HEATING AND VENTILATION. These two subjects are so closely allied that they will be treated together. They deal with the science of maintaining in buildings the proper tempera tures and atmospheric conditions for the health and comfort of human beings.
History.—The earliest known method of artificial heating was the open fire of prehistoric days. As mankind gradually learned to build dwellings, the fireplace with its chimney was developed as a means of enclosing the fire and carrying off the products of combustion. Later the stove was invented and steadily improved, and is still an acceptable method of heating under certain con ditions. The next development was the warm air furnace which made it possible to remove the heating apparatus from the living quarters. As buildings became larger and requirements more exacting, the steam and hot water systems were developed.
Ventilation was not given much attention until relatively recent times. One of the earliest recorded attempts at artificial ventilat ing was a study of the ventilation of the Houses of Parliament by Sir Humphry Davy in 1811. Thorough scientific research into the true principles of ventilating was not carried on until the last decade and, although excellent progress has been made, there is still much to be learned about the physiological effects of atmos pheric conditions as well as the design and operation of the mechanical apparatus.
The General Problem.—Heat is continually being generated by the physiological processes of the human body and must be continually dissipated to the surrounding atmosphere. Comfort. health and working efficiency require that the atmospheric condi tions in occupied buildings be such that the loss of heat from the body will take place at the proper rate. This heat is dissi pated in several ways ; there is direct radiation and conduction from the skin, a considerable amount passes off with the exhaled breath, and much is removed by the evaporation of moisture from the lungs and of perspiration from the skin. The overall cooling effect of the atmosphere upon the body depends upon three conditions—the temperature of the air, its humidity and its rate of motion. They control the heat conduction from the body surface and the rate of evaporation of the perspiration which is almost constantly, though often imperceptibly, being exuded. The higher the temperature and humidity and the slower the movement of the air currents passing over the body, the greater the feeling of warmth. For comfortable conditions the combination of these factors must be correct although each may vary between certain limits. For example, when the atmosphere in a room is very dry the temperature must be maintained considerably higher for comfort than if the atmosphere contained more moisture. The cooling effect of a fan, especially when the skin is bathed in perspiration, illustrates the effect of air motion. The interrelation of these factors has been quantitatively studied, as will be brought out later.
It is the function of the heating and ventilating apparatus to maintain these correct atmospheric conditions within the building. In the great majority of buildings a simple heating system only is provided, and no attempt is made to maintain a definite air move ment or to control the moisture content. But where a large number of people are gathered or when it is desired to maintain the best possible atmospheric conditions, ventilating apparatus, often in combination with the heating apparatus, is provided. The two systems are necessarily parts of the same problem.
The heating system must supply enough heat to replace that which is continually being dissipated from the building in cold weather and must also be adequate to warm up the structure itself after it has been allowed to cool. This heat is lost from the building in several ways. There is direct conduction through the walls, roof and other exposed surfaces, and particularly through the window-glass. There is also a loss due to the infil tration of cold air and a corresponding outward leakage of warm air under the influence of the wind and from other causes. This leakage takes place through the cracks around windows, doors and elsewhere, and even to some extent through the walls them selves. There is a loss of heat occasioned also when windows are left open for ventilation or when ventilating fans discharge air from the building. Besides increasing the infiltration loss, wind movement also increases the heat transmission through the walls and roof, and this fact must be taken into consideration.
The amount of heat required by a building can be computed in advance of its construction, with fair accuracy, by a study of the various sources of heat loss. The heat-conducting properties of all common building materials and types of wall construction are known and are to be found in engineering handbooks. The following are coefficients for some common forms of building construction.
The infiltration loss is computed either by assuming a certain number of air renewals per hour, varying from 3- to 3 for different kinds of rooms, or by computing the length of the cracks around windows and doors and estimating the leakage therefrom, the cracks being the chief source of infiltration. A complete calcula tion of the heat losses with suitable corrections for exposure on the colder sides of the building thus serves as the basis for the selection of the proper size of the boiler, pipes, radiators and other parts of the heating system. Where the building in question is small or where the heating is not important, such calculations are omitted, but in all modern buildings where the heating is at all important the architect or engineer makes a complete study of the requirements.