1845.1 



THE CIVIL ENGINEER AND ARCHITECPS JOURNAL. 



the proportion to each foot of grate, if the rate of combustion be higher or 

 lower than 13 lb., may be found in the same way. This area having been 

 obtained, on the supposilion that no more air is admitted than the quantity 

 chemically required, and that the combustion is complete and perfect in the 

 furnace, it is evident, that this area must be much increased in practiee, 

 where we know these couditions are not fulfilled, hut that a large surplus 

 quantity of air is always admitted. A limit is tlms found for tlie area over 

 the bridge, or the area of the flue immediately behind the furnace, below 

 which it must not be decreased, or the due quantity could not pass off and 

 consequently the due quantity of air could not enter and the combustion 

 would be proportionally imperfect. It will be found advantageous in prac- 

 tice to make the area 2 square inches instead of -510 square inch. The im- 

 perfection of the combustion in any furnace when it is less than I'S square 

 inch will be rendered very apparent, by the quantity of carbon 'vhicb will 

 rise unconsumed along with the hydrogen gas and show itself in a dense 

 black smoke on issuing from the chimney. This would give 26 square inches 

 of area ever the bridge to every square foot of grate, in a furnace in which 

 the rate of combustion is 13 lb. of coal on each square foot per hour and so 

 in proportion for any other rate. Taking this area as tlie proportion for the 

 products of combustion immediately on their leaving the furnace, it may he 

 gradually reduced as it approaches the chimney, on account of the reduction 

 in the temperature aud consequently in the bulk of the gases. Care must, 

 however, be taken that the flues are nowhere so contracted, nor so con- 

 structed, as to cause, by awkward bends or in any other way, any obstruction 

 to the draught, otherwise similar bad consequences will ensue. 



An idea is very prevalent that it is advantageous to make the flame, or hot 

 gases (as they may be termed, because we may look upon flame merely as a 

 stream of gases heated to incandescence) impinge upon, or strike forcibly the 

 plates of a boiler at any bend or change of direction in the flue. The turn 

 in the flue is therefore made with a square end, and with square corners ; but 

 it is difficult to see on what rational grounds the idea of advantage can be 

 upheld. The gases, if they are already in contact with the plate, cannot be 

 brought closer to it, and any such violent action is not necessary to alter the 

 arrangement of the particles of the gases and bring the hotter particles to 

 the outside, while there is a great risk of an eddy being formed and of the 

 gases being thrown back and returned upon themselves, when they strike the 

 flat opposing surface ; thus impeding the draught and injuring the perform, 

 ance of the boiler. That circulation will take place to a very great extent 

 among the particles of heated gar.es, flowing in a stream even in a straight 

 flue, will be apparent from those particles next the surface being retarded by 

 the friction against the sides, and by their tendency to sink into a lower 

 position in the stream from their having been cooled down and become more 

 dense. An easy curve is sutHcient to cause great change in the arrangement 

 of the particles, as those which are towards the outside of the bend, have a 

 much longer course to travel and are thus retarded in comparison with the 

 others. From these causes the hotter particles in the centre of the flowing 

 mass are in their turn brought to the outer surface and made to give out 

 their heat. The worm of a still is never found returning upon itself with 

 square turns, as if the vapour inside would be more rapidly cooled by its im- 

 pinging on the opposite surface ; yet the best form of worm is a subject 

 which has engaged the attention of many able men, and therefore may well 

 be taken by engineers as a guide in the management of a similar process, 

 though carried on at a much higher temperature. 



Another very prevalent practice, and which also would seem to be open to 

 serious objections, is, that the flues are frequently made of much greater area 

 in one part than in another. This arises from a desire to obtain a larger 

 amount of heating surface than is consistent with the proper area of the 

 flue, or with the amount of the heated gases which are passing through it. 

 The flue is thus made shorter in its course than it ought to he in proportion 

 to its sectional area. This is even sometimes done by placing a plate of iron 

 partly across the flue, near the bottom of the chimney, thus suddenly con- 

 tracting the passage for the gases. The effect of this is evidently to cause a 

 very slow and languid current in the larger part of the flue, and the conse- 

 quence is that a deposition of soot rapidly takes place there. In many marine 

 and land boilers, having one internal flue in them, of too large a size, this 

 will be found to be the cBse ; soot being soon deposited, till the flue is so 

 filled up that the area left is only such as is due to the quantity of heated 

 gases passing through it; the value of those parts of the sides of the flue 

 which are covered with soot is thus lost. This is well exemplified in Mr. 

 Dinnen's paper on marine boilers in the Appendix to Weale's edition of 

 Tredgold, where he states, that the flues of the boiler in H. M. Steamer 

 " .Wrican," after she had performed a great deal of work, in the course of 

 five weeks' time, during which period there was no opportunity of sweeping 

 them, were found to be in exactly the same state as after a voyage of 5 days, 

 or probably as they would have been found after a much shorter time,' if 

 they had been examined. These flues are about the same area throughot 

 their whole length, but the chimney is of much less area. In the first por- 

 tion of the flue from the fire no soot was deposited, but the deposit began 

 after the first turn that the flue took, and gradually increased in amount to 

 the foot of the chimney. The inference that may be drawn from this fact 

 appears to be, that the gases, at first highly heated and thereby expanded, 

 filled the first part of the flue, but as they were cooled they became more 

 eontracted in their bulk, regularly towards the chimney, and therefore allowed 

 the soot to he deposited in the space not properly filled by them in their 

 course, and all soot subsequently formed was carried out at the chimney top 

 by the velocity aud power of the curreut. The amount collected near the 



219 



foot of the chimney, and in the portions of the flue furthest from the fire, 

 dimmished the amount of the surface of the boiler exposed to the action of 

 the heated gases and the etficieucy of the boiler was therefore impaired to 

 the same extent. In those boilers in which the flues before reaching the 

 chimney, are very much too large and are contracted, as has been stated, bj 

 a plate put across them, the extent to which their efficiency is thus impaired 

 must evidently be much greater and to a serious extent, as this evU exists in 

 them in a very much greater degree. 



The due amount of heating surface that ought to be given in a flue to 

 carry off the caloric, or to cool down a given quantity of heated gases, has 

 not yet been investigated with any great degree of accuracv, and practice 

 vanes widely under difl-erent circumstances. The largest proportion is al- 

 lowed 111 the Cornish boilers, some of which have not less than 30 feet and 

 even 40 feet of heating surface to one foot of grate. This appears to be more 

 ban IS justified by any corresponding gain, and certainly more than would 

 be advisable m any marine or locomotive boilers. In boilers burnini? 13 lb 

 of coal per hour on each superficial foot of grate, a proportion of 18 feet to 

 each foot of grate will be found to give good results. Where slow combos- 

 tion IS carried on, and where an extra size of boiler is not objectionable some 

 advantage may be gained, by increasing the amount in proportion to the 

 amount of fuel consumed. In calculating this surface, it is usual not to in- 

 clude the bottoms of the square flues in marine boilers, and in circular flues 

 from ith to Jrd of the surface should be deducted as bottom surface and there- 

 fore not efticient as heating surface. It is not usual to make any distinction 

 between horizontal and vertical surfaces, though it is probable that the 

 former are considerably more valuable. The erticiencv, however, of some 

 boilers which have been made with vertical tubes, would rather tend to make 

 It doubtful whether so much difference exists between the value of horizontal 

 and vertical surfaces as has been generally supposed. If the area, instead of 

 being in one large flue, be subdivided into a number of small flues, or pipes 

 so as to expose the gases to the required amount of surface in a short course' 

 the distance traversed between the fire-place and the chimney does not seem 

 to be important. The velocity of the current of gases will not be materially 

 influenced by their subdivision, as the wholeamount of the surface with which 

 the gases must come in contact, tending to impede their course by friction, 

 will be the same in both cases. It is evident that numerous small flues, by 

 subdividing the large stream of gases, which in the other case flow off in one 

 uody, bring the greater proportion of the particles at once into contact with 

 the surfaces and therefore render it unnecessary to pay the same amount of 

 attention to the turning of the stream and the bringing out the hotter parti- 

 cles from the centre of the flowing mass. If these proportions of area through 

 the flues and of heating surface be duly attended to, the results anticipated 

 may be depended upon, whether flues are of large area or are composed of a 

 large number of small tubes. 



The time occupied by the gases in passing through the boiler, from the 

 instant of their generation to that of their leaving the boiler, and the length 

 of the course through which they have travelled, have sometimes been looked 

 upon as matters of great importance. M'here the gases are travelling in one 

 compact mass, it is evident that distance and consequently time (as the ve- 

 locity with which the current flows is the same in all cases) must be allowed, 

 for the different particles of this large mass so to circulate among themselves 

 as that each may have an opportunity of coming into coutact with a cooling 

 medium, to give off its heat ; hut if the large mass of gases is so subdivided 

 that the difli'erent particles are sooner brought into contact with the due 

 amount of cooling medium, then the time the gases remain in the boiler 

 ceases to be of importance. When the gases have reached the foot of the 

 chimney, in a well-proportioned boiler, they will be found to be reduced to a 

 temperature of about 300° Fall., or below it ; their bulk will, in consequence, 

 be reduced byabout Jrd below their bulk on their first leaving the furnace. The 

 reduction in the area of the flue ought not to be in the same proportion, be- 

 cause their velocity is no longer so great. The reduction ought to be made 

 gradually, as has been stated before, and not bv a sudden contraction at the 

 foot of the chimney, as the efl^'ect of this is to cause a slowness of draught in 

 the latter part of the flue, and consequently a deposition of soot ; and then 

 the surface so covered, which had been reckoned upon as eflective heating 

 surface, is lost. The area of a chimney, to allow the products of the com- 

 bustion of each pound of coal consumed in an hour to pass off, should not 

 be less than Jths of 2 square inches, this latter being the area given for the 

 flue immediately behind the fire-place— that is, IS square inch ; and for a 

 boiler burning 13 lb. of coal per hour, on each superficial foot of its grate 

 the area should be |ths of 2(i square inches, or 19.4 square inches. 



Theoretical research not having as yet given us'any valuable assistance in 

 determining the proper height of a chimney, we must again refer to practice 

 as our guide. A good draught may be obtained with a very low chimney, 

 but at a great expenditure of fuel, from the necesesity, that exists in such'a 

 case for allowing the gases to pass off at a much higher temperature than 

 would otherwise be necessary. For a chimney bi ilt of brickwork the height 

 ought not to he less than 20 yards, and may' be increased to 30 yards or 40 

 yards with advantage to the economy of fuel. When chimneys are carried 

 to a still greater height, it is generally for the purpose of carrying off the 

 smoke, or any deleterious gases, from the immediate neighbou'rljood, or to 

 create a good draught with gases at a lower temperature than those from a 

 steam-boiler furnace. On board steam vessels chimneys are limited in their 

 height by the size of the ship, on account of the influe'nce the chimney has 

 on the stability and appearance. It will generally be found advantageous to 

 make the chimney as high as these circumstauces will permit. It will be 



