Building Env. & Human Comf

8/11/2019 Building Env. & Human Comf

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Building Environment

and Human ComfortCT10404

Thermal Comfort

By: Pubudu Kudahetti

Comfort

That condition of mind that expresses satisfaction with

the environment – CIBSE

Environmental Factors Considered for Comfort

Thermal Condition

Comfort

Visual Condition

Comfort

Acoustic Condition

Comfort

Indoor Air Quality (IAQ)

Comfort

Electromagnetic Fields

Static Electricity

Health Aspects

• A state of complete physical, mental and social well-being, not merely the absence of disease and infirmity - World Health Organization

Occupants Experience Symptoms

• mausea

• mucosal dryness or irritation, runny nose, eye problems,

• headaches, skin problems, heavy head and flu-like symptoms,

If a significant proportion of occupants experience these symptoms then, by definition the occupants are suffering from ‘sick building syndrome’

Thermal comfort

• Factors Affecting Thermal Comfort A person’s sensation of warmth is influenced by the following mainphysical parameters, which constitute the thermal environment

air temperature

mean radiant temperature

relative air speed

humidity.

• Besides these environmental factors there are personal Factors that affect thermal comfort :

Metabolic heat production

Clothing.

Thermal comfort

It is also required that there be no local discomfort

(either warm or cold) at any part of the human

body due to followings;

Asymmetric thermal radiation

Draughts

Warm or cold floors

Vertical air temperature differences.

Thermal comfort

Body Temperature

37 C 34 C

Hot Cold

•The normal body core temperature is 37 C.

• We have separate heat and cold sensors.• Heat sensors are located in the skin.

Signals when temperature is higher than 37

• Cold sensors are located in the skin. They

send signals when skin temperature is below34 oC.

• There are more cold sensors that warm

sensors.

• Heating mechanism:

– Reduced blood flow. – Shivering.

• Cooling mechanism:

– Increased blood flow.

– Sweating (Evaporation)

•Heat sensor sends impulses to

the hypothalamus whentemperature exceeds 37 oC.

•Cold sensors sends impulses tothe hypothalamus when skintemperature below 34 oC.

•The bigger temperaturedifference, the more impulses.

•If impulses are of samemagnitude, you feel thermallyneutral.

•If not, you feel cold or warm.

impulsesCold

impulses Activity

Perception of Thermal Environment

The Energy Balance

•Thermal Comfort can only be maintained when heat produced by metabolism equals

the heat lost from body.

Produ-

The Energy Balance

•Parameters influencing the heat loss from a person

The dry heat loss (R+C)

represents ~70% at low Clo-

values and ~60% at higher Clo-values.

The evaporative heat loss (E) represents

~25% at moderate activities

Heat Loss by Conduction (K) and

Respiration (RES) are normally insignificant

compared to the total heat exchange.

Man is a poor machine. The efficiency is

less than 20% even for well-trained

athletes. Normally set to zero in thecomfort equation.

Mapping Physical & Psychological Comfort

Territories

temperature

h u m i d i t y

Mapping Physical & Psychological Comfort

Territories

-- dishealth

conditions the body’s response

Heat Flow to/from Human Body

Conduction (sensible)

Convection (sensible)

Radiation (sensible)

Evaporation/Condensation

(latent)

Conduction

The flow of heat between two adjacentand touching solids (or from one part toanother part within an object) by direct

interaction between moleculesexample: walking on a beach in your barefeet

for comfort, the key environmental variableis: SURFACE TEMPERATURE

Radiation

The flow of heat between objects that are notin direct contact—but that can “see” eachother via electromagnetic radiation; theobjects may be a few inches or a million miles

apartexample: warming yourself in front of afireplace

for comfort, the key environmental variable is:SURFACE TEMPERATURE

Evaporation

The flow of heat that must be provided as amaterial changes state (from a liquid to a gas);this heat represents the energy required tobreak molecular bonds (called the latent heat

of vaporization)example: feeling cool coming out of aswimming pool on a breezy day

for comfort, the key environmental variables

are: RELATIVE HUMIDITY | AIR SPEED

Physical Basis of Thermal Comfort

Fundamentally, comfort involves a heat balance (athermal equilibrium) … where:

heat in ≈ heat out

where “heat in” is provided by metabolism, radiation,conduction, convection

where “heat out” is via radiation, conduction,convection, evaporation

Heat Flow to/from Human Body

Sensible Heat

– Flows via conduction, radiation, and convection

– Flow rate is generally related to space temperatures

Latent Heat

– Flows via evaporation

– Flow rate is generally related to space humidity

Total Heat Flow = sensible + latent flows

Heat Flow Mechanisms

three external “to” mechanisms; four “from”

mechanisms

The Mechanisms Adapt

the body automatically adapts to surrounding environmental conditions in its quest for thermalequilibrium; under high temperatures, evaporation becomes critically important

Measuring Environmental Factors

data logging

air temperature,

RH, wind speed

air speed

surface

temperature

wet and dry bulbtemperatures

MRT stands for mean radiant temperature

MRT is the (hypothetical) uniformtemperature of surrounding surfaces with

which the human body would exchange the

same heat by radiation as occurs in an

actual (non-uniform) environment

Surface temperatures in a typical room areoften not all the same (for example, cold

window glass, warm radiators); the human

body will radiate to/from these differentsurfaces. MRT is the temperature (if all

surfaces were at this one temperature) at

which the body would exchange the sameheat by radiation as occurs in the messy,

many-temperature real space.

Personal Factors Affecting Comfort

• Physical

– Clothing (specifically its insulation value in “clo”)

– Activity level (specifically metabolic heat production in“met”)

• Mental

– State of mind (experiences, expectations, influences ofother conditions, …)

These factors are not controlled through design, butmust be understood by a designer as they will affectoccupant thermal comfort responses

Physical Basis of Thermal Comfort

The potential for thermal equilibrium is:

– Influenced by environmental factors

• Often common to all occupants in a space

• Designer must control these conditions

– Influenced by personal physical factors

• Individual to each occupant in a space

• Designer must be aware of and consider theseconditions

The Designer’s Job

• Understand the physical basis of thermal

comfort and related variables

• Appreciate the influence of the psychologicalaspects of thermal comfort

• Use this understanding and appreciation to designspaces that building users will decide are thermallycomfortable

ASHRAE Thermal Comfort Chart

comfort

zone(s)

addressing operative temperature, relative humidity, and occupant clothing

For 80% occupant acceptability

Combined Heath Effect: Temperature + Humidity

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

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

Humidity - Health Effects

(from Arundel et al., 1986)

Optimum

ZoneBacteria

Viruses

Respiratory Infections*

Allergic Rhini tis and Asthma

Chemical Interactions

Ozone Production

10 20 30 40 50 60 70 80 90

Percent Relative Hu midity

Conditions for Thermal Comfort

•Two conditions must be fulfilled to

maintain Thermal Comfort: – Heat produced must equal heat lost. – Signals from Heat and Cold sensors

must neutralise each other.

•Mean Skin Temp. and Sweat Loss arethe only physiological parameterswhich influence the heat balance at a

given Metabolic Rate•The sweat production is used insteadof body core temperature, as measureof the amount of warm impulses.

•Relation between the parametersfound empirically in experiments.

•No difference between sex, age, race

or geographic origin.

Metabolic Rate

0 1 2 3 4

e a t p r o d .

Comfort Equation

H (Dry Heat Loss)

Ec Evaporative heat exchange at the skin

Cres Respiratory convective heat exchange

Eres Respiratory evaporative heat exchange

Thermal comfort predictive model

Most widely used :

•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 thePMV and PPD Indices and specification of the conditions for thermal comfort”.

Predicted Mean Vote Scale

- +3 Hot

- +2 Warm

- +1 Slightly warm

- +0 Neutral

- - 1 Slightly cool

- -2 Cool

- -3 Cold

The PMV index is used to quantify the degree of

discomfort

Predicted Mean Vote (PMV) Index

• The PMV index is mathematically complex to

compute, so Fanger (1970) provided look-up

tables to help practitioners determine

appropriate thermal conditions.

• Information from these tables, and graphical

representations of comfort conditions, is also

provided in modern thermal comfortstandards (e.g. ASHRAE, 2004: ISO, 1994).

Predicted Mean Vote (PMV) Index

The PMV index predicts the mean response

of a large group of people according to the

ASHRAE thermal sensation scale

Predicted Percentage Dissatisfied (PPD) Index

-0.5 < PMV <0.5 when PPD < 10%

PPD = 100-95 exp[-(0.03353PMV4+0.2179PMV2]

Predicted Percentage Dissatisfied (PPD) Index

PPD = 100-95 exp[-(0.03353PMV4+0.2179PMV2]

-< PMV <0.5 when PPD < 100.5 %

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

environmentL = M - W - [( Csk + Rsk + Esk ) + ( Cres + Eres )]

Predicted Percentage Dissatisfied (PPD)

PPD = 100 - 95 exp [ - (0.03353 PMV4 + 0.2179

PMV2)]

PMV PPD

+- 0.5 20%

+-1.0 50%

Graphical representation Thermal comfort zones?

• ASHRAE 55-2004

– Based on

satisfaction (20%PPD)

– Season dependent

– For Officebuildings- nothomes

• Environmental

Factors:

– Metabolic rate-activity

– Clothing- insulation

– Air temperature

– Radiant temperature

– Air- speed

– HumidityOperative temperature

Operative Temperature

Operative temperature (To):

To = 0.45 Tair + 0.55 Tmrt

Tmrt - Mean radiant temperature

Tmrt = S AiTi / 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)

Graphical representation Thermal comfort zones?

• ASHRAE 55-2004 – Based on

satisfaction (20%PPD)

– Season dependent

– For Officebuildings- not

homes (specificactivity level,clothing level)

– Adjusted comfort

zones for other

Summer

Winter

PMV and PPD

•PMV-index (Predicted Mean Vote) predicts the subjective ratings of the environment in agroup of people.

•PPD-index predicts the number of dissatisfied people.

What should be Estimated?

Parameters to estimate and calculate are:

Met - Estimation of Metabolic Rate

Clo - Calculation of Clo value

Met Value Table

Activity Metabolic Rates [M]

Reclining 46 W/m2 0.8 Met

Seated relaxed 58 W/m2 1.0 Met

Clock and watch repairer 65 W/m2 1.1 Met

Standing relaxed 70 W/m2 1.2 Met

Car driving 80 W/m2

1.4 MetStanding, light activity (shopping) 93 W/m2 1.6 Met

Walking on the level, 2 km/h 110 W/m2 1.9 Met

Standing, medium activity (domestic work) 116 W/m2 2.0 Met

Washing dishes standing 145 W/m2 2.5 Met

Walking on the level, 5 km/h 200 W/m2 3.4 Met

Building industry 275 W/m2 4.7 Met

Sports - running at 15 km/h 550 W/m2 9.5 Met

Met Value Examples

Calculation of Insulation in Clothing

• 1 Clo = Insulation value of 0,155 m2 oC/W

Cl l bl

Clo Values Table

Garment description Iclu Clo Iclu m2 C/W

Jackets VestJacket

0.130.35

0.0200.054

Coats over-trousers

CoatParkaOveralls

0.600.700.52

0.0930.1090.081

Sundries SocksShoes (thin soled)BootsGloves

0.020.020.100.05

0.0030.0030.0160.008

Skirt,dresses

Light skirt, 15cm above kneeHeavy skirt, knee-lengthWinter dress, long sleeves

0.100.250.40

0.0160.0390.062

Sleepwear ShortsLong pyjamasBody sleep with feet

0.100.500.72

0.0160.0780.112

Chairs Wooden or metalFabric-covered, cushioned

Armchair

0.000.100.20

0.0000.0160.032

Adj f Cl V l

Adjustment of Clo Value

1.0 Clo 0.5 Clo

1.2 met

P D ( P

r e d i c t e d P

e r c e n t a g e D i s s a t i s f i e

M R di t T t

Mean Radiant Temperature

•The Mean Radiant Temperature is that uniform temperature of an imaginary

black enclosure resulting in same heat loss by radiation from the person, as theactual enclosure.

•Measuring all surface temperatures and calculation of angle factors is timeconsuming. Therefore use of Mean Radiant Temperature is avoided whenpossible.

O ti d E i l t T t

Operative and Equivalent Temperature

Operative temperature

Equivalent temperature

For given values of humidity, air speed,

metabolic rate, and clothing insulation, a

comfort zone may be determined. The comfort

zone is defined in terms of a range of operative

temperatures that provide acceptable thermal

environmental conditions or in terms of thecombinations of air temperature and mean

radiant temperature that people find thermally

acceptable

O ti d E i l t T t

Operative and Equivalent Temperature

Operative temperature Equivalent temperature

P j t d A F t

Projected Area Factor

tr = 20 C tr = 20 C tr = 20 C

O ti T t

• The Operative temperature to integrates the effect of ta and tr.

• An Operative Temperature transducer must have same heat exchange properties as

an unheated mannequin dummy.

D H t L

Dry Heat Loss

• Dry Heat Loss or equivalent temperature can be measured directly, using a heated

Operative Temperature shaped transducer.

• The Equivalent temperature teq integrates the effect of ta, tr and va .

• The Dry Heat Loss transducer is heated to the same temperature

as the surface temperature of a person’s clothing.

C f t T t

Comfort Temperature

1,7 CLO

2,5 METRH=50%

tco=6oC.

0,8 CLO

2,2 METRH=50%

tco=18oC.

0,5 CLO

1,2 METRH=50%

tco=24,5oC.

General Thermal Comfort

Local Thermal Discomfort

•Draught Radiation

Asymmetry

Vertical Air

Temperature

Differences

Temperature

Draught

•The sensation of

Draught dependson the airtemperature.

•At lower airtemperatures ahigher number

will bedissatisfied.

Mean Air Velocity

Evaluating Draught Rate

•Fluctuations in Air Velocity is describedby Turbulence Intensity (Tu).

•Draught Rate equation is based onstudies of 150 people, and stated in• ISO 7730.

Tu = 100*( SD / va)

SD:Standard Deviation of Air Velocity

va: Local Mean Air Velocity

Radiation Asymmetry

•Radiant Temperature Asymmetry is perceived uncomfortable.•Warm ceilings and cold walls causes greatest discomfort.

Vertical Air Temperature Difference

•Vertical Air Temperature Difference is

the difference between AirTemperature at ankle and neck level.

Radiant asymmetry in the vertical

direction shall be less than 5oC (9oF) under

a warm ceiling and less than 10oC (18oF) in

the horizontal direction from a cool wall.

Workplace Measurements

- 1.1 m

- 0.1 m

- 0.6 m

- 0.1 m

- 1.1 m

- 1.7 m

• Measurements of Vertical Temp. difference and Draught at ankle and neck.

• Other measurements should be performed at persons centre of gravity.

Questions

Building Env. & Human Comf

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