Heat Exchange
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Heat Exchange between the Human Body and the Enviroment

 

1.1    Human Response to heat

Cold Environment:     shivering

Hot Environment:   sweat

 

1.2 Heat Balance Equation

 

(1)

where S = rate of heat storage of human body, W/m2

                M = metabolic rate of human body, W/m2

                W = mechanical work produced by human body, W/m2

                E = rate of total evaporative loss due to evaporation of sweat, W/m2

                Q = total rate of heat loss from skin (dry heat exchange), W/m2

 

1.2.2 Metabolic Rate

(a) degree of muscular activities,

(b) environmental conditions, and

(c) body size.

The unit of the metabolic rate is known as 'met'. which is equivalent to 58.2 W/m2. Some values of metabolic rates for various typical activities are presented in Table 1.

Table 1 Metabolic Rates

 

Activity

Metabolic Rate

(Met)

Activity

Metabolic Rate

(Met)

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

 

1.2.3 Mechanical Work

External work developed by the human body is positive while mechanical impact acted on the human body is said to be negative.

 

1.2.4 Evaporative Heat Loss

Evaporative heat loss consists of two components:

(a) Respired Vapour Loss, Eres

(i) Latent Respiration Heat Loss, Erel

(ii) Convective or Sensible Respiration Heat Loss, Erec

 

(b) Evaporative Heat Loss from Skin Surface, Esk

Evaporative heat loss from skin surface has two parts:

(i) Evaporative Heat Loss by Skin Diffusion, Edif

(ii) Heat Loss due to Regulatory Sweating, Ersw

 

Mathematically, Evaporative heat loss is expressed as:

(2)

 

1.2.5 Dry Heat Exchange

Dry heat exchange represents the heat exchange between the human body and the environment through convective and radiative heat transfer.

 

1.3 Effect of Clothing Insulation

(a) Thermal Insulation of Clothing

The addition of thermal resistance due to clothing affects heat transfer mechanisms between the human body and the environment. "Clo" value is a numerical representation of a clothing ensemble's thermal resistance, 1 Clo = 0.155 m2K/w.

(b) Evaporative Resistance of Clothing

The evaporative resistance is a measure of moisture permeability which affects the latent heat transfer from the skin through the clothing layer and affects evaporative heat loss from skin surface Esk.

Table 2 'Clo' Values for Clothing

 

Clothing combination Clo m2K/W
Naked

Shorts

Typical tropic clothing outfit

Light summer clothing

Working cloths

Typical indoor winter clothing combination

Heavy traditional European business suit

0

0.1

0.3

0.5

0.8

1.0

1.5

0

0.018

0.047

0.078

0.124

0.155

0.233

 

 

Figure 1 shows the heat exchange between clothed and nude occupant and the environment at various operative temperatures (see Section 4.1). Figure 1 is interpreted as follows:

(a) When the metabolic rate is about 1 met (58.2 W/m2) , there is no body cooling nor body heating at an operative temperature of about 25.5 oC for light clothed person and 31 oC for nude person.

(b) When the operative temperature drops to lower values, the dry heat exchange is increased and the evaporative heat loss is mainly respired vapour loss. The skin temperature and the temperature of superficial and deep tissues drop, resulting in a negative heat storage.

(c) When the operative temperature exceeds 29 oC, the rate of evaporative heat loss is significantly increased in order to counterbalance the reduction of dry heat exchange to maintain the thermal equilibrium.

(d) The body temperature tends to rise only when the body is entirely wet, and the evaporative heat loss is inadequate. There exists a positive rate of heat storage.

(e) Body temperature above 43 oC may cause death.

Figure 1 Heat Exchange of Persons with the Environment