Figure 4-39. - High-pressure gun type of oil burner.

FUEL UNIT. - There are many types of fuel units available for oil burners; however, the T-type, two-stage fuel unit is the most commonly used. Figure 4-40 shows this type of unit. It is an oil pump with two strainers mounted on the body of the oil burner and operated by the blower motor shaft.

The T-type, two-stage fuel unit can be used on a single-line or on a two-line system. When Number 1 on the strainer cover is next to the letter marked on the body of the pump, it is correctly arranged for a single-line system. It is set up for a two-line system when the cover is turned so Number 2 is adjacent to the same letter.

A two-line system is necessary when the bottom of the fuel tank is below the level of the pump. The suction line from the tank is connected to the pump port marked "Inlet." The return line is connected to the pump bypass port and is directed back into the tank. With the one-line system, the return line is not used.

Ignition Electrodes. - The heat of a spark jumping between two ignition electrodes ignites the fuel (fig. 4-39). The voltage necessary to cause the spark to jump is much more than the line voltage available. Therefore, an electric transformer is used to step up the line voltage to approximately 10,000 volts.

The wall flame burner has an oil distributor and fan blades mounted on a vertical shaft directly connected to the motor. The oil distributor projects the oil to a flame ring made of either refractory material or metal. Figure 4-41 shows this type of burner. The hot flame ring vaporizes the oil, and the oil vapors mix with air and burn with a quiet blue flame that sweeps the walls of the furnace. Ignition may be electric, gas-electric, or gas. High-grade fuel oil is necessary for satisfactory performance.

Figure 4-40. - A typical T-type, two-stage fuel pump.