harvard school of public health eh522 indoor environmental quality & health lecture 6 part i...

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


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.



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



PART I: Thermal comfort & Environmental Comfort indices



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



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 ?



7

TOTAL COMFORT- IEQ

Physiological

Chemical

Biological

HVAC Systems

Thermal comfort

IAQ

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.



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)



Metabolic Rates- Depend on activity



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



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

Heat dissipation mechanisms

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•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



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?



15http://www.nws.noaa.gov/om/heat/index.shtml

Combined Heath effect: Temperature + Humidity



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



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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

(+)



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



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

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 :



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



Predicted Mean Vote (PMV) + 3 hot

+ 2 warm + 1slightly warm

PMV =0 neutral -1 slightly cool

-2 cool -3 cold

“Thermal sensation” scale



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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



CP191: Intro to Healthy Homes 24

PMV PPD

0 5%

+- 0.5 20%

+-1.0 50%

+1slightly warm-1slightly cool



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



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



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



Example: Effect of air motion



PART II: Air properties and processes(Psychrometrics)



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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%



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



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•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



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




Psycrometric Chart

On the P. Chart:•STATE is a point, •PROCESS (sequence of states) is a line on the Chart.



Psycrometric Chart



TDB W, TDP

vTWB

RH

Properties of Air on Psycrometric chart

RH 100%

RH 100%

RH 100%

RH 100%




Example I:




Example II:



Example III:QUESTION:• In a room:

(condition A)• The windows have

temperature of T= 9 C

• Water Condensation on the window?

Tdp=12 C



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



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???



•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



43

W1

T1

h1

W2

T2

h2

Air Processes

DQ(Heat)=Dh (Energy balance)

Water vapor (Humidity) HEAT & MASS BALANCE



44

Example 1: Sensible heating and cooling

1

2 1

2

heating

cooling

W1 = W2



1

2

heating W2

Tdb1 = Tdb2

W1

Example 2:Latent Heat- Humidification



1

2 W2

Tdb1 Tdb2

W1 1

2

Example 3: Heating- Cooling



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



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

harvard school of public health eh522 indoor environmental quality & health lecture 6 part i...

Documents

health problems

health cp191

health objectives

health human body

thermal comfort conditions

health metabolic rates

health effect issues

health basic definitions