Thermal balance of the body

Heat is continuously produced in the body from most of our bio-chemical processes. Most of the bio-chemical processes involved in tissue building, energy conservation and muscular work are exothermal, i.e. heat producing . All energy and material requirements of the body are supplied from the consumption and digestion of food . The process involved in converting foodstuff in to living matter and useful form of energy are known as metabolism.

Body heat production in different activities

Activity Watts

Sleeping Minimum 70

Sitting, moderate movement, e.g. typing 130-160

Standing, light work at machine or bench 160-190

Sitting, heavy arm and leg movements 190-230

Standing, moderate lifting or pushing 220-290

Walking, moderate work, some walking 290-410

Intermittent heavy lifting, digging 440-580

Hardest sustained work 580-700

Maximum heavy work for 30 minutes duration Max. 1100

(Average values of data published in many sources)

The total metabolism heat production can be divided in to basal metabolism and muscular metabolism. The basal metabolism is meant by heat production of vegetative and automatic processes whish are ever-continuous in body and the muscular metabolism is meant by production of muscles whilst carrying out consciously controlled work. Only about 20% of all energy produced in the body is utilized. The remaining 80% is ‘surplus’ heat and must be dissipated to the environment from human body. This excess heat production varies with the overall metabolism rate and depends on the activity. The deep body temperature must remain balanced at around 37ºC. In order to maintain this body temperature at the steady level, all surplus heat must be dissipated to the environment. If there is some form of simultaneous heat gain from the environment (e.g. solar radiation or warm air) that must also be dissipated

Thermal balance of the body is the centre of focus in the concept of human comfort. The regularly mechanism of the body is such that it maintains constant body temperature from the automatic regulation of the heat and loss factors. The moment, this thermal balance is lost, discomfort will be experienced. Therefore, the importance of the factors that affect the heat dissipating process of the body, is obvious. As one of these factor changes, the condition of comfort changes as well. Therefore the sensation of comfort is relative.

Heat gains:

Met=Metabolism (basal and muscular)

Cv=Convection (if the air is warmer than the skin)

Cd=Conduction (contact with warm bodies)

R=Radiation (from the sun, the sky and hot bodies)

Heat
losses:

E=Evaporation (of moisture and muscular)

Cd=Conduction (contact with cold bodies)

Cv=Convection (if the air is cooler than the skin)

R=Radiation (to night sky and cold surfaces)

It can be expressed by an equation;

M-E±Cv±Cd±R=0

When this sum is more than zero, vasomotor adjustment will take place in side the body to reduce the temperature of the body. The blood circulation to the skin surface is increased, more heat is transported to the surface and the skin temperature is elevated and all forms of heat loss processes are accelerated .

Conversely, if the sum of the above equation is less than zero, the blood circulation to the skin is reduced , skin temperature is lowered and the heat loss processes are slowed down. If the vasomotor regulation is still insufficient, and overheating continuous, sweating will start . The rate of sweating may vary from about 20g / h to 3kg /h during periods of physical effort combined with hot environmental effects .

If in a cold environment, under-heating continuous in spite of vasomotor adjustments, violent shivering may occur, which can cause a ten-fold increase in metabolic heat production for short periods. Long-term endocrine adjustments constitute the acclimatization process .

These may involve the change in the basal metabolic heat production eventually increase in the quality of blood (to produce and maintain vaso-dilation) and an increase in sweat rate.

In a temperature climate, when the air temperature is around 18ºC in indoors, when the air is calm, i.e. air velocity does not exceed 0.25m/s, and when the humidity is between 40% and 60%, a person engaged in sedentary work will dissipate the surplus heat without any difficulty, in the following ways;

By radiation 45%

By convection 30%

By evaporation 25%

Assuming the temperature of bounding surface is approximately the same as the air temperature. The sensation of comfort or discomfort depends primarily on the climate variables, air temperature, wind, humidity and solar radiation.

Thermal preferences

Thermal preferences are however influenced by a number of subjective or individual factors. Following are these factors.

• Clothing

• Acclimatization

• Age and sex

• Body shape

• Subcutaneous fat

• State of health

• Food and drink

• Skin color

Thermal qualities

It is established fact that human sense of comfort is dependence with the thermal conditions. The qualities related to the thermal phenomena have to be defined before going through the analysis . A thermal phenomenon is basically the heat and its movement . It is obvious to give light on the physical facts regarding the nature of heat and the ways of its propagation . It is tried here to define frequently referred qualities in the solar building design.

Temperature is actually not a physical quantity but it can be thought of as a symptom- as the outward appearance of the thermal state of a body.
Heat is a form of energy, appearing as molecule movement in substances or as ‘radiant heat’, a certain wavelength band of electromagnetic radiation in space.
Specific heat of a surface is the amount of heat energy necessary to cause unit temperature increase of a unit mass of the substance. It is measured in: J/kg degC.
Latent heat of a substance is the amount of heat energy absorbed by unit mass of the substance at change of state (from solid to liquid or liquid to gaseous) without any change in temperature. It is measured in : J/kg.
Thermal capacity of a body is the product of its mass and the specific heat of its material. It is measured as the amount of heat require to cause unit temperature increase of the body, in units of J/degC.

Heat flow

Heat energy tends to distribute itself until a perfectly diffused uniform thermal field is achieved. It tends to flow from high temperature to lower temperature zones, by any or all the following ways: Conduction, Convection, Radiation.

Conductivity(k): In conduction through a body or bodies in direct contact, the spread of molecule movement constitutes the flow of heat. The rate at which such molecule movement spreads varies with different material- its thermal conductivity (or ‘k-value’). It is measured as the rate of heat flow (flow of energy per unit time) through unit area of unit thickness of the material, when there is a unit temperature difference between the two sides.

Thermal diffusivity (K)=k/(d*c) Where,

k=conductivity (W/m degC)

d= density (kg/m3)

c=specific heat (J/kg degC)

Resistivity(R) is the reciprocal of this quantity (1/k) measured in units of: m deg C/W. Better insulators will have higher resistivity values.

Conductance(C) whilst conductivity and resistivity are properties of a material, the corresponding properties of a body of a given thickness are described as conductance (c ), or its reciprocal, resistance (R ): C=1/R. Conductance is heat flow rate through a unit area of the body when the temperature difference between the two surface is 1 degC.

Surface conductance

In addition to the resistance of a body to the flow of heat, resistance will be offered by its surfaces, where a thin layer of air film separates the body from the surrounding air. A measure of this is the surface or film -conductance, denoted thus: 1/f(m2 deg C/W). f being the surface or film-conductance (w/m2 deg C).

The overall, air-to-air resistance ( Ra) is the sum of the body’s resistance and the surface resistances:

Ra=1/fi +Rb +1/fo

Where Rb =internal surface resistance 1/fi =resistance of the body

1/fo = external surface resistance

Transmittance(U) is the reciprocal of this air-to-air resistance or the air-to-air transmittance. U=1/ Ra

Its unit of measurement is same as for conductance-W/m2 degC-the only difference being that here the air temperature difference will be taken into account.

• Absorptivity, Emissivity, Reflectivity

• Time log ,Detriment factor 50

Civil Engineering Nepal