Mackean (1977) expresses metabolism as ‘All the chemical changes going on in the cells of an organism.’ In the context of air conditioning the interest lies mainly in the quantities of sensible and latent heat dissipated by the body to its environment as a consequence of these changes and in accordance with equation (4.1). People vary in shape, surface area and mass, thus research workers currently prefer to express heat emission from the body in terms of the met unit, equal to 58.2 W m-2. The related body surface area is difficult to measure but a favoured equation for its expression, developed by Du Bois and Du Bois (1916), defines it as the Du Bois surface area, AD, where
Ad = 0.202m0A25h0125 (4.4)
In which m is the body mass in kg and h its height in m. Using equation (4.4) a man of 70 kg (11 stone) with a height of 1.8 m (5′ 11") has a surface area of 1.9 m2. One met unit is often taken as corresponding to 100 W, approximately, representing the emission from a resting adult.
The metabolic rate depends on the activity and varies from about 84 W for a person of 1.8 m2 surface area, as a basal rate, to a maximum of about 1200 W for a normal, healthy young man. Although trained athletes can work at maxima as high as 2000 W, an ordinary person can only maintain some 50 per cent of his maximum output continuously for any length of time. The maximum possible decays with age to roughly 700 W at the age of 70 years. Women have maximum levels at approximately 70 per cent of these values and children less still, because of their smaller surface areas, but for estimating the emission of heat from a mixed group of people the proportions of the normal male output taken by ASHRAE (1989) for women and children are usually 85 per cent and 75 per cent, respectively. Conservative values, as used for heat gain calculations, are quoted in Table 7.16, but more exact figures for different activities are given in Table 4.1.
Table 4.1 Typical metabolic rates for various activities, according to ASHRAE (1997)
The total heat productions are for continuous activity by an adult having a Du Bois surface area of 1.8 m2. The figures are in good agreement with those published in the CIBSE Guide (1999), Al, Environmental criteria for design.
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