GB2208423A

GB2208423A - Furnace burners with regenerative heat exchangers

Info

Publication number: GB2208423A
Application number: GB8718500A
Authority: GB
Inventors: Brian J Wills; Hans Jasper; Raymond Flore
Original assignee: Fives North America Combustion UK Ltd
Current assignee: Fives North America Combustion UK Ltd
Priority date: 1987-08-05
Filing date: 1987-08-05
Publication date: 1989-03-30
Also published as: GB8718500D0

Abstract

The air supply to a burner is heated by passing it through a heat store which has been heated by contact with combustion products and continuous operation of the burner is possible because the air and combustion products pass alternately through different parts of the heat store. In one embodiment, a rotary valve 15 at the centre of an annular heat store 14 directs continuous streams of combustion air and combustion products radially outwardly then inwardly through different sectors of the heat store in turn, the burner 12 being located at one end of the heat store. Alternatively, the air and combustion products flow longitudinally towards and back form an end of a cylindrical heat store. In a third embodiment, the air and combustion products flow alternately through a number of compartments containing heat storage elements, the flow being controlled by jets of fluid directed to deflect the flows. <IMAGE>

Description

Title: "Furnace and method of operating a burner" Description of Invention The present invention relates to the operation of a burner for burning a fuel in air and to a furnace incorporating the burner.

It is known to provide in a furnace a pair of burners and to use these alternately for discharging a burning mixture of fuel and air into a heating chamber of the furnace. The furnace is operated in a cylical manner. During a first holf of each cycle, fuel and air are fed to a first of the burners and hot products of combustion are exhausted from the chamber through the second burner and a heat store where heat is extracted from the products of combustion, before these are discharged to the atmosphere. During a second part of the cycle, air is heated in the heat store before being fed to the second burner and mixed in that burner with fuel, hot products of combustion being exhausted from the chamber through the first burner to a further heat store where heat is extracted from the products of combustion before these are discharged to the atmosphere.Heat stored in the heat store associated with the first burner is then imparted to air during the first half of the next cycle.

It is an object of the present invention to provide for recovery of heat from the products of combustion without firing alternately the burners of a pair of burners.

According to a first aspect of the invention, there is provided a method of operating a burner wherein there is connected with the burner a heat store, air is delivered to the burner along a feed path which extends through the heat store, fuel is delivered to the burner, is mixed by the burner with the air and burns with the air and wherein hot products of the combustion of the fuel and air are directed through the heat store to impart heat thereto, characterised in that the heat store provides a number of paths for the flow of gases, each of said paths contains air and the products of combustion alternately and in that both the flow of air through the heat store to the burner and the flow of products of combustion through the heat store are substantially continuous throughout a period of operation of the burner.

In a method in accordance with the invention, the period of operation of the burner may be several hours or several days. This contrasts with the known alternate firing of two burners, where each burner is fired for only one half of the cycle and each cycle has a duration of only a few minutes so that there are from ten to fifty cycles per hour.

In a method in accordance with the present invention, the flow of air and of products of combustion within the heat store is changed in a cyclic manner and each cycle may have a duration of less than one minute.

However, firing of the burner is continuous throughout a period of operation which comprises a number of cycles.

In the preferred method, the flow of air and the flow of products of combustion in the heat store are advanced gradually to successive parts of the heat store. At any instant, the airflow rate in the heat store may have a maximum value in a first part of the heat store and a lower value in adjacent parts of the heat store whilst the flow rate of the products of combustion in the heat store has a maximum value in a second part of the heat store and a lower value in parts of the heat store adjacent to the second part. The flowpoths in the heat store may merge with one another. Alternatively, the heat store may define a number of flow paths which are divided from one another.

In a method in accordance with the first aspect of the invention, the flow of products of combustion may be directed to said paths in succession by changes in the fluid pressure at predetermined positions andlor by supplementary fluid flows. This avoids impingement of the hot products of combustion on movable valve members which may deteriorate or become fouled by impingement of the products of combustion. The flow of combustion air along said paths also may be controlled by changes in the fluid pressure at predetermined positions and/or by supplementary fluid flows.

According to a second aspect of the invention, there is provided a furnace defining a chamber and having a burner for discharging a burning mixture of fuel and air into the combustion chamber, feed means for feeding fuel and air to the burner, exhaust means for exhausting products of combustion from the chamber and a heat store for storing heat from the products of combustion which have left the chamber and supplying that heat to air fed to the burner, characterised by flow-controi means for directing the air at respective different times along each of several flowpaths through the heat store and directing the products of combustion at each said time along a different one of the flowpaths.

By several flowpaths, we mean herein at least three flowpaths. The flowpaths may be divided one from another. Alternatively, mutually adjacent flowpaths may merge one into the other so that the boundary between one flowpath and an adjacent flowpath is indistinct. The number of flowpaths provided by the heat store may be indeterminate.

The heat store may be annular and preferably has several air inlets, there being in a central space defined by the heat store a rotary flow-control device having a first port which communicates with the air inlets in turn, the heat store also having several air outlets and the rotary device having a second port which communicates with the air outlets in turn. The rotary device may have a third port which communicates with an exhaust port leading from said chamber and also communicates with the air outlets in turn so that products of combustion are directed into the heat store through one or more of the air outlets communicating with the third port and the rotary device also having a fourth port which communicates with the air inlets in turn so that the products of combustion can leave the heat store through the air inlets as these communicate in turn with the fourth port.

There is also provided in accordance with the invention a unit suitable for use in transferring heat from a stream of hot products of combustion to a stream of cool combustion air, the unit comprising a heat store defining a number of gas flowpaths and flow-control means for directing said streams alternately through each path.

Examples of furnaces in accordance with the second aspect of the invention, incorporating units according to the third aspect and which are heated by operation of a burner in accordance with the first aspect of the invention will now be described, with reference to the accompanying drawings, wherein FIGURE 1 represents diagrammatically the exterior of a furnace; FIGURE 2 is a diagrammatic representation showing internal parts of a burner, some parts of a heat store and a flow-control means incorporated in the furnace of Figure I one part of the store being spaced from the flowcontrol means; FIGURE 3 shows an alternative heat store and flow-control means which may be incorporated in the furnace of Figure 1, the heat store and flow-control means being shown in cross-section on the line Ill-Ill of Figure 4;; FIGURE 4 is a plan view of the heat store of Figure 3; FIGURE 5 is an underneath plan view of the flow control means of Figure 3; and FIGURE 6 is a diagrammatic representation of a further heat store and flow-control means, certain parts being omitted for simplicity of illustration.

The furnace illustrated in Figure I comprises a wall structure 10 which defines a heating chamber and which may be of known construction. The heating chamber may be used for heating workpieces which are conveyed through the chamber in a known manner, for heating workpieces which are placed in the chamber for heating and are subsequently removed, or for heating a heat transfer fluid or other medium which is required to be heated.

The wall structure 10 defines an opening 11 in which there is mounted a burner 12 defining an axis 13 which extends through the wall structure from the heating chamber to the outside of the furnace. The burner may be symmetrical about the axis 13 and the opening Il may be circular, as viewed along the axis. The burner is sealed to the wall structure in a known manner.

There is mounted outside the wall structure 10 a heat store 14. In the example illustrated in Figure 2, the heat store is annular and is arranged coaxially with the burner. In the central space defined by the heat store, there is a rotatable flow-control valve 15 which communicates with the burner 12 and also communicates with a manifold 16 which lies at the end of the valve remote from the burner.

Feed means is provided for feeding a fluent fuel to the burner 12. The fuel-feed means comprises a pipe 17 which extends along the axis 13 through the manifold 16 and the valve IS to a nozzle 18 incorporated in the burner.

The fuel feed means further comprises a control device 8 which may be one or more valves, in a case where the fuel is supplied from a gas main or which may comprise a reservoir of fuel and a pump for pumping fuel from the reservoir to the burner.

There is further provided air feed means for feeding air to the burner through the manifold 16, valve 15 and the heat store 14. The air feed means comprises a fan 19 connected by an air pipe 20 with a central portion of the manifold 16 and a radially inner duct 21 defined by the valve 15. The duct 21 communicates with an air inlet of the burner 12 which leads to a mixing chamber 22 containing the nozzle 18.

The heat store 14 comprises a hollow body having an outer peripheral wall 24 and an inner peripheral wall 25. The body contains a mass of heat storage elements, for example elements formed of a metal or ceramic elerrfents. These may be arranged randomly or packed into the body according to a pattern. An annular baffle 26 extends from the inner wall 25 a part of the way towards the outer wall 24, the baffle being positioned midway between opposite ends of the heat store. The interior of the body may be further divided by a number of longitudinal partitions, each extending from the inner wall 25 to the outer wall 24 so that the interior of the body is divided into a number of segments, two of which are shown in Figure 2.

The inner duct 21 of the valve 15 is divided by a web 23 into an outer end portion remote from the burner and an inner end portion adjacent to the burner. A first port 27 is provided in a circumferential wall of the valve 15 and this port communicates with the outer end portion of the duct 21 via a transverse duct. In the inner wall 25 of the heat store, there is provided a row of air inlet ports 28 spaced from the burner by the same distance which the port 27 is spaced from the burner. Accordingly, the port 27 can be aligned with the ports 28 in turn by rotation of the valve IS about the axis 13.

The inner end portion of the duct 21 communicates via a further transverse duct with a second port 29 in the peripheral wall of the valve. A row of air outlet ports 30 is provided in the wall 25 and these ports can be aligned in turn with the port 29 by rotation of the valve.

At the inside of the peripheral wall of the valve 15, there is defined an outer duct 31 which surrounds the inner duct 21 and is divided therefrom by an intermediate wall 32. The outer duct 31 also is divided by an annular web 33 into an axially inner end portion adjacent to the burner 12 and an axially outer portion remote from the burner. In the peripheral wall of the valve, there is provided a third port 34 which communicates with the inner end portion of the duct 31 and which is aligned with the ports 30 in turn, as the valve is turned. A fourth port 35 communicates with the outer end portion of the duct 31 and with each of the ports 28, as the valve 15 turns. The outer end portion of the duct 31 also communicates with an outer chamber defined by the manifold 16 which leads to an exhaust duct 36.The exhaust duct may lead directly to a chimney stack or to an exhaust fan which provides an induced draft. The inner end portion of the duct 31 communicates with an annular exhaust passage 37 defined by the burner 12 and surrounding the mixing chamber 22. The ports 27 and 29 are diametrically opposite to the ports 34 and 35.

Known seals are provided at the joints between the valve 15 and the burner and at the joints between the valve 15 and the manifold 16. A further seal is provided between the valve and the heat store 14 at a position corresponding to that of the web 33. To inhibit flow of gases along the interface between the valve 15 and the heat store 14, longitudinal seals 38 may be provided at intervals around the axis 13. The longitudinal seals may turn with the valve 15.

During operation of the burner, air is impelled by the fan 19 along the air pipe 20, where it receives some heat from products of combustion leaving the manifold, to the inner part of the manifold and thence to the outer end portion of the inner duct 21 defined by the valve. The air then flows through the port 27 into the heat store 14 via one or more of the ports 28. The air travels radially outwardly through the heat storage elements from the port 28 and then around the radially outermost edge of the baffle 26 to flow to a corresponding one of the ports 29. The heat storage elements impart to the 0 air sufficient heat to raise the temperature to a value in the region of 800 C.

The heated air passes through the port 29 into the valve 15 once more and thence into the mixing chamber 22, where it is mixed with fuel discharged from the nozzle 18. The mixture of fuel and air burns as it travels from the mixing chamber into the heating chamber of the furnace. The products of combustion are exhausted from the heating chamber via the exhaust passage 37 to the inner end portion of the outer duct 31 of the valve.

From this outer duct, the products of combustion pass through the port 34 into a part of the heat store 14 which is diammetrically opposite to that through which air is flowing at the same time. The products of combustion travel radially outwardly through the mass of heat storage elements, around the radially outer extremity of the baffle and then to the port 35. As the products of combustion travel through the heat storage elements, there is imparted to those elements sufficient heat to raise the temperature of the elements to a value in the region of 8000C. The products of combustion flow from the port 35 through the outer end of the outer duct 31 into the manifold and then along the exhaust duct 36. This flow of gases continues without interruption through a period of operation of the burner which may be several hours. During operation of the burner, the valve 15 turns continuously about the axis 13. One revolution of the valve, that is one complete cycle of operation, occurs in less than one minute. Typically, the valve turns at a speed of 10 rpm. As the valve turns, the ports.27 and 29 traverse the internal surface of the heat store, communicating respectively with the ports 28 and the ports 30 in tum. At any instant, there may be one or two ports 28 in communication with the port 27 and one or two ports 30 in communication with the port 29. The flow of air is directed to segments of the heat store in turn until the airflow has been directed through each segment during a complete cycle. In use of the modified apparatus, in which the longitudinal partitions of the heat store are omitted, a similar airflow pattern is established.The air tends to take the shortest route through the heat store from the port 27 to the port 29. Accordingly, there is a concentration of the airflow in a sector of the heat store. However, there will be a minor flow of air in parts of the heat store adjacent to that sector. One quarter of a cycle later, there will be no substantial flow through that sector. Half a cycle after the airflow was concentrated in that sector, the flow of the products of combustion through the heat store will be concentrated in that sector and the elements of that sector will be heated up once more.

For simplicity of illustration, circular ports have been shown in the drawing. However, ports having other shapes may be employed. For example, some of the ports may be elongated in a direction around the axis 13.

For turning the valve 15, there may be provided an electric motor 39 (represented in Figure 1 only) connected with the valve by gearing or by a chain and sprocket drive.

The heat store and flow-control means illustrated in Figures 3, 4 and 5 may be substituted for the heat store and flow-control means illustrated in Figure 2. The apparatus of Figure 3 would then be associated with a furnace wall structure and burner corresponding to the wall structure 10 and burner 12 hereinbefore described. In Figure 3, certain parts corresponding to those hereinbefore described are identified by like reference numerals with the prefix I and the preceding description is deemed to apply to such corresponding parts, except for the differences hereinafter mentioned.

The heat store illustrated in Figures 3 and 4 comprises a hollow, cylindrical housing 140 having a circumferential wall and an end wall 141 which closes one end of the housing, a lower end as represented in Figure 3.

The interior of the housing is divided into a number of segments by a plurality of radial walls. In the example illustrated, there are four segments and four radial walls 142 to 145. Each radial wall extends from the end wall 141 to an opposite end of the housing. Each of the segments is divided into a radially inner part 146 and a radially outer part 147 by a respective partition 148 which extends from the radial wall at one margin of the segment to the radial wall at the opposite margin of the segment. Each of the partitions 148 is spaced by a gap 149 from the end wall 141 so that the inner and outer parts of the segment communicate with each other through this gap. Each partition extends to the end of the housing 140 remote from the end wall 141.

Each of the partitions 148 is so positioned between the centre of the housing and the circumferential wall of the housing that the area of the outer part 147 of the segment, as viewed in a direction along an axis 150 of the housing, is larger than the area of the corresponding inner part 146. Both parts of each segment are occupied by heat storage elements 151 which may be regularly shaped ceramic elements, each of which is small, as compared with the segment containing the element. The elements 151 may be arranged randomly in the segments or stacked in a predetermined manner.Between the elements 151 are interstices through which gases can flow so that each of the segments defined by the radial walls 142 to 145 constitutes a respective flowpath for the flow of gases from the end of the housing remote from the end wall 141 along one part of the segment to the gap 149 and back along the other part of the segment.

The flow-control means illustrated in Figures 3 and 5 comprises a generally disc-like body 152 having a diameter similar to the outside diameter of the housing 140 and arranged to close an end of the housing remote from the end wall 141, except for ports defined by the body 152. In the example illustrated, the body 152 defines four ports which face towards the interior of the housing 140. Each of these ports is arcuate, the centre of the curvature of the port lying on the axis 150. All of the ports lie in one or other of a pair of diammetricolly opposite quadrants. In one quadrant there is an outer port 153 which subtends at the axis 150 an angle within the range 800 to 450 and which is so spaced from the axis that it can communicate with the outer parts 147 of the segments defined by the housing 140 in turn.In the same segment, there is an inner port 154 spaced from the axis 150 by a distance such that it can communicate with the inner parts 147 of the segments in turn. The outer port also subtends at the axis 150 an angle within the range 800 to 450. In the diammetrically opposite segment, there are outer and inner ports 155 and 156 corresponding respectively to the ports 153 and 154.

The body 152 defines a duct 157 through which the port 155 communicates with the burner 112. The body also defines a duct 158 through which the inner port 156 communicates with air supply means 119,120. The port 154 communicates via a duct 159 defined by the body 152 with a chimney stack or an exhaust fan. The outer port 153 communicates via a duct 160 defined by the body 152 with an exhaust passage 137 from the furnace chamber.

The housing 140 is arranged for rotation about the axis 150 relative to the body 152. During operation, the body 152 remains stationary and the heat store is turned by drive means, for example an electric motor 139. Cool combustion air is directed into the heat store through the inner port 156.

When that port is aligned with a single segment defined by the housing 140, the combustion air passes downwardly through the inner part 146 of that segment, through the gap 149 and upwardly through the outer part 147 of that segment to the port 155. From that port, the air passes to the burner 112.

After start-up, the elements 151 are hot and the combustion air is heated whilst flowing through the segment. It will be noted that the air flows firstly through the inner part of the segment, which has a smaller area transverse to the direction of flow, and flows through the larger outer part of the segment when the air has been heated and therefore has expanded.

Products of combustion are exhausted from the heating chamber of the furnace and enter the housing 140 through the port 153. When this port is aligned with a single segment defined by the housing 140, the products of combustion pass round the outer part 147 of that segment, imparting heat to the elements 151 in that part and then pass upwardly through the inner part 146 of that segment. It will be noted that the products of combustion are cooled somewhat, before they pass from the larger to the smaller part of the segment. The cool products of combustion leave the housing through the port 154.

As the housing 140 turns relative to the body 152, the flow of combustion air and the flow of products of combustion are both advanced around the housing so that each flow passes through each segment in turn.

The degree of overlap between each segment and ports defined by the body 152 reduces gradually. Concurrently with this reduction in overlap, there is a gradual increase in overlap of the ports with the next adjacent segment.

Thus, the gas flow is gradually diverted from one segment to the next adjacent segment. Both the flow of combustion air and the flow of products of combustion are continuous during a period of operation of the burner.

The housing 140 and body 152 are formed to co-operate to reduce leakage of gas from the flowpaths. Thus, there is provided at the circumference of the housing 140 a seal I 161, which be of known form, and which prevents gases escaping to the ambient atmosphere from the joint between the housing and the body 152. Adjacent to the ports 153 to 156, the housing and the body 152 are shaped to encourage the flow to occur near to the partitions 148 and away from both the periphery of the housing and the centre of the housing. Some leakage of gas between inner and outer parts of a segment adjacent to the ports is more acceptable than is leakage from one segment to a diammetrically opposite segment or from the interior of the housing to the ambient atmosphere.

Means may be provided to deliver supplementary gas flows, which are small, relative to the flow of combustion air and the flow of products of combustion, in the vicinity of the ports 153 to 156, in order to direct the main flows away from the centre of the housing and away from the periphery of the housing.

In Figure 6, there is illustrated an alternative heat store and flowcontrol means which may be incorporated in the furnace illustrated in Figure 1, in place of the heat store 14 and flow control valve IS. The heat store of Figure 6 comprises a hollow housing 240 which, in the example illustrated, is rectangular, as viewed in plan, and which is divided internally into a number of compartments by a longitudinal partition 242 and transverse partitions 243. In the illustrated example, there are eight compartments but different numbers of compartments may be used.

Each of the compartments defined by the housing 240 has at its lower end a lower port 244 and at its upper end an upper port 245. The compartment is packed with heat storage elements resting partly on a lower wall of the housing 240 and partly on retaining means spanning the lower port 244 to prevent heat storage elements falling through the port. The heat storage elements may be regularly shaped pieces of ceramic material and may be packed in a random manner into the compartment. Alternatively, the elements may be stacked in a regular array. The partitions 242 and 243 prevent flow of gases directly from one compartment to another.

Below the body 240, there is a duct 245 which leads to a stack, possibly via an exhaust fan. Below the duct 245, there is a further duct 246 which leads from a further fan or other air supply means. Each of the lower ports 244 communicates via respective communicating ducts 247 and 248 respectively with both the flue duct 245 and the air supply duct 246. The communicating ducts 247 and 248 may be permanently open. Alternatively, valves or other mechanical means may be provided for closing these ducts and opening them in a predetermined sequence.

Above the body 240, there is an exhaust duct 249 which leads from an exhaust port in the wall structure of the furnace or in the burner. Above the exhaust duct, there is a duct 250 leading to the burner for conveying combustion air to the burner. Each of the upper ports 245 communicates through communicating ducts 251 and 252 respectively with both the exhaust duct 249 and the duct 250. The communicating ducts 251 and 252 are permanently open and no valves are providing for inhibiting the flow of gases through these ducts.

The flow control means associated with the heat store of Figure 6 comprises a number of control jets positioned to direct supplementary fluid flows into the communicating ducts and thereby deflect the flows of combustion air and products of combustion through those ducts. The supplementary flows may be of very brief duration, having the nature of pulses which switch the main flows from one direction to another in a known manner. The junctions between each pair of communicating ducts are formed to facilitate such fluid control of the direction of flow in a known manner.

Means for establishing the control jets are respresented at 253 in Figure 6.

The flow-control means further comprises means (not shown) for delivering supplementary fluid flows in a predetermined sequence to the means 253.

During operation of the furnace, combustion air is supplied to the heat store continuously along the duct 246. At any moment, the flow of combustion air is directed through selected ones only of the compartments defined by the housing 240. The flow is directed from one compartment to another in a accordance with a predetermined programme, so that the flow of combustion air passes through each of the compartments in turn.

Whilst flowing through the compartments defined by the housing 240, the combustion air receives heat from the heat storage elements. The combustion air emerging through the upper port 245 of a compartment is directed by control jets into the duct 250, through which it flows to the burner. In the burner, the combustion air is mixed with fuel and burns to heat the furnace chamber. Products of combustion flow from the furnace chamber along the duct 249 and are directed from that duct through certain of the compartments defined by the housing 240, which compartments are not conveying combustion air at that moment. The flow of products of combustion is directed from one compartment to another in accordance with the programme so that the flow passes through each of the compartments in turn.

The features disclosed in the foregoing description, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately or any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS:-

I. A method of operating a burner wherein there is provided a heat store, air is delivered to the burner along a feedpath which extends through the heat store, fuel is delivered to the burner, is mixed by the burner with the air and burns in the air and wherein hot products of the combustion of the fuel and air are directed through the heat store to impart heat thereto, characterised in that the heat store provides a number of paths for the flow of gases, each of said paths contains air and products of combustion alternately and in that both the flow of air through the heat store to the burner and the flow of products of combustion through the heat store are substantially continuous throughout a period of operation.
2. A method according to Claim I wherein the flow of fuel and air to the burner is substantially continuous throughout a period of operation which includes a number of cycles.
3. A method according to Claim I or Claim 2 wherein the flow of air and the flow of products of combustion in the heat store are advanced gradually to successive parts of the heat store.
4. A method according to any preceding Claim wherein, at any instant, the airflow rate in the heat store has a maximum value in a first part of the heot store and a lower value in adjacent parts of the heat store whilst the flow rate of the products of combustion in the heat store has a maximum value in a second part of the heat store and a lower value in parts of the heat store adjacent to the second part.
5. A method according to Claim 4 wherein the heat store defines a central axis, said first part of the heat store is spaced from the axis and wherein the direction in which the first part of the heat store is spaced from the axis turns around the axis through an angle of 3600 in each cycle of a succession of cycles which together constitute a period of operation of the burner.
6. A method according to any one of claims I to 4 wherein the heat store surrounds an axis and wherein the respective flows of air and of products of combustion are advanced through the heat store in a direction around the axis.
7. A method according to Claim 6 wherein each of the flowpaths includes an inner portion nearer to the axis and an outer portion further from the axis.
8. A method according to Claim 7 wherein hot products of combustion flow along the outer portion of each flowpath in turn and from the outer portion to the inner portion, whereas combustion air flows along the inner portion of each flowpath in turn and from the inner portion to the outer portion.
9. A method according to any preceding Claim wherein the heat store turns.
10. A method according to Claim I or Claim 2 wherein the flow of products of combustion is directed to said paths in succession by changes in the fluid pressure at predetermined positions and/or by supplementary fluid flows.
II. A method according to Claim 2 wherein there is further provided a rotary flow control device which turns to change said flowpaths in the heat store and wherein the rotary flow control device turns substantially continuously through the period of operation.
12. A furnace defining a chamber and having a burner for discharging a burning mixture of fuel and air into the chamber, feed means for feeding fuel and air to the burner, exhaust means for exhausting products of combustion from the chamber and a heat store for storing heat obtained from the products of combustion which have left the chamber and supplying that heat to air fed to the burner, characterised by flow control means for directing the air at respective different times along each of several flowpaths through the heat store and directing the products of combustion at each said time along a different one of the flowpaths.
13. A furnace according to Claim 12 wherein the flow control means is a rotary device.
14. A furnace according to Claim 13 wherein the heat store is annular.
15. A furnace according to Claim 14 wherein the rotary flow control device lies in a central space defined by the heat store.
16. A furnace according to Claim 14 wherein the heat store has several air inlets, the rotary device has a first port which communicates with the air inlets in turn, the heat store has several air outlets and the rotary device has a second port which communicates with the air outlets in turn.
17. A furnace according to Claim 16 wherein the rotary device has a third port which communicates with an exhaust port leading from said chamber and also communicates with the air outlets in turn and the rotary device has a fourth port which communicates with the air inlets in turn.
18. A furnace according to Claim 12 wherein the flow control means and the heat store are arranged for relative movement around an axis.
19. A furnace according to Claim 18 wherein one of the flow control means and the heat store remains stationary and the other of the flow control means and the heat store turns.
20. A furnace according to Claim 18 or Claim 19 wherein each of said flowpaths has an inner portion extending generally longitudinally of the axis and lying nearer to the axis and an outer portion extending generally longitudinally of the axis and lying further from the axis.
21. A furnace according to any one of Claims 18 to 20 wherein each of said flowpaths includes a portion with a larger area transverse to the direction of flow and a portion with a smaller area transverse to the direction of flow.
22. A furnace according to any one of Claims 18 to 21 wherein the heat store comprises a hollow cylinder closed at one end by an end wall of the cylinder and partly closed at the opposite end by the flow control means, wherein the interior of the cylinder is divided into segments by a plurality of radial walls and wherein each segment is divided by a partition lying between the periphery of the cylinder and the centre of the cylinder.
23. A furnace according to Claim 22 wherein each partition is spaced from the end wall of the cylinder by a gap.
24. A furnace according to Claim 22 wherein the flow control means has arcuate ports for communicating with the segments.
25. A furnace according to Claim 12 wherein the flow control means has ports in permanent communication with respective flowpaths through the heat store and include means for delivering supplementary flows of fluid to change the direction of flow of the products of combustion and the direction of flow of combustion air through the heat store.
26. A unit suitable for use in transferring heat from a stream of hot products of combustion to a stream of cool combustion air, the unit comprising a heat store defining a number of gas flowpaths and flow control means for directing said streams alternately through each path.
27. A unit according to Claim 26 wherein the heat store defines at least four paths and the flow control means directs each stream successively to each path.
28. A unit according to Claim 26 or Claim 27 wherein the heat store and the flow control means are arranged for relative movement about an axis.
29. A unit according to Claim 28 further comprising drive means for turning one of the heat store and the flow control means about the axis.
30. A heat store and flow control means substantially as herein described with reference to Figure 2.
31. A heat store and flow control means substantially as herein described with reference to Figures 3, 4 and 5 of the accompanying drawings.
32. A heat store and flow control means substantially as herein described with reference to Figure 6 of the accompanying drawings.
33. Any novel feature or novel combination of features disclosed herein or in the accompanying drawings.