CA2212499A1

CA2212499A1 - Controlling a combustion system

Info

Publication number: CA2212499A1
Application number: CA002212499A
Authority: CA
Inventors: David Michael Sutton
Original assignee: Individual
Current assignee: BG Group Ltd
Priority date: 1995-02-16
Filing date: 1996-02-14
Publication date: 1996-08-22
Also published as: ES2142045T3; GB2298060B; EP0812409A1; EP0812409B1; AU696298B2; DE69606390T2; JPH10504379A; GB2298060A; AU4672596A; GB9603087D0; DE69606390D1; WO1996025629A1

Abstract

Apparatus for controlling a combustion system, particularly a fully-premixed burner combustion system, which incorporates a variable speed fan and means for supplying fuel at a variable rate of flow, the apparatus comprising means operable to vary the rotational speed of the fan progressively from zero to a maximum, means for measuring and storing the speed of the fan and means exposed to the flow of air from the fan for causing the operation of a switch mechanism when the rate of air flow generated by the fan reaches a nominated value, the control means being arranged to control the fuel supplying means so that it supplies fuel at one of a number of predetermined rates of flow and the fan so that it operates at one of a number of predetermined speeds, each fan speed corresponding to a particular predetermined value of the fuel flow rate, the control means being adapted to increase the speed of the fan from zero until the switch mechanism operates and to adjust each predermined fan speed with respect to its corresponding predetermined rate of fuel flow in the event that the fan speed causing the operation of the switch mechanism differs from a reference fan speed deemed suitable for that purpose, but to switch the fan off if the maximum fan speed is reached without the switch mechanism operating.

Description

W 096/25629 PCTIGB9''00~8 CONTROLLING A CO~u~ ON ~iy~

The present invention relates to apparatus for controlling a combustion system, particularly a fully-premixed burner which incorporates a variable speed fan.

According to one aspect of the present invention, apparatus is provided for controlling a combustion system, particularly a fully-premixed burner combustion system, which incorporates a variable speed fan and means for supplying ~uel at a variable rate of flow, the apparatus comprising means operable to vary the rotational speed of the fan progressively from zero to a maximum, means for measuring and storing the speed of the fan and means exposed to the flow of air from the fan for causing the operation of a switch mechanism when the rate o~ air flow generated by the ~an reaches a nominated value and control means arranged to control the fuel supplying means so that it supplies fuel at one o~ a number o~ predetermined rates of flow and the fan so that it operates at one of a number of predetermined speeds, each ~an speed corresponding to a particular predetermined value of the fuel flow rate, the control means being adapted to increase the speed o~ the fan ~rom zero until the switch mechanism operates and to adjust each predetermined ~an speed with respect to its corresponding predetermined rate of ~uel flow in the event that the fan speed causing the operation o~ the switch mechanism di~ers from a reference fan speed deemed suitable ~or that purpose, but to switch the fan off if the maximum ~an speed is reached without the switch mechanism operating.

Preferably the control means is adapted to evaluate and store the ratio of the fan speed causing the operation of the switch mechanism to the reference fan speed and to multiply each predetermined fan speed by this ratio to provide for each rate of fuel flow a corresponding adjusted predetermined fan speed.

According to another aspect of the present invention, apparatus for controlling a combustion system, particularly a fully-premixed burner combustion system, includes a variable speed fan and a ~uel shut-off valve and the apparatus also comprises means operable to vary the speed of the fan from zero to a maximum, 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 and control means arranged to control firstly the fuel shutoff valve so that, when energised, it may cause fuel to be supplied at a predetermined rate and secondly the fan so that it operates at one of a number of predetermined speeds, the control means being adapted to increase the speed of the fan progressively from zero until the switch mechanism operates but to switch the fan off if the maximum fan speed is reached without the switch mechanism operating.

W 096/2~629 PCT/GB9C/0~3~8 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 lA, lB, lC are schematic views of a domesticcombustion 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 lA, there is illustrated a domestic combustion system which comprises a gas boiler 1 located within a room-sealed casing 2 mounted on the inner sur~ace 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 o~ a flanged outlet 10. The horizontal part 8 of the ~lue 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 llbalanced flue" variety.

The burner 4 has a plenum chamber 15 beneath which is located the burner plate 16. Upstream from the plenum chamber lS is a m;~; ng 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 m;~c; ng 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 o~ the ~uel 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 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 ~orm o~ 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. I~ 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 ~allen to some lower value.

A combined igniter and flame ~ailure detector 34, located immediately beneath the burner plate 16, communicates bi-directionally with the control box 26 by means o~ a line 35.
The device 34 is a standard ~eature ~orming no part o~ the present invention, it being mentioned ~or completeness only.

CA 022l2499 l997-08-07 Between the ~an 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 ~10w ~or combustion passes, consequently ~alling in pressure by an amount related in a predictable manner to the rate o~ air ~low The diaphragm is located within a chamber which is thereby divided into two compartments, each of which is connected to a dif~erent side o~ 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 o~ 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 3 7 to the control box 26 ~or purposes to be subsequently described.

A signal indicative of the demand ~or 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 ~orms 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 ~rom the speed sensor, all via the multicore line 27. The control signal is supplied as a train of rectangular pulses o~ 1000 Hz frequency generated by the control box 26, the duration Lcp o~ each 0 -5V
pulse of the train being variable by the control box 26 over the range o.0000 - 0.0010 second to control the speed o~ the fan 18. The time interval between successive pulses ~rom 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 o~ the reference values is reduced to zero by adjustment of the duration of the control pulses supplied to the motor of W 096/25629 PCT/G~9GI'~03~8 the fan 18. In this way the control 26 is able to obtain and maintain the fan speed corresponding to the selected re~erence value. If other factors remain constant, in a combustion system of the type shown in Figure lA the rate of air flow is very nearly proportional to the rotational speed o~ the ~an.
Therefore, provided that the performanc=e 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~ 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.

Re~erring to Table lA, 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 o~
gas flow rates between a m;n;m1lm 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 o~ N in column 1 o~ the look-up table. The rows shown cover fan speeds ranging ~rom 1050 rev/min at N=1 to ~ 1378 rev/min at N=12. The intended air ~low rate in each row is approximately 2.5~ greater than that in the preceding row.

The ~ourth column in the table comprises the respective drive voltage Vmg~ in volts, corresponding to each value o~ N in the table, ~or operating the modulating valve 20.

The fi~th column in the table comprises the nominal duration o~ the ~an speed control pulses in microseconds corresponding to each value of N, as supplied on line 27.

In constructing such a table, each combination o~ gas flow rate and ~an speed is selected to provide a predetermined air/gas ~low rate ratio corresponding to an intended percentage aeration o~ the combustible mixture, given ~uel gas o~ an assumed theoretical air requirement ~or combustion (m3 air/m3 ~uel gas) and a ~an o~ assumed per~ormance characteristics operating normally in a combustion system o~
an assumed ~low resistance characteristic. To secure the maximum possible performance ~rom the combustion system, the intended percentage aeration may be made variable according to - the rate o~ gas ~low. However, this re~inement has not been adopted in the present embodiment. We describe later methods o~ compensating ~or departures ~rom 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 lA 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 ~low 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~ and M, described below. It then reads equal to a preset value Vm~. 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 ~or 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.

I~ the sa~ety checks reveal no hazard, the control box 26 will ~ind ~rom ROM the value of (Fco) , a re~erence ~an speed assumed su~icient ~or actuatlon 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 o~ ~an speed control pulses as described earlier, the duration L~ o~ these pulses being that listed in Column 5 o~
the look-up table, in the row for F = (Fco) . When the speed o~ the ~an 18 has become steady, the control box 26 will determine whether a voltage exists at the high-pressure contact o~ the changeover switch in the assembly 36. I~ these is none, the value o~ L~ in relation to the maximum value o~
0.0010 second is checked; and as L~ will not be at the maximum value at this stage, the control box 26 will increase L~, pause suitably ~or a change in ~an speed to occur and re-~mi ne the high-pressure contact o~ the changeover switch.
This will continue until either a voltage appears at this contact, or the value o~ L~ becomes 0.0010 second. In the latter event, in the interest o~ sa~ety, the control box 26 will set S = O, L~ = O and "lockout", as described above.

In the alternative event, however, the control box 26 will measure the value o~ L~ and find ~rom the look-up table the corresponding listed ~an speed F = (F~)co. This number is then stored into RAM ~or convenience i~ more than one attempt to light the burner should prove necessary, or i~ the ~lame should become extinguished at some time a~ter the burner has come into operation. The control box 26 will then measure the CA 022l2499 l997-08-07 W 096/25629 PCTIC~9G/003~8 12 ~an speed F and store it into RAM as F = Fco~ It will next look-up the value o~ (Fco) and evaluate the ~low switch ~an speed correction ~actor C~ ~rom the Equation:

C~ = Fco . (Fco) (1) The ~actor C~ will be stored into RAM ~or use later, as will be described. I~ the circumstances o~ operation happened to accord exactly with those assumed in constructing the look-up table, C~ would be unity. Clearly, by reason o~ the manner o~
its derivation, the ~actor C~ can only assume values which are compatible with the predetermined values of ~an speed in the look-up table.

A~ter a pause o~ tp seconds during which ~resh air is blown through the combustion system to purge it o~ residual products ~rom previous combustion and of any traces o~ ~uel gas which may have leaked in through the closed valve 21, the control box 26 will estimate, and store into RAM, the ~an speed ~or ~lame ignition F = Fi appropriate under the prevailing conditions to Gm~ the lowest o~ the predetermined rates o~
~uel ~low, and given by the Equation:

Fi = B x C~ x Fmin (2) where B = a constant preset during manu~acture or installation o~ the control box 26 according to the expected degree o~ variation in the properties of the fuel gas to be used by the burner 4.

Fmin = the lowest of the predetermined fan speeds in the look-up table, appropriate to Gmjn 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 o~ 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 o~ Lcp for the fan speed F = F; 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 Fj, the control box 26 will reduce the value of Lcp~ recheck the fan speed when this has become steady and continue the process until the ~an speed attains the target value.

CA 022l2499 l997-08-07 If, however, when first measured the Ian speed is ~ound to be less than Fi, the duration of the control pulses will be measured and compared with the maximum value of 0.0010 second.
If Lcp is less than the maximum, the control box 26 will increase Lcp~ 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, Lcp = o and go into "lockout~'.

Assuming that the target ~an speed is achieved success~ully, however, the control box 26 will measure the value of Lcp arrived at, then find from the look-up table, and store into RAM, the corresponding listed fan speed F = (Fcp)j~ It will next energise firstly the igniter of the device 34 and, a few seconds later, the coil o~ the gas shuto~ valve 21, enabling fuel gas to flow to the burner 4 through the modulating valve which, though unenergised at this stage, sits in a partially-open position against an internal stop. If after a time tj seconds no flame is sensed by the detector of the device 34, the control box 26 will turn of~ 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 CA 022l2499 l997-08-07 W O 96/25629 PCT/GBS'~ 18 will update I to unity and try again to establish a ~lame on the burner 4. To do so it will recall ~rom RAM the ~an speed F = (FCP)CO~ look-up the corresponding value o~ Lcp~ supply control pulses o~ this duration and repeat the steps described above in relation to the initial attempt at ignition. In the course of this, the parameters Fco~ (FCP)CO and C~ will be revised i~ necessary, or alternatively, the control box 26 will establish "lockout" in the manner described above i~ the control pulse duration should rise to its maximum value of 0.0010 second without a voltage appearing at the high-pressure contact o~ the changeover switch. I~ a ~lame ~ails to appear on the second attempt, since now I = 1 the control box 26 will set S = O, Lcp = O and then "lockout". I~ ~lame 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 o~, a ~lame remains present at the detector o~
the device 34. I~ it does not, one attempt will be made to relight the ~lame. To do this the control box 26 will turn o~ the power supply to the valve 21, store a value I = 1 into RAM and go through the remainder o~ the procedure described above ~or a second ignition attempt.

I~ ~lame does exist at the detector, the control box 26 will read the line 38, to establish whether there is still a demand ~or heat. I~, unusually, there is no longer any demand, the control box 26 will turn o~ the supply o~ power to the valve 21, set Lcp = O to stop the ~an and await the emergence o~ a new demand ~or heat. I~, however, the ~m~n~ still exists, the control box 26 Wi11 carry out certain standard sa~ety checks. Should these reveal some hazard, the program will set S = O, de-energise the valve 21, set L~ = O and go to "lockout".

Assuming for the present purpose that the sa~ety checks are completed success~ully, however, the control box 26 Wi11 ~irst start a timer monitoring the length o~ the ~iring period o~
the burner ~or reasons to be explained, and then ~m~ne the value of the parameter M. When the program o~ 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, re~erenced as (N'G)E:, a tentative value oE unity i~or the parameter NG de~ined below, and set out to establish the row number N = N'G in the look-up table which would provide the burner ~iring rate corresponding most nearly to the actual ~em~n~ for heat ~rom the external source 39.

To do this the control box 26 Wi11 first measure and scale the voltage signal on the line 38, on the assumption that the calori~ic value o~ 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 short~all, or alternatively an excess, in a desired temperature in the ~luid (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
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 ~m; ne 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 FN in the row N = N'G Of the look-up table, and therefrom estimate an appropriate target operating speed FOP for the fan 18 using the Equation:

CA 022l2499 l997-08-07 W 096/2~629 PCT/GB96/00~18 18 FOP = B X CFS X FN ( ) Recalling from RAM the values F; and (FCP) ;r restoring them at new addresses denoted by FE~ (FCP) E respectively and then using the values FE~ (F~)E and FOP the control box 26 will estimate, and store into RAM, a target ~an speed (FCP) OP ~or selecting the duration o~ the control pulses, given by:

(FCP) OP = (FCP) E X FOP ~ FE (4 ) Provided (FCP) OP does not exceed Fm~ the highest ~an speed listed in the look-up table, the control box 26 will store into RAM the row number N' G as that ~or setting the drive voltage Vm~ ~or the modulating valve 20; and the stored values F = FOP and F = (FCP) OP will be used to de~ine the desired respective values o~ the actual ~an speed F and the control pulse duration Lcp~ Otherwise, ~or setting the drive voltage ~or the modulating valve 2~ 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 ~an speed (FN) P de~ined by Equation (5) below:

(FN) P = Fm,.,~ ~ ~ (FCP) E: X B X C~S ~ FE] (5) In this case the value (FCP) OP = Fm~ will be stored into RAM ~or setting the control pulse duration. Then, recalling the values O~ (FCP) OP~ FE and (FCP) E, the control box 26 will estimate and store into RAM the target ~an speed FOP given by:

FOP = (FCP) OP X FE ~ (FCP) E = Fm~ X FE ~ (FCP) E ( 6) The control box 26 will now compare the target and existing values (FCP) OP and (FCP) E to determine the re~uired direction o~

W O 96/25629 PCT/~DgG/00348 change in the ~an speed. In the present instance, as the burner is operating at its m; n; mllm rate and the existing and requested values o~ N'G are unequal, by implication an increase in burner heat output is called ~or. By re~erence to the look-up table, the control box 26 will there~ore increment by a number of rows (~or example, four) the pulse duration Lcp~
starting Erom the value corresponding to F = (FCp) E Then, after a pause to allow the change in ~an speed to come partially into being, the control box 26 will increment similarly and by the same number o~ rows the drive voltage V
~or 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 NG arrived at in this manner, compares this with the target value N'G and continues the change process until the respective targets (Fcp)op and N'G are arrived at simultaneously. This stepwise procedure serves to limit any transitory reduction in the air/gas ~low rate ratio which would arise i~ the modulating valve 20 responded more quickly than the ~an 18 to a common change in the row number. A~ter every stage o~ change in the settings of the ~an 18 and modulating valve 20, the control box 26 will check that ~lame continues to exist at the detector o~ the device 34.

Next the control box 26 will measure the actual ~an speed F
and estimate and store into RAM the ratio ~N~ = (Fop - F).
Normally this will be unity and the program will return to the point where it established whether ~lame continued to be present at the detector o~ the device 34 a~ter the igniter had been switched o~f. All the ~oregoing steps will then be performed again in the m~nner described, so that operation will proceed in safety and the control system will become aware o~, and respond to, any change in the heating requirement.

However, i~ [N]l is found to be less than unity, the control box 26 will recall (FCp)op~ the ~an speed regulating the control pulse duration Lcp~ multiply it by the quantity [N]l and store this reduced value o~ (F~)op into RAM. The control box 26 will then look-up, and provide, the corresponding new pulse duration Lcp~ measure the resulting ~an speed F when this had become steady and re-evaluate the ratio [N]~
exceptionally, this were still less than unity, the procedure described would be repeated until [N]l had become equal to unity.

I~, on the contrary, ~N]l is ~ound to be, or to have become, greater than unity the control box 26 will recall (FCp)op~ ~ind ~rom the look-up table the value o~ the m~ mllm possible ~an speed Fm~ estimate the ratio [N] 2 = (Fm~ (Fcp)op) and evaluate a parameter E according to the ~quation:
E = [N] 2 ~ [N]l (7) I~ E is not less than unity, the control box 26 will estimate a new value o~ the parameter (FCp)op = [(FCp)op x [N~l] and store this value into RAM. It will then identi~y ~rom the look-up table the corresponding value o~ the control pulse duration W 096l25629 PCT/GB96/00348 Lcp~ and generate and despatch along the line 27 pulses o~ this duration to increase the speed of the ~an 18. The control box 26 Wi11 again measure the :Ean 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 FOP.

Should E be, or become, less than unity, however, the control box 26 Wi11 recall from RAM the row number N'G~ find f~rom the look-up table the value of the fan speed FN listed in that row, multiply this speed by the amount E and store the reduced value Of FN 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 o~
Vm~ for that row, to lessen the rate of fuel gas flow.
Secondly, using Equation (3) on the basis of the reduced fan speed value FN~ the control box 26 will estimate, and store into R~M, a new value of the target fan speed NOP suitable for the revised value Of N' Gi and thirdly, it will set Lcp to the maximum value of O.0010 second and store into RAM the corresponding fan speed (FCP)OP = Fm~ Next the control box 26 will again measure in due course the new steady fan speed F, recall the reduced value of the target ~an speed FOP and estimate the new ratio ~N]I = (FOP - 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 CA 022l2499 l997-08-07 W 096/25629 PCT/GB~6/00~18 target ~an speed, the control pulse duration r~;n;ng at 0.0010 second. This procedure will continue until the fan speed becomes equal to the reduced target value, the latest t value o~ N' G stored into RAM becoming the working value for setting the drive voltage Vmgr for the modulating valve 20.

With the intended ~low rate ratio attained, the program o~ the control box 26 will read the firing period timer. If the burner firing time exceeds a preset period top (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 Vm~ = O and carry out the procedure for reigniting the flame, as described earlier.
In the course of this the factor C~ 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 ~an 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 ~uel gas properties.

If when checked the burner ~iring time does not exceed the period top~ however, the program of the control box 26 will return to the point, described earlier, where it established whether ~1ame continued to be present at the detector of the CA 02212499 1997W 096/25629 PCT/GB961003~8 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 Vm~ and L~ both to zero to extinguish the flame and go to "standby", awaiting a fresh ~e~n~ 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 Vm~ 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 ~rom 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 dif~erential-pressure-sensing assembly 36. In known combustion controllers, operation o~ the burner would be disallowed unless, when rotating at a prechosen and nominally CA 022l2499 l997-08-07 W 096/25629 PCT/GB9G/~S31X 24 constant speed, the ~an was able to promote a rate o~ air ~low suf~icient to cause operation o~ a switch such as that in the assembly 36.

Finally, because according to the present invention compensation can be applied ~or variations o~ circumstance, including changes in ~uel gas properties, the burner 4 will function, always and automatically, with a rate o~ air supply (relative to the stoichiometric) close to, i~ not identical with, that intended by the designer. This will maximise the li~e o~ the burner and the per~ormance o~ the equipment which it serves, and minimise the generation o~ undesirable by-products o~ the combustion process.

In respect o~ the second embodiment, Figure lB shows a domestic combustion system which is similar to that shown in Figure lA, except that in this case the modulating valve 20 and its associated line 28 are replaced by a ~ixed ~low restrictor ori~ice 20, the size o~ the ori~ice being selected from a predetermined range according to the rate o~ ~uel gas ~low (and so, heat output) desired. The ori~ice 20 may be placed separately ~rom the valve 21 as shown. Alternatively and more conveniently, it may be incorporated within the valve 21.

A signal indicative o~ the demand ~or heat is supplied to the control box 26 along line 38 ~rom a demand signal processor 39, the connections to which are shown schematically in Figure 2B. The processor 39 receives signals ~rom 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 ~orming no intrinsic part o~ the present invention.

Table lB illustrates schematically the ~irst 12 rows o~ a data look-up table which, in this embodiment, is stored in ROM in the control box 26.

The ~irst column o~ the table comprises "N", the row number o~
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 o~ heat output rates between a minimum o~ 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 ~an speed F in revolutions per minute (rev/min) corresponding to each value o~ N in column 1 o~ the look-up table. The rows shown cover ~an speeds ranging ~rom 1050 rev/min at N=l to 1378 rev/min at N=12. The intended air ~low 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 ~low rate) and fan speed is selected to provide a predetermined air/gas ~low rate ratio corresponding to an intended percentage aeration of the combustible mixture, given ~uel 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 o~ explanation, the date in Table lB 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, ~or later purposes, a parameter C~, 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 Vmm. If such a voltage is present, this indicates the existence o~ 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 ~ind ~rom ROM the value of (Fco) , a re~erence fan speed assumed sufficient for actuation o~ the changeover switch in the assembly 3 6 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~ of these pulses being that listed in column 5 o~
the look-up table, in the row ~or F = (Fco) . When the speed of the ~an 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 3 6. If there is none, the value of L~ in relation to the maximum value of 0.0010 second is checkedi and as L~ will not be at the maximum CA 022l2499 l997-08-07 W 096/25629 PCTI~G/00318 value at this stage, the control box 26 Will increase L,p, pause suitably for a change in fan speed to occur and re-~m; ne the high-pressure contact of the changeover switch. t This will continiue until either a voltage appears at this contact, or the value of L~ becomes 0.0010 second. In the latter event, in the interest of safety, the control box 26 will set S = O, L~ = O and "lockout'l, as described above.

In the alternative event, however, the control box 26 will measure the value of L~ and find from the look-up table the corresponding listed fan speed F = (F~)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 2 6 will then measure the ~an speed F and store it into RAM as F = Fco~ It will next look-up the value of (Fco) and evaluate the flow switch fan speed correction factor C~ from the Equation:
C~ = Fco - (Fco) (1) The factor C~ will be stored into RAM ~or use later, as will be described. I~ the circumstances of operation happened to accord exactly with those assumed in constructing the look-up table, C~ would be unity. Clearly, by reason o~ the manner o~
its derivation, the factor C~ can only assume values which are compatible with the predetermined values of fan speed in the look-up table.

CA 022l2499 l997-08-07 W O 96/25629 PCT/GB9CI'~318 29 After a pause of tp seconds during which ~resh 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 = Fop appropriate under the prevailing conditions to G, the predetermined rate of fuel flow, and given by the Equation:

Fop = A x B x C~ x F (2) 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 o~ the ~uel gas to be used by the burner 4.

- Fmm = 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 ~Iow 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 Lcp for the fan speed F = Fopt 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 Fopt the control box 26 will reduce the value o~ Lcp~ 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 Fopt the duration of the control pulses will be measured and compared with the maximum value of 0.0010 second.

:

If Lcp is less than the m~x;mllm, the control box 26 will increase Lcp~ 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, Lcp = O and go into "lockout~.

Assuming that the target fan speed is achieved successfully, however, the control box 26 will measure the value of Lcp arrived at, then find from the look-up table, and store into RAM, the corresponding listed fan speed F = (FCp)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;
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 m~; mllm 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, Lcp~ 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 FCo~ (F~)co and C~ 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 m~;mllm 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~
= 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 r~m~;nq 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 r~m~;n~er o~ the procedure described above for a second ignition attempt.

I~ ~lame does exist at the detector, the control box 26 will read the line 38, to establish whether there is still a ~l~m~nr~

~or 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~ = O to stop the ~an 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 Lcp = 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 tN]I = (Fop - 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 tN]I be found, however, to be less than unity, the control box 26 will recall (FCp)op~ the fan speed regulating the control pulse duration Lcp~ multiply it by the quantity tN]l and store this reduced value of (FCp)op into RAM. The control box 26 will then look-up, and provide, the corresponding new pulse duration Lcp~ measure the resulting fan speed F when this had become steady and re-evaluate the ratio tN]l- If, exceptionally, this is still less than unity, the procedure described will be repeated until ~N]l has become equal to unity.

If, on the contrary, tN~I is found to be, or to have become, greater than unity the control box 26 will recall (FCp)op, ~ind from the look-up table the value of the m~i mllm possible fan speed Fm~ estimate the ratio [N]2 = (Fm~ - (FCp)op) and evaluate a parameter E according to the Equation:
E = [N]2 - [Njl (3) If E is not less than unity, the control box 26 will estimate a new value of the parameter (FCp)op = [(FCp)op x [N]l] and store this value into RAM. It will then identify from the look-up table the corresponding value of the control pulse duration Lcp~ 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 ~an 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 ~an speed Fop~

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~
= 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 top (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 Vm~ = O and carry out the procedure for reigniting the flame, as described earlier.
In the course of this the factor C~ 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 top~ 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 o~ 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 ~m~n~
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 2;, set the parameters vmg. and Lcp both to zero to extinguish the flame and go to "standby", awaiting a fresh demand ~or heat ~rom 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 ~uel ~low and correspondingly, more than one associated rate o~ air ~low.
For example dual-rate ("high/low") operation o~ the burner 4 may be achieved, as shown schematically in Figure lC, by providing two valves 21, 21A, each valve including or otherwise in series with its own flow restrictor ori~ice 20, 20A to provide a particular rate o~ ~uel ~low, the valves being controlled individually by separate lines 29, 29A
respectively ~rom the control box 26, and two values A1 and A2 o~ the constant A being allocated, one value appropriate to each o~ the; ~uel ~low rates. Two levels o~ signal voltage would be supplied on the line 38 from the source 39, the lower of these being at least equal to Vmm, each level being representative o~ a particular one o~ the two requirements ~or heat and causing the control box 26 to select, in accordance with its operating program, the appropriate value Al or A2 ~~
the constant A ~or use in Equation (2).

Considerable advantage to the user derives ~rom the ~acility in the present invention ~or varying the ~an speed during the startup sequence to induce, i~ possible, operation o~ the switch in the di~erential-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.

CA 022l2499 l997-08-07 TABLE lA

(1) (2) (3) (4) (5) N G F Vm~ L~
(m3/h) (rev/min) (volts) (~sec) 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 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 0.44 1311 3.81 45 11 0.45 1344 4.02 48 12 0.46 1378 4.24 52 CA 022l2499 l997-08-07 T~BLE lB

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

Claims

1. Apparatus for controlling a combustion system, particularly a fully-premixed burner combustion system, which incorporates a variable speed fan and means for supplying fuel at a variable rate of flow, the apparatus comprising means operable to vary the speed of the fan progressively from zero to a maximum, means for measuring and storing the speed of the fan, means exposed to the flow of air from the fan for causing the operation of a switch mechanism when the rate of air flow generated by the fan reaches a nominated value and control means arranged to control the fuel supplying means so that it supplies fuel at one of a number of predetermined rates of flow and the fan so that it operates at one of a number of predetermined speeds, each fan speed corresponding to a particular predetermined value of the fuel flow rate, the control means being adapted to increase the speed of the fan from zero until the switch mechanism operates and to adjust each predetermined fan speed with respect to its corresponding predetermined rate of fuel flow in the event that the fan speed causing the operation of the switch mechanism differs from a reference fan speed deemed suitable for that purpose, but to switch the fan off if the maximum fan speed is reached without the switch mechanism operating.

2. Apparatus as claimed in claim 1 in which the control means is adapted to evaluate and store the ratio of the fan speed causing the operation of the switch mechanism to the reference fan speed and to multiply each predetermined fan speed by this ratio to provide for each rate of fuel flow a corresponding adjusted predetermined fan speed.

3. Apparatus for controlling a combustion system, particularly a fully-premixed burner combustion system, which incorporates a variable speed fan and a fuel shutoff valve, the apparatus comprising means operable to vary the speed of the fan from zero to a maximum, 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 and control means arranged to control firstly the fuel shutoff valve so that it may, when energised, cause fuel to be supplied at a predetermined rate and secondly the fan so that it operates at one of a number of predetermined speeds, the control means being adapted to increase the speed of the fan progressively from zero until the switch mechanism operates but to switch the fan off if the maximum fan speed is reached without the switch mechanism operating.

4. Apparatus as claimed in claim 3 in which the control means is adapted to prevent the fuel shutoff valve from being opened unless the switch mechanism has operated.

5. Apparatus as claimed in any of the claims 1 to 4 in which 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.

6. Apparatus as claimed in 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 alter.

7. Apparatus as claimed in 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.

8. Apparatus as claimed in any of the preceding claims in which the value of the fan speed is preadjustable manually by means of a predetermined operating programme.9

9. Apparatus substantially as hereinbefore described with reference to the drawings.