COMBUSTION CONTROL

Combustion control is the process of regulating the mixed flow of air and fuel to a furnace as necessary to supply the demand for steam. A modulating pressuretrol controls the movement of the modutrol motor which, in turn. opens or closes the oil valve and air shutters to adjust the rate of firing to suit the demands of the boiler.

A modulating motor (fig. 1-19) consists of the motor windings, a balancing relay, and a balancing potentiometer, The loading is transmitted to the winding through an oil-immersed gear train from the crank arm. The crankshaft is the double-ended type, and the crank arm may be mounted on either end of the motor. The motor works with the potentiometer coil in the modulating pressuretrol. An electrical imbalance is created by pressure change signals to the pressuretrol. This causes the motor to rotate in an attempt to rebalance the circuit. The crank arm, through linkage, positions the burner air louvers and the oil regulating valve, maintaining a balanced flow of air and oil throughout the burner firing range.

Another process of controlling, combustion air is to use a manually adjusted air damper. A centrifugal blower, mounted on the boiler head and driven by the blower motor, furnishes combustion air. A definite amount of air must be forced into the combustion chamber to mix with the atomized oil to obtain efficient combustion. In operation, a pressure is built up in the entire head and the secondary air is forced through a diffuser to mix thoroughly with the atomized oil as combustion takes place.

The combustion airflow diagram in figure 1-20 shows a cutaway view of those components that influence most the path of the air through the burner assembly. Air is drawn into the motor-driven blower through the adjustable air damper at (A) and forced

Figure 1-19. - A modulating motor.

through openings (B) into the air box. Sufficient pressure is built up to force the air through openings (C) and the diffusor (D). In the area immediately beyond the diffusor (D), combustion is completed. The hot gaseous products of combustion are forced on through the remaining three passes where they give up a large portion of the heat contained to the water which completely envelopes the passes.

The rate at which combustion air is delivered can be changed by throttling the intake to the blower by opening or closing the air damper to obtain the exact rate of airflow required for complete combustion. Since the rate at which fuel is delivered is predetermined by the design and is not readily adjustable, setting of the air damper is the only means of obtaining the correct ratio of fuel to air to ensure the most efficient combustion.

A pressure-regulating valve is built into the pump that controls the fuel. The fuel pump (fig. 1-21) contains a two-stage gear-type pump, a suction strainer, a pressure-regulating valve, and a nozzle cutoff valve, all assembled in a single housing. Knowledge of the functional relationship of the component parts can be gained by studying the internal oil flow diagram shown in figure 1-22. Observe that the two-stage fuel unit consists essentially of two pumps operating in tandem and arranged in a common housing. The first stage develops a pressure below the atmospheric pressure level at its inlet that causes the oil to flow from storage or supply to the strainer chamber reservoir. All air drawn into the unit rises to the top of this chamber. This air and excess oil are drawn into the first-stage-pumping element and pumped back to the fuel oil storage tank. The second stage withdraws air-free oil from the strainer chamber reservoir and raises the oil pressure to that required for proper atomization at the burner nozzles. The second stage, operating against a combination pressure regulating and nozzle cutoff valve, develops atomizing pressure because of the flow restriction imposed by this valve. The pressure-regulating valve also bypasses excess second-stage oil back to the bottom of the strainer chamber reservoir. The atomizing pressure can be varied within a restricted range by adjustment of the spring-loaded pressure-regulating valve. Normal atomizing pressures generally range between 95 and 120 pounds per square inch.

An orifice is included in the fuel line to the main oil burner. as shown in figure 1-22. The orifice serves to keep the oil pressure from experiencing a sudden drop when the solenoid oil valve in that line opens. The orifice is commonly built into the solenoid oil valve (fig. 1-22, item 1). Included in the schematic diagram is a photocell