Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23N—REGULATING OR CONTROLLING COMBUSTION
  • F23N5/00—Systems for controlling combustion
  • F23N5/003—Systems for controlling combustion using detectors sensitive to combustion gas properties
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23N—REGULATING OR CONTROLLING COMBUSTION
  • F23N1/00—Regulating fuel supply
  • F23N1/02—Regulating fuel supply conjointly with air supply
  • F23N1/022—Regulating fuel supply conjointly with air supply using electronic means
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23N—REGULATING OR CONTROLLING COMBUSTION
  • F23N3/00—Regulating air supply or draught
  • F23N3/08—Regulating air supply or draught by power-assisted systems
  • F23N3/082—Regulating air supply or draught by power-assisted systems using electronic means
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23N—REGULATING OR CONTROLLING COMBUSTION
  • F23N5/00—Systems for controlling combustion
  • F23N5/18—Systems for controlling combustion using detectors sensitive to rate of flow of air or fuel
  • F23N5/184—Systems for controlling combustion using detectors sensitive to rate of flow of air or fuel using electronic means
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23N—REGULATING OR CONTROLLING COMBUSTION
  • F23N2223/00—Signal processing; Details thereof
  • F23N2223/08—Microprocessor; Microcomputer
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23N—REGULATING OR CONTROLLING COMBUSTION
  • F23N2225/00—Measuring
  • F23N2225/08—Measuring temperature
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23N—REGULATING OR CONTROLLING COMBUSTION
  • F23N2233/00—Ventilators
  • F23N2233/06—Ventilators at the air intake
  • F23N2233/08—Ventilators at the air intake with variable speed
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23N—REGULATING OR CONTROLLING COMBUSTION
  • F23N2235/00—Valves, nozzles or pumps
  • F23N2235/02—Air or combustion gas valves or dampers
  • F23N2235/06—Air or combustion gas valves or dampers at the air intake
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23N—REGULATING OR CONTROLLING COMBUSTION
  • F23N2235/00—Valves, nozzles or pumps
  • F23N2235/12—Fuel valves
  • F23N2235/16—Fuel valves variable flow or proportional valves
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23N—REGULATING OR CONTROLLING COMBUSTION
  • F23N5/00—Systems for controlling combustion
  • F23N5/24—Preventing development of abnormal or undesired conditions, i.e. safety arrangements

Definitions

• the present invention relates to apparatus for providing an air/fuel mixture particularly an air/fuel gas mixture to a fully premixed burner.
• the fuel gas is usually supplied from a main while the air is supplied by a fan.
• the volume flow rate of air is usually intended to be maintained in excess of the rate theoretically necessary for full combustion of the gas. Typically this excess amounts to 30%, and the burner is then said to be operating with 130% of the stoichiometric air requirement or, for brevity, "at 130% aeration".
• apparatus provides an air/fuel mixture to a fully premixed burner, the apparatus comprising means for providing fuel to the burner, means for supplying air at a variable flow rate to the fuel to form the mixture, means for sensing aeration of fuel combustion products and control means for controlling the air flow rate in dependence upon the aeration sensed such that the air flow rate is sufficient to maintain the aeration at or close to a predetermined value, the controller, in use, maintaining the air flow rate at one of a number of differing predetermined values which are in the form of a geometric series characterised by a constant value of the ratio between successive values.
• the geometric series contains a predetermined number N mMX of terms, each term being in accordance with the following relationship:
• Q N is the air flow rate at the Nth step in the predetermined series of steps
• Q ! is the air flow rate at step one in the series and therefore constitutes the lowest of the permitted rates of flow
• R is a constant term equal to the common ratio of the geometric series, the value of R being chosen according to the resolution desired between successive steps in flow rate, and
• N is a number uniquely identifying any individual step and having a lowermost value of unity and an uppermost value of N, ⁇ , the latter being determined jointly by the chosen value
• R is allocated a value of 1.025.
• the means for supplying fuel at a variable rate comprises a modulating fuel valve having a variable opening to vary fuel flow rate.
• the means for supplying air at a variable rate comprises a variable speed fan.
• the means for supplying air at a variable rate comprises a throttle valve.
• Figure 1 is a schematic view of a domestic combustion system in a gas-fired domestic heating appliance, together with control apparatus therefor, and
• Figure 2 is a schematic circuit diagram illustrating how the heat demand signal is produced.
• a domestic combustion system which comprises a gas boiler 1 located within a room-sealed casing 2 mounted on the inner surface of an outside wall 3 of a dwelling.
• the boiler 1 includes a fully-premixed gas burner 4 mounted on and sealed to an enclosure 5, the gas burner being designed to fire downwardly into an uppermost part of the enclosure 5 which forms a combustion chamber.
• the enclosure 5 terminates in a lowermost flue 6 which has a vertical part 7 immediately beneath the enclosure and a horizontal part 8 connected to the vertical part 7 and extending with a clearance 9 through a hole in the wall 3.
• the clearance 9 is formed by the horizontal part of a flanged outlet 10.
• the horizontal part 8 of the flue has a circumferential flange 11 spaced from the outer surface 12 of the wall 3.
• the flange 11 forms with a flanged guard 13 in the wall surrounding the clearance 9 and the outer surface 14 of the horizontal flue part 8 an air intake of the so-called "balanced flue" variety.
• the burner 4 has a plenum chamber 15 beneath which is located the burner plate 16. Upstream from the plenum chamber 15 is a mixing chamber 17 where the air and fuel gas meet and mix before combustion.
• Air for the burner 4 is provided by a variable-speed fan 18 connected to the mixing chamber 17.
• Fuel gas for the burner 4 is supplied by a gas supply pipe 19 which connects to the mixing chamber 17.
• the gas is supplied from a pressurised main in a conventional manner but the gas flow rate is controlled by a modulating gas valve 20 located in the gas line and shut-off gas valve 21.
• the modulating gas valve 20 has an opening area which is variable to provide variation in the flow rate of the fuel gas.
• Pipework 22 is provided to supply cold water to and remove heated water from the boiler 1, a portion 23 of the piping 22 being in serpentine form and located mainly in the enclosure 5 to enable the water to be heated by the combustion products, the part 23 having finning 24 to improve heat exchange between the combustion gases and the water. Water is pumped through parts 22, 23 and around a hot water and central heating system (not shown) by a water pump 25.
• the combustion system is controlled by a control means or controller in the form of a microelectronic control box 26. This controls the fan 18 via a line 27, the gas modulating valve 20 via a line 28 and the gas shut-off valve 21 via a line 29.
• a hot water temperature sensor 32 located on an external part of the pipe portion 23 delivers a voltage signal to the control box 26 via a line 33. If the water temperature is excessive, the controller 26 will close the valves 20, 21 via the lines 28, 29 respectively, preventing further operation of the burner 4 until the water temperature has fallen to some lower value.
• the device 34 is a standard feature forming no part of the present invention, it being mentioned for completeness only.
• a differential-pressure-sensing assembly 36 comprising a diaphragm-operated switch fitted with changeover contacts and an orifice plate through which the air flow for combustion passes, consequently falling in pressure by an amount related in a predictable manner to the rate of air flow.
• the diaphragm is located within a chamber which is thereby divided into two compartments, each of which is connected to a different side of the orifice plate, but is otherwise sealed.
• the diameter of the diaphragm is chosen to be such that the moving finger of the switch (not shown) will disengage from the zero-pressure (or “rest") contact and engage the pressure contact when the pressure difference across the diaphragm rises to a chosen magnitude; and the diameter of the orifice is selected so that this magnitude will be attained at some predetermined rate of air flow, under some particular set of operating conditions.
• the switch when activated at the predetermined air flow rate delivered by the fan 18 supplies a signal along line 37 to the control box 26 for purposes to be subsequently described.
• a signal indicative of the demand for heat is supplied to the control box 26 along line 38 from a demand signal processor 39, the connections to which are shown schematically in Figure 2.
• the processor 39 receives signals from a room temperature sensor 40 along line 41, a hot water temperature sensor 42 along line 43, a boiler water temperature sensor 44 along line 45, a hot water cylinder thermostat 46 along line 47 and a central heating/hot water programmer 48 along the lines 49 and 50.
• the processor 39 computes an appropriate heat demand signal for transmission to the controller 26 along line 38.
• the processor 39 may be an essentially conventional device: it forms no intrinsic part of the present invention.
• variable-speed fan 18 is an off-the-shelf item incorporating a brushless direct current motor and a sensor for supplying to the control box 26 signal pulses proportional in frequency to the rotational speed of the fan 18.
• the control box 26 supplies power and a control signal to the motor and receives pulses from the speed sensor, all via the multicore line 27.
• the control signal is supplied as a train of rectangular pulses of 1000 Hz frequency generated by the control box 26, the duration L ⁇ , of each 0-5 V pulse of the train being variable by the control box 26 over the range 0.0000 - 0.0010 second to control the speed of the fan 18.
• the time interval between successive pulses from the speed sensor is measured by the control box 26, translated into a rotational speed in revolutions per minute and encoded.
• This value is then compared with a series of similarly encoded reference fan speed values held in ROM in the control box 26, and any difference existing between the sampled and any selected one of these reference values is reduced to zero by adjustment of the duration of the control pulses supplied to the motor of the fan 18. In this way the control 26 is able to obtain and maintain a fan speed corresponding to the selected reference fan speed. In a combustion system of the type shown in Figure 1, if other factors remain constant, the rate of air flow is very nearly proportional to the rotational speed of the fan.
• control box 26 will be able to procure, very nearly, any one of a selection of alternative air flow rates by adjusting the duration L ⁇ of the control pulses, so as to equalise the corresponding reference fan speed value and the actual fan speed value implied by the signal from the sensor on the fan 18.
• this illustrates schematically the first 12 rows of a data look-up table which is stored in ROM in the control box 26.
• the first column of the table comprises "N" , the step number representing the number of a term in the geometric series which forms the basis of flow control in the present invention as described above.
• the second column in the table comprises the respective gas flow rate G in cubic metres/hour (m 3 /h) corresponding to each particular step number N.
• the flow rate at each step is approximately 2.5% greater than that at the preceding step, reflecting the intended value (1.025) of the common ratio of the geometric series.
• the third column in the table comprises the respective fan speed F in revolutions per minute (rev/min) corresponding to each value of N in column 1 of the look-up table.
• the flow rate at each step is approximately 2.5% greater than that at the preceding step.
• the fourth column in the table comprises the respective drive voltage Vmgv in volts, corresponding to each value of N in the table, for operating the modulating valve 20.
• the fifth column in the table comprises the nominal duration L_ of the fan speed control pulses in microseconds corresponding to each value of N, as supplied on line 27.
• each combination of ⁇ gas flow rate and fan speed is selected to provide a predetermined air/gas flow rate ratio corresponding to an intended percentage aeration of the combustible mixture, given fuel gas of an assumed theoretical air requirement for combustion (m 3 air/m 3 fuel gas) and a fan of assumed performance characteristics operating normally in a combustion system of an assumed flow resistance characteristic.
• the intended air/gas flow rate ratio may be made variable according to the rate of gas flow.
• this refinement has not been adopted in the present embodiment. We describe later methods of compensating for departures from the circumstances assumed in constructing the data look-up table, so that the air/gas flow rate ratio may remain as intended.
• Table 1 the data in Table 1 are shown as ordinary numbers. In reality, however, all tabular data are stored in digital form, in keeping with normal practice.
• the gas flow rates in Column 2 are stored as digital voltages representative of these gas flow rates on the basis of a fixed scaling factor.
• columns 3 and 5 may contain entries up to a value of N,,,,, higher than that to which entries in columns 2 and 4 extend.
• the program starts by resetting to zero in RAM, for later program purposes, two parameters C ⁇ and M, described below. It then reads the line 38, to find whether there exists on the line a voltage at least equal to a preset value V, ⁇ . If such a voltage is present, this indicates the existance of a demand for heat from the external source 39, as explained above. In that case, the control box 26 will carry out routine safety checks as in known combustion controllers. If these indicate danger, a value of zero will be stored into RAM for a signpost variable S and all further action will be suspended in a state of "lockout" until the user directs the program back to its startpoint by pressing a conventional "reset” switch on the control box 26, this also causing the program to change the value of S to unity.
• control box 26 will measure the value of L q , and find from the look-up table the associated nominal step number (N q ,) co . This number is then stored into RAM for convenience if more than one attempt to light the burner should prove necessary, or if the flame should become extinguished at some time after the burner has come into operation.
• N, 1 + C re + B (2)
• the index B is a constant preset in the program of the control box 26, in manufacture or during installation of the heating equipment, the value of B being selected to reflect the expected degree of variation in the properties of the fuel gas to be used by the burner 4. If no significant variation is expected, the index B would be preset to zero.
• control box 26 will start an operating-period timer and examine the value of the parameter M.
• the value of M will be zero.
• the program will store into RAM a tentative value of unity for the parameter N G , defined below.
• control box 26 will first measure and scale the voltage signal on the line 38, on the assumption that the calorific value of the fuel gas is at the value assumed in constructing the look-up table. Should this assumption be invalid in a particular case, the temperature sensors connected to the external source 39 will discern this in due course as a shortfall, or alternatively an excess, in a desired temperature in the fluid (water or room air) being heated, and the source 39 will then alter the voltage signal on the line 38 in a sense which will tend to remove the temperature discrepancy.
• the scaled voltage is encoded and compared with the series of encoded voltages stored in Column 2 of the look-up table and representative of rates of gas flow through the modulating gas valve 20.
• the program of the control box 26 will store into RAM a value of unity both for the parameter M and for the parameter N'' G representing the working value of the step number controlling the drive voltage for the valve 20. In either case, the control box 26 will then determine whether the step numbers N' G and (N' G ) E are equal. If they are, the program will return to the point, described earlier, where it established whether flame continued to be present at the detector of the device 34 after the igniter had been switched off. Thereafter all the steps described will be performed again.
• N' G and (N' G ) E are not equal, however, the control box 26 will establish whether the requested value N' G is permissible. To do so, it will recall from RAM the current values of the control pulse step number and the fan speed step number (in the general case, referenced respectively as N q , and (N'' A ) and restore them into RAM at new addresses reference respectively as (N q ,) E , (N" A ) E . Recalling (N q ,) E and (N" A ) E the control box 26 will use Equation (3) below to define an uppermost limiting step number (N' G ) P for controlling the valve 20:
• N' G does not exceed the limiting value (N' G ) P , the control box 26 will adopt the value N' G without modification; otherwise the lesser value (N' G ) P will be adopted instead. In either event the adopted value will be stored into RAM as the step number N'' G to be used for setting the valve 20.
• control box 26 will estimate and store into RAM a corresponding new step number N'' A for controlling the speed of the fan 18, using the Equation: N" A - N" G + C re + B ( 4 )
• N cp (N" A - (N" A ) E ) + (N q ,) E (5)
• the control box 26 will now compare the target and existing values of N q , to determine the required direction of change in the step number. In the present instance, as the burner is operating at its minimum rate and assuming that the existing and adopted values of N'' G are unequal, by implication an increase in burner heat output is called for.
• the control box 26 will therefore increment by a number of steps the pulse duration L q ,, and then by the same number of steps (after a pause to allow the change in fan speed to come partially into being) , the drive voltage V mg ⁇ for the valve 20 to a value corresponding to a step number N G .
• step number N G temporarily controlling the gas flow rate, compare this with the target value N'' G and continue the change process until the respective target step numbers N q , and N'' G are arrived at simultaneously.
• This stepwise procedure serves to limit any transitory reduction in the air/gas flow rate ratio which would arise if the modulating valve 20 responded more quickly than the fan 18 to a given change in the step number.
• the control box 26 After every stage of change in the settings of the fan 18 and modulating valve 20, the control box 26 will check that the flame has not become extinguished.
• control box 26 will again measure the steady fan speed F, identify from the look-up table the corresponding value of N F , recall the reduced value of N'' A and estimate the new difference (N'' A - N F ) . Should (in exceptional circumstances) N F still be less than N'' A , the control box 26 will apply a further reduction in N'' G amounting to the shortfall (N' ' A - N F ) , the control pulse duration remaining at 0.0010 second. This will ensure that N F will become equal to N'' A . The control box 26 will store this latest value of N'' G into RAM and use it as the working value from which to identify and set the drive voltage V ⁇ for the modulating valve 20.
• the control box 26 With the intended flow rate ratio attained, the control box 26 will read the elapsed time t op on the operating cycle timer. If this exceeds a predetermined period t * op (for example, 20 minutes) the control box 26 will reset this timer, turn off the power supply to the shutoff valve 21, set V mg ⁇ ⁇ 0 and follow the procedure described above for relighting an extinguished flame. In doing so the parameters N ⁇ , (N q ,) ⁇ and C p s will be re-evaluated and stored into RAM. The updated value of the factor C*- ⁇ will be adopted thereafter when Equations (2) to (4) are employed. This ensures that the control box 26 will remain in close touch with important factors such as the performance of the fan 18 and the flow resistance characteristic of the combustion system, should these tend to alter during extended periods of continuous heat demand.
• t * op for example, 20 minutes
• the program of the control box 26 will turn off the power supply to the gas shutoff valve 21, set the parameters V mg ⁇ and L q , both to zero to extinguish the flame and go to "standby", awaiting a fresh demand for heat from the source 39.
• control box 26 On receiving this, the control box 26 will once again go through the procedure for burner startup described earlier, and in so doing will re-evaluate the parameters N ⁇ , (N q ,) ⁇ and C re .
• the new value of C ⁇ will be stored into RAM, and adopted when Equations (2) to (4) are next used.
• the burner 4 should function for almost all of its working time at a percentage aeration close to, or identical with, that intended by the designer. This will minimise the generation of undesireable by-products of the combustion process, and maximise the life of the burner and the performance of the equipment which it serves .
• R is the common ratio of the geometric series.
• the percentage change X may, of course, be negative in value, in which case the quantity C will define the number of terms to be traversed from the existing term back towards the beginning of the series.
• the number C may therefore be viewed as an algebraically additive correction factor to the term denoting the existing magnitude in which the change of X% is to be made.
• This is the principle underlying the use of Equations (1) to (6) above.
• estimation operations which are in essence multiplicative are transformed into additive operations, which are simpler to perform in conjunction with data from look-up tables.
• the necessary calculation operations can be carried out with a much lower memory capacity than would be required if, for example, an arithmetic series were used as the basis of control. This saves cost without compromising the flexibility and resolution of the control system.
• I Ignition attempt number having a value of 0 or 1.
• M Program control marker variable having a value of
• N' A Step number corresponding to the desired fan speed, defined by Equ. 4.
• N co Step number corresponding to the fan speed at which a voltage appears at the pressure contact of the switch in the assembly 36.
• N co Nominally sufficient (reference) value of N co .
• Step number used for setting the duration of the fan speed control pulses.
• Step number controlling the duration of the fan speed control pulses when the fan speed step number N co is achieved.
• V mil Minimum value of output voltage from external source 39, indicative of a demand for heat.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Combustion & Propulsion (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Regulation And Control Of Combustion (AREA)

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• 1996
  • 1996-02-14 CA CA002212659A patent/CA2212659A1/en not_active Abandoned
  • 1996-02-14 DE DE69602749T patent/DE69602749T2/de not_active Expired - Fee Related
  • 1996-02-14 WO PCT/GB1996/000349 patent/WO1996025627A1/en active IP Right Grant
  • 1996-02-14 ES ES96902388T patent/ES2135207T3/es not_active Expired - Lifetime
  • 1996-02-14 EP EP96902388A patent/EP0832394B1/de not_active Expired - Lifetime
  • 1996-02-14 JP JP8524771A patent/JPH10504887A/ja active Pending
  • 1996-02-14 GB GB9603079A patent/GB2298059B/en not_active Revoked

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