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358 CHAPTER 17 1949 Guide air is pumped as a spray through the nozzle at a pressure of 2 to 7 psi, i Secondary air is supplied by a fan. Ignition is obtained by means of a high-voltage electric spark used alone, or as primary ignition for a gas pilot. Various features of a low pressure atomizing burner are shown in Fig. 12. The high-pressure atomizing type, illustrated in Fig. 13, is characterized by an air tube, usually horizontal, with oil supply pipe centrally located in the tube and arranged so that a spray of atomized oil is introduced, at about 100 psi, and mixed in the combustion chamber with the air stream emerging from the air tube. A variety of patented shapes is employed at the end of the air tube to influence the direction and speed of the air and thus the effectiveness of the mixing process. This type of burner utilizes a fan to supply the air for combustion, and ignition is established by a high-voltage electric spark that may be operative continuously while the burner is running, or just at the beginning of the'' -Automatic Fuel Burning Equipment - 359 installation within the ash pit of- the boiler or furnace. Various types of ignition are utilized, gas and electric, either spark or hot wire. The air for combustion is supplied partially by natural draft, and partially by fan effect of the central spinner element. Horizontal rotary burners are used principally to bum the heavier oils, Nos. 5 and 6 grades, principally in larger commercial and. industrial in stallations, although domestic sizes, are available.. Such burners are of the mechanical atomizing type, Using rotating cups which throw the oil from the edge of the cup at high velocity into the surrounding stream of air delivered by the blower (Fig. 16). Horizontal rotary burners commonly use a- combination electric-gas ignition system, or are lighted manually. Primary air for combustion is supplied by a blower, and secondary air, often introduced through a Fio. 13. High-Pressure Atomizing Oil Buhner running period. Gun type burners operate on the intermittent on-off principle, and with a luminous flame. The combustion process is completed in a chamber constructed of refractory material, or stainless steel, this being a part of the installation. Pressure-atomizing burners generally use the distillate, oils, No. 1, 2 or 3 grade. (See Chapter 16.) Rotary Type x This class of burners may be divided into two groups: vertical, and horizontal. Most of the smaller rotary burners are of the vertical type, and use the lighter distillate oils, No. 1 or 2 grade. The most distinguishing feature of vertical rotary burners is the principle of flame application. These burners are of two general types: the center flame and wall flame. In the former type (Fig. 14), the oil is atomized by being thrown from the rim of a revolving disc or cup and the flame bums in suspension with a characteristic yellow color. Combustion is supported by means, of a bowl-shaped chamber or hearth. The wall flame burner (Fig. 15) differs in that combustion takes place in a ring of stainless steel or refractory material, which is placed around the hearth. Dependent upon combustion adjustment, these burners may operate with either a 'semi-luminous or non-luminous flame. Both types of vertical rotary burners are further characterized by their Fig. 14. Center Flame Vertical Rotary Burner Fig. 15. Wall Flame Vertical Rotary Burner checkerwork in the combustion chamber, is controlled by chimney draft* These burners operate with a luminous flame, usually on high-low or continuous setting. In larger installations, burners may be installed in multiple in a common combustion chamber. Because of the high viscosity oils used in these burners, it is customary to preheat the oil between the tank and the burner. Preheating when delivering from tank car, or truck, is often required in cold weather. Vaporizing Burners In the vaporizing burner, fuel oil is ignited (manually or electrically) and vaporized in a vessel or pot which is open at the top or one side. Heat for vaporization is supplied by the combustion process. Openings in the side wadis of the burner admit primary air which forms a rich mixture of air and oil vapors in the burner. Adjacent to the outlet opening sufficientadditional, or secondary air is admitted to complete combustion. The openings for admitting air are arranged to obtain' gradual and intimate mixing of air and oil vapor for combustion with a minimum amount of excess air and resulting high combustion efficiency. Fuel is fed by gravity from a constant level control valve and the flow is either on (at rated capacity) or off (at pilot fldw) according to the demand of the thermostat, However, the high fire can be reduced and the pilot'