a plug, the slots being curved at the end to give the necessary rotary motion to the oil as it enters the conical chamber in the burner tip. The position of the slotted plug in relation to the burner tip may be adjusted while the burner is in operation by means of a rod passing through the burner.

Coen Burner. In this burner, which is illustrated in Fig. 109 the oil is delivered to the central chamber by means of small tangential channels, whose area can be altered by means of a rod inside the burner running its full length, operated by the hand-wheel of a special angle valve. It is thus possible to regulate the fire from individual burners without altering the oil pressure.

FIG. 109.-The Coen mechan

The main air supply is controlled by a sliding plate in front of the cylindrical burner chamber, and a sliding cylinder surrounding this chamber controls a secondary air supply.

Draft.-Mechanical atomizing burners operate satisfactorily with natural draft at limited capacities, but if the boilers are to be forced much above their rated capacity it is necessary to equip them with forced draft fans, delivering the air under a slight pressure to the burner air chambers.

Pressure and Temperature of Oil.-The operation of mechanical atomizing burners varies with both the pressure and temperature of the oil. The pressure must be at least 25 lb. in order to produce good atomization and it may be increased up to 200 lb. Pressures higher than 200 lb. are not required and only cause unnecessary ical atomizing stress of the oil piping. The quantity of oil burned is regulated by varying the pressure from 25 lb. up to 200 lb. If more oil is required than can be obtained at the higher pressure it is necessary in most makes of burner to change the nozzles, using tips with larger orifices. If less oil is needed than passes through the burners at 25 lb. pressure it is necessary to shut off some of the burners.

burner.

A temperature of at least 150°F. is required to properly atomize the oil. The atomization is improved by increasing the temperature up to about 200°. Above this temperature no change is made in the atomization but the flame becomes shorter and the

combustion occurs closer to the burner as the oil is heated to a higher temperature. As the oil is heated the first effect is to reduce its viscosity resulting in a greater quantity of oil passing through the burner. At the higher temperatures, however, the increased temperature has little effect on the viscosity, but owing to the increased volume of the oil the capacity of the burner is reduced.

The following table gives the quantity of oil that will flow through a burner having a 16 in. diameter orifice at a pressure of 200 lb., the oil having a gravity of 20° Baumé and a flash point of 220°F.:

Advantages of Mechanical Atomizing. The principal advantage of the mechanical atomizing system is that a large quantity of oil can be burned in a furnace of a given volume.

The steam atomizing burner as applied to stationary boilers produces a flat flame and the combustion occurs on the upper and lower surfaces of this flame, with the result that a considerable proportion of the volume of the furnace is not made use of. With the mechanical atomizing burner, on the other hand, there is what may be called volume combustion, the mixture of air and gases occurring throughout the entire furnace. It is therefore, possible to completely burn more oil in a given size of furnace by this method than by the steam atomizing method, or if the same quantity of oil is burned in both cases the combustion with the mechanical atomizing system will be more complete in the furnace proper, so that the flame will not travel so far among the boiler tubes and the entire efficiency of the boiler will be greater. The cooling of the gases by their coming in contact with the boiler tubes before the combustion is completed is one of the principal causes of low efficiency in boilers that are forced

FIG. 110.-The mechanical fuel oil atomizers installed under two 250 h.p. Heine boilers. The view is the standard equipment at eleven pumping stations of the Shell Company of California oil pipe line, installed by Sanderson and Porter. The operation is entirely automatic, oil being supplied to the burners under the control of the pump governors so as to maintain constant pressure.

up to high capacities, and any method of combustion that prevents this occurring tends to improve the efficiency of the boiler.

The maximum quantity of oil burned by steam atomizing burners amounts to 6 or 7 lb. of oil per hour per cu. ft. of furnace volume, whereas with mechanical atomizing burners as much as 11 or 12 lb. per hour per cu. ft. has been burned.

The mechanical atomizing system also saves the steam that is used in the steam atomizing system, which often amounts to 3 per cent. or 4 per cent. of the total steam generated. While some additional steam is required in the mechanical system for heating the oil up to the higher temperature and for pumping it to the higher pressure, the condensation or exhaust from this steam returns to the feed water heater so that the quantity utilized is not great. The loss of the steam used by the steam atomizing burner is of much greater importance for marine boilers than for stationary boilers, because it means a loss of fresh water as well as a loss of steam. It is for this reason that mechanical atomizing burners have already largely displaced steam atomizing burners on board ship.

Disadvantages of Mechanical Atomizing. The principal disadvantage of the mechanical atomizing burner is that there is aways a tendency for the small orifice in the burner to choke up. This is sometimes due to grit and other solids in the oil which pass through the strainers and sometimes due to carbonization on account of the flame occurring close to the nozzle. To overcome this difficulty it is necessary in some cases to merely slacken back the feed screw of the burner and then readjust it, and in some cases to turn off the oil altogether and blow steam through the burner. Whenever the fire is burning one sided, sparking on one side or showing black streaks it is a sign that the burner needs cleaning. As the number of burners required for mechanical atomization is greater than for steam atomization this difficulty means a good deal of careful attention on the part of the operator. With steam atomizing, three burners are sufficient to operate a large 800 h.p. boiler, whereas the same size boiler would require 6 or 8 mechanical atomizing burners.

Another objection to mechanical atomizing burners for stationary power plant work is the fact that the air supply for each burner must be adjusted separately every time there is a change in the quantity of oil burned. This is not objectionable where there

is a steady load as occurs on board ship, but in a stationary power plant where the load is changing up and down continuously a continual adjustment of the air supply means a large amount of attention on the part of the fireman. This difficulty, however, may be overcome by the use of automatic regulation of the air, applying to mechanical atomizers the same principles that have already proved successful in the automatic regulation of steam atomizing burners.

Until recently the use of mechanical atomizing burners in stationary plants has been confined to small stations operating at a fairly steady load, such as the pumping stations on oil pipe lines. Recent installations have proved that the system can be applied to large boilers, and is desirable especially where high capacities are required, capacities as high as 300 per cent. of the boiler rating having been obtained. It must be realized, however, that the use of this system for variable load boilers is still in the experimental stage, at the time of writing. (April, 1920).

Recent research along the lines of mechanical atomization by D. S. Jacobus and N. E. Lewis is of great importance in future. development of steam electric generation, where oil is used as fuel. As a consequence, in the following pages we shall set forth in full the data presented by these gentlemen before the Pasadena Convention of the National Electric Light Association in May, 1920, on this timely subject:

The practice in central stations of running boilers up to 300 per cent. of rating in carrying peak loads has made it necessary to consider some other means of burning the oil than with steam atomizing burners, as a boiler that would normally be operated over peak load intervals at 300 per cent. of rating or thereabouts with coal could not be operated at over about 200 per cent. of rating with steam atomizing burners.

The limitation of capacity with steam atomizing burners led the Babcock & Wilcox Company to make a series of tests to develop mechanical atomizing burners of the type used in marine practice that would be especially adapted to stationary boilers.

The tests indicated that the economy obtainable with steam atomizing and with mechanical atomizing oil burners at the lower capacities was about the same. At the higher capacities the efficiency with the mechanical atomizing burners was higher than with the steam atomizing burners. Considerably higher capacities could be obtained with the mechanical atomizing burn