1、16.1CHAPTER 16INFRARED RADIANT HEATINGEnergy Conservation 16.1Infrared Energy Sources 16.1System Efficiency 16.3Reflectors . 16.4Controls 16.4Precautions 16.4Maintenance. 16.5Design Considerations for Beam Radiant Heaters 16.5NFRARED radiant heating principles discussed in this chapterI apply to equ
2、ipment with thermal radiation source temperaturesranging from 300 to 5000F. (Equipment with source temperaturesstarting from below the indoor air temperature to 300F is classifiedas panel heating and cooling equipment, discussed in Chapter 6.)Infrared radiant heaters with source temperatures in this
3、 range arecategorized into three groups as follows:Low-intensity source temperatures range from 300 to 1200F. Atypical low-intensity heater is mounted on the ceiling and may beconstructed of a 4 in. steel tube 10 to 80 ft long. A gas burnerinserted into the end of the tube raises the tube surface te
4、mpera-ture, and because most units are equipped with a reflector, ther-mal radiation is directed down to the heated space.Medium-intensity source temperatures range from 1200 to1800F. Typical equipment types include porous matrix gas-firedinfrared heaters or metal-sheathed electric heaters.High-inte
5、nsity source temperatures range from 1800 to 5000F.A typical high-intensity heater is an electrical reflector lamp witha resistor temperature of 4050F.Low-, medium-, and high-intensity infrared heaters are fre-quently applied in aircraft hangars, factories, warehouses, found-ries, greenhouses, and g
6、ymnasiums. They are applied to open areassuch as loading docks, racetrack stands, under marquees, vestibules,outdoor restaurants, carwashes, and around swimming pools. Infra-red heaters are also used for snow and ice melting (see Chapter 51of the 2011 ASHRAE HandbookHVAC Applications), condensa-tion
7、 control, and industrial process heating. Reflectors are fre-quently used to control the distribution of heat flux from thermalradiation in specific patterns.When infrared radiant heating is used, the environment is char-acterized byA directional thermal radiation field created by the infrared heate
8、rsA thermal radiation field consisting of reradiation and reflectionfrom the walls and/or other enclosing surfacesAmbient air temperatures often lower than those found with con-vective systemsThe combined action of these factors determines occupant thermalcomfort and the thermal acceptability of the
9、 environment.ENERGY CONSERVATIONInfrared heaters are effective for spot heating. However, becauseof their efficient performance, they are also used for total heating oflarge areas and entire buildings (Buckley 1989). Radiant heaterstransfer heat directly to solid objects. Little heat is lost during
10、trans-mission because air is a poor absorber of radiant heat. Because anintermediate transfer medium such as air or water is not required,fans or pumps are not needed.As thermal radiation warms floors, walls, and objects, they inturn release heat to the air by convection. Reradiation to surround-ing
11、 objects also contributes to comfort in the area. An energy-savingadvantage is that infrared heaters can be turned off when not needed;when turned on again, they are effective in minutes. Even when theinfrared heater is off, the heated surrounding objects at occupantlevel continue to contribute to c
12、omfort by reradiating heat andreleasing heat by convection.Human thermal comfort is primarily governed by the operativetemperature of the heated space (ASHRAE Standard 55). Operativetemperature may be approximated by the arithmetic average of themean radiant temperature (MRT) of the heated space and
13、 dry-bulbair temperature, if air velocity is less than 1.3 fps and MRT is lessthan 120F. See Chapter 54 of the 2011 ASHRAE HandbookHVAC Applications for further details. In radiant heating, the dry-bulb air temperature may be kept lower for a given comfort levelthan with other forms of heating becau
14、se of increased MRT. As aresult, heat lost to ventilating air and via conduction through theshell of the structure is proportionally smaller, as is energy con-sumption. Infiltration loss, which is a function of dry-bulb air tem-perature, is also reduced.Because of the unique split of radiant and con
15、vective compo-nents in radiant heating, air movement and stratification in theheated space is minimal. This further reduces infiltration and trans-mission heat losses.Buckley and Seel (1987) compared energy savings of infraredheating with those of other types of heating systems. Recognizing thereduc
16、ed fuel requirement for these applications, Buckley and Seel(1988) noted that it is desirable for manufacturers of radiant heatersto recommend installation of equipment with a rated output that is 80to 85% of the heat loss calculated by methods described in Chapters17 and 18 of the 2009 ASHRAE Handb
17、ookFundamentals. BSR/ASHRAE Standard 138P describes a rated output system for ceilingradiant heaters.Chapman and Zhang (1995) developed a three-dimensional math-ematical model to compute radiant heat exchange between surfaces.A building comfort analysis program (BCAP) was developed as partof ASHRAE
18、research project RP-657 (Jones and Chapman 1994).The BCAP program was later enhanced to analyze the effect of radi-ant heaters over 300F on thermal comfort calculations, and to ana-lyze the thermal comfort effect of obstacles in the heated space inASHRAE research project RP-1037 (Chapman 2002).INFRA
19、RED ENERGY SOURCESGas InfraredModern gas-fired infrared heaters burn gas to heat a specific radi-ating surface. The surface is heated by direct flame contact or withcombustion gases. Studies by the Gas Research Board of London(1944), Haslam et al. (1925), and Plyler (1948) reveal that only 10 to20%
20、of the energy produced by open combustion of a gaseous fuelis infrared radiant energy. The wavelength span over which radiationfrom a heated surface is distributed can be controlled by design.The specific radiating surface of a properly designed unit directsThe preparation of this chapter is assigne
21、d to TC 6.5, Radiant Heating andCooling.16.2 2012 ASHRAE HandbookHVAC Systems and Equipment radiation toward the load. Gas-fired infrared radiation heaters areavailable in the following types (see Table 1 for characteristics).Indirect infrared radiation heaters (Figures 1A, 1B, and 1C)are internally
22、 fired and have the radiating surface between the hotgases and the load. Combustion takes place within the radiatingelements, which operate with surface temperatures up to 1200F.The elements may be tubes or panels with metal or ceramic com-ponents. Indirect infrared radiation units are usually vente
23、d andmay require eductors.Porous-matrix infrared radiation heaters (Figure 1D) have arefractory material that may be porous ceramic, drilled port ceramic,stainless steel, or a metallic screen. The units are enclosed, exceptfor the major surface facing the load. A combustible gas-air mix-ture enters
24、the enclosure, flows through the refractory material tothe exposed face, and is distributed evenly by the porous characterof the refractory. Combustion occurs evenly on the exposed sur-face. The flame recedes into the matrix, which adds radiant energyto the flame. If the refractory porosity is suita
25、ble, an atmosphericburner can be used, resulting in a surface temperature approaching1650F. Power burner operation may be required if refractory den-sity is high. However, the resulting surface temperature may alsobe higher (1800F).Catalytic oxidation infrared radiation heaters (Figure 1E) aresimila
26、r to porous-matrix units in construction, appearance, andoperation, but the refractory material is usually glass wool, and theradiating surface is a catalyst that causes oxidation to proceed with-out visible flames.Electric InfraredElectric infrared heaters use heat produced by electric currentflowi
27、ng in a high-resistance wire, graphite ribbon, or film element.The following are the most commonly used types (see Table 2 forcharacteristics).Table 1 Characteristics of Typical Gas-Fired Infrared HeatersCharacteristics Indirect Porous Matrix Catalytic OxidationOperating source temperature Up to 120
28、0F 1600 to 1800F 650 to 700FRelative heat flux,aBtu/hft2Low, up to 7500 Medium, 17,000 to 32,000 Low, 800 to 3000Response time (heat-up) 180 s 60 s 300 sThermal radiation-energy input ratiob0.35 to 0.55 0.35 to 0.60 No dataThermal shock resistance Excellent Excellent ExcellentVibration resistance Ex
29、cellent Excellent ExcellentColor blindnesscExcellent Very good ExcellentLuminosity (visible light) To dull red Yellow red NoneMounting height 9 to 50 ft 12 to 50 ft To 10 ftWind or draft resistance Good Fair Very goodVenting Optional Nonvented NonventedFlexibility Good Excellentwide range of heat fl
30、uxesand mounting possibilities availableLimited to low-heat-flux applicationsaHeat flux emitted at burner surface.bRatio of thermal radiation to energy output to input.cColor blindness refers to absorptance by various loads of energy emitted by different sources.Fig. 1 Types of Gas-Fired Infrared He
31、atersInfrared Radiant Heating 16.3Metal sheath infrared radiation elements (Figure 2A) arecomposed of a nickel-chromium heating wire embedded in anelectrical insulating refractory, which is encased by a metal tube.These elements have excellent resistance to thermal shock, vibra-tion, and impact, and
32、 can be mounted in any position. At fullvoltage, the elements attain a sheath surface temperature of 1200to 1800F. Higher temperatures are attained by configurationssuch as a hairpin shape. These units generally contain a reflector,which directs radiation to the load. Higher radiosity is obtained if
33、the elements are shielded from wind because the surface-coolingeffect of the wind is reduced.Reflector lamp infrared radiation heaters (Figure 2B) have acoiled tungsten filament, which approximates a point sourceradiator. The filament is enclosed in a clear, frosted, or red heat-resistant glass enve
34、lope, which is partially silvered inside to form anefficient reflector. Units that may be screwed into a light socket arecommon.Quartz tube infrared radiation heaters (Figure 2C) have acoiled nickel-chromium wire lying unsupported within an unevac-uated, fused quartz tube, which is capped (not seale
35、d) by porcelainor metal terminal blocks. These units are easily damaged byimpact and vibration but stand up well to thermal shock andsplashing. They must be mounted horizontally to minimize coilsag, and they are usually mounted in a fixture that contains areflector. Normal operating temperatures are
36、 from 1300 to 1800Ffor the coil and about 1200F for the tube.Tubular quartz lamp units (Figure 2D) consist of a 0.38 in.diameter fused quartz tube containing an inert gas and a coiledtungsten filament held in a straight line and away from the tube bytantalum spacers. Filament ends are embedded in se
37、aling materialat the ends of the envelope. Lamps must be mounted horizontally,or nearly so, to minimize filament sag and overheating of the sealedends. At normal design voltages, quartz lamp filaments operate atabout 4050F, while the envelope operates at about 1100F.Oil InfraredOil-fired infrared ra
38、diant heaters are similar to gas-fired indi-rect infrared radiant heaters (Figures 1A, 1B, and 1C). Oil-firedunits are vented.SYSTEM EFFICIENCYBecause many factors contribute to the performance of a spe-cific infrared radiant heating system, a single criterion should notTable 2 Characteristics of Fo
39、ur Electric Infrared ElementsCharacteristic Metal Sheath Reflector Lamp Quartz Tube Quartz LampResistor material Nickel-chromium alloy Tungsten wire Nickel-chromium alloy Tungsten wireRelative linear heat flux Medium, 60 W/in., 0.5 in., diameterHigh, 125 to375 W/spotMedium to high, 75 W/in., 0.5 in.
40、 diameterHigh, 100 W/in., 3/8 in. diameterResistor temperature 1750F 4050F 1700F 4050FEnvelope temperature (in use) 1550F 525 to 575F 1200F 1100FThermal radiation-energy input ratioa0.58 0.86 0.81 0.86Response time (heat-up) 180 s A few seconds 60 s A few secondsLuminosity (visible light) Very low (
41、dull red) High (8 lm/W) Low (orange) High (7.5 lm/W)Thermal shock resistance Excellent Poor to excellent (heat-resistant glass)Excellent ExcellentVibration resistance Excellent Medium Medium MediumImpact resistance Excellent Medium Poor PoorWind or draft resistancebMedium Excellent Medium ExcellentM
42、ounting position Any Any HorizontalcHorizontalEnvelope material Steel alloy Regular or heat-resistant glassTranslucent quartz Clear, translucent, or frost quartz and integral red filter glassColor blindness Very good Fair Very good FairFlexibility Goodwide range of power density, length, and voltage
43、 practicalLimited to 125-250 and 375 W at 120 VExcellentwide range of power density, diameter, length, and voltage practicalLimited1 to 3 W for each V; 1 length for each capacityLife expectancy Over 5000 h 5000 h 5000 h 5000 haRatio of thermal radiation output to energy input (elements only).bMay be
44、 shielded from wind effects by louvers, deep-drawn fixtures, or both.cMay be provided with special internal supports for other than horizontal use.Fig. 2 Common Electric Infrared Heaters16.4 2012 ASHRAE HandbookHVAC Systems and Equipment be used to evaluate comparable systems. Therefore, at least tw
45、o ofthe following indicators should be used when evaluating systemperformance.Thermal radiation-energy input ratio is the thermal energytransferred by radiation in the infrared wavelength spectrum dividedby the total energy input.Fixture efficiency is an index of a fixtures ability to radiate ther-m
46、al energy from the source; it is usually based on total energy input.The housing, reflector, and other parts of a fixture absorb someinfrared energy and convert it to heat, which is lost through convec-tion. A fixture that controls direction and distribution of energyeffectively may have higher fixt
47、ure efficiency.Pattern efficiency is an index of a fixtures effectiveness in direct-ing infrared energy into a specific pattern. This effectiveness, plus ef-fective application of the pattern to the thermal load, influences thesystems total effectiveness (Boyd 1963). Typical thermal radiation-energy
48、 input ratios of gas infrared heaters are shown in Table 1. Lim-ited test data indicate that the amount of thermal radiation from gasinfrared units ranges from 35 to 60% of the amount of convectiveheat. The Stefan-Boltzmann law can be used to estimate thermal ra-diation if reasonably accurate values
49、 of true surface temperature,emitting area, and surface emittance are available (DeWerth 1960).DeWerth (1962) also addresses the spectral distribution of energycurves for several gas sources.Table 2 lists typical thermal radiation-energy input ratios of elec-tric infrared heaters. Fixture efficiencies are typically 80 to 95% ofthe thermal radiation-energy input ratios.Infrared heaters should be operated at rated input. A small reduc-tion in input causes a larger decrease in radiant output because ofthe fourth-power dependence of radiant output on sourc
