Fig. 81.

By these formulæ may be calculated the weight of air inside and outside a chimney. The difference of the two is the pressure to produce draught per foot of chimney height.

Calling D and d the greater and less densities the equivalent height of a column for any chimney of height=h ft. will be L=h() and the velocity of flow per second will be

v=v2gL where L is the equivalent column in feet. In all the foregoing the specific gravity of furnace gas is assumed equal to that of air of the same temperature, the steam balancing the carbonic acid more or less closely.

Seeing that draught is of less importance with liquid fuel, it is permissible to reduce the furnace products to a lower temperature if facilities can be had for doing this. The smaller excess of air with which perfect combustion can be secured is a factor in rendering more efficient the heating surfaces of the boiler, and reduced flue gas temperatures are a natural consequence of liquid fuel.

A chimney must be large enough to pass all the products of a furnace at a certain given velocity of flow. The calculation of chimney area is thus simple. Assuming the velocity of flow of gas to be 30 feet per second, it is simply necessary to divide the volume of gas produced per second by 30. The result is the area in square feet of the chimney. To find the volume of gas produced per second, the coal consumption per second is first found as follows in pounds-

P=

W × 2,240
H x 3,600

VA,

where W is the daily coal consumption in tons and H the daily hours. Then P×20=pounds of gas G. At ordinary temperatures one pound of gas measures 13 cubic feet very closely. At the chimney temperature it will measure 20 to 25 feet. Let 22 be assumed: then G x 22÷30 will give the area of the chimney inside=A. The chimney will measure, if square, on each side, or, if round, its diameter will be D=1.128 A. Chimneys for liquid fuel are similarly calculated from the weight of fuel burned, and the ratio of P to G will still be 1:20, for though a pound of oil uses up more air than a pound of coal, the excess supplied will be less, and in each case the furnace gases will weigh about twenty times the fuel burned.

When coal is burned it rests upon a grate surface, the bars of which and the fuel resting upon and obstructing the spaces offer great resistance to the inflow of air. A fan or a chimney is necessary to draw in the air which is necessary to burn the coal.

With oil it is quite different. Air is either blown in with the oil or induced through free openings by the action of the steam or air atomizer, and a very small draught will draw in enough air for perfect combustion, and it is usually considered necessary rather to check the flow of the gases through the flues, only sufficient draught being required to remove the products of combustion as formed. Chimneys of small altitude will do this, for they do not require to overcome any grate or fuel

bed resistance. In locomotives, for example, the steam-jet may be considerably reduced in the chimney, and on the Great Eastern Railway of England the MacAllan variable blast-pipe is enlarged from 5 inches with coal to 5 inches diameter with oil to the reduction of the back pressure on the pistons and economy of steam in consequence. In foreign locomotive practice it is usual to employ caps over the chimney-top in order to save the loss of heat when running down grade or standing idle. Mr. Urquhart continued to use this cap with his oil-fired engines, and though it presents an odd appearance to English eyes, the cap has advantages. Applied to stationary work it is represented ordinarily by a damper at the chimney-base, and is thus recognized as good, but it is not used in locomotive work. It affords, however, a ready means of regulating the fires, and cannot quite be replaced by the ash-pit damper, which is heavier to work and is by no means always so tight-shutting as it should be.

A

very usual remedy for a bad draught in coal-fired furnaces is a steam jet. This is very effective in moving the gases forward to a poor chimney, which then appears able to deal with them. In oil-firing this aid to draught is inevitably present in the atomizer, which really replaces the need for a certain chimney or fan effect. The area for chimneys must not be calculated from the horse-power to be developed at a station. The actual fuel consumption should be worked from. The fuel per horse-power hour will vary according to the load-factor and other conditions, and large stations will use less fuel per horse-power hour than will small stations with smaller load-factors. Each case must stand by itself. A very small draught will give a velocity of 30 feet per second. The resistance to draught is chiefly the fuel on the grate and the long flues, especially if narrow, and the economizer.

Ordinary rules for chimneys provide for areas that will reduce the velocity of flow to much less than the foregoing 30 feet per second, but it is doubtful if such large areas are necessary with liquid fuel, and it is certain that a chimney hitherto used for solid fuel will serve well when a change is made to liquid fuel. Experience so far is lacking on the question of chimney practice for liquid fuel work, but the subject may be approached from the standpoint above, viz., that with liquid fuel not only is the resistance of the fuel on the grate eliminated but there is added a propelling force in the atomizer which, if applied to a poor draught in a coal-burning furnace, would render such draught good and sufficient. Bearing these points in mind, the ordinary treatises on draught may be studied with advantage as regards the effect of height upon velocity of flow. But the ordinary rules otherwise have very little application to liquid fuel conditions.

Chapter XXIX

THE ATOZIZING OF LIQUID FUEL

INCE liquid fuel of the heavy varieties cannot be burned except by the system

SING

of atomizing, the burner, injector, sprayer or atomizer, as it is variously termed, is an important detail in any oil burning system.

The object aimed at is the pulverizing of the liquid fuel, so that, mixed with air in the act of pulverization, and supplied with any further amount of air that may be necessary, the liquid atoms may burn like vapour. The spray must not be so directed that an intense blow-pipe flame impinges severely upon any small area of furnace plate.

It is sought to fill the furnace with a full soft voluminous flame which shall envelope the whole interior of the furnace. Given a sufficiently long space in front of the burner, a spray jet directed straight ahead and gradually coning out would doubtless produce a satisfactory effect, but the space between the point of the burner and that part of the cone of flame which first touched the furnace plate would be of little use as heating surface. What should be aimed at is such a burner and spray device as will produce a certain disrupture and outward expanding effect, so as at once to spread the oil to a considerable extent normally to the axis of the burner as well as parallel; to give a sort of balloon effect, so that, in a locomotive boiler for example, there shall be flame well to the back of the box as well as forward under the arch. Various forms of atomizers will be found illustrated here or elsewhere, including—

The Holden (figs. 21, 24 and 27).

The Baldwin (fig. 37).

The Urquhart (fig. 45).

The Guyot (fig. 89).

The Rusden and Eeles (figs. 7 and 8la).
The Billow (fig. 47).

The Oil City Boiler Works Co. (figs. 9 and 13, The Williams (fig. 102).

Appendix 3).

The Hayes (fig. 11, Appendix 3).

The Grundell-Tucker (fig. 5, Appendix 2).
The Hydroleum Co. (fig. 81).

The Swensson (fig. 86).

The Aerated Fuel Co. (fig. 50).

The F. M. Reed (fig. 14, Appendix 3).
Kermode's Burner (fig. 82).

Orde's (fig. 10).

Körting's (figs. 19, 19a).

The Holden Atomizer.

The Holden Injector (figs. 21, 27, 24) consists of a gun-metal casing with oil, air and steam inlets. Air comes in at the back, preferably hot, and is delivered at the point where the oil escapes to the inner nozzle. Steam comes between the oil and air, and the mixed jet escapes forward and slightly laterally by two orifices. A further air supply is directed upon the spray by a ring of several fine jets of steam.

The atomized fuel is directed along the plane of the fire when the fire-bars are retained, as this gives the best action. Mr. Holden does not confine himself to the use of steam as an atomizing agent, but recognizes that air may be preferable for chemical reasons. Two burners deal with about six pounds of oil each per mile, or, say, 240 pounds per hour.

Rusden and Eeles.

In this burner (fig. 7 and fig. 81a), steam escapes by a central annular jet, and

is directed outwards on a fine annular jet of oil, which is heated also by a steam jacket. This disposition gives a balloon flame. The burner is largely used in marine work.

The Urquhart.

This burner (fig. 45), one of the earliest of the successful atomizers, employs central steam, external air, and an annular oil jet between the two, the expansion of the steam atomizing the oil into the air and mixing the two.

For tyre heating, Mr. Urquhart used the arrangement of fig. 112a, the oil simply dropping into a jet of air under pressure, which induces also a supply of air round the oil supply.

The Baldwin Company's Burner (fig. 37).

The burner is very simple, being simply a broad thin jet of steam which is directed upon oil escaping from a parallel passage. It could not well be simpler, but it is claimed to act well, and there appears no reason to doubt this.

The Aerated Fuel Company's Burner.

This is of the central air jet type, as shown in fig. 50.

The Oil City Boiler Company's Burner.

Fig. 9, Appendix 3, shows this Company's air spray burner, air being admitted

centre.

outside a central oil jet and directed inwards upon the oil which escapes at the This burner was used in the tests of the Bureau of Steam Engineering, Nos. 1 to 8. These burners were arranged in pairs, so that they inclined towards each other, and produced impinging flames. Each burner dealt with about 140 pounds of oil per hour. The same company supplied the steam actuated burner (fig. 13, Appendix 3), the oil being here admitted by a cone or needle valve, and steam being admitted outside. Both these burners appear to have given satisfaction, and in both it was found that the best results followed on the highest oil pressure, which in tests 10, 11, 12, was 20, 30, and 45 pounds per square inch respectively. High steam pressure was also conducive to efficiency, but more steam was used. It appears to be thought also that higher air pressure would be an improvement in the case of air burners.

The F. M. Reed Burner.

Fig. 14, Appendix 3, is a combined air and steam burner, not recommended as economical in steam. Here the provision for air is very large, and in no way resembles the small air passage of the Holden burner, and it would appear as though the ring jets of the Holden burner were more efficient than the air bulbs of this burner.

The Grundell-Tucker Burner.

This, as used in the ss. Mariposa trials, Appendix 2, is given in fig. 5 of Appendix 2, the mixed oil and air being gyrated by special blades and ballooned out. Air was used at 40 lb. pressure and heated. The oil enters the air stream through several small holes drilled normal to the air jet. Twenty-four burners appear to deal with 3,000 to 3,400 pounds of oil per hour, or 125 to 140 pounds per hour each.

What apparently is necessary to good operation is an air pressure not under 15 lb. by gauge, while even better results will accompany higher pressures. High pressure of the oil also appears good, probably because of the self-atomizing tendency of the oil as it escapes through the fine openings necessary where the pressure is considerable.

The Hayes Burner.

Fig. 11 of Appendix 3 was not found to be a success, probably because the long pipe nozzle destroyed the atomizing effect. Atomizing, to be a success, apparently requires to be performed close to the point of escape into the furnace. Fig. 81b shows the nozzle of the Hydroleum Company's burner. Oil is cen

trally regulated by a needle, and issues from a mouthpiece flared out externally