climate- thermal comfort

8/4/2019 Climate- Thermal Comfort

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Thermal Comfort

By

Dr. Anupama Sharma


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Optimum thermal conditions

Temperature, humidity, air movement etc.

Visual conditions

Light quality and levels, pleasant environment

Acoustical conditions

Low noise levels and disturbances

Indoor air quality

Required amount of fresh air, control of odours and pollutants

Indoor Environment

Thermal Comfort


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A condition of mind that expresses satisfactionWith the thermal environment

Definition

Thermal Comfort

Dissatisfaction may be caused by warm or cool discomfortof the body as a whole

ormay be caused by an unwanted heating or cooling of oneparticular part of the body

It is impossible to specify a thermal environment that will satisfyeverybody. There will always be a percentage of dissatisfiedoccupants.


4/16

Thermal Comfort

Sensible & Latent heat lossfrom respiration

Radiation gains

Low RH of ambient air aids inevaporation of perspiration

Air movement aids heat loss byconvection and evaporation

Moisture evaporation(sweating)

Conduction to & fro fromthe surrounding air

Heat productionby metabolism

Internal body temp. (37 C)

Radiation losses

Interaction of the human body with the environment

Skin surface temp. (32 C)

Heat production within a bodyThe human body is like a complex internal combustion engine

Metabolism = Basal +

Muscular

37

CONDUCTION

CONVECTION

RADIATION

EVAPORATION


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Thermal Comfort

Heat Balance

Human body must maintain heat balance to survive

If it generates more heat that needed, it must loose heat to the

surroundings or its temperature will rise.

If it looses too much heat, its temperature will lower

When conditions surrounding the body are not ideal, the adaptation

mechanism starts.

When the body takes adaptation measures, it is under thermal stress

The rate of heat production within the body is called metabolic rate.


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Thermal Comfort

Like the machine, the conversion ofthe fuel (oxidation of food) is not100% efficient. The energy which isnot converted to do work is in form of

heat, and if not needed to maintainthe body temperature, it is brought tosurfaces by blood flow. Then it isrejected to the surroundingenvironment.

The heat is rejected in two forms:Sensible and Latent heat transfer

Heat transfer


7/16Thermal Comfort

Bodys thermoregulatory system

Hypothalamus is the bodys thermostat (set point 37.7 C).

Sensors in the skin: for inflow and outflow of heat.

Body must be protected from the excessive high temperatures.

Vasodilatation and skin wettedness


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Thermal Comfort

Bodys thermoregulatory system

Gain:Met = metabolism (basal & muscular)Cnd = conduction (contact with warm

bodies)

Cnv = convection (if air is warmer

than the skin)

Rad = radiation (from the sun, sky &

hot bodies)Loss:Cnd = conduction (contact with cold bodies)

Cnv = convection (if air is cooler than the skin

Rad = radiation (to night sky & cold surfaces)Evp = evaporation (of moisture & sweat)

Then thermal balance exists when;MetEvp +_ Cnd +_ Cnv +_ Rad = 0


9/16

Thermal Comfort

Parameters influencing TC

Physical variables

Air temperature, MRT of the walls,Relative humidity, Air velocity, Atmosp.pressure, Odours, Light intensity, noiselevels, colour of the environment

Organic parameters

Age, Sex, National characteristics ofoccupants

External parameters

Human activity level (Metabolism), Typeof clothing (clo), Social conditions Six primary variables


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Thermal Comfort

Representation of the variables

Milne - Givoni Diagram

Bioclimatic charts (Olgyay)

Psychrometric charts(comfort zones)


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Thermal Comfort

Metabolic rates

Activity

Metabolic Rate

[watts)]

Sleeping Min. 70

Seated/Standing 130-190

Sitting, heavy leg & arm movements

Standing, moderate work,some walking

Walking, moderate lifting, pushing

190-230

220-290

290-410

Intermediate heavy lifting, digging440-580

Hardest sustained work 580-700

Maximum heavy work for 30 min. duration Max. 1100


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Thermal Comfort

Clothing levels

Clothing Icl (clo)*Nude 0.0

Shorts 0.1

Light summer clothing 0.5

Light working ensemble 0.6

Typical business suite 1.0

Typical business suite and cotton overcoat 1.5

Light outdoor sports wear 0.9

Heavy traditional European suite 1.5

Heavy wool pile ensemble 3.0 4.0

* Thermal resistance of the clothing expressed in clo units1 clo = 0.155 m2.C/ W


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Thermal Comfort

Thermal comfort indices

Rationally Derived Indices Mean Radiant temperature (tmrt)

Operative Temperature (top)

Heat Stress (HS)

Thermal Stress (TS)

Direct Indices Ambient Temperature (ta or dbt)Dewpoint Temperature (tdp or dpt)

Wetbulb Temperature (twb or wbt)

Relative Humidity (rh)

Air movement (V)

Empirical Indices Effective Temperature (teff or ET)

Standard Effective Temperature (SET)

Equatorial Comfort (EC)


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Thermal Comfort

Prediction of Thermal Comfort

PMV Index

-3 Cold

-2 Cool

-1 Slightly cool0 Neutral

+1 Slightly warm

+2 Warm

+3 Hot

Predicted Mean Vote (PMV)

Predicted percent of Dissatisfied People (PPD)

Metabolic rate Mechanical efficiency

Clothing insulation Clothing factor

Vapour pressure Ambient air temperature

Temperature of clothing Mean radiant temperature


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Thermal Comfort

Passive and active controls

Building in passive mode. Surfaces at 23 C during some parts of the year.No need to intervene with an active system.

Daily outdoor temp. rises, passive strategies like night ventilation or

changing envelope resistance work effectively.

Temp. rises further and passive strategies become ineffective. Heat isdriven inwards, and some of the inside surfaces have heated up above theupper limits of comfort.

The design of the passive systems should attempt to maintain conditions as near as

comfort temp. as possible. The longer these conditions are maintained, the better

the passive design. Extending this period of comfort is usually accomplished using

the thermal mass. During this period, the indoor temp. is the result of surrounding

surface temps. Therefore controlling the surface temperature (rather that air

temperature) is the best strategy.


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Thermal Comfort

Project ideas

Surveys among the subjects living in non-air-conditioned buildings in the Bhopal

region

Questionnaires to be prepared;

Selection of sample size and type (rich, middle class, poor);

Analysis of collected data;

Result (comfortable temperatures for various categories of people); andTo be repeated in different seasons.

Relationship of the comfort conditions with other parameters

Age;

Sex;

Food and clothing habits; and

Professions.

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