On-Off Burner Control by Cycle Time Variation

1. Field of the Invention

The present invention relates to the field of the control of burners of a fuel-fired system and, more specifically, to the control of the input rate of ON-OFF burners by varying the cycle time (hereinafter referred to as "pulse spacing").

2. Prior Art

It has been state of the art to vary a directly controlled variable by shifting the trailing edge of the ON-pulse to vary the duty factor correlating the pulse duration and the pulse separation, keeping the pulse spacing constant. If such a control method is adopted, both the pulse duration and the pulse separation are changed. However, if said control method is applied to control ON-OFF-controlled burners of a fuel-fired system, it is a disadvantage that such burners can only be operated at medium load, since burners will only emit oxides of nitrogen at a relatively low rate, and other combustion conditions will only be satisfactory, if said burners remain ON for a first minimum period, depending on burner design, forming part of the total firing cycle, and remain OFF for a second minimum period also depending on burner design. The useful (medium-load) range for the operation of such burners could be widened by increasing the time length of the total firing cycle, but such an increase would cause the response of the control system to be slower. The present invention has remedied said disadvantage.

It is an object of the present invention to improve the rangeability of burners used in a fuel-fired system while achieving satisfactory burner operating and combustion conditions and keeping the total firing cycle short in order not to affect system response.

The present invention describes a system for the control of ON-OFF-controlled burners for controlling the temperature in a fuel-fired system. For temperature control, the burners of said fuel-fired system are controlled by a feedback control system. Said control system comprises temperature measuring means, a controller for temperature control and final controlling means translating the controller output value into an appropriate burner input rate as well as a digital central processing unit determining pulse spacing, the duration of the burner ON-pulse and ON-pulse separation. A minimum pulse spacing, being the sum of a minimum pulse duration and a minimum pulse separation, is preset and used as an input rate reference value. Using said input rate reference value for said minimum pulse spacing, the burner input rate may be controlled in accordance with process requirements by varying said pulse spacing through varying either said pulse separation or said pulse duration. To obtain an input rate lower than said reference value, pulse separation is increased, and to obtain an input rate higher than said reference value, pulse duration is increased, keeping pulse separation constant at its minimum value, thereby substantially improving the rangeability of an ON-OFF-controlled burner, minimizing pulse spacing and thus ensuring fast response.

Unlike conventional control systems for ON-OFF-controlled burners, the control system divulged by the present invention varies either pulse separation or pulse duration whenever the input rate is varied. Whenever any such input rate variation is made, the second portion of the pulse spacing (either the pulse duration or the pulse separation) is maintained at its preset minimum value. Pulse spacing will thence be altered and no longer be equal to the minimum pulse spacing whenever an input rate variation is made. It is an important advantage of the present invention, proposing the simultaneous variation of pulse spacing and duty factor, that the rangeability of the ON-OFF-controlled burners controlled in accordance with the teachings of the present invention is fully exploited without affecting combustion behavior, pulse spacing always being minimized for the input rate set. If said input rate is high, pulse separation is at a minimum and if said input rate is low, pulse duration is at a minimum. It is a further advantage of the present invention that the ON-time or the OFF-time will never be shorter than the minimum ON-time or the minimum OFF-time, as the case may be, even if the deviation output signal representing deviations between the measured values and the set point values changes suddenly.

A particularly fast response is achieved in an embodiment of the present invention by presetting a minimum pulse spacing equal to the sum of a minimum pulse duration and a minimum pulse separation adapted to the burner being controlled. Said presetting, and the input rate reference value allocated thereto, correspond substantially to the middle of the useful range in which said burner may be operated, with pulse frequency being highest at said presetting. Whenever a higher input rate is required for adequate process operation, pulse separation is kept at its minimum value and pulse duration is increased, and whenever a lower input rate is required for adequate process operation, pulse duration is kept at its minimum value and pulse separation is increased.

In an alternative embodiment of the present invention, a maximum pulse duration is preset, the input rate reference value being the maximum input rate, and the actual input rate is controlled by varying pulse separation, keeping said maximum pulse duration constant.

According to another aspect of the present invention, the method divulged herein may easily be adapted to different fuel-fired systems by selecting pulse duration and pulse separation presettings individually for the final controlling means being used and/or for the fuel-fired process being controlled and storing the values so preset in a memory, a microprocessor being used to form actual pulse durations and actual pulse separations, employing the parameters so stored and thereby controlling the input rate. In a preferred embodiment of the present invention the control of all final burner controlling means, respectively valves, of a fuel-fired system is synchronized by a microprocessor with respect to pulse positions and duty factors.

A fuel-fired system so controlled may be employed for heating or alternately heating and cooling, and the control of the valves of all burners or other valves of such a fuel-fired system may be out of phase but synchronized if separate control circuits are provided.

In a preferred embodiment of the present invention, the central processing unit is mounted on a plug-in unit and coupled with the control means and the final burner controlling means via input interface means and output interface means respectively. Said plug-in unit is provided with a front panel on which display means, input means and operator control means are arranged, said input means and said operator control means being used to input parameter values and, more specifically, minimum pulse durations and minimum pulse separations, the phase angles of valves controlled separately, data to select one of a multitude of set point generating devices and/or data to select one of a multitude of channels used to control several fuel-fired systems being operated by one common central processing unit.

A central processing unit as proposed by the present invention may be used virtually for any fuel-fired system provided that the parameters stored in the programmable memory are changed as may be required. The same plug-in unit may thus be used for different fuel-fired systems and for any input rate. The present invention may hence be exploited, for example, to operate a furnace designed for a temperature of 1400.degree. C. at, for instance, a temperature of 140.degree. C. without affecting combustion quality and thus to use said furnace at such a low temperature as a reheating furnace.

Further details of the present invention are revealed in the claims hereinbelow, and it is apparent to any person versed in the art that any such details released in said claims may be combined in accordance with the teachings of the present invention.

The present invention will now be described with the help of preferred embodiments illustrated in the accompanying drawings in which:

FIG. 1 is a block schematic diagram showing an embodiment of the present invention as used for controlling a furnace equipped with burners controllable by an ON-OFF control system;

FIGS. 2A-2D are pulse diagrams representative of a first mode of operation;

FIGS. 3A-3D are pulse diagrams representative of a second mode of operation; and

FIG. 4 is a view of a central processing unit mounted on a plug-in unit provided with a front plate carrying operator control means, input means and display means.

A system for the improved control of ON-OFF-controlled burners or burner groups used in a fuel-fired system is described. In the following description, numerous specific details are set forth, such as numbers of burners or other components, burner rangeabilities and details of the configuration and arrangement of display means and operator control means, etc., in order to provide a thorough understanding of the present invention. It will be obvious, however, to one skilled in the art that the present invention may be practiced without these specific details. In other cases, well-known circuits and structures have not been described in detail in order not to unnecessarily obscure the present invention.

Referring first to FIG. 1, a block schematic diagram of a control loop for a fuel-fired system 1 is shown. Said loop comprises temperature measuring means 3, a controller 5, comparing an actual temperature T.sub.act derived from said measuring means with a preset set point temperature T.sub.set and generating an output signal .DELTA.T representing the deviation between said actual temperature and said set point temperature, and final burner controlling means 7. Said means 7 controls the ON-time and the OFF-time of the eight ON-OFF-controlled burners 10 with which the furnace 1 is provided in the embodiment of the present invention depicted in FIG. 1 in response to the deviation .DELTA.T so that the actual temperature T.sub.act measured in said furnace 1 is maintained at the preset set point temperature T.sub.set. An arrangement of such a kind is known from prior art and is used in conventional furnace engineering design.

According to the teachings of the present invention, a central processing unit 9 is included in the control loop between the output of controller 5 and the control signal input (line 20) of the final controlling means 7 controlling burners 10. In the embodiment of the present invention depicted in FIG. 1, said central processing unit 9 comprises a microprocessor 11 with an integrated electrically erasable programmable read-only memory, a read-only memory 13 serving as a program memory, an input interface 14 coupled with controller 5 and microprocessor 11, an output interface 15 with an appropriate output driver coupled with said microprocessor 11 and the final burner controlling means 7, a remote control input interface 16, display means 17 and appropriate input and operator control means 18 for parameter input. Said display, input and operator control means are coupled with the programmable memory and the central processing unit and mounted on a control panel 19.

Referring now to FIGS. 2 and 3, the pulse diagrams presented therein provide an understanding of the control method proposed by the present invention.

FIG. 2 depicts a method of controlling the ON-OFF-controlled burners 10 using a central processing unit 9 acting upon final burner controlling means 7 by varying pulse spacing t.sub.ps to obtain different input rates over a wide range. FIGS. 2A through 2D show pulse trains representing burner ON-times tON and burner OFF-times tOFF for different input rates corresponding to different pulse spacings t.sub.ps. The pulses in the diagrams correspond to electric signals sent by central processing unit 9 via output interface 15 through line 20 to final controlling means 7, said final controlling means 7 controlling fuel flow to burners 10. The times of pulse duration and pulse separation depicted schematically in said FIGS. 2A through 2D correspond to the times in which such electric signals are present or absent and thence to the ON-times or OFF-times of said burners 10.

FIG. 2A represents the minimum input rate for the described embodiment equal to 1% of the maximum input rate, the pulse duration being set for the minimum ON-time tON.sub.min being a characteristic preset value for each ON-OFF-controllable burner 10. The OFF-time tOFF corresponding to the separation between two consecutive pulses is at a maximum for a minimum input rate, the pulse spacing t.sub.ps in FIG. 2A, representing the minimum input rate, being equal to the sum of tON.sub.min and tOFF.sub.max.

Referring now to FIG. 2B, the burner input rate is increased relative to the burner input rate set by the pulse sequence depicted in FIG. 2A. To achieve said increase, the OFF-time tOFF between two consecutive pulses is decreased, whereas the ON-time corresponding to the pulse duration is maintained at the minimum ON-time level tON.sub.min. The pulse spacing t.sub.ps is hence also reduced by a time equal to said decrease in said OFF-time.

Referring to FIG. 2C next, the diagram shows the pulse train with the minimum pulse spacing. For the minimum pulse spacing t.sub.ps.sbsb.min equivalent to an input rate of XX %, both the ON-time and the OFF-time are set at the shortest intervals allowed by the burner manufacturer. The operating conditions depicted in FIG. 2C are preset in the programmable memory and serve as reference conditions. If the input rate is increased relative to the above XX %, then the control system will not vary the OFF-time tOFF, but vary, that is increase, the pulse duration corresponding to the ON-time tON, whereas the OFF-time is maintained at the minimum OFF-time level tOFF.sub.min, the pulse spacing t.sub.ps increasing by the same amount as the ON-time tON (see FIG. 2D). The methods of varying pulse separation or pulse duration depicted schematically in FIG. 2 hence allow the variation of input rates over a wide range hitherto not achieved without decreasing ON-times and OFF-times below the minimum ON-times and the maximum OFF-times admitted for a specific burner and the consistent minimization of pulse spacing, thereby keeping response times short.

Referring now to FIG. 3, the mode of operation presented therein is typical of the application of the present invention to a fuel-fired system used for heating and cooling, the air valves and the fuel valves being controlled separately, but in a synchronized mode. The input rates depicted in FIGS. 3A through 3D are similar to the input rates depicted in FIGS. 2A through 2D. In both applications shown to provide a clear understanding of the present invention, the input rate is initially at its lower limit (as in the case depicted in FIG. 3A) and then first increased by decreasing the OFF-time tOFF, and thereafter, upon reaching an input rate reference value set at 50% of the maximum input rate in the example presented in FIG. 3, said OFF-time having been decreased to the minimum OFF-time tOFF.sub.min is kept constant and the input rate is increased further by increasing pulse duration. Unlike the mode of operation depicted in FIG. 2, the operating mode shown in FIG. 3 provides for the separate control of the air valves and the fuel valves. The air valve of each burner is opened at time t.sub.1 prior to burner ignition, ignition being then initiated, whereas fuel valve opening is delayed by a constant delay t.sub.d. Fuel flow is further interrupted before air flow.

As FIGS. 3A through 3C show, the input rate is increased to an input rate of 50% of the maximum input rate by decreasing pulse separation both between consecutive air pulses 22 and consecutive fuel pulses 23, the durations of pulses 22 and 23 remaining at their minimum values as the input rate is being increased. Upon reaching the input rate reference value, as shown in FIG. 3C, the OFF-time is at its minimum and the pulse train frequency is highest for the burners used in the application, the train frequency and the pulse spacing t.sub.ps being in agreement in the case of synchronized air and fuel control. As process input rate requirements increase further beyond said 50%, the OFF-time is kept at a minimum, while the ON-time is increased.

Further modes of operation, such as heating with variable pulse durations and pilot burner/main burner operation and heating with variable pulse separations and separate air valve and fuel valve control, may be implemented using pulse configurations similar to those presented in FIG. 3, with only the delays of the synchronized pulses 22 and 23 being different.

Under certain conditions, it may be appropriate to control heating, heating/cooling and/or pilot burner/main burner functions using a fixed (maximum) pulse duration and a variable pulse spacing corresponding to a variable pulse separation. Rangeability would then substantially correspond to the difference between FIG. 2C and FIG. 2A, the fixed pulse ON-time being tON.sub.max rather than tON.sub.min.

If an appropriate multi-channel arrangement is selected, the central processing unit 9 may furnish different pulse trains for the control of different groups of burner valves or other control valves. All pulses forming part of pulse trains derived from the central processing unit 9 and carried by one channel are synchronized with each other irrespective of any phase shifts, as the principle depicted in FIG. 3 demonstrates.

The central processing unit with all associated input means, display means and operator control means is mounted on a plug-in unit 30 shown in FIG. 4. Said plug-in unit is provided with a main card frame 32 carrying the power supply unit 31 and the main functional assemblies represented by the blocks in FIG. 1. The different interfaces 14 through 16 are cards known from other applications and inserted in appropriate slots of card frame 32. The microprocessor 11, 12, the read-only memory 13 and the power supply unit 31 are mounted directly on said frame 32. The interfaces and the driver circuitry for the operator control, input and display means visible on the control panel 19 are integrated in board 40 on the rear side of said panel 19. The display means includes a four-digit display window 33 which shows the actual input rate when the central processing unit is in the operational mode and the applicable parameter values while parameters are being inputted and may also be used for check display during maintenance work. A set point input display unit 34 is arranged below said four-digit display window 33, said display unit 34 having light-emitting diodes to indicate the set point source providing set point input. If "C" is lit, then controller 5 is a computer and the set point temperature is being inputted via serial interface 16, if "mA" is lit, then controller 5 is a continuous controller, if the third light-emitting diode is lit, then set point input is by a three-step controller, and if "M" is lit, then set point input is manual. A further display arrangement 35 is provided with a total of eight light-emitting diodes to indicate what output driver is being addressed by the central processing unit. A rocker switch 36 is also provided in the front panel for manual set point input rate input and for the input or modification of parameters such as minimum pulse durations and minimum pulse separations. Depending on the direction in which the switch is actuated, the parameter or value selected is increased or decreased by increments. A selector switch 37 is provided to select the set point input rate input mode or to select parameters. The push buttons may also be used to select the mode of operation.

The different operator control and input functions described hereinabove allow the adaptation of the central processing unit 9 mounted on the plug-in unit 30 for any ON-OFF-controlled burner or group of burners. Said adaptation may be made easily by any user actuating switches 36 and 37 provided for parameter input. For protection against unauthorized parameter input, the system may be safeguarded by a security code, blocking programmable memory 12.