ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 33-2012 FURNACES.pdf

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1、33.1CHAPTER 33FURNACESComponents . 33.1Heat Source Types 33.4Commercial Equipment 33.5Controls and Operating Characteristics . 33.5Equipment Selection 33.6Calculations . 33.8Technical Data . 33.9Installation . 33.10Agency Listings 33.11URNACES are self-enclosed, permanently installed majorFappliance

2、s that provide heated air through ductwork to thespace being heated. In addition, a furnace may provide the indoorfan necessary for circulating heated or cooled air from a split orsingle-package air conditioner or heat pump (see Chapter 9). Fur-naces may be used in either residential or commercial a

3、pplications,and may be grouped according to the following characteristics: Heat source: electricity, natural gas/propane (natural draft or fanassisted, and condensing or noncondensing), or oil (forced draftwith power atomizing burner)Installation location: within conditioned space (indoors), oroutsi

4、de conditioned space either outdoors, or inside the structurebut not within the heated space (isolated combustion systems)Mounting arrangement and airflow: horizontal forced-air, vertical(natural convection, forced-air upflow, forced-air downflow, orforced-air lowboy), or multiposition forced-airFur

5、naces that use electricity as a heat source include one or moreresistance-type heating elements that heats the circulating air eitherdirectly or through a metal sheath that encloses the resistance ele-ment. In gas- or oil-fired furnaces, combustion occurs in the heatexchanger sections or in a combus

6、tion chamber. Circulating airpasses over the outer surfaces of a heat exchanger so that it does notcontact the fuel or the products of combustion, which are passed tothe outdoor atmosphere through a vent.In North America, natural gas is the most common fuel suppliedfor residential heating, and the c

7、entral-system forced-air furnace(Figure 1) is the most common way of heating with natural gas. Thistype of furnace is equipped with a blower to circulate air through thefurnace enclosure, over the heat exchanger, and through the duct-work distribution system. The furnace is categorized as follows:He

8、at source: GasCombustion system: Induced-draft manifold burnerInstallation location: Inside the structure but not within the con-ditioned spaceMounting: VerticalAirflow: UpflowCOMPONENTSA typical furnace consists of the following basic components:(1) a cabinet or casing; (2) heat exchangers; (3) com

9、bustion systemsand other heat sources, including burners and controls; (4) ventingcomponents, such as an induced-draft blower or draft hood; (5) a cir-culating air blower and motor; and (6) an air filter and other acces-sories such as a humidifier, electronic air cleaner, air-conditioningcoil, or a

10、combination of these elements.Casing or CabinetThe furnace casing is most commonly formed from paintedcold-rolled steel. Access panels on the furnace allow access tothose sections requiring service. The inside of the casing adjacentto the heat exchanger or electric heat elements is lined with a foil

11、-faced blanket insulation and/or a metal radiation shield to reduceheat losses through the casing and to limit the outer surface tem-perature of the furnace. On some furnaces, the inside of the blowercompartment is lined with insulation to acoustically dampen theblower noise. Commercial furnace cabi

12、nets may also include theindoor and outdoor air-conditioning or heat pump components.Heat ExchangersFurnaces with gas-fired burners have heat exchangers that aretypically made either of left/right sets of formed parts that arejoined together to form a clamshell, finless tubes bent into a com-pact fo

13、rm, or finned-tube (condensing) heat exchangers. Standardindoor furnace heat exchangers are generally made of coated oralloy steel. Common corrosion-resistant materials include alumi-nized steel, ceramic-coated cold-rolled steel, and stainless steel.Furnaces certified for use downstream of a cooling

14、 coil must havecorrosion-resistant heat exchangers.Some problems of heat exchanger corrosion and failure havebeen encountered because of exposure to halogen ions in flue gas.These problems were caused by combustion air contaminated bysubstances such as laundry bleach, cleaning solvents, and haloge-n

15、ated hydrocarbon refrigerants.Research has been done on corrosion-resistant materials for usein condensing (secondary) heat exchangers (Stickford et al. 1985).The presence of chloride compounds in the condensate can cause aThe preparation of this chapter is assigned to TC 6.3, Central Forced AirHeat

16、ing and Cooling Systems. Fig. 1 Induced-Draft Gas Furnace33.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)condensing heat exchanger to fail, unless a corrosion-resistantmaterial is used.Several manufacturers produce liquid-to-air heat exchangers inwhich a liquid is heated and is either evapor

17、ated or pumped to a con-denser section or fan-coil, which heats circulating air.Heat exchangers of oil-fired furnaces are normally heavy-gagesteel formed into a welded assembly. Hot flue products flow throughthe inside of the heat exchanger into the chimney, and conditionedair flows over the outside

18、 of the heat exchanger and into the air sup-ply plenum.Electric Heat Elements. Elements for electric furnaces are gen-erally either open wire, open ribbon, or wire enclosed in a tube.Current is applied to the element and heats it through resistance ofthe material.Burners and Internal Controls. Gas b

19、urners are most frequentlymade of stamped sheet metal, although cast iron is also used. Fabri-cated sheet metal burners may be made from cold-rolled steel coatedwith high-temperature paint or from a corrosion-resistant materialsuch as stainless or aluminized steel. Burner material must meet thecorro

20、sion protection requirements of the specific application. Gasfurnace burners may be of either the monoport or multiport type; thetype used with a particular furnace depends on compatibility with theheat exchanger.Gas furnace controls include an ignition device, gas valve, fancontrol, limit switch, a

21、nd other components specified by the manu-facturer. These controls allow gas to flow to the burners when heat isrequired. The most common ignition systems are (1) standing pilot,(2) intermittent pilot, (3) direct spark, and (4) hot-surface ignition(ignites either a pilot or the main burners directly

22、). (Standing-pilotignition systems are not typically available from manufacturerstoday because federally mandated efficiency standards precludetheir use.) The section on Technical Data has further details on thefunction and performance of individual control components.Oil furnaces are generally equi

23、pped with pressure-atomizingburners. The pump pressure and size of the injection nozzle orificeregulate the firing rate of the furnace. Electric ignition lights theburners. Other furnace controls, such as the blower switch and thelimit switch, are similar to those used on gas furnaces.Combustion Ven

24、ting ComponentsNatural-draft indoor furnaces are equipped with a draft hoodconnecting the heat exchanger flue gas exit to the vent pipe or chim-ney. The draft hood has a relief air opening large enough to ensurethat the exit of the heat exchanger is always at atmospheric pressure.One purpose of the

25、draft hood is to make certain that the natural-draft furnace continues to operate safely without generating carbonmonoxide if the chimney is blocked, if there is a downdraft, or ifthere is excessive updraft. Another purpose is to maintain constantpressure on the combustion system. Residential furnac

26、es built since1987 are equipped with a blocked-vent shutoff switch to shut downthe furnace in case the vent becomes blocked.Fan-assisted combustion furnaces use a small blower to induceflue products through the furnace. Induced-draft furnaces may ormay not have a relief air opening, but they meet th

27、e same safetyrequirements regardless.Research into common venting of natural-draft appliances (waterheaters) and fan-assisted combustion furnaces shows that nonposi-tive vent pressure systems may operate on a common vent. Refer tomanufacturers instructions for specific information.Direct-vent furnac

28、es use outdoor air for combustion. Outdoorair is supplied to the furnace combustion chamber by direct connec-tions between the furnace and the outdoor air. If the vent or the com-bustion air supply becomes blocked, the furnace control system willshut down the furnace.ANSI Standard Z21.47/CSA 2.3 cla

29、ssifies venting systems.Central furnaces are categorized by temperature and pressureattained in the vent and by the steady-state efficiency attained by thefurnace. Although ANSI Standard Z21.47/CSA 2.3 uses 83% as thesteady-state efficiency dividing central furnace categories, a generalrule of thumb

30、 is as follows:Category I: nonpositive vent pressure and flue loss of 17% ormore.Category II: nonpositive vent pressure and flue loss less than17%.Category III: positive vent pressure and flue loss of 17% or more.Category IV: positive vent pressure and flue loss less than 17%.Furnaces rated in accor

31、dance with ANSI Standard Z21.47/CSA2.3 that are not direct vent are marked to show that they are in oneof these four venting categories.Ducted-system, oil-fired, forced-air furnaces are usually forceddraft.Circulating Blowers and MotorsCentrifugal blowers with forward-curved blades of the double-inl

32、et type are used in most forced-air furnaces. These blowersovercome the resistance of furnace air passageways, filters, and duc-twork. They are usually sized to provide the additional air require-ment for cooling and the static pressure required for the cooling coil.The blower may be a direct-drive

33、type, with the blower wheelattached directly to the motor shaft, or it may be a belt-drive type,with a pulley and V-belt used to drive the blower wheel.Electric motors used to drive furnace blowers are usually customdesigned for each furnace model or model series. Direct-drivemotors may be of the sh

34、aded-pole or permanent split-capacitor type.Speed variation may be obtained by taps connected to extra wind-ings in the motor. Belt-drive blower motors are normally split-phaseor capacitor-start. The speed of belt-drive blowers is controlled byadjusting a variable-pitch drive pulley. Electronically

35、controlled, variable-speed motors are also avail-able. This type of motor reduces electrical consumption when oper-ated at low speeds.Filters and Other AccessoriesAir Filters. An air filter in a forced-air furnace removes dustfrom the air that could reduce the effectiveness of the blower andheat exc

36、hanger(s), and may also help provide cleaner air for theindoor environment (see ASHRAE Standard 52.2). Filters installedin a forced-air furnace are often disposable. Permanent filters thatmay be washed or vacuum-cleaned and reinstalled are also used.The filter is always located in the circulating ai

37、rstream ahead of theblower and heat exchanger. Because the air filter keeps airflow com-ponents of the furnace clean, it should be cleaned or replaced regu-larly to extend the life of the furnace components. See Chapters 10and 29 for further information on air filters.Humidifiers. These are not incl

38、uded as a standard part of the fur-nace package. However, one advantage of a forced-air heating sys-tem is that it offers the opportunity to control the relative humidityof the heated space at a comfortable level. Chapter 22 addresses var-ious types of humidifiers used with forced-air furnaces.Elect

39、ronic Air Cleaners. These air cleaners may be much moreeffective than the air filter provided with the furnace, and they filterout much finer particles, including smoke and pollen. Electronic aircleaners create an electric field of high-voltage direct current inwhich dust particles are given a charg

40、e and collected on a plate hav-ing the opposite charge. The collected material is then cleaned peri-odically from the collector plate by the homeowner. Electronic aircleaners are mounted in the airstream entering the furnace. Chapter29 has detailed information on filters.Automatic Vent Dampers. This

41、 device closes the vent openingon a draft hood-equipped natural-draft furnace when the furnace isnot in use, thus reducing off-cycle losses. More information aboutthe energy-saving potential of this accessory is included in the sec-tion on Technical Data.Furnaces 33.3Airflow VariationsThe components

42、 of a forced-air furnace can be arranged in a vari-ety of configurations to suit a residential heating system. The relativepositions of the components in the different types of furnaces are asfollows:Upflow or “highboy” furnace. In an upflow furnace (Figure 2),the blower is located beneath the heat

43、exchanger and dischargesvertically upward. Air enters through the bottom or the side of theblower compartment and leaves at the top. This furnace may beused in closets and utility rooms on the first floor or in basements,with the return air ducted down to the blower compartmententrance. Downflow fur

44、nace. In a downflow furnace (Figure 3), the bloweris located above the heat exchanger and discharges downward.Air enters at the top and is discharged vertically at the bottom.This furnace is normally used with a perimeter heating system ina house without a basement. It is also used in upstairs furna

45、ceclosets and utility rooms supplying conditioned air to both levelsof a two-story house.Horizontal furnace. In a horizontal furnace, the blower is locatedbeside the heat exchanger (Figure 4). Air enters at one end, travelshorizontally through the blower and over the heat exchanger, andis discharged

46、 at the opposite end. This furnace is used for loca-tions with limited head room such as attics and crawl spaces, or issuspended under a roof or floor or placed above a suspended ceil-ing. These units are often designed so that the components may berearranged to allow installation with airflow from

47、left to right orfrom right to left.Multiposition (multipoise) furnace. This furnace can beinstalled in more than one airflow configuration (e.g., upflow orhorizontal; downflow or horizontal; or upflow, downflow, or hor-izontal). In some models, field conversion is necessary to accom-modate an altern

48、ative installation.Basement or “lowboy” furnace. The basement furnace (Fig-ure 5) is a variation of the upflow furnace and requires less headroom. The blower is located beside the heat exchanger at thebottom. Air enters the top of the cabinet, is drawn down throughthe blower, is discharged over the

49、heat exchanger, and leavesvertically at the top. This type of furnace has become less popu-lar because of the advent of short upflow furnaces.Gravity furnace. These furnaces are no longer available, andthey are not common. This furnace has larger air passages throughthe casing and over the heat exchanger so that the buoyancy forcecreated by the air being warmed circulates the air through theducts. Wall furnaces that rely on natural convection (gravity) arediscussed in Chapter 34.Fig. 2 Upflow Category I Furnace with Induced-Draft BlowerFig. 3 Downflow (Counterflow) Category I Furnace wi