Mobile asphalt production machine

A mobile asphalt production machine mixes bituminous concrete at the job site through a mixing auger mechanism mounted in an insulated housing. Propane burners provide heat within the mixing auger mechanism and within the insulated housing to produce a hot-mix bituminous concrete in large or small batches. The components of the asphalt mixture are stored in separate bins that can be re-filled at the job site to provide a continuous supply of asphalt. Hot liquid bitumen is added to heated aggregate within the mixing auger mechanism. Recycled asphalt can be added through a port in the mixing auger for incorporation into the mixture. Controls permit the rate of flow of each individual component to be selectively varied in order to change the recipe for the mixture and to provide calibration of the component, while a master control will maintain the pre-established flow rates through a variable speed of operation.

BACKGROUND OF THE INVENTION

The present invention is directed to machines that pave roads and, more particularly, to a self-contained mobile vehicle that is operable to create hot-mix bituminous concrete, asphalt, at the job site.

Roadways are typically formed with either a Portland cement concrete roadway or a roadway formed from bituminous concrete, which is often referred as asphalt. When the roadway surfaces deteriorate, the repair thereto is typically done with an asphalt patch, even the Portland cement concrete road surfaces. Very often, small repairs are done with cold-mix asphalt, which does not have a long life span and deteriorates quickly. Large repairs, even complete re-surfacing of the roadway, are accomplished with large paving machines that receive supplies of hot-mix asphalt from a central mixing plant. Large trucks carry the supply of hot-mix asphalt to the paving machines, which spread the asphalt into a wide swath having a uniform thickness and a smooth uniform surface.

The central asphalt mixing plant is a fixed asset that cannot be easily moved from one location to another. The distance from the job site to the central mixing plant, as well as the ambient temperature, will define the temperature of the hot-mix asphalt at the time of delivery to the job site. Once hot-mix asphalt is cooled, the material hardens and cannot be spread easily or compacted at the job site. The shortness of time during which the bituminous concrete material remains pliable makes the material difficult to be utilized for small repair jobs, or for jobs that are a substantial distance from the central mixing plant. Such problems are particularly difficult to overcome when the ambient temperatures are low, as will be found in the Northern states during the winter months.

In U.S. Pat. No. 3,820,914, granted to Harold M. Zimmerman on Jun. 28, 1974, a self-contained mobile asphalt mixing and applying apparatus was disclosed. This machine depicted in the '914 patent was expensive to manufacture and utilized a mixing drum that was rotated to mix and heat the aggregate for discharge into a mixing auger where the hot liquid bitumen was added to manufacture the bituminous concrete mixture. While this arrangement was successful in mixing small patches of hot-mix bituminous concrete, the volume required for large repairs and/or re-surfacing projects could not be produced. Furthermore, the machine disclosed in the '914 patent was not capable of recycling old bituminous concrete material.

Accordingly, it would be desirable to provide an improved mobile asphalt missing machine that could provide substantial volume of bituminous concrete at job sites.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a mobile asphalt production machine that is capable of producing large quantities of bituminous concrete at a job site.

It is another object of this invention to provide a mobile asphalt production machine that carries supplies of the component material needed to produce bituminous concrete to be mixed at the job site.

It is a feature of this invention that the bituminous concrete is produced at the job site and does not have to be transported from a central mixing plant to the job site.

It is an advantage of this invention that the bituminous concrete can be discharged and placed at the job site immediately after being mixed.

It is another advantage of this invention that the temperature loss of the hot-mix bituminous concrete mixture is minimized from the moment of production to the moment of utilization at the job site.

It is another feature of this invention that old bituminous concrete material can be recycled into the on-site production of the bituminous concrete mixture.

It is still another feature of this invention that a mixing auger mechanism is utilized to produce continuously a bituminous concrete mixture.

It is still another feature of this invention that the machine incorporates a hopper for accumulating a limited supply of bituminous concrete mixture before being discharged to the job site.

It is still another advantage of this invention that the production of bituminous concrete is continuous to meet the demand of a high volume project.

It is yet another feature of this invention that the mixing auger mechanism is mounted within an insulated housing that is heated independently of the mixing auger mechanism to maintain temperature control of the asphalt mixture.

It is still another feature of this invention that the gaseous discharge from the mixing auger mechanism and from the heated housing is passed through a bag housing to control emissions.

It is yet another feature of this invention that the component supplies can be replenished at the job site to provide continuous long-term production of bituminous concrete at a job site.

It is a further feature of this invention to provide a control mechanism that allows for individual adjustment of the component materials for flow into the mixing auger mechanism to permit the mixture to be varied as desired.

It is yet another advantage of this invention that the individually adjustable flow controls can be pre-set for operation through a master control device.

It is still a further feature of this invention that the individual operative components of the mobile asphalt production machine can be calibrated for on-site production certification as needed.

It is a further advantage of this invention that different mix recipes can be accommodated with the individually adjustable flow controls for the operative components providing sand, stone, liquid bitumen (AC), and optional additives into the mixing auger.

It is still a further feature of this invention that the mixing auger mechanism utilizes a ribbon conveyor.

It is yet another object of this invention to provide a mobile asphalt production machine that can produce small batch of bituminous concrete at a job site for use in the repair of potholes on an existing roadway surface.

It is another feature of this invention that the mobile asphalt production machine can be equipped with a dispensing auger and chute for filling potholes with bituminous concrete manufactured at the job site.

It is another advantage of this invention that the cost of pothole repairs can be reduced by the manufacture of bituminous concrete at the job site, rather than producing the bituminous concrete at a remote location and transporting to the job site.

It is still another feature of this invention that a waste material receptacle is provided for disposal of debris accumulated in the pothole to be cleaned-out before being repaired.

It is a further object of this invention to provide a mobile asphalt production machine that is durable in construction, inexpensive of manufacture, carefree of maintenance, facile in assemblage, and simple and effective in use.

These and other objects, features and advantages are accomplished according to the instant invention by providing a mobile asphalt production machine that mixes bituminous concrete at the job site through a mixing auger mechanism mounted in an insulated housing. Propane burners provide heat within the mixing auger and within the insulated housing to produce a hot-mix bituminous concrete in large or small batches. The components of the asphalt mixture are stored in separate bins that can be re-filled at the job site to provide a continuous supply of asphalt. Hot liquid bitumen is added to heated aggregate within the mixing auger mechanism. Recycled asphalt can be added through a port in the mixing auger for incorporation into the mixture. Controls permit the rate of flow of each individual component to be selectively varied in order to change the recipe for the mixture and to provide calibration of the component, while a master control will maintain the pre-established flow rates through a variable speed of operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to theFIGS. 1-7, a mobile asphalt production machine incorporating the principles of the instant invention can best be seen. The asphalt production machine10is mounted on a truck chassis5, shown in phantom inFIG. 1, to provide mobility for the machine10. The truck chassis5is preferably a tri-axle configuration, due to the weight of the machine10and of the aggregate and other components stored in the machine10, as will be described in greater detail below. The truck chassis5includes a conventional engine and drive transmission (not shown) for affecting mobility of the machine10. The arrangement is somewhat similar to the mobile concrete mixers of the type seen in U.S. Pat. No. 4,538,916, issued to Harold M. Zimmerman, on Sep. 3, 1985, which carry discrete quantities of the various materials and components used to produce Portland cement concrete. Such concrete mixing units have been marketed under the trademark ZIM-MIXER by Zimmerman Industries, Inc. of Ephrata, Pa.

The mobile asphalt production machine10is provided with at least four storage bins or tanks12,14,16,17that are enclosed structures with an access opening that permits a re-filling thereof on the job site. Storage bin12is located at the forward end of the machine10and can be used to provide an optional supply of fine material, additives, or other materials desired for the production of the asphalt mixture to be created. The lateral aggregate storage bins14,16are located behind the first storage bin12and are best seen inFIGS. 4-6. These laterally disposed storage bins14,16stored large supplies of aggregate and sand for use in the production of the asphalt mixture. The fourth storage bin17is located behind the lateral bins14,16and is used to contain a supply of liquid bitumen. A burner or heater (not shown) is used to keep the bitumen at a desired temperature to maintain liquidity in the bitumen.

The lateral storage bins14,16are sloped, as is seen inFIGS. 4 and 6, to direct the material within the bins14,16toward a chain and slat conveyor20,22located at the bottommost portion of the bins14,16. The conveyors20,22are preferably driven by hydraulic motors (not shown) and are independently adjustable as to speed of operation to permit the flow rate of aggregate and sand to be adjusted as desired according to the mixture to be created. The conveyors20,22pull materials from the lateral storage bins14,16to direct material into a bucket elevator25located between the first storage bin12and the lateral storage bins14,16. Similarly, the first storage bin12can have a chain and slat conveyor24to deliver material therefrom rearwardly into the bucket elevator25. The conveyors20,22can be constructed to have a return run that passes through the respective storage bins14,16, as is depicted inFIG. 6. The details of the chain and slat conveyors20,22are described in aforementioned U.S. Pat. No. 4,538,916, the contents of which are incorporated herein by reference. The last storage bin17has a discharge pipe19through which the liquid bitumen is pumped into the mixing auger mechanism30, as is described in greater detail below.

The mixing auger mechanism30, as can be seen inFIGS. 1-4, is mounted at the top of the machine10above and between the lateral storage bins14,16. As opposed to the mixing drum disclosed in the '916 patent, the mixing auger mechanism30, as described herein, can be oriented substantially horizontally. The mixing auger mechanism30includes a longitudinally extending ribbon auger32that cooperates with a semi-circular auger tube35against which the ribbon auger32works. The ribbon auger32is also provided with fins or paddles33mounted on the periphery of the ribbon auger32. The fins or paddles33operate to throw aggregate and other material within the auger tube35upwardly to enhance mixture thereof within the mixing auger mechanism30. The top part of the auger tube35is preferably closed by an insulated lid37that is hinged to open the auger tube35for inspection, servicing and clean-out, as needed. The auger tube35is set within a similarly shaped housing40that is insulated and spaced outwardly from the auger tube35to provide an annular space therebetween. The annular space and the insulation provided by the housing40helps to keep the heat, as will be described in greater detail below, within the auger tube35.

A pair of burners42,44is mounted at the forward end of the mixing auger mechanism30to provide a source of heat for the mixture of hot-mix bituminous concrete within the mixing auger mechanism30. The burners42,44can be fired from oil or, preferably, by propane. The burners42,44are fueled from storage tanks43for the propane and supply a substantial amount of heat energy into the mixing auger mechanism30. The first burner42is positioned to fire into the auger tube35to provide a direct heat source into the mixing auger mechanism30, while the second burner44is positioned to fire into the insulated housing40below the auger tube35. The combustion gases from the burners42,44can be trapped within the auger tube35and the housing40, respectively, and discharged at the rear of the mixing auger mechanism30into a bag housing45to eliminate unwanted emissions into the environment. However, since the combustion gases from the burner44do not mix directly with the bituminous concrete mixture being produced, the combustion gases from the second burner44can be discharged through a flue49positioned in front of the bag housing45while the combustion gases from the first burner42are exhausted through the bag housing. Water for emergency use can be stored in a tank15positioned below the first storage tank12opposite the propane storage tanks43.

The bag housing45can be positioned on either side or on both sides of the mixing auger mechanism30and can include one or more exhaust fans46that draw the combustion gases and airborne materials from the production of the hot-mix bituminous concrete mixture upwardly through a filter47before being discharged to the atmosphere. The filter47removes the contaminants in the combustion gases before being discharged. An access door (not shown) is provided at the rear of the machine10to permit access to the filter47for servicing and changing the filter as needed to maintain air quality controls.

The lid37of the mixing auger mechanism30is provided with an access port38that can be opened for the introduction of recycled bituminous concrete material. The access port38is located approximately mid-way along the auger tube35to give adequate opportunity for the recycled asphalt materials to become melted and incorporated into the mixture. The access port38is preferably closed during normal operation of the mixing auger mechanism30, but can be opened and connected to a delivery mechanism (not shown) that would be operable to introduce into the mixing auger mechanism30pieces of bituminous concrete that is being recycled into the new mixture.

The liquid bitumen is introduced into the mixing auger mechanism30at about the mid-point of mixing auger mechanism30, downstream of the access port37, via the discharge tube19delivering the heated liquid bitumen from the storage tank17. The liquid bitumen is sprayed into the auger tube35for mixing with the heated aggregate being thrown about by the ribbon auger32and the outwardly projecting fins33. The flow rate of the liquid bitumen is established through the control mechanism described in greater detail below and is proportionate with the flow rates of the aggregate and other materials delivered from the storage bins12,14, and16to create the desired asphalt mixture.

The mixing auger mechanism30discharges the mixed asphalt mixture out of the rearward end of the auger tube35into an enclosed, insulated accumulation hopper50that is operable to receive a supply of asphalt mixture from the missing auger mechanism30and accumulate the asphalt mixture until a conveyor52is actuated to remove the asphalt mixture from the hopper50into a distribution mechanism55. The distribution mechanism55can be in the form of a spreader56, as is depicted inFIG. 1, or an elongated, pivoted dispensing conveyor apparatus70, such as an auger conveyor71as depicted inFIG. 8, that would receive the asphalt mixture from the hopper conveyor52and direct the discharge thereof as needed within the reach of the auger conveyor (not shown). When operating at peak efficiency, the machine10would provide a continuous flow of mixed asphalt discharged from the rearward end of the auger tube35onto the hopper conveyor52for distribution externally of the machine10.

All of the operative components of the asphalt production machine10are preferably driven hydraulically, such as through the hydraulic motor34, and operatively controlled through the control mechanism60. One skilled in the art will recognize that other operative power could be used for the components of the machine10, but the use of hydraulic drives is convenient and easy to control selectively. The control mechanism60is best seen inFIG. 7and includes a first valve61for operation of the hydraulic motor34for varying the speed of rotation of the ribbon auger32. The speed of rotation of the ribbon auger32is directly related to the flow rate of the output from the mixing auger mechanism30.

Similarly, the control mechanism60also includes a second valve62for controlling the speed of operation of the conveyor24for the front storage bin12, a third valve63for controlling the speed of operation of the conveyor20in the lateral storage bin14, a fourth valve64for controlling the speed of operation of the conveyor22in the other lateral storage bin16, a fifth valve65for controlling the flow rate of the pump18for discharging the liquid bitumen through the discharge pipe19out of the storage tank17into the mixing auger mechanism30, and possibly a sixth valve (not shown) for the control of the operative speed of the bucket elevator25delivering materials into the mixing auger mechanism30. The two burners42,44are controlled by controls66,67, including on/off function and flow rate of propane thereto.

One of the operative functions described above, such as the pump rate of the liquid bitumen could be maintained as a constant parameter, so that the rate of delivery of the conveyors20,22and24could be varied selectively to change the ratio of the respective components for the asphalt mixture. Thus, the sixth control valve (not shown) is preferably optional.

A master control valve68overrides all of the other control valves61-65in that the master control valve controls the rate of flow of hydraulic fluid to the respective valves for operative control of the corresponding component. Thus, the control valves61-65set proportionate flows for controlling the operation of the corresponding operative component for delivery of the component for the asphalt mixture into the mixing auger mechanism30. Accordingly, if the second control valve62were set to deliver a first flow rate of 10 and the second control valve62were set to deliver a second flow rate of 5, twice as much aggregate in the first lateral bin14would be delivered to the bucket elevator as the sand, or other aggregate, in the second lateral bin16. The master control valve68, however, dictates the overall speed at which the conveyors deliver material to the bucket elevator25, even though twice as much aggregate from storage bin14will be delivered as compared to the material in storage bin16, according to the example set forth herein.

As is seen inFIG. 7, the control mechanism60is a banded hydraulic system that delivers a flow of hydraulic fluid under pressure from a reservoir79to the master control valve68which is operable to divert a selected flow rate into the system, as described below, and return the remaining hydraulic fluid to the reservoir79. The hydraulic fluid delivered into the system passes into a first control valve61which diverts a preselected flow rate to the hydraulic motor34driving the rotation of the mixing auger mechanism30, passing the remaining hydraulic fluid on to the second control valve62along with the return hydraulic fluid from the hydraulic motor34. Similarly, the second control valve62diverts the preselected flow rate to the hydraulic motor driving the operation of the conveyor24, which can be set anywhere from a zero flow rate to a maximum flow rate. The remaining hydraulic fluid along with the return hydraulic fluid from the conveyor24is then received at the third control valve63which diverts hydraulic fluid at the preselected rate set for the operation of the conveyor20, and then to the fourth control valve64which diverts fluid at the preselected flow rate to the conveyor22, and so on through all the control valves until the final control valve in the series returns the hydraulic fluid to the reservoir79.

Each operative component and the associated control valve61-65has a counter69associated with the operation of that particular component so that the counter records the number of revolutions of the corresponding hydraulic motor which is then calibrated to a corresponding quantity of material delivered by the operative component. For example, an reading of10on the counter for the conveyor20may indicate that 100 pounds of sand has been delivered into the mixing auger mechanism30. Repeated calibrations of the counter readings to the volume of material delivered can be obtained by opening a test door (not shown) corresponding to the component being calibrated and weighing the amount of material delivered for a given counter reading. Various recipes for bituminous concrete can then simply be dialed into the control valves. If stone is being delivered by the conveyor22and, as an example, no stone is desired for the particular recipe being mixed, the setting on the control valve64for the conveyor22would be zero, thus no stone would be delivered to the elevator25.

Accordingly, the throughput of the machine10, i.e. the rate of production of the asphalt mixture discharged from the auger tube35, can be set through the master control valve68. Furthermore, the operation of the first valve61controlling the operative speed of the ribbon conveyor32can be tied directly to the master control valve68, instead of being an independent control. The higher the desired output of the machine, the greater the amount of component materials that need to be delivered into the mixing auger mechanism30, and the greater the rotational speed of the ribbon conveyor32is needed.

In operation, the asphalt production machine10mounted on the truck chassis5is driven to the selected job site, which can be either a small or large repair patch or a re-surfacing of the roadway surface. The mixture ratios of aggregate types to the flow rate of liquid bitumen is selected through the manipulation of the control valves62-65to provide a selected “recipe” for the asphalt mixture. Generally, the proportional valves62-65are pre-set and not normally changed, unless the “recipe” needs to be adjusted. The master control valve68is manipulated to start the operation of the various operative components of the machine10to deliver aggregate and sand from the storage bins12,14and16into the bucket elevator25. The bucket elevator25raises the combined aggregate materials to the forward end of the mixing auger mechanism30and discharges the aggregates into the beginning of the auger tube35.

The burners42,44have been ignited to pre-heat the mixing auger mechanism30and the housing40and operate to heat the aggregates being delivered into the front of the auger tube35. The rotating ribbon conveyor32both transports the heated aggregates along the auger tube35and tosses the aggregates into the air to obtain a greater exposure of the individual aggregate components to the heated air. Re-cycled bituminous concrete materials can be introduced into the aggregate mixture through the access port38. Liquid bitumen is then pumped from the storage tank17through the discharge pipe19into the central part of the mixing mechanism30. The tossing action of the fins33on the ribbon conveyor flights32substantially enhances the mixing of the aggregate particles with the liquid bitumen to effectively coat the individual aggregate particles.

The mixed aggregates and liquid bitumen forms an asphalt mixture that is discharged from the rearward end of the auger tube35into the accumulation hopper50. If the operator is ready to utilize the discharged asphalt mixture, the operator then actuates the discharge conveyor52in the bottom of the hopper50to discharge the accumulated asphalt mixture into the selected distribution device55.

For a continuous large supply of bituminous concrete from the asphalt production machine10for utilization at the selected job site, the operator can have supplies of the component aggregates, sand, bitumen and propane delivered to a location adjacent the job site. A loader can then replenish the supplies of aggregate and other materials into the asphalt production machine10so that the production of bituminous concrete can continue without interruption.

Referring now toFIG. 8, a second embodiment of the mobile asphalt production machine10can be seen. In this second embodiment, the general components will be substantially the same as described above, except as noted below. This second embodiment of the mobile asphalt production machine10is particularly adapted to the repair of potholes and is specifically intended to be somewhat smaller than the first embodiment described above since continuous production of bituminous concrete is not a desired aspect of a machine dedicated to the repair of potholes. In fact, small batches of bituminous concrete are the production norm for such a machine, which aspect can be delivered by the instant invention.

The supplemental storage bin12at the forward end of the first embodiment of the machine10is replaced by a waste storage bin78which preferably has hinged doors discharging the waste material from the machine at the end of the day. Waste material would be the loose debris and the debris obtained during the cleanout of the pothole before patching material is delivered thereto. The insulated accumulation chamber50at the rear of the first embodiment of the machine10is also not needed as large batches of bituminous concrete is not anticipated for use in the repair of potholes. Thus, the insulated accumulation chamber50is eliminated, permitting the overall length of the vehicle10to be shortened. Furthermore, the sizes of the lateral sand and stone bins14,16do not need to be as large as desired for the first embodiment of the machine10, thus further reducing the size of the machine.

At the rear of the second embodiment of the machine10, as shown inFIG. 8, a surge hopper59is mounted to receive the discharge of mixed bituminous concrete from the mixing auger mechanism30through a discharge chute75. The surge hopper59is mounted in flow communication with a dispensing apparatus70that is pivotally mounted for movement through approximately 180 degrees of rotation about a vertical pivot74. The dispensing apparatus70includes a dispensing auger71receives bituminous concrete from the surge hopper59and conveys the asphalt to a delivery chute73through which the material is dropped into the pothole (not shown) to be repaired. The control of the hydraulic motor72powering the rotation of the dispensing auger71is preferably independent of the master control valve68so that material already mixed and received in the surge hopper59can be delivered into the pothole for use in the repair thereof without running the mixing auger mechanism30and other operative components, although alternatively each of those control valves could be reset at zero to prevent their respective operation while running the dispensing auger71from the banded hydraulic system that includes a control valve for operating the dispensing auger71, according to the principles of the control system described above.

This smaller version of the asphalt production machine is ideally adaptable for use as a pothole repair machine. As can be seen inFIG. 9, a pothole repair apparatus can be mounted at the rearward end of the machine10to enable an operator to produce a limited supply of hot mix bituminous concrete for use in repairing poleholes, even in the winter time, i.e. times of cold weather. The pothole repair apparatus80includes the dispensing auger71receiving freshly mixed bituminous concrete from the mixing apparatus30through the surge hopper59. As described above, the dispensing auger71is pivotally mounted to be capable of delivering a desired supply of bituminous concrete directly into the pothole.

The pothole repair apparatus80includes a heat shield81that is supported by frame supports82beneath the chassis5to be capable of moving vertically from an elevated transport position onto the surface of the ground G, as is indicated in phantom. The heat shield81is also preferably formed with a center pan member83that is not only positionally adjustable vertically, but also horizontally in a transverse direction so that the heat shield can be accurately placed over top of the pothole to be repaired. The center pan member83has mounted thereon a burner84that is operable to inject heat energy beneath the center pan member83to soften the existing bituminous concrete surrounding the pothole.

After the area around and at the bottom of the pothole has been softened through the application of heat energy, a tack hose85can be extended from the reel86on which the tack hose85is stored to spray liquid bitumen, or other suitable tack material onto the bottom surface of the pothole. The dispensing auger71can then be positioned to deliver the required amount of bituminous concrete into the pothole. After tamping suitably, the operator can swing a vibratory compactor88from a storage location on the chassis5by using a movable support89, such as a winch, to lower the compactor to the ground G where the bituminous concrete in the pothole can be compacted and properly repaired.

The pothole repair apparatus80can also include an air hose87that can be used to deliver a stream of compressed air to help clean out the fine debris in the bottom of the pothole before initiating the repair process as described above. The loose debris collected from the cleaning of the pothole to be repaired can be accumulated in a waste storage bin78suitably positioned on the chassis5, as is described above with respect toFIG. 8. Accordingly, the smaller version of the mobile asphalt production machine10can be utilized in a manner heretofore unknown. This pothole repair apparatus80can be utilized to provide a hot mix pothole repair at times of cold weather when normal asphalt production is not being conducted. As a result, conventional cold weather repairs are made with cold mix asphalt material. A similar process for repairing potholes is shown and described in Applicant's co-pending U.S. patent application Ser. No. 10/944,219, now U.S. Pat. No. 6,988,849, granted on Jan. 24, 2006, the contents thereof being incorporated herein by reference.

For example, a microprocessor (not shown) could be employed in conjunction with the control system60to provide an automated and/or remote control of each hydraulic control valve61-65,68. Furthermore, a microprocessor could be programmed with all the different desired recipes for bituminous concrete so that the necessary proportional changes to the control valves61-65would be made automatically when the recipe was selected on the microprocessor. A microprocessor can automate other functions on the machine10, including a monitoring of the temperature within the mixing auger mechanism30and adjust the firing of the burners42,44to maintain a desired temperature and to more efficiently utilize energy resources.