Metabolism and comfort

The human body differs from the insentient machine in tolerating only a narrow band of temperature (36°C to 38°C) and in having an appreciation of comfort. Defining the latter is not easy because of its subjective nature: in the long run a person’s comfort can only be assessed by posing the question, ‘Are you comfortable?’ Even then the individual may have difficulty in phrasing a satisfactory answer. Establishing a numerical scale of comfort, interpreted from simple physical measurements, has not proved straightforward although Fanger (1972) has proposed a comfort equation (see section 4.7) that allows the determination of thermal comfort in terms of activity, clothing, dry-bulb temperature, the mean radiant temperature of the surrounding surfaces, relative humidity and air velocity.

The body eats and digests food which, through the intake of oxygen by breathing, is converted into energy for useful work with the liberation of heat and the emission of waste products. The chemical changes occurring in the liver and other parts of the body, and muscular contraction, release thermal energy which is transported throughout the body by the circulation of the blood so that it can be lost to the outside world and the temperature of the deep tissues kept at comparatively constant value (about 37.2°C) if good health is to be preserved. The efficiency of the body—defined as W/(M — 44), where W is the rate of working, M the metabolic rate of energy production for the particular activity and 44 W m-2 is the basal metabolic rate per unit area of bodily surface—is not very great, varying from 0 per cent when at rest to a maximum of about 20 per cent when walking up a one in four gradient at 10 km h_1. So most of the energy produced by the bodily metabolism must be dissipated as heat to the environment.

In good health, the thermo-regulatory system of the body exercises automatic control over the deep tissue temperature by establishing a correct thermal balance between the body and its surroundings. Apart from the matter of good health the comfort of an individual also depends on this balance and it might therefore be interpreted as the ease with which such a thermal equilibrium is achieved. ASHRAE Comfort Standard 55-74 goes further and defines comfort as ‘… that state of mind which expresses satisfaction with the thermal environment…’ but points out that most research work regards comfort as a subjective sensation that is expressed by an individual, when questioned, as neither slightly warm nor slightly cool.

4.2 Bodily mechanisms of heat transfer and thermostatic control 81

Posted in Engineering Fifth Edition