As to the difficulty arising from the tendency of the gases to follow the path of least resistance and to flow, for instance, with too great velocity at the center of the space and too little at the sides, that can always be checked by means of retarders placed so as to equalize the velocity over the cross section of the current. The difficulty, therefore, reduces itself to the mere trouble of finding out where to place the retarders, and this is obviously a question to be settled by experiment. What is true in this matter of the combustion space is also largely true of the tube space. The process of so important to combustion, continues after the combustion is complete, and must have a good deal to do with the rate at which heat is abstracted from the gases by the heating surfaces. As affecting the necessary amount of draft pressure, a tube space short in the direction of flow of the gases and of large cross-sectional area is better than one of small area and long in the direction of flow ; but on account of the lesser velocity of flow through the short space the gases within it will be less thoroughly mixed by eddying, and the importance of ar ranging the heating surfaces so as to permeate all parts of the space will be increased." The following essential requirements govern boiler and fur nace design, according to the U. S. Bureau of Minesa : "(1) The heating surfaces must be arranged in such a way that the gas passages are long and of small cross section so as to give a small hydraulic mean depth, the hydraulic mean depth being defined as "the quotient of the area of the cross section of the gas stream divided by the perimeter formed by the boiler heating surface touched by gases." An increase of the ratio of the length of gas path to the hydraulic mean depth of the cross section of the path increases the efficiency of the boiler, because the hot molecules of gas will strike the heating surface oftener and will have to travel smaller distances to reach this surfaCe. The amount of heat given up to this surface by a given volume of gases will therefore be greater, and both boiler and furnace ef ficiency will be higher. This ratio can be increased, either by increasing the length of the gas path, or by reducing the hydraulic mean depth. The length of the gas path can be increased by either increasing the length of the boiler or by placing baffles and thus putting parts of the heating surface in series with one an (2) The heating surface should "see" as much of the fur nace as possible in ordg-, to increase the amount of heat imparted to it. This effect should not be so pronounced that the heat will be radiated to the heating surface too rapidly, for the furnace temperature would then be reduced below that required to support combustion. (3) The combustion space of the furnace must he so constructed that the burning particles of fuel shall be com pletely consumed before they can touch the relatively cold boiler surface ; also this space should enlarge in the direction of the flow of the heated and expanding gases, as the capacity of a fur nace for burning oil is limited almost entirely by the furnace volume. The furnace shetild be lined with refractory brick, which when very hot radiate heat and assist the combustion of the fuel." Mr. K. L. Martin, writing in Oil News,a discusses furnace design for burning fuel oil as follows : "An authority on oil burn ing recently stated that the selection of a burner, while important, was secondary to the proper design of the furnace. Nine-tenths of the trouble experienced in the installation of oil burners could be avoided if the proper attention were paid to getting the com bustion space large enough and to locating the walls opposite the burner far enough away so the flame does not strike them. A study of the best stationary boiler practice using steam atomizing furnaces would indicate that a ratio of one cubic foot of furnace volume should be provided for every boiler horsepower to be de veloped. In other words, a 500-horsepower boiler which is ex pected to run at 200 percent of rating should have approximately 1,000 cubic feet of space below the tubes. Furnaces have unques tionably been operated with proportionately smaller combustion space but the constant tendency of all furnace design, not only for oil but for powdered coal, and modern stokers is decidedly for larger combustion space.
This has resulted in higher boiler settings, fourteen feet from the floor to the bottom of the front header being common, and in the moving back of the bridge wall to a point ten, eleven, or even more feet from the front wall. As installations are frequently made in boilers where the height is fixed and usually too low, the most common way to get the necessary furnace volume is to move back the bridge wall. Until recently this has made neces sary the laying of a horizontal shelf of T-tile on the lower row of tubes to joint the old cross baffle with the top of the bridge wall in its new position. This practice had several objections : 1. The T-tile must necessarily be small in order to get them in place and the resulting mosaic is full of open joints through which a quantity of hot gases short circuit directly from the fur nace chamber to the third pass and escape up the stack.
2. Those gases which do not escape travel along underneath the baffle until they meet the elbow formed by the horizontal and vertical baffles. The tubes at this point are already exposed to the radiant heat of the flame and to the gases rising directly from the front of the furnace, and the resulting concentration of the heat is often too much for the tubes and failures are frequent.
3. The horizontal baffle forms a shelf on which the soot is deposited and while this deposit is not as troublesome as in coal burning boilers, it still has to be reckoned with. These troubles been remedied by of a baffle wall so constructed that, while absolutely gas tight, it can be built at any desired in clination and the horizontal baffle entirely eliminated. One of the first applications is shown in fig. 44. This boiler was originally coal burning and was converted to oil burning in a manner all too frequent—by taking out the grates, laying a checker work and inserting a couple of burners through the front wall. At the end of six months they had replaced over a hundred tubes, the remainder were bent so that the tubes in some rows were down on the tube in the next, the furnace linings had been replaced several times and complaints from the authorities as to the smoke were insistent. Neither the ratings nor the economies anticipated had been obtained. Realizing the opportunity for better design made possible by the new type of baffle wall, the bridge wall was moved back to a point ten feet from the front wall so that the flame no longer played upon it. The horizontal shelf and right angle baffle was replaced by a long inclined baffle wall starting from the top of-the wall and making an angle of 45° with the tubes.. At the same time the floor of the furnace was lowered 42 inches.
This furnace has now been in continuous service for nearly three years and the original liningS are in the furnace. No tubes have been renewed except about three months ago. A few of the worst of the bent ones left in when the change was made, with the expectation they would soon burn out, were replaced. High ratings and satisfactory economy have been realized. No repairs to the baffles have been necessary. It will be noted that the wide open throat of the first pass gives every opportunity for the radiant heat from the flame and the reflected heat from the fur nace walls and floor to strike the tubes. The wide opening also means a low velocity for the gases and abundant time for their heat to be transmitted through the steel walls of the tubes to the water inside.
The gases are cooled as they pass by the tubes and naturally shrink in volume and tend to draw away from the front header, leaving a dead space, at its top. The inclined wall contracts the space as the gases cool, so that they need every cubic inch of space to get through and every square inch of heating is flooded with hot gas. This action is continued through the second and third passes. The result is shown in fig. 45.
In another installation a low setting had been used in connec tion with coal fires. Before the existence of the new baffle was known, the bridge wall was moved back, a horizontal shelf built and a back shot burner installed. At the end of 54 days they had been unable at any time to develop more than rating for the boiler, and they had lost 12 tubes. The inclined baffle (fig. 46) was installed in a similar boiler alongside the first as an experi ment and at the end of 57 days no tubes had been replaced and they had carried a load averaging 200% of rating. As this meant a development of 100,000 more horsepower per year per boiler, the first boiler was immediately rebaffled and has since given equally good results." The application of fuel oil burners to any type of furnace is easily performed. Fig. 47 shows an oil burner under a vertical tubular boiler. Fig. 48 shows an oil-burning system for Scotch Marine boilers. Fig. 49 shows an oil-burning system applied to the Stirling water-tube boiler and fig. 50 to a return-tubular boiler. Fig. 51 shows a Babcock and Wilcox Oil Furnace, patented.
The same procedure is gone through in the design for stacks for oil fuel firing as for coal burning. The required draft in the furnace at maximum overload in each case is obtained by -the necessary height and the maximum volume of gases generated letermines the proportion of the area of the stack. When coal is burned there is seldom any danger of too much draft, but the economy of oil-fired furnaces is greatly affected by excessive draft and for this reason the various draft losses through the boiler and breaching must be estimated very carefully. A bed of coal on the grate occasions loss of draft, but with oil fuel this loss is negligible and in addition on account of the smaller volume of gases discharged per boiler horsepower hour, the pressure loss through the boiler will be less than with coal. To a more or less degree the action of the burner itself acts as a forced draft. For this reason both the height and area of a stack for any given capacity of boiler will be less for oil firing than for coal firing. Mr. C. R. Weymoutha has prepared the most authoritative table for proportioning stacks for oil fuel. The data Mr. Weymouth are given in Table 14.