ASHRAE FUNDAMENTALS IP 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 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 cont

10、rast, internal tempera-tures rise with 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 tempera

11、ture greater than 110F can cause irreversiblebrain 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 characteriz

12、e metabolic activity in terms of heatproduction 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

13、 of 17.2 ft2. Systematic differences 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, lo

14、cated in the brain, is the central 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 o

15、f the aver-age internal body temperature. 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 temp

16、erature. Its control behavior is 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 bl

17、ood is directed tothe skin. This vasodilation 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

18、flow is reduced (vasoconstricted) 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 visib

19、le shivering,which can increase 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 oth

20、er animals and is increasinglynecessary 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 ofsw

21、eating. At skin conditions less 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 evaporatio

22、n rate is termed skin wettedness (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 per

23、son to beThe preparation of this chapter is assigned to TC 2.1, Physiology andHuman Environment.9.2 2017 ASHRAE HandbookFundamentals comfortable with a skin wettedness greater than 25%. In addition tothe perception of skin moisture, skin wettedness increases the fric-tion between skin and fabrics, m

24、aking clothing feel less pleasant 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 proportiona

25、l gain or tempera-ture sensitivity 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 tha

26、n interstitial body fluid or 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 vapo

27、r pressure of water and thereby 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 increase

28、d blood flowand sweating in 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 (86 to 100F) for 30

29、s with the subjectat different 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

30、observations during transient 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 interact

31、ion of the human body withits 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 n

32、et heat production M W is transferred 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 pr

33、oduction, Btu/hft2W = rate of mechanical work accomplished, 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 conve

34、ctive heat loss from respiration, Btu/hft2Eres= rate 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 ex

35、change: sensible heat flow C + R from theskin; latent 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 th

36、e skinmay be a complex mixture of conduction, convection, 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

37、 of environmental factors, skin temperature tsk,and 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 wa

38、tervapor pressure pa. The independent personal variables 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 Mode

39、l sectionunder Prediction of Thermal Comfort). The 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=

40、 specific heat capacity of body = 0.834 Btu/lbFAD= 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.3. THERMAL EXCHANGES WITH ENVIRONMENTFanger (1967, 1

41、970), Gagge and Hardy (1967), Hardy (1949),and Rapp 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, C

42、res,Fig. 1 Thermal Interaction of Human Body and Environmenttr1 skmcpb,AD-dtcrd-skmcpb,AD-dtskd-mblThermal Comfort 9.3Eres). Terms describing the heat exchanges associated with the ther-moregulatory control mechanisms (qcr,sk, Mshiv, Ersw), values forthe coefficients, and appropriate equations for M

43、actand ADare pre-sented in later sections.Mathematical 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 h

44、eat exchange, and empirical expressions are used to 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 term

45、s in Equation (1) have units of power per unit area 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 cor

46、rection factor fcl= Acl/ADmust be applied to the heat 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 eq

47、uations are expressed per unitDuBois surface area.Sensible 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 clothin

48、g insulation, to the outer clothing surface,and (2) 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 tclof

49、the outer surface of the clothed body and the appropriate 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