ASHRAE FUNDAMENTALS IP CH 9-2013 Thermal Comfort.pdf

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1、9.1CHAPTER 9THERMAL COMFORTHuman Thermoregulation . 9.1Energy Balance . 9.2Thermal Exchanges with the Environment 9.2Engineering Data and Measurements . 9.6Conditions for Thermal Comfort 9.11Thermal Comfort and Task Performance 9.13Thermal Nonuniform Conditions and Local Discomfort 9.14Secondary Fac

2、tors Affecting Comfort . 9.16Prediction of Thermal Comfort . 9.17Environmental Indices . 9.20Special Environments 9.23Symbols . 9.28principal purpose of HVAC is to provide conditions for humanA thermal comfort, “that condition of mind that expresses satis-faction with the thermal environment” (ASHRA

3、E Standard 55).This definition leaves open what is meant by “condition of mind” or“satisfaction,” but it correctly emphasizes that judgment of comfortis a cognitive process involving many inputs influenced by physical,physiological, psychological, and other processes. This chaptersummarizes the fund

4、amentals of human thermoregulation and com-fort in terms useful to the engineer for operating systems and design-ing for the comfort and health of building occupants.The conscious mind appears to reach conclusions about thermalcomfort and discomfort from direct temperature and moisture sen-sations f

5、rom the skin, deep body temperatures, and the efforts nec-essary to regulate body temperatures (Berglund 1995; Gagge 1937;Hardy et al. 1971; Hensel 1973, 1981). In general, comfort occurswhen body temperatures are held within narrow ranges, skin mois-ture is low, and the physiological effort of regu

6、lation is minimized.Comfort also depends on behaviors that are initiated consciouslyor unconsciously and guided by thermal and moisture sensations toreduce discomfort. Some examples are altering clothing, alteringactivity, changing posture or location, changing the thermostat set-ting, opening a win

7、dow, complaining, or leaving the space.Surprisingly, although climates, living conditions, and culturesdiffer widely throughout the world, the temperature that peoplechoose for comfort under similar conditions of clothing, activity,humidity, and air movement has been found to be very similar(Busch 1

8、992; de Dear et al. 1991; Fanger 1972).HUMAN THERMOREGULATIONMetabolic activities of the body result almost completely in heatthat must be continuously dissipated and regulated to maintain nor-mal body temperatures. Insufficient heat loss leads to overheating(hyperthermia), and excessive heat loss r

9、esults in body cooling(hypothermia). Skin temperature greater than 113F or less than64.5F causes pain (Hardy et al. 1952). Skin temperatures associatedwith comfort at sedentary activities are 91.5 to 93F and decreasewith increasing activity (Fanger 1967). In contrast, internal temper-atures rise wit

10、h activity. The temperature regulatory center in thebrain is about 98.2F at rest in comfort and increases to about 99.3Fwhen walking and 100.2F when jogging. An internal temperatureless than about 82F can lead to serious cardiac arrhythmia anddeath, and a temperature greater than 110F can cause irre

11、versiblebrain damage. Therefore, careful regulation of body temperature iscritical to comfort and health.A resting adult produces about 350 Btu/h of heat. Because mostof this is transferred to the environment through the skin, it is oftenconvenient to characterize metabolic activity in terms of heat

12、production per unit area of skin. For a resting person, this is about18.4 Btu/hft2(50 kcal/hm2) and is called 1 met. This is based onthe average male European, with a skin surface area of about19.4 ft2. For comparison, female Europeans have an average surfacearea of 17.2 ft2. Systematic differences

13、in this parameter may occurbetween ethnic and geographical groups. Higher metabolic rates areoften described in terms of the resting rate. Thus, a person workingat metabolic rate five times the resting rate would have a metabolicrate of 5 met.The hypothalamus, located in the brain, is the central co

14、ntrolorgan for body temperature. It has hot and cold temperature sensorsand is bathed by arterial blood. Because the recirculation rate ofblood is rapid and returning blood is mixed together in the heartbefore returning to the body, arterial blood is indicative of the aver-age internal body temperat

15、ure. The hypothalamus also receives ther-mal information from temperature sensors in the skin and perhapsother locations as well (e.g., spinal cord, gut), as summarized byHensel (1981).The hypothalamus controls various physiological processes toregulate body temperature. Its control behavior is prim

16、arily propor-tional to deviations from set-point temperatures with some integraland derivative response aspects. The most important and often-usedphysiological process is regulating blood flow to the skin: wheninternal temperatures rise above a set point, more blood is directed tothe skin. This vaso

17、dilation of skin blood vessels can increase skinblood flow by 15 times (from 0.56 L/hft2at resting comfort to8.4 L/hft2) in extreme heat to carry internal heat to the skin fortransfer to the environment. When body temperatures fall below theset point, skin blood flow is reduced (vasoconstricted) to

18、conserveheat. The effect of maximum vasoconstriction is equivalent to theinsulating effect of a heavy sweater. At temperatures less than the setpoint, muscle tension increases to generate additional heat; wheremuscle groups are opposed, this may increase to visible shivering,which can increase resti

19、ng heat production to 4.5 met.At elevated internal temperatures, sweating occurs. This defensemechanism is a powerful way to cool the skin and increase heat lossfrom the core. The sweating function of the skin and its control ismore advanced in humans than in other animals and is increasinglynecessa

20、ry for comfort at metabolic rates above resting level (Fanger1967). Sweat glands pump perspiration onto the skin surface forevaporation. If conditions are good for evaporation, the skin canremain relatively dry even at high sweat rates with little perception ofsweating. At skin conditions less favor

21、able for evaporation, the sweatmust spread on the skin around the sweat gland until the sweat-covered area is sufficient to evaporate the sweat coming to the sur-face. The fraction of the skin that is covered with water to account forthe observed total evaporation rate is termed skin wettedness (Gag

22、ge1937).Humans are quite good at sensing skin moisture from perspira-tion (Berglund 1994; Berglund and Cunningham 1986), and skinmoisture correlates well with warm discomfort and unpleasantness(Winslow et al. 1937). It is rare for a sedentary or slightly active per-son to be comfortable with a skin

23、wettedness greater than 25%. InThe preparation of this chapter is assigned to TC 2.1, Physiology andHuman Environment.9.2 2013 ASHRAE HandbookFundamentalsaddition to the perception of skin moisture, skin wettednessincreases the friction between skin and fabrics, making clothing feelless pleasant and

24、 fabrics feel more coarse (Gwosdow et al. 1986).This also occurs with architectural materials and surfaces, particu-larly smooth, nonhygroscopic surfaces.With repeated intermittent heat exposure, the set point for theonset of sweating decreases and the proportional gain or tempera-ture sensitivity o

25、f the sweating system increases (Gonzalez et al.1978; Hensel 1981). However, under long-term exposure to hot con-ditions, the set point increases, perhaps to reduce the physiologicaleffort of sweating. Perspiration as secreted has a lower salt concen-tration than interstitial body fluid or blood pla

26、sma. After prolongedheat exposure, sweat glands further reduce the salt concentration ofsweat to conserve salt.At the surface, the water in sweat evaporates while the dissolvedsalt and other constituents remain and accumulate. Because saltlowers the vapor pressure of water and thereby impedes its ev

27、apo-ration, the accumulating salt results in increased skin wettedness.Some of the relief and pleasure of washing after a warm day isrelated to the restoration of a hypotonic sweat film and decreasedskin wettedness. Other adaptations to heat are increased blood flowand sweating in peripheral regions

28、 where heat transfer is better.Such adaptations are examples of integral control.Role of Thermoregulatory Effort in Comfort. Chatonnet andCabanac (1965) compared the sensation of placing a subjects handin relatively hot or cold water (86 to 100F) for 30 s with the subjectat different thermal states.

29、 When the person was overheated (hyper-thermic), the cold water was pleasant and the hot water was veryunpleasant, but when the subject was cold (hypothermic), the handfelt pleasant in hot water and unpleasant in cold water. Kuno (1995)describes similar observations during transient whole-body expo-

30、sures to hot and cold environment. When a subject is in a state ofthermal discomfort, any move away from the thermal stress of theuncomfortable environment is perceived as pleasant during thetransition.ENERGY BALANCEFigure 1 shows the thermal interaction of the human body withits environment. The to

31、tal metabolic rate M within the body is themetabolic rate required for the persons activity Mactplus the meta-bolic level required for shivering Mshiv(should shivering occur).Some of the bodys energy production may be expended as externalwork W; the net heat production M W is transferred to the envi

32、ron-ment through the skin surface (qsk) and respiratory tract (qres) withany surplus or deficit stored (S), causing the bodys temperature torise or fall.M W = qsk+ qres+ S= (C + R + Esk) + (Cres+ Eres) + (Ssk+ Scr)(1)whereM = rate of metabolic heat production, Btu/hft2W = rate of mechanical work acc

33、omplished, Btu/hft2qsk= total rate of heat loss from skin, Btu/hft2qres= total rate of heat loss through respiration, Btu/hft2C + R = sensible heat loss from skin, Btu/hft2Esk= total rate of evaporative heat loss from skin, Btu/hft2Cres= rate of convective heat loss from respiration, Btu/hft2Eres= r

34、ate of evaporative heat loss from respiration, Btu/hft2Ssk= rate of heat storage in skin compartment, Btu/hft2Scr= rate of heat storage in core compartment, Btu/hft2Heat dissipates from the body to the immediate surroundings byseveral modes of heat exchange: sensible heat flow C + R from theskin; la

35、tent heat flow from sweat evaporation Erswand from evapo-ration of moisture diffused through the skin Edif; sensible heat flowduring respiration Cres; and latent heat flow from evaporation ofmoisture during respiration Eres. Sensible heat flow from the skinmay be a complex mixture of conduction, con

36、vection, and radiationfor a clothed person; however, it is equal to the sum of the convec-tion C and radiation R heat transfer at the outer clothing surface (orexposed skin).Sensible and latent heat losses from the skin are typicallyexpressed in terms of environmental factors, skin temperature tsk,a

37、nd skin wettedness w. Factors also account for thermal insulationand moisture permeability of clothing. The independent environ-mental variables can be summarized as air temperature ta, meanradiant temperature , relative air velocity V, and ambient watervapor pressure pa. The independent personal va

38、riables that influ-ence thermal comfort are activity and clothing.The rate of heat storage in the body equals the rate of increase ininternal energy. The body can be considered as two thermal com-partments: the skin and the core (see the Two-Node Model sectionunder Prediction of Thermal Comfort). Th

39、e storage rate can be writ-ten separately for each compartment in terms of thermal capacityand time rate of change of temperature in each compartment:Scr= (2)Ssk= (3)wheresk= fraction of body mass concentrated in skin compartmentm = body mass, lbcp,b= specific heat capacity of body = 0.834 Btu/lbFAD

40、= DuBois surface area, ft2tcr= temperature of core compartment, Ftsk= temperature of skin compartment, F = time, hThe fractional skin mass skdepends on the rate of blood flow-ing to the skin surface.THERMAL EXCHANGES WITH ENVIRONMENTFanger (1967, 1970), Gagge and Hardy (1967), Hardy (1949),and Rapp

41、and Gagge (1967) give quantitative information on calcu-lating heat exchange between people and the environment. This sec-tion summarizes the mathematical statements for various terms ofheat exchange used in the heat balance equations (C, R, Esk, Cres,Eres). Terms describing the heat exchanges assoc

42、iated with the ther-moregulatory control mechanisms (qcr,sk, Mshiv, Ersw), values forFig. 1 Thermal Interaction of Human Body and Environmenttr1 skmcpb,AD-dtcrd-skmcpb,AD-dtskd-mblThermal Comfort 9.3the coefficients, and appropriate equations for Mactand ADare pre-sented in later sections.Mathematic

43、al description of the energy balance of the humanbody combines rational and empirical approaches to describingthermal exchanges with the environment. Fundamental heat transfertheory is used to describe the various mechanisms of sensible andlatent heat exchange, and empirical expressions are used to

44、deter-mine the values of coefficients describing these rates of heatexchange. Empirical equations are also used to describe the thermo-physiological control mechanisms as a function of skin and coretemperatures in the body.Body Surface AreaThe terms in Equation (1) have units of power per unit area

45、andrefer to the surface area of the nude body. The most useful measureof nude body surface area, originally proposed by DuBois andDuBois (1916), is described byAD= 0.108m0.425l0.725(4)whereAD= DuBois surface area, ft2m =mass, lbl = height, in.A correction factor fcl= Acl/ADmust be applied to the hea

46、t transferterms from the skin (C, R, and Esk) to account for the actual surfacearea Aclof the clothed body. Table 7 presents fclvalues for variousclothing ensembles. For a 68 in. tall, 154 lb man, AD= 19.6 ft2. Allterms in the basic heat balance equations are expressed per unitDuBois surface area.Se

47、nsible Heat Loss from SkinSensible heat exchange from the skin must pass through clothingto the surrounding environment. These paths are treated in seriesand can be described in terms of heat transfer (1) from the skin sur-face, through the clothing insulation, to the outer clothing surface,and (2)

48、from the outer clothing surface to the environment.Both convective C and radiative R heat losses from the outer sur-face of a clothed body can be expressed in terms of a heat transfercoefficient and the difference between the mean temperature tclofthe outer surface of the clothed body and the approp

49、riate environ-mental temperature:C = fclhc(tcl ta)(5)R = fclhr(6)wherehc= convective heat transfer coefficient, Btu/hft2Fhr= linear radiative heat transfer coefficient, Btu/hft2Ffcl= clothing area factor Acl/AD, dimensionlessThe coefficients hcand hrare both evaluated at the clothing surface.Equations (5) and (6) are commonly combined to describe the totalsensible heat exchange by these two mechanisms in terms of anoperative temperature toand a combined heat tr