ATOMIZATION OF THE OIL DELIVERED TO THE FURNACE For the perfect combustion of any fuel, it is neces sary that each particle be surrounded by an atmos phere of oxygen, as is the case in calorimetric deter minations. In approaching this condition when using oil, it is necessary to spray the fluid as much as possible to enable the air supporting combustion to inter mingle with the minute particles of oil. This is commonly accomplished in one of two ways.
Steam as an Atomizing Agent: Under no circumstances can the admission of steam into the fire-box of a boiler be anything but detrimen tal to the temperature. If it is reduced to water vapor it gives up its latent heat only to absorb that heat again upon subsequent evaporation, and will absorb further heat when passing from the furnace as superheated steam. The original heat in the steam was taken from the boiler and should be considered lost as far as any possibility of regaining it is con cerned.
It is necessary, however, to provide some means of properly atomizing the oil ; no agent lends itself so readily to this purpose as steam from the boiler and consequently it is generally used. It should be taken from the dome, or from a point where the dryest steam can be secured, and piped direct to the burners; with no low places in the piping which would permit of the gathering of moisture, the piping being well protected by a suitable covering to reduce con densation to a minimum.
It is usual to reduce the steam pressure to the burners by means of a regulating valve. When boiler pressures of less than 125 pounds are carried it is customary to eliminate the regulating valve.
Air as an Atomizing Agent: When compressed air is available, or in large plants where the installation of an air-blowing unit would be warranted, this method of atomizing gives excellent results. The combustion is more complete than with steam and higher temperatures can be obtained, with a smaller percentage of steam used for the blowing unit than is ordinarily used through the burners.
Air pressures as low as twelve ounces may be used sat isfactorily, but oftentimes a pressure of four or five pounds is desirable. The air has the effect not only of atomizing the oil, but at the same time aids combustion.
The oil flame, when using low pressure air to atomize, is shorter than when steam is used and is of an intense white color indicating that combustion is more rapid and complete. Unless the oil is heated to a temperature approaching its flash point before admission to the burners, the atomization is apt to be incomplete; particles of carbon will be seen dropping to the furnace floor forming a carbon deposit there and producing a smoky flame.
In plants, such as cement plants, brick plants, which cover a wide area and which require heat at points distant from the central steam supply, etc., the use of high or low pressure air for vaporizing the oil is especially desirable. In this case the installation should cost no more than where steam is used, and it eliminates condensation losses, frozen steam lines, etc.
Pressure of Oil at the Burners.
It has been observed in many plants where oil is being used under the boilers that pressures of 75 lbs. to 125 lbs. of oil are maintained by the oil pumps and the regulation of oil is effected by a valve near the burners. It is very obvious that such pressures are not necessary, if the piping is of proper size to carry the amount of oil required, as it is usually only neces sary in such cases to "crack" the valve between the pump and the burner in order to secure sufficient oil. This means considerable reduction in pressure between the valve and the burner, consequently the effect of the pump pressure for atomization is not obtained.
With "internally mixed" burners, where the oil and steam come together inside the burner, it is necessary to maintain sufficient pressure of oil to overcome the back pressure of the steam, and at the same time supply the required amount of oil. This is usually about 30 lbs. to 50 lbs. With "externally mixed" burners, however, it is only necessary to have an oil pressure sufficient to insure the free passage of oil through the piping, which will be (with the proper size pipe) about 4 lbs. or 5 lbs.
Pre-Heating the Oil.
It is desirable to heat the oil to a temperature of approximately 150° F. A variation from this temperature may be required to meet local con ditions and on account of the character of the oil used. 20% from the temperature stated will prob ably meet the majority of cases.
At no time should the oil be heated above the flash point.
In cities where the insurance rules require the use of pressure pumps rather than a gravity system, this heating can easily be done by passing the exhaust steam from the pumps through coils in a heater, connected in series between the pressure pumps and the burners. The preheating of the oil is sometimes accomplished by passing it through coils in the uptake of the boiler or in the furnace proper. This is not to be recommended as the action of the hot flue gases or the high temperature in the furnace is very detri mental to the coils and they are sure to fail in the course of time, with the possibility of fatal results.
In case exhaust steam is not available, the oil should be heated by live steam. The specific heat of oil fuel is only about one-half that of water, hence very little steam will be required.
A slightly better economy is obtained by preheating, as it increases the fluidity of the oil and consequently does not require as much steam or air to effect the atomization.
The "flash" point of oil is that temperature at which it gives off sufficient gas to flash but not burn con tinuously. The latter being the "fire" point.
When oil fuel is heated beyond the flash point, decomposition begins to occur, causing carbon to be precipitated; this will clog the oil lines and burners, causing no little inconvenience as well as producing an unnecessary hazard in case of a broken oil line. If, however, the heater does not raise the temperature of the oil above the flash point, there will be no trouble or danger from this source.