2. THERMAL COMFORT
• Our life cycle comprises Activity, fatigue and recovery.
• Recovery is essential to counter balance against mental and
physical fatigue through recreation, rest and sleep.
• This can be affected by unfavorable climatic conditions and
the resulting stress on body and mind causes discomfort, loss
of efficiency and breakdown of health.
• Thermal comfort is the condition of mind that expresses
satisfaction with the thermal environment and is assessed by
subjective evaluation (ANSI/ASHRAE Standard 55).
• The task of the designer is to create the best possible indoor
climate or even the environment for the users as they judge
the quality of design based on physical and emotional point of
view.
4. THERMAL COMFORT
• Interest in establishing thermal comfort criteria dates back to
Europe to the beginning of the 19th century.
• It started during the Industrial revolution when significant
reforms to industry and housing were being introduced.
• This was done as heat and humidity stress led to illnesses and
accidents.
• Factors affecting thermal comfort:
• Air temperature
• Humidity
• Radiation
• Air movement
8. BODY THERMAL BALANCE
• Heat is continuously produced in the body. Most biochemical
processes involved in tissue building, energy conversion and
muscular work are exotherm.
• The energy needed is obtained from consumption and
digestion of food and this process by which food is converted
into living matter and useful form of energy is called
metabolism.
• Metabolic heat production:
• Basal metabolism: Heat production of vegetative, automatic
process.
• Muscular metabolism: Heat production due to consciously
controlled work.
10. BODY THERMAL BALANCE
• Only 20% of the heat produced in the body is utilized and
surplus heat must be dissipated.
• Thermoregulation is the process that allows the human body
to maintain its core internal temperature.
• The state of having an even internal temperature is called
homeostasis. All thermoregulation mechanisms are designed
to return the body to homeostasis.
• A healthy, safe temperature has a very narrow window –
between 98.6°F (37°C) and 100°F (37.8°C). (Skin temp. is 31 to
34 deg.)
• The human body releases heat to the environment by
evaporation, radiation, convection and to a lesser extent by
conduction.
15. HEAT LOSS BY HUMAN BODY
• Convection
• is due to heat transmission from the body to the air in contact
with the skin or clothing which then rises and is replaced by
cooler air.
• Rate of convective heat loss is increased with faster air
movement, a lower temperature and a higher skin temperature.
• Radiant heat loss depends upon temperature of the body
surface and temperature of opposing surface.
• Conduction: Depends on temperature difference of body
surface and the object the body is in direct contact with.
• Evaporation
• is governed by the rate of evaporation which in turn depends on
the humidity of the air (dryer the air, the faster the evaporation)
and the amount of moisture available for evaporation.
• Evaporation takes place through perspiration and sweating and in
lungs through breathing.
22. CALM, WARM AIR,
MODERATE HUMIDITY:
• In the indoors of temperate climate, when the
• Air temperature is 18 deg.
• Air velocity does not exceed 0.25 m/s and when
humidity is 40% to 60%, a person engaged in sedentary
work will easily dissipate heat as
• By radiation: 45%
• Convection: 30%
• Evaporation: 25 %
(If temperature of bounding surface is same as air
temperature)
23. HOT AIR AND
CONSIDERABLE RADIATION:
• The human body temp. is 37 deg. but skin temp. is 31‐34
deg.
• As the air temp. approaches skin temp., heat loss by
convection gradually decreases and the body makes
vasomotor adjustments to increase temperature to the
higher limit (34 deg.) but when air temp. reaches this point,
there is no more heat loss by convection.
• Body can gain substantial heat by radiation: sun, radiator,
bonfire.
• When heat loss is negligible in the above situations, it can
still take place through evaporation only if thee air is
sufficiently dry.
25. HOT AIR, RADIATION AND
APPRECIABLE AIR MOVEMENT
• When the air is hot (equal to or above skin temperature),
surroundings objects are hot (no heat loss by radiation) and
when air is humid (but less than 100% RH), air movement
will accelerate evaporation even if air temp. > skin temp.
• Moving air continuously replaces surrounding saturated air.
• Poorly designed houses can create a fatal situation where
the air is completely saturated, no air flow and air is
warmer than skin which may lead to heat stroke.
26. SATURATED STILL AIR, ABOVE
BODY TEMPERATURE
• When the situation is adverse such as
• air is hot (over 34 deg.)
• Unappreciable air movement (<0.25 m/s)
• Humidity is near 100 %.
• This leads to profuse sweating and no evaporation and
body temperature begins to rise.
• A body temperature of 40 deg. Will cause heat stroke.
(Failure in circulation system, followed by rapid increase in
body temp.)
• At 41 deg. , coma sets in and death is imminent.
• At 45 deg., death is unavoidable.
27.
28. EFFECTS OF PROLONGED
EXPOSURE
• A high wind velocity provides immediate relief, but causes
irritation and discomfort with long duration.
• Even perfectly comfortable conditions may produce adverse
effects if there are no change over prolonged periods.
• Change and variation is a basic need of a human.
• The designer must aim at a range of comfort conditions
within which considerable variations are permitted.
29. SUBJECTIVE VARIABLES
• Thermal comfort depends upon four climatic variables, but
preferences can vary among individuals.
• Clothing: A person wearing a suit will require a temperature about 9
deg. lower than a naked body.
• Acclimatization: A person adjusts to a new set of climatic conditions
in about 30 days. (A person in London might be comfortable at 18
deg., but will be comfortable in India at 25 after spending a month.
• Age and sex.: Women have higher metabolic rates and their
preference is 1 deg. Higher than men.
• Body Shape: A corpulent person with same weight as a thin person
dissipates more heat due to larger surface area.
• Fat: Excellent insulator.
• State of health.
• Food and drink.
• Skin colour: Dark skin has melanin which prevents higher UV
penetration than lighter skin.
