The oil-pressure regulator on the fuel pump is generally factory-set to give nozzle oil pressures of 100 psig. The firing rate is indicated on the nameplate and can be obtained with standard nozzles by adjusting the pump pressures as follows:
1. Turn the adjusting screw clockwise to increase pressure.
2. Turn the adjusting screw counterclockwise to decrease pressure.
3. Never exceed the pressures indicated in Table 1-2.
The capacity of an oil burner fuel pump should be sufficient to handle
the total vacuum in the system. The vacuum is expressed in
inches and can be determined by the following procedure:
• 1 inch of vacuum for each foot of lift
• 1 inch of vacuum for each 90° elbow in either the suction or return lines
• 1 inch of vacuum for each 10 feet of horizontal run (3?8-inch OD line)
• 1 inch of vacuum for each 20 feet of horizontal run (1?2-inch OD line)
After you have calculated the total vacuum, you can use these data to select the most suitable pump for the burner. Table 1-1 lists various vacuums and suggests appropriate pump capacities.
A single-stage pump contains only one set of pumping gears (see Figure 1-23). These pumps are commonly used in single-pipe gravityfeed installations or two-pipe installations under low-lift conditions with up to 10 inches of vacuum. The following are the principal components of a single-stage fuel pump:
1. Pumping gears.
2. Cutoff valve.
3. Strainer.
4. Shaft seal.
5. Noise-dampening device.
6. Shaft bearing.
7. Body.
8. Bleed valve.
Fuels Used in Oil Burners
No. 1 and No. 2 fuel oil are both commonly used for residential heating purposes. The No. 2 is slightly more expensive, but the fuel oil gives more heat per gallon used.The lighter No. 1 fuel oil is used in vaporizing, or pot-type, oil burners.The No. 2 fuel oil is used in both atomizing and rotary oil burners.The manufacturer of the oil burner will generally stipulate the grade of fuel oil to be used. If this information is unavailable, the label of Underwriters Laboratories, Inc., and the Underwriters Laboratories of Canada will stipulate the correct grade of fuel oil to be used.
The heavier the grade of fuel oil used in an oil burner, the greater the care that must be taken to ensure that the oil is delivered for combustion at the proper atomizing temperature. If the oil is not maintained at this temperature prior to delivery for combustion, the oil burner will fail to operate efficiently. An efficient oil burner is one that burns the fuel oil completely using the smallest amount of air necessary for combustion.
The fuel oil first enters the unit by passing through the strainer, where foreign particles such as dirt and line filter fibers are removed. The fuel oil then moves through the hydraulically balanced pumping gears and is pumped under pressure to the valve (see circuit diagram in Figure 1-24). The pressure forces the piston away from the nozzle cutoff seat, and the fuel oil then flows out the nozzle port. Oil in excess of nozzle capacity is bypassed through the valve back to the strainer chamber in a single-pipe system or is returned to the tank in a two-pipe system. Pressure is reduced on the head of the piston when the pump motor is shut off. At this point, the piston snaps back, causing the nozzle port opening to close. A bleeder valve opening in the piston provides for automatic air purging on a two-pipe system, providing for fast cutoff.
Gun-type, high-pressure atomizing oil burners are sometimes called sprayers or atomizing burners because they spray the fuel oil instead of vaporizing it. They are also referred to as gun or pressure oil burners because the oil is forced under pressure through a special gun-like atomizing nozzle. The liquid fuel is broken up into minute liquid particles or globules to form the spray.
The principal components and parts of a gun-type, high-pressure atomizing oil burner used in residential and light commercial oil heating systems are illustrated in Figures 1-6 and 1-7. The construction details of gun-type oil burners will vary somewhat in different makes and models, but the overall design of these burners is now nearly standardized. The components and parts of a typical gun-type oil burner can be divided into the following categories:
1. Burner control.
2. Primary safety control.
3. Gun assembly.
4. Ignition transformer.
5. Burner motor and coupling.
6. Fuel pump.
7. Combustion air blower.
Burner Control
The burner control is the operational control center of the burner. As shown in Figures 1-6 and 1-7, it is located on the right side of the burner assembly directly above the combustion air blower housing. It operates in conjunction with the primary control and a bimetallic temperature sensor. When the room thermostat calls for heat and the ignition cycle begins, the burner control will start the burner only when the cad cell detects (proves) a flame. The burner control shuts off the burner if the cad cell fails to prove the flame or if the bimetallic sensor detects a temperature too high for safe operation.
Primary Safety Control
The primary safety control is an automatic safety device designed to stop the flow of fuel oil at the burner should ignition or flame failure occur. Modern oil-fired furnaces and boilers use a cad cell as the primary control to prove the flame; older ones were equipped with a stack detector primary control. The former is mounted inside the burner behind the access door (see Figure 1-8), and the latter is located in the stack.
Gun Assembly
The oil burner gun assembly consists of a burner nozzle, the electrodes, and a tube connecting the electrodes to the fuel pump (see Figure 1-9). The burner nozzle changes the fuel oil into a form that can be burned in the combustion chamber. It accomplishes this by forcing the oil under pressure through a small hole at the end of the nozzle. The atomized fuel oil is ignited by spark from the electrodes.
Ignition Transformer
A step-up ignition transformer located on top of the burner assembly produces the voltage used by the electrodes to ignite the fuel oil. This type of transformer is designed to increase the voltage of a high-voltage (110 VAC) circuit to the ultrahigh 14,000 volts required to ignite the fuel oil.
Burner Motor and Coupling
As shown in Figure 1-5, the burner motor is located on the right side of the oil burner assembly. The drive shaft of the burner motor is connected to both the fuel pump and the combustion air blower by a coupling that functions as the drive shaft for both of these units. A burner motor is also sometimes called an oil pump motor or a pump motor because it is connected to and drives the fuel (oil) pump.
Fuel Pump
The fuel pump (also called an oil pump or a fuel unit) is used to draw fuel oil from the storage tank and deliver it under high pressure (100 to 140 psi) to the nozzle assembly (see Figure 1-11). It is driven by the burner motor and coupling and is located on the left side of the oil burner.
Combustion Air Blower
The combustion air blower is also driven by the burner motor and coupling. It is located between the burner motor and the fuel pump. Its function is to introduce the required amount of air for the combustion process. The amount of air can be manually adjusted by an air adjustment gauge located between the blower wheel and the inlet air scoop (see Figure 1-7). Depending on the oil burner manufacturer, a combustion air blower is also sometimes called a blower wheel, a burner motor fan, or an induction blower. Do not confuse the combustion air blower with the furnace indoor blower. The former delivers air to the oil burner for combustion. The latter delivers the heated air to the rooms and spaces inside the structure.