GB1590918A - Ceramic heat exchange unit - Google Patents

Description

(54) CERAMIC HEAT EXCHANGE UNIT (71) We, HAGUE INTERNATIONAL, of 3, Adams Street, South Portland, Maine, United States of America, a Corporation organised under the laws of the State of Maine, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- It is known to use ceramic heat exchange tubes in heat transfer apparatus. However, there have been difficulties in providing ceramic heat exchange tubes suitable for use in recuperators for high temperature applications i.e. above 1500 degrees F, which tubes can be made at reasonable cost in long lengths with good heat transfer properties and good mechanical properties.There also have been difficulties in obtaining ceramic heat exchange tubes for use in high temperature applications which tubes can be sealed to provide effective pressure seals between internal flow and external flow of fluids.

It is an object of this invention to provide heat transfer apparatus which makes effective use of ceramic heat exchange tubes.

The invention consists in a heat transfer apparatus comprising a plurality of generally parallel, ceramic, heat exchange tubes mounted to provide heat transfer between fluid passing within said tubes and fluid passing over an exterior surface of said tubes, each said tube being formed of a cast ceramic material, support means being provided for mounting said heat exchange tubes in said heat transfer apparatus, said support means comprising on each said tube an encircling body element, corresponding body elements of the tubes cooperating to define an end wall, each body element having a first polygonal perimeter formed by an end surface of said element and a second polygonal perimeter defined by a second end surface of said element, said perimeters being angularly displaced with respect to each other to form a three-dimensional surface, the three dimensional surfaces of co-operating body elements being in interlocking cooperation.

In order to make the invention clearly understood, reference will now be made to the accompanying drawings which are given by way of example and in which: Fig. 1 is a side view of a recuperator, mounted on a slot furnace; Fig. 2 is a cross sectional view taken along line 2-2 of Fig. 1; Fig. 3 is a left-hand end view of the recuperator shown in Fig. 2; Fig. 4 is a partial cross sectional view taken along line 44 of Fig. 2; Fig. 5 is a cross sectional view taken along line 5-5 of Fig. 6; Fig. 6 is a cross sectional view taken along line 66 of Fig. 2; Fig. 7 is a front view of a group of heat exchange tubes in a heat exchange apparatus of the present invention; Fig. 8 is a side view thereof; Fig. 9 is a detail view thereof; Fig. 10 is another side view of an alternate embodiment thereof.

With reference now to the drawings and more particularly to Figs. 1 to 6, a recuperator 12 is shown in conjunction with an energy conserving process furnace system generally indicated at 10 in Fig. 1 having a furnace 11, an exhaust gas stack 13, a recuperator 12 and a burner 15 to heat work 17 in a combustion zone 16 of the furnace which is preferably in the form of a slot furnace. The furnace 11 can be used for conventional forging operations where a burner such as 15 and an accompanying burner (not shown) at an opposite side of the furnace are preferably of the recirculating type. Combustion air in heated form is fed through lines 42 (Fig. 2) the recuperator 12 to enable recuperation of substantial heat from the escaping products of combustion in the furnace stack 13.For example, the furnace may be operated at temperatures of from 1200"F to 2600"F as at 2500"F with the recuperator recouping heat from the gas flow 39 coming out of the slot 35 at a rate such that combustion air passing through the recuperator and then into the tubes 42 is heated as for example from 80"F or essentially ambient temperature to 1500 F as a result of passing through the recuperator.

The particular slot furnace and exhaust gas system used will not be fully described herein since the recuperator and heat exchange units of this invention can be used in accordance with the teachings of this invention in many different environments.

Generally the exhaust gas path of the furnace system 10 comprises the slot 35 which is an exhaust port from the combustion chamber 16, an exhaust passageway 33 for hot gas above which and in the path of which is mounted the recuperator 12. Above the recuperator 12 is an exhaust stack 13 leading to an upper exhaust port as suggested at 34. A second upwardly extending exhaust passageway is provided by duct work as indicated at 36 to provide passageway 36 having a bottom opening substantially level with the slot 35 and a top opening 38 in the stack above the recuperator. A door to the environment is provided at 73 and damper 37 is located in the passageway 36. Passageway 36 can be considered a single vertical passageway for purposes of function in the system although two such passageways are preferably used.

In the illustrated recuperator substantial recuperation of heat is carried out. Using a 20 ft. high exhaust stack with 160 to 200 Ibs.

of exhaust gas/hour/sq. ft. of recuperator passage area, the desired temperature can be maintained at a total fuel usage rate of 15 gallons/hr. of fuel oil when 1500 lbs. of steel per hour is treated in the furnace. When operating an identical furnace with nonrecirculating burners and no recuperator, 30 gallons of fuel oil/hr. are required to maintain the desired furnace temperature.

The use of the parallel vertical passageways in the exhaust system provides for safe and efficient operation of the slot furnace. Although other systems could be used, this system of exhaust is found to be extremely beneficial.

When a cold furnace operation is initiated, the exhaust gases flow into passageway 33 and then overflow into passageway 36. As the temperature in the stack increases causing the natural draft or aspiration of the stack to increase, the exhaust gases flow through passageway 33 and thus through the recuperator. This is assured by the operator observing the flow pattern of the exhaust gas and adjusting the damper 37 to increase the pressure drop of passageway 36 so that these passageways draw air from the room only and permit essentially all of the hot gases to flow through the recuperator 12.The furnace can then be operated with the furnace front completely opened and the furnace operator can have ready access to the work area of the furnace through slot 35 for insertion and removal of work in process 17 to be heated and later forged without resorting to a closure means such as doors at 73, in the furnace front. In conventional slot furnaces, doors are generally employed at 73 and substantial flow of exhaust gases out of the slot 35 can occur. The operator can be faced with hot blasts of gas when opening doors at 73 to gain access to the work in process 17.

Particularly in the present embodiment, if the second air passageway 36 is not used, hot exhaust gas 39 would tend to pass to the environment through 73 rather than through passage 33 to the recuperator 12 during transient operation. Therefore the preferred arrangement also protects the operator.

Doors can be used to close 73 if desired in some embodiments.

In one configuration of the recuperator 12 (FIGS. 1-6), the internal exhaust gas passageway 79 has a generally rectangular horizontal cross section with a length of 80 inches, a width of 17 inches and a height of 40 inches formed by the end tube sheets (FIG. 2) and side walls 82, 83 which are heat insulated. End walls 80, 81 form a rectangular cross sectioned casing with side walls 82, 83 and all are joined together by suitable welded joints which can include structural supports. Heat exchangers within the recuperator comprise ceramic tubes 90 arranged in 10 rows of4 tube assemblies each arranged in a staggered, triangular matrix array.Each tube assembly is preferably identical to the others and as best seen in Fig. 6, consists of two 42 inch long ceramic heat exchange tubes 90 with a finned outer surface, joined by male to female sockets (105, 106) therebetween and two ceramic end adapter pieces 91 and 92 with a female socket on one end and a male socket piece at the opposite end.

The socket joints between each of the heat exchange units 90 and end pieces 91, 92 are close fitting, spherical surfaces under approximately 750 pounds of compression to prevent leakage of the combustion air passing therethrough. Compression springs preferably located at the male adapter end of the tube assembly maintain essentially constant pressure on the socket joints while permitting differential thermal expansion of the tube assembly in operation. The male adapters can be supported by a metal tube sheet 93, shown in FIG. 6, if desired. The tube assemblies are preferably supported by three vertically extending ceramic tube sheets 140 located at each of the socket joints. The entire tube bundle, including the ceramic tube sheets, is contained within the insulated metal casing structure consisting of the side walls and end walls which form a rectangular frame. Structural ceramic insulating liner sheets 94 (Fig. 2) on either side of the assemblies provide vertical positioning slots 96 for the ceramic tube sheets. Sheets 94 in conjunction with end ceramic insulation 95 provide insulation to protect the metal frame.

FIGS. 3 and 6 illustrate the end mounting of each tube assembly. Inlet metal tube 240 has a surrounding ceramic insulating sleeve 241' and passes into end adapter 91 ending short of male spherical end 241. Compression spring 242 presses against washer 243 to form a seal as will be described. An end of spring 242 is held in place by slotted washer 244 which is in turn held in place on the end wall 80 or 81 by washer 245 bound by nuts 246 on studs 247. An outlet pipe 248 is similarly positioned against axial movement by an identical washer and nut holding arrangement.

The heat exchange units have presented a problem in the art due to the fact that no high temperature operable elongated and extended heat transfer surfaces capable of containing a pressurized fluid without leaking excessively have been previously accepted for widespread use in the art insofar as known to applicants. Each unit 90 (Fig. 6) is a key element in the design of the heat exchanger. All of the units 90 used are preferably identical. These units are in the form of tubes and have a configuration designed to obtain a maximum, overall heat transfer coefficient at minimum pressure drop and loss of the pressurized combustion air to the exhaust gas.The units have a finned external surface and preferably this consists of I and 3/4 fins per inch with the basic unit cross section in the shape of a teardrop 101 with a blunt leading edge 102 and a relatively sharp trailing edge 103 known to be optimum configuration in minimizing the pressure losses of the fluid flowing over the unit. The preferred embodiment has an overall outside diameter at the fins 100 of 3 1/2 inch and an inner through bore diameter of the central bore 104 of I inch. The solid material formed at 102 and 103 between the fins along with the helical configuration of the fins aid in providing mechanical strength to elongated heat exchange units 90.In another preferred embodiment each unit 90 has a wall thickness of 1/2 inch, fin tip thickness of .1 inch, fin root thickness of .32 inches, bore diameter of 1.55 inches, fin height of .62 inches with 2-3/8 fins per inch in a 48-inch long unit.

Each unit has a male end 105 and a female end 106 which act as joining sockets as will be described.

The material of each unit 90 is ceramic in order to accommodate the high temperatures encountered in the gas stream which can be from 1200 F to 2800"F and above yet provide for good heat transfer. Most ceramics have low thermal conductivity with the exception of silicon carbide and silicon nitride which are preferred for use in the ceramic heat exchange units of the present invention.

Other ceramics having a thermal conductivity of at least 2 BTU and preferably 3 BTU/hr/ft2/ F/ft. can be used. Preferably the silicon carbide is used in the form of a casting. For example, commercially avail- able castable silicon carbide such as Carbofrax 11 a product of the Carborundum Company of Niagara Falls, New York, can be mixed with water and cast to a desired shape such as the shape of unit 90. The shape is then fired to temperatures over 1800 to develop strength and good thermal conductivity.

In a specific example of forming a heating unit 90, Carborundum formula 3069-8 (or Carbofrax 11L1) a commerically available form of Carbofrax 11 made up of castable silicon carbide using calcium-aluminate as a binder, is cast into the desired shape in a pliable plastic mold having a wax base mold release. In formulas the silicon carbide powder used comprises 85% by weight silicon carbide, 15% by weight calcium aluminate binder. The amount and type of binder can vary greatly as known in the art. Preferably the silicon carbide material is cast at room temperature and allowed to cure for 12 to 24 hours at room temperature. It is then removed from the mold, preheated for about 4 hours at 200"F and then fired at 2100"F for 4 to 8 hours.The working strength of the resulting product has a 3500 psi modulus of rupture and the thermal conductivity is about 5 BTU/hr/ft2/ F/ft.

In general, mold release substances such as silicone materials, waxes and polytetrafluorethylene substances are used to provide mold release from the molds which are preferably steel or plastic. Positive draught is used in all molding cases. Cores to form the central passageway 104 have applied to them a layer of material that can easily be liquefied, vaporized or otherwise removed such as for example a low melting point wax. Other mold release materials can also be used. The silicon carbide castable material is preferably vigorously vibrated when poured into the mold to assure easy flow and reduction of large air bubbles. Pin or fiber reinforcement may be used in the mix. The fibers add mechanical strength to the final product.

In a second specific example of forming a unit 90 of this invention, castable silicon carbide, having 2% by weight steel pins 1 inch long by .009 inch diameter, as previously described is positioned in a mold for the element 90 using a centrally located steel core with a 0.015 inch thick paraffin wax coating. Drying is accomplished for 12 hours at room temperature and the mold is then removed. The rod or tube forming the core is removed after first raising the temperature above the melting point of the wax. The unit is then preheated for 2 hours at 160 F to complete drying. The casting is then thrust directly into a kiln and fired at 300 to 500"F for 4 hours, removed and then air cooled.

Firing temperatures can be 2150F + 50 for 4 to 8 hours thereafter.

In some cases, the metal pins can adversely interact with the silcon carbide and/or binder and their use is preferably avoided in cases where adverse interreaction occurs.

It should be understood that many variations of the heat exchange units 90 can be used. In all cases, it is desirable to maximize surface area for good heat exchange. Preferably the materials are cast into molds which provide sufficient resiliency to enable castings to be obtained in large sizes and great axial length as for example 30 inches or more. The firing temperatures for the molding material are preferably in the range of from 2000"F to 2400"F or higher.

Preferable the density range of the material is carefully selected to provide substantial mechanical protection while increasing heat transfer over that obtained by conduction through the material. For example, by selecting proper density and porosity and pore size within the material, due to radiational heat transfer through spaces in the material, heat transfer is enhanced at elevated temperatures. Thus, in the preferred embodiments, the ceramic materials preferably have porosity in the range of from 10 to 25% by volume pores with substantially uniform pore sizes ranging from 0.0001 to 0.015 inch with a small number of larger pores.

To optimize the heat exchanger assembly containing units 90, the configuration of the units 90 is preferably arranged so that the external surface area (Ao) multiplied by the external heat transfer coefficient H,, is equal to the internal surface area Al, times the internal heat transfer coefficient H,. The unit 90 design is such that A1 or the internal surface area may be adjusted to accommodate the state and nature of the fluids contained by the unit 90 and flowing over the external surface of the unit 90.The range of A0/A1 would be from 2 to 8 in practice and the range of H0/H1 from 0.03 to 8 so that the possible range of AoHo/A,H, would be from 0.06 to 64 whereas ideally the ratio of AoHo/A,H, would be unity.

In certain uses the internal bore 104 can have an enhanced surface as by fin formation to increase exposure to the gas passed internally thereof and enhance heat transfer.

When internal fins are used there is a substantially unobstructed cross section from end to end of each tube to, enable ease of cleaning and ease of removal from a core about which the tubes are cast. For example, internal fins can extend in parallel relation to each other parallel with the elongated axis of each tube, and may be interrupted along their length to "break up" the internal fluid boundary layer and further enhance the internal heat transfer coefficient of the unit 90.

Preferably the heat exchange units have internal diameters of from 1/2 inch to 3 inches and more preferably 1 inch to 1 1/2 inches. Fin heights of from 3/4 to 1/2 inch are preferably used with unit wall thickness of 1/4 inch to 2/3 inch. Preferably 1--1/2 to 3 fins per inch at heights of 1/2 to 3 inches are used. Sloping sided trapezoidal cross section fins are preferably used. The fins 100 preferably vary from .075 to .15 inch thickness at the tip to .25 to .6 at the root. The root thickness to height ratio of the fins is preferably .4 to 0.6. The fins 100 can be single helically or double helically arranged.

Other configurations can also be used. The provision of the fins not only adds mechanical strength, but also when aerodynamically designed, provides optimum heat transfer with minimized pressure drop of the gas flowing over the surface of the tube in the direction of arrow 201.

Preferably all wall thickness forming the heat exchange units, aside from the tapered fins are equal in order to allow uniform drying during the curing and firing of the units.

Where there is a pressure difference between the inside and outside of from 1 to 10 atmospheres or as high as 350 atmospheres, it is often desirable to seal the units not only at joints as will be described, but also through the body sections of the units. Thus, glazes or seal coatings of various sorts can be used. In all cases the glaze or seal coating that is used to prevent leakage through the heat exchange unit walls is compatible with the materials used in the fabrication of units 90 and the intended operating temperature range of the units. Preferably the seal coating used is formed with a thickness of from .001 to .050 inch and has a coefficient of expansion matched to that of the material of the heat exchange units.For example when the heat exchange tube has a coefficient of thermal expansion of 2.4 10-6 in/in/ F as a mean over a range of from 75"F to 2200 F, the seal coating preferably has a coefficient of thermal expansion which is + 10% of the coefficient of the tube body material. Preferably, in all cases the glaze does not delaminate after 10,000 temperature cycles of from 75"F to 2300"F where the glaze temperature substantially coincides with the tube body material temperature.

Sealing can be accomplished by many methods as for example glazing, sintering, or vapor deposition.

In one specific method, a mixture of water and a glass frit is applied to the units 90 by spreading the mixture thereover and then drying and heating to a temperature just above the melting point of the frit which may be for example 1800 F.

In another method, zirconium oxide can be used as a sealing material by spreading a coating of 20 parts by weight zirconium oxide with 3 parts by weight of water glass over a silicon carbide unit formed as described above and then heating to 2400"F for 12 hours.

Conventional vapor deposition methods can be used where seal coatings are deposited from various materials such as tin oxide, metal carbonyls B4C, WC, Si3N4, A1N, TiO2, A1203, SiC and the like.

In some cases, sealing is unnecessary as in low pressure operations such as in the slot furnace application of the invention where the pressure difference between the inside and outside is typically from 1/2 to 1-1/2 psi.

The sealing is preferably carried out to seal the heat exchange units so that they can maintain a pressure difference of from 1 to 350 atmospheres at room temperature between the tube inside and the outside gas pressure with a leakage through the tube walls corresponding to from .0005 to .002 darcys. The leakage in the tube wall before sealing the tube is preferably in the range of from .03 to 0.05 darcys. The sealing of the internal bore 104 is particularly desirable when the heat exchange tubes are used in gas turbines where large pressure differences between inside and outside pressures are commonly encountered.

As will be seen from FIG. 6, two types of joint seals are necessary to contain the cold air taken in from the atmosphere within the units 90 and tube assemblies. The first seal is the male to female socket seals as between the end pieces 91 or 92 and the units 90 and between adjacent units 90. The seal is accomplished by the spring loading as previously described which urges the male and female sockets toward each other.

It is clear that the ceramic tubes are not flexible because of the nature of the material and they cannot be welded not should they be permanently joined in any way since they are exposed to some vibration and some degree of flexibility is desired. Moreover, any attempt to seal the units by use of mortar or other permanent joining often fails because of thermal distortions of the assembly.

In the preferred method of sealing the female and male socket joints, the male parts are dipped in molten wax or other transient material that will form a thin coating and can later be removed by heating. A conventional sealing compound is then coated on the adjacent spherical female portion ofthejoint as at 106. The sealing compound can be water glass, silicates or other materials which are resistant to the temperatures encountered in use. The tubes are then assembled with the sockets in engaging relationship while the seal material is still moldable to assure good conformance of the male end to the female end in each socket. The assembled units can then be heated to set the sealable material such as the water glass. Additional heating can be carried out to remove the wax or other material by melting or vaporizing.This leaves the joint free to flex since the male and female portions now closely conform in surface configuration to each other yet are not rigidly joined together. A slight degree of flexing can occur, yet, a good gas seal is provided. It should be understood that air is passed through the central bore of the heat exchange units under a pressure so that if any leakage occurs, it will be outwardly of the joint to prevent contamination of the combustion air with exhaust gases. When air is fed through the heat exchanger as by a pump (not shown) under a pressure of 2 psig or higher, excellent sealing occurs through the female and male joints.

It should be noted that when internal fins are not provided, the aligned bores 104 of the heat exchange units and end members may be provided with a helically twisted steel or other metal strip 111. The strip 111 can be of any metal preferably being 12 gauge stainless steel which is resistant to heat and yet provides some degree of rigidity. The strip serves a dual function in that it creates a turbulence in the combustion air which is desirable to aid in the transfer of heat from the ceramic tube bore to the air and in addition, should fracture of a heat exchange tube 90 occur, the strip 111 has sufficient rigidity to hole the pieces of the tube in place until corrective measures can be taken by the operator. The strip 111, suitably reduced in width, preferably extends into the end adapters 91 and 92.The units 90 preferably have an axial length of at least 30 inches to minimize the number of units and seals in each assembly.

Heat exchange units are arranged to provide a single pass of the outside exhaust gas thereover in upward passage to the exhaust stack with a double pass of the incoming air through a central bore of the units. Thus a two pass configuration is used on the air side which permits good recuperator effectiveness without resorting to an excessively complex configuration. More passes may be used; however, such two pass configurations as suggested in Fig. 3 are known for use in recuperators of various types. The two pass configurations require the use of an inlet header 119, an inlet series of tubes indicated generally at 240 and suitable interconnection of pairs of units at 248 as shown in Fig. 3 and an outlet header 121 with interconnecting tubes 240' leading to conduit system 42 to pass the combustion air to the burners 15.

Thus, a second type of high temperature resistant seal is required in each heat exchange assembly, that is, a seal between a metal tube such as 240 or 248 and the inlet and outlet adapter pieces 91 and 92. In the preferred embodiment, the seal between the male and female spherical ceramic surfaces 105 and 106 are as previously described.

Tube 240 extends in a loose slip fit through a bore in end piece 91, with the outer diameter of the tube being I inch and the diameter of said bore in 91 being 1.030 inch. A 3-1/2 inch outer diameter steel ring 243 is welded to the tube 240 as at 131. Spring 242 maintains sufficient pressure on the ring 243 to form a seal between it and a flat end surface 132 of the end piece 91. The steel ring 243 can be countersunk at 250 to permit the ring to distort thereby conforming to surface 132 of part 91 assuring a seal under the action of the spring 242. Washer 260 is similarly urged against end surface 261 to provide a seal at the outlet end piece 92.

Other seals can be used between the metal tubes 240, 248 and the ceramic end pieces 91 and 92. For example, at the cold air inlet, high temperature seals need not be used and a number of sealing materials such as silicone rubber may be used.

In addition to the spring mounting of each assembly of heat exchange units, the units are further supported by ceramic tube sheets 140 as best shown in FIG. 4. Each tube sheet 140 can be formed of castable silicon carbide or alumina and has preformed holes 141 though which the assemblies of heat exchange units 90 pass and are supported thereby. Because of the environment in which the process furnaces are used, i.e., vibration due to the operation of forges and/or drop hammers, it is preferred to vibration mount the tube assemblies. This is accomplished by vibration mounting of the tube sheets in conjunction with spring mounting of the ends of the assembly. The tube sheets 140 each have an outwardly extending solid support rod 142 from which they hang in the recuperator.Each rod end 142 is mounted in a vertically extending elongated slot 143 preventing side to side movement while allowing up and down movement. The rod ends each rest on a spring supported stud 144. The stud 144 slides within a bore 145 and a second bore 146 of a flanged end support plate 147. The spring 148 with stop nut 180 provides a flexible action to the support. Damping action is obtained by a friction sliding action as by designing stud 144 with a diameter such that it is in frictional sliding engagement with bore 145 or 146 and by the friction of the tube sheet 140 sliding snugly in the grooves 96 of the recuperator side walls 94.

In extreme cases, conventional automobile shock absorbers can be added or other damping means for improved absorption of the energy of vibration can be used.

When the units 90 are in a staggered array the ceramic wall 94 preferably includes horizontally extending ribs 194 corresponding to discontinuities in the unit arrangement ro prevent the gas from by-passing the units by flowing along the wall of the heat exchanger casing. The wall 94 can be ceramic such as alumina. Materials of higher thermal insulation value with adequate strength are desired.

The recuperator can be used in recuperation of energy from exhaust gases and other gas flows in other environments such as from the exhaust gases of gas turbines and other heat engines. In all cases, ceramic heat exchanger units are of advantage. The units can find use in a variety of recuperator designs as well as in other heat transfer environments and constructions. While shock mounting is preferred, this can be eliminated in certtain embodiments. Conversely, the concept of shock mounting of ceramic heat exchange units can be applied in other environments. Castable silicon carbide or similar material can be used for the adapter pieces 91 and 92 although high temperature resistant material such as alumina not having high heat transfer properties preferably is used.Preferably the ceramic heat insulation throughout is alumina which has high structural strength when structural strength is required, however, other insulation materials may be used when appropriate.

Turning now to the tube sheet nesting means of the present invention and with particular reference to FIGS. 7-10, a tube sheet arrangement is generally indicated at 310 with a plurality of ceramic heat exchange units 90 maintained in position with respect to each other by locking or nesting elements 311. The elements 311 when locked to each other in a nest act as a tube sheet.

Conventional tube sheets are known for use to support heat exchanger tubes mounted in parallel in heat exchangers in the path of fluid flow to effect heat transfer to or from the fluid on the inside of the tubes.

When using ceramic heat exchange units of the type shown lateral support must be provided by some sort of tube sheet or nesting means for the units. When integral tube sheets are used the heat exchanger units must be assembled axially passing through the holes 141 in the tube sheets. The assembly and disassembly procedure is time consuming and therefore costly. The nesting element 311 is useful to replace integral tube sheets 140 enabling lateral assembly and disassembly of the units in a heat exchanger assembly such as a recuperator or other heat exchanger assembly incorporating a plurality of heat exchange units.

The nesting means is preferably formed of individual elements used about each unit 90 to provide a locking nest that can be easily assembled or disassembled. Each heat exchange tube 90 carries an element 311. The element 311 is a polygonal section that may be extremely short as in the nature of an inch or less or up to several inches long. The element 311 has a first surface defined by a polygonal perimeter 312 and a second surface defined by a polygonal perimeter 313. In the preferred embodiment the polygons are hexagons in a staggered or triangular pitch tube array. When square or in line pitch tube arrays are desired the polygons are preferably square or octagonal.The perimeter 312 is formed as if twisted at from 7 to 30 degrees between surfaces 312 and 313 so as to provide locking or nesting surfaces 314 which nest and interlock with like surfaces when a plurality of units are axially aligned as shown in Figs. 7 and 9. The polygonal surfaces prevent sliding in the planes of the polygonal perimeters since they can be mounted between walls 315, 316 of a recuperator or other heat exchanger. The walls are provided on an interior surface with projections 317, 318 and indentations 320, 321 for interlocking purposes as will be obvious from a viewing of FIGS. 7 and 9.

The polygonal surfaces normally prevent movement in the planes of the polygon, or ends 322, 323 of element 311, and also act to retard movement axially of the elements and units due to the twist in the elements. In effect, the elements 311 interlock with each other to form a supporting nest. The tubes 90 can be provided with reduced diameter portions 331 at each end to provide a mounting for the elements 311. The width of the elements 311 and the surfaces 322, 323 can act to prevent axial movement of the tubes 90 since the surfaces can abut adjacent shoulders of the tubes if desired. Alternatively, the end piece mounting of the tubes can provide axial stability.

These elements are preferably formed of a high temperature material and in the case of mounting for ceramic heat exchange units, are formed of similar ceramics as for example silicon carbide.

The elements are placed in position by merely aligning them as shown in FIG. 7 and twisting them individually into the position shown. The first row of units may be supported from the bottom until locked in position as shown by the lowermost elements in FIG. 7. The twisting movement in effect screws or unscrews polygonal-shaped elements from each other and provides positive locking yet easy assembly and disassembly.

The assembled elements 311 form a tube sheet which supports the units and can act as a guide wall directing the fluid flowing outside the units. The size and dimensions of elements 311 can be varied to predetermine the spacing of the heat exchange units. Two or more units 90 can be aligned in rows by elements 311 formed substantially in two parallel planes. The ends of each row of units can be mounted by conventional end pieces and assemblies of the type shown in Fig. 6.

In the preferred embodiment, element 311 has an axial length of 2 inches with a twist of 30 degrees between the two ends of the hexagon which have a maximum diameter of 4.94 inches to provide for a unit center to center distance of 4.5 inches. Silicon carbide is used and is the same material as the heat exchange units 90.

In the preferred embodiment silicon carbide positioning rings or discs such as 330 are used in conjunction with element 311. These rings slip over the male end onto a reduced diameter portion 331 to give greater axial length to elements 311. The rings can have an outer circular diameter smaller than the smallest diameter of elements 311 or can have their outer rims corresponding to the polygonal outer rims of the elements 311.

While hexagons have been used in the preferred embodiment, it will be obvious to those skilled in the art that other polygonally-shaped elements having twists between two polygonal defined faces can be used to form in-line and other heat exchange unit arrangements in heat exchanger assemblies.

In some cases, the nesting elements 311 can be keyed at their inner diameters 332 with corresponding keys formed in the heat exchange units to positively prevent turning of the units within the nesting elements as suggested by the dotted outline 400 in FIG.

7. Similarly the inner diameters of the nesting elements need only correspond substantially to the tube and need not be round as shown but can be other shapes. In some cases, the nesting elements can be formed directly on the heat exchange tubes and be an integral cast part thereof. The number of nesting elements used for each tube can vary and in some cases two or more elements can be used on each tube length.

While it is preferred that the elements be formed of substantially the same material as the heat exchange tubes, this is not always required. In some cases other materials can be used which provide suitable mechanical support and heat insulating or conducting properties as needed.

In some cases, the nesting elements interlock with each other to provide when nested with other corresponding elements, support for at least two tubes arranged across a space to prevent dropping of the tubes and preferably provide against axial movement of the tubes by the twist formed surfaces 314.

Moreover, in all cases, it is preferred that the locking elements enable removal of individual units as may be required without necessarily removing all units held by a group of elements forming a functional tube sheet.

While many specific embodiments have been described above, it should be understood that many variations are possible. It is preferred that the porosity of each silicon carbide unit is in the range of from 10 to 25% by volume (more preferably about 15%) with the pores preferably being substantially uniform and having a diameter of approximately from 0.0001 to about 0.015 inch.

Some small number of larger pores up to 0.1 exist in the material but are in small enough number as to not affect the structural strength and heat transmission properties.

These values are preferred for optimum thermal shock resistance and provision of suitable crack stoppers at the pores. The porosity may also aid in reducing carbonaceous and other material deposits on the units in those applications where no seal coating is used because of the outward leakage of the fluid flowing in the units. The overall silicon carbide material when used preferably has a thermal shock resistance such that it will withstand a 250"F per minute change at a temperature of from room temperature to at least 2350"F for at least 2000 temperature cycles.

WHAT WE CLAIM IS: 1. A heat transfer apparatus comprising a plurality of generally parallel, ceramic, heat exchange tubes mounted to provide heat transfer betwen fluid passing within said tubes and fluid passing over an exterior surface of said tubes, each said tube being formed of a cast ceramic material, support means being provided for mounting said heat exchange tubes in said heat transfer apparatus, said support means comprising on each said tube an encircling body element, corresponding body elements of the tubes cooperating to define an end wall, each body element having a first polygonal perimeter formed by an end surface of said element and a second polygonal perimeter defined by a second end surface of said element, said perimeters being angularly displaced with respect to each other to form a threedimensional surface, the three dimensional surfaces of co-operating body elements being in interlocking cooperation.

2. A heat transfer apparatus as claimed in claim 1, wherein said element is formed of a ceramic.

3. A heat transfer apparatus as claimed in claim 2, wherein said polygonal perimeters are hexagons.

4. A heat transfer apparatus as claimed in claim 1, 2 or 3, wherein said element is integrally formed with a cast ceramic heat exchange tube.

5. A heat transfer apparatus as claimed in any one of claims 1 to 4, comprising a plurality of said elements each supporting a heat exchange tube, and side walls defining interlocking means for supporting sides of an adjacent horizontally arranged row of elements so that said elements can act as a tube sheet for supporting individual heat transfer tubes.

6. A heat transfer apparatus as claimed in claim 5, wherein said elements are formed of a cast ceramic material having a thermal coefficient of expansion closely matched to the thermal coefficient of expansion of said heat exchange tubes.

7. A heat transfer apparatus as claimed in claim 6, comprising a plurality of said elements arranged in side by side interlocking nested relationship with each element supporting a heat exchange tube in a heat exchange assembly, the plurality of elements acting as a tube sheet.

8. A heat transfer apparatus as claimed in any one of claims 1 to 7, wherein the heat exchange tubes define a seal coating for preventing gas passage through the body of each tube so that gas leakage through said body to the external surface thereof is in the range of from 0.0005 to 0.002 darcys.

9. A heat transfer apparatus as claimed in claim 8, wherein said ceramic material has a porosity of from 10 to 25% by volume with pores in said material having diameters of from 0.0001 inch to 0.015 inch diameter.

10. A heat transfer apparatus as claimed in claim 9, wherein said body has a thermal shock resistance such that it is resistant to damage at thermal cycling of from 250"F per minute for at least 2000 cycles within a temperature range of from ambient to 2350'F.

11. A heat transfer apparatus as claimed in claim 1 and substantially as hereinbefore described with reference to the Figs. 7 to 10 of the accompanying drawings.

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

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. preferred that the porosity of each silicon carbide unit is in the range of from 10 to 25% by volume (more preferably about 15%) with the pores preferably being substantially uniform and having a diameter of approximately from 0.0001 to about 0.015 inch. Some small number of larger pores up to 0.1 exist in the material but are in small enough number as to not affect the structural strength and heat transmission properties. These values are preferred for optimum thermal shock resistance and provision of suitable crack stoppers at the pores. The porosity may also aid in reducing carbonaceous and other material deposits on the units in those applications where no seal coating is used because of the outward leakage of the fluid flowing in the units. The overall silicon carbide material when used preferably has a thermal shock resistance such that it will withstand a 250"F per minute change at a temperature of from room temperature to at least 2350"F for at least 2000 temperature cycles. WHAT WE CLAIM IS:

1. A heat transfer apparatus comprising a plurality of generally parallel, ceramic, heat exchange tubes mounted to provide heat transfer betwen fluid passing within said tubes and fluid passing over an exterior surface of said tubes, each said tube being formed of a cast ceramic material, support means being provided for mounting said heat exchange tubes in said heat transfer apparatus, said support means comprising on each said tube an encircling body element, corresponding body elements of the tubes cooperating to define an end wall, each body element having a first polygonal perimeter formed by an end surface of said element and a second polygonal perimeter defined by a second end surface of said element, said perimeters being angularly displaced with respect to each other to form a threedimensional surface, the three dimensional surfaces of co-operating body elements being in interlocking cooperation.

2. A heat transfer apparatus as claimed in claim 1, wherein said element is formed of a ceramic.

3. A heat transfer apparatus as claimed in claim 2, wherein said polygonal perimeters are hexagons.

4. A heat transfer apparatus as claimed in claim 1, 2 or 3, wherein said element is integrally formed with a cast ceramic heat exchange tube.

5. A heat transfer apparatus as claimed in any one of claims 1 to 4, comprising a plurality of said elements each supporting a heat exchange tube, and side walls defining interlocking means for supporting sides of an adjacent horizontally arranged row of elements so that said elements can act as a tube sheet for supporting individual heat transfer tubes.

6. A heat transfer apparatus as claimed in claim 5, wherein said elements are formed of a cast ceramic material having a thermal coefficient of expansion closely matched to the thermal coefficient of expansion of said heat exchange tubes.

7. A heat transfer apparatus as claimed in claim 6, comprising a plurality of said elements arranged in side by side interlocking nested relationship with each element supporting a heat exchange tube in a heat exchange assembly, the plurality of elements acting as a tube sheet.

8. A heat transfer apparatus as claimed in any one of claims 1 to 7, wherein the heat exchange tubes define a seal coating for preventing gas passage through the body of each tube so that gas leakage through said body to the external surface thereof is in the range of from 0.0005 to 0.002 darcys.

9. A heat transfer apparatus as claimed in claim 8, wherein said ceramic material has a porosity of from 10 to 25% by volume with pores in said material having diameters of from 0.0001 inch to 0.015 inch diameter.

10. A heat transfer apparatus as claimed in claim 9, wherein said body has a thermal shock resistance such that it is resistant to damage at thermal cycling of from 250"F per minute for at least 2000 cycles within a temperature range of from ambient to 2350'F.

11. A heat transfer apparatus as claimed in claim 1 and substantially as hereinbefore described with reference to the Figs. 7 to 10 of the accompanying drawings.