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Harvard School of Public Health
EH522INDOOR
ENVIRONMENTAL QUALITY & HEALTH
Lecture 6PART I
Thermal comfort & Environmental Comfort indices
PART II
Air properties and processes (Psychrometrics)
Philip Demokritou, Ph.D
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
Reading Materials
2
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.
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
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).
3
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
PART I: Thermal comfort & Environmental Comfort indices
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
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)
5
•Human body obeys the first law of thermodynamics: Energy balance for human body
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
That condition of mind in which satisfaction is
expressed with the thermal environment.”
ANSI/ASHRAE Standard 55-2004:Thermal Environmental Conditions for
Human Occupancy
What is Thermal Comfort ?
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
7
TOTAL COMFORT- IEQ
Physiological
Chemical
Biological
HVAC Systems
Thermal comfort
IAQ
Health
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
CP191: Intro to Healthy Homes 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.
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
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)
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
Metabolic Rates- Depend on activity
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
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
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
From: Lechner N. Heating, Cooling, Lighting, 3rd Edition. Hoboken, NJ: John Wiley & Sons, 2009.
Heat dissipation mechanisms
12
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
•Air temperature ~ Convection•Relative humidity ~ Evaporation•Air velocity near a human body, V ~ Convection•Surface temperature of the enclosure and other
objects ~ Radiation
Heat dissipation and Environmental factors
The way heat dissipates depends on EF and what else??? CLOTHING
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
From: Lechner N. Heating, Cooling, Lighting, 3rd Edition. Hoboken, NJ: John Wiley & Sons, 2009. 14
Heat dissipation from human body- 2
• Question:
• Why are we sweating more in the Summer?
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
15http://www.nws.noaa.gov/om/heat/index.shtml
Combined Heath effect: Temperature + Humidity
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
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
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
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
(+)
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
Humidity - Health Effects
(from Arundel et al., 1986)
OptimumZone
BacteriaViruses
FungiMites
Respiratory Infections*Allergic Rhinitis and Asthma
Chemical InteractionsOzone Production
10 20 30 40 50 60 70 80 90
Percent Relative Humidity
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
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!
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
20
Energy production – Mechanical work = Heat losses
M - W = Qsksk + QresM - W = ( Csk + Rsk + Esk ) + ( Cres + Eres )
M - Rate of metabolic heat production (W/m2 body surface area)
W - Rate of mechanical workQ - Heat losses
C - Convective heat lossesR - 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 !!!
Can we predict thermal comfort?Thermal comfort modeling- Energy balance on body :
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
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”.
Thermal comfort predictive model
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
Predicted Mean Vote (PMV) + 3 hot
+ 2 warm + 1slightly warm
PMV =0 neutral -1 slightly cool
-2 cool -3 cold
“Thermal sensation” scale
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
23
PMV = [0.303 exp ( -0.036 M ) + 0.028 ] LL - Thermal load on the body
L = Internal heat production - heat loss to the actual environmentL = M - W - [( Csk + Rsk + Esk ) + ( Cres + Eres )]
Predicted Percentage Dissatisfied (PPD)PPD = 100 - 95 exp [ - (0.03353 PMV4 + 0.2179 PMV2)]
PMV/PPD method
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
CP191: Intro to Healthy Homes 24
PMV PPD
0 5%
+- 0.5 20%
+-1.0 50%
+1slightly warm-1slightly cool
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
Graphical representation Thermal comfort zones?
• ASHRAE 55-2004– Based on satisfaction
(20% PPD)– Season dependent– For Office buildings-
not homes
• Environmental Factors:– Metabolic rate- activity– Clothing- insulation– Air temperature– Radiant temperature– Air- speed– Humidity
Operative temperature
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
Operative temperature (To):
To = 0.45 Tair + 0.55 Tmrt
Tmrt - Mean radiant temperature
Tmrt = AiTi / 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)
Operative Temperature
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
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
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
Example: Effect of air motion
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
PART II: Air properties and processes(Psychrometrics)
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
30
Air Composition (two components)Moist air = Dry air + water vapor
Moist Air and its Properties
Dry air composition (volume fraction):•Nitrogen 78.084% •Oxygen 20.948% •Argon 0.934% •Carbon dioxide 0.031%•Minor gases 0.003%
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
31
)/( airdrrya
w KgKgm
mW
%100s
w
p
pRH
•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
temperatureEXAMPLE:Dry air: RH=0% Saturated air: RH=100%Difference between W and RH: W : water content RH: saturation degree
Fundamental Parameters I
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
32
•Dewpoint Temperature, Tdp
Temperature that “air saturation” occurs(Condensation on window and wall surfaces will occur)
•Wet Bulb Temperature, TwbThe temperature measure of moisture content in the air
Fundamental Parameters II
Enthalpy=enthalpy of the dry air + enthalpy of the water vapor (Enthalpy is energy per unit mass KJ/Kgda)
•Enthalpy, hSling psychrometer
cloth wick
water
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
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
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
From: Lechner N. Heating, Cooling, Lighting, 3rd Edition. Hoboken, NJ: John Wiley & Sons, 2009.
Psycrometric Chart
On the P. Chart:•STATE is a point, •PROCESS (sequence of states) is a line on the Chart.
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
Psycrometric Chart
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
TDB W, TDP
vTWB
RH
Properties of Air on Psycrometric chart
RH 100%
RH 100%
RH 100%
RH 100%
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
From: Lechner N. Heating, Cooling, Lighting, 3rd Edition. Hoboken, NJ: John Wiley & Sons, 2009.
Example I:
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
From: Lechner N. Heating, Cooling, Lighting, 3rd Edition. Hoboken, NJ: John Wiley & Sons, 2009.
Example II:
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
Example III:QUESTION:• In a room:
(condition A)• The windows have
temperature of T= 9 C
• Water Condensation on the window?
Tdp=12 C
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
HEATING, VENTILATION, & AIR-CONDITIONING(HVAC) SYSTEMS
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
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???
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
•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)
Basic Air Conditioning Processes
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
43
W1
T1
h1
W2
T2
h2
Air Processes
DQ(Heat)=Dh (Energy balance)
Water vapor (Humidity) HEAT & MASS BALANCE
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
44
Example 1: Sensible heating and cooling
1
2 1
2
heating
cooling
W1 = W2
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
1
2
heating W2
Tdb1 = Tdb2
W1
Example 2:Latent Heat- Humidification
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
1
2 W2
Tdb1 Tdb2
W1 1
2
Example 3: Heating- Cooling
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
Example 4: Adiabatic Mixing
The heat balance for the mixture can be expressed asmA 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
WA
TA
hA
mA
WB
TB
hB
WC
TC
hC
mCA
B
C
Harvard School of Public Health
EH:522 : Indoor Environmental Quality and Health
A
C
WB
TdbA TdbB
WA
Example 4: Adiabatic Mixing (P. Chart)
B Wc
hB
hc
hA
TdbC
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.
Thank you for your attention