On the Recent Improvements in Regenerating Hot Blast

REGENERATIVE HOT BLAST STOVES. 309

No. 1,282.--'. On Recent Imnrovements in Regenerative Hot Blast Stoves for BlastFurnace&' By EDWAR;ALFRED Cowmn, M. Inst. C.E. THEeconomical manufacture of iron is of so much importance, that no apology is needed for presenting a short account of some details closely bearing on the improved economy now obtained in the blast furnace, in the production of pig iron. Up to t'he year 1829, all blast furnacesmere blown with cold air, and the working of such furnaces was more or less irregular, depending upon a number of circumstances-one being the state of the atmosphere, as a furnace was more regular in its operation when the air was dry and frosty, than in damp weather, when it never worked well. Blast furnaces could not be made of large size when blownby cold blaat, as the cold air chilled the materials as it entered the furnace. In the year 1525, thelate Mr. James Bea.umontNeilson (M. Inst. C.E.), introduced the plan of heating the air employed as Ijlast, by means of iron pipes placed in or near a fire, the air being blown through the pipes on its way from the blowing engine to the furnace. Mr. Neilson at first began with an increase of temperature of from 60" to 100" Fahr. ; and even this small rise in tempera- tureat once toldupon the blast furnace, causing it to work as though it mere supplied with dry air, and effecting some saving of fuel. Subsequently, Mr. Neilsonimproved the cast-iron pipe stoves, until a blast wasobtained of a temperature of 600" or 650"; and the pipestoves have since beenurged upto 90Oo, ad in somefew cases to 1000". The wear andtear, however, with such temperatures of blast are considerable, and great care is requisite in the management of the stoves, or they would soon melt, or bedestroyed, whenever thecurrent ofcold air through the pipes is stopped, as, for instance, at the time of tapping the furnace. It will be readily understood that, when cast-iron pipes are used for heating the blast, they must be considerably hotter than the air passing through them, or the conduction of heat would bevery slow. Then again the heat of the fire, or of the products of combustion, must be considerably higher than the pipes, in order that they may be heated wit11 sufficient rapiditmyto produce the necessary rcsnlt.There arethus twolosses by conduction(besides that throughthe metalitself), and thenatural result is, thatthe

Downloaded by [ Imperial College London Library] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 310 REGENERATIVE HOT BLAST STOVES. products of combustion generally pass away from the stoves at about 1250", causing one great loss of heat, besides failing to heat the blast to the desired degree. The friction of the air through the pipe stoves, or the reduction in the pressure of the blast (or pillar of blast as it is commonly termed) is always considerable, and the leakage of air, or loss of blast, is likewise an item with pipe stoves ; and when they get out of repair,from the warping and twisting of the pipes, and con- sequent straining of the numerous joints, the leakage becomes so considerable, that the stoves have to be laid off for some time for heavy repairs. This is such a serious matter, that pipe stoves are oft,enworked in a leaky condition, necessitating the expendi- ture of engine power for blowing air uselessly, in place of its being utilized in the blast furnace. The stoves now about to be described are based upon the principle of theregenerative furnace introduced hy Mr. Siemens (M. Inst. C.E.). The Authorearly appreciated the fact, that by certain new and judicious arrangements,a considerable pressure of blast might be retainedin properly-constructed stove, whilst the casing or shell of the stove was protected from the great heat of the fire, and of the blast inside it, by a fireproof lining. Thus the full pressure of air required for a blast furnace was retained within an air-tight wrought-iron skin, and the skin wm kept cool by a lining of firebrick. The principle of Mr. Siemens' regenerativefurnace is nom so well understood, that only a brief allusion to it will be necessary in describing the construction of these stoves. Each stove of a pair consists of a wrought-iron cylindrical casing of light boiler work (A A Figs. 1 and 2, Plate 19), having a flat bottom standing on the ground, and a dome atthe top. It is lined with brickwork throughout, and is provided with a circular central shaft or flue (N), which extends to within a few feet of the inside of the brick dome. Around this shaft there are a number of compartments, or boxes (B B), formed of bricks so placed that those in one course arenot exactly coincident in position with those inthe courses either above or below, though a passage is left open from the top tothe bottom of the mass of brickwork. Thus a new kind of regenerator has been introduced, which is supported on cast-iron gratings, at the bottom, or cool part, of the stove. The wrought-iron casing (A A Figs. 1 and 2) is provided with several valves, three being for the admission ofcold blast (E), of gas (K), and of air for combustion (L), and two being for the exit of the products of combustion (C), and of the hot blast (F). These valves are all of simple construction, except the hot blast valve (F), which has a small circulation of water through it, to keep it suf- ficiently cool, just as the temperature in a tuyere is kept down by

Downloaded by [ Imperial College London Library] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. REGENERATIVE ROT BLAST STOVES. 311 a circulation of wateraround it. Thishas always answered the purpose very well. Theopening and shutting of the valves, at intervals of several hours, are matters of simple routine, and are in factall the attention the stoves require beyond an occasional observation, to see thatthe stove hasgas enough. Thereis no fear that these stoveswill get too hot, as it is firebrick that is being heated, andnot castriron pipes. Supposing, then, that a stove has been regularly at work, heating blast, and it is wished to heat the stove up again, the first thing to be done is to put another stove on, and then shut the hot and the cold blast valves (F and E) and allow the air in the stove to blow out at a small valve (c), to reduce it to atmospheric pressure. The gas, air,and chimney valves (E, L and C) are then opened, and the gas at once ignites as it enters, and gives a large volume of flame, right up the central shaft,and over andinto the regenerator, thusheating the top course of brickworkconsiderably,-the next course, rather less, -the next still less,-and the lower part of the regenerator not at all,-the products of combustion passing away to the chimney at a temperature of about 300'. Then,as the heating goes on, and large quantitiw of heat are taken up by each course of brickwork, the heat penetrates by degrees lower and lower into the regenerator, until a good red heat has, in the course of several hours, reached nearly to 6he bottom, thus storing up a large amount of heat in the bricks forming the regenerator. The gas and the air are next shut off; the chimney valve is also shut, the cold blast is put on, and lastly the hot blast valve (F) is opened. The stove then again does duty in heating blast to full red heat, viz., a temperature of 1400" to 1500". Allthe hot blast pipesfrom the stove to the furnace are of wrought iron and of large size, so as to allow of several rings of brickwork lining, to prevent loss of heat. The condition of theregenerator after it has been supplying blast for some time, and again after it has been heated, is perhaps most clearly shown byaslip of paper,coloured red nearly throughoutits height onone side, with a graduated partat the bottom, shaded off to black, and on theother side coloured black nearlythroughout its height,with agraduated part at the top, shaded off to red. It may be observed thatthe fluc- tuationin thetemperature of the blast for two hours is only about 100" or 150". This is notsurprising when it is known, that in a little experimentalstove, only 1ft. 3 in. high, the chimney at the top was quite cool, whilst after the blast hadbeen heated

by its passage through- the stove, lead could readily be melted at the botiom. When Messrs. Cochrane & Co. adopted the regenerative hot blast stoves aome years ago, at their works at Orm&by, it was at first

Downloaded by [ Imperial College London Library] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 312 REGENERATIVE HOT BLAST STOVES. contemplated to use the waste gas from the top of the blast furnace for heating the stoves. On consideration, however, it was thought that the dust mixed with the gas might choke up the regenerators, as they were at that time always filled with chequered work ; the bricks being so placed (Fig. 3) that in every case a brick stood over a narrow slit or passage (though a little above it), thus stopping and splitting the current of air, and effectually preventing ally brush from being passed through the slit. Again, a blast of air or steam through the slit was inefffctive for blowing out dust, because the existed free horizontal openings in all directions, by which the force of a blast applied to a slit was at once dispersed and lost. J2essrs. Cochrane therefore erected Mr. Siemens’ gas producers, and for some years worked the stoves most successfully with gas so produced, and which cont.ained nodust. After this, they built large brick chambers, with an extensive series of shelves inside, for tllc purpose of catching the dust entering with the gas from the top of the Slast furuacc, whichwas passed through suchdust- catchers before being used in the hot-blast stoves. Now, however, by the use of the later improve~nents, inventedby Mr. Siemens, Nr. Cochrane, andthe Author, and embodied in the stoves just described, the cost of such dust-catchers is avoided, and the expense of producing gas is also saved, as the gas is used direct from the top of the blast furnace, and the stoves can be cleaned out with the greatest facility. The arrangement of blast pipes, for air or steam under pressure, for cleaning out the dust from each compartment, or set of boxes, seriatim, is shown at (P) and at (Q Fig. 1). That at (P) consists of a wrought-iron pipe, jointed to a central pipecapable of revolving by a worm and worm wheel, so as to bring the pipe over each box in succession, and blow violently down itto clear outall dust. The central pipe has a slight vertical motion given to it, each time the pipe (P) is broughtnearer to the centre of the stove. The blast pipe (Q), shown in dotted lines at the bottom part (0)of the stove, has a small sheet-iron cone, or umbrella, attached to it, to keep the dust off the workman when he applies the pipe to Mow upwards through the boxes. The bottom part of the stove, where the cold blast enters,is always very cool, and can atany time be made quite cold,by running the cold blast through for sonw time longer than usual, and then a man can enter at the chimney valve atany time. The construction of the regenerator in compartments or boxes,connected together vertically, butnot hori- zotltully, gives the power of applyingthe blast with efficiency (inasmuch as the wholeforce of the blast is confined to the one pssage that is being blown at the time), and admits of agood brus11, like a chimney-sweeper’s brush,being passcd up or down

Downloaded by [ Imperial College London Library] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. REGENERATIVE BOT BLAST STOVES. 31 3 through the boxes to brush out the dust if preferred: the brush being provided with a long jointed or flexible handle. The form and proportion of the passages have been found, after numerous experiments, to produce an excellent effect in stirring up and mixing the air passing through, inasmuch as the current is caught by the two contiguous sides of the box that overhang the one below it, and thus is, so to speak, turned over and over, first on one side and then on the other, producing a most intimat,e mixing of the air, and therefore quick and good conduction of heat from the hot bricks to the air, or vice vers$ from the products of combustion tothe bricks. It is obvious thatthe same surfaces that tilke up the heat are those that again give it out to the blast, so that there is but little loss from bad conduction; and the cold blast is thus heatedto a high temperature, and the products ofcom- bustion are cooled to a low temperature, in fact nearly exchanging their temperatures,instead of the products of combustion, as in the old casbiron pipe stoves, going away far hotter thm the tern- perature of the blast that then obtttined. As a rule, a small stick of lead was employed m a test, to as- certain if the blast was hot enough in the pipe stoves, as it would melt the led if hot enough; then zinc was used in like manner ; but with the regenerative stows antimony is required, and that is ‘cut,’ or melted, in three or fourseconds. Glass rods are melted easily, as will be seen by the smlples. The results obtained by Messrs. Cochrane from the adoption of these! stoves, as regards the quality of the iron, the increased make of iron,and the large saving of coke in the blast furnace,have hem most satisfactory. Hithertothere has onlybeen one draw- back to the use of gasdirect from the top of the blast furnace, viz., the dust, which has prevented the extensiveemployment of the regenemtive stoves. As that difficulty has now been overcome, manyironmasters are contemplatingtheir immediateuse, and several arebeing erected at Barrow in Furness, of a larger size than any made previously ; besides others of a smaller size at Ebbw Vale and at Pontypool. It is a fact, however, which will be interesting to the owners of srnall blast furnaces, that they can, by the application of a high temperature of blast, make their funaces equal in economy large nlodern blast furnaces, as is already evidenced by its application to furnaces of only 7,000 cubic feet capacity. But it must be borne in mind that large modern furnaces can also have their economy still further improved, by the application of the hotter blast. Although these stoves have been described as applicable to heating the blast to 1,400’, or more, they are equally applicable to heatiug blast to only 1,0003, should any ironmaster not wish to go further. They can likewise be uscd to heat steam or gtscs to a high temperature.

Downloaded by [ Imperial College London Library] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 314 REGENERATIVE HOT BLAST STOVEB. The results of several years' working withthe old stoves, and with the regenerative stoves, have been communicated to the Author by Mr. Charles Cochrane, who has drawn out a curve (Fig. 7)showing the saving of coke, according to the temperatures to which the blast was heated. It will be seen from this curve, that about 4 cwt. of coke is saved per ton of iron, by the use of the regenerative stoves, when compared with good cast-iron pipe stoves, giving blast of' 1,000", and that the saving is larger ifcompared with ordinary pipe stoves, which usually give a much lower temperature. The Author has prepared the following statement to ahom the comparative economy of blast of ordinary and of high temperature : S. The first cost of the improved regenerative stoves for from 8,000 to 9,060 cubic feet of air per minute, at 1,400" to 1,5003, including royalty . , -3,367 The common casbiron pipe stoves for from 8,000 to 9,000 cubic fcet of air per miuute, at 1,0003 ...... 4,000 The annual cost of the common cast-iron pipe stoves for from 8,000 to - 9,000 cubic feet of air per minute : 1ntereb.t at 5 per cent. on S4,OOO ...... 200 Cleansing of stoves same as regenerative stoves ...... Iienewal of stoves once in 10 years...... - 400 X600 The annual cost of the improved regenerative stoves for from 8,000 to - 9,000 cubic feet of air per minute : Interest at 5 per cent. on 23,367 ...... 168 Cleansing of stoves same as pipe stoves ...... Repairs ...... - 20 -flS8 By theuse of the improved regencrative stoves, a saving would be eEectcd per ton of Con made, of 4 cwt. of coke, at say 11s. 6d. per ton X 475 tons per week X 52 weeks ...... 2,810 The increased make of iron would amount to 50 tons pcr week X 52 weeks X say 7s. per ton profit ...... 910 Less cost of working stoves GO0 - 188 ...... 412 -. Increased profit per annum ...... -24, I62 From this statement it will be seen that, with a large furnace, producing 475 tons a week, the first cost of the stoves is somewhat less than for the pipe stoves, while the annual cost of' working is less, and the profit, takingthe whole into account, is about %4,162a year. Mr. Charles Cochrane has drawn another, and a very interesting curve (Fig. S), showing the economy resulting from the use of large blast furnaces,a matter so nearly connected with hot blast that it has been appended to this communication. The Paper is accompanied by a series of diagrams, from which plate 19 has been compiled. __ [Nr. C. W. SIENIW