Abstract:
A gas burner and method of controlling burning, the gas burner including a controller disposed in a control cabinet, a burner cabinet, an actuator and a blower, the blower being fluidly coupled to the burner cabinet. The gas burner has an air valve that is operably, fluidly coupled to both the burner cabinet and the blower for controlling the flow of air from the blower to the burner cabinet. A gas valve is fluidly coupled to a source of gas for controlling the flow of gas from the source of gas. An actuator is communicatively coupled to the controller and is linearly coupled to the air valve and the gas valve for simultaneous linear actuation thereof responsive to commands from the controller.

Description:
TECHNICAL FIELD 
     The present invention is a gas burner. More particularly, the present invention is a gas burner with a high turndown capability, permitting the burner to operate between less than 5% and 100% of the maximum firing rate. 
     BACKGROUND OF THE INVENTION 
     A gas burner is the fire producing device used in a warm air furnace, a heat exchanger, a boiler, an oven, and the like. Typically, the gas burner controls the flow rate and mixing of air and gas and includes the controls that do the ignition and safety monitoring of the flame. For many applications of a gas burner, the amount of heat required is not constant. The amount of heat required may vary according to the weather, the process load, and other conditions. To deal with varying loads, banks of multiple burners have been used. The banks of multiple burners may be sequenced to produce the required amount of heat. Alternatively, a burner with a variable firing rate may also be used. A burner with a variable firing rate can be a staged burner, capable of operating either at a low fire or high fire, or it can be a modulating burner. A modulating burner is capable of being controlled to operate at any firing rate within a range between its minimum and maximum firing rates. That range is typically 50%-100%, with some of the better burners being capable of 33%-100%. That means that when the heat requirement is less than the minimum firing rate of the burner, 33% in the case of the better burners, the only alternative is to periodically cycle the burner on and off at the minimum rate in order to produce a lesser amount of heat than is produced at the minimum rate. Unfortunately, this results in fluctuating temperatures and therefore less than ideal control when operating in this mode. 
     Accurate and consistent temperature control is improved if a burner is capable of operating at a very low minimum firing rate. A great deal of effort in the industry has been expended toward achieving the goal of having a very low minimum firing rate. Typically, efforts at providing such capability have concentrated on control of the gas flow and control of the secondary air. 
     With respect to control of the gas flow, the maximum fire rate of a gas burner is typically controlled by the sizing of the main gas orifice. The size is typically set when the burner is manufactured and is invariable thereafter. The maximum firing rate occurs when a specified gas pressure is present at the fixed orifice. To effect the minimum firing rate on a modulating gas burner, it is common practice to control a butterfly gas valve or other similar device to cause a reduction in the gas pressure to the fixed orifice. Reducing the gas pressure causes a reduction in the gas flow rate through the fixed orifice, thereby reducing the firing rate of the burner. Typically, a control actuator is mechanically linked to the butterfly gas valve to also control a combustion air damper, such that both the gas and the combustion air are simultaneously reduced to achieve the minimum firing rate. Alternatively, the combustion air damper only is controlled. Such control reduces the air pressure within the burner. A suitable pressure regulator is then used to sense the reduced air pressure and to control the gas pressure proportionately. 
     Because the flow rate to a fixed orifice varies as the square of the pressure across it, there are practical limits as to how low the flow can be reduced using either of the foregoing techniques. As an example, if the burner utilizes 4.0 inches water column orifice pressure at the maximum firing rate, the pressure would have to be reduced to unmanageably low levels to operate in the region below approximately 20% of the maximum firing rate. Such levels are indicated in Table 1 below. 
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 100%  
                 4.00 In. W.C. 
                   
               
               
                 50% 
                 1.00 In. W.C. 
               
               
                 33% 
                 .44 In. W.C. 
               
               
                 20% 
                 .16 In. W.C. 
               
               
                 10% 
                 .04 In. W.C. 
               
               
                  5% 
                 .01 In. W.C. 
               
               
                   
               
             
          
         
       
     
     As indicated above, secondary air may be also controlled to achieve a minimum firing rate. Secondary air is that air which is introduced directly into the combustion zone. Typically the combustion air to a modulating gas burner is controlled by a pivoting damper blade. A pivoting damper blade is inadequate for a burner that is going to be modulated down to a minimum firing rate that is less than 25% of the maximum firing rate. A pivoting damper blade simply does not allow precise enough control near and at the desired minimum firing rate. 
     On gas burners that control secondary air to proportion combustion air, primary air is not presently varied in any fashion in order to affect the minimum and maximum burning rates. Primary air is that air that is mixed directly with the gas stream before it enters the combustion zone. Having a source of primary air is common practice with many types of gas burners. 
     As previously indicated, there is a need in the industry for a gas burner that is capable of operating efficiently at very low minimum firing rate. Such firing rate should be in the range of less than 25% of the maximum firing rate. In order to achieve such a low minimum firing rate, a new means of accurate and consistent temperature control is required. 
     SUMMARY OF THE INVENTION 
     The present invention substantially meets the aforementioned needs of the industry. The apparatus of the present invention maintains a relatively constant pressure on the gas flow orifice but varies the area of the orifice. This is accomplished by having a square orifice and controlling the open area of the orifice by positioning a tapered plug at various positions within the orifice. Generally, the valve will have a specific stroke length for the tapered plug and the taper of the tapered plug will be defined for a particular capacity profile along that stroke. Accordingly, valves sized for lower capacity will have less taper and therefore there will be less open area at the maximum capacity position. Although a square orifice has been described, the present invention may also utilize round or other shaped orifices with an appropriate shaped plug. Additionally, the profile of the tapered plug can be characterized so that a specific flow rate will occur at specific stroke positions. In this manner, the plug can have a linear rate of change or with a compound face of the taper the plug can have a slow rate of increase at the minimum firing rate end of the stroke and a fast rate of increase toward the maximum firing rate end the stroke. 
     The gas burner of the present invention meters secondary air using a sliding blade under a plate that had characterized openings responsive to the need of the burner from the minimum firing rate to the maximum firing rate. Accordingly, the apertures admitting the secondary air can be precisely determined along the stroke of the blade. 
     The aforementioned sliding blade also controls air to a port that supplies the primary air to the burner. Preferably, at the minimum firing rate, a specific amount of primary air is mixed with the gas. As the amount of gas increases when a higher firing rate is commanded, the amount of primary air is also increased. When the firing rate increases beyond a certain point, the primary air is cut off. At this point, the primary air is not needed for good combustion and the addition of the primary air needlessly adds to the gas port pressure drop in the burner gun. 
     For the gas burner of the present invention, a new source of air is utilized to enhance the combustion of the gas. At very low firing rates, good combustion requires that the combustion air be greatly reduced and that the flame receives that air at the correct location relative to the gas. Toward this end, a source of base air is supplied directly into the burner gun assembly. The base air and the gas are mixed proximate the point at which the gas emerges from the burner gun. 
     A further advantage of the present invention is that both the sliding blade of the air valve and the wedge of the gas valve are linearly actuated. Accordingly, they can be directly connected to a single linearly actuated rod, thus eliminating the need for crank arms, adjustable linkage, and the like typically employed in present gas burners to coordinate an air damper and a gas valve linked together. 
     The present invention is a gas burner and method of controlling burning, the gas burner including a controller disposed in a control cabinet, a burner cabinet, an actuator and a blower, the blower being fluidly coupled to the burner cabinet. The gas burner has an air valve that is operably, fluidly coupled to both the burner cabinet and the blower for controlling the flow of air from the blower to the burner cabinet. A gas valve is fluidly coupled to a source of gas for controlling the flow of gas from the source of gas. An actuator is communicatively coupled to the controller and is linearly coupled to the air valve and the gas valve for simultaneous linear actuation thereof responsive to commands from the controller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the gas burner of the present invention with portions of the burner cabinet broken away; 
     FIG. 2 is an exploded perspective view of the gas burner of the present invention; 
     FIG. 3 is a sectional perspective view of the gas valve of the gas burner; 
     FIG. 4 is an exploded perspective view of the gas valve of the gas burner; 
     FIG. 5 is a sectional side view of the gas valve of the gas burner; 
     FIG. 6 is a perspective view of the tapered plug and the orifice of the gas valve; 
     FIG. 7 a  is a side elevational view of an alternative embodiment of the tapered plug; 
     FIG. 7 b  is a side elevational view of a further alternative embodiment of the tapered plug; 
     FIG. 8 is a sectional side view of the burner gun of the gas burner; 
     FIG. 9 is an elevational end view of the burner gun of the gas burner; 
     FIG. 10 is an elevational end view of the center portion of the burner plate and the burner gun of the gas burner; 
     FIG. 10 a  is side sectional view of the burner plate and burner gun of FIG. 10; 
     FIG. 11 is a perspective view air valve of the gas burner with portions of the air valve broken away; 
     FIG. 12 is a sectional side view of the air valve of the gas burner; 
     FIG. 13 is a elevational front view of the profile plate and sliding plate of the air valve; 
     FIG. 14 a  is front sectional view of the primary air aperture at the minimum fire position; 
     FIG. 14 b  is front sectional view of the primary air aperture at the maximum flow position; 
     FIG. 14 c  is front sectional view of the primary air aperture at the off position; 
     FIG. 14 d  is a diagrammatic of the flow of primary air as indicated in FIGS. 14 a - 14   c.    
     FIG. 15 is front sectional view of the primary air aperture; 
     FIG. 16 is a front elevational view of the actuator of the gas burner; 
     FIG. 17 is a perspective, exploded view of the actuator arm coupled to the air valve and the gas valve; and 
     FIG. 18 is an enlarged front elevational view of the actuator coupled to the air valve and the gas valve taken at oval  18  of FIG.  17 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The gas burner of the present invention is shown generally at  10  in FIGS. 1 and 2. Gas burner  10  has four major components: control cabinet  12 , burner cabinet  14 , control actuator  16 , and blower  18 . 
     The control cabinet  12  contains timers, relays and wiring necessary to control the gas burner  10 . At the lower portion of the control cabinet  12  is a switch compartment  20 . A pair of interlock switches, the maximum fire switch  22  and the minimum fire switch  24 , are spaced apart within the switch compartment  20  and are utilized to control the prepurge of the furnace combustion chamber prior to ignition of the gas burner  10 . The interlock switches  22 ,  24  are also depicted in FIG.  16 . 
     The burner cabinet  14  is generally parallelepiped shaped and has a burner gun aperture  30  and air inlet  32 , and a gas valve aperture  33 . A face cover  34  is positioned in place on the burner cabinet  14  during burner operations to make the burner cabinet  14  generally air tight. The burner cabinet  14  has three major components therein; gas valve  36 , burner gun  38 , and air valve  40 . 
     The gas valve  36  of the burner cabinet  14  is depicted in FIGS. 1 through 7 b . The gas valve  36  has a generally cylindrical housing  42 . A first end of the cylindrical housing  42  has threads  44  cut therein. The threads  44  facilitate fluidly coupling the gas valve  36  to a pipe having a source of gas under pressure. A mounting plate  48  is fixedly coupled to the cylindrical housing  42  in a substantially orthogonal relationship to the center line of the cylindrical housing  42 . Mounting plate  48  is designed to fixedly couple the gas valve  36  to the side of the burner cabinet  14 . The cylindrical housing  42  has a gas flow passageway  49  defined therein. 
     A gas-air outlet  50  is fixedly coupled to the cylindrical housing  42 . The gas-air outlet  50  is preferably disposed at an acute included angle with respect to the cylindrical housing  42 . A gas-air passageway  51  is defined within the gas-air outlet  50 . The gas-air passageway  51  is in flow communication with the gas flow passageway  49 . A primary air inlet  52  is fixedly coupled to the gas-air outlet  50 . The primary air inlet  52  is fluidly coupled to the gas-air passageway  51  defined in the gas-air air outlet  50  for the mixing of primary air and gas therein. The primary air inlet  52  is fluidly coupled to the air valve  40  by a primary air tube  53 . 
     An orifice plate  54  is disposed within the gas flow passageway  49  at a point approximately midway along the cylindrical housing  42 . Preferably, the orifice plate  54  is held in place be a press fit. A pressure tap  56  is formed in the cylindrical housing  42  upstream of the orifice plate  54 . An orifice  58  is defined in the orifice plate  54 . In the preferred embodiment, the orifice  58  is rectangular in shape. Other shapes, such as a circular or oval opening, could also be used for the orifice  58 . A tapered plug  60  is translatably disposed within the orifice  58 . The shape of the tapered plug  60  is designed to match that of the orifice  58 . Accordingly, the tapered plug  60  has a rectangular cross-section for use with a rectangular orifice  58 . The tapered plug  60  has a circular cross-section for use with a circular orifice  58 . In the preferred embodiment, tapered plug  60  has an upwardly directed tapered face  62 . 
     As indicated in FIGS. 7 a  and  7   b , the slope of the tapered face  62  can be adjusted to accommodate greater or lesser gas flow rates required of the particular usage of the gas burner  10 . As depicted in FIG. 7 a , the tapered face  62  having a taper indicated at  66 A is utilized for a lower capacity gas valve  36 , while the taper indicated at  66 B is utilized for a relatively higher capacity gas valve  36 . 
     As indicated in FIG. 7 b , the slope of the tapered face  62  can be compounded having a first slope  64   a  for use at relatively low burn rates and a great slope  64   b  for use as the gas burner  10  approaches its maximum burn rate. In translation, the tapered plug  60  is supported by its lower surface  67  riding on the lower margin  65  of the orifice  58 . 
     Referring to FIGS. 3,  4  and  6 , an actuator bore  68  is defined in an end of the tapered plug  60 . A cross-bore  70  intersects the actuator bore  68 . An end of an actuator rod  72  is disposed within actuator bore  68  and coupled thereto by pin  74  passing through the cross-bore  70  and a bore (not shown) defined in the actuator rod  72  that is in registry with the cross-bore  70 . 
     The actuator rod  72  preferably has a first inflexible segment  76  and a second flexible segment  78 . The inflexible segment  76  is preferably made of a slender metallic rod. The flexible segment  78  is preferably made of a twisted metallic cable. A threaded connector  80  is fixedly coupled to an end of the flexible segment  78 . 
     A generally circular bearing  82  is inserted into an end of the cylindrical housing  42 . The bearing  82  is preferably formed of a plastic material having a very low coefficient of friction. The bearing  82  has a bearing bore  84  defined therein. The bearing bore  84  has a slightly greater inside diameter than the outside diameter of the inflexible segment  76  of the actuator rod  72 , such that the actuator rod  72  is freely translatable within the bearing bore  84 . 
     An O-ring groove  86  is defined circumferential to the bearing  82 . An O-ring  88  is disposed within the O-ring groove  86 . The bearing  82  is preferably pressed into the cylindrical housing  42  with the O-ring  88  providing a gas-air seal. The bearing  82  is retained in position by set screw  90 . 
     Turning now to the burner gun  38  as depicted in FIGS. 1 and 2, and  8 - 10 , a blast tube  94  is mounted to the rear wall of the burner cabinet  14  with a gasket  95  interposed therebetween. When the gas burner  10  is mounted to a furnace or the like, the blast tube  94  projects to the combustion chamber of the furnace he innermost projection of the blast tube  94  is typically mounted flush with the wall of the combustion chamber of the furnace. The blast tube  94  has an outer wall  96  and an inner wall  97 , with a cast refractory material  99  deposited therebetween. 
     The burner gun  38  has a generally circular burner plate  100 , as depicted in FIGS. 1,  9  and  10 . The diameter of the burner plate  100  is slightly smaller than the inside diameter of the inner wall  97  of the blast tube  94  such that the burner plate  100  may be disposed within the inner wall  97 . 
     The burner plate  100  has a plurality of secondary air orifices  102  defined therein. Some of the secondary orifices  102  are defined peripheral to the burner plate  100 , while other secondary air orifices  102  are defined in the mid-region of the burner plate  100 . A nozzle bore  103  is defined at the very center of the burner plate  100 . A nozzle  104  is disposed within the nozzle bore  103  and is fixedly joined to the burner plate  100 . The nozzle  104  has a central axis that is disposed generally orthogonal to the plane of the burner plate  100 . 
     Referring to FIG. 8, the nozzle  104  has a tubular body  106 . An end plate  108  caps the distal end of the tubular body  106 . A plurality of radial orifices  110  are defined in the tubular body  106  proximate the end plate  108 . The proximal end of the tubular body  106  is fixedly coupled to the inside diameter of a gas-air pipe  111 . 
     A base air shroud  112  is disposed circumferential to and spaced apart from the nozzle  104 . A circumferential base air passageway  113  is defined between the base air shroud  112  and the tubular body  106  of the nozzle  104 . A first end of the base air shroud  112  is fixedly joined to the burner plate  100  and a second end of the base air shroud  112  is fixedly joined at the outside diameter of the gas-air pipe  111 . A plurality of base air orifices  114  are defined in the burner plate  100  and are fluidly coupled to the base air passageway  113 . Preferably, a base air orifice  114  is disposed adjacent to each of the radial orifices  110  of the nozzle  104 . 
     A base air inlet  116  is defined in the wall of the base air shroud  112 . The base air inlet  116  is fluidly coupled to the base air passageway  113  and to a base air tube  118 . The base air tube  118  is fluidly coupled to the air valve  40  for receiving air under pressure therefrom. An orifice  117  is defined in the base air inlet  116  to control the amount of base needed for the particular application of the gas burner  10  and is typically increased in size for the higher output applications. In an application, the orifice  117  may be a sixteenth of an inch in diameter. 
     The gas air pipe  111  is fluidly coupled to an elbow  120  and a union  122  to the gas-air outlet  50  of the gas valve  36 . A flame rod  124  is mounted on the burner gun  38 . The sensor tip  126  of the flame rod  124  projects through a bore defined in the burner plate  100  to sense the presence of a flame. 
     The third component of the burner cabinet  14  is the air valve  40 . The air valve  40  is depicted in FIGS. 1 and 2 and  11 - 15 . The air valve  40  is fixedly, sealingly coupled to the floor of the burner cabinet  14 , overlying the air inlet  32  defined therein. 
     The air valve  40  has an air box enclosure  130  having a generally triangular cross-section, as seen in FIGS. 11 and 12. The air box enclosure  130  has a front profile plate  132  and a back plate  134 . The profile plate  132  and the back plate  134  are joined at the upper margins thereof and sealed by the two opposed end plates  136   a ,  136   b.    
     Referring to the profile plate  132 , a secondary air aperture  138  is defined in the profile plate  132 , fluidly coupling the space defined within the air box enclosure  130  and the space defined within the burner cabinet  14 . Secondary air aperture  138  is defined by the aperture margin  140  of the profile plate  132  in cooperation with the end plate  136   a . A connector slot  142  is preferably defined in a corner of the aperture margin  140 . 
     A moveable restrictor plate  144  is positioned over a portion of the secondary air aperture  138 . The restrictor plate  144  is positionable relative to the secondary air aperture  138  by an elongated slot  146  defined therein and a set screw  148  threaded into the profile plate  132 . 
     A second secondary air aperture, termed a characterized aperture  150 , is defined in the profile plate  132 . The shape of the characterized aperture  150  is preferably unique to the specific application that the gas burner  10  is to be used in. 
     A primary air aperture  152  is defined in the profile plate  132 . The primary air aperture  152  is fluidly coupled to a primary air housing  153 . The primary air housing  153  is fixedly, sealingly coupled to the profile plate  132 . The primary air housing  153  is threadably coupled to the primary air tube  53 . 
     A third secondary air aperture, termed the secondary air bore  155 , is also defined in the profile plate  132 . In the embodiment depicted, the secondary air bore  155  is open for the initial translation of the sliding plate  156  from the minimum fire position and is closed off by the sliding plate  156  as the sliding plate  156  approaches the maximum fire position. Alternatively, the secondary air bore  155  may be formed in the back plate  134 . In such a disposition, the secondary air bore  155  is always open between the space defined within the air box enclosure  130  and the space defined in the burner cabinet  14 . 
     The sliding plate  156  is positioned beneath the profile plate  132 . The sliding plate  156  is slidably borne in tracks  157 . The sliding plate  156  has a leading edge  158  and a trailing edge  160 . The leading edge  158  defines the size of the secondary air aperture  138  that is open to the space defined within the air box  130  and defines the portion of the characterized aperture  150  that is open to the space defined within the air box enclosure  130 . Similarly, the trailing edge  160  defines when the secondary air bore  155  is open to the space defined within the air box enclosure  130  as a function of the translational position of the sliding plate  156  relative to the profile plate  132 . 
     Referring to FIGS. 14 a - 14   c  and  15 , a primary air slot  161 , defined in the sliding plate  156 , is partially or fully in registry with the primary air aperture  152  or closes off the primary air aperture  152  as a function of the translational position of the sliding plate  156  relative to the profile plate  132 . 
     A bolt  164  couples the sliding plate  156  to a flexible actuator  166 . A threaded connector  168  is fixedly coupled to the flexible actuator  166 . 
     The fourth component of the gas burner  10  is the control actuator  16 . The control actuator  16  is depicted in FIGS. 1 and 2 and  16 - 18 . The control actuator  16  has an actuator enclosure  180  that is preferably fixedly joined to the burner cabinet  14 . 
     A reversible gear motor  182 , comprising a rotary actuator, is disposed within the actuator enclosure  180 , as depicted in FIG.  2 . An output shaft  184  of the motor  182  projects through the side of the actuator enclosure  180 . A rotary actuator arm  186  is fixedly coupled to the output shaft  184 . The sliding bearing  188  is rotatably coupled to the rotary actuator arm  186  by a bolt  190 . A bearing bore  192  is defined in sliding bearing  188 . The sliding bearing  188  is preferably made of a plastic material having a very low coefficient of friction. 
     A generally L-shaped linear actuator arm  194  has a first arm  195  that is slidably disposed within the bearing bore  192 . 
     The second arm  197  of the linear actuator arm  194  is substantially longer than the first arm  195 . The second arm  197  passes through the burner cabinet  14  and terminates in the switch compartment  20  of the control cabinet  12 . The second arm  197  is borne in bearings  198  positioned in actuator bores  196  in the two side panels of the burner cabinet  14 . 
     A slidable sleeve  200  is positioned on the second arm  197  within the burner cabinet  14 . Sleeve  200  is positioned as desired on the second arm  197  and then set in position by set screws  202 . 
     An air control arm  204  is fixedly adjoined to a first end of the sleeve  200 . A gas control arm  206  is fixedly joined to the second end of the sleeve  200 . Both the air control arm  204  and the gas control arm  206  have a bore  208  defined therein. The threaded connector  168  that is joined to the sliding plate  156  is positioned within the bore  208  of the air control arm  204  and fixed in place by nuts  210 . The threaded connector  80  coupled to the tapered plug  60  of the gas valve  36  is positioned in the bore  208  defined in the gas control arm  206  and fixed in place by nuts  210 . In this manner, translation of the second arm  197  of the linear actuator arm  194  simultaneously linearly controls both the gas valve  36  and the air valve  40 . 
     A switch actuator  212  is disposed proximate the distal end of second arm  197  and held in position by a set screw  214 . The switch actuator  212  is designed to make the maximum fire switch  22  when the linear actuator arm  194  is in the maximum fire position and to make the minimum fire switch  24  when the linear actuator arm  194  is in the minimum fire position. FIG. 1 depicts the gas burner  10  in the minimum fire position. 
     The blower  18  of the gas burner  10  is depicted in FIGS. 1,  2 , and  12 . Blower  18  has a helical housing  220  having a discharge port  222 . When the blower  18  is mated to the underside of the burner cabinet  14 , the discharge port  222  is in registry with the air inlet  32  of the burner cabinet  14 . A gasket  224  is positioned between the helical housing  220  and the surface of the burner cabinet  14 . 
     An electric blower motor  226  is positioned on a first side of the helical housing  220 . The blower motor  226  is rotatably coupled to a rotor  228 . An inlet cone  230  and grill  232  are positioned on the opposite side of the helical housing  220  from the blower motor  226 . 
     The gas burner  10  of the present invention has a control system housed within the control cabinet  12 . The control system uses a microprocessor flame safeguard control. A typical sequence of operation commences with the control system calling for burner operation. Prior to ignition of the gas burner  10 , a pre-purge operation is performed. The pre-purge period is necessary to clear the combustion chamber of the furnace and the burner cabinet  14  of any combustibles that may have accumulated there since the last operation of the gas burner  10 . It should be noted that no gas flow in the gas valve  36  occurs during the pre-purge period. Prior to initiation of the timed pre-purge period, the control system sends a signal to the control actuator  16  commanding the maximum fire position and also initiates operation of the blower  18 . As indicated in FIG. 16, the rotary actuator arm  186  preferably rotates through an arc of 90° commencing at a minimum fire position that is approximately 10% below a level position. 
     Responsive to the command from the control system, the bi-directional rotary stepper motor  182  energizes and rotates the rotary actuator arm  186  from the minimum fire position to the maximum fire position. Such rotation causes the sliding bearing  188  to slide downward on the first arm  195  of the linear actuator arm  194  at the same time that the linear actuator arm  194  is moved to the left as depicted in FIG.  16 . When the rotary actuator arm  186  has rotated through 90°, the linear actuator arm  194  is in the position depicted in phantom in FIG. 16, which is the maximum fire position. The stroke of the linear actuator arm  194  is preferably 3.5 inches or 4.5 inches, depending on the application of the gas burner  10 . The stroke may be any selected length. 
     Linear translation of the linear actuator arm  194  through the full stroke length from the minimum fire position to the maximum fire position simultaneously fully opens the gas valve  36 , fully opens the air valve  40 , unmakes the minimum fire switch  24 , and makes the maximum fire switch  22 . The stroke length of the tapered plug  60 , the stroke length of the sliding plate  156 , and the distance between the minimum fire switch  24  and the maximum fire switch  22  are substantially equal to the stroke of the linear actuator arm  194 . Thus, the tapered plug  60 , the sliding plate  156  and the distance between making the two interlock switches  22 ,  24  all have the same linear stroke length between the respective minimum fire and maximum fire positions. 
     In the maximum fire position, the sliding plate  156  of the air valve  40  is in its full open position. Secondary air under pressure is flooding the burner cabinet  14  and base air under pressure is being provided to the burner gun  38 . 
     Air flow from the blower  18  is sensed by a pressure switch (not shown) with the air valve  40  in the full open position is indicated to the control system by the making of the maximum fire switch  22  and with air pressure sensed indicating that blower  18  is in operation, the timed pre-purge period is commenced by the control system. This operating condition continues for a selected timed period, preferably approximately twenty seconds. 
     At the conclusion of the above timed period, the control system sends a command to the control actuator  16  to return to the minimum fire position. Responsive thereto, the control actuator  16  rotates the rotary actuator arm  186  back to the minimum fire position as indicated in FIG.  16 . Such rotation causes the linear actuator arm  194  to translate to the right. When the linear actuator arm  194  reaches the minimum fire position, the profile plate  132  of the air valve  40  is in closed position. A small amount of secondary air is provided to the burner cabinet  14  through the secondary air bore  155 . Also, the translation of the linear actuator arm  194  to the right causes the switch actuator  212  to make the minimum fire switch  24  when the minimum fire position is reached. Making of the minimum fire switch  24  indicates to the control system that the gas burner  10  is in the minimum fire position. Approximately ten seconds after the minimum fire switch  24  is made, the pre-purge period concludes and the gas burner  10  is ready for ignition. 
     In the minimum fire position, with the blower  18  in operation, pressurized secondary air is being provided to the burner cabinet  14  via the secondary air bore  155 . Additionally, base air is passing through the base air aperture  170  of the air valve  40  to the base air passageway  113  of the burner gun  38 . Further, as indicated in FIGS. 14 a  and  15 , an initial quantity of primary air is passing through the primary air aperture  152  of the air valve  40  through the primary air inlet  52  of the gas valve  36  and thence to the nozzle  104  of the burner gun  38 . No gas is at this point being provided to the gas burner  10 . When the control system completes the pre-purge cycle and receives the signal from the minimum fire switch  24  indicating that the gas burner is in the minimum fire position, the control system opens a gas valve (not shown) permitting gas to flow into the gas flow passageway  49  defined in the gas valve  36 . Simultaneously, spark ignition is provided by spark igniter  101  at the face of the burner plate  100  to ignite the gas-air mixture. The minimum fire position corresponds to a fire rate that is 5% or less than the maximum firing rate of the gas burner  10 . 
     The gas-air being combusted at the minimum burn position is a mixture of gas passing around the tapered plug  60  at the orifice  58  combined with the minimum amount of primary air as indicated in FIGS. 14 a  and  15 . The gas and primary air are discharged via the radial orifices  110  defined in the nozzle  104  into the blast tube  94  to be consumed in the combustion chamber of the furnace. As the gas-primary air mixture emerges from the radial orifices  110 , the mixture is combined with the base air emerging from the base air orifices  114 . 
     A flame safeguard sensor  124  is positioned proximate the interior face of the burner plate  100 . After spark ignition at spark igniter  101  is energized, a short trial period for ignition occurs. If the flame safeguard sensor  124  does not detect flame at the end of the trial period, the flame safeguard sensor  124  provides a signal to the control system. The control system goes into safety lockout and must be manually reset before an attempt at burner ignition will occur. If the flame safeguard detects ignition, a signal is sent to the control system and the gas burner  10  will continue to operate as long as the control system requires it and as long as the flame safeguard sensor  124  is detecting flame. 
     At this point, the control system may command a higher burn rate for the gas burner  10 . Such command is sent to the control actuator  16  which causes the rotary actuator arm  186  to rotate out of the minimum fire position toward the maximum fire position. Such rotation causes the linear actuator arm  194  to translate to the left as depicted in FIG.  16 . This translation simultaneously causes a number of events to occur. The first such event is the switch actuator  212  unmakes the minimum fire switch  22 . The tapered plug  60  is partially withdrawn from the orifice  58 . This increases the area in the orifice  58  that is open to the passage of gas. Accordingly, an increased volume of gas flows to the burner gun  38 . The increased volume of gas flow requires an increased volume of airflow as well. Accordingly, the sliding plate  156  of the air valve  40  also translates to the left. Such translation does not affect the flow of secondary flow out of the secondary air bore  155  and does not affect the flow of base air out of the base air aperture  170 . 
     Translation of the sliding plate  156  progressively opens the secondary air aperture  138 . Additionally, the characterized aperture  150  is also progressively opened. Secondary air then flows through the secondary air aperture  138  and through the characterized aperture  150  to flood the interior of the burner cabinet  14  and to flow into the furnace for combustion via the secondary air orifices  102  defined in the burner plate  100 . Simultaneously, the volume of primary air is increased as indicated in the schedule depicted in FIG. 14 d . As the sliding plate  156  continues to the left, the primary air is increased. When the firing rate increases beyond a certain point as indicated in FIGS. 14 c  and  14   d , the primary air is cut off. Primary air is not needed beyond the cut off point for good combustion and the primary air needlessly adds to the pressure drop at the radial orifices  110  defined in the nozzle  104 . 
     As commanded by the control system, the linear actuator arm  194  may continue to the left to the maximum fire position. In the maximum fire position, the switch actuator  212  on the linear actuator arm  194  makes the maximum fire switch  22 , however, the signal from the maximum fire switch is used only during the pre-purge operation. Additionally, the tapered plug  60  has been withdrawn from the orifice  58  to the maximum extent possible, thereby opening the area for the passage of gas through the orifice  58  to the maximum, creating the maximum area of the orifice  58  for the flow of gas. The air valve  40  is also in its full open position. In such position, primary air is cut off, the base air is flowing, the secondary air aperture  138  and the characterized aperture  150  are fully open, admitting the maximum amount of secondary air into the burner cabinet  14 . 
     Numerous characteristics and advantages of the invention have been set forth in the foregoing description, together with details of the structure and function of the invention, and the novel features thereof are pointed out in the appended claims. The disclosure, however, is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts, within the principal of the invention, to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.