1. HEAT TRANSFER FROM HUMAN BODY
Mechanism of heat loss from the human body

2. The metabolic heat generated in the body is dissipated to the environment
through the skin and the lungs by convection and radiation as sensible heat
and by evaporation as latent heat.
Sensible heat transfer includes-
→ The warming of the inhaled air represents sensible
heat transfer in the lungs and is proportional to the temperature rise of
inhaled air.
→convection and radiation heat transfer from skin.
latent heat transfer includes-
→Latent heat represents the heat of vaporization of water as it evaporates in the lungs and on the skin
by absorbing body heat.
Thus, The total rate of heat loss from the body can be expressed as-
·
Depends upon-
- temp. Of skin. Depends upon-
- temp .of environment. -skin wettedness. Depends upon-
-temp.of surrounding surfaces. -relative humidity. -frequency of -air
motion. breathing and the
volume of the lungs.
-air motion.
-temp.of environment.
Thus heat transfer from human body depends upon various factor & clothing further Complicates the
situation . Thus we must rely on experimental data for heat transfer from human body.
(skin)
Clothing serves as insulation and reduces both the sensible and latent forms of heat loss.
Sensible heat loss from = heat transfer through + heat loss from outer
Clothed skin the clothing surface of clothing.
Heat transfer through the clothing-
------------(1)

3. where Rclothing is the unit thermal resistance of clothing in m2 · °C/W,
which involves the combined effects of conduction, convection, and radiation
between the skin and the outer surface of clothing. The thermal
resistance of clothing is usually expressed in the unit clo where 1 clo = 0.155 m2 ·
°C/W.
Heat transfer from outer surface of clothing-
Where
hconv = convection heat transfer coefficient.
Values of convection heat transfer coefficient are experimentally determined
and are given in following table. These are at pressure 1 atm. To find hconv at
Pressure other than 1 atm these values are multiplied by p0.55 where is in atm.
Types of clothing Resistance of clothing (clo)
trousers,
long-sleeve shirt, long-sleeve sweater, and
T-shirt.
1
Summer clothing such as light slacks and
short-sleeved shirt.
0.5
winter clothing such as heavy
slacks, long-sleeve shirt, and a sweater or
jacket.
0.9

4. hrad = radiation heat transfer coefficient, 4.7 W/m2 · °C for typical indoor
conditions; the emissivity is assumed to be 0.95.
Aclothing = outer surface area of a clothed person
Tclothing = average temperature of exposed skin and clothing
Tambient = ambient air temperature
Tsurr = average temperature of the surrounding surfaces.
Qconv & Qrad can be combined as-
------------(2)
Where
Toperative = Tambient + Tsurr
2
Combining (!) & (2) total sensible heat loss through the skin can be expressed as-

5. Where 1/hcombined=Rcombined.
Latent heat loss from skin
-latent heat loss from the skin is proportional to the difference
between the water vapour pressure at the skin and the ambient air,
and the skin wettedness, which is a measure of the amount of moisture on
the skin.
-It is due to the combined effects of the evaporation of sweat and
the diffusion of water through the skin, and can be expressed as-
where
m vapor = the rate of evaporation from the body, kg/s
hfg = the enthalpy of vaporization of water 2430 kJ/kg at 30°C.
→Heat loss by evaporation is maximum when the skin is completely wetted.
→The maximum evaporation rate for an average man is about 1 L/h (0.3 g/s),
which represents an upper limit of 730 W for the evaporative cooling rate.
→A person can lose as much as 2 kg of water per hour during a workout on a
hot day, but any excess sweat slides off the skin surface without evaporating.
Sensible & latent heat loss from lungs ( )

6. →During respiration, the inhaled air enters at ambient conditions and exhaled
air leaves nearly saturated at a temperature close to the deep body
Temperature.
→The body loses both sensible heat by convection and latent heat by evaporation from the lungs.
→The rate of air intake to the lungs is directly proportional to the metabolic
rate Qmet.
The rate of total heat loss from the lungs through respiration can
be expressed approximately as-
where Pv is the vapor
pressure of ambient air in kPa.

7. THERMAL COMFORT AND HEAT TRANSFER
FROM HUMAN BODY
Heat transfer describes the exchange of thermal energy, between physical systems depending on the
temperature and pressure, by dissipating heat. The fundamental modes of heat transfer are conduction,
convection and radiation.
HEAT TRANSFER IN HUMAN BODY
Heat is produced in the body by the continuous metabolism of nutrients which provides energy for the
systems of the body. The human body must maintain a consistent internal temperature in order to maintain
healthy bodily functions. Therefore, excess heat must be dissipated from the body to keep it from
overheating. When a person engages in elevated levels of physical activity, the body requires additional
fuel which increases the metabolic rate and the rate of heat production. The body must then use additional
methods to remove the additional heat produced in order to keep the internal temperature at a healthy level.
It means there should not be any heat accumulation within the body. Thermal comfort is basically the
maintenance of equilibrium between heat generated by metabolism and heat loss from human body by
various processes.
WHY TO STUDY HEAT TRANSFER IN HUMAN BODY?
→THERMAL COMFORT
Thermal comfort is that condition of mind that which expresses satisfaction with the thermal
environment. Thermal environment is those characteristics of the environment which affects a person's
heat loss.
In terms of bodily sensations, thermal comfort is a sensation of hot, warm, slightly warmer, neutral,
slightly cooler, cool and cold.
From the physiological point of view, thermal comfort occurs when there is a thermal equilibrium in
the absence of regulatory sweating between the heat exchange between the human body and the
environment.
Thus, to know thermal comfort first we have to study process of heat transfer through human body.
→TEMPERATURE INFLUENCE

8. Temperature influences the functioning of biological (sub) systems. For humans, the core
temperature varies within narrow bounds around about 37°C. Changes in temperature can have significant
consequences for the behaviour of individual cells and the body as a whole. The temperature dependence of
biological processes can be used to clinical effect. Examples are hyperthermia treatment against cancer;
cooling of the head to prevent hair loss as a side effect of chemotherapy; and cooling of patients during
major surgery to protect the brain.
Temp. In body depends upon heat loss from it thus to know mechanism of heat transfer from human body
is vital to us.
→Air conditioning (AC)
The term air-conditioning is usually used in a restricted sense to imply cooling, but in its broad sense it
means to condition the air to the desired level by heating, cooling, humidifying, dehumidifying, cleaning,
and deodorizing.
The purpose of the air-conditioning system of a building is to provide complete thermal comfort for
its occupants. Therefore, we need to understand the thermal aspects of the human body in order to design
an effective air-conditioning system.
Before studying heat transfer from human body first of all we should know following terms:
Metabolism
Metabolism is the set of life-sustaining chemical transformations within the cells of living organisms.
These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their The high level of
chemical activity in the cells that maintain the human body temperature at a temperature of 37.0°C
(98.6°F) while performing the necessary bodily functions is called the metabolism.
So, metabolism simply refers to the burning of foods such as carbohydrates, fats and proteins.
Basal metabolic rate
The rate of metabolism at the resting state is called the basal metabolic rate, which is the rate of
metabolism required to keep a body performing the necessary bodily functions such as breathing and blood
circulation at zero external activity level. The metabolic rate can also be interpreted as the energy
consumption rate for a body.
The unit of metabolic rate is known as ‘met’, which is equivalent to 58.2 W/m2.
Some values of metabolic rates for various typical activities are given in the following table:
Activity Metabolic Rate
(Met)
Activity Metabolic Rate
(Met)

9. Thus metabolism results in heat generation in body which ultimately lost to surroundings.
Our thermal comfort depends upon relative amount of heat generation and heat loss.
RESTING
Sleeping
Reclining
Seated, quiet
Standing, relaxed
WALKING
DOMESTIC WORK, WOMEN
House cleaning
Cooking
Washing by hand and ironing
Shopping
OFFICE WORK
Typing
Miscellaneous office work
Drafting
LEISURE ACTIVITIES
Stream fishing
Calisthenics exercise
Dancing, social
Tennis, singles
Squash, singles
Basketball, half court,
intramural
Wrestling-competitive or
intensive
Golf, swimming and walking
Golf, swinging and golf cart
0.7
0.8
1.0
1.2
2.0-3.8
2.0-3.4
1.6-2.0
2.0-3.6
1.4-1.8
1.2-1.4
1.1-1.3
1.1-1.3
1.2-2.0
3.0-4.0
2.4-4.4
3.6-4.6
5.0-7.2
5.0-7.2
7.0-8.7
1.4-2.6
1.4-1.8
MISCELLANEOUS OCCUPATIONS
Bakery
Brewery
Carpentry
Machine sawing, table
Sawing by hand
Planning by hand
Foundry work
Using a pneumatic hammer
Tending furnaces
Garage work
General laboratory work
Machine work
Light
Heavy
Shop Assistant
Teacher
Watch repairer, seated
Vehicle driving
Car
Motorcycle
Heavy vehicle
Aircraft flying routine
Instrument landing
Combat flying
1.4-2.0
1.2-2.4
1.8-2.2
4.0-4.8
5.6-6.4
3.0-3.4
5.0-7.0
2.2-3.0
1.4-1.8
2.0-2.4
3.5-4.5
2.0
1.6
1.1
1.5
2.0
3.2
1.4
1.8
2.4

10. Some practical examples related to thermal
comfort:
→In cold environment, there is excessive heat loss from the body which may exceed the rate of metabolic
heat generation which will eventually lead to thermal discomfort. The best preventive measure to increase
the metabolic heat generation is shivering. Shivering begins slowly in small muscle groups and may
double the rate of metabolic heat production of the body at its initial stages.
The rate of metabolic heat generation
may go up for six times the resting
level during total body shivering in
cold weather. . `
→ Second measure to avoid thermal discomfort in winter is to wear clothes with high value Of
Rclothing, as it will decrease heat loss from body & helps in maintaining thermal equilibrium between
heat generated & heat loss from body.

11. For eg- trousers, long-sleeve shirt, long-sleeve sweater, and T-shirt as these clothes have high value of
clo and as an insulation to heat loss.
→In hot environment, the rate of heat loss from the body may drop below the rate of metabolic heat
generation. In this case the body activates the opposite mechanism. First the body increase the blood flow
and thus transports heat to the skin causing the temperature of skin to rise and approach the deep body
temperature, which will make feel thermally uncomfortable.
The best way to overcome this problem it to release water from the sweat glands i.e. to dissipate
a large amount of metabolic heat by sweating.
During sweating, the sweat absorbs the heat from the body and evaporates. Also, for better
evaporative cooling the humidity should be low.
That’s why human should drink more water in summer so that more water
Can be dissipated from sweat glands to dissipate large amount of metabolic heat
Without the scarcity of water in body.
→ In summer we wear clothing with low resistance value to have more heat loss.
For eg- light slacks and short-sleeved shirt.
As these clothes have clo=o.5 which is half of winter clothes.

12. Specific work done by each member
1. SAJAL GUPTA UE119061 -ANALYSIS OF HEAT TRANSFER
FROM HUMAN BODY
-PRACTICAL PROBLEMS RELATED TO
THERMAL COMFORT.
2. RAJKRISHNA UE119053 -THERMAL COMFORT & METABOLISM.
-PRACTICAL PROBLEMS RELATED TO
THERMAL COMFORT.
3. RISHABH UE119055 -EFFECT OF CLOTHING.

13. HEAT TRANSFER FROM
HUMAN BODY &THERMAL
COMFORT