Many persons imagine that the fire-box of a furnace connects with the hot-air flues, because smoke, coal-gas, and fine dust from ashes sometimes come through the registers. In reality, however, the smoke and dust escape into the cellar when the furnace doors are opened to put on fresh coal, are sucked through cracks in the cold-air box, and then drawn up through the registers. Few persons understand the mechanism of a furnace, because it is boxed in with brick or galvanised iron, so that the interior parts are never exposed to view. Out of sight is out of mind. An ordinary furnace consists of a fire-pot of cast-iron, lined with firebrick, with a wrought-iron dome or cover set in a groove filled with sand so as to permit of alternate expansion and contraction of the metal when heated and cooled. This is enclosed in galvanised iron or brick, with a small intervening space. Cold air is brought from outdoors through a box of wood or galvanised iron, and, after being warmed by contact with the fire-pot, is carried up through the flues constructed in the walls of the house. These flues open into living-rooms and halls through registers which can be closed by valves. To increase the efficiency of the furnace, various designs have been made to enlarge the heating-surface by the addition of flanges or vertical ribs with deep corrugations which are sometimes undulated, and into which the gases and smoke penetrate, thus increasing the heating-surface very materially. Merely enlarging the space between the fire-pot and the outer casing would admit too large a volume of cold air in proportion to the heating-surface. Only the inner stratum of air would come in contact with the surface of the fire-pot, and the outer stratum would scarcely be warmed at al]. The heavier the casting, and the more numerous the convolutions, the greater heat will be evolved. Another device which has proved successful has been the addition of an elaborate system of pipes— upright, horizontal, circular, conical, and cylindrical—which can be con nected or disconnected at will, and which carry smoke and heated gases into the chimney. Most modern furnaces combine both features, being constructed with a fire-pot flanged and corrugated to increase the heating surface, and having also a complication of smoke-flues so as to create direct and indirect draughts.
When the fire is started, it connects directly with the chimney by a short cut, or direct-draught damper, as slightly warmed air cannot be expected to be drawn through a roundabout system of flues. :\ tier the chimney is warm enough to maintain a permanent draught, the direct-draught damper is closed, and the circuitous route through the system of pipes is utilised, bringing the entire heating-surface into action, In addition to these provisions for controlling a furnace fire, sliding doors are provided in the upper and lower doors of the fire-pot. A check-draught damper is also placed in the main smoke-pipe, or in the indirect-draught pipe, to control the entrance of cold air. Various devices are employed to make this action automatic, so that a furnace will become self-regulating. A furnace gives the best results, and is most saving of fuel if the fire-pot in very cold weather is kept filled as high as possible, with the draughts regulated so as to keep a steady fire. A large heating-surface kept at a moderate temperature will prove far more effective than a smaller fire-box intensely heated by forced draught, which is wasteful of fuel and liable to crack the fire-pot.
The Kitchen Range.—_1 range differs from a stove in three features. The main object of a stove is to create warmth, while a range is designed to cook food and to heat water for household purposes. The grate-surface is made as large as possible, and is supplied with movable lids, so that the red-hot coals can be uncovered for broiling and frying. Other openings serve to admit pots and kettles. These methods of cooking require a bright and hot fire, whereas baking demands a steady and uniform temperature. The oven must therefore be heated on all sides, and this is accomplished by carrying the smoke and hot gases from the fire-box around the oven before they enter the chimney. If there are separate ovens, two smoke pipes are provided which join before entering the chimney. The fire-box is made oblong and deep to hold plenty of coal, and is lined with fire brick.
The water-back, an iron box corrugated on the side next the fire to increase the heating-sur face, has two pipes con necting with the kitchen boiler through which cold water enters and, when warmed, flows back into the boiler, and then circulates through the house to the bath-room, laundry, and butler's pantry. In some ranges the water-back is divided likewise by a diaphragm which separates the inflowing cold water from the out flowing hot stream, thus greatly increasing the heating capacity of the range. A brass water-back is more expensive, but no more serviceable, than one of iron. The water-back is sometimes placed in front of the range, but it acts just the same in either case. A coil of heavy copper tubing is sometimes used instead of cast-iron. (See Fig. 364.) Unless the range is kept free from ashes and clinkers, it will be impos sible to broil chops and steaks. Clearing the grate-bars with a poker is a slow and annoying process. The " Smythe " or " triangular bar " grate has three-sided bars which revolve and interlock, and are turned in opposite directions by a handle operating cog-wheels, thus removing ashes and clinkers with little dust and trouble. Other forms of grates have been devised with the same object. It is highly important to keep the grate clear, as otherwise the bars may melt with the excessive heat, or clinkers may stick to the fire-bricks. The pan should also be kept free from ashes if a good fire is to be maintained.
The best range will not work if there is a poor draught. This may be due to a crack in the oven door or top ; or a broken cover or open joint in the smoke-pipe may admit enough cold air to check the draught even if the chim ney-flue is ample. The latter should not he less than 8 by io inches, and, better still, 8 by 12 inches. If a boiler, laundry stove, or ventilating hood connects with the range-flue, it will interfere with the draught unless fitted with dampers which can be closed on occasion. The hot-air flues around the oven collect dust and fine ashes, and thus check the draught ; they should therefore be care fully cleaned. If the smoke-pipe from a range is connected with the chimney near the ceiling, the draught will be better, hut the room will be uncomfortable during warm weather.