Abstract:
A burner control system for controlling gas flow to a burner in response to a pressure differential indicative of air flow to the burner, the pressure differential being impressed across a diaphragm which actuates a bleed valve controlling the pressure on one side of the diaphragm actuator of a fuel valve. The bleed chamber is connected to the outlet of the gas valve and the low pressure side of a pressure differential through separate flow restrictors.

Description:
BACKGROUND OF THE INVENTION 
     The invention set forth herein relates generally to modulating fuel/air controls for gas burners, and more particularly to a system of the type in which heat output of a burner is varied by varying air flow to the burner and in which a substantially constant fuel/air ratio or other fuel/air characteristic is maintained by varying fuel gas flow in response to the air flow. 
     For a variety reasons in apparatus for heating space and/or hot water for domestic use, it is frequently desirable to employ mechanical means such as a fan or blower for inducing or forcing air flow through a combustion chamber. In such systems, it is also frequently desirable to modulate the heat output of the burner. A common system arrangement for accomplishing these objectives utilizes a variable speed fan or blower under thermostatic control. A signal indicative of the air flow through the combustion chamber is used to modulate the output of a fuel gas regulator valve which supplies gas to the burner. The air flow signal may be a pressure differential generated across an orifice or by means of a venturi section in the air flow passage. An objective of the system design is to for example maintain a substantially constant fuel to air ratio at the burner so as to provide a fuel mixture in which combustion is easily started and high efficiency combustion is maintained. 
     Apparatus of this general type is disclosed in European Patent Application 0 390 964. More specifically, the differential pressure indicative of air flow is applied across a large area control diaphragm of an amplifying pneumatic control module. Displacement of the large area diaphragm is communicated to a smaller area regulating diaphragm by means of a spring. The regulating diaphragm carries a closure member of a bleed valve which affects a control pressure in a main gas valve containing a secondary servo valve whose closure member is spring biased to limit the maximum output gas pressure. Adjustable biasing of the large and small diaphragms in the control module is accomplished by means of a spring and an associated adjustable retainer screw. 
     Somewhat similar control apparatus is disclosed in European Patent Application 0 326 880 in which a first diaphragm chamber of a control module is connected to a venturi nozzle in the combustion air passage of a burner system, and an opposing second diaphragm chamber is connected to a pressure port downstream from the venturi nozzle. The control module directly actuates the main gas valve by means of a valve rod connecting the control module diaphragm to the gas valve closure member. 
     Another burner control arrangement is shown in German utility model publication 83 00 157 in which gas and air flow to a burner are controlled by separate controllers or valves. The air flow controller includes a thermostatically controlled pressure regulator. The regulated output pressure of the air flow controller is supplied to the gas valve through a pneumatic amplifier therein as its control signal. 
     Although the previously described systems provide modulating operation and are capable of achieving a desired fuel/air ratio for a particular type of gas under relatively constant pressures and other parameters, additional adaptability for use with other types of gasses and under more variable conditions would be desirable. The applicant has devised a burner control system which provides improvements in meeting these objectives. 
     SUMMARY OF THE INVENTION 
     The invention is a burner control system in which a differential pressure signal proportional to the flow rate of combustion air is directly pneumatically compared with the gas pressure at the outlet of a gas control valve. This comparison is used to derive a pneumatic control signal for controlling the diaphragm operator of a main gas valve. 
     In particular, the air flow rate is indicated by a pressure differential between high and low pressure ports respectively connected to control and bleed chambers on opposite sides of a diaphragm in a control module, of which the bleed chamber is connected through a bleed valve actuated by the control module diaphragm to a control chamber on one side of a diaphragm in the main gas valve, the control chamber also being connected through a first flow restrictor to a gas inlet of the gas valve. The other side of the diaphragm in the gas valve is exposed to the pressure of gas supplied through a gas outlet to the fuel nozzle. A second flow restrictor connects the bleed chamber of the control module to the gas outlet of the gas valve and a third flow restrictor is provided in the passageway connecting the bleed chamber of the control module to the low pressure port. 
     The second and third flow restrictors are preferably sized so that the ratio of the fuel gas pressure and air pressure equals the sum of the resistances of the first and second restrictors divided by the resistance of the second restrictor. System operation may be enhanced by including a fourth flow restrictor in the passageway between the gas valve outlet and the gas nozzle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The single FIGURE of the drawing is a functional schematic representation of the preferred embodiment of a system in accordance with the applicant&#39;s invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the FIGURE, reference numeral 1 identifies a closed combustion chamber of a gas-heating apparatus. Combustion chamber 1 contains a heat exchanger 2 and a burner 3, which is supplied with a fuel and air mixture as set forth in detail hereinafter. Heat exchanger 2 is illustrated as a gas to water heat exchanger which is connected via a supply pipe 4 and a return pipe 5 to a load (not shown). A temperature sensor 6 measures the supply temperature of the hot water supplied to the load and provides a corresponding signal to a measured value input 7 of a temperature controller 8. Controller 8 also receives a setpoint signal at a setpoint input 9. The setpoint signal, which may be manually adjusted, corresponds to the desired temperature. Controller 8 controls the energy supply to a motor 10 driving a blower 11 which supplies combustion air to burner 3 via an air passageway 12. The exhaust gases leave combustion chamber 1 via a stack 13. As shown, a gas nozzle 14 is provided in passageway 12 and is supplied with gas from a gas control valve 15. Although a particular arrangement of the air supply, gas supply and a mixing chamber 14a is illustrated, these elements may be designed or positioned differently. For example, blower 11 may be provided in stack 13. 
     Gas control valve 15, which functions as the main gas valve, is provided between a gas inlet 16 and gas outlet 17. Main gas valve 15 includes a closure member 18 spring biased toward a closed position by means of a spring 19. Closure member 18 cooperates with a valve seat 20 in a wall 21 of the valve housing. Closure member 18 is operated by a diaphragm 22 via a valve rod 23. Diaphragm 22 and portions of the valve housing define first and second control chambers 24a and 24b on opposite sides of the diaphragm. Control chamber 24a is connected to gas inlet 16 via a first flow restrictor 25, and to a bleed valve provided in a control module 26. 
     Control module 26 includes a closure member 27 carded by a diaphragm 28. Closure member 27 cooperates with a valve seat 29 to form a bleed valve 27, 29 which is connected to control chamber 24a via a passageway 30. A spring 31 on one side of diaphragm 28 is arranged to bias closure member 27 toward an open position, and a spring 32 between the opposite side of the diaphragm and an adjustment screw 33 acts in the opposite direction. 
     The combustion air flow rate generated by blower 11 is measured by means of a differential pressure measuring device provided in air passageway 12, which device includes an orifice 34 in the passageway, a first measuring passageway 35 porting into air passageway 12 at the upstream or high pressure side of orifice 34, and a second measuring passageway 36 porting into air passageway 12 at the downstream or low pressure side of the orifice. 
     Measuring passageway 35 is connected to a control chamber 37 in control module 26 on one side of diaphragm 28. Control module 26 also contains a bleed chamber 38 on the opposite side of diaphragm 28. Bleed chamber 38 is connected to an outlet port 39 of main gas valve 15 via a passageway 40 containing a second flow restrictor 41, and to low pressure measuring passageway 36 via a third flow restrictor 42. Flow restrictors 41 and 42 preferably are both adjustable. A fourth flow restrictor 43, which may be adjustable, is shown in the gas outlet 17 between passageway 40 and gas nozzle 14. 
     The speed of blower 11, and therefore the flow rate of combustion air is controlled by means of controller 8 according to the heat demand. As the air flow rate increases, the pressure in measuring passageway 35 increases. The pressure increase is transmitted to control chamber 37, thereby deflecting diaphragm 28 in a downward direction. This tends to close bleed valve 27, 29 and increase the pressure in control chamber 24a, which tends to open main valve 18, 20. Accordingly, the increased air flow rate results in an increased gas flow rate. 
     Flow restrictors 41 and 42, which communicate with bleed chamber 38 through a passageway 44, function to convert the previously described operation into closed loop control. Passageway 40 containing flow restrictor 41 couples bleed chamber 38 of control module 26 with output port 39 of main gas control valve 15. If for any reason the gas pressure at output port 39 increases, then the pressure in passageway 44 also increases. This increases the pressure acting on the lower side control diaphragm 28, which tends to open bleed valve 27, 29 and decrease the pressure in control chamber 24a. As a result, spring 19 tends to close gas valve 18, 20 and reduce the gas pressure at output port 39. In this manner the air flow rate and the gas flow rate are pneumatically linked to provide a feed forward control. 
     Flow restrictor 42 between bleed chamber 38 and low pressure measuring passageway 36 both enables pressure to be built up in passageway 44, and permits bleeding off the pressure within bleed chamber 38 when bleed valve 27, 29 is closed. 
     The dependency of the gas pressure P g  within gas nozzle 14 from the air pressure P a  generated by blower 11 can be described by the following formula if flow restrictor 43 is ignored: 
     
         P.sub.g =(R.sub.41 +R.sub.42)/R.sub.42 ×P.sub.a 
    
     R 41  and R 42  are the flow resistances of the flow restrictors 38 and 40, respectively. The pneumatic gain of the control module 26 is assumed to be unity. For the pressure P 38  within bleed chamber 38 the following formula applies: 
     
         P.sub.38 =R.sub.42 /(R.sub.41 +R.sub.42)×P.sub.g 
    
     If the pressure within mixing chamber 14a is designated P m , the following pressure differences appear: 
     
         dP.sub.g =P.sub.g -P.sub.m 
    
     
         dP.sub.a =P.sub.a -P.sub.m 
    
     
         dP.sub.38 =P.sub.38 -P.sub.m 
    
     It follows that: 
     
         dP.sub.g =(R.sub.41 +R.sub.42)/R.sub.42 ×dP.sub.a 
    
     
         dP.sub.38 =R.sub.42 /(R.sub.41 +R.sub.42)×dP.sub.g 
    
     The gain or proportionality factor, by which a change of the gas pressure dP g  is linked to a change of the air pressure dP a , therefore, can be determined in a desired manner by means of flow restrictors 41 and 42. By repositioning adjusting screw 33 for spring 32, the system offset can be adjusted. A fine adjustment of the gas/air ratio can be accomplished by means of flow restrictor 43. 
     Although a variable speed blower is shown and described in the disclosed embodiment for varying air flow rate in response to heat demand, other implementations are equally satisfactory. The air flow rate could, for example, be controlled by a damper or air valve.