ASHRAE HVAC APPLICATIONS SI CH 54-2015 RADIANT HEATING AND COOLING.pdf

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1、54.1CHAPTER 54RADIANT HEATING AND COOLINGLow-, Medium-, and High-Intensity Infrared Heating. 54.1Panel Heating and Cooling 54.1Elementary Design Relationships 54.1Design Criteria for Acceptable Radiant Heating 54.3Design for Beam Radiant Heating. 54.4Radiation Patterns 54.5Design for Total Space Hea

2、ting . 54.6Testing Instruments for Radiant Heating . 54.7Applications 54.8Symbols 54.9ADIANT heating and cooling applications are classified asR panel heating or cooling if the panel surface temperature isbelow 150C, and as low-, medium-, or high-intensity radiant heat-ing if the surface or source t

3、emperature exceeds 150C. In thermalradiation, heat is transferred by electromagnetic waves that travel instraight lines and can be reflected. Thermal radiation principallyoccurs between surfaces or between a source and a surface. In a con-ditioned space, air is not heated or cooled in this process.

4、Because ofthese characteristics, radiant systems are effective for both spot heat-ing and space heating or cooling requirements for an entire building.Sensible heating loads may be reduced by 4 to 16% comparedto ASHRAE standard design load. Percent reduction increases withthe air change rate (Suryan

5、arayana and Howell 1990).1. LOW-, MEDIUM-, AND HIGH-INTENSITY INFRARED HEATINGLow-, medium-, and high-intensity infrared heaters are compact,self-contained direct-heating devices used in hangars, warehouses,factories, greenhouses, and gymnasiums, as well as in areas such asloading docks, racetrack s

6、tands, outdoor restaurants, animal breed-ing areas, swimming pool lounge areas, and areas under marquees.Infrared heating is also used for snow melting and freeze protection(e.g., on stairs and ramps) and process heating (e.g., paint baking anddrying). An infrared heater may be electric, gas-fired,

7、or oil-firedand is classified by the source temperature as follows:Low-intensity (source temperatures to 650C)Medium-intensity (source temperatures to 980C)High-intensity (source temperatures to 2800C)The source temperature is determined by such factors as thesource of energy, the configuration, and

8、 the size. Reflectors can beused to direct the distribution of thermal radiation in specific pat-terns. Chapter 16 of the 2012 ASHRAE HandbookHVAC Systemsand Equipment covers radiant equipment in detail.2. PANEL HEATING AND COOLINGPanel heating and cooling systems provide a comfortable envi-ronment

9、by controlling surface temperatures and minimizing airmotion within a space. They include the following designs:Ceiling panels Embedded hydronic tubing or attached piping in ceilings, walls, orfloorsAir-heated or cooled floors or ceilingsElectric ceiling or wall panelsElectric heating cable or wire

10、mats in ceilings or floorsDeep heat, a modified storage system using electric heating cableor embedded hydronic tubing in ceilings or floorsIn these systems, generally more than 50% of the heat transferbetween the temperature-controlled surface and other surfaces is bythermal radiation. Panel heatin

11、g and cooling systems are used in resi-dences, office buildings, classrooms, hospital patient rooms, swim-ming pool areas, repair garages, and in industrial and warehouseapplications. Additional information is available in Chapter 6 of the2012 ASHRAE HandbookHVAC Systems and Equipment.Some radiant p

12、anel systems, referred to as hybrid HVAC systems,combine radiant heating and cooling with central air conditioning(Scheatzle 2003). They are used more for cooling than for heating(Wilkins and Kosonen 1992). The controlled-temperature surfacesmay be in the floor, walls, or ceiling, with temperature m

13、aintained byelectric resistance or circulation of water or air. The central stationcan be a basic, one-zone, constant-temperature, or constant-volumesystem, or it can incorporate some or all the features of dual-duct,reheat, multizone, or variable-volume systems. When used in com-bination with other

14、 water/air systems, radiant panels provide zonecontrol of temperature and humidity.Metal ceiling panels may be integrated into the central heatingand cooling system to provide individual room or zone heating andcooling. These panels can be designed as small units to fit the build-ing module, or they

15、 can be arranged as large continuous areas foreconomy. Room thermal conditions are maintained primarily bydirect transfer of radiant energy, normally using four-pipe hot andchilled water. These systems have generally been used in hospitalpatient rooms. Metal ceiling panel systems are discussed in Ch

16、apter6 of the 2012 ASHRAE HandbookHVAC Systems and Equipment.3. ELEMENTARY DESIGN RELATIONSHIPSWhen considering radiant heating or cooling for human comfort,the following terms describe the temperature and energy character-istics of the total radiant environment:Mean radiant temperature (MRT) is the

17、 temperature of animaginary isothermal black enclosure in which an occupant wouldexchange the same amount of heat by radiation as in the actualnonuniform environment.Ambient temperature tais the temperature of the air surroundingthe occupant.Operative temperature tois the temperature of a uniform is

18、other-mal black enclosure in which the occupant would exchange thesame amount of heat by radiation and convection as in the actualnonuniform environment.For air velocities less than 0.4 m/s and mean radiant tempera-tures less than 50C, the operative temperature is approximatelyequal to the adjusted

19、dry-bulb temperature, which is the average ofthe air and mean radiant temperatures.Adjusted dry-bulb temperature is the average of the air temper-ature and the mean radiant temperature at a given location. Theadjusted dry-bulb temperature is approximately equivalent to theoperative temperature for a

20、ir motions less than 0.4 m/s and meanradiant temperatures less than 50C.The preparation of this chapter is assigned to TC 6.5, Radiant Heating andCooling.t r54.2 2015 ASHRAE HandbookHVAC Applications (SI)Effective radiant flux (ERF) is defined as the net radiant heatexchanged at ambient temperature

21、tabetween an occupant, whosesurface is hypothetical, and all enclosing surfaces and directionalheat sources and sinks. Thus, ERF is the net radiant energyreceived by the occupant from all surfaces and sources whosetemperatures differ from ta. ERF is particularly useful in high-intensity radiant heat

22、ing applications.The relationship between these terms can be shown for an occu-pant at surface temperature tsf, exchanging sensible heat Hmin aroom with ambient air temperature taand mean radiant temperature. The linear radiative and convective heat transfer coefficients arehrand hc, respectively; t

23、he latter is a function of the relative move-ment between the occupant and air movement V. The heat balanceequation isHm= hr(tsf ) = hc(tsf ta)(1)During thermal equilibrium, Hmis equal to metabolic heat minuswork and evaporative cooling by sweating. By definition of opera-tive temperature,Hm= (hr+ h

24、c)(tsf to) = h(tsf to)(2)The combined heat transfer coefficient is h, where h = hr+ hc. UsingEquations (1) and (2) to solve for toyieldsto= (3)Thus, tois an average of and ta, weighted by their respective heattransfer coefficients; it represents how people sense the thermallevel of their total envir

25、onment as a single temperature.Rearranging Equation (1) and substituting h hrfor hc,Hm+ hr( ta) = h(tsf ta)(4)where hris, by definition, the effective radiant flux (ERF)and represents the radiant energy absorbed by the occupant from allsources whose temperatures differ from ta.The principal relation

26、ships between , ta, to, and ERF are asfollows:ERF = hr( ta)(5)ERF = h(to ta6= ta+ ERF/hr(7)to= ta+ ERF/h (8)= ta+ (h/hr)(to ta)(9)to= ta+ (hr/h)( ta) (10)In Equations (1) to (10), the radiant environment is treated as ablackbody with temperature . The effect of the emittance of thesource, radiating

27、at absolute temperature in kelvins, and the absorp-tance of skin and clothed surfaces is reflected in the effective valuesof or ERF and not in hr, which is generally given byhr= 4feff(11)wherehr= linear radiative heat transfer coefficient, W/(m2K)feff= ratio of radiating surface of the human body to

28、 its total DuBois surface area AD= 0.71 = Stefan-Boltzmann constant = 5.67 108W/(m2K4)The convective heat transfer coefficient for an occupant dependson the relative velocity between the occupant and the surroundingair, as well as the occupants activity:If the occupant is walking in still air,hc= 8.

29、6 V0.530.5 ta, ERF adds heat to the body; when ta , heat islost from the body because of thermal radiation. ERF is independentof the occupants surface temperature and can be measured directlyby a black globe thermometer or any blackbody radiometer or fluxmeter using the ambient air taas its heat sin

30、k.In these definitions and for radiators below 925C (1200 K), thebody clothing and skin surface are treated as blackbodies, exchang-ing radiation with an imaginary blackbody surface at temperature. The effectiveness of a radiating source on human occupants isgoverned by the absorptance of the skin a

31、nd clothing surface forthe color temperature (in K) of that radiating source. The relation-ship between and temperature is illustrated in Figure 1. Values for are those expected relative to the matte black surface normallyfound on globe thermometers or radiometers measuring radiantenergy. A gas radi

32、ator usually operates at 925C (1200 K); a quartzlamp, for example, radiates at 2200C (2475 K) with 240 V; and thesuns radiating temperature is 5530C (5800 K). The use of in esti-mating the ERF and tocaused by sources radiating at temperaturesabove 925C (1200 K) is discussed in the section on Testing

33、 Instru-ments for Radiant Heating.trtrhrtrhcta+hrhc+- tahrh-trta+=trtrtrtatrtrtrtrtrtrtrtrta+2-273+3trtrtrFig. 1 Relative Absorptance and Reflectance of Skin and Typical Clothing Surfaces at Various Color TemperaturesRadiant Heating and Cooling 54.34. DESIGN CRITERIA FOR ACCEPTABLE RADIANT HEATINGPe

34、rceptions of comfort, temperature, and thermal acceptabilityare related to activity, body heat transfer from the skin to the envi-ronment, and the resulting physiological adjustments and bodytemperature. The split between convective and radiant heat transferfrom or to a body is also a matter of subj

35、ective human comfort. Theoptimum split is about 60% by thermal radiation and 40% by ther-mal convection. Heat transfer is affected by ambient air tempera-ture, thermal radiation, air movement, humidity, and clothing worn.Thermal sensation is described as feelings of hot, warm, slightlywarm, neutral,

36、 slightly cool, cool, and cold. An acceptable environ-ment is defined as one in which at least 80% of the occupants per-ceive a thermal sensation between “slightly cool” and “slightlywarm.” Comfort is associated with a neutral thermal sensation dur-ing which the human body regulates its internal tem

37、perature withminimal physiological effort for the activity concerned. In contrast,warm discomfort is primarily related to the physiological strainnecessary to maintain the bodys thermal equilibrium rather than tothe temperature sensation experienced. For a full discussion of theinterrelation of phys

38、ical, psychological, and physiological factors,refer to Chapter 9 of the 2013 ASHRAE HandbookFundamen-tals.ANSI/ASHRAE Standard 55-1992 shows a linear relationshipbetween clothing insulation worn and the operative temperature tofor comfort (Figure 2). Figure 3 shows the effect of both activity andcl

39、othing on the tofor comfort. Figure 4 shows the slight effecthumidity has on the comfort of a sedentary person wearing averageclothing. Figures 2, 3, and 4 are adapted from ANSI/ASHRAEStandard 55, Thermal Environmental Conditions for HumanOccupancy.A comfortable toat 50% rh is perceived as slightly

40、warmer ashumidity increases, or as slightly cooler as humidity decreases.Changes in humidity have a much greater effect on warm and hotdiscomfort. In contrast, cold discomfort is only slightly affected byhumidity and is very closely related to a cold thermal sensation.Determining the specifications

41、for a radiant heating installationdesigned for human occupancy and acceptability involves the fol-lowing steps:1. Define the probable activity (metabolism) level of, and clothingworn by, the occupant and the relative air velocity. The followingare two examples:Case 1: Sedentary (65 W/m2)Clothing ins

42、ulation = 0.09 (m2K)/W; V = 0.15 m/sCase 2: Light work (walking) (116 W/m2)Clothing insulation = 0.14 (m2K)/W; V = 0.5 m/s2. From Figure 2 (sedentary) or 3 (active), determine the optimum tofor comfort and acceptability:Fig. 2 Range of Thermal Acceptability for Sedentary People with Various Clothing

43、 Insulations and Operative TemperaturesFig. 3 Optimum Operative Temperatures for Active People in Low-Air-Movement EnvironmentsFig. 4Fig. 1 ASHRAE Comfort Chart for Sedentary OccupantsFig. 4 ASHRAE Comfort Chart for Sedentary Occupants54.4 2015 ASHRAE HandbookHVAC Applications (SI)Case 1: to= 24C; C

44、ase 2: to= 18C3. For ambient air temperature ta, calculate the mean radiant tem-perature and/or ERF necessary for comfort and thermalacceptability.Case 1: For ta= 15C and 50% rh and assuming = 30.5C,Solve for hrfrom Equation (11):hr= 4 5.67 108 0.71(30.5 + 15)/2 + 2733= 4.16Solve for hcfrom Equation

45、 (12c):hc= 3.1Then,h = hr+ hc= 4.16 + 3.1 = 7.26 W/(m2K)= 0.57From Equation (6), for comfort,ERF = 7.26(24 15) = 65.3 W/m2From Equation (9),= 15 + (7.26/4.16)(24 15) = 30.7CCase 2: For ta= 10C and assuming = 29.5C,hr= 4 5.67 108 0.71(29.5 + 10)/2 + 2733= 4.04hc= 8.6(0.5)0.53= 5.96h = 4.04 + 5.96 = 1

46、0 W/(m2K)= 0.40ERF = 10(18 10) = 80 W/m2From Equation (7),= 10 + 80/4.04 = 29.8CThe tofor comfort, predicted by Figure 2, is on the cool sidewhen the humidity is low; for high humidities, the predicted toforcomfort is warm. This effect on comfort for sedentary occupants canbe seen in Figure 4. For e

47、xample, for high humidity at tdp= 15C, thetofor comfort isCase 1: to= 23C, compared to 24C at 50% rhWhen thermal acceptability is the primary consideration in aninstallation, humidity can sometimes be ignored in preliminarydesign specifications. However, for conditions where radiant heat-ing and the

48、 work level cause sweating and high heat stress, humid-ity is a major consideration and a hybrid HVAC system should beused.Equations (3) to (12) can also be used to determine the ambientair temperature tarequired when the mean radiant temperature MRTis maintained by a specified radiant system.When c

49、alculating heat loss, tamust be determined. For a radiantsystem that is to maintain a MRT of , the operative temperature tocan be determined from Figure 4. Then, tacan be calculated byrecalling that tois approximately equal to the average of taand tr.For example, for a toof 23C and a radiant system designed to main-tain an MRT of 26C, the tawould be 20C.When the surface temperature of outside walls, particularly thosewith large areas of glass