Fig. 28. Peabody Oil Burner
In the outside mixing class the steam flows through a narrow slot or horizontal row of small holes in the burner nozzle; the oil flows through a similar slot or hole above the steam orifice, and is picked up by the steam outside of the burner and is atomized. Fig. 28 shows a type of the Peabody burner of this class, which has [Pg 219] given eminent satisfaction. The construction is evident from the cut. It will be noted that the portions of the burner forming the orifice may be readily replaced in case of wear, or if it is desired to alter the form of the flame.
Where burners of the spray type are used, heating the oil is of advantage not only in causing it to be atomized more easily, but in aiding economical combustion. The temperature is, of course, limited by the flash point of the oil used, but within the limit of this temperature there is no danger of decomposition or of carbon deposits on the supply pipes. Such heating should be done close to the boiler to minimize radiation loss. If the temperature is raised to a point where an appreciable vaporization occurs, the oil will flow irregularly from the burner and cause the flame to sputter.
On both steam and air atomizing types, a by-pass should be installed between the steam or air and the oil pipes to provide for the blowing out of the oil duct. Strainers should be provided for removing sludge from the fuel and should be so located as to allow for rapid removal, cleaning and replacing.
Mechanical burners have been in use for some time in European countries, but their introduction and use has been of only recent occurrence in the United States. Here as already stated, the means for atomization are purely mechanical. The most successful of the mechanical atomizers up to the present have been of the round flame type, and only these will be considered. Experiments have been made with flat flame mechanical burners, but their satisfactory action has been confined to instances where it is only necessary to burn a small quantity of oil through each individual burner.
This system of oil burning is especially adapted for marine work as the quantity of steam for putting pressure on the oil is small and the condensed steam may be returned to the system.
The only method by which successful mechanical atomization has been accomplished is one by which the oil is given a whirling motion within the burner tip. This is done either by forcing the oil through a passage of helical form or by delivering it tangentially to a circular chamber from which there is a central outlet. The oil is fed to these burners under a pressure which varies with the make of the burner and the rates at which individual burners are using oil. The oil particles fly off from such a burner in straight lines in the form of a cone rather than in the form of a spiral spray, as might be supposed.
With burners of the mechanical atomizing design, the method of introducing air for combustion and the velocity of this air are of the greatest importance in securing good combustion and in the effects on the character and shape of the flame. Such burners are located at the front of the furnace and various methods have been tried for introducing the air for combustion. Where, in the spray burners, air is ordinarily admitted through a checkerwork under the burner proper, with the mechanical burner, it is almost universally admitted around the burner. Early experiments with these air distributors were confined largely to single or duplicate cones used with the idea of directing the air to the axis of the burner. A highly successful method of such air introduction, developed by Messrs. Peabody and Irish of The Babcock & Wilcox Co., is by means of what they term an “impeller plate”. This consists of a circular metal disk with an opening at the center for the oil burner and with radial metal strips from the center to the periphery turned at an angle which in the later designs may be altered to give the air supply demanded by the rate of combustion.
The air so admitted does not necessarily require a whirling motion, but experiments show that where the air is brought into contact with the oil spray with the right “twist”, better combustion is secured and lower air pressures and less refinement of adjustment of individual burners are required.
Mechanical burners have a distinct advantage over those in which steam is used as the atomizing agent in that they lend themselves more readily to adjustment under wider variations of load. For a given horse power there will ordinarily be installed a much greater number of mechanical than steam atomizing burners. This in itself is a means to better regulation, for with the steam atomizing burner, if one of a number is shut off, there is a marked decrease in efficiency. This is due to the fact that with the air admitted under the burner, it is ordinarily passing through the checkerwork regardless of whether it is being utilized for combustion or not. With a mechanical burner, on the other hand, where individual burners are shut off, air that would be admitted for such burner, were it in operation, may also be shut off and there will be no undue loss from excess air.
Further adjustment to meet load conditions is possible by a change in the oil pressure acting on all burners at once. A good burner will atomize moderately heavy oil with an oil pressure as low as 30 pounds per square inch and from that point up to 200 pounds or above. The heating of the oil also has an effect on the capacity of individual burners and in this way a third method of adjustment is given. Under working conditions, the oil pressure remaining constant, the capacity of each burner will decrease as the temperature of the oil is increased though at low temperatures the reverse is the case. Some experiments with a Texas crude oil having a flash point of 210 degrees showed that the capacity of a mechanical atomizing burner of the Peabody type increased from 80 degrees Fahrenheit to 110 degrees Fahrenheit, from which point it fell off rapidly to 140 degrees and then more slowly to the flash point.
The above methods, together with the regulation possible through manipulation of the boiler dampers, indicate the wide range of load conditions that may be handled with an installation of this class of burners.
As has already been stated, results with mechanical atomizing burners that may be considered very successful have been limited almost entirely to cases where forced blast of some description has been used, the high velocity of the air entering being of material assistance in securing the proper mixture of air with the oil spray. Much has been done and is being done in the way of experiment with this class of apparatus toward developing a successful mechanical atomizing burner for use with natural draft, and there appears to be no reason why such experiments should not eventually produce satisfactory results.
Steam Consumption of Burners—The Bureau of Steam Engineering, U. S. Navy, made in 1901 an exhaustive series of tests of various oil burners that may be considered as representing, in so far as the performance of the burners themselves is concerned, the practice of that time. These tests showed that a burner utilizing air as an atomizing agent, required for compressing the air from 1.06 to 7.45 per cent of the total steam generated, the average being 3.18 per cent. Four tests of steam atomizing burners showed a consumption of 3.98 to 5.77 per cent of the total steam, the average being 4.8 per cent.
Improvement in burner design has largely reduced the steam consumption, though to a greater degree in steam than in air atomizing burners. Recent experiments [Pg 221] show that a good steam atomizing burner will require approximately 2 per cent of the total steam generated by the boiler operated at or about its rated capacity. This figure will decrease as the capacity is increased and is so low as to be practically negligible, except in cases where the question of loss of feed water is all important. There are no figures available as to the actual steam consumption of mechanical atomizing burners but apparently this is small if the requirement is understood to be entirely apart from the steam consumption of the apparatus producing the forced blast.
Capacity of Burners—A good steam atomizing burner properly located in a well-designed oil furnace has a capacity of somewhat over 400 horse power. This question of capacity of individual burners is largely one of the proper relation between the number of burners used and the furnace volume. In some recent tests with a Babcock & Wilcox boiler of 640 rated horse power, equipped with three burners, approximately 1350 horse power was developed with an available draft of .55 inch at the damper or 450 horse power per burner. Four burners were also tried in the same furnace but the total steam generated did not exceed 1350 horse power or in this instance 338 horse power per burner.
From the nature of mechanical atomizing burners, individual burners have not as large a capacity as the steam atomizing class. In some tests on a Babcock & Wilcox marine boiler, equipped with mechanical atomizing burners, the maximum horse power developed per burner was approximately 105. Here again the burner capacity is largely one of proper relation between furnace volume and number of burners.
Furnace Design—Too much stress cannot be laid on the importance of furnace design for the use of this class of fuel. Provided a good type of burner is adopted the furnace arrangement and the method of introducing air for combustion into the furnace are the all important factors. No matter what the type of burner, satisfactory results cannot be secured in a furnace not suited to the fuel.
The Babcock & Wilcox Co. has had much experience with the burning of oil as fuel and an extended series of experiments by Mr. E. H. Peabody led to the development and adoption of the Peabody furnace as being most eminently suited for this class of work. Fig. 29 shows such a furnace applied to a Babcock & Wilcox boiler, and with slight modification it can be as readily applied to any boiler of The Babcock & Wilcox Co. manufacture. In the description of this furnace, its points of advantage cover the requirements of oil-burning furnaces in general.
The atomized oil is introduced into the furnace in the direction in which it increases in height. This increase in furnace volume in the direction of the flame insures free expansion and a thorough mixture of the oil with the air, and the consequent complete combustion of the gases before they come into contact with the tube heating surfaces. In such a furnace flat flame burners should be used, preferably of the Peabody type, in which the flame spreads outward toward the sides in the form of a fan. There is no tendency of the flames to impinge directly on the heating surfaces, and the furnace can handle any quantity of flame without danger of tube difficulties. The burners should be so located that the flames from individual burners do not interfere nor impinge to any extent on the side walls of the furnace, an even distribution of heat being secured in this manner. The burners are operated from the boiler front and peepholes are supplied through which the operator may watch the flame while regulating the burners. The burners can be removed, inspected, or cleaned and replaced in a few minutes. Air is admitted through a checkerwork of fire brick supported on the furnace floor, the openings in the checkerwork being so arranged as to give the best economic results in combustion.
[Pg 222]
[Pg 223] With steam atomizing burners introduced through the front of the boiler in stationary practice, it is usually in the direction in which the furnace decreases in height and it is with such an arrangement that difficulties through the loss of tubes may be expected. With such an arrangement, the flame may impinge directly upon the tube surfaces and tube troubles from this source may arise, particularly where the feed water has a tendency toward rapid scale formation. Such difficulties may be the result of a blowpipe action on the part of the burner, the over heating of the tube due to oil or scale within, or the actual erosion of the metal by particles of oil improperly atomized. Such action need not be anticipated, provided the oil is burned with a short flame. The flames from mechanical atomizing burners have a less velocity of projection than those from steam atomizing burners and if introduced into the higher end of the furnace, should not lead to tube difficulties provided they are properly located and operated. This class of burner also will give the most satisfactory results if introduced so that the flames travel in the direction of increase in furnace volume. This is perhaps best exemplified by the very good results secured with mechanical atomizing burners and Babcock & Wilcox marine boilers in which, due to the fact that the boilers are fired from the low end, the flames from burners introduced through the front are in this direction.
Fig. 29. Babcock & Wilcox Boiler, Equipped with a Peabody Oil Furnace
Operation of Burners—When burners are not in use, or when they are being started up, care must be taken to prevent oil from flowing and collecting on the floor of the furnace before it is ignited. In starting a burner, the atomized fuel may be ignited by a burning wad of oil-soaked waste held before it on an iron rod. To insure quick ignition, the steam supply should be cut down. But little practice is required to become an adept at lighting an oil fire. When ignition has taken place and the furnace brought to an even heat, the steam should be cut down to the minimum amount required for atomization. This amount can be determined from the appearance of the flame. If sufficient steam is not supplied, particles of burning oil will drop to the furnace floor, giving a scintillating appearance to the flame. The steam valves should be opened just sufficiently to overcome this scintillating action.
Air Supply—From the nature of the fuel and the method of burning, the quantity of air for combustion may be minimized. As with other fuels, when the amount of air admitted is the minimum which will completely consume the oil, the results are the best. The excess or deficiency of air can be judged by the appearance of the stack or by observing the gases passing through the boiler settings. A perfectly clear stack indicates excess air, whereas smoke indicates a deficiency. With properly designed furnaces the best results are secured by running near the smoking point with a slight haze in the gases. A slight variation in the air supply will affect the furnace conditions in an oil burning boiler more than the same variation where coal is used, and for this reason it is of the utmost importance that flue gas analysis be made frequently on oil-burning boilers. With the air for combustion properly regulated by adjustment of any checkerwork or any other device which may be used, and the dampers carefully set, the flue gas analysis should show, for good furnace conditions, a percentage of CO2 between 13 and 14 per cent, with either no CO or but a trace.
In boiler plant operation it is difficult to regulate the steam supply to the burners and the damper position to meet sudden and repeated variations in the load. A device [Pg 224] has been patented which automatically regulates by means of the boiler pressure the pressure of the steam to the burners, the oil to the burners and the position of the boiler damper. Such a device has been shown to give good results in plant operation where hand regulation is difficult at best, and in many instances is unfortunately not even attempted.
Efficiency with Oil—As pointed out in enumerating the advantages of oil fuel over coal, higher efficiencies are obtainable with the former. With boilers of approximately 500 horse power equipped with properly designed furnaces and burners, an efficiency of 83 per cent is possible or making an allowance of 2 per cent for steam used by burners, a net efficiency of 81 per cent. The conditions under which such efficiencies are to be secured are distinctly test conditions in which careful operation is a prime requisite. With furnace conditions that are not conductive to the best combustion, this figure may be decreased by from 5 to 10 per cent. In large properly designed plants, however, the first named efficiency may be approached for uniform running conditions, the nearness to which it is reached depending on the intelligence of the operating crew. It must be remembered that the use of oil fuel presents to the careless operator possibilities for wastefulness much greater than in plants where coal is fired, and it therefore pays to go carefully into this feature.
[Table 48] gives some representative tests with oil fuel.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Burning Oil in Connection with Other Fuels—Considerable attention has been recently given to the burning of oil in connection with other fuels, and a combination of this sort may be advisable either with the view to increasing the boiler [Pg 225] capacity to assist over peak loads, or to keep the boiler in operation where there is the possibility of a temporary failure of the primary fuel. It would appear from experiments that such a combination gives satisfactory results from the standpoint of both capacity and efficiency, if the two fuels are burned in separate furnaces. Satisfactory results cannot ordinarily be obtained when it is attempted to burn oil fuel in the same furnace as the primary fuel, as it is practically impossible to admit the proper amount of air for combustion for each of the two fuels simultaneously. The Babcock & Wilcox boiler lends itself readily to a double furnace arrangement and Fig. 30 shows an installation where oil fuel is burned as an auxiliary to wood.
Fig. 30. Babcock & Wilcox Boiler Set with Combination Oil and Wood-burning Furnace
Water-gas Tar—Water-gas tar, or gas-house tar, is a by-product of the coal used in the manufacture of water gas. It is slightly heavier than crude oil and has a comparatively low flash point. In burning, it should be heated only to a temperature which makes it sufficiently fluid, and any furnace suitable for crude oil is in general suitable for water-gas tar. Care should be taken where this fuel is used to install a suitable apparatus for straining it before it is fed to the burner.
[Pg 226]
Babcock & Wilcox Boilers Fired with Blast Furnace Gas at the Bethlehem Steel Co., Bethlehem, Pa. This Company Operates 12,900 Horse Power of Babcock & Wilcox Boilers