Method and system for heat recovery from products of combustion and charge heating installation including the same

Abstract

A plurality of independently flow rate-controlled flows of fuel may be preheated at a heat exchanger (or both oxidant and fuel at separate heat exchangers) by heat exchange with a hot shell-side (heat transfer) fluid. The separate flows of hot fuel are directed to associated separate burners where they combust with flows of fuel to produce hot combustion gases. The hot combustion gases are used to preheat the hot shell-side fluid at a recuperator or regenerator.

Claims

1. A charge heating installation utilizing pre-heated fuel, comprising: a system for recovering heat from a furnace comprising: a source of gaseous fuel; a recuperator or regenerator, said recuperator or regenerator adapted and configured to exchange heat between a flow of cool shell-side fluid and a flow of hot combustion gases from a furnace to produce a flow of hot shell-side fluid; a first shell and tube heat exchanger comprising: a shell, a hot shell-side fluid inlet and a cool shell-side fluid outlet formed in the shell thereof, first and second fuel inlets receiving first and second main flows of the fuel, respectively, first and second fuel inlet interior spaces receiving said first and second main flows of the fuel, respectively, said first and second fuel inlet interior spaces being disposed within said shell and being divided from one another by a divider, first and second sets of one or more fuel tubes each, the first and second sets receiving the first and second main flows of fuel, respectively, from the first and second fuel inlets via said first and second fuel inlet interior spaces, respectively, each of the fuel tubes extending through an interior of the shell, an upstream tubesheet dividing said first and second fuel inlet interior spaces from the interior portion of said shell that each of the fuel tubes extends through, first and second fuel outlet interior spaces receiving said first and second main flows of the fuel, respectively, from said first and second sets of one or more fuel tubes each, respectively, said first and second fuel inlet outlet spaces being disposed within said shell and being divided from one another by a divider, first and second fuel outlets receiving the first and second main flows of fuel, respectively, from the first and second sets of fuel tubes, respectively, via said first and second fuel outlet interior spaces, respectively, a downstream tubesheet or a downstream imperfectly sealed tubesheet-like divider dividing said first and second fuel outlet interior spaces from the interior portion of the shell that each of the fuel tubes extends through, the first shell and tube heat exchanger being adapted and configured to transfer heat from the flow of hot shell-side fluid to the main flows of fuel flowing through the fuel tubes; a first cool fuel feed conduit fluidly communicating between the source of fuel and the first fuel inlet; a second cool fuel feed conduit fluidly communicating between the source of fuel and the second fuel inlet; first and second fuel flow control devices disposed in the first and second cool fuel feed conduits, respectively; a first controller adapted and configured to control flow rates of fuel from the fuel source and through the first and second cool fuel feed conduits with the first and second fuel flow control devices, respectively, wherein each of the first and second fuel flows through the cool fuel feed conduits may be controlled by said first controller independently and separately from control of one other; and first and second hot fuel feed conduits receiving the first and second main fuel flows, respectively, from the first and second fuel outlets, respectively; first and second burners receiving first and second flows of hot fuel, respectively, from the first and second hot fuel feed conduits, respectively; and a melting furnace containing a charge, each of the burners being operatively associated with the furnace such that the charge is heated through combustion of an oxidant and the hot fuel injected by the burners, wherein the recuperator or regenerator receives a flow of hot combustion gases from the combustion of the hot fuel and the oxidant in the furnace to produce the flow of hot shell-side fluid.

2. The charge heating installation of claim 1, wherein none of the first and second fuel flows bypass the heat exchanger so that the first and second fuel flows become the first and second main fuel flows, respectively.

3. The charge heating installation of claim 1, further comprising first and second bypass valves disposed in the first and second cool fuel feed conduits, respectively, wherein: each of the bypass valves is adapted and configured to split an associated one of the fuel flows into first and second portions; the first portion split by the first bypass valve being the first main fuel flow and the second portion split by the first bypass valve being a first bypass fuel flow; the first portion split by the second bypass valve being the second main fuel flow and the second portion split by the second bypass valve being a second bypass fuel flow; the first and second bypass flows flowing through first and second bypass flow conduits disposed entirely outside the shell; the first hot fuel feed conduit receiving the first bypass flow from the first bypass flow conduit at which the first bypass flow is combined with the first main fuel flow; and the second hot fuel feed conduit receiving the second bypass flow from the second bypass flow conduit at which the second bypass flow is combined with the second main fuel flow.

4. The charge heating installation of claim 1, wherein each of the splits of the first and second fuel flows into the respective first and second portions is controlled by the controller separately and independently of one another.

5. The installation of claim 1, wherein none of the first and second fuel flows bypass the heat exchanger so that the first and second fuel flows become the first and second main fuel flows, respectively.

6. The installation of claim 1, further comprising first and second bypass valves disposed in the first and second cool fuel feed conduits, respectively, wherein: each of the bypass valves is adapted and configured to split an associated one of the fuel flows into first and second portions; the first portion split by the first bypass valve being the first main fuel flow and the second portion split by the first bypass valve being a first bypass fuel flow; the first portion split by the second bypass valve being the second main fuel flow and the second portion split by the second bypass valve being a second bypass fuel flow; the first and second bypass flows flowing through first and second bypass flow conduits disposed entirely outside the shell; the first hot fuel feed conduit receiving the first bypass flow from the first bypass flow conduit at which the first bypass flow is combined with the first main fuel flow; and the second hot fuel feed conduit receiving the second bypass flow from the second bypass flow conduit at which the second bypass flow is combined with the second main fuel flow.

7. The installation of claim 6, wherein each of the splits of the first and second fuel flows into the respective first and second portions is controlled by the controller separately and independently of one another.

8. A method for recovering heat from a melting furnace containing a charge, comprising the steps of: providing the charge heating installation of claim 1; injecting oxidant and a first flow of hot fuel from the first burner; injecting oxidant and a second flow of hot fuel from the second burner; combusting the injected oxidant and hot fuel to heat a charge in the furnace and produce hot combustion gases; exchanging heat with the recuperator or regenerator between a flow of cool shell-side fluid and a flow of the hot combustion gases to produce a flow of hot shell-side fluid; independently and separately controlling flow rates of first and second flows of fuel flowing in first and second cool fuel feed conduits upstream of the first and second fuel inlets; feeding the independently and separately flow rate-controlled first and second flows of fuel to the first and second fuel inlets, respectively; receiving, from the first and second fuel inlets, the independently and separately flow rate-controlled first and second flows of fuel into the first and second fuel inlet interior spaces; receiving, from the first and second fuel inlets via said first and second fuel inlet interior spaces, respectively, the first and second main flows of fuel into the first and second sets of one or more fuel tubes each, respectively; heating the first and second flows of fuel through heat exchange with the hot shell-side fluid across the first and second sets of one or more fuel tubes each, respectively to produce the first and second flows of hot fuel; and receiving, from said first and second sets of one or more fuel tubes each, respectively, said first and second main flows of the fuel into the first and second fuel outlet interior spaces, respectively.

9. The method of claim 8, wherein the shell-side fluid is air, carbon dioxide, helium, nitrogen, other inert gas, or mixtures thereof.

10. The method of claim 8, further comprising the step of producing the cool shell-side fluid through heat exchange between the hot shell-side fluid and the first and second main flows of fuel at the shell and tube heat exchanger.

11. The method of claim 8, wherein none of the first and second fuel flows bypass the heat exchanger so that the first and second fuel flows become the first and second main fuel flows, respectively.

12. The method of claim 8, further comprising the steps of: splitting each of the first and second fuel flows into first and second portions with first and second bypass valves, respectively, wherein: the first portion split by the first bypass valve being the first main fuel flow and the second portion split by the first bypass valve being a first bypass fuel flow, and the first portion split by the second bypass valve being the second main fuel flow and the second portion split by the second bypass valve being a second bypass fuel flow, the first and second bypass flows flowing through first and second bypass flow conduits disposed entirely outside the shell; combining the first bypass flow with the first main fuel flow downstream of the shell and tube heat exchanger; and combining the second bypass flow with the second main fuel flow downstream of the shell and tube heat exchanger.

13. The method of claim 12, wherein each of the splits of the first and second fuel flows into the respective first and second portions is controlled by the controller separately and independently of one another.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

(2) FIG. 1 is a schematic of one prior art heat recovery solution in a furnace.

(3) FIG. 2 is a schematic of another prior art heat recovery solution in a furnace.

(4) FIG. 3 is a schematic of yet another prior art heat recovery solution in a furnace.

(5) FIG. 4 is a schematic of a non-limiting example of heat recovery in a furnace involving one oxidant heat exchanger and one fuel heat exchanger for every four burners.

(6) FIG. 5 is a schematic elevation end view (from an oxidant feed end), of one non-limiting example of a shell and tube heat exchanger of the present invention showing interior features.

(7) FIG. 6 is a schematic elevation side view of the shell and tube heat exchanger of FIG. 5 showing interior features.

(8) FIG. 7 is a schematic elevation side view of a shell and tube heat exchanger similar to that of FIG. 5 that shows alternative interior features.

(9) FIG. 8 is a process flow diagram of a bypass scheme for controlling the temperature of one of the oxidant streams downstream of the heat exchanger.

(10) FIG. 9 is a process flow diagram for the invention.

DETAILED DESCRIPTION

(11) In conventional furnaces, a single source of oxidant and a single source of fuel are typically split up into multiple streams (at ambient temperature) for injection by multiple burners. If an operator wishes to control the power of a particular burner or burners independently of the powers of other burners, it is relatively simple matter of adjusting the flow rate of the fuel and/or oxidant for the burner(s) at issue using a flow control device upstream of or at the burner level.

(12) In the cases of a conventional furnace including burners using pre-heated oxidant and/or pre-heated fuel, each of the reactants (fuel and oxidant) is directed to a heat exchanger as a single stream oxidant at which it is heated through heat exchange with a hot fluid. The heated reactant(s) is then split up into multiple streams in parallel for injection by multiple burners. If an operator wishes to control the power of an individual burner or burners independently of the powers of other burners, the flow rate of the fuel and/or oxidant for the burner(s) at issue hypothetically could be adjusted using a flow control device(s) in the line(s) split off from the pre-heated oxidant stream and/or line(s) split off from the pre-heated fuel stream.

(13) However, this hypothetical approach suffers three significant disadvantages. First, the tolerances in between the internal components of the flow control device are increased due to thermal expansion of those components caused by heat transfer from the hot reactant to those components. The increased tolerances result in the formation of leaks from the device and thus poor control over the flow rate if not creation of a safety risk. Second, any change in the flow rate of a single one of the multiple streams will affect the pressures in the lines conveying the other of the streams because those lines are in parallel and are in flow communication with one another. This effect is exacerbated by the fact that the preheated reactant is at a higher pressure after heating than the ambient temperature reactant. Because the flow rates in the other lines are subject to significant pressure variations and must be simultaneously controlled, this leads to a very complex and difficult control scheme. Third and in the case of hot oxidant in particular, hot oxidative attack of internal components of the flow control device can cause premature or even catastrophic failure due to the enhanced reaction rate of oxidation of the material comprising the device.

(14) The invention overcomes these disadvantages by: a) splitting up the reactant, upstream of heating, into multiple streams, b) using a novel heat exchanger, and c) controlling the flow rates of the reactant streams with flow control devices disposed upstream of the heat exchanger. The flow control devices are not subjected to heating and thus remain relatively leak-proof because the tolerances in between the internal components are not affected. Because the pressure of the reactant source (oxidant or fuel) is at a much higher pressure than those of the multiple reactant streams upstream of the heat exchanger, an adjustment of the flow rate of one of the streams upstream of the heat exchanger does not make as significant an impact in the pressure in the other of the streams. Thus, there is longer a need to simultaneously control the flow rates of the other streams, or alternatively, such simultaneous control is more easily done due to the relatively smaller pressure variations at ambient temperature.

(15) In general, the system and method includes a recuperator or regenerator that is used to transfer heat from hot combustion gases produced in a furnace (containing a charge) to a heat transfer fluid. A shell and tube heat exchanger is used to transfer heat from the hot heat transfer fluid to multiple reactant (oxidant or gaseous fuel) streams. The heat transfer fluid is called the shell-side fluid because it flows through an interior of the heat exchanger on the shell-side, i.e., in the space between the interior surface of the shell and the exterior surfaces of tubes extending in the space. Hence, the oxidant or fuel is called the tube-side fluid because it flows on the tube-side (i.e., through tubes extending through the heat exchanger. As best shown in FIG. 9, control devices D.sub.A, D.sub.B, D.sub.C, D.sub.D equal in number to the number of oxidant and/or fuel streams M1.sub.A, M1.sub.B, M1.sub.C, M1.sub.D are disposed upstream of the heat exchanger HE and are used to control the flow rates of the oxidant and/or fuel streams M1.sub.A, M1.sub.B, M1.sub.C, M1.sub.D flowing to the heat exchanger HE. These flow control devices D.sub.A, D.sub.B, D.sub.C, D.sub.D are controlled with a controller C separately and independently of one another. This means that adjustment of the flow rate of one of the oxidant (or fuel) streams M1.sub.A, M1.sub.B, M1.sub.C, M1.sub.D does not require concomitant adjustment of the flow rates of the other of the oxidant (or fuel) streams M1.sub.A, M1.sub.B, M1.sub.C, M1.sub.D.

(16) The shell and tube heat exchanger HE includes multiple oxidant (or fuel) inlets and outlets equal in number to the number of oxidant (or fuel) streams being pre-heated. Extending in flow communication between an associated inlet and outlet is a set of tubes termed oxidant tubes in the case of oxidant pre-heating or fuel tubes in the case of fuel pre-heating. Each oxidant (or fuel) stream is first received by an oxidant (or fuel) inlet and divided into a plurality of sub-streams that flow through the tubes of the associated set. Typically, the tubes extend between upstream and downstream tube sheets in order to separate the oxidant or fuel from the hot shell-side fluid. The sub-streams of oxidant (or fuel) for a given set of tubes are then recombined into the thus-heated flow of oxidant (or fuel) which flows out of the heat exchanger HE at an associated oxidant (or fuel) outlet. Thus, it is seen that each flow of oxidant (or fuel) to be pre-heated is associated with one of the oxidant (or fuel) inlets, with one of the sets of oxidant (or fuel) tubes, and one of the oxidant (or fuel) outlets. The number of tubes per set is non-limiting and may be selected based upon space, design, and material limitations. Typically, the number of tubes per set ranges from 2-12.

(17) The shell and tube heat exchanger HE may optionally include conventional baffles oriented perpendicular to, and outside of, the oxidant (or fuel) tubes so that the hot shell-side fluid follows a serpentine path that allows heat transfer between the hot shell-side fluid and a first portion of the oxidant (or fuel) tubes, then with second portions of the oxidant (or fuel) tubes adjacent to the first portions, and so on. In this manner, the hot shell-side fluid acts to equalize the temperatures of the oxidant (or fuel) tubes, and therefore equalize the temperatures of the oxidant (or fuel) flowing through the tubes. The shell and tube heat exchanger HE may have a cross-sectional configuration conventionally used in the heat exchanger art, including but not limited to: circular, oval, rectangular, and square.

(18) While the heat exchanger HE may be made of out of a wide variety of materials, typically it is made of a material that is recognized as being suitable for handling hot oxidants (in the case of oxygen pre-heating) or hot gaseous fuels (in the case of fuel pre-heating). Additionally, each set of oxidant (or fuel) tubes may optionally be separated from one another by walls running parallel to the tubes. In this case, the hot shell-side fluid is split into a plurality of sub-streams equal in number to the number of sets where each single sub-stream of hot shell-side fluid is caused to flow alongside only one set of oxidant (or fuel) tubes.

(19) The oxidant has an oxygen concentration higher than that of air. Typically, it is oxygen-enriched air or industrially pure oxygen. In the case of no fuel pre-heating, the fuel may be any fuel conventionally used in burners associated with furnaces for heating a charge, including pulverent, particulate, or crushed solid fuels, liquid fuels, or gaseous fuels. In the case of fuel pre-heating, the fuel is gaseous. Typically, the fuel is natural gas, methane, or propane. The furnace may be any conventional furnace designed for heating and/or melting a charge, such as ceramic, glass, or metal. Typically, it is a melting furnace, such as a glass melting furnace. The shell-side fluid may be air, carbon dioxide, helium, other inert gas, or mixtures thereof.

(20) The burner may be any burner suitable for the combustion of a fuel with an oxidant in a furnace for heating and/or melting a charge (such as metal or glass), for example, those disclosed by U.S. Pat. No. 6,910,879, US 2007-0172781, and US 2007-0281254.

(21) In operation, the ratio of the flow rate of shell-side fluid to the flow rate of the oxidant stream or fuel stream is dependent in a trivial way upon a variety of factors, including the type of shell-side fluid, the type of oxidant, the temperature of the shell-side fluid, the temperature of the oxidant before pre-heated, the temperature of the fuel before pre-heating, the desired hot oxidant and hot fuel temperatures, process requirements, and the particular configuration of the heat exchanger. Typically, the ratio is at least 2:1.

(22) The temperature of the shell-side fluid and the hot combustion gases are also dependent in a trivial way upon a variety of factors, including the type of shell-side fluid, the type of combustion gases, the temperature of the shell-side fluid before heat exchange at the recuperator or regenerator, the temperature of the hot combustion gases, process requirements, and the particular configuration of the recuperator or regenerator. While higher temperatures are possible, typically the hot shell-side fluid is at a temperature up to about 730 C. Typically, the oxidant and fuel before pre-heating are at ambient temperature. After pre-heating, the oxidant is typically at a temperature of up to about 700 C., but higher temperatures are still possible. After pre-heating, the fuel is typically at a temperature of up to about 450 C. After heat exchange between the hot shell-side fluid and the oxidant and fuel streams, the cooled shell-side fluid is typically at a temperature of about 200-300 C.

(23) Optionally, each of the oxidant streams is pre-heated at a first heat exchanger while each of the fuel streams is pre-heated at a second heat exchanger. The flow of hot shell-side fluid may be arranged in parallel whereby two streams of the hot shell-side fluid are directed to the two heat exchangers. The flow hot shell-side fluid may instead be arranged in series whereby one of the oxidant and fuel streams is pre-heated at the first heat exchanger through heat exchange with the hot shell-side fluid, and the now-somewhat cooled hot shell-side fluid exiting the first heat exchanger is used to pre-heat the other of the oxidant and fuel streams at the second heat exchanger.

(24) Optionally, the shell-side fluid may be recirculated. By recirculated, we meant that after heat exchange is performed between the shell-side fluid and the oxidant and/or fuel streams, it is returned to the regenerator or recuperator to complete a circuit. In this case, shell-side fluids other than air become more cost-effective. The shell-side fluid may be chosen so as to optimize heat transfer between conduits, for example, by choosing a fluid of high thermal conductivity such as helium. Alternatively, overall heat transfer may be optimized by choosing a fluid of high heat capacity such as carbon dioxide. Optionally, the shell-side fluid is any other inert gas or mixtures of any of helium, carbon dioxide, and the other inert gas.

(25) The overall design of the heat exchanger HE is optimized based upon the total power of the combined burners receiving pre-heated oxidant (and/or fuel). This means that the diameter of the oxidant (or fuel) tubes, the number of oxidant (or fuel) tubes, the oxidant (or fuel) tube pitch (i.e., the tube to tube spacing), and the oxidant (or fuel) length to diameter ratio are optimized based upon the total combined power of the burners receiving the pre-heated oxidant (or fuel). Once these variables are optimized, the heat exchanger is provided with a single shell. Then, the oxidant (or fuel) tubes are divided into sets based upon the number of oxidant (or fuel) streams to be pre-heated by the heat exchanger where each set receives a separate oxidant (or fuel) stream. This design optimization can be distinguished from a combination of heat exchangers each one of which has been individually optimized based upon the burners it supplies with pre-heated oxidant or fuel where the combination includes a number of shells equal to the number of heat exchangers combined. A combination of heat exchangers is less efficient than the optimized heat exchanger of the invention.

(26) The flow rate of each individual, separately controlled, oxidant (or fuel) stream M1.sub.A, M1.sub.B, M1.sub.C, M1.sub.D is typically varied over time in response to process requirements. If the flow rate of one or less than all of the oxidant (or fuel) streams M1.sub.A, M1.sub.B, M1.sub.C, M1.sub.D is lowered, the slower oxidant (or fuel) stream flow rate causes that slower-rate stream to be heated to a relatively higher temperature than other faster-rate streams. This is because the longer residence time of the oxidant (or fuel) inside the heat exchanger HE allows greater heat transfer between the hot heat transfer fluid to the slower-rate stream. Conversely, a higher oxidant (or fuel) stream flow rate causes that faster-rate stream to be heated to a relatively lower temperature than other slower-rate streams because of the shorter residence time of the faster rate stream.

(27) Because the individual oxidant (or fuel) streams M1.sub.A, M1.sub.B, M1.sub.C, M1.sub.D may have higher or lower flow rates (and therefore the oxidant (or fuel) tubes have correspondingly lower or higher temperatures), the thermal expansion or thermal contraction of each oxidant (or fuel) tube conveying that higher or lower flow rate stream may be greater or lesser than those of the other oxidant (or fuel) streams. In order to avoid the possibility that the differing thermal expansions and/or contractions may place undue stresses on the oxidant (or fuel) tubes and the shell, each set of oxidant (or fuel) tubes may be provided with a separate thermal expansion joint. In this manner, the separate joints may allow the differing expansions and contractions of the different sets of tubes without subjecting the heat exchanger HE to undue stresses.

(28) It is desirable to maintain the oxidant (or fuel) temperatures of the various pre-heated oxidant (or fuel) streams as close as possible. However, and as discussed above, when individual oxidant (or fuel) streams have higher or lower flow rates, their temperatures may be lower or higher than the other lower or higher flow rate streams. There are several ways to compensate for these different temperatures.

(29) Under one approach and where appropriate, thermally conductive packing materials may be used to facilitate heat transfer, for example alumina packing may be used. When using packing materials, it is important to have a sufficiently loose packing so that the pressure drop is minimized, while still achieving good thermal contact with the hot and cold surfaces of the heat exchanger. Also, thermal conduction between the oxidant (or fuel) streams is maximized, for example, by using thermally conductive plates to connect the oxidant (or fuel) tubes to one another another. Thus heat transfer occurs between streams via the plates. By facilitating heat transfer from tube to tube, differences in temperature between the various oxidant (or fuel) streams may be compensated for.

(30) Under another approach, the oxidant (or fuel) tubes of a given set of oxidant (or fuel) tubes are not disposed alongside one another as described above. Thus, after division of a given oxidant (or fuel) stream into a plurality of sub-streams, the oxidant (or fuel) tubes for the various oxidant (or fuel) streams are interleaved with one another. For example and in the case of three streams of oxidant (or fuel) each one of which is divided amongst three oxidant (or fuel) tubes, a first tube of the first stream extends alongside the first tube of the second stream which in turn extends alongside the first tube of the third stream. A second tube of the first stream extends alongside the second tube of the second stream which in turn extends alongside the second tube of the third stream. Finally, a third tube of the first stream extends alongside the third tube of the second stream which in turn extends alongside the third tube of the third stream. In each case, the corresponding first tubes (of the first, second, and third streams) are closer to one other than they are to the second tubes or third tubes of the set stream.

(31) Under yet another approach and as best illustrated by FIG. 8, one or more flows of oxidant (or fuel) M1.sub.A, M1.sub.B, M1.sub.C, M1.sub.D are split into a main flow M2.sub.A, M2.sub.B, M2.sub.C, M2.sub.C and a bypass flow M3.sub.A, M3.sub.B, M3.sub.C, M3.sub.C with a control valve V.sub.A, V.sub.B, V.sub.C, V.sub.D. The main flow M2.sub.A, M2.sub.B, M2.sub.C, M2.sub.C is directed into the heat exchanger HE where it is heated through heat exchange with the hot shell-side fluid. The bypass flow M3.sub.A, M3.sub.B, M3.sub.C, M3.sub.C completely bypasses the heat exchanger HE and is recombined with the now-heated main flow. A controller C controls the ratio of the main M2.sub.A, M2.sub.B, M2.sub.C, M2.sub.D and bypass flow M3.sub.A, M3.sub.B, M3.sub.C, M3.sub.C rates via the control valve V.sub.A, V.sub.B, V.sub.C, V.sub.D to a value within the range of 1:0 to 0:1. Typically, the value is in the range of 9:1 to 7:3. While any known process control scheme may be utilized to control this ratio, generally speaking, when a temperature of the stream (downstream of where the main flow and bypass flows are recombined) exceeds a set maximum temperature, the controller C commands the control valve to increase the bypass flow rate and decrease the main flow rate. When a temperature of the stream (again, downstream of where the main and bypass flows are recombined) goes below a set minimum temperature, the controller C commands the control valve to decrease the bypass flow rate and increase the main flow rate.

(32) The overall flow M1.sub.A, M1.sub.B, M1.sub.C, M1.sub.D of cold oxidant is split between an associated interior stream M2.sub.A, M2.sub.B, M2.sub.C, M2.sub.C and an exterior stream M3.sub.A, M3.sub.B, M3.sub.C, b. The interior stream is directed to the feed end of the heat exchanger, heated in the oxidant tubes in heat exchange with the hot heat transfer fluid, and discharged out the hot oxidant outlet. The exterior stream remains outside the heat exchanger and is recombined with the interior stream to again provide the overall flow M1. A temperature setpoint is predetermined for the recombined flow of hot oxidant. By measuring T2 and adjusting the allocation of the overall flow M1 into interior and exterior stream flows M2, M3, the temperature of the hot oxidant after recombination of the streams may be controlled. In other words, if T2 is higher than the setpoint temperature, M3 is increased and M2 is decreased at a valve accomplishing the split until T2 reaches the setpoint. Preferably, a butterfly valve is used to do this. T1 will increase as M1 decreases and will eventually approach a limit close to the hot heat transfer fluid temperature. Close to this limit, T1 will increase slowly as M1 decreases. On the other hand, T2 will decrease through dilution of the interior stream with the cold, unheated exterior stream (having flow M3). This temperature decrease is more rapid close to the limit. In this way, we can achieve a desirable temperature irrespective of overall flow rates.

(33) One generalized and illustrative arrangement of the invention is shown in FIG. 4. Hot combustion gases 1 preheat a heat transfer fluid (i.e., the shell-side fluid) 3 at a recuperator or regenerator 5. The resultant hot shell-side fluid 7 flows to a heat exchanger 9 for preheating oxidant where it exchanges heat with flows of cold oxidant 11.sub.A, 11.sub.B, 11.sub.C, 11.sub.D. The resultant flows of hot oxidant 13.sub.A, 13.sub.B, 13.sub.C, 13.sub.C are directed to burners 23.sub.A, 23.sub.B, 23.sub.C, 23.sub.C. The flows of fuel 19.sub.A, 19.sub.B, 19.sub.C, 19.sub.C are directed to the burners 23.sub.A, 23.sub.B, 23.sub.C, 23.sub.C where the fuel combusts with the hot oxidant to produce the hot combustion gases 1. The hot shell-side fluid is cooled at heat exchanger 9 and is optionally recirculated to the recuperator or regenerator 5 as the shell-side fluid 3 to complete a loop.

(34) Another generalized and illustrative arrangement of the invention is shown in FIG. 5. Hot combustion gases 1 preheat the shell-side fluid 3 at a recuperator or regenerator 5. The resultant hot shell-side fluid 7 flows to a heat exchanger 17 for preheating fuel where it exchanges heat with flows of cold fuel 19.sub.A, 19.sub.B, 19.sub.C, 19.sub.D. The resultant flows of hot fuel 21.sub.A, 21.sub.B, 21.sub.C, 21.sub.D are directed to the burners 23.sub.A, 23.sub.B, 23.sub.C, 23.sub.D where the hot fuel combusts with the flows of oxidant 11.sub.A, 11.sub.B, 11.sub.C, 11.sub.D. The hot shell-side fluid is cooled at heat exchanger 17 and is optionally recirculated to the recuperator or regenerator 5 as the shell-side fluid 3 to complete a loop.

(35) Another generalized and illustrative arrangement of the invention is shown in FIG. 6. Hot combustion gases 1 preheat the shell-side fluid 3 at a recuperator or regenerator 5. The resultant hot shell-side fluid 7 flows to a heat exchanger 9 for preheating oxidant where it exchanges heat with flows of cold oxidant 11.sub.A, 11.sub.B, 11.sub.C, 11.sub.D. The resultant flows of hot oxidant 13.sub.A, 13.sub.B, 13.sub.C, 13.sub.D are directed to burners 23.sub.A, 23.sub.B, 23.sub.C, 23.sub.D. The hot shell-side fluid is cooled at heat exchanger 9 and is directed to a heat exchanger 17 for preheating fuel where it exchanges heat with flows of cold fuel 19.sub.A, 19.sub.B, 19.sub.C, 19.sub.D. The resultant flows of hot fuel 21.sub.A, 21.sub.B, 21.sub.C, 21.sub.D are directed to the burners 23.sub.A, 23.sub.B, 23.sub.C, 23.sub.D where the hot fuel combusts with the hot oxidant to produce the hot combustion gases 1. Optionally, the shell-side fluid 3 (before heating at the recuperator or regenerator 5) may be the cooled shell-side fluid after heat exchanger at the heat exchanger 17.

(36) While FIGS. 4-6 illustrate one heat exchanger for every four streams of oxidant 11.sub.A, 11.sub.B, 11.sub.C, 11.sub.D and one heat exchanger for every four streams of fuel 19.sub.A, 19.sub.B, 19.sub.C, 19.sub.D, the invention is not limited in such a manner. Rather, each heat exchanger may handle as few as two or three oxidant streams 11.sub.A, 11.sub.B, 11.sub.C, 11.sub.D or fuel streams 19.sub.A, 19.sub.B, 19.sub.C, 19.sub.D or it may handle more than four. Also, while FIGS. 4-6 illustrate only four burners, there may be as few as two or three or as many as several dozen. In the case of a glass melting furnace, typically all of the burners (utilizing pre-heated oxidant and/or fuel) on one side of a furnace receive pre-heated oxidant and pre-heated fuel from a pair of heat exchangers (one of oxidant and one for fuel) while all of the burners on the opposite side receive pre-heated oxidant and pre-heated fuel from a different pair of heat exchangers (again, one for oxidant and one for fuel). Also, while FIG. 6 illustrates pre-heating of the oxidant before the shell-side fluid 3 is used to pre-heat the fuel, this order may be reversed.

(37) A non-limiting example of a shell and tube exchanger for use in the invention is best shown in FIG. 7A-F, the flows of cold oxidant 11A, 11B, 11C, 11D are received in respective oxidant channels/nozzles 33.sub.A, 33.sub.B, 33.sub.C, 33.sub.D formed in shell 36. The hot flows of oxidant 13.sub.A, 13.sub.B, 13.sub.C, 13.sub.D exit the heat exchanger from respective oxidant channels/nozzles 51.sub.A, 51.sub.B, 51.sub.C, 51.sub.D also formed in shell 36. The flow of hot shell-side fluid 7 is directed into an interior of the shell 36 via hot fluid inlet 35. The cooled shell-side fluid exits the heat exchanger from cold fluid outlet 37.

(38) The interior spaces 41.sub.A, 41.sub.B, 41.sub.C, 41.sub.D of the housing adjacent the oxidant (or fuel) channels/nozzles 33.sub.A, 33.sub.B, 33.sub.C, 33.sub.D are divided by dividers 39 to keep the flows of oxidant (or fuel) 13.sub.A, 13.sub.B, 13.sub.C, 13.sub.D (21.sub.A, 21.sub.B, 21.sub.C, 21.sub.D) separate from one another. The interior spaces 49.sub.A, 49.sub.B, 49.sub.C, 49.sub.D adjacent the oxidant (or fuel) channels/nozzles 51.sub.A, 51.sub.B, 51.sub.C, 51.sub.D are similarly divided by dividers 59 to keep the flows of hot oxidant (or fuel) 13.sub.A, 13.sub.B, 13.sub.C, 13.sub.D (21.sub.A, 21.sub.B, 21.sub.C, 21.sub.D) separate from one another.

(39) Each flow of oxidant (or fuel) 13.sub.A, 13.sub.B, 13.sub.C, 13.sub.D (21.sub.A, 21.sub.B, 21.sub.C, 21.sub.D) is split into a plurality of sub-streams that flow from a corresponding chamber 41.sub.A, 41.sub.B, 41.sub.C, 41.sub.D and into a corresponding set of oxidant tubes (or fuel) 45.sub.A, 45.sub.B, 45.sub.C, 45.sub.C. Each of the sub-streams flowing through a given set of oxidant (or fuel) tubes 45.sub.A, 45.sub.B, 45.sub.C, 45.sub.D recombines into a single stream of hot oxidant (or fuel) 13.sub.A, 13.sub.B, 13.sub.C, 13.sub.D (21.sub.A, 21.sub.B, 21.sub.C, 21.sub.D) in an associated chamber 49.sub.A, 49.sub.B, 49.sub.C, 49.sub.D. In this manner, the flows of hot oxidant (or fuel) 13.sub.A, 13.sub.B, 13.sub.C, 13.sub.D (21.sub.A, 21.sub.B, 21.sub.C, 21.sub.D) do not comingle with one another but are kept separate by the dividers 39 and oxidant tubes 45.sub.A, 45.sub.B, 45.sub.C, 45.sub.D, and dividers 59.

(40) With continuing reference to FIGS. 7A-F, the oxidant (or fuel) and shell-side fluid are prevented from contacting one another by virtue of tubesheets 43, 47 separating the interior portion of the heat exchanger through which the hot shell-side fluid flows and through which the oxidant tubes 45.sub.A, 45.sub.B, 45.sub.C, 45.sub.D extend from end portions constituting interior spaces 41.sub.A, 41.sub.B, 41.sub.C, 41.sub.D, 49.sub.A, 49.sub.B, 49.sub.C, 49.sub.D. In other words, the feed ends and discharge ends of the oxidant (or fuel) tubes 45.sub.A, 45.sub.B, 45.sub.C, 45.sub.D are sealed from the hot shell-side fluid by the presence of tube sheets 43, 47.

(41) A variant of the shell and tube exchanger of FIGS. 7A-7F is best illustrated in FIG. 7G where features are included that compensate for differences in thermal expansion between the shell and the various sets of oxidant (or fuel) tubes. When one or more of the oxidant flows, for example oxidant flow 11.sub.A, is decreased through the heat exchanger while the other oxidant flows, for example oxidant flows 11.sub.B, 11.sub.D, 11.sub.D, is kept unadjusted or increased, one of ordinary skill in the art will recognize that the lower flow of oxidant 11.sub.A (through associated oxidant tubes 45.sub.A), will be heated to a relatively higher temperature than the higher flow of oxidant 11.sub.B, 11.sub.C, 11.sub.D (and associated oxidant tubes 45.sub.B, 45.sub.C, 45.sub.D). The higher temperature oxidant tubes 45.sub.A will experience thermal expansion greater than that of the lower temperature oxidant tubes 45.sub.B, 45.sub.C, 45.sub.D. One of ordinary skill in the art will also recognize that the converse situation applies with equal force, namely: the flow rate of oxidant flow 11.sub.A is higher while that of oxidant flows 11.sub.B, 11.sub.C, 11.sub.D are lower and oxidant tube 45.sub.A experiences less thermal expansion than that of oxidant tubes 45.sub.B, 45.sub.C, 45.sub.D). If the differences in thermal expansion (and/or contraction) exceed the degree to which the heat exchanger can withstand the resultant stresses, the seal between the oxidant tubes and tube sheets may burst or leak.

(42) To compensate for the above differences in thermal expansion, the shell 36 may be provided with an expansion joint 59. Also, each of the oxidant (or fuel) tubes 45.sub.A, 45.sub.B, 45.sub.C, 45.sub.D in a set associated with flow of oxidant (or fuel) 11.sub.A, 11.sub.B, 11.sub.C (19.sub.D, 19.sub.A, 19.sub.B, 19.sub.C, 19.sub.D) discharges into an associated collection space that is enclosed and sealed with a bonnet 53.sub.A, 53.sub.B (the bonnets associated with the other flows are not illustrated in FIG. 7G). Each of the bonnets 53.sub.A, 53.sub.B (including those for the other flows) is connected to the shell 36 via an associated expansion joint 55.sub.A, 55.sub.B in order to accommodate the differing thermal expansion/contraction.

(43) In the variant of FIG. 7G, the heat exchanger is not provided with the downstream tubesheet 47. Rather, the heat exchanger includes a tube sheet-like divider 48 that provides an imperfect seal in between, on one hand, the interior portion of the shell through which the hot shell-side fluid flows, and on the other hand, the space in between the divider 48 and the downstream end of the shell 36. The divider 48 includes orifices having a cross-section approximating those of the oxidant (or fuel) tubes 45.sub.A, 45.sub.B, 45.sub.C, 45.sub.D only with wider dimensions so that the oxidant (or fuel) tubes 45.sub.A, 45.sub.B, 45.sub.C, 45.sub.D may slide through expansion or contraction through the orifices. Instead of using a tubesheet 47 to separate the hot shell-side fluid from the oxidant (or fuel) in the collection spaces at the discharge end of the heat exchanger, the combination of the oxidant (or fuel) tubes 45.sub.A, 45.sub.B, 45.sub.C, 45.sub.D and the divider 48 keeps the oxidant (or fuel) and hot shell-side fluid separate.

(44) One of ordinary skill in the art will recognize that, while FIGS. 7A-7G illustrate each set as including only four oxidant (or fuel) tubes 45.sub.A, 45.sub.B, 45.sub.D, 45.sub.D each, each set may include any number that is only limited by the complexity of manufacturing and/or cost of manufacture.

(45) In one variation, each burner may also receive pre-heated oxidant (or fuel) from two heat exchangers. This enables a larger variation in overall oxidant (or fuel) flow to the burner without imposing a high variation in temperature at the burner. For example, for four burners each consuming 200 Nm.sup.3/hr of oxygen (or fuel), and two heat exchangers each configured to pre-heat four oxidant (or fuel) flows, each heat exchanger may deliver 100 Nm.sup.3/hr of pre-heated oxidant (or fuel) to each burner from each heat exchanger. Then, if it is required to reduce the oxidant (or fuel) flow to one burner to 100 Nm.sup.3/hr, one oxidant (or fuel) stream to that burner is shut off, and the flow of hot air to the corresponding heat exchanger is reduced so as to maintain the temperature of the remaining three flows. In this way the flow to one burner can be reduced by a large factor without impacting the temperature of the oxidant (or fuel) flowing to any burner.

(46) While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

(47) The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

(48) Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing i.e. anything else may be additionally included and remain within the scope of comprising. Comprising is defined herein as necessarily encompassing the more limited transitional terms consisting essentially of and consisting of; comprising may therefore be replaced by consisting essentially of or consisting of and remain within the expressly defined scope of comprising.

(49) Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

(50) Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

(51) Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

(52) All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.