GB2138610A

GB2138610A - Fuel burner control systems

Info

Publication number: GB2138610A
Application number: GB08310834A
Authority: GB
Inventors: Brendan Kemp; Paul Nichols
Original assignee: Autoflame Engineering Ltd
Current assignee: Autoflame Engineering Ltd
Priority date: 1983-04-21
Filing date: 1983-04-21
Publication date: 1984-10-24
Also published as: GB8310834D0; GB2138610B; EP0124330A1; CA1317356C

Description

1 GB 2 138 610 A 1

SPECIFICATION

Impr ovements in or Relating to Fuel Burner Control Systems The invention relates to a fuel burner control system capable of controlling the supply of air and fuel to- a burner, and to a combustion process control system including such a fuel burner control system.

The quantity of air and fuel supplied to a fuel burner should be controlled in such a manner that the fuel is burned completely without havi - ng a significant quantity.of excess air. The supply of too little air results in incomplete combustion and the waste of fuel whilst the supply of too much air results in the absorption of some heat by the excess air. For efficient combustion the ratio of the quantity of the fuel supplied to the burner to the quantity of air supplied to the burner should be constant at a value 10 which provides just enough oxygen for complete combustion to take place. However, because of the behaviour of the fuel and the air flowing through the respective control valves, the ratio of the extent of opening of the fuel valve to the extent of opening of the air valve to provide a constant fuel:air ratio is not constant over the heat supply range of the burner.

Known burner control systems employ mechanically linked air and fuel supply valves and suffer 15 from the disadvantage that they are capable of achieving efficient combustion of fuel over a small part only of the burner heat supply range.

It is an object of the present invention to provide a burner control system capable of improved performance compared to existing burner control systems.

The present invention provides a fuel burner control system which includes a memory holding 20 values for air valve and fuel valve settings, and the said burner control system is arranged to provide, in response to each of a plurality of input signal values representing values of a first variable, respective values for air valve and fuel valve settings.

The said variable may be a variable (for example, temperature) of the installation controlled by the burner control system. The variable may, for example, be the temperature or a function of the temperature of a heat exchange medium (for example, water) of a boiler in the installation.

The fuel burner control system may include (i) means capable of determining a setting value of one valve for each value of the said variable, (ii) means capable of finding the said valve setting value in the said memory, (iii) means capable of finding, in the said memory, the corresponding setting value for the other 30 valve, and, (iv) means capable of effecting readout of the said fuel valve and air valve setting values from the memory.

Preferably, the fuel burner control system includes (i) means capable of determining a setting value for the fuel valve for each value of the said 35 variable, memory.

(ii) means capable of finding the said fuel valve setting value in the said memory, (iii) means capable of finding, in the said memory, the corresponding setting for the air valve, and, (iv) means capable of effecting readout of the said fuel valve and air valve setting values from the In one form of the burner control system according to the invention, the means capable of determining the said fuel valve setting is arranged to select increasing values of fuel valve setting with increasing values of the said first variable over a limited range of values of the first variable, and beyond the said limited range, to select a fixed value of fuel valve setting, the said fixed value being the fuel 45 valve setting at the end of the said limited range.

Preferably, the said limited range of the said first variable lies between five and twenty percent of the possible range of the said variable.

In one form of the burner control system according to the invention, the means capable of determining a fuel valve setting value for each value of the said first variable is arranged to select a 50 non-zero value of fuel valve setting at the zero value of the said first variable.

The memory may be so organised that the address of each fuel valve setting value points to the address of the corresponding air valve setting value in order to facilitate the location of air valve setting data once the fuel valve setting has been determined.

In one form of the fuel burner control system, the first variable is the difference between second 55 and third variables.

A fuel burner control system in accordance with the invention may include:- (i) means capable of determining a first variable as the difference between the values of second and third variables, (ii) first and second input ports capable of receiving signals representative of values of the second 60 variable and the third variable, (iii) means capable of calculating a fuel valve setting value for each value of the first variable, (iv) a memory holding values for fuel valve and corresponding air valve settings at respective addresses having a fixed relationship whereby the address of a fuel valve setting value points to the address of the corresponding air valve setting value for efficient combustion, 2 GB 2 138 610 A 2 (v) means capable of finding the calculated fuel valve setting and the corresponding air valve setting in the memory, and, (vi) first and second output ports capable of providing the said calculated fuel valve setting and the corresponding air valve setting as output signals from the burner control system.

In a heating system employing the burner control system, the second and third variables ma be 5 the actual and desired operating temperatures of a medium arranged to be heated by a burner controllable by the burner control system.

Preferably, the fuel burner control system includes display means capable of displaying the second and third variables alternately on common display elements.

A fuel burner control system according to the invention may be incorporated in a combustion process control system capable of operating in a first mode to effect the entry of values for fuel valve settings and corresponding air valve settings of a burner in a memory at respective locations having a fixed relationship, and in a second mode to provide, in respect of each of a plurality of input signals representative of respective values of the said first variable, the appropriate stored values for air valve and fuel valve settings.

Preferably, the said first variable is the difference between second and third variables.

In a heating system employing the combustion process control system the second and third variables are advantageously the actual and desired operating temperatures of a medium arranged to be heated by a burner controllable by the burner control system.

Preferably, the combustion process control system includes display means capable of displaying 20 the second and third variables alternately on common display elements.

In one form of combustion process control system according to the invention, the memory includes data as to the number of valve settings it is intended to accommodate and the system is capable of operating in the second mode only when all the air valve and fuel valve setting values which it can accommodate have been entered in the memory.

A combustion process control system may be arranged -to provide a nonzero start value of the fuel valve setting for a zero value of the first variable, to provide a plurality of intermediate values of the fuel valve setting increasing linearly from the start value for increasing values of the first variable from zero up to a maximum value for fuel valve setting for a set limit value of the said variable, and to maintain the fuel valve setting at the said maximum value setting for values of the said first variable in 30 excess of the said limit value, and, advantageously, the set limit value of the said first variable is adjustable.

More specifically, the combustion process control system may be arranged to provide a non-zero start value of the fuel valve setting at a desired temperature, to provide a plurality of intermediate values of fuel valve setting increasing linearly from the start value for temperatures decreasing from the 35 desired temperature up to a maximum value for the fuel valve setting for a set deviation from the desired temperature, and to maintain the fuel valve setting at the said maximum value setting for temperatures differing from the desired value by more than the set deviation, and, the set deviation may be adjustable.

In one form of combustion process control system according to the invention, the memory holds 40 data corresponding to the open and shut positions of the fuel and air valves. A memory suitably programmed with data for open, shut, start, intermediate and maximum fuel and air valve settings performs a specific function as a data store in a burner control system according to the invention.

A burner control system in accordance with the present invention and a combustion process control system including the burner control system will now be described byway of example only and with reference to the accompanying drawings, in which:

Fig. 1 is a schematic representation of a combustion process control system arranged as the central unit of an electrical system capable of controlling a boiler, Fig. 2 is a block schematic representation of the combustion process control system of Fig. 1, Fig. 3 is an illustration of a control panel of the combustion process control system of Fig. 1, 50 Fig. 4 is a flow chart representation of the operation of the combustion process control system of Figs. 1 and 2, Fig. 5 is a graphical representation of the relationship between the fuel valve setting and, (i) the deviation of the actual temperature from the thermostat setting (the upper abscissa scale), and, Oi) the temperature relative to the thermostat setting TIC (the lower abscissa scale), for a burner control system according to the invention, and, Fig. 6 is a diagrammatic representation of the arrangement of fuel and air valve setting data in an addressable data store, for a burner control system according to the invention.

Referring to Fig. 1, an electrical system capable of controlling a boiler includes a combustion process control system 1, an air supply control valve 2, a fuel supply control valve 3, an air control valve motor 4, a fuel control valve motor 5, position indicating potentiometers 6 and 7, a thermostat 8, and a fuel selector switch 9. The combustion process control system 1 includes a plurality of input ports by means of which it receives information from its sensors and output ports by means of which it provides information to actuators and the like. The combustion process control system 1 includes input 65 3 GB 2 138 610 A 3 ports F 1, F2 one of which is energ.ised by means of the fuel selector switch 9 to signal the type of fuel in use, a temperature sensor input port T1/T2 for receiving information as to an actual temperature, a remote load control input port 10 for receiving information as to a desired temperature, a boiler thermostat input port S 10, an open/start switch position-sensing input port S1 3, switch position sensing ports S 14 and S1 5, a load control switch sensing port S7, an air valve position sensing input 5 portA, and a fuel valve position sensing port F. Also included are output portsA+ and A- for controlling the air control valve motor 4 and output ports F+ and F- for controlling the fuel valve control motor 5.

Referring to Fig. 2, the combustion process control system 1, of Fig. 1, inclo -des a microprocessor 100, a serial timer interrupt controller 101, an electrically erasable memory 102, a plurality of displays 10 103, input/output controllers 104 and 105, a fixed programme memory 106, a random access memory 107, and an analogue-to-digital converter 108. The microprocessor is a Type Z80 integrated circuit which, underthe direction of the fixed programme memory 106, reads the signals at the various input ports and executes the actions for providing control signals at the appropriate output ports in addition to providing information for the displays 103. The serial timer interrupt controller 101, which 15 is a Type MK 3801 integrated circuit, is a multifunction device providing a USART (Universal Synch ronous/Asynch ronous Receiver/Transmitter), four timers (two binary and two full function), and eight bidirectional input/output lines with individually programmable interrupts. The random access memory 107 acts as a short term store for the signals received from input ports and the signals to be presented to output ports. The random access memory 107 acts also as a scratchpad memory for the 20 microprocessor 100. The input/output 104 and 105 control the activation and deactivation of the ports as instructed by the microprocessor 100 and the serial timer interrupt controller 101. The signals from the temperature sensor port (T1-T2) and the valve motor position indicator ports (F, A) are subjected to analogue-to-digital conversion by the analogue-to-digital converter 108. The signals from the remote load sensing port 10 and other ports in its group (S7, S1 0, S1 3, F1, F2) are each subject to 25 modification by means of a level-translating circuit 109 which also provides electrical isolation by means of optical coupling. There are provided manual controls capable of effecting the operations listed below. The manual controls are identified on the front panel represented in Fig. 3. The manual controls are switches connected to a plurality of control input ports shown in Fig. 2. The operations referred to above are:- 1. Placing the combustion process control system in either the commissioning mode or the run mode, and, in the commissioning mode-2. Increasing or decreasing the fuel supply.

3. Increasing or decreasing the air supply.

4. Increasing or decreasing the desired temperature.

5. Signalling to the system the positions of the air and fuel valves relative to their respective open and closed positions.

Referring to Fig. 4, the operations carried out by the combustion process control system commence with switch-on and the selection of fuel (1). The system then checks whether or not it has a look-up memory with information for the fuel. selected (2) and, if not, places itself in the commissioning 40 mode permitting control by means of the manual controls shown in Fig. 3 and illuminating the CLOSE POSITION and ENTER MEMORY displays at the control panel (3). The manual controls for-the air and fuel valve motors are then used by the operator to close both valves (indications of the positions of the valves are given at the control panel) and the operator presses ENTER MEMORY on the front panel when he is satisfied that the valves are closed (4). The system then illuminates a SET STAT display, 45 indicating that the operator should enter a temperature setting at which the burner is to be extinguished in order to prevent a further rise in the temperature of the medium being heated e.g.

water in a boiler. The OPEN POSITION and ENTER MEMORY displays on the control panel are next illuminated (7) and the operator uses the manual controls to open both valves fully and presses ENTER MEMORY on the first panel when he is satisfied that both valves are open (8). The system next purges 50 waste gases from the combustion chamber (9) after which it illuminates the START POSITION display on the control panel (10, 11, 12). The manual controls are then used by the operator to open partially both valves to allow ignition and combustion of fuel and he then presses START POSITION 0 3) to initiate boiler operation. The system then illuminates the HIGH POSITION and ENTER MEMORY displays on the front panel (14). The manual controls are used by the operator to obtain, from the burner, a maximum heat output sutiable for the installation in which it is being used while ensuring efficient combustion at the maximum heat demand (15). This part of the operation is executed with the aid of combustion analysis equipment and requires an operator skilled in the use of such equipment.

When the operator is satisfied that efficient combustion is taking place at the high heat demand setting he presses ENTER MEMORY (15). The system then decides whether subsequent operation is to be for 60 the entry of intermediate or start data 0 7), and, for the entry of intermediate data, illuminates the INTER POSITION and START displays on the front panels (16). For the entry of intermediate data, the operator presses INTER (18), selects some fuel valve setting below the maximum value set previously, adjusts the air valve to provide efficient combustion at this new intermediate heat demand setting, and when he is satisfied that the combustion is efficient he presses ENTER MEMORY (19). The system 65 4 Gb 2 138 610 A.4 continues to illuminate the INTER and START displays (return to 16) until the required number of locations in the look-up memory are filled with values for intermediate fuel valve and air valve settings. On completion of the entries for intermediate settings the START and ENTER MEMORY displays are illuminated (20), the operator uses the manual controls to set a selected START position for the fuel valve, adjusts the air valve for efficient combustion and then presses the ENTER MEMORY display/switch to effect.entry of the settings into the memory (21). The system then illuminates the RUN display on the front panel to indicate that it is -ready for operation (22) which is effected by pressing RUN (23).

When the RUN control is operated at the end of the commissioning phase the combustion process control system deactivates all of the front panel controls with the exception of the COM (commission) and RUN controls and thereafter functions as a burner control system capable of providing its stored valve setting data in response to a remote load control input.

Following the operation of the RUN control as described above, the system waits for 20 second (24) and then responds to the remote control, checking periodically for a change in demand (25).

Further shown in Fig. 4, the combustion process control system may be reprogrammed by switching it off and on (return to 1), and then operating the COM control on the front panel which returns it to the commissioning cycle via check point (27) and decision (28).

Referring still to Fig. 4, should the flame be extinguished by external influences, the system switches off (29) and will restart when the pilot flame is reestablished (30 to 37).

The programmer/operator is required to set, by means of a presettable control forming part of the 20 apparatus, an -offset- temperature difference to be used by the apparatus in normal operation. The function of the---offset-temperature difference and the relationship between the START, INTERMEDIATE, and HIGH settings will now be explained with reference to Fig. 5.

In Fig. 5, the relationship between the fuel valve setting and the deviation of the actual temperature from the thermostatically set temperature is represented by a graph having two straight 25 portions, one (the first) portion rising at a constant rate to meet the other portion which has zero slope.

The first portion of the graph represents an increasing fuel valve setting, that is, the extent of opening of the fuel valve, from the START value to the HIGH value. The increase in the fuel valve setting from the START value to the HIGH value occurs over a change from 01C to 1 01C in the deviation of the actual temperature from the thermostatically set temperature. The fuel valve setting then remains constant at the HIGH value for temperature deviations in excess of 1 011C. It will be appreciated that the thermostatically set temperature TOC is represented by a OOC temperature deviation and T-1 01C is represented by a 100 C deviation, as s.hown in the alternative temperature scale of Fig. 5. The "offset" temperature difference referred to above is, in Fig. 5, the 1 OOC difference at which the change occurs in the slope of the graph. Values of fuel valve setting which lie on the rising part of the graph are the intermediate fuel valve setting values.

The equipment is capable of "constructing" the graph of Fig. 5 by calculation, since it is given the START value, the HIGH value, and the "offset" temperature difference.. As stated above the HIGH value represents the setting for the maximum heat output which may be used with the particular installation, e.g. a boiler, which incorporates the burner control system.

The fuel burner control system, according to the invention, in operation, monitors the actual temperature of a medium e.g. water in a boiler, which is being heated by the fuel burner and compares the said actual temperature with a thermostatically set temperature for the medium. The fuel burner control system is capable of calculating the deviation of the actual temperature from the thermostatically set temperature and also of performing the operations necessary to obtain a value for fuel valve setting for any temperature deviation value in accordance with the relationship represented by Fig. 5. Therefore the fuel burner control system selects the START value of fuel valve setting if the temperature deviation is zero and selects the HIGH value of fuel valve setting if the temperature deviation is 1 01C or more. For a temperature deviation between OOC and 1 01C, the fuel burner control system calculates the fuel valve setting (angular position in degrees) in accordance with the relationship:- HIGH fuel START fuel temp. deviation Fuel valve position= 1 valve setting-valve setting 1 X 1 Also shown in Fig. 5 are alternative forms of the relationship between fuel valve settings and temperature deviation having break points at XIC (less than 1 OOC) and YOC (more than 1 OIC), respectively. The fuel burner control system -shuts off the fuel supply if the temperature deviation 55 becomes negative.

Referring now to Fig. 6, data required by the fuel burner control system in its operation is stored ap fuel valve settings in a first addressable data store, represented diagrammatically on the left in Fig.

6, and as air valve settings in a second addressable data store, represented diagrammatically on the right in Fig. 6. Once the-fue - 1 burner control system has determined a fuel valve setting, as described above with reference to Fig. 5, it locates the said fuel valve setting in the fuel valve setting data store GB 2 138 610 A (or the fuel valve setting closest to the said valve setting), notes the address at which the relevant fuel valve setting was located, and selects the air valve setting data at a corresponding address in the air valve setting data store. Once the fuel burner control system has acquired both fuel valve and air valve setting da ' ta it proceeds to apply the fuel valve setting data to its fuel valve control output port and to apply the air valve setting data to its air valve control output port.

Referring to Fig. 6, the fuel valve setting data available in the first data store includes control data giving the following positions of the fuel valve:

CLOSED, at which the fuel valve is shut.

OPEN, at which the fuel valve is open fully.

HIGH, at which the fuel valve is open to a position which provides the maximum heat output which the installation, e.g. a boiler, can use.

INTERMEDIATE 1 to INTERMEDIATE N, a set of positions along the sloping part of Fig. 5 representing progressive opening of the fuel valve for a minimum heat output START position to the maximum heat output HIGH position. There may be 25 such INTERMEDIATE positions chosen along the sloping part of Fig. 5 (N=25).

START, at which the fuel valve is slightly open to provide enough heat to compensate for heat losses of the system in order to maintain the medium being heated at the thermostatically set temperature (Tin Fig. 1).

The data store also includes an indication of the value of N (the number of INTERMEDIATE positions of the fuel valve available in the data store), so that the system can check on whether or not it 20 holds a full set of INTERMEDIATE data.

Referring again to Fig. 1, the accuracy of control of the air and fuel valves 2 and 3 is of the order of a quarter degree and the valve positions are read as those of the motors 4 and 5 by the feedback provided by the potentiometers 6 and 7. The positions of the motors are checked eight times per second.

Referring again to Fig. 3, the front panel displays include "fuel selected" indicators, "commission" and "run" indicators, and a temperature indicator which displays the desired and actual temperatures alternately. The front panel also includes an 02 display and setting control for establishing an optimum level of oxygen in the exhaust gases during commissioning. The system may be arranged to maintain a boiler to provide the optimum oxygen level in the exhaust gases by fine control of the valves (over and 30 above the fixed control set on commissioning). The 02 display is arranged to display the actual and desired values alternately.

In the equipment described above the temperature (or more precisely the difference between the actual and desired temperatures) of the boiler water is used as a variable control quantity. It is also possible to use other variables; for example the steam pressure of the boiler, the temperature of the products of combustion of the boiler, the process or output temperature of the boiler, or a variable related to the heat load requirements of, for example, a building heated by the boiler.

Claims

1. A fuel burner control system which includes a memory holding values of air valve and fuel valve settings, the fuel burner control system being arranged to provide, in response to each of a 40 plurality of input signal values representing values of a first variable, respective values for air valve and fuel valve settings.

2. A fuel burner control system as claimed in claim 1, including (I) means capable of determining a setting value of one valve for each value of the said variable, (ii) means capable of finding the said valve setting value in the said memory, (iii) means capable of finding, in the said memory, the corresponding setting value for the other valve, and, (iv) means capable of effecting readout of the said fuel valve and air valve setting values from the memory.

3. A fuel burner control system as claimed in claim 1 or claim 2, including (I) means capable of determining a setting value for the fuel valve for each value of the said variable, (ii) means capable of finding the said fuel valve setting value in the said memory, (Iii) means capable of finding, in the said 50 memory, the corresponding setting for the air valve, and, (iv) means capable of effecting readout of the said fuel valve and air valve setting values from the memory.

4. A fuel burner control system as claimed in claim 3, wherein the means capable of determining the said fuel valve setting is arranged to select increasing values of fuel valve setting with increasing values of the said first variable over a limited range of values of the first variable, and beyond the said 55 limited range, to select a fixed value of fuel valve setting, the said fixed value being the fuel valve setting at the end of the said limited range.

5. A fuel burner control system as claimed in claim 4, wherein the said limited range of the said first variable lies between five and twenty percent of the possible range of the said variable.

6. A fuel burner control system a claimed in any one of claims 3 to 5, wherein the means capable of determining a fuel valve setting value for each value of the said first variable is arranged to select a non-zero value of fuel valve setting at the zero value of the said first variable.

6 GB 2 138 610 A 6

7. A fuel burner control system as claimed in any one of claims 3 to 6, wherein the memory is so organised that the address of each fuel valve setting value points to the address of the corresponding air valve setting value.

8. A fuel burner control system as claimed in any one of claims 1 to 7, in which the first variable is the difference between second and third variables.

9. A fuel burner control system including:

(i) means capable of determining a first variable as the difference between the values of second and third variables, (ii) first and second input ports capable of receiving signals representative of values of the second variable and the third variable, (ill) means capable of calculating a fuel valve setting value for each value of the first variable, (iv) a memory holding values for fuel valve and corresponding air valve settings at respective addresses having a fixed relationship whereby the address of a fuel valve setting value points to the address of the corresponding air valve setting value for efficient combustion, (v) means capable of finding the calculated fuel valve setting and the corresponding air valve setting in the memory, and, (vi) first and second output ports capable of providing the said calculated fuel valve setting and the corresponding air valve setting as output signals from the burner control system.

10. A burner control system as claimed in claim 8 or 9, in which the second and third variables are the actual and desired operating temperatures of a medium arranged to be heated by a burner 20 controllable by the burner control system.

11. A burner control system as claimed in any one of claims 8 to 10 and including display means capable of displaying the second and third variables alternately on common display elements.

12. A burner control system substantially as herein described with reference to, and as illustrated by, the accompanying drawings.

13. A combustion process control system capable of operating in a first mode to effect the entry of values for fuel valve settings and corresponding air valve settings of a burner in a memory at respective locations having a fixed relationship, and in a second mode to provide, in response to each of a plurality of input signal values representative of respective values of a first variable, the appropriate stored values for air valve and fuel valve settings.

14. A combustion process control system as claimed in claim 13, wherein the said first variable is the difference between second and third variables.

15. A combustion process contol system as claimed in claim 14, wherein the second and third variables are the actual and desired operating temperatures of a medium arranged to be heated by a burner controllable by the burner control system.

16. A combustion process control system as claimed in claim 14 or claim 15, and including display means capable of displaying the second and third variables alternately on common display elements.

17. A combustion process control system, as claimed in any one of claims 13 to 16, wherein the memory includes data as to the number of valve settings it is intended to accommodate and the 40 system is capable of operating in the second mode only when all the air valve and fuel valve setting values which it can accommodate have been entered in the memory.

18. A combustion process control system as claimed in any one of claims 13 to 17, wherein the system is arranged to provide a non-zero start value of the fuel valve setting for a zero value of the first variable, to provide a plurality of intermediate values of fuel valve setting increasing linearly from the start value for increasing values of the first variable from zero up to a maximum value for fuel valve setting for a set limit value of the said variable, and to maintain the fuel valve setting at the said maximum value setting for values of the said.first variable in excess of the said limit value.

19. A combustion process control system as claimed in claim 18, wherein the set limit value of the said first variable is adjustable.

20. A combustion process control system as claimed in any one of claims 13 to 17, wherein the system is arranged to provide a non-zero start value of the fuel valve setting at a desired temperature, to provide a plurality of intermediate values of fuel valve setting increasing linearly from the start value for temperatures decreasing from the desired temperature up to a maximum value for the fuel valve setting fora set deviation from the desired temperature, and to maintain the fuel valve setting at the 55 said maximum value setting for temperatures differing from the desired value by more than the set deviation.

2 1. A combustion process control system as claimed in claim 20, wherein the set deviation is adjustable.

22. A combustion process control system as claimed in any one of claims 1 to 2 1, wherein the 60 memory holds data corresponding to the open and shut positions of the fuel and air valves.

23. A memory having data for open, shut, start, intermediate, and maximum fuel and air valve settings for a burner control system as claimed in any one of the preceding claims.

24. A combustion process control system substantially as herein described with reference to, and as illustrated by, the accompanying drawings.

A 45. 9 7 GB 2 138 610 A 7

25. A boiler installation including a burner control system as claimed in any one of claims 1 to 12.

26. A boiler installation including a combustion process control system as claimed in any one of claims 13 to 21 or claim 24.

Printed in the United Kingdom for Her Majesty's Stationery Office, Demand No. 8818935, 10/1984. Contractor's Code No. 6378. Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.