EP0812409B1

EP0812409B1 - Controlling a combustion system

Info

Publication number: EP0812409B1
Application number: EP96902387A
Authority: EP
Inventors: David Michael Sutton
Original assignee: BG PLC; British Gas PLC
Current assignee: BG Group Ltd; British Gas PLC
Priority date: 1995-02-16
Filing date: 1996-02-14
Publication date: 2000-01-26
Anticipated expiration: 2016-02-14
Also published as: DE69606390T2; GB2298060B; JPH10504379A; ES2142045T3; WO1996025629A1; GB2298060A; GB9603087D0; AU4672596A; AU696298B2; DE69606390D1; CA2212499A1; EP0812409A1

Description

The present invention relates to an apparatus for controlling a combustion system, particularly a fully-premixed burner which incorporates a variable speed fan.
FR-2662751 discloses an apparatus for controlling a combustion system. The apparatus incorporates a variable speed fan, means for supplying fuel at a variable rate of flow, means operable to vary the speed of the fan progressively from zero to a maximum and means exposed to the flow of air from the fan for causing the operation of a switch mechanism when the rate of flow of air generated by the fan reaches a nominated value.
EP-A-0124330 discloses a fuel burner control system including a memory and a processing means. Under the influence of an operator the processing means generates output values for setting a fuel valve and an air valve respectively controlling fuel and air to be supplied to the burner.
According to a first aspect, the present invention provides an apparatus for controlling a combustion system as set out in claim 1.
According to a second aspect, the present invention provides an apparatus for controlling a combustion system as set out in claim 3.
Preferably the control means is adapted to prevent the fuel shut-off valve from being opened unless the switch mechanism has operated.
Suitably the means for causing the operation of the switch mechanism comprises a flow metering orifice through which the air supplied to the burner passes and to which, in use, the switch mechanism is connected.
Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:-
Figures 1A, 1B, 1C are schematic views of a domestic combustion system in a gas-fired domestic heating appliance, together with control apparatus therefor, and
Figures 2A, 2B are schematic circuit diagrams illustrating how the heat demand signal is produced in each embodiment.
Referring to Figure 1A, there is illustrated 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 30 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.
A combined igniter and flame failure detector 34, located immediately beneath the burner plate 16, communicates bidirectionally with the control box 26 by means of a line 35. The device 34 is a standard feature forming no part of the present invention, it being mentioned for completeness only.
Between the fan 18 and the mixing chamber 17 there is mounted 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 a predetermined air pressure delivered by the fan 18 delivers 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 2A. 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.
From the various signals received 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.
In the present embodiment, the 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_cp of each 0 -5V 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 values held in ROM in the control box 26, and any difference existing between the sampled and any selected one of the 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 the fan speed corresponding to the selected reference value. If other factors remain constant, in a combustion system of the type shown in Figure 1A the rate of air flow is very nearly proportional to the rotational speed of the fan. Therefore, provided that the performance of the fan is sufficient under the given conditions, the control box 26 will be able to procure any one of a selection of alternative air flow rates by adjusting the duration L_cp 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.
Referring to Table 1A, 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 row number of the various entries in the table.
The second column in the table comprises the respective gas flow rate G in cubic metres/hour (m³/h) corresponding to each particular row number N. The entries shown cover a range of gas flow rates between a minimum of 0.35 m³/hr and 0.46 m³/hr at row N=12. The flow rate in each row is approximately 2.5% greater than that in the preceding row.
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 rows shown cover fan speeds ranging from 1050 rev/min at N=1 to 1378 rev/min at N=12. The intended air flow rate in each row is approximately 2.5% greater than that in the preceding row.
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 of the fan speed control pulses in microseconds corresponding to each value of N, as supplied on line 27.
In constructing such a table, 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 (m³ air/m³ fuel gas) and a fan of assumed performance characteristics operating normally in a combustion system of an assumed flow resistance characteristic. To secure the maximum possible performance from the combustion system, the intended percentage aeration may be made variable according to the rate of gas flow. However, 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 percentage aeration of the combustible mixture may remain as intended.
For ease of explanation, the data in Table 1A are shown as ordinary numbers. In reality, however, all tabular data are stored in digital form, in keeping with normal practice. In particular, 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. Furthermore, it will be appreciated that columns 3 and 5 may extend to a row number higher than that to which columns 2 and 4 extend.
The program followed by the control box 26 in the present embodiment will now be described in outline.
The program starts by resetting to zero in RAM, for later purposes, two parameters C_FS and M, described below. It then reads equal to a preset value V_min . If such a voltage is present, this indicates the existence 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.
If the safety checks reveal no hazard, the control box 26 will find from ROM the value of (F_CO )*, a reference fan speed assumed sufficient for actuation of the changeover switch in the assembly 36 when the look-up table was constructed. The control box 26 will then generate and supply along the line 27 a train of fan speed control pulses as described earlier, the duration L_cp of these pulses being that listed in Column 5 of the look-up table, in the row for F = (F_CO )". When the speed of the fan 18 has become steady, the control box 26 will determine whether a voltage exists at the high-pressure contact of the changeover switch in the assembly 36. If these is none, the value of L_cp in relation to the maximum value of 0.0010 second is checked; and as L_cp will not be at the maximum value at this stage, the control box 26 will increase L_cp, pause suitably for a change in fan speed to occur and re-examine the high-pressure contact of the changeover switch. This will continue until either a voltage appears at this contact, or the value of L_cp becomes 0.0010 second. In the latter event, in the interest of safety, the control box 26 will set S = O, L_cp = O and "lockout", as described above.
In the alternative event, however, the control box 26 will measure the value of L_cp and find from the look-up table the corresponding listed fan speed F = (F_cp)_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. The control box 26 will then measure the fan speed F and store it into RAM as F = F_CO . It will next look-up the value of (F_CO )* and evaluate the flow switch fan speed correction factor C_FS from the Equation: CFS = FCO ÷ (FCO)*
The factor C_FS will be stored into RAM for use later, as will be described. If the circumstances of operation happened to accord exactly with those assumed in constructing the look-up table, C_FS would be unity. Clearly, by reason of the manner of its derivation, the factor C_FS can only assume values which are compatible with the predetermined values of fan speed in the look-up table.
After a pause of t_p seconds during which fresh air is blown through the combustion system to purge it of residual products from previous combustion and of any traces of fuel gas which may have leaked in through the closed valve 21, the control box 26 will estimate, and store into RAM, the fan speed for flame ignition F = F_i appropriate under the prevailing conditions to G_min , the lowest of the predetermined rates of fuel flow, and given by the Equation: Fi = B x CFS x Fmin where

B =: a constant preset during manufacture or installation of the control box 26 according to the expected degree of variation in the properties of the fuel gas to be used by the burner 4.
F_min =: the lowest of the predetermined fan speeds in the look-up table, appropriate to G_min under the conditions assumed in constructing the table.

If no significant variation in fuel gas properties is expected to occur, the constant B would be preset at unity. If, however, an increase of up to 10% in Wobbe Number is considered possible, a value B = 1.05 might be selected, assuming that the look-up table is constructed for fuel gas of the lowest Wobbe Number likely to be distributed. The rate of air flow in relation to the stoichiometric requirement would then remain within ± 5% of the intended value as the Wobbe Number of the fuel gas varied.
The value of the constant B is chosen from a range of values compatible with the predetermined values of fan speed stored in the look-up table.
The control box 26 will now look-up in the table the nominal value of L_cp for the fan speed F = F_i and supply pulses of this duration on the line 27. Next it will measure the steady fan speed F resulting in due course. If this is greater than F_i, the control box 26 will reduce the value of L_cp , recheck the fan speed when this has become steady and continue the process until the fan speed attains the target value.
If, however, when first measured the fan speed is found to be less than F_i, the duration of the control pulses will be measured and compared with the maximum value of 0.0010 second. If L_cp is less than the maximum, the control box 26 will increase L_cp , measure the fan speed when this has become steady and continue the process until either the fan speed attains the target value, or the control pulse duration becomes 0.0010 second. In the latter event, the control box 26 will set S = O, L_cp = O and go into "lockout".
Assuming that the target fan speed is achieved successfully, however, the control box 26 will measure the value of L_cp arrived at, then find from the look-up table, and store into RAM, the corresponding listed fan speed F = (F_cp)_i . It will next energise firstly the igniter of the device 34 and, a few seconds later, the coil of the gas shutoff valve 21, enabling fuel gas to flow to the burner 4 through the modulating valve 20 which, though unenergised at this stage, sits in a partially-open position against an internal stop. If after a time t_i seconds no flame is sensed by the detector of the device 34, the control box 26 will turn off the power supply to the igniter and to the valve 21.
Next the control box 26 will recall from RAM the value of I, an ignition attempt index which may be allocated a value of zero or unity by the program, as circumstances require. In the present instance, as no previous attempt at ignition had been made the stored value of I will be zero, so the program will update I to unity and try again to establish a flame on the burner 4. To do so it will recall from RAM the fan speed F = (F_cp)_CO , look-up the corresponding value of L_cp , supply control pulses of this duration and repeat the steps described above in relation to the initial attempt at ignition. In the course of this, the parameters F_CO, (F_cp)_CO and C_FS will be revised if necessary, or alternatively, the control box 26 will establish "lockout" in the manner described above if the control pulse duration should rise to its maximum value of 0.0010 second without a voltage appearing at the high-pressure contact of the changeover switch. If a flame fails to appear on the second attempt, since now I = 1 the control box 26 will set S = O, L_cp = O and then "lockout". If flame is established n either attempt, however, the igniter will be de-energised and a value I = O will be stored into RAM.
For safety, the control box 26 will now check whether, with the igniter off, a flame remains present at the detector of the device 34. If it does not, one attempt will be made to relight the flame. To do this the control box 26 will turn off the power supply to the valve 21, store a value I = 1 into RAM and go through the remainder of the procedure described above for a second ignition attempt.
If flame does exist at the detector, the control box 26 will read the line 38, to establish whether there is still a demand for heat. If, unusually, there is no longer any demand, the control box 26 will turn off the supply of power to the valve 21, set L_cp = O to stop the fan and await the emergence of a new demand for heat. If, however, the demand still exists, the control box 26 will carry out certain standard safety checks. Should these reveal some hazard, the program will set S = O, de-energise the valve 21, set L_cp = O and go to "lockout".
Assuming for the present purpose that the safety checks are completed successfully, however, the control box 26 will first start a timer monitoring the length of the firing period of the burner for reasons to be explained, and then examine the value of the parameter M. When the program of the control box 26 has come into operation from its start-point, the value of M will be zero. In this event the program will store into RAM, referenced as (N'_G)_E , a tentative value of unity for the parameter N_G defined below, and set out to establish the row number N = N'_G in the look-up table which would provide the burner firing rate corresponding most nearly to the actual demand for heat from the external source 39.
To do this the 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 encloded 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. This comparison will identify the entry in the table most nearly suitable, on the basis of the assumed calorific value, to meet the particular demand for heat. Therefrom the control box 26 will identify from Column 1 of the same table, and store into RAM, the corresponding row number N'_G for setting the drive voltage V_mgv for the modulating valve 20.
The control box 26 will then compare the stored numbers N'_G and (N'_G)_E . If these are equal, the program of the control box 26 will return to the point, describeed earlier, where it established whether flame continued to be present at the detector of the device 34 after the igniter had been switched off. From there all the foregoing steps will then be performed again in the manner described.
If N'_G and (N'_G)_E are not equal, however, the control box 26 will examine the value of M again. Should it be zero, the program will store into RAM a value M = 1. Next, or in the alternative event, the control box 26 will determine and store into RAM the fan speed F_N in the row N = N'_G of the look-up table, and therefrom estimate an appropriate target operating speed F_op for the fan 18 using the Equation: Fop = B x cFS x FN
Recalling from RAM the values F_i and (F_cp)_i, restoring them at new addresses denoted by F_E , (F_cp)_E respectively and then using the values F_E, (F_cp)_E and F_op the control box 26 will estimate, and store into RAM, a target fan speed (F_cp) _op for selecting the duration of the control pulses, given by: (Fcp)op = (Fcp)E x Fop ÷ FE
Provided (F_cp)_op does not exceed F_max , the highest fan speed listed in the look-up table, the control box 26 will store into RAM the row number N'_G as that for setting the drive voltage V_mgv for the modulating valve 20; and the stored values F = F_op and F = (F_cp)_op will be used to define the desired respective values of the actual fan speed F and the control pulse duration L_cp . Otherwise, for setting the drive voltage for the modulating valve 20 the control box 26 will look-up and store into RAM the lesser (but largest permissible) row number (N'_G)_P , corresponding to a reduced look-up table fan speed (F_N)_P defined by Equation (5) below: (FN)P = Fmax ÷ [(Fcp)E x B x CFS ÷ FE]
In this case the value (F_cp)_op = F_max will be stored into RAM for setting the control pulse duration. Then, recalling the values of (F_cp)_OP, F_E and (F_cp)_E , the control box 26 will estimate and store into RAM the target fan speed F_op given by: Fop = (Fcp)op x FE ÷ (Fcp)E = Fmax x FE ÷ (Fcp)E
The control box 26 will now compare the target and existing values (F_cp)_op and (F_cp)_E to determine the required direction of change in the fan speed. In the present instance, as the burner is operating at its minimum rate and the existing and requested values of N'_G are unequal, by implication an increase in burner heat output is called for. By reference to the look-up table, the control box 26 will therefore increment by a number of rows (for example, four) the pulse duration L_cp, starting from the value corresponding to F = (F_cp)_E . Then, after a pause to allow the change in fan speed to come partially into being, the control box 26 will increment similarly and by the same number of rows the drive voltage V_mgv for the valve 20, in this case starting from the value listed in the look-up table row N = (N'_G)_E . It then notes the new row number N_G arrived at in this manner, compares this with the target value N'_G and continues the change process until the respective targets (F_cp)_op 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 common change in the row number. After every stage of change in the settings of the fan 18 and modulating valve 20, the control box 26 will check that flame continues to exist at the detector of the device 34.
Next the control box 26 will measure the actual fan speed F and estimate and store into RAM the ratio [N]₁ = = (F_op ÷ F). Normally this will be unity and the program will return to the point where it established whether flame continued to be present at the detector of the device 34 after the igniter had been switched off. All the foregoing steps will then be performed again in the manner described, so that operation will proceed in safety and the control system will become aware of, and respond to, any change in the heating requirement.
However, if [N]₁ is found to be less than unity, the control box 26 will recall (F_cp)_op , the fan speed regulating the control pulse duration L_cp, multiply it by the quantity [N]₁ and store this reduced value of (F_cp)_op into RAM. The control box 26 will then look-up, and provide, the corresponding new pulse duration L_cp , measure the resulting fan speed F when this had become steady and re-evaluate the ratio [N]₁ . If, exceptionally, this were still less than unity, the procedure described would be repeated until [N]₁ had become equal to unity.
If, on the contrary, [N]₁ is found to be, or to have become, greater than unity the control box 26 will recall (F_cp)_op, find from the look-up table the value of the maximum possible fan speed F_max, estimate the ratio [N]₂ = (F_max ÷ (F_cp)_op) and evaluate a parameter E according to the Equation: E = [N]2 ÷ [N]1
If E is not less than unity, the control box 26 will estimate a new value of the parameter (F_cp)_op = [(F_cp)_op x [N]₁] and store this value into RAM. It will then identify from the look-up table the corresponding value of the control pulse duration L_cp , and generate and despatch along the line 27 pulses of this duration to increase the speed of the fan 18. The control box 26 will again measure the fan speed F when this had become steady and repeat the process if, exceptionally, this proves necessary, so that F may become equal in due course to the required fan speed F_op.
Should E be, or become, less than unity, however, the control box 26 will recall from RAM the row number N'_G, find from the look-up table the value of the fan speed F_N listed in that row, multiply this speed by the amount E and store the reduced value of F_N into RAM. Using this value, the control box 26 will then determine from the look-up table, and store into RAM, the corresponding reduced row number N = N'_G and further identify from the look-up table, and set, the listed value of V_mgv for that row, to lessen the rate of fuel gas flow. Secondly, using Equation (3) on the basis of the reduced fan speed value F_N , the control box 26 will estimate, and store into RAM, a new value of the target fan speed N_op suitable for the revised value of N'_G; and thirdly, it will set L_cp to the maximum value of 0.0010 second and store into RAM the corresponding fan speed (F_cp)_op = F_max . Next the control box 26 will again measure in due course the new steady fan speed F, recall the reduced value of the target fan speed F_op and estimate the new ratio [N]₁ = = (F_op ÷ F). Should (in exceptional circumstances) this still be greater than unity, the control box 26 will apply a further reduction in N'_G as described above and estimate a correspondingly reduced new target fan speed, the control pulse duration remaining at 0.0010 second. This procedure will continue until the fan speed becomes equal to the reduced target value, the latest value of N'_G stored into RAM becoming the working value for setting the drive voltage V_mgv for the modulating valve 20.
With the intended flow rate ratio attained, the program of the control box 26 will read the firing period timer. If the burner firing time exceeds a preset period t_op (for example, twenty minutes), the control box 26 will simulate a loss of flame at the detector of the device 34 by interrupting the signal on the line 35. This will cause the program to stop and reset the firing period timer, set V_mgv = 0 and carry out the procedure for reigniting the flame, as described earlier. In the course of this the factor C_FS will be re-evaluated from Equation (1) and stored into RAM, for use when Equations (2), (3) and (5) are next employed. By this means the control box 26 becomes able to take account, at regular intervals and before igniting the burner 4, of any change in the fan performance or in the system flow resistance characteristic which may be relevant. Via the constant B, a preset allowance may also be made for any expected fluctuations in fuel gas properties.
If when checked the burner firing time does not exceed the period t_op , however, the program of the control box 26 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. From there all the foregoing steps will then be performed again in the manner described.
Should the safety checks at this point show that the demand for heat has ceased, or that the temperature at the sensor 32 on the pipe portion 23 has become excessive, the program of the control box 26 will turn off the power supply to the gas shutoff valve 21, set the parameters V_mgv and L_cp both to zero to extinguish the flame and go to "standby", awaiting a fresh demand for heat from the source 39. On receiving this, the control box 26 will respond as described earlier.
Although there will be some loss of heat service, if, to obtain the desired air/gas flow rate ratio, the control box 26 reduces the drive voltage V_mgv from the requested setting, the user will find this approach preferable to conventional practice: therein, operation of the burner 4 would be prevented altogether if the fan 18 became unable to support, at an intended air/gas flow rate ratio, the maximum rate of fuel flow allowed by the valve 20.
Further advantage to the user derives from the facility in the present invention for varying the fan speed during the startup sequence to induce, if possible, operation of the switch in the differential-pressure-sensing assembly 36. In known combustion controllers, operation of the burner would be disallowed unless, when rotating at a prechosen and nominally constant speed, the fan was able to promote a rate of air flow sufficient to cause operation of a switch such as that in the assembly 36.
Finally, because according to the present invention compensation can be applied for variations of circumstance, including changes in fuel gas properties, the burner 4 will function, always and automatically, with a rate of air supply (relative to the stoichiometric) close to, if not identical with, that intended by the designer. This will maximise the life of the burner and the performance of the equipment which it serves, and minimise the generation of undesirable by-products of the combustion process.
In respect of the second embodiment, Figure 1B shows a domestic combustion system which is similar to that shown in Figure 1A, except that in this case the modulating valve 20 and its associated line 28 are replaced by a fixed flow restrictor orifice 20, the size of the orifice being selected from a predetermined range according to the rate of fuel gas flow (and so, heat output) desired. The orifice 20 may be placed separately from the valve 21 as shown. Alternatively and more conveniently, it may be incorporated within the valve 21.
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 2B. The processor 39 receives signals from a room temperature thermostat 40 along line 41, a hot water temperature thermostat 42 along line 43 and a central heating/hot water programmer 48 along the lines 49 and 50. The processor 39 is a conventional device forming no intrinsic part of the present invention.
Table 1B illustrates schematically the first 12 rows of a data look-up table which, in this embodiment, is stored in ROM in the control box 26.
The first column of the table comprises "N", the row number of the various entries in the table.
The second column in the table comprises P, the respective heat output in kilowatts (KW) corresponding to each particular row number N. The entries shown cover a range of heat output rates between a minimum of 3.5KW and 4.6KW at row N=12. The heat output in each row is approximately 2.5% greater than that in the preceding row.
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 rows shown cover fan speeds ranging from 1050 rev/min at N=1 to 1378 rev/min at N=12. The intended air flow rate in each row is approximately 2.5% greater than that in the preceding row.
The fourth column in the table comprises the nominal duration of the fan speed control pulses in microseconds corresponding to each value of N, as supplied on line 27.
In constructing such a table, each combination of heat output rate (and hence 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 (m³ air/m³ fuel gas) and a fan of assumed performance characteristics operating normally in a combustion system of an assumed flow resistance characteristic. We describe later methods of compensating for departures from the circumstances assumed in constructing the data look-up table.
For ease of explanation, the date in Table 1B are shown as ordinary numbers. In reality, however, all tabular data are stored in digital form, in keeping with normal practice. In particular, the heat output rates in column 2 are stored as digital voltages representative of these rates on the basis of a fixed scaling factor. It will be appreciated that columns 3 and 4 may extend to a row number higher than that to which column 2 extends.
The program followed by the control box 26 in this embodiment will now be outlined.
The program starts by resetting to zero in RAM, for later purposes, a parameter C_FS, 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_min. If such a voltage is present, this indicates the existence 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 in 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 the conventional "reset" switch on the control box 26, this also causing the program to change the value of S to unity.
If the safety checks reveal no hazard, the control box 26 will find from ROM the value of (F_CO)*, a reference fan speed assumed sufficient for actuation of the changeover switch in the assembly 36 when the lookup table was constructed. The control box 26 will then generate and supply along the line 27 a train of fan speed control pulses as described earlier, the duration L_cp of these pulses being that listed in column 5 of the look-up table, in the row for F = (F_CO)*. When the speed of the fan 18 has become steady, the control box 26 will determine whether a voltage exists at the high-pressure contact of the changeover switch in the assembly 36. If there is none, the value of L_cp in relation to the maximum value of 0.0010 second is checked; and as L_cp will not be at the maximum value at this stage, the control box 26 will increase L_cp , pause suitably for a change in fan speed to occur and re-examine the high-pressure contact of the changeover switch. This will continiue until either a voltage appears at this contact, or the value of L_cp becomes 0.0010 second. In the latter event, in the interest of safety, the control box 26 will set S = O, L_cp = O and "lockout", as described above.
In the alternative event, however, the control box 26 will measure the value of L_cp and find from the look-up table the corresponding listed fan speed F = (F_cp)_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. The control box 26 will then measure the fan speed F and store it into RAM as F = F_CO. It will next look-up the value of (F_CO)* and evaluate the flow switch fan speed correction factor C_FS from the Equation: CFS = FCO ÷ (FCO)*
The factor C_FS will be stored into RAM for use later, as will be described. If the circumstances of operation happened to accord exactly with those assumed in constructing the look-up table, C_FS would be unity. Clearly, by reason of the manner of its derivation, the factor C_FS can only assume values which are compatible with the predetermined values of fan speed in the look-up table.
After a pause of t_p seconds during which fresh air is blown through the combustion system to purge it of residual products from previous combustion and of any traces of fuel gas which may have leaked in through the closed valve 21, the control box 26 will estimate, and store into RAM, the fan speed for flame ignition F = F_op appropriate under the prevailing conditions to G, the predetermined rate of fuel flow, and given by the Equation: Fop = A x B x CFS x Fmin where

A =: a constant preset during manufacture or installation of the control box 26 according to the predetermined rate of fuel flow to be provided by the restrictor orifice within, or otherwise in series with, the valve 21, any such rate of flow being compatible with one of the predetermined values of fan speed stored in the look-up table.
B =: a constant preset during manufacture or installation of the control box 26 according to the expected degree of variation in the properties of the fuel gas to be used by the burner 4.
F_min =: the lowest of the predetermined fan speeds in the look-up table appropriate, under the conditions assumed in constructing the table, to the rate of fuel gas flow corresponding to the value A = 1.

If no significant variation in fuel gas properties is expected to occur, the constant B would be preset at unity. If, however, an increase of up to 10% in Wobbe Number is considered possible, a value B = 1.05 might be selected, assuming that the look-up table is constructed for fuel gas of the lowest Wobbe Number likely to be distributed. The rate of air flow in relation to the stoichiometric requirement would then remain with ± 5% of the intended value as the Wobbe Number of the fuel gas varied.
The value of the constant B is chosen from a range of values compatible with the predetermined values of fan speed stored in the look-up table.
The control box 26 will now look-up in the table the nominal value of L_cp for the fan speed F = F_op, supply pulses of this duration on the line 27 and measure the steady fan speed F resulting in due course. If this is greater than F_op , the control box 26 will reduce the value of L_cp, recheck the fan speed when this had become steady and continue the process until the fan speed attains the target value.
If, however, when first measured the fan speed is found to be less than F_op, the duration of the control pulses will be measured and compared with the maximum value of 0.0010 second.
If L_cp is less than the maximum, the control box 26 will increase L_cp , measure the fan speed when this had become steady and continue the process until either the fan speed attains the target value, or the control pulse duration becomes 0.0010 second. In the latter event, the control box 26 will set S = O, L_cp = O and go into "lockout".
Assuming that the target fan speed is achieved successfully, however, the control box 26 will measure the value of L_cp arrived at, then find from the look-up table, and store into RAM, the corresponding listed fan speed F = (F_cp)_op . Thereafter it will energise firstly the igniter of the device 34 and, a few seconds later, the coil of the gas shutoff valve 21, enabling fuel gas to flow to the burner 4. If after a time t_i seconds no flame is sensed by the detector of the device 34, the control box 26 will turn off the power supply to the igniter and to the valve 21.
Next the control box 26 will recall from RAM the value of I, an ignition attempt index which may be allocated a value of zero or unity by the program, as circumstances require. In the present instance, as no previous attempt at ignition had been made the stored value of I will be zero, so the program will update I to unity and try again to establish "lockout" in the manner described above if the control pulse duration should rise to its maximum value of 0.0010 second without a voltage appearing on the second attempt since now I = 1 the control box 26 will set S = O, L_cp, supply control pulses of this duration and repeat the steps described above in relation to the initial attempt at ignition. In the course of this, the parameters F_CO, (F_cp)_CO and C_FS will be revised if necessary, or alternatively, the control box 26 will establish "lockout" in the manner described above if the control pulse duration should rise to its maximum value of 0.0010 second without a voltage appearing at the high-pressure contact of the changeover switch. If a flame fails to appear on the second attempt, since now I = 1 the control box 26 will set S = O, L_cp = O and then "lockout".
If flame is established in either attempt, however, the igniter will be de-energised and a value I = O will be stored into RAM.
For safety, the control box 26 will now check whether, with the igniter off, a flame remains present at the detector of the device 34. If it does not, one attempt will be made to relight the flame. To do this the control box 26 will turn off the power supply to the valve 21, store a value I = 1 into RAM and go through the remainder of the procedure described above for a second ignition attempt.
If flame does exist at the detector, the control box 26 will read the line 38, to establish whether there is still a demand for heat. If, unusually, there is no longer any demand, the control box 26 will turn off the supply of power to the valve 21, set L_cp = O to stop the fan and await the emergence of a new demand for heat. If, however, the demand still exists, the control box 26 will carry out certain standard safety checks. Should these reveal some hazard, the program will set S = O, de-energise the valve 21, set L_cp = O and go to "lockout".
Assuming for the present purpose that the safety checks are completed successfully, however, the control box 26 will start a timer monitoring the length of the firing period of the burner, then measure the actual fan speed F and estimate and store into RAM the ratio [N]₁ = (F_op ÷ F). Normally this will be unity and the program will return to the point where it established whether flame continued to be present at the detector of the device 34 after the igniter had been switched off. All the foregoing steps will then be performed again in the manner described, so that operation will proceed safely and the control system will respond rapidly if the heating requirement ends.
Should [N]₁ be found, however, to be less than unity, the control box 26 will recall (F_cp)_op , the fan speed regulating the control pulse duration L_cp , multiply it by the quantity [N]₁ and store this reduced value of (F_cp)_op into RAM. The control box 26 will then look-up, and provide, the corresponding new pulse duration L_cp, measure the resulting fan speed F when this had become steady and re-evaluate the ratio [N]₁ . If, exceptionally, this is still less than unity, the procedure described will be repeated until [N]₁ has become equal to unity.
If, on the contrary, [N]₁ is found to be, or to have become, greater than unity the control box 26 will recall (F_cp)_op , find from the look-up table the value of the maximum possible fan speed F_max , estimate the ratio [N]₂ = (F_max ÷ (F_cp)_op) and evaluate a parameter E according to the Equation: E = [N]2 ÷ [N]1
If E is not less than unity, the control box 26 will estimate a new value of the parameter (F_cp)_op = [(F_cp)_op x [N]₁] and store this value into RAM. It will then identify from the look-up table the corresponding value of the control pulse duration L_cp , and generate and despatch along the line 27 pulses of this duration to increase the speed of the fan 18. The control box 26 will again measure the fan speed F when this had become steady and repeat the process if, exceptionally, this proves necessary, so that F may become equal in due course to the required fan speed F_op.
Should E be, or become, less than unity, however, the control box 26 will turn off the power supply to the valve 21, set L_cp = O, S = O and "lockout".
With the intended flow rate attained, the program of the control box 26 will read the firing period timer. If the burner firing time exceeds a preset period t_op (for example, twenty minutes), the control box 26 will simulate a loss of flame at the detector of the device 34 by interrupting the signal on the line 35. This will cause the program to stop and reset the firing period timer, set V_mgv = O and carry out the procedure for reigniting the flame, as described earlier. In the course of this the factor C_FS will be re-evaluated from Equation (1) and stored into RAM, for use when Equation (2) is next employed. By this means the control box 26 becomes able to take account, at regular intervals and before igniting the burner 4, of any change in the fan performance or in the system flow resistance characteristic which may be relevant. Via the constant B in Equation (2), a preset allowance may also be made for any expected fluctuations in fuel gas properties.
If, when checked, the burner firing time does not exceed the period t_op , however, the program of the control box 26 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. From there all the foregoing steps will then be performed again in the manner described.
Should the safety checks at this point show that the demand for heat has ceased, or that the temperature at the sensor 32 on the pipe portion 23 has become excessive, the program of the control box 26 will turn off the power supply to the gas shutoff valve 21, set the parameters V_mgv and L_cp both to zero to extinguish the flame and go to "standby", awaiting a fresh demand for heat from the source 39. On receiving this, the control box 26 will repeat the entire procedure described earlier.
It will be apparent to one skilled in the art that the apparatus described may be adapted in another embodiment to provide more than one predetermined rate of fuel flow and correspondingly, more than one associated rate of air flow. For example dual-rate ("high/low") operation of the burner 4 may be achieved, as shown schematically in Figure 1C, by providing two valves 21, 21A, each valve including or otherwise in series with its own flow restrictor orifice 20, 20A to provide a particular rate of fuel flow, the valves being controlled individually by separate lines 29, 29A respectively from the control box 26, and two values A₁ and A₂ of the constant A being allocated, one value appropriate to each of the fuel flow rates. Two levels of signal voltage would be supplied on the line 38 from the source 39, the lower of these being at least equal to V_min , each level being representative of a particular one of the two requirements for heat and causing the control box 26 to select, in accordance with its operating program, the appropriate value A₁ or A₂ of the constant A for use in Equation (2).
Considerable advantage to the user derives from the facility in the present invention for varying the fan speed during the startup sequence to induce, if possible, operation of the switch in the differential-pressure-sensing assembly 36. In known combustion controllers, operation of the burner would be disallowed unless, when rotating at a prechosen and nominally constant speed, the fan was able to promote a rate of air flow sufficient to cause operation of a switch such as that in the assembly 36.

Finally, because according to the present invention compensation can be applied for variations of circumstance, including changes in fuel gas properties, the burner 4 will function, always and automatically, with a rate of air supply (relative to the stoichiometric) close to, if not 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.

(1)	(2)	(3)	(4)	(5)
N	G	F	V_mgv	L_cp
	(m³/h)	(rev/min)	(volts)	(µsec)
1	0.35	1050	0.00	23
2	0.36	1076	0.54	25
3	0.37	1103	1.09	27
4	0.38	1131	1.66	29
5	0.39	1159	2.24	31
6	0.40	1188	2.83	33
7	0.41	1218	3.20	36
8	0.42	1248	3.39	39
9	0.43	1279	3.60	41
10	0.44	1311	3.81	45
11	0.45	1344	4.02	48
12	0.46	1378	4.24	52

(1)	(2)	(3)	(4)
N	P	F	L_cp
	(KW)	(rev/min)	(µsec)
1	3.5	1050	23
2	3.6	1076	25
3	3.7	1103	27
4	3.8	1131	29
5	3.9	1159	31
6	4.0	1188	33
7	4.1	1218	36
8	4.2	1248	39
9	4.3	1279	41
10	4.4	1311	45
11	4.5	1344	48
12	4.6	1378	52

Claims

Apparatus for controlling a combustion system, particularly a fully-premixed burner combustion system, which incorporates a variable speed fan (18) and means (20, 21) for supplying fuel at a variable rate of flow, the apparatus comprising means (26) operable to vary the speed of the fan progressively from zero to a maximum, means (26) for measuring and storing a value of the speed of the fan, sensing and switching means (36) exposed to the flow of air from the fan (18) for causing the operation of an associated switch mechanism when the rate of air flow generated by the fan (18) reaches a nominated value and control means (26) adapted to increase the speed of the fan (18) from zero until the switch mechanism operates, and the apparatus being characterised in that the control means (26) is adapted firstly to control the fuel supplying means (20, 21) so that it supplies fuel at one of a number of predetermined rates of flow and the fan (18) so that it may operate at one of a number of predetermined speeds, each fan speed corresponding to a particular predetermined value of the fuel flow rate such that the resulting flow rate ratio of air and fuel is at an intended value, secondly to evaluate and store the ratio of the fan speed causing the operation of the switch mechanism to a predetermined reference fan speed, thirdly to multiply each predetermined fan speed by this ratio to define for each rate of fuel flow a corresponding adjusted predetermined fan speed so that the intended value of the air/fuel flow rate ratio may still be exactly or very substantially achievable should the fan speed causing the operation of the switch mechanism differ under particular conditions from the predetermined reference fan speed and fourthly to switch the fan (18) off if the maximum fan speed is reached without the switch mechanism operating.
An apparatus according to claim 1, wherein the fuel supplying means (20, 21) includes a fuel shut-off valve (21).
Apparatus for controlling a combustion system, particularly a fully-premixed burner combustion system, which incorporates a variable speed fan (18) and a fuel supplying means (21), the apparatus comprising means operable to vary the speed of the fan (18) from zero to a maximum, means (36) exposed to the flow of air from the fan (18) for causing the operation of an associated switch mechanism when the rate of flow of air generated by the fan (18) reaches a nominated value and control means (26) arranged to control the fuel supplying means (21) and the fan (18) and being adapted to increase the speed of the fan (18) progressively from zero until the switch mechanism operates, the apparatus being characterised in that the control means (26) is adapted firstly to control the fuel supplying means (21) so that it may, when energised, cause fuel to be supplied at a predetermined rate and the fan (18) so that it may operate at one of a number of predetermined speeds, such that the resulting flow rate ratio of air and fuel is at an intended value, secondly to evaluate and store the ratio of the fan speed causing the operation of the switch mechanism to a predetermined reference fan speed, thirdly to multiply each predetermined fan speed by this ratio to define a corresponding adjusted predetermined fan speed so that the intended value of the air/fuel flow rate ratio may still be exactly or very substantially achievable should the fan speed causing the operation of the switch mechanism differ under particular conditions from the predetermined reference fan speed and fourthly to switch the fan (18) off if the maximum fan speed is reached without the switch mechanism operating.
An apparatus according to claim 3, in which the fuel supplying means (21) is a fuel shut-off valve.
An apparatus according to claim 2 or claim 4, in which the control means (26) is adapted to prevent the fuel shut-off valve (21) from being opened unless the switch mechanism has operated.
An apparatus according to any of the preceding claims in which the means (36) for causing the operation of the switch mechanism comprises a flow metering orifice through which the air supplied to the burner passes and to which, in use, the switch mechanism is connected.
An apparatus according to any of the preceding claims in which the predetermined value of fan speed associated with any predetermined value of fuel gas flow rate is automatically variable to maintain the rate of air flow and the rate of gas flow at, or substantially at, an intended ratio, should the resistance to flow or the performance of the fan (18) alter.
An apparatus according to any of the preceding claims in which the predetermined value of fan speed associated with any predetermined value of fuel gas flow rate is preadjustable manually according to an expected degree of variation in the properties of the fuel gas to minimise the change in the aeration of the fuel/air mixture should the expected variation in fuel gas properties occur.
An apparatus according to any of the preceding claims in which the value of the fan speed is preadjustable automatically by means of a predetermined operating programme.