1902 Encyclopedia > Heating

Heating




HEATING. In temperate latitudes the climate is gene-rally such as to necessitate in dwellings, during a great portion of the year, a temperature warmer than that out of doors, and, similarly, tropical plants growing in temperate climates require artificial heat in the house in which they are preserved. Thus heating is required for health and comfort : the object of the application of science is to obtain these with the greatest degree of economy. In its aspect as to health it may be assumed that no system of heating is advisable which does not provide for a constant renewal of the air in the locality warmed. In climates such as that of the United Kingdom, the temperature of living rooms should be maintained at from 54° to 68° Fahr. in the daytime; the night temperature may be lower, but should not fall below 40°; and the humidity of the air as measured by the wet and dry bulb thermometers should show a difference of not less than 4° nor much exceeding 8° between the two thermometers, although with an ample supply of air a greater degree of dryness would probably not be found objectionable.

All heating apparatus depends upon the transference of heat from the fire to the various parts of the building which 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 as 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. The 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, hut 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 ; and 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 wdil 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-J 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 unequaf. 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 generating 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 ic 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 of water in pipes. When the boiler or source of heat is very -near the level of the pipes for heating the air, the average temperature which can be obtained in the pipes will be lower than when the vertical column is long. The heating surface must be regulated with reference to this difference of level. It may further bo assumed that with small pipes, the temperature being constant, the velocity of flow in the pipe necessary to furnish a given amount of heat will vary in the ratio of the length of the pipe. When the water circu-lates through the pipes by virtue of the difference of temperature of the flow and return currents only, it is impossible to count upon a greater mean temperature of the pipes than from 160° to 180°, because above that temperature the water in the boiler begins to boil. To obtain a sufficient velocity of circulation for long dis-tances, or with small differences of level, a forced circulation may be resorted to. This has been done by Messrs Easton and Anderson at the county lunatic asylum at Banstead, in the following manner. The whole hot-water service is supplied from boilers placed at one end of the asylum buildings, which extend to a distance of several hundred yards. There ate two pipes : one of them, which may be called the flow pipe, is connected directly with the boiler, ter-minating at the point furthest from the boiler in a dead end ; the other, which may be termed the return pipe, is parallel to the first, and terminates at one end in a cistern which is placed about 6 feet above, and supplies the boiler. At the other end furthest from the cistern the second pipe also terminates in a dead end. At each pavilion or place to which hot water is required to be conveyed, there is a connexion between the two pipes, which can be closed or opened at will; when it is opened, the water can pass from the flow to the return pipe. In the second, or return pipe, near the point where it ascends to the cistern, is placed a rotatory pump, or fan wheel, which is always kept revolving. When the openings are all closed between the two pipes, this pump or fan simply slips through the water ; but as soon as the return pipe obtains a supply of water from any of the openings between it and the flow pipe, a circulation is established.

_200 X 150 "°
wo '2

The following diagram, resulting from Mr Anderson's experi-ments, published in the Journal of the Institution of Civil Engineers for 1877, shows the total units of heat given out by cast-iron and wrought-iron pipes per square foot of surface per hour for various differences of temperature applicable either to hot-water or steam pipes. Suppose, for example, it is required to know how much heat will be given out by 4-inch cast-iron or 2-inch wrought-iron pipes at 190° in a room, the temperature of which is 60°; the difference of temperature is 130°, and corresponding to this will be found 232'7 units for 4-inch pipes and 356 units for 2-inch wrought-iron pipes per square foot per hour.

== TABLE ==

Diagram for ascertaining the heat given out by 4-inch cast-iron and 2-inch wrought-fron pipes at various differences of temperature. The dotted line shows the 4-inch, and the solid line the 2-inch pipes.





The amount of heating surface to be afforded with hot-water pipes depends mainly upon the volume of air to be admitted and removed, and the temperature desired to be maintained, but in any given building there are other circumstances to be taken into account, viz., the position, aspect, subsoil, temperature of locality, thickness of walls, size and form of windows, and other influences affecting the temperature of the incoming air, or causing loss of heat. An empirical rule has been laid down that in a dwelling-house 1 square foot of heating surface is required for every 65 cubic feet of space to be warmed, and in a greenhouse 1 square foot to every 24 cubic feet. This empirical rule does not take into account the sanitary considerations as to the renewal of air.

Steam-heated pipes present important advantages in some cases over hot-water pipes for heating purposes, because of the higher temperature to which the pipes can be raised, their consequent smaller size, and the facility of conveying the heat to a distance. Steam heating may be applied directly; and th» waste steam from an engine is also applicable for heating.

The direct application of steam heating on a large scale has been made at Lockport, New York. About 200 houses in the city are heated from a central supply through about 3 miles of piping, radiating from a boiler-house, which contains two boilers 16 feet by 5 feet, and one boiler 8 feet by 8 feet. These boilers are fired during the winter to a pressure of 35 lb to the inch, with a con-sumption of 4 tons of anthracite coal in twenty-four hours. The boiler pressure of 35 K> in winter and 25 lb in summer is maintained through a total length of 3 miles of piping up to the several points of consumption, where there is a cut-off under the control of the consumers. The first 600 feet of mains from the boilers are 4 inches in diameter. There are 1400 feet of 3-ineh pipes, 1500 feet of 24-inch pipes, and 2000 feet of 2-inch pipes. The supply pipes from these mains to the houses are 1J inches in diameter, and within each house f-inch pipes are used. In addition to the cut-off tap from the main under the control of the consumer, there is a pres-sure valve regulated to a 5-lb pressure under the control of the company ; and beyond this is an ingeniously constructed meter, which indicates, not only the total consumption in cubic feet of steam, but also the quantity of steam in each apartment. At each 100 feet of main an expansion valve, like an ordinary piston and socket, is inserted, allowing an expansion in each section of 100 feet of If inches for the heat at 35-Ib pressure. No condensation occurs in the mains. They are covered with a thin layer of asbestos paper next the iron, then a wrapping of Russian felt, and finally manilla paper, and the whole is encased in timber bored out three-quarters of an inch larger than the felt-covered pipes, and laid along the street like gas-pipes. The distribution of heat in the apartments is by means of radiators consisting of inch pipes, 30 inches long, placed vertically either in a circle or as a double row, and connected together at top and bottom, with an outlet pipe for the condensed water, which escapes at a temperature a little below boiling, and is sufficient for all the domestic purposes of the house, or it may be used as accessory heating power for horticultural and other purposes. The steam has also been applied at a distance of over half a mile from the boilers for motive power, and two steam engines of 10-horse and 14-horse power are worked from the boilers at a distance of half a mile with but a slightly increased consump-tion of fuel. The laid-on steam is also used for cooking purposes, for boiling, and even for baking. As in the ease of gas supply, the steam supply company lay their pipes up to the houses, the con-sumer paying for all internal pipes, fittings, and radiators. In a moderately-sized eight-roomed house the expense of these amounted to $150, and in larger houses with costlier fittings to §500.

Boulton's system of heating with exhaust or wTaste steam is devised to cause the steam from a steam engine to travel long distances without any back pressure on the engine. It is especially applicable to drying rooms in which 150° Fahr. has been obtained by a large heating surface ; for a lower temperature less heating surface is required. The capacity of heating by exhaust steam is nearly in a ratio wdth the fuel expended in the boiler. There is some cooling in passing through the engine, and in the conveyance along the pipes to the rooms to be heated, but this loss is compara-tively small if the pipes and the cylinder are covered with a good non-conductor, and the condensed water is taken back hot into the boiler. Thus, if the steam be taken from the boiler direct to the pipes at five atmospheres, the temperature would be 307°, and if a comparative capacity of steam were allowed to pass through the engine to create power, and discharged into the pipes at one atmo-sphere, it would decrease in temperature to 213° ; but it w-ould increase in bulk according to the expansion ; and thus to obtain nearly the same temperature in the room the heating surface should be increased.

With an engine of 17-inch cylinder and 25-horse power nominal, the exhaust steam has been made to travel 200 yards in a direct line, as well as to pass into various branches, amounting in ';he aggregate to about 2386 yards (or 1J miles) of lj-inch pipes. After this it warms the water for the boiler, and the steam is not all used up. The whole efficiency of the system depends upon so arranging the pipes as to prevent back pressure. Mr Boulton assumed that one-horse power if properly applied should warm about 30,000 cubic feet of space, subject to reductions for window space, wall space, the number of cubic feet of air allowed to escape for ventilation, and other considerations, and lays down the follow-ing empirical rule, viz.:—1 square foot of steam pipe is allowed for each 6 square feet of glass in the window, 1 for every 6 cubic feet of air escaping for ventilation per minute, and 1 for every 120 feet of wall, roof, or ceiling, adding about 15 per cent, for contingencies.

Wrought-iron pipes lj-inch bore are the most economical for steam, as they afford a large heating surface with small area. In heating living rooms by steam, the high temperature of the pipes affords one of the advantages of an open fire, viz., warmth by radiation, and combines with this the advantage which hot-water pipes possess of directly warming the air. (D. (I.)




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