Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/625

HEATING 591 it is intended to warm, and this transfer may be effected by radiation, by conduction, or by convection. Radiant heat is emitted and absorbed in an accelerating ratio in proportion as the difference of temperature between the radiant and the recipient increases, and, with the same difference of temperature between the recipient and the radiant, the effect of the radiant will be greater according to the increased temperature of the recipient. In other words, the ratio of the emission of heat increases with the temperature. It is thus easier to effect the warming of a given space by means of a highly-heated surface than by a surface emitting a lower temperature. An open fire acts by radiation ; it warms the air in a room by first warming the walls, floor, ceiling, and articles in the room, and these in their turn warm the air. There fore in a room with an open fire the air of the room is, as a rule, less heated than the walls. In this case the warm ing of the air depends on the capacity of the surfaces to absorb or emit heat ; except that the heat received by the walls may be divided into two parts, one part heating the air in contact with the wall, and the other passing through the wall to the outer surface, where it is finally dissipated and wasted. Fireplaces are sometimes constructed to assist the warming of the air of a room. For instance, in Sylvester s grate iron bars of which one end terminates under the fire are laid so aa to form a projecting radiating hearth. The ventilating fireplace warms the fresh air before its admission into the room by means of gills cast on the back of the grate. < In a close stove, heated to a moderate temperature, the heat, as it passes from the fire, warms the surface of the materials which enclose and are in contact with the fire and with the heated gases. Th e materials next transfer the heat to the outer surface in contact with the air ; and the air is warmed by the agency of this outer surface. If heated to high temperatures a stove gives out radiant heat, which passes through the air to warm the objects on which the rays impinge. With hot- water pipes, the heat from the water heats the inner surface of the pipe, and this surface transfers its heat to the outer surface through the material of the pipes. The rate at which the heat can pass from the inner to the outer surface, and be thus utilized instead of passing away straight into the chimney, depends on the heat evolved by the fire, on the extent of surfaces exposed to the heat and their capacity to absorb and emit heat, and on the quality of the material between the inner and the outer surface as a good or bad conductor of heat. This passage of heat through a body by conduction varies directly with the quality of material, and with the difference between the temperature of the inner surface exposed to the heat and the outer sur face exposed to a cooling influence, and inversely as the thickness between the surfaces. Other things being equal, copper is a better material than iron for conveying the heat from the fire to water or air ; and coverings of brickwork, wood, or woollen fabrics are better adapted than iron for re taining the heat. The property which appears more than any other to make materials good non-conductors of heat is their porosity to air, and the retention of the air in their pores. The direct warming of the air may be effected by stoves with brick or iron flues, or -by hot- water or steam pipes. The sizes of the heat ing surfaces for this object must be proportioned to the volume of air required to be warmed for ventilation, and the degree of heat to be maintained, the thickness of the material, and its capacity for absorbing and radiating heat and for transferring heat from one surface to the other. When a large volume of air is supplied and removed for ventilation, rapidity in transferring the heat from the fuel to the air is an important consideration. Brick stoves and flues are worse conductors of heat than iron stoves or flues, but the surface of a brick stove parts with the heat which reaches it somewhat more rapidly than do the surfaces of an iron flue. The slow conducting power of the material and the greater thickness of a brick stove prevent alternations which may take place in the fire from being felt so much as with iron stoves or flues ; atfd therefore the brick stove warms the air more equably, without sudden variations ; the air so warmed is free from objectionable elements ; and where they can be conveniently applied, it is advis able to use brick stoves for warming air for ventilating purposes. With an iron flue pipe from a stove, almost the whole heat which any fuel is capable of developing may be utilized by using a sufficiently long pipe, horizontal for the greater part of its length, to convey the products of combustion to the outer air. The heat given out by a stove pipe varies with the temperature from end to end, being of course greatest at the end next the stove, where the emission of heat is very rapid ; and the amount of heat given out per square foot will vary at each point as the distance from the stove increases. The proportions also into which the heat divides itself between radiation and convection vary greatly with the tem perature. Thus, with a stove pipe heated at the end nearest the stove to a dull red heat of 1230 Fahr. , and of sufficient length to allow the heat to be diminished to 150 at the further end, it would be found that at the stove end of the flue pipe 92 per cent, of the total heat emitted by the pipe is given out by radiation to the walls and only 8 per cent, to the air ; but at the exit end the heat is nearly equally divided, the walls receiving 55 and the air 45 per cent. Taking the whole length of such a pipe, the walls would receive 74 per cent, and the air 26 per cent, of the heat emitted. But with a flue pipe heated to lower temperatures, the air might receive half the heat or even more. When therefore the object is to heat the walls rather than the air, the temperature of the pipes should be high ; and for this purpose stove pipes are more effective than hot-water or low-pressure steam pipes. At high temperatures there will be practically little difference of effect between horizontal and vertical flue pipes, because the heat given out is principally that due to radiation, which is independent of the form and position of the radiant. An adequate proportion of flue pipes to the form and size of the stove involves a large surface for the flue pipe ; with a careful observance of proportion, as much as 94|- per cent, of the heat in the fuel has been utilized. There are, however, several serious objections to iron stoves, especially for small rooms : a long flue pipe is unsightly, and on that account often inadmissible ; iron stoves heat rapidly, and easily become red-hot, and the effect produced therefore is unequal. Carbonic oxide, too, has been found in air warmed by iron stoves very highly heated. It is alleged that highly-heated iron may take oxygen from the carbonic acid in the air in contact with its surface, and thus reduce the acid to carbonic oxide. Whenever iron stoves or cockles are used for heating air, care should be taken to prevent the iron from attaining a high tempera ture, and with this object all iron stoves should have a lining of fire-brick, so as to prevent the fire from coming in direct contact with the iron ; such an arrangement preserves greater regularity in the heating of the air. This object may be also attained by giving the stove a large surface in proportion to the fire by means of flanges or gills to carry off the heat as fast as it is generated. Iron coated with a surface of glazed enamel would enable the heat to pass rapidly from the fire to the surface, while the enamel surface would emit the heat more rapidly than the iron surface. Hot-water pipes for warming air are free from many of the objections arising from the direct application of heat to iron, because the heat can be regulated with exactness. A high temperature may be obtained from water without gene rating steam by heating it under pressure. In Perkins s high- pressure system, a continuous iron tube, about 1 inch diameter, is filled with water ; about one-sixth of the length of the tube is coiled and placed in a furnace, and the remainder, forming the heating surface, is heated by the circulation of the water. At the highest level to which the tube is carried it is enlarged so as to allow of a space for expansion of the heated water equal to 5 per cent, of the contents of the small tube. Pipes may be heated by either hot water or by steam. The higher the temperature, the greater is the comparative effect in warming air ; therefore, with a small heating surface, steam pipes are more efficient than hot-water pipes, and steam at a high pressure more efficient than low-pressure steam. The efficient action of hot- water pipes depends upon the upward flow of the heated and expanded water as it passes from the boiler, the passage being made as direct as possible, and so protected as to lose little heat between the boiler and the place where the heat is to be utilized. The return pipe, which brings back the water after it has been cooled down by the abstraction of heat in warming the air, should be passed into the bottom of the boiler as directly and in as uniform a line from the place where the heat has been used as possible. The velocity of flow in the pipes will depend upon the temperature at which the water leaves the boiler, the height to which the heated water has to rise, and the temperature at which it passes down the return pipe back into the boiler. The efficiency of a hot-water apparatus will be regulated by these conditions, by

the size of the pipe, and by such other conditions as affect the flow