ASHRAE FUNDAMENTALS SI 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 45C or less than18C causes pain (Hardy et al. 1952). Skin temperatures associatedwith comfort at sedentary activities are 33 to 34C and decrease withincreasing activity (Fanger 1967). In contrast, internal temperaturesrise with activ

10、ity. The temperature regulatory center in the brain isabout 36.8C at rest in comfort and increases to about 37.4C whenwalking and 37.9C when jogging. An internal temperature less thanabout 28C can lead to serious cardiac arrhythmia and death, and atemperature greater than 43C can cause irreversible

11、brain damage.Therefore, careful regulation of body temperature is critical to com-fort and health.A resting adult produces about 100 W of heat. Because most ofthis is transferred to the environment through the skin, it is oftenconvenient to characterize metabolic activity in terms of heatproduction

12、per unit area of skin. For a resting person, this is about58 W/m2and is called 1 met. This is based on the average maleEuropean, with a skin surface area of about 1.8 m2. For compari-son, female Europeans have an average surface area of 1.6 m2. Sys-tematic differences in this parameter may occur bet

13、ween ethnicand geographical groups. Higher metabolic rates are oftendescribed in terms of the resting rate. Thus, a person working atmetabolic rate five times the resting rate would have a metabolicrate of 5 met.The hypothalamus, located in the brain, is the central controlorgan for body temperature

14、. 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 temperature. The hypothalamus also rece

15、ives 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 primarily propor-tional to deviatio

16、ns 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 vasodilation of skin blood vessels

17、can increase skinblood flow by 15 times from 1.7 mL/(sm2) at resting comfort to25 mL/(sm2) 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 conserveheat. The effect of ma

18、ximum 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 resting heat production to 4.5 met.

19、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 increasinglynecessary for comfort at metabolic ra

20、tes 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 favorable for evaporation, the swea

21、tmust 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 (Gagge1937).Humans are quite good

22、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 wettedness greater than 25%. I

23、nThe preparation of this chapter is assigned to TC 2.1, Physiology andHuman Environment.9.2 2013 ASHRAE HandbookFundamentals (SI)addition to the perception of skin moisture, skin wettednessincreases the friction between skin and fabrics, making clothing feelless pleasant and fabrics feel more coarse

24、 (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 of the sweating system inc

25、reases (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 plasma. After prolongedheat

26、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 evapo-ration, the accumulat

27、ing 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 where heat transfer is b

28、etter.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 (30 to 38C) for 30 s with the subjectat different thermal states. When the person was overh

29、eated (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-sures to hot and cold envi

30、ronment. 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 total metabolic rate M withi

31、n 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 environ-ment through the skin

32、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, W/m2W = rate of mechanical work accomplished, W/m2qsk= total rate

33、 of heat loss from skin, W/m2qres= total rate of heat loss through respiration, W/m2C + R = sensible heat loss from skin, W/m2Esk= total rate of evaporative heat loss from skin, W/m2Cres= rate of convective heat loss from respiration, W/m2Eres= rate of evaporative heat loss from respiration, W/m2Ssk

34、= rate of heat storage in skin compartment, W/m2Scr= rate of heat storage in core compartment, W/m2Heat dissipates from the body to the immediate surroundings byseveral modes of heat exchange: sensible heat flow C + R from theskin; latent heat flow from sweat evaporation Erswand from evapo-ration of

35、 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, convection, and radiationfor a clothed person; however, it is equal t

36、o 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,and skin wettedness w. Factors also account for thermal insulationa

37、nd 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 variables that influ-ence thermal comfort are activity and clothing.

38、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). The storage rate can be writ-ten separately for each compartment in

39、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, kgcp,b= specific heat capacity of body = 3490 J/(kgK)AD= DuBois surface area, m2tcr= temperature of core compartment, Ctsk

40、= temperature of skin compartment, C = time, sThe 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 and Gagge (1967) give quantitative information on calcu-lating heat

41、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 associated with the ther-moregulatory control mechanisms (qcr,sk, Mshiv,

42、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.Mathematical description of the energy balance of the humanbody combines ratio

43、nal 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 deter-mine the values of coefficients describing these rates of heat

44、exchange. 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 andrefer to the surface area of the nude body. The most useful measu

45、reof nude body surface area, originally proposed by DuBois andDuBois (1916), is described byAD= 0.202m0.425l0.725(4)whereAD= DuBois surface area, m2m =mass, kgl =height, mA correction factor fcl= Acl/ADmust be applied to the heat transferterms from the skin (C, R, and Esk) to account for the actual

46、surfacearea Aclof the clothed body. Table 7 presents fclvalues for variousclothing ensembles. For a 1.73 m tall, 70 kg man, AD= 1.8 m2. Allterms in the basic heat balance equations are expressed per unitDuBois surface area.Sensible Heat Loss from SkinSensible heat exchange from the skin must pass th

47、rough 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) from the outer clothing surface to the environment.Both convective C and ra

48、diative 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 appropriate environ-mental temperature:C = fclhc(tcl ta)(5)R = fclhr(6)wherehc= c

49、onvective heat transfer coefficient, W/(m2K)hr= linear radiative heat transfer coefficient, W/(m2K)fcl= 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 transfer coefficient h:C + R = fclh(tcl to)(7)whereto= (8)h = hr+ hc(9)Based on Equation (8),