Human beings are born into a hostile environment, but the degree of hostility varies with the season of the year and with the geographical locality. This suggests that the arguments for air conditioning might be based solely on climatic considerations, but although these may be valid in tropical and subtropical areas, they are not for temperate climates with industrialised social structures and rising standards of living.
Briefly, air conditioning is necessary for the following reasons. Heat gains from sunlight, electric lighting and business machines, in particular, may cause unpleasantly high temperatures in rooms, unless windows are opened. If windows are opened, then even moderate wind speeds cause excessive draughts, becoming worse on the upper floors of tall buildings. Further, if windows are opened, noise and dirt enter and are objectionable, becoming worse on the lower floors of buildings, particularly in urban districts and industrial 2 The need for air conditioning
Areas. In any case, the relief provided by natural airflow through open windows is only effective for a depth of about 6 metres inward from the glazing. It follows that the inner areas of deep buildings will not really benefit at all from opened windows. Coupled with the need for high intensity continuous electric lighting in these core areas, the lack of adequate ventilation means a good deal of discomfort for the occupants. Mechanical ventilation without refrigeration is only a partial solution. It is true that it provides a controlled and uniform means of air distribution, in place of the unsatisfactory results obtained with opened windows (the vagaries of wind and stack effect, again particularly with tall buildings, produce discontinuous natural ventilation), but tolerable internal temperatures will prevail only during winter months. For much of the spring and autumn, as well as the summer, the internal room temperature will be several degrees higher than that outside, and it will be necessary to open windows in order to augment the mechanical ventilation. See chapter 16.
The design specification for a comfort conditioning system is intended to be the framework for providing a comfortable environment for human beings throughout the year, in the presence of sensible heat gains in summer and sensible heat losses in winter. Dehumidification would be achieved in summer but the relative humidity in the conditioned space would be allowed to diminish as winter approached. There are two reasons why this is acceptable: first, human beings are comfortable within a fairly large range of humidities, from about 65 per cent to about 20 per cent and, secondly, if single glazing is used it will cause the inner surfaces of windows to stream with condensed moisture if it is attempted to maintain too high a humidity in winter.
The major market for air conditioning is to deal with office blocks in urban areas. Increasing land prices have led to the construction of deep-plan, high-rise buildings that had to be air conditioned and developers found that these could command an increase in rent that would more than pay for the capital depreciation and running cost of the air conditioning systems installed.
Thus, a system might be specified as capable of maintaining an internal condition of 22°C dry-bulb, with 50 per cent saturation, in the presence of an external summer state of 28°C dry-bulb, with 20°C wet-bulb, declining to an inside condition of 20°C dry-bulb, with an unspecified relative humidity, in the presence of an external state of-2°C saturated in winter.
The essential feature of comfort conditioning is that it aims to produce an environment which is comfortable to the majority of the occupants. The ultimate in comfort can never be achieved, but the use of individual automatic control for individual rooms helps considerably in satisfying most people and is essential.
Posted in Air Conditioning Engineering