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 1-19