First, the body heat is generated by metabolic processes to maintain body temperature. Metabolic processes are influenced by factors such as age, health condition and level of activity. For example, a given range of environmental conditions may be quite acceptable in the space occupied by a healthy person, but unacceptable to those who are ill. When the people are ready to adjust their dress habits depending on the season, they find that they are comfortable in a wide range of environmental conditions than they could expect.
The body is constantly a source of heat that must be eliminated to maintain a constant body temperature. Various mechanisms to control the temperature is described in Sec. For a person in a state of rest or do light work in air-conditioned comfort, dissipates heat of the body, mainly by convection (carried away by the atmospheric air and radiation (for the surrounding surfaces, which have a lower temperature than on the surface of the body). Each of these components heat dissipation is approximately 30 percent of heat loss. Evaporation breath and sweat, account for the remaining 40 percent. As environmental conditions or change in activity level, these percentages will vary. For example, if a person does the heavy work, the main heat dissipation mechanism will evaporation.....
Four environmental factors influence the body's ability to dissipate heat: air temperature, the temperature of the surrounding surfaces, humidity, and air velocity. The amount and type of clothing and the activity levels of the occupants interact with these factors. In designing an air-conditioning system we turn our attention to the control of these four factors. If a person is wearing appropriate clothing, the following ranges should usually be acceptable:
Operative temperature. 20 to 26°C Humidity. A dew-point temperature of 2 to 17°C Average air velocity. Up to 0.25 m/s. The operative temperature is approximately the average of the air dry bulb temperature and the mean radiant temperature as long as the mean radiant temperature is less than 50°C and the average air velocity is less than 0.4 m/s. The mean radiant temperature is the uniform surface temperature of an imaginary black enclosure with which an occupant would have the same radiant energy exchange as in the actual nonuniform space. A person wearing heavy clothing may be comfortable at lower temperatures; conversely, lighter clothing and higher air velocity may provide comfort despite higher temperatures. The temperatures of surrounding surfaces have an influence on comfort as great as that of the air temperature and can not be neglected.
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