1. HCB-3 Chap 3A: Thermal Comfort1 Chapter 3A: THERMAL COMFORT Agami Reddy (July 2016) 1.Indoor environment quality 2.Basics of comfort: Definition and categories of factors 3.Heat balance 4.Metabolic rates: met 5.Environmental indices: Direct and Indirect 6.Mean radiant and operative temperatures 7.Clothing insulation: clo 8.ASHRAE Thermal sensation scale model: PMV and PPD 9.ASHRAE comfort chart- graphical method 10.Adjustments to ASHRAE comfort chart 11.Adaptive comfort model 12.Other considerations

2. HCB-3 Chap 3A: Thermal Comfort2 Indoor Environment Quality What is Indoor Environment Quality? –Thermal Comfort –Indoor Air Quality –Other Environment Issues Visual, Acoustic, Access to daylight,… Why do we need to care? –90 % of our lives are spent indoors –70 % of US work force (90 million persons) –Comfort - productivity –Health

3. HCB-3 Chap 3A: Thermal Comfort3 From Kolderup, 2009

4. HCB-3 Chap 3A: Thermal Comfort4 Fig. 3.1 Factors influencing human comfort

5. HCB-3 Chap 3A: Thermal Comfort5 Objective criteria From Kolderup, 2009

6. HCB-3 Chap 3A: Thermal Comfort6 Basics of Thermal Comfort - Definition: ASHRAE 55-2013 Thermal comfort is that condition of mind that expresses satisfaction with the thermal environment Task of AC system is to maintain a thermally comfortable environment by simultaneous control of temperature, humidity, cleanliness and air circulation.

7. HCB-3 Chap 3A: Thermal Comfort7 Metabolic Rates: unit of “Met” 1 M = 1 met = 58.2 W/m 2 =18.4 Btu/h.ft 2

8. HCB-3 Chap 3A: Thermal Comfort8 Heat Balance of Body Thermal comfort is maintained by heat transfer –Human body generates heat (about 100 W under sedentary conditions with body area = 1.5 to 2 m 2 ) –For comfort to be maintained: heat generated = heat loss –Fundamental trends: Heat flows from body to ambient air Heat flow rate is proportional to temperature difference which is affected by clothing More layers of clothing=more insulation= less heat loss More surface area = More heat transfer

9. HCB-3 Chap 3A: Thermal Comfort9 Heat Balance Network Skin temperature conducive to comfort (Eq. 3.14): where M and W are in W Heat flow can be represented by a thermal network diagram:

10. HCB-3 Chap 3A: Thermal Comfort10 Importance of different heat transfer mechanisms at diff temperatures Heat generated and lost by a person at rest (RH fixed at 45%) Convection Conduction Radiation Evaporation

11. HCB-3 Chap 3A: Thermal Comfort11 Environmental Indices for Measuring Comfort- Direct Indices (a) Dry-bulb temperature: single most important index, especially influential when RH is in the range 40-60% (b) Moisture: three measures - Dew point temperature: good single measure but is of limited usefulness for comfort - Wet bulb temperature: useful for describing comfort conditions in regions of high temperature and where dry-bulb has less significance - Relative humidity: has no real meaning in terms of comfort unless accompanying dry-bulb temp. is also known, Very low or very high values associated with discomfort (c) Air movement: most difficult of the direct indices to describe. It affects only convective heat exchange from body.

12. HCB-3 Chap 3A: Thermal Comfort12 Environmental Indices for Measuring Comfort- Derived Indices (a) Mean radiant temperature (MRT): surface temperature of an imaginary black body (or enclosure) to which a person, also assumed to be a black body, exchanges the same amount of heat by radiation as in the actual environment (b) Operative temperature (OT): uniform temperature of a radiantly black enclosure in which an occupant exchanges the same amount of heat by radiation plus convection as in the actual non-uniform indoor environment. Numerically, it is close to the average of indoor dry- bulb and mean radiant temperatures (c) Effective temperature (ET): the operative temperature of an enclosure at 50% RH that would cause the same sensible plus latent heat exchange from a person as would the actual environment (combines temperature and humidity in one index)

13. HCB-3 Chap 3A: Thermal Comfort13 (a) Mean radiant temperature (MRT): surface temperature of an imaginary black body (or enclosure) to which a person, also assumed to be a black body, exchanges the same amount of heat by radiation as in the actual environment Basic index to describe radiative conditions in space (cold walls, sunlight walls) Simplified methods From Bobenhausen, 1994

14. HCB-3 Chap 3A: Thermal Comfort14 Instrument to Measure MRT Vernon’s globe thermometer (Hollow sphere 6” diameter painted black with a thermocouple at the center) One measures globe temperature T g, ambient temperature T a and the air velocity v (which determines h c ) from there T mrt can be deduced (globe emissivity close to 1)

15. HCB-3 Chap 3A: Thermal Comfort15

16. HCB-3 Chap 3A: Thermal Comfort16 (b) Operative Temperature

17. HCB-3 Chap 3A: Thermal Comfort17

18. HCB-3 Chap 3A: Thermal Comfort18 If we had used the simple arithmetic average: (21+17.5)/2=19.25 o C

19. HCB-3 Chap 3A: Thermal Comfort19 Note: Clothing acts as an insulation layer. The insulation value is measured in unit: clo (1.0 clo is equivalent to the typical American Man’s Business suit in 1941) 1 clo = 0.88 ft 2.h. o F/Btu (0.155 m 2.K/W) Clothing Insulation

20. HCB-3 Chap 3A: Thermal Comfort20 Example 3.2: Sensible Heat Loss Consider the same conditions as in Example 3.1. The person is active (met level of 2.6) wearing trousers and a long-sleeve shirt. Calculate the total (convective plus radiative) sensible heat loss. Given: Top = 18.9°C (66°F) Assumption: Steady-state condition; skin area Ask = 1.8 m 2 (19.6 ft 2 ) (DuBois skin surface area) and work rate = 0; M = 2.6 Lookup value: From Table 3.3, the total thermal resistance I T = 1.21 clo and Also, A cl /A sk = 1.20.

21. HCB-3 Chap 3A: Thermal Comfort21.9.9 2.16 x (31.54 – 18.9) / 0.188 = 145.2 W

22. HCB-3 Chap 3A: Thermal Comfort22 ASHRAE Thermal Sensation Scale Empirical approach which captures occupant psychological aspects, i.e., subjective differences between individuals. - Thermal comfort is characterized by: air temperature, MRT, air humidity and air velocity with M and Clo kept constant - Additional criteria for discomfort: local draught, high turbulence, high radiant temperature asymmetry and unacceptably high vertical air temperature difference

23. HCB-3 Chap 3A: Thermal Comfort23 A thermal sensation index, called the predicted mean vote (PMV), has been proposed to represent occupant acceptability of the indoor environment. - It can be calculated through a complex mathematical correlation - The PMV index is used to quantify the degree of discomfort and ranges from +3 to −3, with zero indicating neutral or comfort condition.

24. HCB-3 Chap 3A: Thermal Comfort24 Predicted Mean Vote (PMV) Model

25. HCB-3 Chap 3A: Thermal Comfort25

26. HCB-3 Chap 3A: Thermal Comfort26

27. HCB-3 Chap 3A: Thermal Comfort27 Percentage of People Dissatisfied (PPD) Method widely used in studies investigating tradeoffs between energy use and human comfort Due to individual preferences, one would expect a distribution of votes for the PMV. An index meant to characterize this variability is PPD. It has been found that PPD is empirically correlated to PMV. Note: even under optimal conditions (i.e., mean vote = 0) approximately 5% may be dissatisfied with the thermal environment.

28. HCB-3 Chap 3A: Thermal Comfort28 ASHRAE Comfort Chart Valid for: -Sedentary or slightly active person -Summer: light slacks & Short sleeve shirt (0.5 clo) -Winter: Heavy slacks & long sleeve & sweater or jacket (1.0 clo) - Air motion: < 30 ft/min in winter < 50 ft/min in summer - No direct solar or other radiation When application conditions are not standard, use eq. (3.20) given in a later slide For example: For each 0.1 clo increase, decrease the comfort zone borders by 1 °F and visa- versa -Why two regions? - Why tilt in comfort regions?

29. HCB-3 Chap 3A: Thermal Comfort29

30. HCB-3 Chap 3A: Thermal Comfort30 Comfort Conditions Comfort Standard applies to sedentary conditions: Temperature - Dry Bulb Temperature (70 – 75 0 F) Humidity – Relative Humidity (Up to 60%) Air Motion – air velocity –0 – 50 ft/min: Still –50 – 250 ft/min : Noticeable –> 250 ft/min Drafty –Activity plays a role (metabolic or met level) M=1 (sedentary) to M=5 (heavy activity) –Clothing plays a role (Table of clo): 0.5 (summer) to 1.0 (winter) –ASHRAE Standard 55-2013: Thermal Environmental Conditions for Human Occupancy

31. HCB-3 Chap 3A: Thermal Comfort31 Instead of using direct indices, comfort stated in terms of Operative temperature

32. HCB-3 Chap 3A: Thermal Comfort32 Adjustments to Standard Conditions Where clo is the clothing insulation M is the metabolic value For different clo values and met values between 1.2 and 3, ASHRAE 55 (2013) recommends that the comfort operative temperature be determined from Eq. 3.21:

33. HCB-3 Chap 3A: Thermal Comfort33 Example 3.6: Operative Temperature for Other Conditions Consider a gymnasium where the metabolic rate of the occupants is 3.0 and the clothing level is 0.3. If the occupants are to experience the same level of comfort as when sedentary, what should be the operative temperature in this space? Given: M = 3.0, clo = 0.5 Find: T opt,active Solution From Equation 3.21: Note that the gymnasium need not be kept at this low temperature since typically people do not stay longer than about an hour and do not exercise continuously, while the ASHRAE comfort chart applies to steady-state conditions (occupancy of 3 h)

34. HCB-3 Chap 3A: Thermal Comfort34 Figure 3.6 Clothing insulation necessary to be within ASHRAE 80% acceptability limits The effect of clothing insulation on the ASHRAE comfort recommendations. - Note the rather wide uncertainty bands reflective of the large uncertainties inherent in the comfort relations. - For more than an hour, the minimum operative temperature should not be below 18°C (65°F). Corrections To ASHRAE Comfort Chart Conditions

35. HCB-3 Chap 3A: Thermal Comfort35 Figure 3.7 Recommended operative temperatures for active people Note inverse relationship between operative temperature and activity level (ASHRAE Fundamentals, 2013)

36. HCB-3 Chap 3A: Thermal Comfort36

37. HCB-3 Chap 3A: Thermal Comfort37 - Air movement plays a role because the convective heat transfer from the body depends on air velocity. -An excess may be perceived as draft, and too little as stuffiness. -In hot weather, upper range of temperatures could be extended if the airspeed is increased (upper limit 160ft/min (0.8 m/s)- loose paper can be blown away From Kolderup, 2009

38. HCB-3 Chap 3A: Thermal Comfort38 Adaptive Comfort Model (ASHRAE Fundamentals, 2013)

39. HCB-3 Chap 3A: Thermal Comfort39 (ASHRAE Fundamentals, 2013) Local temperature variations: ASHRAE allows up to 10% PPD for such considerations Other Considerations

40. HCB-3 Chap 3A: Thermal Comfort40 Field study results (Roelofsen, 2001) PPD FIGURE 3.12 Illustration of how loss of indoor office occupant productivity closely tracks PPD.

41. HCB-3 Chap 3A: Thermal Comfort41 (ASHRAE Fundamentals, 2013 ) FIGURE 3.14 Variation of rate of unsolicited thermal complaints with mean indoor environment temperature (based on filed study of six commercial buildings in 3 cities) This is important for O&M staff

42. Outcomes Understand the basics of human comfort & health and the various factors which affect them Familiarity with the metabolic rates, unit of "met", unit of "Clo“ Be able to analyze simple cases to predict response of the human body to different environments using the thermal network model Familiarity with the various environmental indices for measuring comfort: direct and indirect indices Understanding of the concepts of mean radiant and operative temperatures Familiarity with the ASHRAE thermal sensation scale and the concepts of PMV and PPD Be able to use the standard ASHRAE chart to determine acceptable range of comfort temperature and relative humidity Be able to use correlations and associated charts to analyze non-standard indoor conditions Understanding of the applicability of the adaptive comfort model Familiarity with how occupant productivity and complaints rate are affected by indoor conditions HCB-3 Chap 3A: Thermal Comfort42