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

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

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