ASHRAE FUNDAMENTALS SI CH 9-2017 Thermal Comfort.pdf

1、9.1CHAPTER 9 THERMAL COMFORTHuman Thermoregulation 9.1Energy Balance 9.2Thermal Exchanges with Environment 9.2Engineering Data and Measurements . 9.6Conditions for Thermal Comfort . 9.12Thermal Comfort and Task Performance 9.14Thermal Nonuniform Conditions and Local Discomfort . 9.14Secondary Factor

2、s Affecting Comfort 9.17Prediction of Thermal Comfort 9.17Environmental Indices . 9.21Special 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 and is assessed

3、 by subjectiveevaluation” (ASHRAE Standard 55). This definition leaves openwhat is meant by “condition of mind” or “satisfaction,” but it cor-rectly emphasizes that judgment of comfort is a cognitive processinvolving many inputs influenced by physical, physiological, psy-chological, and other proces

4、ses. This chapter summarizes the funda-mentals of human thermoregulation and comfort in terms useful tothe engineer for operating systems and designing for the comfort andhealth of building occupants.The conscious mind appears to reach conclusions about thermalcomfort and discomfort from direct temp

5、erature and moisture sen-sations from 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, an

6、d the physiological effort of regulation 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

7、thermostat set-ting, opening a window, 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 bee

8、n found to be very similar(Busch 1992; de Dear et al. 1991; Fanger 1972).1. 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(hyp

9、erthermia), and excessive heat loss results 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,

10、 internal temperaturesrise with activity. 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 gre

11、ater than 43C can cause irreversible 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 often con-venient to characterize metabol

12、ic activity in terms of heat productionper unit area of skin. For a resting person, this is about 58 W/m2andis called 1 met. This is based on the average male European, with askin surface area of about 1.8 m2. For comparison, female Europeanshave an average surface area of 1.6 m2. Systematic differe

13、nces in thisparameter may occur between ethnic and geographical groups.Higher metabolic rates are often described in terms of the restingrate. Thus, a person working at metabolic rate five times the restingrate would have a metabolic rate of 5 met.The hypothalamus, located in the brain, is the centr

14、al controlorgan 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 tem

15、perature. 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

16、 primarily 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

17、 vasodilation of skin blood vessels 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 (vasoconstricte

18、d) to 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

19、 resting 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 increasinglyn

20、ecessary 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

21、 favorable 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 wettednes

22、s (Gagge1937).Humans are quite good at sensing skin moisture from perspiration(Berglund 1994; Berglund and Cunningham 1986), and skin moisturecorrelates well with warm discomfort and unpleasantness (Winslowet al. 1937). It is rare for a sedentary or slightly active person to becomfortable with a ski

23、n wettedness greater than 25%. In addition toThe preparation of this chapter is assigned to TC 2.1, Physiology andHuman Environment.9.2 2017 ASHRAE HandbookFundamentals (SI)the perception of skin moisture, skin wettedness increases the fric-tion between skin and fabrics, making clothing feel less pl

24、easant andfabrics feel more coarse (Gwosdow et al. 1986). This also occurswith architectural materials and surfaces, particularly smooth, non-hygroscopic surfaces.With repeated intermittent heat exposure, the set point for theonset of sweating decreases and the proportional gain or tempera-ture sens

25、itivity of 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 o

26、r blood plasma. After prolongedheat exposure, sweat glands further reduce the salt concentration ofsweat to conserve salt.At the skins surface, the water in sweat evaporates while the dis-solved salt and other constituents remain and accumulate. Becausesalt lowers the vapor pressure of water and the

27、reby impedes its evap-oration, 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

28、 peripheral regions 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 (30 to 38C) for 30 s with the subjectat differe

29、nt thermal states. 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 transien

30、t whole-body expo-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.2. ENERGY BALANCEFigure 1 shows the thermal interaction of the human body withit

31、s environment. The total 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 tr

32、ansferred to the environ-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, W/m2W = rate of me

33、chanical work accomplished, W/m2qsk= total rate 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

34、evaporative heat loss from respiration, W/m2Ssk= 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

35、 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, convection, and radiat

36、ionfor 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,and skin wettedness

37、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 variables that influ-

38、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). The storage rate can

39、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, kgcp,b= specific heat capacity of body = 3490 J/(kgK)AD= DuBois surface are

40、a, m2tcr= temperature of core compartment, Ctsk= temperature of skin compartment, C = time, sThe fractional skin mass skdepends on the rate of blood flow-ing to the skin surface.3. THERMAL EXCHANGES WITH ENVIRONMENTFanger (1967, 1970), Gagge and Hardy (1967), Hardy (1949),and Rapp and Gagge (1967) g

41、ive 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 associated with theFig.

42、 1 Thermal Interaction of Human Body and Environmenttr1 skmcpb,AD-dtcrd-skmcpb,AD-dtskd-mblThermal Comfort 9.3thermoregulatory control mechanisms (qcr,sk, Mshiv, Ersw), valuesfor the coefficients, and appropriate equations for Mactand ADarepresented in later sections.Mathematical description of the

43、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 deter-mine the values

44、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 andrefer to the surfac

45、e area of the nude body. The most useful measureof 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 s

46、kin (C, R, and Esk) to account for the actual 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 SkinSen

47、sible 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) from the outer clothing surfa

48、ce 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 appropriate environ-mental temperat

49、ure:C = fclhc(tcl ta)(5)R = fclhr(6)wherehc= convective 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)wher