ASHRAE HVAC SYSTEMS AND EQUIPMENT IP CH 6-2012 PANEL HEATING AND COOLING.pdf

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1、6.1CHAPTER 6PANEL HEATING AND COOLINGPRINCIPLES OF THERMAL RADIATION. 6.1General Evaluation 6.1Heat Transfer by Panel Surfaces 6.2General Design Considerations. 6.6Panel Design 6.8HEATING AND COOLING PANEL SYSTEMS 6.9Hydronic Panel Systems . 6.10Hydronic Metal Ceiling Panels 6.13Distribution and Lay

2、out . 6.14Electrically Heated Panel Systems. 6.16Air-Heated or Air-Cooled Panels. 6.18Controls 6.19Hybrid (Load-Sharing) HVAC Systems 6.20ANEL heating and cooling systems use temperature-controlledPindoor surfaces on the floor, walls, or ceiling; temperature ismaintained by circulating water, air, o

3、r electric current through acircuit embedded in or attached to the panel. A temperature-controlled surface is called a radiant panel if 50% or more of thedesign heat transfer on the temperature-controlled surface takesplace by thermal radiation. Panel systems are characterized by con-trolled surface

4、 temperatures below 300F. Panel systems may becombined either with a central forced-air system of one-zone,constant-temperature, constant-volume design, or with dual-duct,reheat, multizone or variable-volume systems, decentralized con-vective systems, or in-space fan-coil units. These combined sys-t

5、ems are called hybrid (load-sharing) HVAC systems.This chapter covers temperature-controlled surfaces that are theprimary source of sensible heating and cooling in the conditionedspace. For snow-melting and freeze-protection applications, seeChapter 51 of the 2011 ASHRAE HandbookHVAC Applications.Ch

6、apter 16 covers high-temperature panels over 300F, which maybe energized by gas, electricity, or high-temperature water.PRINCIPLES OF THERMAL RADIATIONThermal radiation (1) is transmitted at the speed of light, (2) trav-els in straight lines and can be reflected, (3) elevates the temperatureof solid

7、 objects by absorption but does not noticeably heat the airthrough which it travels, and (4) is exchanged continuously betweenall bodies in a building environment. The rate at which thermal radi-ation occurs depends on the following factors:Temperature of the emitting surface and receiverEmittance o

8、f the radiating surfaceReflectance, absorptance, and transmittance of the receiverView factor between the emitting and receiver surfaces (viewingangle of the occupant to the thermal radiation source)ASHRAE research project RP-876 (Lindstrom et al. 1998) con-cluded that surface roughness and texture

9、have insignificant effectson thermal convection and thermal radiation, respectively. Surfaceemittance (the ratio of the radiant heat flux emitted by a body to thatemitted by a blackbody under the same conditions) for typicalindoor surfaces, such as carpets, vinyl texture paint, and plastic,remained

10、between 0.9 and 1.0 for panel surface temperatures of86 to 131F.The structure of the radiation surface is critical. In general, roughsurfaces have low reflectance and high emittance/absorptancecharacteristics. Conversely, smooth or polished metal surfaces havehigh reflectance and low absorptance/emi

11、ttance.One example of heating by thermal radiation is the feeling ofwarmth when standing in the suns rays on a cool, sunny day. Some ofthe rays come directly from the sun and include the entire electro-magnetic spectrum. Other rays are absorbed by or reflected fromsurrounding objects. This generates

12、 secondary rays that are a combi-nation of the wavelength produced by the temperature of the objectsand the wavelength of the reflected rays. If a cloud passes in front ofthe sun, there is an instant sensation of cold. This sensation is causedby the decrease in the amount of heat received from solar

13、 radiation,although there is little, if any, change in the ambient air temperature.Thermal comfort, as defined in ASHRAE Standard 55, is “thatcondition of mind which expresses satisfaction with the thermalenvironment.” No system is completely satisfactory unless the threemain factors controlling hea

14、t transfer from the human body (radia-tion, convection, and evaporation) result in thermal neutrality.Maintaining correct conditions for human thermal comfort by ther-mal radiation is possible for even the most severe climatic condi-tions (Buckley 1989). Chapter 9 of the 2009 ASHRAE HandbookFundamen

15、tals has more information on thermal comfort.Panel heating and cooling systems provide an acceptable thermalenvironment by controlling surface temperatures as well as indoorair temperature in an occupied space. With a properly designed sys-tem, occupants should not be aware that the environment is b

16、eingheated or cooled. The mean radiant temperature (MRT) has astrong influence on human thermal comfort. When the temperatureof surfaces comprising the building (particularly outdoor exposedwalls with extensive fenestration) deviates excessively from theambient temperature, convective systems someti

17、mes have difficultycounteracting the discomfort caused by cold or hot surfaces. Heat-ing and cooling panels neutralize these deficiencies and minimizeradiation losses or gains by the human body.Most building materials have relatively high surface emittanceand, therefore, absorb and reradiate heat fr

18、om active panels. Warmceiling panels are effective because heat is absorbed and reflectedby the irradiated surfaces and not transmitted through the construc-tion. Glass is opaque to the wavelengths emitted by active panelsand, therefore, transmits little long-wave thermal radiation outside.This is s

19、ignificant because all surfaces in the conditioned space tendto assume temperatures that result in an acceptable thermal comfortcondition.GENERAL EVALUATIONPrincipal advantages of panel systems are the following:Because not only indoor air temperature but also mean radianttemperature can be controll

20、ed, total human thermal comfort maybe better satisfied.Because the operative temperature for required human thermalcomfort may be maintained by primarily controlling the meanradiant temperature of the conditioned indoor space, dry-bulb airtemperature may be lower (in heating) or higher (in cooling),

21、The preparation of this chapter is assigned to TC 6.5, Radiant Heating andCooling.6.2 2012 ASHRAE HandbookHVAC Systems and Equipmentwhich reduces sensible heating or cooling loads (see Chapter 15for the definition and calculation of operative and mean radianttemperatures).Hydronic panel systems may

22、be connected in series, followingother hydronic heating or cooling systems (i.e., their return watermay be used), increasing exergetic efficiency.Comfort levels can be better than those of other space-conditioningsystems because thermal loads are satisfied directly and airmotion in the space corresp

23、onds to required ventilation only.Waste and low-enthalpy energy sources and heat pumps may bedirectly coupled to panel systems without penalty on equipmentsizing and operation. Being able to select from a wide range ofmoderate operation temperatures ensures optimum design forminimum cost and maximum

24、 thermal and exergetic efficiency.Seasonal thermal distribution efficiency in buildings may behigher than in other hydronic systems.In terms of simple payback period, ceiling cooling panels andchilled beams have the highest technical energy savings potential(DOE 2002).Part or all of the building str

25、ucture may be thermally activated(Meierhans and Olesen 2002).Space-conditioning equipment is not required at outdoor exposedwalls, simplifying wall, floor, and structural systems.Almost all mechanical equipment may be centrally located, sim-plifying maintenance and operation.No space within the cond

26、itioned space is required for mechanicalequipment. This feature is especially valuable in hospital patientrooms, offices, and other applications where space is at a premium,if maximum cleanliness is essential or legally required.Draperies and curtains can be installed at outdoor exposed wallswithout

27、 interfering with the space-conditioning system.When four-pipe systems are used, cooling and heating can besimultaneous, without central zoning or seasonal changeover.Supply air requirements usually do not exceed those required forventilation and humidity control.Reduced airflow requirements help mi

28、tigate bioterrorism risk,especially in large buildings.Modular panels provide flexibility to meet changes in partitioning.A 100% outdoor air system may be installed with smaller penal-ties in refrigeration load because of reduced air quantities.A common central air system can serve both the interior

29、 andperimeter zones.Wet-surface cooling coils are eliminated from the occupied space,reducing the potential for septic contamination.Modular panel systems can use the automatic sprinkler systempiping (see NFPA Standard 13, Section 3.6). The maximum watertemperature must not fuse the heads.Panel heat

30、ing and cooling with minimum supply air quantitiesprovide a draft-free environment.Noise associated with fan-coil or induction units is eliminated.Peak loads are reduced as a result of thermal energy storage in thepanel structure, as well as walls and partitions directly exposed topanels.Panels can

31、be combined with other space-conditioning systems todecouple several indoor requirements (e.g., humidity control,indoor air quality, air velocity) and optimally satisfy them withoutcompromises.In-floor heating creates inhospitable living conditions for housedust mites compared to other heating syste

32、ms (Sugawara et al.1996).Disadvantages include the following:Response time can be slow if controls and/or heating elements arenot selected or installed correctly.Improper selection of panel heating or cooling tube or electricalheating element spacing and/or incorrect sizing of heating/coolingsource

33、can cause nonuniform surface temperatures or insufficientsensible heating or cooling capacity.Panels can satisfy only sensible heating and cooling loads unlesshybrid panels are used see the section on Hybrid (Load-Sharing)HVAC Systems. In a stand-alone panel cooling system, dehu-midification and pan

34、el surface condensation may be of primeconcern. Unitary dehumidifiers should be used, or a latent air-handling system should be introduced to the indoor space.HEAT TRANSFER BY PANEL SURFACESSensible heating or cooling panels transfer heat throughtemperature-controlled (active) surface(s) to or from

35、an indoor spaceand its enclosure surfaces by thermal radiation and natural convection.Heat Transfer by Thermal RadiationThe basic equation for a multisurface enclosure with gray,diffuse isothermal surfaces is derived by radiosity formulationmethods (see Chapter 4 of the 2009 ASHRAE HandbookFunda-men

36、tals). This equation may be written asqr= Jp Jj(1)whereqr= net heat flux because of thermal radiation on active (heated or cooled) panel surface, Btu/hft2Jp= total radiosity leaving or reaching panel surface, Btu/hft2Jj= radiosity from or to another surface in room, Btu/hft2Fpj= radiation angle fact

37、or between panel surface and another surface in room (dimensionless)n = number of surfaces in room other than panel(s)Equation (1) can be applied to simple and complex enclosureswith varying surface temperatures and emittances. The net heat fluxby thermal radiation at the panel surfaces can be deter

38、mined bysolving the unknown Jjif the number of surfaces is small. Morecomplex enclosures require computer calculations.Radiation angle factors can be evaluated using Figure 15 inChapter 4 of the 2009 ASHRAE HandbookFundamentals. Fanger(1972) shows room-related angle factors; they may also be devel-o

39、ped from algorithms in ASHRAEs Energy Calculations I (1976).Several methods have been developed to simplify Equation (1)by reducing a multisurface enclosure to a two-surface approxima-tion. In the MRT method, the thermal radiation interchange in anindoor space is modeled by assuming that the surface

40、s radiate to afictitious, finite surface that has an emittance and surface tempera-ture that gives about the same heat flux as the real multisurface case(Walton 1980). In addition, angle factors do not need to be deter-mined in evaluating a two-surface enclosure. The MRT equationmay be written asqr=

41、 Fr (2)where = Stefan-Boltzmann constant = 0.1712 108Btu/hft2R4Fr= radiation exchange factor (dimensionless)Tp= effective temperature of heating (cooling) panel surface, RTr= temperature of fictitious surface (unheated or uncooled), RThe temperature of the fictitious surface is given by an area emit

42、-tance weighted average of all surfaces other than the panel(s):Tr= (3)whereAj= area of surfaces other than panels, ft2j= thermal emittance of surfaces other than panel(s) (dimensionless)Fpjj=1nTp4Tr4AjjTjj=pnAjjj =pn-Panel Heating and Cooling 6.3When the surface emittances of an enclosure are nearl

43、y equal,and surfaces directly exposed to the panel are marginally unheated(uncooled), then Equation (3) becomes the area-weighted averageunheated (uncooled) temperature (AUST) of such surfaces exposedto the panels. Therefore, any unheated (uncooled) surface in thesame plane with the panel is not acc

44、ounted for by AUST. For exam-ple, if only part of the floor is heated, the remainder of the floor isnot included in the calculation of AUST, unless it is observed byother panels in the ceiling or wall.The radiation interchange factor for two-surface radiation heatexchange is given by the Hottel equa

45、tion:Fr= (4)whereFpr= radiation angle factor from panel to fictitious surface(1.0 for flat panel)Ap, Ar= area of panel surface and fictitious surface, respectivelyp, r= thermal emittance of panel surface and fictitious surface,respectively (dimensionless)In practice, the thermal emittance pof nonmet

46、allic or paintedmetal nonreflecting surfaces is about 0.9. When this emittance is usedin Equation (4), the radiation exchange factor Fris about 0.87 formost indoor spaces. Substituting this value in Equation (2), Frbecomes 0.15 108. Min et al. (1956) showed that this constantwas 0.152 108in their te

47、st room. Then the equation for heat fluxfrom thermal radiation for panel heating and cooling becomesapproximatelyqr= 0.15 108(tp+ 459.67)4 (AUST + 459.67)4(5)wheretp= effective panel surface temperature, FAUST = area-weighted temperature of all indoor surfaces of walls, ceiling,floor, windows, doors

48、, etc. (excluding active panel surfaces), FEquation (5) establishes the general sign convention for thischapter, which states that heating by the panel is positive and cool-ing by the panel is negative.Radiation exchange calculated from Equation (5) is given inFigure 1. The values apply to ceiling,

49、floor, or wall panel output.Radiation removed by a cooling panel for a range of normallyencountered temperatures is given in Figure 2.In many specific instances where normal multistory commercialconstruction and fluorescent lighting are used, the indoor air tem-perature at the 5 ft level closely approaches the AUST. In structureswhere the main heat gain is through the walls or where incandescentlighting is used, wall surface temperatures tend to rise considerablyabove the indoor air temperature.Heat Transfer by Natural