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Harvard School of Public Health

EH522 INDOOR ENVIRONMENTAL QUALITY & HEALTH. Lecture 6 PART I Thermal comfort & Environmental Comfort indices PART II Air properties and processes (Psychrometrics) Philip Demokritou, Ph.D. Harvard School of Public Health. Reading Materials. ANSI/ASHRAE Standard 55-2004:

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Harvard School of Public Health

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  1. EH522 INDOOR ENVIRONMENTAL QUALITY & HEALTH Lecture 6 PART I Thermal comfort & Environmental Comfort indices PART II Air properties and processes (Psychrometrics) Philip Demokritou, Ph.D Harvard School of Public Health

  2. Reading Materials ANSI/ASHRAE Standard 55-2004: Thermal Environmental Conditions for Human Occupancy CHAPTER 4: Thermal Comfort From: Lechner N. Heating, Cooling, Lighting, 3rd Edition. Hoboken, NJ: John Wiley & Sons, 2009.

  3. Objectives of the Lecture • Discuss Thermal comfort and its environmental indices. • Discuss current standards and guidelines related to thermal comfort (ASHRAE 55 standard) • Discuss health effect issues related to thermal comfort conditions. • Introduce students to air properties and the processes taking place in the indoor environment. (Psychrometrics).

  4. PARTI: Thermal comfort & Environmental Comfort indices

  5. Basic Definitions I: Heat Transfer • Three types of heat transfer • Conduction: • Whenever there is a temperature gradient in a solid medium • Movement of “free electrons” and atom oscillations • Convection: • Heat is transferred by the “bulk flow” of air/liquid medium. • Radiation: • Infrared radiation or thermal radiation. Movement through space from warm to cold surfaces (No medium is required) • Human body obeys the first law of thermodynamics: • Energy balance for human body

  6. What is Thermal Comfort ? That condition of mind in which satisfaction is expressed with the thermal environment.” ANSI/ASHRAE Standard 55-2004: Thermal Environmental Conditions for Human Occupancy

  7. TOTAL COMFORT- IEQ Physiological Health Thermal comfort Biological Chemical IAQ HVAC Systems

  8. Why should we care about comfort? • Health and well-being : many thermal comfort conditions can cause health problems • Optimize performance (for work or leisure) • Improve perceived quality of life • People will do whatever it takes to be comfortable-changing the micro-clima was always a high priority for humans. CP191: Intro to Healthy Homes

  9. Human body: A bio-engine • Our body burns “food” and generate energy through its metabolic activities. • Metabolism: Transformation of chemical energy to heat and work. • Metabolic rate is expressed in met units (1 met: 58.2 w/m2 of human surface (Human surface = 1.8 m2)

  10. Metabolic Rates- Depend on activity

  11. Thermal Comfort & Health Effects • Human body: Requires constant temp. (96.8F, 36.7C) • Slight body temp. variation can cause stress • +- 20 F body temperature diff. can cause death (hypothermia, hyperthermia) • Hypothalamus in brain –autonomic system responsible to control temperature • Skin contains nerve ends that can sense heat flow and humidity (not temperature!!!) • Autonomic system declines with age but also infants have less developed system

  12. Heat dissipation mechanisms From: Lechner N. Heating, Cooling, Lighting, 3rd Edition. Hoboken, NJ: John Wiley & Sons, 2009.

  13. Heat dissipation and Environmental factors • Air temperature ~ Convection • Relative humidity ~ Evaporation • Air velocity near a human body, V ~ Convection • Surface temperature of the enclosure and other objects ~ Radiation The way heat dissipates depends on EF and what else??? CLOTHING

  14. Heat dissipation from human body- 2 • Question: • Why are we sweating more in the Summer? From: Lechner N. Heating, Cooling, Lighting, 3rd Edition. Hoboken, NJ: John Wiley & Sons, 2009.

  15. Combined Heath effect: Temperature + Humidity http://www.nws.noaa.gov/om/heat/index.shtml

  16. Humidity indoors • Indoor humidity is a function of • Outdoor humidity • Indoor sources: • Unvented cooking, • Unvented bathrooms • Showering • Number of Occupants • Humidifier use • Air conditioner use • Clothes drying--mechanical or air drying

  17. Humidity – Heath Effects • DIRECT • Eye irritation • Respiratory • mucociliary clearance • asthma • Dermal • skin dryness • Comfort perception • INDIRECT • Biological • Dust mites (+) • Molds (+) • Cockroaches (+) • Infectious agents • Bacteria (+/-) • Viruses [+ adenoviruses] • Chemical • Ozone (-) • Formaldehyde, SO2, NO2 (+)

  18. Humidity - Health Effects (from Arundel et al., 1986)

  19. Environmental conditions affecting thermal comfort • Primary factors: • Metabolic rate • Clothing insulation • Air temperature • Radiant temperature • Air- speed • Humidity Personal factors Non uniformity over body!

  20. Can we predict thermal comfort? Thermal comfort modeling- Energy balance on body : Energy production – Mechanical work = Heat losses M - W = Qsk + Qres M - W = ( Csk + Rsk + Esk ) + ( Cres + Eres ) M - Rate of metabolic heat production (W/m2 body surface area) W - Rate of mechanical work Q - Heat losses C - Convective heat losses R - Radiative heat losses E - Evaporative heat losses(sk – Skin, res – Respiration) What happens if the heat dissipated to the Environment is less than the M-W??? Body thermal load=not dissipated to the environment heat from body !!!

  21. Thermal comfort predictive model • Most widely used : • Prof. Fanger’s famous methods: • Comfort equation method (heat balance method) • (Links environmental conditions to body thermal load) • Predicted Mean Vote method (PMV model). • (links body thermal load to a Thermal sensation scale) • Predicted percentage of dissatisfied (PPD). • (Empirically PMV is related to PPD) • Standards: • ASHRAE Standard 55-2004: “Thermal Environmental conditions for Human Occupancy.” • ISO Standard 7730: “Moderate thermal environments- Determination of the PMV and PPD Indices and specification of the conditions for thermal comfort”.

  22. “Thermal sensation” scale Predicted Mean Vote (PMV) + 3 hot + 2 warm + 1slightly warm PMV =0 neutral -1 slightly cool -2 cool -3 cold

  23. PMV/PPD method PMV = [0.303 exp ( -0.036 M ) + 0.028 ] L L - Thermal load on the body L = Internal heat production - heat loss to the actual environment L = M - W - [( Csk + Rsk + Esk ) + ( Cres + Eres )] Predicted Percentage Dissatisfied (PPD) PPD = 100 - 95 exp [ - (0.03353 PMV4 + 0.2179 PMV2)]

  24. +1slightly warm -1slightly cool CP191: Intro to Healthy Homes

  25. Graphical representation Thermal comfort zones? • Environmental Factors: • Metabolic rate- activity • Clothing- insulation • Air temperature • Radiant temperature • Air- speed • Humidity • ASHRAE 55-2004 • Based on satisfaction (20% PPD) • Season dependent • For Office buildings- not homes Operative temperature

  26. Operative Temperature Operative temperature (To): To = 0.45 Tair + 0.55 Tmrt Tmrt - Mean radiant temperature Tmrt = SAiTi / S Ai Ti - Surface temperature of enclosure i Ai - Area of surface i NOTE: Operative temperature is the same as dry bulb temperature if there is no radiant heat!!! ( cos Tair =Tmrt)

  27. Graphical representation Thermal comfort zones? • ASHRAE 55-2004 • Based on satisfaction (20% PPD) • Season dependent • For Office buildings- not homes (specific activity level, clothing level) • Adjusted comfort zones for other conditions (ie. air speed, clothing etc) Summer Winter

  28. Example: Effect of air motion

  29. PARTII: Air properties and processes (Psychrometrics)

  30. Moist Air and its Properties Air Composition (two components) Moist air = Dry air + water vapor • Dry air composition (volume fraction): • Nitrogen 78.084% • Oxygen 20.948% • Argon 0.934% • Carbon dioxide 0.031% • Minor gases 0.003%

  31. Fundamental Parameters I • Pressure • Temperature (Dry Bulb Temperature), Tdb • Humidity Ratio, W • Relative Humidity RH pw = partial pressure of the water vapor in the air ps = partial pressure of the water vapor in a saturated mixture under the same temperature EXAMPLE: Dry air: RH=0% Saturated air: RH=100% Difference between W and RH: W : water content RH: saturation degree

  32. Fundamental Parameters II • Dewpoint Temperature, Tdp • Temperature that “air saturation” occurs • (Condensation on window and wall surfaces will occur) • Wet Bulb Temperature, Twb • The temperature measure of moisture content in the air cloth wick water Enthalpy=enthalpy of the dry air + enthalpy of the water vapor (Enthalpy is energy per unit mass KJ/Kgda) Sling psychrometer • Enthalpy, h

  33. Moist air properties- Graphical representation • For a given atmospheric pressure, two air properties define ALL “thermodynamic properties” of moist air. • Graphical representation: Psychrometric Chart, Mollier diagram

  34. Psycrometric Chart • On the P. Chart: • STATE is a point, • PROCESS (sequence of states) is a line on the Chart. From: Lechner N. Heating, Cooling, Lighting, 3rd Edition. Hoboken, NJ: John Wiley & Sons, 2009.

  35. Psycrometric Chart

  36. TDB W, TDP TWB RH v Properties of Air on Psycrometric chart RH 100% RH 100% RH 100% RH 100%

  37. Example I: From: Lechner N. Heating, Cooling, Lighting, 3rd Edition. Hoboken, NJ: John Wiley & Sons, 2009.

  38. Example II: From: Lechner N. Heating, Cooling, Lighting, 3rd Edition. Hoboken, NJ: John Wiley & Sons, 2009.

  39. Example III: • QUESTION: • In a room: (condition A) • The windows have temperature of T= 9 C • Water Condensation on the window? Tdp=12 C

  40. HEATING, VENTILATION, & AIR-CONDITIONING(HVAC) SYSTEMS

  41. OBJECTIVES OF HVAC SYSTEMS • Temperature Control • Humidity Control • Air Distribution • Air Motion • Building Pressurization (0.05 in. w.) • Indoor Air Quality (IAQ) • Dilution ventilation • Air cleaning (e.g., filtration) • GENERATE/DISTRIBUTE contaminants???

  42. Basic Air Conditioning Processes • Sensible Heating / Cooling • Cooling and dehumidification • Heating and humidification • Humidification • Adiabatic Mixing of Air • On the P. Chart: • STATE is a point, • PROCESS (sequence of states) is a line on the Chart. • EXAMPLES: • Sensible Heat (change TDB, constant W) • Latent Heat (constant TDB, change W)

  43. W1 T1 h1 W2 T2 h2 Air Processes DQ(Heat)=Dh (Energy balance) Water vapor (Humidity) HEAT & MASS BALANCE

  44. 1 2 DH heating W1 = W2 2 1 cooling Example 1: Sensible heating and cooling

  45. 2 DH heating W2 1 Example 2:Latent Heat- Humidification W1 Tdb1 = Tdb2

  46. 2 1 DH W2 Example 3: Heating- Cooling 1 W1 2 Tdb1 Tdb2

  47. WA TA hA mA WB TB hB Example 4:Adiabatic Mixing The heat balance for the mixture can be expressed as mA hA + mC hC = (mA + mC)hB  (1) where  m = mass flow of the air  h = enthalpy of the air  The moisture balance for the mixture can be expressed as: mA wA + mC wC = (mA + mC) wB   (1) where  w = humidity ratio in the air B WC TC hC mC A C

  48. C A DH WB Example 4: Adiabatic Mixing (P. Chart) hc • When mixing air of condition A and air of condition C, then • mixing point will be on the straight line between the two conditions in point B. • The position of point B depends on the volume of air A to the volume of air C. hB hA Wc B WA TdbA TdbB TdbC

  49. Thank you for your attention

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