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HEAT

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HEATING AND VENTILATION

melted, the water can be changed in temperature until you reach its boiling-point. After that, all the heat applied is spent in converting the water into steam, at the same temperature. The same phenomenon happens in the case of nearly all solids and all liquids, provided the pressure remains constant while the heat is applied.

CHANGE OP TEMPERATURE

As everyone knows, the effect of heat is, in general, to increase the temperature of a body. Two exceptions to this rule, melting and boiling, we have just noted.

QUANTITY  OF  HEAT

The arbitrary unit of heat employed in physics is denned as that quantity necessary to raise the temperature of one gram of water from o° to i° Centigrade. This is called a calorie. It was early discovered that equal amounts of heat applied to equal masses of different substances do not produce the same rise in temperature.

The ratio between the amount of heat required to raise the temperature of any body one degree and that required to raise the temperature of an equal mass of water one degree is, accordingly, known as the specific heat of the substance. The amount of heat, H, required to heat a mass, m, from the temperature ti° to temperature ta°, is described as follows:

H = w C (t2°—tj°) calories, where C is the specific heat of the substance. The amount of heat required just to melt one gram of a solid substance, without changing its temperature, is called the heat of fusion. This is'the quantity which used to be known as latent heat. The heat of fusion of ice is 79 calories per gram. In a strictly analogous manner, experiment shows that it takes a definite amount of heat just to vaporize one gram of any liquid without changing the temperature. This quantity is called the heat of vaporization. To evaporate one gram of water without changing its temperature, 536 calories are required. For an account of the nature of heat and of the evidence for thinking heat a form of energy see THERMODYNAMICS and STEAM-ENGINE.

HENRY CREW.

Heat'ing and Ven'tila'tion, a branch of engineering dealing with the practical application of the laws of heat to the designing, construction and installation of apparatus for the warming and ventilation 01 buildings. Ventilation requires a regular and continuous change of air in a room in order to maintain the air at a required standard of purity, and is evidently intimately connected with the subject of heating. Practical systems of ventilation are always installed in connection with the heating system. The requirements of a heating system are that it shall be capable of maintaining a certain temperature in the space considered under

all conditions of weather. In Europe the temperature demanded for living rooms is about 59° F. In the United States it is fully 10° higher. A heating system must consider, first, the production of the necessary heat as in a fireplace, a stove, a steam or hot-water boiler or in an electric radiator; second, the means of transmitting and diffusing the heat economically to the room which it is desired to heat. Stoves and fireplaces are placed in the rooms to be heated, and of course require no means of transmission. Steam and hot water may be transmitted in pipes to radiators in the rooms to be heated (direct radiation system), or the radiators may be placed at a convenient point and used to heat the air which is to pass into the rooms to be warmed (indirect radiation system).

The common hot-air furnace system is an indirect radiation system, in which the air is heated by contact with the iron walls of the furnace and then led by ducts through the house. In indirect systems the diffusion of the air may be by the tendency of hot and lighter air to rise or by some mechanical means as by fans and "blowers. Fans and blowers are used in all successful indirect radiation methods of warming large buildings. The quantity of heat that must be produced depends mostly upon the losses in transmission through the walls and windows of the building and on the heat required to warm the fresn air required for ventilation. The losses in transmission and by conduction might be reduced to a very small quantity by construction, but healthy animal life requires a continual supply of fresh air. A common rule is to supply 30 cubic feet of fresh air per minute for each occupant of a room. In ordinary dwellings and in other buildings not occupied by large numbers, sufficient fresh air is ordinarily supplied by so-called natural ventilation, that is, by the ordinary drafts through doors, windows etc.; for buildings occupied by large numbers of persons, such as school-houses, theaters etc., some method of forced ventilation must be used. This may be done by drawing out the impure air (the aspirating system) or by forcing in fresh air by a blower (the plenum system). The latter method is much more successful. One of the advantages of indirect radiation systems is that the heating and the ventilation demands are both satisfied.

Of the three systems of heating by hot-air furnace, steam and hot water, the hot air is the least costly to install and the steam system next. Hot water requires more radiating surface, and hence costs from 25 to 30 per cent. more. In economy of fuel hot water is cheaper than steam, and steam in turn cheaper than the hot-air furnace. A direct radiation system is in general considerably cheaper to operate than the indirect, but costs more to install. Heating by