Burners are also classified by the means employed to prepare the fuel for combustion. The oil may be atomized or vaporized. In the latter type, oil is prepared for combustion by first being heated. The vapour is mixed with the air just before or during the combustion process. Burners of this type are especially flexible, with low oil capacities, and moving mechanical parts can be eliminated. In the atomizing burner, high or low pressure air or steam, mechanical pressure or rotating parts break up the fuel into small particles which are quickly vaporized by the heat of combustion. In commercial installations, where oil fuel is used in the heating of factories, office buildings, schools, theatres, apartments, etc., atomizing burners are used almost to the exclu sion of other types because of their ability to burn the heavier and cheaper grades of fuel oils. The modern oil burner is auto matic. A room thermostat electrically controls the burner so that the heat of the house can be kept constant within two degrees on either side of the desired temperature. A boiler or furnace con trol prevents overheating of the boiler and, in the case of a steam boiler, prevents abnormal pressures being developed. A burner safety control establishes a time limit within which the oil must be ignited every time the burner is started up; if the oil is not ignited the burner mechanism is shut down. When ignition is electric, it is accomplished by means of a spark 'from a high tension transformer or by a hot resistance coil. The spark type of ignition is more widely used. Some burners require a gas pilot.
An oil burner can be installed in practically any hot water or steam boiler, or in any warm-air heater.
The fuel oil used in automatic oil heating equipment varies in accordance with the needs of the individual burner; ranging from very light, untreated kerosene or distillate for burners that pre pare the fuel for combustion by vapourization to heavier, straw coloured distillates and dark coloured gas or Diesel oils for burn ers that employ atomization in preparing the fuel 'for combustion. The Commercial Standards Specifications for fuel oils as pub lished by the U.S. Bureau of Standards are the most widely recognized specifications covering fuel oils. The grade numbers and names, together with their old designations and approximate A.P.I. (American Petroleum Institute) gravity range are: The utility of a fuel oil for domestic heating depends also upon its flash point. The flash point of any oil is the lowest temperature at which the vapours on the surface of the oil will ignite and burn momentarily on application of a spark or flame. About 2o° F. above the flash point is the fire point of the oil. This is the temperature at which the oil will ignite and burn continuously. It is desirable that this fire point should be sufficiently low so that the oil will ignite from a cold start without hesitation. A flash point as low as oo° F. or no° F. is allowable, and an oil with a flash point above 23o° F. should not be used with most burners because of the difficulty of initiating ignition.