This invention relates to a method of, and apparatus for controlling heating of a furnace by a regenerative arrangement.

Regenerative arrangements are known for pre-heating combustion air for a burner, in which exhaust (or flue gases) and air are passed alternately through a heat retentive matrix.

One known form of regenerative furnace heating arrangement comprises two burners, each with its own regenerative arrangement, and a common changeover valve operable to connect each burner, through its own regenerative arrangement to a respective one of a forced supply of fresh air, and means for drawing exhaust gases from the interior of the furnace or a radiant tube, through the respective burner and regenerative arrangement. Hence one of the burners is fired using fresh air fed to it via its regenerative arrangement so that it is heated, that burner directing a stream of products of combustion into the

interior of the furnace or radiant tube, while the other burner serves as an exhaust flue for the furnace or radiant tube, products of combustion within the interior of the furnace or radiant tube being drawn through the other burner and the respective regenera¬ tive arrangement so that the latter is heated.

Previous processes for controlling the heating of a furnace interior using a regenerative heating arrange- ment, either where the regenerative burners are adapted to fire directly into the interior of the furnace or where they are arranged to fire into either end of a radiant tube, have incorporated means responsive to the forced supply of combustion air and the fuel supply means, as well as being responsive to the temperature of the interior of the furnace, such that the heating of the interior of the furnace is controlled by variation of the fuel supply pressure. Such arrangements have been complicated. An object of this invention is to simplify the control of heating of a furnace by a regenerative heating arrangement.

According to one aspect of this invention, there is provided a method of controlling heating of a furnace by a regenerative arrangement which comprises a

plurality of burners, each of the burners having one mode in which it is operated to direct hot products of combustion into the furnace and another mode in which it serves as a flue through which hot products are vented from the furnace interior, combustion within each burner being supported by a supply of fresh combustion air which is pre-heated by passage through a heat retentive matrix of the respective burner and the heat retentive matrix of each burner being heated by passage through it of the hot products which are vented from the furnace interior through the respective burner when it is serving as a flue and at least one selected one of the other burners is being operated to direct hot products of combustion into the furnace interior, the method including controlling the plurality of burners such that the state of the burners changes periodically from one condition in which at least one selected one of the burners is being operated to direct hot products of combustion into the furnace interior and at least one other selected one of the remaining burners is serving as a flue, and a second condition in which said at least one selected one of the burners is serving as a flue and at least one selected one of the remaining burners is being operated to direct hot products of combustion

into the furnace, wherein such operation of the burners whereby they collectively operate so that hot combustion products are fed continuously to the furnace interior and so that hot products are vented continuously from the furnace interior, can comprise one phase of a two phase furnace heating cycle, the other phase being a phase within which no heat is supplied to the furnace interior by any of the burners, the frequency and duration of either'phase being controlled whereby to control heating of the furnace.

According to another aspect of this invention there is provided apparatus for carrying out the method of said one aspect of this invention.

Another object of this invention is to provide a form of regenerative furnace heating means which is more flexible in its application for regenerative furnace heating than is the known form of regenerative furnace heating arrangement referred to above.

According to a further aspect of this invention there is provided regenerative furnace heating means, comprising:

1} a burner having its own heat retentive regenerative arrangement, combustion chamber and conduit by which the combustion chamber is placed in direct communication with the interior of the furnace; ii) combustion air supply means; iii) flow control means having; a) a first mode in which fresh combustion air is caused or allowed to be fed from said combustion air supply means through the regenerative arrangement of the burner to the combustion chamber of the burner so that fresh combustion air that enters that combustion chamber is pre-heated, and b) a second mode in which it causes or allows a stream of hot products to be drawn from the interior of the furnace through the conduit, combustion chamber and regenerative arrangement of the burner whereby to heat the latter; and iv) actuating means operable: a) to effect alternation of the flow control means between its first and second modes during operation of the burner so that fresh combustion air and the stream of hot products are passed alternately through the heat retentive regenerative arrangement of the burner, and b) to effect operation of the burner so that

it is fired substantially in sychronism with the supply of pre-heated combustion air to its combustion chamber; characterised in that the burner has its own, independent combustion air supply means and its own flow control means and actuating means.

The heat retentive regenerative arrangement of the burner provides low resistance to exhaust flow so that there is a low pressure drop across it in operation and conveniently comprises a loosely packed bed of heat storage packing material which may be in the form of pebbles, balls or like particles of refractory material accommodated within an outer casing. It is also desirable that the eductor arrangement associated with the burner be designed for high efficiency operation whereby to maximise the suction effect being created.

Conveniently the burner may be arranged so that substantially all combustion occurs within its combustion chamber and hot products of combustion are directed as a stream through its conduit into the interior of the furnace to heat the furnace.

Preferably the burner has fuel supply means operable to feed fuel to the combustion chamber of the burner at a certain predetermined flow rate for the duration of each firing phase only, the fuel supply being terminated at the end of each firing phase whereby the burner alternates between firing and non-firing phases, the firing taking place in a series of pulses between non-firing, and the heat output of the burner is governed by the frequency of the firing pulses and the duration of each pulse.

The flow control means conveniently has a third mode in which the supply of fuel at said predetermined flow rate to the burner is inhibited, the supply of fresh combustion air to the combustion chamber at a flow rate sufficient for combustion of fuel supplied at said predetermined flow rate is inhibited, and passage of hot products from the interior of the furnace through the conduit, combustion chamber and regenerative arrangement to heat the latter is inhibited.

A trickle of air is conveniently supplied by said forced supply through the regenerative arrangement to the combustion chamber when the flow control means is

in said third mode, whereby to assist in transporting combustion products resulting from operation of the pilot burner into the furnace.

The burner may be one of a plurality of similar burners each having its own, independent combustion air supply means and its own flow control means and actuating means, and further including common control means responsive to certain operating parameters of the furnace and operatively associated with the actuating means of the burners so that, when the flow control means of at least one selected one of the burners is in its first mode, the flow control means of at least one of the other burners is in its second mode.

One form of regenerator in which this invention is embodied and a control system operable to carry out a method in which this invention is embodied are described now by way of example with reference to the accompanying drawings in which:-

Figure 1 is a longitudinal section through a regenerator incorporating a pulse-fired type burner.

Figure 2 is a section along lines II - II of Figure 1 showing, in particular, details of the pilot burner/ fuel injector and associated components, and

Figure 3 is a schematic illustration of a electrical control system for a regenerative heating system incorporating two regenerators of the type described in Figure 1.

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Referring to the drawings and in particular to Figure 1, the regenerator 1 incorporates at one end a pulse- fired burner 2 and at the other end a butterfly-type valve 3 for closing off the other end of the regenerator 1 together with a gas permeable heat storage and release means 4 disposed between the ends of the regenerator 1 and an air injection means 5 located between the heat storage and release means 4 and the valve 3 for injecting main combustion air into the regenerator 1.

The pulse-fired type burner 2 comprises two adjoining refractory lined ducts 5 and 7, the outer casing of the ducts 6 and 7 being of steel. The duct 7 forms a cylindrical combustion chamber θ for receiving fuel from a device 9 serving both as a pilot burner and a

fuel injector respectively during non-firing and firing phases.

The duct 7 has an inlet 10 for receiving main combustion air for combusting fuel supplied by the device 9 during a firing phase. The duct 6 forms an outlet 11 for discharge of the combustion products to a furnace (not shown), the duct β, in use, being located within an aperture in the wall of the furnace.

Since the burner described is forming part of a regenerator, it can also serve as a flue for the discharge of waste gas from the furnace during a fluing cycle as conventional. In this case the duct 6 provides an inlet from the furnace for the waste gas and the duct 7 provides an outlet for discharging the waste gas to the remainder of the regenerator 1. The waste gas serves to reheat or preheat respectively the previously cooled or unheated heat storage and release means 4 which can then release its stored heat to pre¬ heat main combustion air for supporting combustion of the main fuel during a subsequent firing phase.

The duct 7 adjoins a further duct 12 and the inlet 10 has a bore tapering inwardly towards the combustion chamber 8 to terminate in a diameter less than that of the combustion chamber 8 for increasing the velocity at which the main combustion air is supplied to the combustion chamber 8. Similarly the outlet 11 has a bore tapering away from the combustion chamber 8 to accelerate the rate at which combustion products are _ supplied to the furnace.

The combustion chamber 8 is formed by a cylindrical bore part of the duct 7 adjoining the inlet 10 in the duct 7 and the outlet 11 in the duct 6, the diameter of the outlet 11 being similar to that of the combustion chamber 8 where it adjoins the chamber 8.

During a firing cycle the butterfly valve 3 at the other end of the regenerator is closed causing air from the air injection means 5 to flow towards the burner 2 by way of the heat storage and release means 4 which preheats the air for combustion with fuel in the combustion chamber 8.

During a fluing cycle the butterfly valve 3 can be opened (to assume the position shown in Figure 1} to permit the waste gas and the main combustion air to escape through the other end of the regenerator 1 so that none reaches the burner 2.

The remainder of the regenerator 1 will be described in more detail subsequently.

Referring now to Figure 2, the device 9 comprises a pipe 20 having a side branch 21 which is internally threaded at its end to receive an externally threaded end of an inlet pipe 22 for receiving fuel premixed with a quantity of pilot air. The main portion 23 of the pipe 20 is internally threaded at one end to receive an externally threaded end of an outlet pipe 24 and at its other end to receive the threaded mounting 25 of an ignition electrode 26.

The outlet pipe 24 extends into a bore 27 in the wall of the duct 6 permitting the outlet 24 to communicate with the combustion chamber 8. The outlet 24 is held within a sleeve 28 welded to the outer steel casing 29 of the duct 6.

A further bore 30 extending through the wall of the duct 6 terminates in the bore 27 at right angles thereto at a position lying between the outlet 24 and the wall of the combustion chamber 8 to provide a port for visual flame inspection, as is conventional, by suitable positioning of the bore 30.

Located within the bore 30 is a viewing tube 31 of known type to permit any flame present at the outlet end of the device 9 or in the combustion chamber 8 to be detected both visually and in the well known manner by means of an ultra-violet sensor 32 suitable aligned with that end of the tube 31 located outside the chamber 8.

Shown schematically in Figure 2 is a line 40 for supplying a premix of fuel and pilot level air to the inlet 22 of the device 9.

Fuel, in this case natural gas, is supplied to this line 40 from a fuel gas supply 41 by way of a main fuel gas line 42 and the outlet 43 of a mixing chamber 44 in which fuel supplied at a pilot rate and air supplied at a pilot rate are pre ixed.

Fuel gas is also supplied from the supply 41 by way of a pilot fuel gas line 45 to one inlet 46 of the chamber 44.

Pilot rate air is supplied from an air supply 47 by way of a pilot air line 48 to a further inlet 49 of the chamber 44.

The main fuel gas line 42 comprises an open-close type solenoid valve 50 and a regulator valve 51 which can be adjusted to set the main fuel gas pressure for providing a main flame. Valve 50 is open during firing phases and is closed during non-firing eg. fluing phases.

The pilot fuel gas line 45 comprises an open-close type solenoid valve 52, a gas pressure governor 53 and a regulator valve 54 which can be adjusted to set the pilot rate gas pressure for providing a pilot flame. Valve 52 is open during both firing and non-firing phases. Both valves 50 and 52 close of course in the event that during operation no flame is sensed by the sensor 32.

SUBSTITUTE SHEET

The pilot air line 48 comprises only a regulator valve 55 to adjust the pilot air pressure. This is held at a constant pressure to hold the air supply rate constant. It will be seen therefore that pilot air is supplied continuously during operations and even when the burner 2 has been shut down following flame failure.

The chamber 44 is of the venturi-type to promote turbulent mixing of the pilot fuel with the air. The pilot fuel/pilot air mixture after leaving the chamber outlet 43 is mixed with the main fuel gas supplied by the line 42 (during firing) at- a junction 56 in the line 40.

As we have previously mentioned the volume of air supplied by the line 48 should be sufficient to support combustion of the pilot fuel gas to permit the establishment of a pilot flame at the outlet of the pilot burner/fuel injector device 9. However, the volume of air must be insufficient to support combustion of the main fuel gas supplied during firing by the line 42. Combustion of fuel gas supplied at this rate can only proceed in the presence of main

combustion air present in the burner combustion chamber 8.

We have found that in order to suppress inherent combustion of fuel, particularly natural gas, the fuel/air premix supplied by the line 40 during firing should contain no more than 5% by volume of air based on the total volume of fuel and air. The lower limit is about 2% . Obviously for the production of k pilot flame there should be sufficient air to satisfy stoiche etric combustion requirements of the fuel supplied at the pilot rate.

The ratio of the rates at which the fuel is supplied during the firing phase (when fuel from both lines 42 and 46 is present) and the non-firing phase (when only fuel from line 46 is present) may lie between 30:1 and 50:1 and preferably is 33:1 with the rate of supply of premix pilot air being of course correspondingly chosen to fulfil the requirements described for the establishment of a pilot flame.

Referring to Figure 2, located within the outlet pipe 24 of. the device 9 is a member 57 for enabling the

pilot flame produced by the device 9 during non-firing to be retained close to the end of the outlet pipe 24.

The pilot flame retention member 57 comprises a stub pipe 58 located within and spaced with clearance from the inner wall of the outlet pipe 24. The rearward end of the stub pipe 58 is circumvented by an annular flange 59 which engages with the inner wall of the oulet pipe 24 and is provided with a number of ^1" σirσumferentially spaced apertures 60. The fuel gas and air mixture therefore reaches the outlet by way both of the stub pipe 58 and the flange 59.

Ignition of the fuel gas/air mixture is provided by the electrode 26 which extends through the pipes 23 and 24 and has a tip terminating within the stub pipe 58, the tip providing a spark discharge to the stub pipe 24 which forms an earth return for the discharge.

In use of the device 9, the flame established at the outlet of the pipe 24 is modulated between a pilot level and a main level by the periodic supply of main fuel by line 42 during firing phases. The main flame will therefore during firing extend into the burner combustion chamber 8 but only if main combustion air

is simultaneously being supplied to the combustion chamber 8. The flame therefore will alternately expand and contract between its main flame and pilot flame levels.

The need to extinguish the pilot flame during non- firing phases is therefore obviated together with the need for sophisticated pilot interrupt controls to provide out of phase operation of a pilot burner with an adjacent fuel injector since the role of the pilot burner and fuel injector are in effect provided by one device.

As will subsequently be described in detail with reference to Figure 3, the operation of the main gas solenoid valve 50 and pilot gas solenoid valve 52 is controlled by the ultraviolet flame sensor 32. When the sensor 32 detects no flame either at the outlet 24 of the device 9 (the pilot flame) during a non-firing phase or in the combustion chamber 8 (the main flame) during a firing phase, both valves 50 and 52 are closed to prevent any gas entering the burner 2. In other words the burner shuts down.

Referring to Figure 1 again, the other end of the regenerator comprises a duct 70 of steel having an opening 71 serving as an outlet for the discharge of waste gas (and air) from the regenerator 1.

The duct 70 has a vertical portion 72 including the outlet 71 and a horizontal portion 73 connected to the remainder of the regenerator.

The vertical portion 72 includes an uppermost cylindrical part 74 including the opening 71 and the butterfly valve 3, a lowermost cylindrical part 75 connected to the horizontal portion 73, a central cylindrical part 76 of narrower cross section than the parts 74 and 75, a lowermost frusto-coniσal part 77 connecting the lowermost cylindrical part 75 to the central part 76 and an uppermost frustconical part 78 connecting the uppermost cylindrical part 74 to the central part 76.

The butterfly valve 3 has wings 79 and 80 and is mounted for rotation within the cylindrical part 74. When the valve 3 is in the closed position, the wings 79 and 80 engage respectively with a pair of semi- circular seals 81 and 82 which are secured to the wall

of the part 74, the uppermost seal 82 being slightly vertically offset from the lowermost seal 81. In the closed position, gases are prevented from discharging through the opening 71.

When the valve 3 is rotated to the open position shown in Figure 1, the valve 3 provides a substantially uninterrupted path for gases flowing through the duct 70 towards and out of the opening 71. The valve 3 may be rotated between the open and closed positions by means of an electrical solenoid actuator (not shown) although other suitable means for actuating the valve 3 may be used.

The air injection means 5 comprises a pipe 63 having a vertically directed portion 84 extending into the vertical duct portion 72 through a horizontal wall of the horizontal portion 73, the pipe 83 terminating in a nozzle or jet 85 which has a bore whose diameter is restricted in comparison to the bore of the pipe 84. The tip 86 of the nozzle 85 (the outer surface of which tapers upwardly away from the body 87 of the nozzle 85 fitted over the end of the pipe 84) terminates in the duct frusto-conical portion 77. The constricted diameter of the bore of the nozzle 85

causes air supplied to the pipe 84 to be accelerated as it leaves the pipe 84. During a fluing phase, this air together with the venturi-type throat formed in the duct 70 by the duct parts 77, 76 and 78 creates a suction effect tending to suck any furnace waste gas entering the burner 2 towards the duct 70 to provide an accelerated removal of waste gas from the furnace and accelerated transportation of the waste gas to the outlet 71. Once the waste gas has entered the narrow throat part 76, the air discharged from the nozzle 85 tends to blow the waste gas out through the outlet 71 in which the valve 3 has assumed its open position.

The pipe 83 is supplied with air from a suitable supply 87A eg. a compressed air source. This source may also provide the air supply 47 for the pilot air regulator valve 55 shown in Figure 2. The air supply 87A is connected to the pipe 83 by means of a line 88 which incorporates an open-close solenoid valve 89. Connected across the solenoid valve 89 is a by-pass 90 incorporating a regulator valve 91.

When the solenoid valve 89 is open, air is supplied to the pipe 83 at a high rate suitable to provide sufficient main combustion air to support combustion

of the main fuel supplied by the device 9 during a firing phase. This high air flow rate is also maintained during a fluing cycle to assist in the expulsion of waste gas entering the burner 2 to reheat or preheat the heat storage and release means 4 as previously described.

When the solenoid valve 89 is closed, the air is supplied by the by-pass line 90 at a rate controlled by the setting of the regulator valve 91. This rate may be low in comparison to the main flow rate and represents a so-called "standby" rate when the regenerator is in a standby state at which state the burner 2 is held between a firing or a fluing phase but at any rate is neither firing nor fluing and is "standing by" to fire or flue.

In this mode of operation the burner device 9 provides a pilot flame and the valve 3 is closed. The low volume of air from the air nozzle 85 serves to transport the waste gas produced by the pilot flame to the furnace.

The heat storage and release means 4 comprises a loosely packed bed of heat storage packing material 100 which may, as is shown, be in the form of balls or like particles of refractory material. The nature of this packing allow gaseous fluid to pass through the heat storage bed 100 in either direction.

The bed 100 is contained within a refractory lined

__ steel duct 101, which is connected at its lowermost end to a manifold 102 and at its uppermost end to a connector duct 12.

The duct 101 has a lowermost bore portion 103 which is cylindrical and actually houses the bed 100. The bore portion 103 communicates with a conically tapering intermediate bore portion 104 which terminates upwardly in an uppermost bore portion 105 which is cylindrical and has a diameter which is reduced in size compared to the bore portion 103.

The outer steel casing of the duct 101 terminates at its lowermost end in an inturned flange 106 which is connected to the manifold 102 and upon which is seated a steel ring 107. The ring 107 is recessed into the inner face of the refractory lining 108 and supports a

grid 109 of metal eg. steel bars, the grid 109 itself supporting the bed 100 thereabove.

The manifold 102 comprises a plenum chamber 110 leading to a generally horizontal duct 111 which is connected to and communicates with the horizontal portion 73 of the duct 70. The chamber 110 has an opening 112 for communicating with the lowermost bore portion 103 of the duct 101.

The connector duct 12 connects the burner duct 7 to the heat storage duct 101 and comprises a refractory lined right angled steel duct 113 having a vertical portion 114 connected to the duct 101 and a horizontal portion 115 connected to the burner duct 7. The duct portion 114 has a bore portion 116 the diameter of which is similar to the adjacent bore portion 105 of the duct 101 while the duct portion 115 has a bore 117 the diameter of which is similar to the widest diameter of the bore portion 10 of the duct 7.

Referring to Figure 3, similar components to those shown in Figures 1 and 2 bear the same reference numerals, except that those components forming part of or controlling one regenerator have an "A" suffix

after the respective number while those components forming part of or controlling the other regenerator have a "B" suffix after the respective number.

In Figure 3, OR gates 130A and 130B control the pilot gas solenoid valves 52A and 52B, AND gates 131A and 131B control the main gas solenoid valves 50A and 50B while further OR gates 132A and 132B control the main air solenoid valves 89A and 89B.

An enabling signal from the OR gates 130 will open their respective pilot gas valves 55. An enabling signal from the AND gates will open respective main gas valves 50. Finally an enabling signal from the OR gates 132 will open their respective main air valves 89.

The pilot gas OR gates 130A and B are enabled by a signal from the respective ultra-violet sensor 32A and B OR from a respective start-up override 133A and 133B.

The main gas AND gates 131A and B are enabled when there is a signal from the respective ultraviolet

sensor 32A and B and from the respective line (A or B) from the firing interval timer 134.

The main air OR gates 132 are enabled when a signal is present either on line A or line B of the timer 134.

The start-up overrides 133A and B are conventional and both must be set to initiate operations. Apart from controlling an OR gate, each override 133 when ^' set causes a respective burner electrode 26A or B to produce sparks to ignite the pilot mixture emitted from the outlet of the device 9A or 9B. After a predetermined period each override 133 is reset automatically.

The firing interval timer 134 is also conventional and controls the firing interval of each burner. The firing interval for the regenerator IA is controlled by the length of time during which a signal is placed on line A of the timer 134 while the firing interval for the regenerator IB is controlled by the length of time a signal is placed on line B of the timer 134. The arrangement is such that when there is a signal on one of the timer lines there is no signal on the other line. In this event one regenerator will be firing

while the other will be fluing as conventional. Periodically the regenerator roles are reversed by reversing the signal from one line to the other. This reversal process takes place automatically according to a predetermined firing sequence. However, in addition, the timer 134 can be manually or automatically set to a state where there is no signal either on line A or line B. In this case each regenerator 1 will assume the standby mode previously described in which neither regenerator is firing or fluing but each device 9 is emitting a pilot flame.

Each regenerator valve 3 is controlled by a valve actuator 135A (for regenerator IA) and 135B (for regenerator IB). The valve actuators 135 can be a solenoid or similar device.

In the arrangement shown in Figure 3, the valve actuator 135A is controlled by line B of the timer 134 such that when there is a signal on line B the valve actuator 135A is actuated to cause valve 3A to open. Similarly the valve actuator 135B is controlled by line A of the timer 134 such that when there is a signal on line A the valve actuator 135B is actuated to cause valve 3B to open. Valves 3 close of course

when there is no signal on their respective timer control lines.

Air both for the pilot burner/fuel injector devices 9A and 9B and for the air injection means 5A and 5B is supplied by a central supply source 136.

Fuel gas for the devices 9A and 9B is supplied by a central fuel source 137.

To initiate operation of the system described, both overrides 132A and 132B must be set. This action causes both pilot gas valves 52A and 52B to open permitting pilot gas and pilot air to mix in the respective chambers 44A and 44B. Each chamber 44A and 44B is of course supplied continuously with pilot air by means of its own pilot air regulator valve 55A or 55B.

Setting of the overrides 132A and B causes a current to flow on lines 138A and 138B respectively to activate the electrodes 26 in the respective devices 9A and 9B to spark and ignite the pilot fuel/pilot air mixture entering the devices 9A and B from the respective chambers 44A and B. Hence, both devices 9A

and 9B produce pilot flames. The overrides 132A and B then automatically reset.

Suppose, that the regenerator IA is to fire and regenerator IB is to flue, then there will be a signal on line A of the timer 134 but no signal on line B of the timer 134.

Since ultraviolet sensor 32A is sensing a pilot flame from its device 9A and there is a signal on timer line A, the AND gate 131A will be enabled to open the main gas valve 50A to enable main gas to be mixed with the pilot gas/pilot air mixture downstream of the chamber 44A.

In addition since there is a signal on line A of the timer 134, both OR gates 132A and B will be enabled to open the respective main air valves 89A and B in each regenerator.

Finally, since there is a signal on line A of the timer 134 but no signal on line B of the timer 134, the valve actuator 135B will be actuated to open its respective valve 3B but the valve actuator 135A will not be actuated so that its valve 3A will be closed.

Consequently the main air in regenerator IA will be forced to flow towards the burner combustion chamber by way of the heat storage bed and the air will support combustion of the main gas/pilot air mixture leaving the outlet of the device 9A. The pilot flame will therefore expand to the level of a main flame in the co ubstion chamber. The combustion products so produced will enter the furnace to provide heating of the furnace enclosure.

Since valve 3B is open, the main air in regenerator IB will leave the regenerator IB by way of the opening 71B. Waste gas (produced by the burner of regenerator IA) will enter the regenerator IB and will be drawn through the heat storage bed by the suction effect produced by the injection of main air into the venturi formed by the duct parts 76, 77 and 78 in regenerator IB. The bed will therefore be preheated by the waste gas which will eventually be expelled through the opening 71B with the assistance of the main air.

When regenerator B is to fire, regenerator A must flue, and in this case there will be a signal on line B but no signal on line A.

In this case of course, main gas valve 50B is opened by an enabling signal from the AND gate 131B while main gas valve 50A closes due to the lack of an enabling signal on AND gate 131A.

Both main air valves 89A and B remain open however, since the OR gates 132A and B are now enabled by a signal from timer line B.

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However, valve 3B is now closed since its valve actuator 135B is no longer actuated by a signal on timer line A while valve 3A is now open since its valve actuator 135A is actuated by a signal on timer line B.

In this case therefore, the pilot flame in device 9B expands to a main flame level which provides combustion products for the furnace, while the flame in device 9A contracts to a pilot level.

The main air for combustion of the main fuel/pilot air mixture in device 9B is of course preheated by heat released from the previously heated heat storage bed in regenerator IB.

Waste gas entering regenerator IA preheats the heat storage bed before leaving the opening 71A in regenerator IA.

Should it be necessary to terminate firing temporarily during operation because for instance the furnace interior has temporarily at least reached a desired temperature, both regenerators IA and B can be placed in the standby mode previously described.

In this case there will be no signal on either of the timer lines A and B so that neither of the AND gates 131A or B is enabled and consequently both main gas valves 50A and B are closed.

If a pilot flame is detected by the sensors 32A and B both the pilot gas valves 52A and B will be open since the OR gates 130A and B will be enabled.

Main air valves 89A and B will both be closed since the OR gates 132A and B will not be enabled since there is no signal on timer line A or B.

U STITULTEhSiiSET

Similarly both valves 3A and 3B will be closed since the valve actuators 135A and 135B will not be activated.

Consequently both device 9A and B will be operating in the pilot mode and only a trickle of air will reach the regenerators IA and B from their respective air injectors 5 by way of the by-pass valves 91A and B. This trickle level air will serve merely to assist in transporting the combustion products resulting from the pilot flames into the furnace.

If during firing, the main flame in the respective regenerator is extinguished for any reason, the respective main gas valve will close automatically as the AND gate 131 will no longer be enabled. Similarly, the pilot gas valve 52 will also close since the OR gate 130 will no longer be enabled. If no flame is sensed during a fluing or standby phase, the pilot gas valve 52 will also close since the OR gate 130 will no longer be enabled.

In order to restart operations the relevant start up override 132 must be reset.

SUBSTI

A regenerative furnace heating system may incorporate more than two burners. It is convenient to arrange the burners in pairs although that is not necessarily essential. Where there are two or more pairs of burners, the number of burners on standby at any one instant, when a steady state operating condition of the furnace is established, may vary depending upon whether the system is operating at its highest possible heat output setting or whether it is i'n a 'turn-down* mode. In the former case it may be there will always be at least one burner firing, although the burners of any one pair may be on standby and the timing of firing of the burners may be staggered to follow a predetermined sequence which is not related to the pairing of the burners. On the other hand all the burners could be on standby for certain periods during a ^* turn-down* mode of operation.