Steam, Its Generation and Use

by Babcock & Wilcox Company

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.