1、31.1CHAPTER 31AUTOMATIC FUEL-BURNING SYSTEMSGeneral Considerations 31.1Gas-Burning Appliances . 31.3Oil-Burning Appliances 31.11Solid-Fuel-Burning Appliances. 31.16Controls. 31.19UEL-BURNING systems provide a means to mix fuel and airFin the proper ratio, ignite it, control the position of the flame
2、envelope within the combustion chamber, and control a fuel flowrate for safe combustion-heat energy release for space conditioning,water heating, and other processes. This chapter covers the designand use of automatic fuel-burning systems. The fuel can be gaseous(e.g., natural or liquefied petroleum
3、 gas), liquid (primarily thelighter grades of fuel oil or biodiesel), or solid (e.g., coal, or renew-able items such as wood or corn). For discussion of some of thesefuels, their combustion chemistry, and thermodynamics, see Chap-ter 28 of the 2009 ASHRAE HandbookFundamentals.GENERAL CONSIDERATIONST
4、ERMINOLOGYThe following terminology for combustion systems, equipment,and fuel-fired appliances is consistent with usage in gas-fired appli-ance standards of the American National Standards Institute (ANSI)and Canadian Standards Association, the National Fire ProtectionAssociations National Fuel Gas
5、 Code (ANSI Z223.1/NFPA 54),and the Canadian Standards Associations Natural Gas and Pro-pane Installation Code (CSA Standard B149.1).Air, circulating. Air distributed to habitable spaces for heating,cooling, or ventilation.Air, dilution. Air that enters a draft hood or draft regulator andmixes with
6、flue gas.Air, excess. Air that passes through the combustion chamber inexcess of the amount required for complete (stoichiometric) com-bustion.Air, primary. Air introduced into a burner that mixes with fuelgas before the mixture reaches the burner ports.Air, secondary. Air supplied to the combustion
7、 zone down-stream of the burner ports.Appliance. Any device that uses a gas, a liquid, or a solid as afuel or raw material to produce light, heat, power, refrigeration, orair conditioning.Draft. Negative static pressure, measured relative to atmosphericpressure; thus, positive draft is negative stat
8、ic pressure. Draft is theforce (buoyancy of hot flue gas or other form of energy) that producesflow and causes pressure drop through an appliance combustion sys-tem and/or vent system. See Chapter 34 for additional information.Equipment. Devices other than appliances, such as supply pip-ing, service
9、 regulators, sediment traps, and vents in buildings.Flue. General term for passages and conduit through which fluegases pass from the combustion chamber to the outdoors.Flue gas. Products of combustion plus excess air in applianceflues, heat exchangers, and vents.Input rate. Fuel-burning capacity of
10、 an appliance in kilowatts asspecified by the manufacturer. Appliance input ratings are markedon appliance rating plates.Vent. Passageway used to convey flue gases from appliances ortheir vent connectors to the outdoors.Vent gas. Products of combustion plus excess air and dilution airin vents.SYSTEM
11、 APPLICATIONThe following considerations are important in the design, speci-fication, and/or application of systems for combustion of fossil fuels.Safety. Safety is of prime concern in the design and operation ofautomatic fuel-burning appliances. For more information, see thesections on Safety and C
12、ontrols. Appliance standards and installa-tion codes (e.g., ANSI Standard Z21.47/CSA Standard 2.3 for gas-fired central furnaces and ANSI Z223.1/NFPA 54, National FuelGas Code, in the United States) provide minimum safety require-ments. Appliance manufacturers may include additional safetycomponents
13、 to address hazards not covered by appliance standardsand installation codes.Suitability for Application. The system must meet the require-ments of the application, not only in heating capacity, but also in itsability to handle the load profile. It must be suitable for its environ-ment and for the s
14、ubstance to be heated.Combustion System Type. System operation is very much afunction of the type of burner(s), means for moving combustionproducts through the system, proper combustion air supply, andventing of combustion gases to the outdoors.Efficiency. Efficiency can be specified in various ways
15、, depend-ing on the application. Stack loss and heat output are commonmeasures, but for some applications, transient operation must beconsidered. In very high-efficiency appliances, heat extraction fromcombustion products may cool vent gas below its dew point, so con-densation of water vapor in the
16、combustion products must be han-dled, and venting design must consider corrosion by combustionproducts, as well as their lack of buoyancy.Operating Control. Heat load or process requirements mayoccur in batches or may be transient events. The burner control sys-tem must accommodate those requirement
17、s, and the combustionsystem must be able to respond to the controls.Emissions. For safety and air quality reasons, combustion prod-ucts must not contain excessive levels of noxious materials, notablycarbon monoxide, oxides of nitrogen, unburned hydrocarbons, andparticulate material such as soot.Fuel
18、 Provision. Liquid and solid fuels, liquefied gases, and somegaseous fuels require space for storage. Gaseous and liquid fuelsrequire appropriate piping for the fuel-burning system. Fuel storageand delivery provisions must be of adequate capacity and must bedesigned to ensure safe operation.Sustaina
19、bility. Fuel-burning appliances consume fuel resourcesand produce combustion products that are emitted to the atmosphere.The effect of these inherent characteristics can be minimized byusing highly efficient systems with very low emissions of undesir-able substances. (See the section on Efficiency a
20、nd Emission Rat-ings.) Appliances using environmentally neutral (nonfossil) fuel,such as biofuels processed from vegetable oils and biomass materialThe preparation of this chapter is assigned to TC 6.10, Fuels and Com-bustion.31.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)(e.g., forestry wa
21、ste), are available. New technologies are alsoemerging. Through photosynthesis, biomass chlorophyll capturesenergy from the sun by converting carbon dioxide from the atmo-sphere and water from the ground into carbohydrates, complex com-pounds composed of carbon, hydrogen, and oxygen. When thesecarbo
22、hydrates are burned, they are converted back into carbon diox-ide and water, and release the suns energy. In this way, biomassstores solar energy and is renewable and carbon-neutral (UCS 2007).Venting, Combustion Air Supply, and Appliance Installation.Combustion product gases must be handled properl
23、y to ensuresafety and satisfactory system operation. Adequate air supply mustbe provided for combustion and ventilation. Appliances must belocated to provide safe clearance from combustible material and forconvenient service.Standards and Codes. Building codes typically require thatfuel-burning appl
24、iances be design-certified or listed to comply withnationally recognized standards. Appliance construction, safe oper-ation, installation practices, and emissions requirements are oftenspecified. In some locations, codes require special restraint for seis-mic or high wind conditions.Cost. The choice
25、 of fuel-burning system is often based on itbeing the least expensive way to provide the heat needed for a pro-cess. The basic cost of energy tends to narrow the choices, but thetotal cost of purchase and ownership should dictate the final deci-sion. Initial cost is the cost of the appliance(s), ass
26、ociated equipmentand controls, and installation labor. Operating cost includes the costof fuel, other utilities, maintenance, depreciation, and various ongo-ing charges, taxes, fees, etc. Energy cost analysis may indicate thatone fuel is best for some loads and seasons, and another fuel is bestfor o
27、ther times. Substantial operating cost is incurred if skilled per-sonnel are required for operation and maintenance. Sometimesthese costs can be reduced by appliances and control systems thatautomate operation and allow remote monitoring of system perfor-mance and maintenance requirements. Warrantie
28、s should be con-sidered. See Chapter 37 of the 2011 ASHRAE HandbookHVACApplications for a thorough discussion of costs.SAFETYAll appliance systems must either operate safely or have a way tosense unsafe operation, and safely and promptly shut off the fuelsupply before injury or property damage occur
29、s. Safe and unsafeoperation sensing and control is generally designed into the com-bustion control system. Examples of what controls must detect,evaluate, and act on include the following:Time to achieve fuel ignitionSufficient combustion air and/or flue gas flow ratesFuel flow rate (e.g., gas orifi
30、ce pressure)Loss of flameHeat exchange operation (e.g., circulating air blower operatingspeed and timing for furnaces)Flame containment (flame rollout)Appliance component temperaturesLoss of control power supplyEFFICIENCY AND EMISSION RATINGSHeating capacity may be the primary factor in selecting fu
31、el-burning appliances, but efficiency and emission ratings are often ofequal importance to building owners and governmental regulators.Steady-State and Cyclic EfficiencyEfficiency calculations are discussed in Chapter 28 of the 2009ASHRAE HandbookFundamentals. Boiler and furnace efficien-cies are di
32、scussed in Chapters 32 and 33 of this volume.Stack Efficiency. Stack efficiency is a widely used rating ap-proach based on measurement of the temperature and compositionof gases exhausted by fuel-burning appliances. Knowing the oxygenor carbon dioxide concentration of the flue gases and the fuelshyd
33、rocarbon content provides a measure of stack mass flow. In con-junction with flue gas temperature, these data allow determination ofenergy loss in the flue gas exiting the stack. The difference betweenstack loss and energy input is assumed to be useful energy, and stackefficiency is the ratio of tha
34、t useful energy to energy input, expressedas a percentage. Generally, the rating is applied to steady-state com-bustion processes. Flue gas carbon dioxide and oxygen concentra-tions are affected mostly by the fuels hydrocarbon content and bythe appliances combustion system design. Flue gas temperatu
35、re ismostly affected by the appliances heat exchanger design.Heat Output Efficiency. Some rating standards require actualmeasurement of heat transferred to the substance being heated. Heatoutput measurement accounts for all heat losses, not just those in theflue gases. Nonstack heat loss, often call
36、ed jacket loss, is difficult tomeasure and may be quite small, but can be accounted for by mea-suring heat output. The ratio of heat output to energy input,expressed as a percentage, is the heat output efficiency.Load Profile Efficiency. U.S. Federal Trade Commission rulesrequire that some types of
37、residential appliances be rated under pro-tocols that consider load profile. Residential and commercial space-heating furnaces and boilers, for example, are rated by their annualfuel utilization efficiency (AFUE), which considers steady-stateefficiency, heat-up and cooldown transients, and off-seaso
38、n energyconsumption by gas pilot burners. Residential storage-type waterheaters are rated under a protocol that requires measurement ofenergy consumption over a 24 h period, during which prescribedamounts of heated water are drawn. A water heater energy factor Efis calculated from the measurements.
39、The Efrating accounts forstandby losses (i.e., energy loss through the tank and fittings thatdoes not go into the water). Ratings and discussion of AFUE andenergy factors can be found in ASHRAE Standard 103 and in prod-uct directories by the Gas Appliance Manufacturers Association.EmissionsRegulated
40、 Flue Gas Constituents. Appliance safety standardsand environmental regulations specify limits for various substancesthat may be found in combustion flue gases. Substances most oftenregulated are carbon monoxide, oxides of nitrogen, and soot. Limitsfor sulfur oxides, unburned hydrocarbons, and other
41、 particulatematter may also be specified. Rules vary with location, type ofinstallation, and type of combustion appliance. Regulations thatrestrict fuel sulfur content are generally intended to reduce sulfuroxide emissions, which may also reduce particulate emissions undercertain conditions.Flue Gas
42、 Concentration Limits. Standards and codes oftenspecify maximum concentration levels permitted in flue gases.Because flue gases may be diluted by air, requirements invariablyspecify that measured concentration levels be adjusted by calcula-tion to a standard condition. In appliance certification sta
43、ndards,that condition is typically the air-free level (i.e., what the concentra-tion would be if there were no excess air). Carbon monoxide, forexample, is often limited to 400 parts per million air-free. Air qual-ity regulations sometimes specify the maximum level for a fixeddegree of excess air. F
44、or oxides of nitrogen, the level is often spec-ified in parts per million at 3% oxygen. The calculation, in effect,adds or subtracts dilution air to reach a condition at which oxygenconcentration is 3% of the exhaust gas volume.Emission per Unit of Useful Heat. In some U.S. jurisdictions,regulations
45、 for gas-fired residential furnaces and water heatersrequire that emission of oxides of nitrogen not exceed a specifiedlevel in nanograms per joule of useful heat. Measured flue gas emis-sion levels are compared with the benefit in terms of heat output.Under this rating method, high efficiency is re
46、warded. Regulationsfor new installations may differ from those for existing systems andmay be more stringent.Automatic Fuel-Burning Systems 31.3Mass Released to Atmosphere. Emissions from large fuel-firedappliances are often regulated at the site in terms of mass released tothe atmosphere. Limits ma
47、y be expressed, for example, in terms ofkilograms per gigajoule, or megagrams per year.GAS-BURNING APPLIANCESGAS-FIRED COMBUSTION SYSTEMSGas-burning combustion systems vary widely, the most signifi-cant differences being the type of burner and the means by whichcombustion products are moved through
48、the system. Gas input ratecontrol also has a substantial effect on combustion system design.BurnersA primary function of a gas burner is mixing fuel gas and com-bustion air in the proper ratio before their arrival at the flame. In apartially-aerated burner (Bunsen burner), only part of the nec-essar
49、y combustion air is mixed with the gas ahead of the flame. Thisprimary air is typically about 30 to 50% of the stoichiometric air(i.e., that amount of air necessary for complete combustion of thegas). Combustion occurs at the point where adequate secondary airenters the combustion zone and diffuses into the mixture. In mostcases, secondary air entry continues downstream of the burner andheat release is distributed accordingly. The total of primary and sec-ondary air typically ranges from 140 to 180% of the stoichiometricair (i.e., 40 to 80% excess air).Most often, partially aerate
