Numerous attempts have been made at different times to lay down rules for the area and height of chimneys that would produce sufficient draught for the consumption of a certain quantity of fuel in a given time, but such formulæ have more frequently failed, than succeeded, in giving satisfactory results, which is due probably to the want of knowledge of the requirements in each individual case, and of the location and surroundings. Attempts are, in many instances, made to produce a good draught by carrying the chimney above all surrounding objects and buildings, but it frequently occurs that shorter chimneys of the same area and internal dimensions have a better draught. It is claimed by some engineers that chimneys ought to increase in area from bottom to top, to be capable of producing a good draught, while others assert just the reverse, and claim that they ought to decrease from bottom to top. It has been found by experiment that both arrangements produced a good draught under some circumstances, but neither of them under all circumstances. The area of any chimney should increase slightly from bottom to top, in order to provide for the increased volume of the heated air and gases resulting from their expansion. It has been found that round flues produced a better draught, as a general thing, than either square or oval ones of the same area and height. This doubtless arises from the fact that air, rushing through or up a flue or funnel, has a tendency to assume the form of a screw, which is due probably to some natural cause. Adverse currents and capping winds frequently interfere with the draught in short chimneys, but the same effect is frequently produced on tall ones during some kinds of weather and at certain seasons of the year; certain it is, that very tall stacks do not produce a corresponding draught in proportion to the height, and it has been demonstrated by observation that there is nothing to be gained by raising chimneys very high. It often occurs that chimneys of apparently sufficient height are incapable of producing sufficient draught. This, in many instances, arises from the fact that the quantity of fuel consumed in the furnace will not produce sufficient heat in the flue to rarefy the air and cause draught, while in other chimneys of ample height and area, in consequence of the air and heated gases having to pass through a long, cold flue between the boiler and chimney, the draught is sluggish and unsatisfactory. There is no lack of formulæ for proportioning chimneys, which have been furnished by Wye Williams, Rankine, Weisbach, Trowbridge, Steel, Watt, and others, but each is only applicable in certain cases; and indeed it appears that Watt knew as much about proportioning the flue as any of our modern engineers, which may be inferred from the fact that modern writers on the subject refer to him as frequently as to any one else. This goes to show that we have not made such rapid advances in mechanical science, so far as regards proportioning chimneys to produce good draught under all circumstances, as might have been expected, considering the intelligence of the present generation and the progressive ingenuity of the age. There are always individuals to be found who can tell how to proportion a chimney or a flue that will produce a draught sufficient to carry off the smoke and waste gases resulting from the consumption of a certain quantity of fuel, but they rarely ever explain all the conditions under which this may be accomplished; such as the distance between the furnace and chimney; whether the flue is perfectly straight, or contains a number of bends; and whether in its course it ascends or descends. Such information is akin to that which tells engineers that a pound of coal will evaporate 8 or 9 lbs. of water, but never gives the conditions under which it may be done, which include the type or design of boiler, the quality of the iron, the condition of the boiler for cleanliness, etc., the purity of the fuel, and the intelligence and experience of the care and management. It is well known to most experienced engineers that the boiler that will evaporate 9 lbs. of water per lb. of coal under some circumstances, will not evaporate over 5 lbs. of water per lb. of coal under others, and the results will be about the same in regard to draught. A forced draught may be produced by various mechanical ar rangements, such as blowing-engines, fan-blowers, steam-jets, etc.; but, although it may be suitable, and even an absolute necessity in the prosecution of many branches of mechanical industries, a forced draught is objectionable in assisting the combustion of fuel for the generation of steam in ordinary steam-boilers, and never fails to induce mischievous effects, and consequently a good natural draught is very much to be preferred when attainable. Any flue ought to be as smooth on the inside as circumstances will permit, in order to diminish the friction between the walls of the flue and the escaping air and gases. And in regard to the height of chimneys and proportions of flues, it is always better to be governed by such practice as has given satisfaction in that locality, and with a particular kind of fuel, than to be guided by any theory, however scientific. The sectional area of the flue is what is termed the calorimeter of the boiler, and the calorimeter, divided by the length of the flue in feet, is termed the vent. The flues of all boilers diminish in their calorimeter as they approach the chimney, as the smoke contracts in volume in proportion as it passes through the heat. Funnels. The area of the funnels of steamships, tug-boats, and ferry-boats varies considerably with different builders and in different countries. The number of circular inches per nominal horsepower is given in the following table, for several makers. Highest, 15 14 Highest, 12.96 | Highest, 14:45 | Highest, 16:40 | Highest, 14:06 13.94 Mean, 12.17 15.94 Mean, 13.12 15.14 Low, Mean Total, 13.78 These are all for low pressures. For high pressure, the number of inches varies from 911 to 6·02, mean 7:07. The funnel should evidently bear a proportion to the amount of heated air and smoke passing through it, which must bear a nearer proportion to the horse-power than to the surface of the fire-grate. Where the fire-grate is small, a large quantity must be burned per square foot. If, in one case, 20 lbs. of coal are burned per square foot per hour, and in another 40 lbs., and the funnels are propor tioned to the fire-grate, they will not be proportioned to their requirements. Rule for finding the required area for the chimneys of stationary boilers.- Multiply the nominal horse-power of the boiler by 112, and divide. the product by the square root of the height of the chimney in feet. The quotient will be the required area in square inches. A well-proportioned and moderately high smoke-stack is to be preferred for sea-going steam-vessels, as tall ones are difficult to steady on account of the oscillation of the vessel, arising from the disturbance of the water and the resistance of the wind. Superheaters. Superheaters are steam-chambers located in the uptakes of marine-boilers or at the base of the funnel, and so arranged that the waste heat from the furnaces may pass around and through them, prior to escaping up the chimney. They are used for drying the steam in its transit from the boilers to the steam-cylinders of the engines. The heat or flame passes through the tubes and around the shell, the steam being inside. They are fitted with a stop-valve, and arrangements for mixing the superheated and saturated steam, or using either independently; they also have safety-valves similar to those used on steam-boilers. There is no definite size for superheaters, as they are not intended for a receptacle for any large amount of steam, but simply as a means of drying it. The proportionate area of superheating to heating surface in modern marine-boilers is about 1 to 10 square feet. An intercepter or separator is a chamber attached to marineboilers for the purpose of intercepting the water carried out by the steam. The steam enters at the top and strikes against a partition plate, then passes under it and escapes to the cylinder; the water which enters with the steam is collected in the bottom of the box and drawn off through a valve. Smoke. Smoke once formed in a furnace, flue, or chimney can never be burned by any mechanical device or arrangement, nor can there be any advantage in incurring much expense in the attempt, except to abate a nuisance, as very little economy in fuel would result from the adoption of any such device. A very general idea prevails that, when we see large volumes of smoke issuing from the mouths of the chimneys of stationary boilers, smoke-stacks of locomotives, and funnels of marine-boilers, whenever fresh fuel has been applied, a great waste of fuel is taking place; this, however, is a mistake, as about of the volume is steam resulting. from the moisture expelled from the coal, wood, or shavings by the application of heat; besides, sulphur and other earthy matters which, like the steam, are incombustible, enter into and increase the volume. This may be easily explained by stating that ton of water is converted into steam in the furnace for every ton of bituminous coal consumed, which is an actual benefit, because, if the carbon had not been thoroughly mixed with such a great mass of steam, it would have fallen in the shape of a black cloud of dust in the locality where the furnace was situated, and have become a more insufferable nuisance than the smoke. Smoke contains about 20 per cent. of combustible and 80 per cent. of incombustible matter. Such being the case, the question would naturally arise, Would it be advisable to incur much expense in an attempt to consume 80 per cent. of incombustible matter, for the purpose of gaining 20 per cent.? Feed-Water Heaters. The benefits to be derived from heating the feed-water for boilers by exhaust steam may be explained as follows: A pound of feed-water entering a steam-boiler at a temperature of 50° Fah., and evaporated into steam of 60 lbs. pressure per square inch, requires as much heat as would raise 1157 pounds of water 1 degree. A pound of feed-water raised from 50° Fah. to 220° Fah. requires 987 thermal units of heat, which, if absorbed from exhaust steam passing through a heater, would be a saving of 15 per cent. in fuel. Feed-water, at a temperature of 200° Fah., entering a boiler, |