BOILERS. The origin of vessels for generating steam to sup ply the motive power of an engine was contemporaneous with the evolution of the steam engine itself, and even in the earliest days of steam it was recognized that they should be fitted with safety valves and have a considerable margin of safety. They were made of cast-iron with leaden or even wooden tops and often with flat surfaces. These iron tanks were set in brickwork and the fire was lighted underneath. The Watts "wagon" boiler marked a striking improvement in practice, for it was constructed of wrought iron with an internal furnace flue passing through it from back to front. As this was flat sided it became generally known as the "marine box boiler." The interior surfaces were stayed with iron rods, which added to its strength, but the average working pressure was very low, being generally in the neighbour hood of io lb. The advent of the compound engine revolution ized the construction of boilers, especially for marine use, as pres sures were increased to 25 lb. and upwards. A further impetus was given to progress when in the 185o's the steamship "Lima," of the Pacific steam navigation company, by using higher steam pressure, reduced her coal consumption from i 4 tons to one ton per hour.
The "Usco" hand-fired forced draught furnace (fig. I) con sists essentially of entirely closed furnace fronts with suitable trunking leading to a brick conduit passing along under the boiler footplates, at the end of which is a forced draught fan. The firing is carried out as usual, and when the firedoors are opened the blast is automatically shut off. Part of the air passes up through the furnace fronts behind the liners of the firedoors, thus keeping the whole arrangement extremely cool, while at the same time supplying a comparatively small amount of secondary heated air over the top of the fires, the main part of the volume, of course, passing underneath the bars in the ordinary way. A large amount of fuel can be burned per square foot of grate area and the forced draught is completely controlled by the fireman by means of a simple damper arrangement.
Forced draught has the advantage not only of supplying air in a cool condition, but also of "local izing" the heat in the boiler, which is of particular importance for many inferior fuels of low specific gravity, such as wood material, brown coal, peat, and general factory refuse. The de sign of the air heater consists essentially of a series of flat mul tiple thin metal plates, fixed close together in a casing, the outgoing hot gases and the incoming cold air, flowing in opposite directions on the contra-flow prin ciple, being split up vertically into alternate extremely long segments or channels of cold air and hot gases about tin. wide, travelling at high speed. This gives an extremely intimate con tact between the air and the hot gases through the metal walls, which are about 12 wire gauge thick, resulting in high efficiency of heat transmission, while there is a negligible resistance to the passage of both the air and the gases, the flow throughout— including the entry and discharge—being on the stream line principle without any eddies or back pressure. The saving in coal obtained depends, of course, on the size of the heater and the saving already made by the feed water economizers and superheaters, if these are fitted. Even under the latter conditions, however, the average additional saving is from IQ to 12% in the fuel bill. This combination of air heater, mechanical forced draught with closed fronts, and hand firing, is highly advantageous for a large proportion of Lancashire boiler plants, extremely ef ficient results being obtained in continuous operation.
It has always been a principle of Scotch boiler practice that the furnaces, combustion chamber, and nests of tubes which return the hot gases to the f ront of the boiler should be surrounded by water—an admirable feature. But in spite of its many advantages and the great work which this class of boiler has done f or the shipping industry, it has certain imperfections, which would al most seem to be impossible of elimination. Combustion is never perfect, as witness the volume of smoke which issues from the funnels of steamers, the unconsumed fuel which passes into the tubes, and in some cases the re-ignition of the gases at the base of the funnel. All this suggests waste, and in spite of many im. provements and exhaustive tests the waste continues. Oil firing, however, has done much to reduce these shortcomings.
The specifications for the materials for a Yarrow boiler are very severe. Steel plates, bars, and rivets must satisfy all the tests and inspections laid down in accordance with the British engineering standards specification, while the conditions respecting the manu facture of the tubes are very exacting. They must be straight, smooth, cylindrical, and of uniform sectional thickness and equal diameter throughout, while they are also required to be free from scale, longitudinal seaming, grooving, or blistering either inter nally or externally. They must successfully stand temper, flatten ing, bell-mouthing, and maximum pressure tests, together with a water pressure test in the case of a seamless tube of 2in. or less external diameter, of 2,5oo lb. per square inch. All plates which may be subject to stress in the boilers must be pickled in a dilute liquid of hydrochloric acid to remove oxide or scale. Hydraulic flanging is insisted upon, and great importance is attached to sub sequent annealing. Ultimate tensile strength and the percentage of elongation are carefully measured. It is theref ore very rarely that trouble is experienced with boiler tubes, and great attention is also given to the U-shaped tubes used in the superheaters. High pressures and temperatures have been rendered possible by this care in manufacture, with the result that steam pressures of soo to 600 lb. per sq. in. and superheat of 7oo° to 75o° Fahr. are com mon in land practice, and there seems no reason why they should not be extensively adopted at sea. There are certain fundamental factors common to both problems, among them being suitable materials and workmanship. The better and more uniform ma terials now in use and the improved metallurgical and manufac turing methods adopted remove from the designer's mind any anxiety as to materials. Copper, wrought-iron, and cast-iron have been discarded in the manufacture of water tube boilers of high pressures. Steel is in common use, and its uniformity leaves little to be desired. Certain of the non-ferrous alloys which are now used also contribute to that security which characterizes modern installations. There is, however, one difference between land and marine practice, viz., that in the latter there is always the risk of a leaky condenser allowing salt to enter the system, although it must be remembered that there are many electric power stations where the condensing water is by no means entirely free from salt.
The design of the boiler which is illustrated (fig. 2) has been passed by the Board of Trade, whose officers have offered many valuable suggestions. The boiler has a total heating surface of .3,42osqlt., of which the superheating surface is 87osq.ft.; in addition, there are 2,2oosq.ft. of air-heating surface f or preheat ing the air prior to its admission to the closed ashpit. The safety valves are set to 575 lb. pressure, and the final temperature will be from 7oo° to 75o° Fahrenheit. The test pressure is 913 lb. The design generally, while not unlike that used in the navy, has been based on mercantile practice, and more closely follows the design of the Yarrow land type boiler now in use at a number of electric power stations. The boiler consists of a f orged steam drum connected to the three f orged water drums by means of straight tubes expanded and bell-mouthed in accordance with the usual pra.ctice. Between the two water drums on the right-hand side is a superheater, consisting of a forged drum with a number of U tubes and placed between the two generating elements. The gases all pass up one side of the boiler and through the air-heater situated above the boiler to the funnel. The reason for adopting a single-flow type of boiler, where all the gases pass through one side, is that this design makes in this particular case a somewhat better arrangement in the ship than the double-flow type, where the gases pass equally through each side. The generating element on the left-hand side of the boiler absorbs a considerable portion of the heat from the furnace by direct radiation, and it will be noted that the proportion of the total surface of the boiler sub ject to direct radiation is considerable--an important feature in modern water-tube boiler design, not only increasing the output and efficiency of the unit, but also providing a large amount of comparatively cool surface adjacent to the combustion chamber, which lengthens the life of the brickwork. The admission of air for combustion is secured by the cool air in the stokehold enter ing an opening between the inner and outer casings at the front of the boiler about 6f t. up from the firing-floor, passing up the double casing through the air-heater, down the double casing at the back of the boiler into the closed ashpit, and so through the fire-bars. The efficiency of the unit is naturally increased by the air-heater, which extracts heat from the flue gases ; moreover, the air in its passage to the combustion chamber takes up a certain amount of heat which would otherwise be lost owing to radiation, and incidentally keeps the stokehold cool. Also, the circulation of air in close proximity to the furnace lining helps to keep the brick work at a temperature which ensures low cost of upkeep.
For controlling the supply of steam—especially as, unlike an oil-burning boiler, the supply of fuel cannot be quickly cut off— various means of regulation have been provided. A damper is fitted in the uptake at the side of the top of the air-heater. When this is in its horizontal position the air passes through the air heater to the combustion chamber. When, however, the damper is brought to a vertical position the air from the fan passes straight up to the funnel, thereby short-circuiting the air-heater and entirely stopping the supply of air for combustion. This ar rangement would be used when the engines are stopped suddenly. Provision is also made for controlling the speed of the forced draught fan, thereby limiting the amount of air for combustion. A still further control is to by-pass the steam direct from the boiler to the condenser by the silent blow-off.
Two vertical rows are fitted to each other and are staggered. The headers are connected with the steam drum by short tubes ex panded into a cross box, which in turn is connected to the drum. Each tube is accessible through individual handhole openings. These openings are elliptical in shape in the vertical headers be cause of the inclination of the tubes, the shape being necessary to provide for the insertion and removal of the tubes. The elliptical openings are closed by inside fitting forged covers, held in position by steel clamps and bolts. The circular openings—where circular handholes are used in incline headers—are closed on the outside by forged steel caps, milled and ground and held in place by clamps and bolts. Jointing rings or gaskets are required with the inside elliptical covers, but not with the outside circular plates. The main tubes are inclined at an angle of 22° with the horizontal. The rear headers are connected at the bottom to a rectangular forged steel mud-drum by means of nipples expanded into counterbored seats. The boiler is supported by steel girders rest ing on suitable columns independent of the brick setting. The feed water enters the front of the drum. All these water-tube boilers are constructed on the sectional principle, i.e., they may be shipped in sections and erected at the power station. In the marine boiler, to save weight, it is usual to fit only one (or some times two) rows of 4in. tubes, the remainder being i 14 in. in diameter. The land type boiler is fitted with revolving mechani cal chain grate stoker automatically fired and with economizer feedheater and superheater.
The Babcock and Wilcox boiler is by far the most popular land type boiler of the present day. At sea it has been fitted in battleships and battle cruisers and also in certain mercantile ships, but it is not, owing to its circulation, recognized as a boiler that can be forced greatly. With water-tube boilers it is of the greatest importance to use nothing but distilled water, and great care must be exercised in the selection of material for condenser tubes. It has recently been shown that a cupro-nickel tube is likely to be the tube of the future. Pure nickel would be the most reliable metal, but the difficulty of manufacture has not yet been overcome. A composition, however, of 7o parts of copper to 3o of nickel gives excellent results. (See fig. 3.) Cochran Boiler.—The Cochran boiler is remarkably efficient, durable, and adaptable. It is manufactured in 2 2 standardized sizes up to 8ft. 6in. diameter and i,000sq.ft. heating surface. It takes up comparatively little space and is economical to install. A special feature is the patent seamless furnace, which has a large ratio of heating surface to grate area, and provides easy facilities for cleaning or inspection purposes. The bricklined combustion chamber has a large thermal storage capacity, and the boiler has the additional recommendation of being readily adapt able to any class of fuel—coal, oil, coke, or wood, town, pro ducer, or water gas.
through the suction strainer by the oil fuel pump and discharged to the heater in which its temperature is raised to reduce the viscosity of the oil sufficiently to enable a very fine spray to be obtained at the burner. After leaving the heater the oil is passed through a discharge strainer in which it is very finely strained. The fittings and their accessories are generally mounted on an oil tight tray to form a complete unit. From the pumping and heating unit the oil travels through the discharge pipe to the burners, one of which is illustrated. The burner (fig. 4) is extremely simple and strong, and consists of only f our parts—viz., the body, cap, nozzle, and diaphragm. Holes are drilled nearly tangentially through the diaphragm, causing the oil to spin rapidly in the swirl ing chamber, so that on issuing from the nozzle it opens out in the form of a fine conical mist-like spray, due to the centrifugal effect of the spinning motion. The burner is held in its carrier by a clamp, a mitre joint being formed between the burner and an adapter screwed into the carrier. This makes a perfectly oil The "elesco" fire tube superheater consists of loops of tubing forming units which extend into the boiler flues.
In the Wallsend-Howden liquid fuel burning system, the furnace arrangement allows (r) burning coal only (2) burning oil only or (3) burning coal and oil together (fig. 5) .
The Howden-Ljungstrom air preheater (fig. 6) differs from others, inasmuch as it is constructed as a continuous regenerator and uses the contra-flow principle. The air enters the preheater through the forced draught fan and passes downwards through the rotor to be delivered through steel ducts to the boiler furnace. The flue gases pass through the rotor in the opposite direction, being drawn by the induced draught fan and delivered to the chimney. These fans are of the axial type, the upper part of the preheater shell being adapted as the fan casting.
Successful experiments have been carried out by the United States shipping board, which show there is no difficulty not only in burning pulverized coal under boilers, but in performing the requisite pulverizing on board ship. The advantages of the system are higher temperature of combustion; complete combustion and therefore no carbon loss in ash; ability to use any class of coal; increased flexibility of operation, so that the fuel supply can be immediately cut off or automatically adjusted to the load; re duced costs; and elimination of wastage from smoke.
Advances in boiler practice in the United States during the ten years ending in 1928 have been made primarily in the size of boiler units used, in the pressures for which they have been built and operated, in the ultimate steam temperatures used and in the capacities or rates of evaporation developed from a given amount of heating surface.
Boiler Sizes.—Twenty years ago the average boiler in power plant service contained some 2,5oo sq.ft. of heating surface while the maximum sized unit contained approximately 6,000 sq.ft. The aN,Trage sized boiler in central station work now contains from 12,000 10 14.000 sq.ft. of heating surface, the largest boiler yet built containing 42,37o sq.f t. This unit is of the inclined, curved tube design (fig. 7); the largest boiler of the straight tube type maximum output per boiler unit may be limited by the class of coal used, as affecting slag troubles either on the furnace walls or on the boiler tubes. Much has been done in the development of the so-called slag screens, formed by certain of the boiler tubes, to minimize the latter trouble. This has been accomplished by the use of an arrangement of the boiler tubes which the products of combustion first strike in such manner as to give greater gas flow areas, and lower gas velocities, into the boiler heating sur faces. A sufficient number of tubes are so arranged as to cool the gases below the temperature at which the ash will fuse to the tubes before the gases strike the main portion of the heating sur face of the boiler. The development of water-cooled furnaces. discussed hereafter, has tended to minimize any slagging trouble with the furnace walls.
In another plant, where refractory furnaces are used, the lim iting factor may be the life of the furnace brickwork as affected by the rate of driving. This factor too is becoming of less im portance through the increased use of water walls in high duty boilers. In a third plant the load factor may have the greatest bearing on the rate of output; i.e., at peak load periods, rates might be justifiable from the standpoint of slag or brickwork trouble that would not be allowable in steady operation.
contains 35,449 sq.ft. of heating surface exclusive of furnace wall cooling tubes. This unit is of the horizontal sectional heater water tube type (fig. 8).
The increase in size of individual boiler units has followed naturally the increase in size of prime movers, the desire to cut down boiler-room labour and to reduce the unit cost of power. The limitation on the physical size of boiler units would seem to be set by a combination of boiler design and boiler-shop equip ment.
Capacities.—While the maximum amount of heating surface possible in a single unit is a question of physical size and the ability to build any particular design of boiler to such maximum, the maximum amount of steam to be generated by any given amount of heating surface is dependent upon a great number of factors. Almost regardless of the design of any particular com bination of furnace, boiler, economizer or air heater, or both, because of the great number of these factors involved, it is im possible to state what rate of output per sq.ft. of surface repre sents the best rate for the best commercial return. The limita tion is set not by the maximum thermal efficiency obtainable so much as by the return on the capital invested and by problems of operation. With first cost eliminated, it would always be possible to obtain efficiencies at very high rates of boiler output comparable with those obtainable at normally high rates simply by the addition of economizer or air pre-heater surface. It is the operating factor that has the greater bearing. In one plant the Twenty years ago rates of evaporation of 5 or 6 lb. per sq.ft. of boiler heating surface were considered high. Such rates were limited not so much by the lack of ability of the boiler to absorb heat as by the limitations of the combustion apparatus available. With the developments in combustion apparatus, improvements in furnace design, and the increase in use of water-cooled fur naces, such rates to-day are considered moderate and continu ous rates of 9 or io lb. per sq.ft. of boiler heating surface are common in central station practice, while somewhat lower rates are common in industrial plants.
The highest rates of evaporation expressed in pounds per square foot of heating surface have been obtained with a boiler of a late design in which a large proportion of total heat absorption is through direct radiation. A boiler of this type, illustrated in fig. 9, is divided in heating surface as follows:—Boiler, sq.ft. ; furnace walls and floor, 2,46o sq.ft.; economizer, 8,365 sq.ft.; total water surface, 1 6, 763 sq.ft., air heater, 41,700 sq.ft.
This boiler has been operated at a capacity of somewhat over 290,000 lb. output per hour. Such a rate would mean an evapora tion per square foot of boiler and furnace wall surface of 34.5 lb. or of total water surface including economizer of 17.3 lb. While the economizer is designed to permit steaming, the amount of steam made in the economizer, even at the high rates of out put, is not great, and the actual evaporation per square foot of boiler and furnace wall surface is much closer to the first figure than the second. Another design of boiler in which the major proportion of the total heat absorption is through direct radia tion, and for which a rate of evaporation of 24.6 lb. per sq.ft. of surface per hour is reported, is shown in fig. so. Of the boilers absorbing heat largely through convection, the maximum total evaporation is largely a function of the fuel burning equipment, while the maximum evaporation per square foot of heating sur face will decrease as the heating surface per foot of furnace width increases. Of the convection absorption boilers of the general design illustrated in fig. 8 the maximum evaporation per square foot of heating surface is from 15.1 lb. per hour for a boiler 18 tubes high to 22.5 lb. per hour for a boiler i 1 tubes high.
The maximum evaporation reported with multidrum boilers of the design illustrated in fig. 7 is 20.5 lb. per hour. With con tinued developments in furnace design, combustion apparatus, and possibly some modification in the arrangement of heating surface relative to furnace, these rates may be exceeded. It is to be understood that these high rates of steam output are only possible with the very best feed water, and it is becoming more and more thoroughly appreciated in the United States that the best of feed water is essential to proper central station operation.
Pressures.—Twenty years ago the average boiler pressure used in central station work was 200 or 225 lb., though some few stations were equipped with 35o lb. boiler units. In 1926, two 650-lb. pressure plants were placed in operation. In 1928 there were 18 different stations either in operation or in course of erec tion, utilizing pressures of 65o lb. or over. There are to-day, in operation or in course of erection, seven different stations utiliz ing pressures of from 1,200 to 1,400 lb., the latter being the high est pressure for which boilers have as yet been built for com mercial service in the United States. Experimental boilers have been built for pressures higher than 1,400 pounds.
Riveted drums have been used for pressures up to 73o lb. per sq.in. Boilers built for pressures above this have been equipped with seamless forged steel drums. In the more or less unstable state of the art it is useless to attempt to predict what pressure will ultimately be accepted as representing the best practice. The efficiency of the steam cycle increases very slowly with in creased steam pressures above 600 lb., and the problems of re heating—it is generally accepted that reheating is necessary at pressures of about 55o lb. and above—feed pumps and the like must be carefully weighed against the increased first cost and the increased skill necessary in operation resulting from added complication. Maintenance costs are apparently no higher with high than with moderate pressures. The number of plants in service and in contemplation perhaps best indicate that under proper conditions the so-called super-pressures are justifiable. Most engineers have felt that the use of such pressures could only be justified for a base load plant where fuel costs were high. On the other hand at least one plant is being built for 1,400 lb. that will not be a base load plant and where fuel costs are rea sonably low, and the engineers responsible for the design have been able to justify the installation.
An attractive field for the use of the very high pressure boiler would seem to be in the older and less efficient plants. Here the high pressure turbine can be made to exhaust at existing line pressure and the overall plant efficiency be raised appreciably.
The tendency toward the use of higher steam pressures is not limited to central station practice. Industrial plants within the past few years have also been adopting higher pressures though not the super-pressures. Many industrial plants are utilizing 450 lb. pressure and one plant has gone to Soo lb. The use of high pressures in industrial plants is particularly of advantage where process steam is used, the steam being bled from the turbine at one or more stages at such pressure or pressures as are used in process work. See the article STEAM, GENERATION OF, and related subjects, including MERCURY-VAPOUR BOILER.
Both convection and radiant heat superheaters are in use in the United States and in some instances a combination of the two. Radiant heat superheaters show a falling superheat curve; i.e.,; a decrease in superheat with increased rates of steam out put. A properly designed combination convection and radiant heat superheater should give the desirable flat superheat curve; i.e., a constant degree of superheat regardless of rate of output. The principal objection to the combination type as so far de signed has been a total pressure drop that at high rates of output might be considered excessive.
The so-called interdeck superheater such as is shown in fig. 8, where the superheater is placed closer to the furnace than was formerly standard practice with this type of boiler, is in effect a combination convection and radiant heat superheater in that the gases pass over it and it absorbs a certain amount of heat by radiation through the lanes between the tubes below it.
Two different systems of the reheat cycle are at present in use in the United States. In the first installation made, steam was taken from the primary superheaters of all the boilers serv ing a single machine, was exhausted at approximately 150 lb. and still being slightly superheated, was all returned to a special re heater boiler unit, reheated in this unit to approximately the original ultimate temperature, and then returned to the same turbine at the stage next beyond that from which it was ex hausted. Fig. i t represents a design of reheat boiler of this type.
In the second reheat system, which was developed with the superpressure boilers, a high pressure turbine is used simply as a reducing valve. In this system, steam is taken from the primary superheater to a high pressure turbine which exhausts at a pres sure of some 350 or 400 lb. per sq.in., or if the installation is made in an existing plant at the existing main steam line pres sure. From the high pressure turbine exhaust the steam is re turned to a reheater element integral with the high pressure boiler, is reheated and returned to a lower pressure machine, or in the case of existing plants in which this class of installation is made, to the main station steam lines. Reheaters of this design have been of both the convection and radiant heat types. Fig. 12 and fig. 13 illustrate convection and radiation reheater units as used in the United States.
Live steam reheaters have also been used to a limited extent. The use of convection or radiant heat reheaters lead to a some what better overall plant thermal efficiency due to the fact that any desired reheat temperature may be obtained, whereas with live steam reheating the ultimate temperature is limited by the steam pressure available. On the other hand, the live steam reheater brings a simplification of piping and a saving in space that may more than offset the lower overall thermal efficiency.
In the convection and radiant heat reheaters installed up to the present time the ultimate reheat temperatures have ordinarily been approximately the same as the initial temperature to the turbine-725 or 750° F. Higher reheat temperatures are being considered. With live steam reheaters the ultimate reheat tempera ture possible without going to very special construction is some 20° lower than the temperature of the high pressure steam used as a heating medium.
Furnaces.—Improvements in combustion apparatus and the high duties demanded and required from a given amount of heat ing surface brought about changes in furnace design much more radical than in boiler design. This has been true with all classes of fuel and methods of firing, but the most radical changes have come with increased size and capacity of stokers and particularly with the development of pulverized fuel burning equipment.
With the increased amounts of fuel that had to be burned to give the desired rates of steam output, furnace volumes have been greatly increased. In 192o the average furnace volume per io sq.ft. of boiler heating surface in 24 representative stoker fired installations was 1.9 cu.ft. In 1926 a similar average for 24 stoker installations had increased to 3.85 cu.ft., while in 14 large pulverized fuel installations the average had increased to 8•o cu.ft. In several of the most modern stoker fired installations the furnace volume supplied has been as great as is general with pulverized fuel. In the earlier pulverized coal furnaces, while the volumes used were large the rate of heat liberation per cubic foot was relatively low—from i o,000 to 12,000 British thermal units. In present practice the large volumes are retained but such volumes are utilized much more effectively. This has been made possible by the develop ment in pulverized fuel burners. By the use of the principle of turbulence in mixture of coal and air, combustion takes place with much greater rapidity than in early practice and a B.T.U. liber ation of 30,00o to 35,00o per cubic foot of furnace volume is now common practice. It is pos sible that as further improve ments are made in burner design this B.T.U. release per cubic foot may be still further increased. This would result in a greater steam output from a given boiler and furnace or an equal steam output with a reduction in fur nace volume.
The same improvements in stoker and pulverized coal burner design that made possible the higher rates of B.T.U. liberation per cubic foot of furnace volume also greatly reduced the amount of excess air necessary to com plete combustion. This in turn led to higher furnace tempera tures and corresponding higher efficiencies. The increased use of air preheaters also tended toward higher furnace temperatures.
With the possibility of developing these higher furnace tern peratures, the limiting factor in boiler output became generally accepted as furnace refractory upkeep cost, and because of the desirability of such high furnace temperatures it became neces sary to develop some method of reducing furnace upkeep cost.
The remedy developed was the use of furnace wall cooling, either by air or by water-cooled surface. The earlier cooled furnaces were air-cooled and to an extent air-cooling is used satisfactorily to-day though generally with the smaller boiler units and where the B.T.U. liberation is appreciably lower than that demanded in central station practice. Water-cooled furnaces developed rapidly. As now used they consist either of bare tubes, so-called fin tubes, and tubes protected by refractory lined metal blocks.
In the case of plain bare tubes, these are usually set in re cesses in the brickwork of the furnace walls though in some in stances the tubes have been set out from the inside face of the walls. In one design of bare tube wall, the tubes are made to enter the headers in such manner that adjacent tubes touch, offering an all metal surface to the furnace. The fin tube fur nace is made up of tubes on the sides of which steel fins are welded longitudinally and the tubes are so placed that the fins of one tube touch the fins of adjacent tubes presenting an all metal furnace wall. Another type of all metal furnace wall is made up of tubes on which blocks are cast.
Of the refractory lined water walls the Bailey wall has been most highly developed. This consists of vertical tubes to which are clamped metal blocks so ground as to give an absolute con tact between block and tube. The inside face of the blocks is lined with refractory material, this refractory being used as the bottom of the mould in which the block is cast. Experience would seem to indicate that the refractory protected tube would stand a greater amount of punishment than would bare tubes, and higher rates of heat liberation in the furnace are possible with the former than with the latter. In most designs of water-cooled furnaces the combination of tubes and headers are connected di rectly to the circulation of the boiler thus becoming an integral part of the boiler. In a few instances the furnace water cooling surface has been made a separate boiler with its own feed line and water level. For the insulation of water walls various types of tile or commercial cements are being used with satisfactory results. The earlier water-cooled furnaces were comparatively small in volume and because of a lowering of flame temperature combustion could not be completed before the products of com bustion entered the boiler heating surface proper. The flame tem peratures were lowered, owing to the presence of a large amount of comparatively cold surface in the furnace.
As has been indicated furnace volumes were very generally increased due to the demands for increased total B.T.U. liberation corresponding to increased rates of steam output. With such in crease in volume, together with improvements in stoker design and pulverized fuel burning equipment, the effect of furnace cooling surface on temperature was lessened. The development of refractory covered cooling tubes also tended toward increased furnace temperature. Further, a better understanding of the laws of heat absorption through radiation by furnace cooling tubes and boiler tubes exposed to radiation made it possible to properly correlate radiant heat absorbing surface and convection absorption surface. This was particularly true in the case of pulverized fuel where turbulent burners gave a great rapidity of combustion.
With the introduction and development of bleeder steam feed heating in multiple stages, the initial feed temperature to econ omizers became higher and higher, and the temperature range through which the economizer could function, if the feed tem perature leaving the economizer was limited as described, became less. The amount of economizer surface that could be installed thus became small and the justification for such installation be came questionable.
As pressures increased above 35o lb.—to 650-80o and 1,200 and above—the steam temperatures due to such pressures correspond ingly increased. With such pressures the upper limit of feed tem perature leaving the economizer was raised to a point such that, even with the maximum incoming feed temperature to the econ omizer resulting from three or four stage bleeder heating, the temperature range through which the economizer could function became sufficient in many instances to warrant the installation of an economizer of normal size. More recently a design of econ omizer has been developed in which steaming is permissible. Such a design with its boiler is shown in fig. 9. Economizers as now in use are in general of the counterflow type and with either plain tubes or extended surface tubes, this extended surface being in the nature of fins. In one of the latter designs the surface is lead coated to minimize exterior corrosion. Transfer rates to be expected in economizers for a given set of temperature condi tions and a given amount of surface are largely dependent upon gas velocities over the surface. The gas velocity in turn deter mines the draft loss through the economizer. The allowable draft loss therefore is in most instances the governing factor in the amount and arrangement of economizer surface to be installed for a given set of conditions. Obviously there is some maximum gas velocity with its corresponding maximum heat transfer rate above which the added gross efficiency due to the economizer installation would be more than offset by the power required to overcome the additional frictional resistance of the gases over the surface. No general statement can be made as to where this point of maximum allowable transfer rate should be set, and it is necessary to consider each individual set of conditions' as a separate engineering problem.
The ideal air heater is one that will give a maximum heat trans fer rate, minimum draft loss, minimum leakage, occupy the least space and be most readily cleaned. The first two of these factors are intimately related. While the form of gas and air channels have a bearing on the transfer rate, such rate is primarily a func tion of gas and air velocities, and such velocities in turn govern the draft loss on both the gas and air sides. As in the case of economizers there is some velocity and corresponding transfer rate that cannot be exceeded because of excessive power required to produce the draft. The questions of space occupied, ability to clean and tightness, are features of mechanical design and vary in the preheaters of different manufacturers. Air heaters as now used in the United States may be classed under two types— tubular and plate. The regenerative design is a form of plate heater.
The question of maximum temperature of air for combustion advisable or allowable is intimately connected with that which furnace refractories or stoker or other fuel burning apparatus can withstand from the standpoint of upkeep costs—outage and replacements. The development in water-cooled furnaces has largely solved the problem of trouble with refractories resulting from high furnace temperatures, which in part, at least, are the result of the use of preheated air. Just what the limit of preheat that may safely be used with stokers has not as yet been definitely determined. Temperatures as high as soo° have been used with one type of stoker and no particular trouble reported. With an other type and a particular grade of coal trouble was encountered with temperatures considerably lower than this, though such trouble was due to the action of the coal on the grates and not stoker maintenance. With pulverized coal the limit of air tem perature, assuming a properly cooled furnace, would appear to be the limit of temperature under which the metal of the air heater would stand. This statement applies to the secondary air since the primary air temperature will be limited by the mills and the burners. Theoretically, the increase in overall efficiency due to the use of preheat should be higher than that represented by the gas temperature drop through the air heater. This results from the fact that the increased furnace temperatures and more rapid combus tion resulting from the use of preheat will add to the efficiency represented by the temperature drop. Further, with stokers, the use of preheat has been shown to result in a reduction in the loss through unconsumed carbon in the ash.
The amount of air heater surface installed in terms of boiler heating surface has varied widely. Such amounts where econ omizers have been used, except in the case of steaming econ omizers, has ordinarily been from 8o to 12o%. In one installa tion where no economizer was used and the air heater set directly next the boiler this percentage has been as high as 353%. In the case of the steaming economizer unit designs, the percentage of air heater surface to boiler and water-cooled furnace surface has been as high as 495%. (A. D. P.)