218 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[July, 



These plans must all cease to be necessary or useful, if a furnace can be 

 so constructed, and tlie combustion of the coal in it so managed, that a very 

 small proportion only of uncombined inflammable gases would pass off, as in 

 this case no economy would result from their combustion, owing to the large 

 excess of air, which must be supplied and heated as before explained. 



The admission of a large quantity of air into the flue, at a distance from 

 the furnace, though advocated by some authorities, cannot be advantageous, 

 unless in extreme cases, when the temperatute in the flue is very high, and 

 where the combustion in the furnace has been more than usually imperfect. 



As the carburetted hydrogen gases are generated rapidly, on the applica- 

 tion of heat to the coal, and are in themselves much lighter, than the car- 

 bonic acid gas, or the nitrogen gas, formed at the same time, it is sometimes 

 assumed, that they rise nearly unmixed to the top of the space over the fur- 

 nace, and thence it is considered more advantageous to supply the air at this 

 place than in the flue. The cooling effect of air, if admitted into the fur- 

 nace, has been stated to be more injurious than if admitted into the flue ; 

 but the correctness of this statement may be doubted, especially if the gases 

 be unmixed, as this would render a much less quantity of air sufficient. 



The bars in this case should be placed at least 25 feet or 3 feet below the 

 boiler, or the crown of the furnace, to allow the principle to be more fully 

 carried out. An increase of space over the bars to this extent has always 

 been found to be advantageous, and ought to be particularly attended to. 

 The system of admitting the air to the gases in a subdivided form, in what- 

 ever part of the boiler the admission of it may take place, is very eflicaciouB 

 in procuring a thorough and speedy mixture of the particles. It has been 

 very extensively and successfully introduced by Mr. C. W. Williams in sup- 

 plying air behind the bridge of the furnace. 



An opinion is entertained that a sufficient supply of air for the gases may 

 be obtained, through the fire-bars ; and it is obvious that a partial supply, at 

 least, may be obtained in this manner, by a judicious management of the 

 fire. This may be effected by keeping the fires thin and open, feeding by 

 small quantities at a time, or by a system of coking the coal, allowing the 

 combustion of it to be slow at first, by which means the coal is formed into 

 masses of coke, between which tlie air has a passage. The air which passes 

 through is not vitiated further than in being mechanically mixed with the 

 carbonic acid and nitrogen gases, caused by the combustion of the coal on 

 the bars. 



The perfect combustion of the whole ingredients of coal being entirely de- 

 pendent, chemically considered, on the supply of the due quantity of atmo- 

 spheric air, it is evident that the velocity with which the air flows into the 

 fire will materially affect the result. According as this velocity is greater or 

 less, so in proportion must the quantity of coal that is to be consumed on a 

 given area of grate be increased or diminished, and there is no limit to the 

 quantity that may be so consumed, beyond the difficulty of supplying the air 

 sufficiently rapid. Tlie various circumstances which affect the velocity of 

 the entering air, have placed this question, as yet, completely beyond the 

 reach of theory, so that practical experiments must be taken as the only 

 guide, in determining what quantity of air can be made to enter into a given 

 furnace, and consequently, what amount of coal can be properly consumed 

 in it in a given time. 



Mr. Parkes has stated, as the result of a long series of experiments made 

 by him, (Vide Trans. Inst. C.E., vol. iii.) that the rate of combustion should 

 not exceed 7 lb. per superficial foot of grate bar per hour, and that this quan- 

 tity may with advantage be reduced as low as 4 lb. or even 3 lb. General 

 experience would tend to prove that these latter quantities are unnecessarily 

 low, and can only be advantageous when the arrangements for supplying the 

 air, or for carrying off the products of combustion, are defective or inefficient. 

 It is evident that if the area of any part of the passage, for either of these 

 currents, be too limited, the velocity at this contracted spot cannot rise 

 higher, than is due to the weight of the ascending column of heated gases in 

 the chimney. The quantity passing through is therefore diminished in pro- 

 portion as the area is limited ; and a good draught at a particular place, as 

 at the bridge of a boiler, may here be quite compatible with an insufficient 

 supply of air, and imperfect combustion of the coal. The draught in every 

 other part of the flues is, at the same time, rendered slow and languid, and 

 deposition of soot takes place in them. This fault is apparent in a great 

 number of boilers at present in use, and in some cases, especially in tubular 

 boilers, it is attended with very injurious results, by stopping up the tubes 

 and decreasing the amount of heating surface to such an extent, as to render 

 the boilers incapable of generating the required amount of steam. 



The furnaces of the boilers in general use in Cornwall, are upon the com- 

 mon principle of construction, and as in them it is not usual to apply any of 

 the peculiar patented arrangements, for the supply of air to the gases, behind 

 the bridge, it follows, either that these gases are not consumed, or that they 

 are consumed by air admitted through the bars. In the Cornish system of 

 raising steam, slow combustion is adopted in its fullest extent ; the fires are 

 kept thin and open, the fuel is supplied in small quantities and frequently, 

 and it is well spread over the whole surface. As the result is highly favour- 

 able in the economy of fuel, it may be presumed that the combustion of the 

 gases, as well as of the solid carbon, is comparatively perfect. When more 

 air is admitted into the furnace than can be made to enter through tlie bars, 

 it is generally done by apertures in the furnace doors. 



The average rate of combustion throughout the country is far above even 

 the largest quantity named by Mr. Parkes, and may be stated to be about 

 13 lb. per superficial foot of grate per hour. With due care in the construc- 

 tion of the furnaces and flues, there seems to be no reason why this quantity 



may not be as perfectly consumed, and the heat as thoroughly extracted fro 

 the products of combustion, before they leave the boiler, as with the smaller 

 quantity. Whether this be so or not, it is necessary in practice to prepare 

 for many cases, as on board of steam-vessels, where it is impossible to allow 

 a larger amount of fire grate, or boiler room, and when it would cease to be 

 ultimately economical to obtain a small saving of fuel, by great additional 

 expense in boilers and their fittings, and in space for them. 



To determine the velocity wiht which the products of combustion pass off 

 from the furnace, or from the boiler, is attended with much difiicultv, on 

 account of the great number of extraneous circumstances, which do so easily 

 and so constantly affect it. Some experiments on this subject were made by 

 Dr. Ure, and an account of them was read before the Royal Society, (Read 

 June 16, 1S36,) when he stated, that he considered the velocity ruight be 

 taken at about 36 feet per second, and as this result has been corroborated 

 by others, it may be assumed, in the absence of better data, as nearly cor- 

 rect. 



The subject, in a theoretical point of view, is surrounded by many difficul- 

 ties — in discovering the allowance which must he made for friction, and 

 other circumstances, similar to those affecting the flow of water through 

 pipes; and though this latter has engaged much more of the attention of 

 scientific men, no very definite results, to bear accurately upon practice, can 

 yet, even in this case, be obtained hy calculation. 



The practical question of the proper proportions of the eifierent parts of 

 boilers is then proceeded with in the paper, the leading chemical and physical 

 features connected with the combustion of coal in their furnaces having been 

 considered. 



The supply of the requisite quantity of air to the fuel on the bars being of 

 the utmost importance, it is usual to make the ash-pit, and the entrance to 

 lt„ as large, and as free, as the situation will allow. In marine boilers, or 

 wherever it is necessary to limit the size of asp-pit, the area for the entrance 

 of the air into it should never be less than one-fourth part of the area of the 

 grate ; and in order to facilitate the supply to the back part of the grate, the 

 bars should be made to incline downwards to the extent of about 1 inch in a 

 foot. No advantageous results will be obtained rrom increasing the ash-pit, 

 as is sometimes done in land boilers, to a very great extent, by making it 5 

 or 6 feet deep ; about 21 feet is sufficiently deep, even supposing that the 

 ashes are not cleared out oftener than once a day. 



The extent of " dead plate" in front of the furnace is not material, as re- 

 spects combustion ; in marine boilers it is generally not more than about 6 

 inches broad, which is the width of the water space, between the fire and 

 the front of the boiler ; but in land boilers it is frequently required lo be very 

 broad, to support the brickwork, especially in those cases where the flue is 

 carried across the front. 



The amount of the opening between the bars should be about ^ths of an 

 inch, but this must be regulated by the kind of coal to be burnt upon them ; 

 but for any kind of coal, it should not be less than f ths of an inch, nor more 

 than i an inch. If the space were made larger, the waste from the amount 

 of cinders, or of small pieces of coke, which would fall through in a state of 

 incandescence, would he considerable; otherwise it would be prefejahle to 

 have a larger space. In order to facilitate the supply of air, each bar should 

 be as thin as is consistent with the strength required. The bars in general 

 use this country are 1 inch or 1| inch in thickness, but it would be much 

 more advantageous to use them thinner, as in France, where they are fre- 

 quently used not more than J inch thick. 



The advantage of a considerable amount of space in the furnace, over the 

 fire-bars, has been already mentioned, but no very decisive experiments have 

 been made on this subject. Three cubic feet of space to each superficial foot 

 of grate bar surface, may be stated as a good proportion, where there is 

 nothing to prevent this amount being obtained. When the space is reduced 

 below one foot and a half of grate, it will be found to be attended with a 

 marked disadvantage. 



The area of the flue, and subsequently of the chimney, through which the 

 the products of combustion must pass off, must be regulated by their bulk 

 and their velocity. The quantity of air chemically required for the combus- 

 tion of 1 lb. of coal, has been shown to be 150-35 cubic feet, of which 44-64 

 enter into combination with the gases, and 105-71 with the solid portion of 

 the coal. From the chemical changes which take place in the combination 

 of the hydrogen with oxygen, the bulk of the products is found to be to the 

 bulk of the atmospheric air required to furnish the oxygen, as 10 is to 11. 

 The amount is therefore 49-104. This is without taking into account the 

 augmentation of the bulk, due to increase of the temperature. In the com- 

 bination which takes place between the carbon and the oxygen, the resultant 

 gases (carbonic acid gas and nitrogen gas) are of exactly the same bulk as 

 the amount of air, that is, 105-71 cubic feet, exclusive, as before, of the aug- 

 mentation of bulk from the increase of temperature. The total amount of 

 theproducts of combustion inacool state would therefore be 49-104 + 105-71 = 

 154-814 cubic feet. 



The general temperature of a furnace has not been very satisfactorily as- 

 certained, but it may be stated at about 1000°Fah., and at this temperature, 

 the products of combustion would be increased, according to the laws of the 

 expansion of auriform bodies, to about three times their original bulk. Tlie 

 bulk, therefore, of the products of combustion which must pass off, must be 

 154-814x3 = 404-442 cubic feet. At a velocity of 36 feet per second, the 

 area, to allow this quantity to pass off in an hour, is "516 square inch. In a 

 furnace in which 13 lb. of coal are burnt on a square foot of grate per hour, 

 the area to every foot of grate would be -516 x 13 = 6-709 square inches; and 



