fuel.TERMINOLOGYIn this chapter, appliance refers to any furnace, boiler, or incin-erator (including
6、the burner). Unless the context indicates other-wise, the term chimney includes specialized vent products such asmasonry, metal, and factory-built chimneys; single-wall metal pipe;type B gas vents; special gas vents; or masonry chimney liner sys-tems. (NFPA Standard 211). Draft is negative static pr
7、essure, mea-sured relative to atmospheric pressure; thus, positive draft isnegative static pressure. Flue gas is the mixture of gases dischargedfrom the appliance and conveyed by the chimney or vent system.Appliances can be grouped by draft conditions at the applianceflue gas outlet as follows (Ston
8、e 1971):1. Those that require draft applied at the appliance flue gas outlet toinduce air into the appliance2. Those that operate without draft applied at the appliance flue gasoutlet (e.g., a gas appliance with a draft hood in which the com-bustion process is isolated from chimney draft variations)
9、3. Those that produce positive pressure at the appliance flue gasoutlet collar so that no chimney draft is needed; appliances thatproduce some positive outlet pressure but also need some chim-ney draftIn the first two configurations, hot flue gas buoyancy, induced-draft chimney fans, or a combinatio
10、n of both produces draft. Thethird configuration may not require chimney draft, but it should beconsidered in the design if a chimney is used. If the chimney systemis undersized, draft inducers in the connector or chimney maysupply draft needs. If the connector or chimney pressure requirescontrol fo
11、r proper operation, draft control devices must be used.Vented gas-fired appliances have been grouped by draft and fluegas conditions as follows by installation codes in Canada (CSAB149.1) and in the United States (ANSI/NFPA 54/ANSI/AGAZ223.1):1. Category I appliances operate with nonpositive vent st
12、atic pres-sure and a vent gas temperature that avoids excessive condensateproduction in the vent.2. Category II appliances operate with nonpositive vent static pres-sure and a vent gas temperature that may cause condensate pro-duction in the vent.3. Category III appliances operate with positive vent
13、 static pressureand a vent gas temperature that avoids excessive condensate pro-duction in the vent.4. Category IV appliances operate with positive vent static pressureand a vent gas temperature that may cause condensate produc-tion in the vent.Category I venting systems are typically sized using ve
14、ntingtables for unobstructed vent systems, as listed in the installationcodes; they are provided for fan-assisted appliances and natural-draft appliances as well as multiappliance system vent arrange-ments. Although these categories are intended for gas-fired appli-ances, they could apply to other a
15、ppliances (e.g., oil- or coal-fired).DRAFT OPERATING PRINCIPLESAvailable draft Dais the draft supplied by the vent system, avail-able at the appliance flue gas outlet. It can be shown asDa= Dt p Dp+ Db(1)whereDa= available draft, PaDt= theoretical draft, Pap = flow losses, PaDp= depressurization, Pa
16、Db= boost (increase in static pressure by fan), PaThis equation can account for a nonneutral (nonzero) pressuredifference between the space surrounding the appliance or fire-place and the atmosphere. If the surrounding space is at a lowerpressure than the atmosphere (space depressurized), the pressu
17、redifference Dpshould also be subtracted from Dtwhen calculatingavailable draft Da, and vice versa. This equation applies to allthree appliance draft conditions at the vent system inlets; forexample, in the second condition with zero draft requirement atthe appliance outlet, available draft required
18、 is zero, so theoreticaldraft of the chimney equals the flow resistance, if no depressuriza-tion or boost is present.Operational consequences of various values of Daare describedas follows:The preparation of this chapter is assigned to TC 6.10, Fuels andCombustion.35.2 2012 ASHRAE HandbookHVAC Syste
19、ms and Equipment (SI)Positive available draft (negative vent pressure); Dais posi-tive. Category I fan-assisted and draft hood-equipped appliancesand category II appliances can operate satisfactorily when servedby venting systems having positive available draft at the applianceflue gas outlet, if th
20、e positive draft is sufficient to convey all fluegas from the appliance flue gas outlet to the outdoors and if thepositive available draft does not aspirate excessive excess air tocause flame lifting or other detriments to combustion perfor-mance. Category III and IV appliances can operate satisfact
21、orilywhen served by venting systems having positive available draft ifthe appliance flue gas discharge pressure plus positive draft is suf-ficient to convey all flue gas from the appliance flue gas outlet tothe outdoors. If positive available draft and/or appliance flue gasdischarge pressure is insu
22、fficient to convey all flue gas from theappliance flue gas outlet outdoors, incomplete combustion,flame rollout, and/or flue gas spillage can occur at the appliance.Zero available draft (neutral vent pressure); Da= 0. CategoryI fan-assisted and draft hood-equipped appliances and CategoryII appliance
23、s can operate satisfactorily when served by ventingsystems having zero available draft (neutral draft) at the appli-ance flue gas outlet, if the venting system creates sufficient the-oretical draft to convey all flue gas from the appliance flue gasoutlet to the outdoors. If the venting system create
24、s insufficienttheoretical draft to convey all flue gas from the appliance fluegas outlet to the outdoors, incomplete combustion, flame rollout,and/or flue gas spillage can occur at the appliance. Category IIIand IV appliances can operate satisfactorily when served byventing systems having zero avail
25、able draft, if appliance fluegas discharge pressure is great enough to overcome the ventflow pressure-drop loss.Negative available draft (positive vent pressure); Dais nega-tive. Category I and II appliances cannot operate satisfactorilywhen served by venting systems having negative available draft.
26、Category III and IV appliances can operate satisfactorily whenserved by venting systems having negative draft, if appliance fluegas discharge pressure plus the theoretical draft created by theventing system is sufficient to overcome the vent negative draft atthe vent inlet and the vent flow pressure
27、-drop loss.If chimney height and flue gas temperatures provide surplusavailable draft Da(excessive excess air), draft control is required.Theoretical draft Dtis the natural draft produced by thebuoyancy of hot gases in the chimney relative to cooler gases inthe surrounding atmosphere. It depends on
28、chimney height, localbarometric pressure, and the mean chimney flue gas temperaturedifference tm, which is the difference in temperature between theflue gas and atmospheric gases. Therefore, cooling by heat transferthrough the chimney wall is a key variable in chimney design. Pre-cise evaluation of
29、theoretical draft is not necessary for most designcalculations because of the availability of chimney design charts,computer programs, capacity tables in the references, buildingcodes, and vent and appliance manufacturers data sheets.Chimney temperatures and acceptable combustible materialtemperatur
30、es must be known in order to determine safe clearancesbetween the chimney and combustible materials. Safe clearancesfor some chimney systems, such as type B gas vents, are determinedby standard tests and/or specified in building codes.Losses from Flow p represent the friction losses imposed onthe fl
31、ue gas by flow resistance through the chimney.Depressurization Dpis negative pressure in the space surround-ing the appliance with respect to the atmosphere into which thechimney discharges. Dpcan be caused by other appliances and fansoperating in the building that remove or add air to the space sur
32、-rounding the appliance, by building stack effect, by outdoor atmo-spheric effects such as wind impacting the chimney exit or the sideof the building facing or leeward to the wind, and other buildingphenomena.Boost Dbis the pressure boost from a mechanical-draft fan. Achimney with an forced-draft fa
33、n at the inlet of the chimney wouldhave positive boost (increased static pressure). A chimney with aninduced-draft fan at the outlet of the chimney would have negativeboost (decreased static pressure).The following sections cover the basis of chimney design foraverage steady-state category I and III
34、 appliance operating condi-tions. For other appliance operating conditions, a rigorous cyclicevaluation of the flue gas and material surface temperatures in thechimney vent system can be obtained using the VENT-II computerprogram (Rutz and Paul 1991). For oil-fired appliances, chimneyflue gas and ma
35、terial surface temperature evaluations can be ob-tained using the OHVAP computer program (Krajewski 1996).CHIMNEY FUNCTIONSThe proper chimney can be selected by evaluating factors such asdraft, configuration, size, and operating conditions of the appliance;construction of surroundings; appliance usa
36、ge classification; resi-dential, low, medium, or high heat (NFPA Standard 211); and build-ing height. The chimney designer should know the applicable codesand standards to ensure acceptable construction.In addition to chimney draft, the following factors must be con-sidered for safe and reliable ope
37、ration: adequate air supply for com-bustion; building depressurization effects; draft control devices;chimney materials (corrosion and temperature resistance); flue gastemperatures, composition, and dew point; wind eddy zones; and par-ticulate dispersion. Chimney materials must resist oxidation and
38、con-densation at both high and low fire levels at all design temperatures.Start-UpThe equations and design charts in this chapter may be used todetermine vent or chimney size for average category I and III ventsystem operating conditions based on steady-state operating con-ditions. The equations and
39、 charts do not consider modulation,cycling, or time to achieve equilibrium flow conditions from a coldstart. Whereas mechanical draft systems can start gas flow, gravitysystems rely on the buoyancy of hot flue gases as the sole force todisplace the cold air in the chimney. Priming follows Newtonslaw
40、s of motion. The time to fill a system with hot flue gases, dis-place the cold air, and start flow is reasonably predictable and isusually a minute or less; however, unfavorable thermal differen-tials, building/chimney interaction, mechanical equipment (e.g.,exhaust fans), or wind forces that oppose
41、 the normal flow of ventgases can overwhelm the buoyancy force. Then, rapid priming can-not be obtained solely from correct system design. The VENT-IIcomputer program contains detailed analysis of gas vent and chim-ney priming and other cold-start considerations and allows forappliance cycling and p
42、ressure differentials that affect performance(Rutz and Paul 1991). A copy of the solution methodology (Rutz1991) for VENT-II, including equations, may be ordered fromASHRAE Customer Service.Air IntakesAll rooms or spaces containing fuel-burning appliances musthave a constant supply of combustion air
43、 from outdoors (eitherdirectly or indirectly) at adequate static pressure to ensure propercombustion. In addition, air (either directly or indirectly) is requiredto replace the air entering chimney systems through draft hoods andbarometric draft regulators and to ventilate closely confined boilerand
44、 furnace rooms.The U.S. National Fuel Gas Code (ANSI/NFPA 54/ANSI/AGAZ223.1) and the Canadian Natural Gas and Propane InstallationCode (CSA Standard B149.1), along with appliance manufacturers,provide requirements for air openings. Any design must considerflow resistance of the air supply, including
45、 register-louver resistance,air duct resistance, and air inlet terminations. Compliance with theseChimney, Vent, and Fireplace Systems 35.3codes or the appliance manufacturers instructions accounts for theair supply flow resistance.Vent SizeSmall residential and commercial natural-draft gas applianc
46、esneed vent diameters of 75 to 300 mm. U.S. and Canadian codesrecommend sizes or input capacities for most acceptable gas appli-ance venting materials. These sizes also apply to gas applianceswith integral automatic vent dampers, as well as to appliances withfield-installed automatic vent dampers. F
47、ield-installed automaticvent dampers should be listed for use with a specific appliance by arecognized testing agency and installed by qualified installers.Draft ControlPressure, temperature, and other draft controls have replaceddraft hoods in many residential furnaces and boilers to attain highers
48、teady-state and seasonal efficiencies. Appliances that use pulsecombustion or forced- or induced-draft fans, as well as thosedesigned for sealed combustion or direct venting, do not have drafthoods but may require special venting and special vent terminals. Iffan-assisted burners deliver fuel and ai
49、r to the combustion chamberand also overcome the appliance flow resistance, draft hoods orother control devices may be installed, depending on the design ofthe appliance. Vent category II, III, and IV and some category Iappliances do not use draft hoods; in such cases, the listed appliancemanufacturers vent system design requirements should be fol-lowed. The section on Vent and Chimney Accessories has informa-tion on draft hoods, barometric regulators, draft fans, and other draftcontrol devices.Frequently, a chimney must produce excess flow or draft. Forexample, dangerously
