Book - School of Science and Technology

General principles 375In summer. A supply of air which has been cleaned and cooled. As the cooling is normallyaccomplished by exposing the air to cold surfaces or cold spray, the excess moisture iscondensed and the air is left nearly saturated at a lower temperature. Inherent in thisprocess therefore is a measure of dehumidification which counters the increase inrelative humidity that results from cooling the air. The temperature of air has then tobe increased, to give a more agreeable relative humidity, which can be done by warmingor by mixing with air which has not been cooled.Dehumidifying may also be brought about by passing the air over certain substanceswhich absorb moisture. Thus, in laboratories, a vessel is kept dry by keeping a bowl ofstrong sulphuric acid in it or a dish of calcium chloride, both of which have a strongaffinity for moisture. Silica-gel, a form of silica in a fine state of division exposing a greatabsorbing surface, is used also for drying air on this principle, but this process iscomplicated by the need for regeneration of the medium by heat and subsequent cooling,and is not generally used in comfort air-conditioning applications.Establishment of needThe application of air-conditioning may be considered necessary to meet a variety ofcircumstances:. Where the type of building and usage thereof involves high heat gains from sources suchas solar effects, electronic equipment, computers, lighting and the occupants.. In buildings which are effectively sealed, for example where double glazing is installedto reduce the nuisance caused by external noise.. The core areas of deep-planned buildings where the accommodation in the core isremote from natural ventilation and windows, and is subject to internal heat gains fromequipment, lights and occupants.. Where there is a high density of occupation, such as in theatres, cinemas, restaurants,conference rooms, dealing rooms and the like.. Where the process to be carried out requires close control of temperature and humidity,such as in computer suites, or where stored material, or artifacts, require stable andclose control of conditions, such as in museums and paper stores.. Where work has to be carried out in a confined space, the task being of a high precisionand intensive character, such as in operating theatres and laboratories.. Where the exclusion of air-borne dust and contaminants is essential, such as in microchipassembly and animal houses.In tropical and sub-tropical countries, air-conditioning is required primarily to reducethe high ambient temperature to one in which working and living conditions are moretolerable. In the temperate maritime climate of the British Isles and in similar parts of theworld, long spells of warm weather are the exception rather than the rule, but modernforms of building and modern modes of living and working have produced conditions inwhich, to produce some tolerable state of comfort, air-conditioning is the best answer.Thus we find buildings of the present day incorporating to a greater or lesser extent,almost as a common rule, some form of air-conditioning. This great variety of applicationshas produced an almost equally great variety of systems, although all are fundamentallythe same in basic intention: that is, to achieve a controlled atmospheric conditionin both summer and winter, as referred to earlier, using air as the principal medium ofcirculation and environmental control.