GB1571906A - Air fuel gas ratio controls for burners - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO AIR FUEL GAS RATIO CONTROLS FOR BURNERS (71) We, BRITISH GAS CORPORATION, of 59 Bryanston Street, London, W1A 2AZ, a British Body Corporate, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to controllers for the supply of air and gaseous fuel to burners.

Efficient combustion of gas in a furnace or burner requires accurate control of the ratio of air and gas supplied to the burner. If insufficient air is supplied, there will be incomplete combustion of the gas, giving rise to production of carbon monoxide and consequent reduction in efficiency, and too much air will result in an increase in the heat wasted in the fuel gases.

Air/gas ratio control on small modulating burners is often achieved by means of linked valves in the air and gas supplies, where the linkage between the valves is adjusted to give the desired air/gas ratio throughout the burner range. Such systems can be set to give the optimum ratio at the conditions prevailing at the time of commissioning, but can take no account of changes which occur subsequently, such as changes in temperature or pressures.

Such changes can cause changes in the mass flow ratio of air to gas of up to fifteen per cent, resulting in significantly increased flue losses. Also the performance of open loop ratio control systems, of the linked valve type, is critically dependent on the quality of the linkage as there can be significant changes in air/gas ratio if there is play in the coupling. On larger plants these problems are generally overcome by flow metering of the air and gas supplies to the burner, with an automatic electric or pneumatic control system which maintains the air/gas ratio at the desired value. Such systems may also incorporate trimming of the air/gas ratio by oxygen analysis of the flue products.However, conventional ratio control systems based on flow metering techniques are very expensive and have therefore only been applied to very large plant where the fuel saving can justify the increased capital cost.

In order to overcome the disadvantages of existing flow metering control techniques, a novel system based on shunt flow meters has been devised.

According to the present invention there is provided apparatus for the flow control of air and gaseous fuel ratio to a burner comprising a first and a second supply line respectively for said air and said fuel gas, each of said supply lines having restrictor means to impede the flow of the air or fuel gas therein, first and second bypass lines respectively mounted about said first and second restrictor means, first and second electrical flow sensing means respectively mounted in said bypass lines, said flow sensing means having an electrical transducer having an output parameter dependent on the rate of flow of fuel gas or air in the respective bypass line, comparator means for comparing the output of said transducers and a valve controlled by said comparator to adjust the flow of fuel gas or air in one of said first and second supply lines.

Preferably, the control apparatus also includes means to shut down the burner in the event of a fault causing the gas/air ratio to depart from a predetermined ratio.

The invention will now be particularly described with reference to the accompanying drawings, in which: Figure 1 is a schematic drawing of a fuel gas/air ratio controller in accordance with an embodiment of the invention, Figure 2 is a block diagram showing the principal components in the electrical control system shown in Figure 1, Figure 3 is a detailed circuit diagram showing a fuel gas/air ratio controller in accordance with a specific embodiment of the invention, Figure 4 is a detailed circuit diagram illustrating auxiliary circuits, and Figure 5 a to c is a schematic diagram illustrating a thermistor mounting block suitable for use with the above embodiment.

An air/fuel gas ratio controller according to the invention is operated by metering the air and fuel gas flows to a burner and actuating a throughput control valve situated in either the air or the gas line so as to maintain the air/fuel gas ratio at a preset value. It employs an electrical sensing system comprising, for example, inexpensive thermistor anemometers (hot films or wires could also be used) to obtain pressure and temperature compensated electrical measurements of the air and gas flows.

Referring now to Figure 1 of the drawings, thermistor anemometers 1,2 are mounted in a holder 3 connected to bypass lines 4,5 around constrictions 6,7 in the main air and gas supply lines to a burner. The constrictions in the main lines may be fixed orifices, adjustable control valves or similar components to give a pressure drop at the maximum rating of the burner of at least one inch water gauge. On the inlet of each bypass line there is a filter 8, 9 and a fine control valve 10, 11 which determines the ratio of the flow through the bypass to the flow through the main line.In order to achieve good compensation for variations of air and gas temperature, the bypass valves are fitted close to the mainlines 12, 13 so that the fluid passing through the bypass valve is substantially at the same temperature as the fluid in the main line.

The thermistor anemometers are mounted remote from the main lines. The lengths of the bypass lines are such that the air and gas flowing down them reaches ambient temperature before passing over the thermistors. Typically this requires a length of at least three feet of 3/8 inch diamter copper tubing.

The thermistors are maintained at a constant temperature (i.e. constant resistance), using a standard feedback bridge circuit. An electronic controller 15 compares the voltages required to maintain the air and gas anemometers at constant temperature. The voltage difference is amplified, and any difference greater than a preset margin causes a signal to be sent out to an electric motor driven ratio control valve 14,25, the sense of the signal being such as to restore the balance between the two anemometer outputs. The circuit preferably includes a facility to vary the controlled air/gas ratio with burner firing rate. This gives an independent setting for the air/gas ratio at high and low fire, with a linear extrapolation between the two set values for intermediate firing rates. The circuit is designed for actuation of a well known reversing motor.No feedback potentiometer is required on the motor, as the circuit receives feedback from the flow signal of the anemometer. The ratio control valve is situated in either the air line for 'gas lead' control, or in the gas inlet for 'air lead' control. In order to prevent transient off ratio conditions during modulation between high and low fire it is important that the motor used for the lag valve should be faster than that used on the lead valve.

The ratio controller incorporates a safe start check, and a safety lock-out. These are wired into the burner control unit to hold the light-up programme, or initiate shut-down, in the event of an unsafe condition existing, such as loss of air or gas pressure. In addition to saving the cost of the motor limit switches, these additional features considerably reduce on-site wiring requirements. Included in the gas line are a throughput control valve 16 and two safety shut-off valves 17.

The voltage output from a thermistor is given bv:

where V is the output voltage QI is a constant amplification factor k is the thermal conductivity of the fluid T is the controlled temperature of the thermistor To is the ambient temperature A and B are constants of the thermistor p is the density of the fluid u is the fluid velocity past the thermistor is the viscosity of the fluid The amplification factors (d for the air and gas anemometers are adjusted so that at zero flow (u = 0) the outputs from the two anemometers are equal.

thus Cgkg (Tg - Tog)Ag = Q)aka (Ta- Toa)Aa (2) where the symbols have the meaning defined above, and the suffixes "g" and "a" represent the values for gas and air respectively.

Note the equation (2) will remain satisfied for all values of ambient temperature because: (a) the anemometers are controlled to the same temperature i.e. Tg = Ta (b) the anemometers are subject to the same ambient temperature, i.e. Tog = Toa (c) the thermal conductivities of air and gas vary in a similar manner with temperature.

Thus the velocity independent term of equation (1) is the same for the air and the gas anemometers at all ambient temperatures.

In operation the electronic controller maintains the voltage outputs from the two anemometers equal, and hence for equilibrium the velocity dependent term of equation (1) must be equal for the air and gas anemometers.

Hence Q)Bkg (Tg - Tog)Bg (pgug/,ug)+ = Qlaka(Ta - Toa) Ba(paua/,Ma)* ........(3) (3) but as stated above: Tg- Tog + Ta - Toa also the thermal conductivities (k), and the viscosities (,u) for the air and gas vary similarly with temperature, hence equation (3) can be simplified to : pgug = Constant x paua (4) Thus the controller maintains the mass flows past the air and gas anemometers in a constant proportion.Since the flows in the bypass arms are a fixed proportion of the flows in the main air and gas lines, it therefore follows that the flows in the main lines are in a constant ratio, The air/gas ratio is not affected by changes in the ambient temperature, and temperature compensation of the anemometers is not necessary, but can usefully be added where the anemometer outputs are used to indicate the air and gas flow rates. Furthermore, provided the restrictions in the air and gas bypass lines are at the same pressure and temperature as the respective restrictions in the main lines, the system automatically compensates for any changes in air and gas pressure or temperature.

Thus the system gives mass ratio control. The controlled air/gas ratio can be set to any desired value by adjustment of the restriction in the main or bypass lines.

The electronic circuit of the ratio controller is shown schematically in Figure 2 and consists essentially of two feedback bridge circuits 21, 22 to maintain the thermistor anemometers at a constant temperature, a comparator 23 which compares the voltage output from the two thermistor circuits, and a motor drive circuit 24 which provides power to the electric motor 14 of the ratio control valve 25 when the output from the comparator circuit exceeds a preset value. In addition to providing the three functions listed above, this circuit may incorporate a low fire ratio setting adjustment 26 and a safety lockout device 27 which can be used to shut the burner down if a fault leads to off-ratio firing. The various circuit elements which provide the five functions listed above are illustrated in Figures 3 and 4.

The circuit shown in Figure 3 is designed for use with ITT thermistors (type P15) TH1, TH2 which form the anemometers for measurement of the gas and air throughputs. The thermistors are maintained at a constant resistance of 1K (i.e. 1800C), by the bridge circuits controlled by integrated circuits IC 1, IC 8. In commissioning the circuit the variable resistors VR1 and VR4 are set so that Va and Vg are both zero at no flow condition. (This adjustment must be made with gas surrounding the gas flow thermistor). As the flow increases, the output voltages from the integrated circuits rise so as to maintain the thermistor at 1800C. Thus the voltages Va and Vg are a measure of the air and gas throughputs.

The voltages Va and Vg are fed to the difference amplifier IC2, which gives a fixed gain of 6.

The output is fed to variable gain amplifier IC3 and a lockout circuit via follower IC9.

Resistor R11 is included between integrated circuit IC2 and the +1 5v rail to ensure that the lockout circuit will be activated if integrated circuit IC2 becomes open circuited. Variable gain amplifier IC3 has a gain of from 1 to 19 dependent on the setting of variable resistor VR2. This enables the sensitivity of the control system to be adjusted, and is set when commissioning the complete control system to achieve stable control.

The output from the variable gain amplifier is fed to two Schmitt triggers IC4, IC10. When the output from integrated circuit IC3 exceeds + lv the output of integrated circuit IC4 switches from being saturated positive to saturated negative. This turns on transisotr TR6 causing triac TC2 to fire. Similarly, when the output from integrated circuit IC3 falls below -lv, triac TC1 is fired. These two triacs activate the "open" and "close" terminals of a 24v reversing motor 14 for the control valve 25 via an auto-manual switch S2a, S2b and output terminals B1, B2. For gas-lead ratio control, triac TC2 is connected to the motor "open" terminal and triac TC1 is connected to the motor "close" terminal. For air-lead ratio control these connections are reversed.Manual control of the motor 14 is effected by the changeover switch S2a, S2b and selective operation of the switch S1.

In order to set the air/gas ratio at low fire independently of the setting at high fire, a small voltage AV, from the output of integrated circuit IC7 is added to the air signal Va at the difference amplifier IC2. The voltage AVis derived from the output of the potential divider R30, VR3 and R3 1, and can be set to give the required air/gas ratio at low fire by adjustment of variable resistor VR3. The potential divider R30, VR3, R3 1 is fed from the outputs of the difference amplifiers IC5,.IC6 which give an output equal to the difference between the air anemometer output and 7.5v.Where a temperature compensation circuit is provided, the temperature corrected voltage of the air anemometer is fed to the input of integrated circuits IC5, IC6 but where no temperature compensation is required, the subtraction circuits are fed directly from the output of the air anemometer. The potential divider is thus fed at one end with the voltage + (7.5 - Va)v and at the other end with -(7.5 - V1)v. The output from the potential divider is therefore always zero when Va = 7.5v, and this condition corresponds to the maximum firing rate.The voltage added to the air signal is thus set on variable resistor VR3 to give the required air/gas ratio a low fire, and this voltage gradually falls to zero at the high fire rate, so the setting of variable resistor VR3 does not alter the air/gas ratio at high fire, which is determined solely by the ratio of the pneumatic restrictions in the bypass lines containing the anemometers. Where the air/gas ratio at low fire is different from that set at high fire, the controller will give an approximately linear interpolation between the set air/gas ratios at intermediate firing rates.

The safety lockout circuit is fed from the output of follower IC9. In the balanced condition this output will be zero, and any significant drift from this should cause actuation of the valve so as to restore the zero condition. With zero volts on the output of integrated circuit IC9 transistors TR3, TR4 are both held on, and both relays RL1 and RL2 are energised. A positive voltage on the output of integrated circuit IC9 which indicates a 'gas-rich' condition drives the base of transistor TR4 positive, thus reducing the current through relay RL2, causing it to de-energise. Similarly a negative voltage, which indicates a 'lean' condition causes relay RL1 to de-energise. The sensitivity of the lockout circuit is controlled by variable resistor VR5.At maximum sensitivity the circuit will trip with approx. + 1 volt on the output of integrated circuit IC9, which is equivalent to + 1/6 volt between the air and gas anemometer outputs. At minimum sensitivity, the circuit will trip with approximately +6v on the output of integrated circuit IC9, equivalent to +1v difference between the air and gas anemometer outputs. The safety lockout circuit protects against any of the following failures: 1. Loss of air or gas pressure to the burner.

2. Open or short circuit failure of either of the thermistors or their drive circuits.

3. Open or short circuit failure of the comparator circuit.

4. Failure in the motor drive circuit or motor.

5. Loss of the + 15v or -15v from the dc power supply.

6. A failure in the low firing circuit which causes the output from integrated circuit IC9 to saturate in the positive of negative sense.

A feature of this electronic ratio controller is that electrical measurements indicative of the air and gas flows can easily be extracted from the controller and displayed on meters. Such meters are a considerable aid in commissioning and monitoring the performance of plant.

However, variations in ambient temperature will cause significant changes in the output from the anemometer circuits, and although such changes do not affect the accuracy of the air/gas ratio control, they are inconvenient when it is desired to maintain a continuous display of the air and gas flows. In such cases it is desirable to correct the anemometer outputs for the effects of ambient temperature.

Figure 4 shows a circuit designed to correct both the zero and span of the anemometer outputs for the variations in ambient temperature, in which case the terminals A5, A6 and A8 from the controller 15 (Figure 3) are respectively connected to terminals B5, B6 and B8, the normally connected terminals A7, A8 then being disconnected. The temperature is sensed by thermistor TH3, which is located near the flow sensing thermistors TH1, TH2. Corrections for changes in zero of the anemometers are obtained by subtraction of the appropriate voltage from Va and Vg at the difference amplifiers IC12, IC14 respectively.To correct for changes in span, the zero corrected voltages are then multiplied by the appropriate factor, which is set by means of voltage controlled amplifiers TR7 with IC12 and TR8 with IC14.

The temperature corrected air and gas flow rates are displayed on meters M2 and M3 respectively.

After commissioning, the meter outputs are approximately independent of ambient temperature for all values between the maximum and minimum temperatures used in the calibration procedure. For intermediate ambient temperatures where the temperature compensation Is not exact, accurate readings can be taken by zeroing the meters immediately prior to taking readings. Adjustment of the meter zeros will not affect the air/gas ratio.

An additional advantage that can be obtained when temperature correction is applied to the anemometer outputs, is that the voltage outputs can then be used to check the air and gas flows prior to ignition. Thus the air anemometer can be used to check the high fire pre-purge and the return to low fire before ignition. This eliminates the need for high and low limit switches on the air damper. Also, since the main ratio control circuit includes a lockout facility which will trip the burner in the event of loss of air or gas, there may be no need for air or gas low pressure switches.

The circuit shown in Figure 4 includes four simple comparator circuits (IC16, IC17, IC18, IC19) which are used to check both the air and gas outputs prior to ignition.

Integrated circuits IC17 and IC19 control relay RL3, which is energised when the air flow is normally at maximum, and the gas flow is nominally at zero, i.e. RL3 is energised if Va > 4.5v (adjustable on VR15 up to 5.6v) Vg < 0.15v (adjustable on VR16 up to 0.3v) where Va and Vg are the temperature corrected air and gas anemometer outputs respectively.

Integrated circuits IC16 and IC18 control relay RM, which is energised if the air flow is at the preset low fire position, and the gas anemometer output is greater than -3v. (The latter merely checks that there is no short circuit in the gas anemometer). That is, RL4 is energised if Va < 0.5v (adjustable on VR14 up to 3v) Vg > -3.2v The relays RL3 and RL4 are wired into the burner programme controller in place of the air damper high and low position indicator switches. The check circuit then ensures that 1. The air flow is at a maximum for the duration of the purge.

2. The air flow is reduced to the low fire setting prior to the ignition attempt.

3. The gas flow is nominally zero throughout the air purge (i.e. < 2 % maximum flow at the most sensitive setting < 3 % maximum flow at the least sensitive adjustment).

4. There is no open or closed circuit failure in the thermistor anemometers, their control circuits, temperature compensating circuits or valve drive circuits.

Also the low fire setting can be adjusted at the ratio controller, which facilitates the commissioning and gives more consistent setting than can normally be obtained with a mechanical stop on the air damper.

The flow sensors used with the electronic ratio controller are based on ITT type P15 thermistors. As supplied the thermistor bead is mounted by its connecting wires between two splayed CUNIFE wires which protrude from a lead glass probe. Connection to the thermistors is made via the CUNIFE wires.

The two flow sensing thermistors, and the ambient temperature compensating thermistor are integrally mounted in a plastics block, 101. The thermistor wires are taken to a terminal block from which the connections are made to the controller.

The design of the thermistor mounting block is shown in Figures 5a to Sc. The thermistors are into light alloy holders 103. The holders are held in the plastics block 101 by means of standard compression fittings 104. The pneumatic connections for the air and gas bypass lines are also made via i inch stud couplings. The block has been designed to minimise air turbulence at the thermistors and ensure high sensitivity combined with a low pressure drop across the block. The air flow through the block for a 7.5v output from the anemometer is approximately 3 cubic feet per hour and this gives a pressure drop across the block of 0.05 inches w.g.

WHAT WE CLAIM IS: 1. Apparatus for the flow control of the air and gaseous fuel ratio to a burner comprising a first and a second supply line respectively for said air and said fuel gas, each of said supply lines having restrictor means to impede the flow of the air or fuel gas therein, first and second bypass lines respectively mounted about said first and second restrictor means, first and second electrical flow sensing means respectively mounted in said bypass lines, said flow sensing means having an electrical transducer having an output parameter dependent on the rate of flow of fuel gas or air in the respective bypass line, comparator means for comparing the output of said transducers, and a valve controlled by said comparator to adjust the flow of fuel gas or air in one of said first and second supply lines.

2. Apparatus as claimed in Claim 1, further including first means for adjusting the ratio of said fuel gas and air flow rates and independent means for adjusting said ratio at a different absolute flow rate.

3. Apparatus as claimed in either Claim 1 or Claim 2, including first and second display means connected to said flow sensing means to provide a visual indication of the absolute flow rate of said air and fuel gas.

4. Apparatus as claimed in Claim 3, including a transducer to sense the ambient temperature and connected to said display means to correct said visual indication for variations in ambient temperature.

5. Apparatus as claimed in any one of the preceding Claims having safety cutout means to shut down said burner in the event of failure of the fuel gas or air supply.

6. Apparatus for the control of flow of fuel gas and air ratio to a burner substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. Integrated circuits IC17 and IC19 control relay RL3, which is energised when the air flow is normally at maximum, and the gas flow is nominally at zero, i.e. RL3 is energised if Va > 4.5v (adjustable on VR15 up to 5.6v) Vg < 0.15v (adjustable on VR16 up to 0.3v) where Va and Vg are the temperature corrected air and gas anemometer outputs respectively. Integrated circuits IC16 and IC18 control relay RM, which is energised if the air flow is at the preset low fire position, and the gas anemometer output is greater than -3v. (The latter merely checks that there is no short circuit in the gas anemometer). That is, RL4 is energised if Va < 0.5v (adjustable on VR14 up to 3v) Vg > -3.2v The relays RL3 and RL4 are wired into the burner programme controller in place of the air damper high and low position indicator switches. The check circuit then ensures that 1. The air flow is at a maximum for the duration of the purge. 2. The air flow is reduced to the low fire setting prior to the ignition attempt. 3. The gas flow is nominally zero throughout the air purge (i.e. < 2 % maximum flow at the most sensitive setting < 3 % maximum flow at the least sensitive adjustment). 4. There is no open or closed circuit failure in the thermistor anemometers, their control circuits, temperature compensating circuits or valve drive circuits. Also the low fire setting can be adjusted at the ratio controller, which facilitates the commissioning and gives more consistent setting than can normally be obtained with a mechanical stop on the air damper. The flow sensors used with the electronic ratio controller are based on ITT type P15 thermistors. As supplied the thermistor bead is mounted by its connecting wires between two splayed CUNIFE wires which protrude from a lead glass probe. Connection to the thermistors is made via the CUNIFE wires. The two flow sensing thermistors, and the ambient temperature compensating thermistor are integrally mounted in a plastics block, 101. The thermistor wires are taken to a terminal block from which the connections are made to the controller. The design of the thermistor mounting block is shown in Figures 5a to Sc. The thermistors are into light alloy holders 103. The holders are held in the plastics block 101 by means of standard compression fittings 104. The pneumatic connections for the air and gas bypass lines are also made via i inch stud couplings. The block has been designed to minimise air turbulence at the thermistors and ensure high sensitivity combined with a low pressure drop across the block. The air flow through the block for a 7.5v output from the anemometer is approximately 3 cubic feet per hour and this gives a pressure drop across the block of 0.05 inches w.g. WHAT WE CLAIM IS:

1. Apparatus for the flow control of the air and gaseous fuel ratio to a burner comprising a first and a second supply line respectively for said air and said fuel gas, each of said supply lines having restrictor means to impede the flow of the air or fuel gas therein, first and second bypass lines respectively mounted about said first and second restrictor means, first and second electrical flow sensing means respectively mounted in said bypass lines, said flow sensing means having an electrical transducer having an output parameter dependent on the rate of flow of fuel gas or air in the respective bypass line, comparator means for comparing the output of said transducers, and a valve controlled by said comparator to adjust the flow of fuel gas or air in one of said first and second supply lines.

2. Apparatus as claimed in Claim 1, further including first means for adjusting the ratio of said fuel gas and air flow rates and independent means for adjusting said ratio at a different absolute flow rate.

3. Apparatus as claimed in either Claim 1 or Claim 2, including first and second display means connected to said flow sensing means to provide a visual indication of the absolute flow rate of said air and fuel gas.

4. Apparatus as claimed in Claim 3, including a transducer to sense the ambient temperature and connected to said display means to correct said visual indication for variations in ambient temperature.

5. Apparatus as claimed in any one of the preceding Claims having safety cutout means to shut down said burner in the event of failure of the fuel gas or air supply.

6. Apparatus for the control of flow of fuel gas and air ratio to a burner substantially as herein described with reference to the accompanying drawings.