To obtain the full heat value of a liquid fuel it must be provided with sufficient air to complete the combustion, it must be in a very finely subdivided state, or in the form of a vapor at the time of ignition, and it must be thoroughly mixed with the air so that every part of the oil is in contact which its chemical equivalent of oxygen. Failure to comply with any of these conditions will not only result in a waste of fuel but will also be the cause of troublesome carbon deposits and soot, which eventually will interfere with the operation of the burner.
Complete combustion is much more easily attained with the lighter hydrocarbons such as gasoline or naptha than with crude oil or the heavier distillates, for they are more readily vaporized and mix more thoroughly with the oxygen. Only a slight degree of heat and pressure is required with gasoline while with crude oil a high atomizing pressure and high temperature are required to obtain a satisfactory flame. In the majority of cases where heavy oils are used the fuel is not even completely vaporized but enters the combustion chamber in the form of a more or less finely atomized spray. The methods by which the liquid fuel is broken up divides the burners into three primary classes.
(1) LOW PRESSURE BURNERS in which the fuel is atomized by a blast of low pressure air which also supplies a considerable percentage of the air required for combustion.
(2) HIGH PRESSURE BURNER in which a small jet of high pressure air or steam is used to atomize the oil, the air for combustion being supplied from a source external to the burner.
(3) COMBINED HIGH AND LOW PRESSURE BURNER in which the fuel is atomized by high pressure air or steam, and the greater part of the air for combustion is furnished by a blower at a comparatively low pressure.
In class (1) the oil is supplied to the burner under pressure and by means of a specially designed jet is thrown against hot baffle plates or gauze screens where the partially broken up liquid is caught by the high velocity air and reduced to a still finer spray by its impact against other screens or baffles further on in the burner. This system is applicable only to the light and intermediate grades of oils, such as gasoline, naptha or kerosene, unless heat is applied to the external casing to aid in the vaporization. In some cases the projection of the burner into the furnace gives satisfactory results, but with such an arrangement there is a tendency to deposit carbon in the burner and for the flame to “strike back” should the velocity of the air fall below a certain critical point. Better results were had with this type of burner, by the author when the air blast was preheated by passing several long lengths of the intake air pipe over a hot part of the furnace, instead of entering the burner nozzle into the combustion chamber proper.
A well known modification of this type is the gasoline torch used by electricians and plumbers in which the gasoline is sprayed into a perforated hot tube by air pressure in the tank. When the spray formed at the needle valve strikes the surrounding hot tube it is instantly vaporized and is mixed with the air passing through the perforations in the tube. While the air entering the tube is not forced through the openings by external pressure it attains sufficient velocity to aid in the vaporization because of the vacuum established by the jet. This however is only enough for the more volatile fuels—such as gasoline or benzine.
The high pressure which is by far the most commonly used with low grade fuels may be divided into five principal types (a) ATOMIZER burner, (b) The INJECTOR burner, (c) DRIP feed burner, (d) CHAMBER OR INTERNAL burner, (e) EXTERNAL BLAST burner. All of these burners break up the fuel by high pressure air or steam, the types given being different only in the way that the pressure is applied to the fuel.
The atomizer acts on the same principle as the medical or perfumery atomizer, the high pressure jet playing directly across the open end of the oil passage as shown by Fig. A. As the vacuum created by the blast is very low, and has little effect in lifting the fuel to the burner, the oil either is made to flow by gravity or by a pump. In the figure the oil in the upper passage is shown pouring down in front of the air or steam jet issuing from the lower port. Both ports are supplied by the pipes shown by the circular openings at the right. The steam and oil are controlled by independent valves placed in the two passages.
In practice the oil and steam openings at the end of the burner may be either single or multiple round openings or long thin slots, the former style being the most common. Since only a small amount of air is admitted through the blast nozzle, far too little to completely consume the oil, the air for the combustion is admitted through openings in the combustion chamber proper, this air being supplied by natural draft or by blower. In some cases the burner is entered into the furnace through an opening that is much larger than the burner itself. The atmospheric air enters through baffle plates in this opening which impart a whirling motion to the air that passes over the burner. This is of considerable aid in maintaining complete combustion in the furnace, and also tends to prevent deposits in the burner.