CA1042418A - Heat transfer structure - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Disclosed is a heat exchange system for transferring heat between first and second fluid media through a thermally conductive structure which incorporates a passageway for the first medium and a plurality of flow paths for the second medium. The thermally conductive structure surrounds a central plenum which has a burner for producing combustion. The paths are defined between a plurality of layers of bodies which are bonded together and to the portions of the structure forming the passageway in regions of contact there-with forming a matrix and whose surfaces, which form the elemental surface areas of the flow paths, are predominantly convexly curved in all directions.
The average length of the flow paths is not greater than 15 times the average radius of curvature of the elemental surface areas.

Description

1~342~18 This is a divisional application of application serial No. 038383 filed on December 27, 1968.
The efficient transfer of lleat as well as the efficient and economi-cal conversion of the~nal energy between flowing fluids and media to be heated or cooled is highly desirous in the present day art The areas of interest reside in the home, for example, in cooking and heating, and in industry in numerous industrial processes such as condensation~ distillation and heating.
In the heat transfer art the completeness of extraction of thermal energy between a heated flowing fluid and another medium ;s the parameter of primary concern. In normal fuel burners, for example, with limited heat transfer area the exhaust temperatures which may be a few hundred degrees indicate that a considerable amount of available heat in the fuel is not utilized and is transported through the chimney flue. Efficiencies of between 50 percent to 60 percent are, therefore, quite conventional in present day thermal energy conversion devices.
Increasing the transfer area between the flowing fluid medium to be heated in applicable devices through baffles, plates, tinsel or other obstruc-tions has not met with marked success in improvement of heat transfer efficien-cies. An expression often utilized in the art to describe the heat transfer characteristics is "power density" which denotes the thermal energy per unit of time flowing through a unit of area of a body to be heated. Prior art devices have normally observed power densities in the order of 100 watts per square inch of transfer area. This indicates that with the numerous high thermal energy sources available such as, for example, a direck flame having a 7 kilowatt output capabillty higher efficiencies will be realized if the power density characteristic of the transfer structure can be suitably en-hanced. New and novel structures to achieve much higher efficiencies with power densities 10 to 100 times that normally achieved in the transfer of ~hermal energy will be described in accordance with the teachings of the -- 1 -- .
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present invention.
A compact structure for rapid transfer of thermal energy and vast improvement in the power density factor is provided by the arrangement of a plurality of thermal conducting bodies in a bonded porous barrier matrix. The interstices between the contiguous surfaces of the bodies in the matrix define a tortuous path for a fluid heating or cooling medium. A heat transfer inter-face surface arranged adjacent to the barrier matrix provides for the passage of a second medium at a higher or lower temperature differential relative to the fluid medium within the barrier matrix structure. The porosity and dens-ity of the barrier matrix composed of individual thermally conductive membersis of a predetermined design parameter to provide f'or efficient heat transfer betweenthe media. In accordance with this invention, an optimum requirement for the depth and porosity of the barrier matrix is that the average size of the thermal conducting bodies be substantially the size which will produce an optically dense path in substantially the shortest distance along a passageway or restricted path for a flowing fluid. For the purposes of the description of the invention the term "optically dense" is defined as relating to the packing of the individual thermal conductive bodies in such a manner that a beam of light directed through the resultant structure will not be directly visible but small traces of light~ay be noted in the interstices between the individual bodies because of internal reflections and light scatter. The heat transfer barrier matrix may be provided by joining together the thermally conductive members through conventional brazing, sintering or soldering tech-niques by coating the individual members with suitable materials having characteristics for such metallurgical processes.
Another term useful in the understanding of the present invention and description of the parameters of the individual thermal conducting bodies and maximum heat flow paths is the"characteristic dimension." This term shall be interpreted to denote the distance between adjacent transfer interface . -.... . :
.. .
.. . .
. ' , . ~' ,' :' boundaries of a passageway occupied by the optically dense barrier matrix throllgh which one of the fluid media flows. ~n a circular configuration with an internally contained matrix st mcture the characteristic dimension will be the diameter of the passageway containing the fluid medium means. In the flat or planar configuration having spaced parallel thermally conductive interface boundaries ~rith the barrier matrix structure disposed therebetween the term shall denote the distance between the parallel boundary means. In configura-tions providing fluid medium circulating means embedded in an external barrier matrix configuration the term shall define the distance between adJacent inter-face conduit means. If circular conduits are involved then the distance maybe derived by averaging the separation dimensions at preselected points.
Numerous embodiments of the present invention will be described in-cluding coiled fluid passage means embedded within an optically dense barrier matrix. Such a structure will provide an efficient domestic hot water source and may be advantageously disposed at any desired utilization point. Another embodiment of the invention incorporates the disposition of thermal conductive bodies within as well as surrounding the medium conducting path to accommodate heat power densities as high as lOgOOO watts per square inch for applications such as, for example, boilers for furnaces. The high efficiencies realized with the disclosed embodiments will result in substantial reductions in space and cost of heat transfer modules.
Thus, in accordance with the invention, there is provided a heat exchange system, comprising a thermally conductive structure in structural and heat exchange association with a conduit for a first fluid and a plurality of passages for directing the flow of a second and hotter fluid through the thermally conductive structure, thereby to heat said first fluid, the therm-ally conductive structure surrounding a central plenum, the conduit comprising a plurality of elongated conduit portions spaced around the central plenum and the thermally conductive structure rigidly interconnecting at least some of :... : . . ..

the conduit portions~ the length ot` said passages being less than twice the average spacing be-tween said conduit portions, a burner supported outside said plen~ for producing combustion within said plenum, a blower connected to the burner and arranged for supplying a mixture of fuel and air thereto, and a pressure regulator for supplying said fuel in gaseous form to the input of said blower.
The invention, as well as the specific illustrative embodiments, will now be described, reference being directed to the accompanying drawings in which:
Figure 1 is a vertical cross-sectional view of an illustrative embodiment of the invention for heating of a circulating liquid by an external matrix structure;
Figure 2 is an enlarged fragmentary view of a portion of the exter-nal matrix included within the line 2-2 in Figure l;
Figure 3 is a sectional view taken along the line 3-3 in Figure 1 viewed in the direction of the arrows;
Figure 4 is a diagrammatic representation cf the principal optimum parameters of the illustrative embodiment;
Figure 5 is a diagrammatic representation of a flat planar heat transfer configuration;
Figure 6 is a diagrammatic representation of the embedded external barrier matrix structure illustrative of one of the conceptual configurations of the present invention;
Figure 7 is a schematic representation of a complete system utilizing the illustrative heat transfer module illustrated in Figures 1 through 3 inclu~ive;
Figure 8 is a vertical sectional view of an alternative embodiment of the present invention;
Figure 9 is a horizontal sectional view taken along the line 9-9 in , . .
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Fi~lre 8) ~ lgnre 10 is a vertical sectional view of another alternative embodiment of the invention to provide high power density parameters;
Figure 11 is a vertical sectional view along the line 11-11 in Figure 10; and Figure 12 is an exploded fragmentary partially sectioned view illustrative of an alternative embodiment of the invention.
In the drawings, Figures 1, 2, and 3 illustrate a preferred embodiment of the invention. Before proceeding to the detailed description, however, it will be of assistace to refer to ~ , . . . . . . . . . .............................. : .

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~L~)4~418 Figurcs 4, 5 and 6 and a dcscription of the important conceptual aspec~s of the invention.
A heat transfer arrangement which provides for a high thermal transfer rate utili~ing a structure to provide optimum power density along a heat flow path is illustrated in Figure 4.
Thermally conductive bodies are metallurgically bonded along contiguous surfaces to define an optically dense barrier matrix 11 along a fluid flow path. The interstices between the bodies de~ine a tortuous heat transfer path. Spherical members, such as shot or ball bearingsJ have been shown although similar results are attainable with other similaTly oriented members th~ configuration and dimensions of whlch meet the critical parameters required by the ~n~ention. Suitable thermally conductive materials include copper, brass, stainless steol, carbon steel, aluminum, as well as any of the plastic materials embedded with metallic particles. Each of the ferTous or copper body members may be coated with a copper-silver eutectic solder and the uverall matrix structure may be - conglomerated by any of the well known processes including brazing, sintering or welding. A dip brazing technique is required for aluminum conductive members.
Ach~evement of the maximum average size for the body members to secure an optically dense matrix in the shortest distancs possible along the direction cf fluid flow is one of the design criteria to be followed in the practice of the invention. A flowing fluid along a path indicated by arrow 13 will encounter the optically dense structure which is joined to a surface of a conducting boundary interface 12. Through the provision of a number of body members 10 arranged to provide tortuousheat transfer paths in the optically dense barrier matrix the total overall efficiency of th~ heat trans-fer device is enhanced. The pa~h of the thermal energy flow from the , , . .: . , ;' ' '" '' ' '' . . . .
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flowlng fluid through thc matrix 11 and interface surface 12 is indicated by th~ arrow 14 to result in transfer to the media contacting the opposing sur-face 15.
Another criteria required for the provision of an ef~icient heat transfer structure in the shortest distance possible along the heat flow path conce~ns the number of bonded joints or junctions in any direction from the point of thermal contact along a heat path to the nearest adjacent conducting interface. Referring to Figure 5, a matrix structure 16 is shown disposed between spaced interface boundary surfaces 17 and 18. Such surfaces may be provided between the walls within a conduit or between the outer walls of spaced conduits as will hereinafter be described. The fluid flow path is indicated by the arrow l9. It has been discovered that optimu~ results will be obtained ~ith an optically dense arrangement when the number of contiguous bonded joints in a desired heat path direction from the point of contact to th~ nearest adjacent interface surface is in the order of two such brazed joints. The barrier matrix structure disclosed herein is of the internally mounted configuration and may be practicedin circular or rectangular conduits as well as between flat planar plates.
The Temaining criteria hereinbefore defined is the characteristic transrerse dimension shown in Figure 5, and designated by the arrow C.D. as the dlstance between the parallel boundary surfaces 17 and 18. To achieve t~e hi~hest heat transfer rates with an optically dense barrier structure the heat path to the nearest boundary interface surface for aflowing fluid directed along path 19 may be depicted by perpendicularly directed arrows 20 and 21. The maximum length of the heat path through the matrix ~rom the ~luid to the nearest boundary interface then may be defined as one-half of the characteristic transverse dimension of the device. For barrier struFtures employing discrete bodies the present invention discloses that the average size of each of the ,.;.; . - . . ,., . . . :
... ~ . . . . ;

:lV~ 8 bodies shall preferably bo ~pproximatcly o~e-third of the character~s-tic dimension of the device. Bodies of subs~antially lar~er dimen-sions, for example, above on~-half of the characteristic dimension, would not collectively define a sufficiently optically dense arrange-~ent. In fact, such a device would ble highly inefficicnt in the transfer of incremental quanta of thermal energy. At the other oxtreme of the range, thermally conductive bodies of smaller diameters b~low one_sixth of the characteristic dimension violate the number of brazed joints requirement and thereby lower the thermal conductivity eficiency of the heat transfer tevice.
Figure 6 illustrates an embodiment of the invention wherein the spaced conduit means for directing the flow of a ~luid are embedded in the barrier matrix and a seeond flowing fluid is directed ~n the region between ~he conduit means as indicated by arrow 22.
This confi~uration is referred ~o as the external type and again *he design criteria of the number of bonded joints as well as optical density of the ~atrix are applicable. A circular conduit 23 which may comp~ise a linea~ array of parallel members or a ~elical coil is encased in the barrier matrix 24 of thermally conductive bodies fabricated in accordance with the invention.
The characteris.tic dimension of this configuration is calculated between the conduit wall surfaces and is derived by averaging the dimension A as well as the dimension B which represents ~he furthermost spacing between the conduit means. Thermal energy directed along path 22 will ~raverse heat paths indicated by - ~rrows 25 and 26 to the adjacent conduit ;~alls. Again, as in the example shown in Figure 5, the heat path maximum is desirably one-half the characteristic dimension or average distance between the spaced conduit walls. The number of brazed joints are in the order of two from point of impact and the average size of ~he '' :
;' ~ ' ' ' bodies will ~c bctwc~ll on~-halr to onc sixth of the ch~rackeristic d~.nlcnsion for thc reguisit~ opt~c~1 density. In such cascs with the one-hRlf body dimension an average Or one brazed ~oint will inh~rently result and in tho case of the one-sixth dim~nsion an a~era~e or a~out three brazed joints will lnherently result.
The hi~h heat transfer rate or increased pOWer density achi~ved by the invention is believed to be attributable to the large number of surfaces provided by the conducti~e area o~ each of the matrix body members, turbulent ~luid ~low and the ~ery short heat flow path through the matri~ from the fluid to the interface surface. Relatively high power densities are attainable in embodiments o~ the invention to be hereinafter described and may be as high ~s 10,000 watts per square inch of the area of the face of the m~trix body initially im~inged by the flame. Compared with embodiments of conventional prior art structures capable of handling power densities of only 100 watts per area per unit o~ time it ~s apparent that an improvement of several orders of magnitude have resulted. A use~ul equation in th~ determination of the design criteria incorporating the teachings of the invention are as ~ollo-~s:
(1) Heat path (L) = (t~ dr~(condUcti~ity of ~ater~a~

~0 Ihe term "heat flux" referq to input thermal energy and may be expressed in terms o~ British thermal units per hour per square feet o~ ~all area of ~he ~nterface boundary through ~Jhich the heat is trans~erred. Since the power dens~ty on the face o~ the matrix is trans~erred to the wall area, the heat flux ~riIl be approxi~ately one-tenth of the above 10,000 watts or 1,00~ watts.
$he~mal conducti~ity of the material is a constant value and is readily determined from tables for that purpose. This term indicates the quantity Or heat that 1ill flow aFross a unit area of the body heated if the temper-ature ~radient is unity. As stated previously, the heat path then re-presents one-half Or the characteristic dimension. The m.atrix body member 3D dimensions can then be rea~;ly computed from this value Or characteristic :~0~43L~3 dimcnsion. Tho a~plication of this equation will be de~ans~rated hereinafter in relation to one of the described embodiments.
In Figures 1, 2 and 3 a highly efficient and practical embodiment of the present invention is illustrated and will now be described. ~elical conduit 30 is embedded in and surrounded by an external sintered barrier matrix 31. Ihe matrix 31 is composed of discrete thermally conductive bodies to provide for the optical density in accordance with the teachings of thc i~vention as have hereinbefore been enumerated. The thermal conductivity, pressure drop limits and power density will determine the piteh, diame~er snd overall Iength of the conduit to arrive at the maximum allow-~ble heat path and this in turn will determine the charactesistic dimensions. The matrix design criteria a~e then determined from the characteristic dimension value. An inlet 32 and outlet 33 are, respectively, connected to the water source and egress means for the util~ization cf the fluid ~edium. The embedding of the ~ondui~ in the barrier matrix can be achieved by positioning the helical conduit 30 within a cylindrical space defined by two con-centrically disposed tubular jig ~embers of a material which will not bond to the body members whose dimensions are related to the cha~acteristic, dimension Yalue. The jig members have differen~ -diameters and the circular spa~e between these members may be filled ~ith the individual body members. Shaking and ~ibrating of the over-all assembly will provide for the desired array of the bodies around each of the conduit turns. The entire assembly is then metal-~rgically treated at the requisite temperature and the jig members may be separated. The combined externa~ barrier matrix structure and conduit is then assembled in the embodiment and a central combustion chamber 38 is defined by the disclosed hea~ transfer ma~rix.

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t. ' , ' ' ' ~ , ~ '........ ` ' ' . " ': , , ~4i~8 Typically, a b-lrner platc member 34 may be provided with a plurality of passagew~ys 35 for the admittance of an air-gas ~ixture under pressure from a source coupled to conduit 36 and fitting 37 into the combustion chamber 38. Angularly and laterally disposed within the burner plate mem'ber 34 is an ignition means 40 of any well known construction such ias a spark plug to provide the ~ecessary ignition of ~he gaseous fuel mixture, Outer wall member 41 s~rTounds the heat transfer structure and a flue 4Z for the passage of the exhaust gases extends to a conventional chimney tnot shohn~. Top plate member 43 is suitably secured to the heat transfer structure and conduit such as by nut and bolt means 44J
part of which may also be embedded in the matrix.
In an ex~mplary working embodiment a heat transfer unit as - described in Figures 1 ~hrough 3 inclusive having dimensions of about ive inches in diameter and about five inches in leng~h was utilized to provide a:continuous ho~"water ~low'of approximately three gallons per minute. The burner driving'the heat transfer ', unit and all the electrical controls including a thermostat~ air filter and safety regulating devices were incorporated into a stsucture having a hsight of about six inches, a width of about twelve inches~and an overall len~h of about eighte~n inches.
Such a haat transfer module can replace conventional present day hot water heaters of the storage tank variety having diameters of approximately two feet and heights of approximately six feetO
TSe new improved structure can be very conveniently mounted adjacent to the final utilization point. In view of the exceeding-ly low cost many such devices can also be incorporated with resul-, tant savings in cost of piping and plumbing necessary with present J' day centralized domestic hot water heating systems.
' 30 Referring now to Figure 7, the embodiment of the invention .

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Z4~8 shown in Figurcs 1-3 inclusivo together with the appurtenant structures, is collectively referred to as a heat transfer ~odule tesignated by tlle numeral 50. An air blower Sl is coupled through the fitting 37 IO feed the air and gas mixture into combustion chamber 38, A gas from source 53 which may be any commercially available natural or tank type, is fed through a solenoid control Yalve 54 and regulator 55 to the inlet 52 in blower 51~ Any small si~e blower of the inexpensive varie~y should suffice for most applications. The ven~ 42 extending laterally from ~he hea~
transfer module S0 will provide for the egress of the combustion gases to a convenient outlet. Due to the efficiency of the heat transfer and the fact ~hat the exhaust temperature is exceedingly low a small vent opening in a wall may be used similar ~o ~he type employed in home clothes dryers. No natural draft type chimney is required, which also results in savings in construction costs. The : ~ water supply is indicated by numeral 56 and the heated water medium is fed through line 57 to the outlet tap 58 for instant usage. A
temperature and pressure relief valve 59 may be disposed in line 57. It is thus noted that large storage tanks or boilers utilized in present day hot water generation sources are completely elimina-ted. A compact and unique cource is thus disclosed which may be - seadily installed directly in the area where the use is intended, for example, the bathroom or kitchen.
Associated wiring for the control of the blower as well as the thermosta~ and ignition controls toge~her with the main solenoid ~alve for the gas source have not been specifically described since they are readily commercially available and normal techniques incor porating such means will be fo11Owed.
In Figures 8 and 9 a linear array of fluid conduits 61 is embedded within barrier matrix 62 composed of thermally conductive .

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bodies as havc hcreinbcforc been dcscribcd. Upper plat~ ~cmber 63 supports fluid inlet passagc means 64 and is secured by fastening means 65 to screws 66 embedded in collar member 67. The optically densc matrix structure 62 surrounds the linear conduits 61 which are embedded therein, ~he ends of these conduits and the inlet 64 all communicating with a channel 68 in the inner side of collar member 67. A similar end arrangement is disposed at the opp~site end of the matrix structure including a lower pl~te member 71 and adjacent coilar member 70 and communicates with an inner channel 68a in collar member 70 with which the adjacent ends of conduits 61 also communicate. A fluid outlet means ?2 is supported by ~he low~r collar member 70. Plate member 71 further defines a plurality ~f passages 73 for a gas-air mixture fed into the device through ~ -- conduit 74. The ignition means for the combustible fuel within the chamber 75 is furnished by spark plug member 76 supported by upper :: plate member 63.~
Figures lO and ll are directed to an embodiment for very high ~ower density applica~ions. In 5uch embodiments conduits 77 and 7g are disposed about a common axis. Outer conduit 77 is closed by conductive pla~e means 79 and 80 at opposing ends. Inlet member 81 proYides f~r the ingress of a fluid medium and ou~let member 82 - provides for the egress of the medium in the Yaporized or heated state. The inner conduit 78 is open at the ends for the flow of a heated medium such as the gases from a direct oxygen-gas flame along the inner passage B3 of this conduit, with the direction of , flow being ~ndicated by the arrow 84. An optically dense barrier~atrix structure 85 comprises a plurality of thermally conductive spherical members joined ~ogether to define the thermal transfer paths in accordance with the teachings of the invention. The barri~r struc~ure 85 occupies a major portion of the cross-sectional . , _ 13 -i .,: , .
:~ . .

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arca of the conduit 78 and the characteristic dimension of this member will be the inner diameter of the circ~lar conduit as designated by the arrow 86 and symbol C.D.
A similar barrier matrix 87 occupies the cross-sectional area of the outer conduit 77. With the internal barrier ma~rix structure 85 occupying only a portion of the overall length of the passage 83 for concentration of the heated medium, the heat trans-fer a~ea between the medium in the respective conduits will occur substantially in the region indicated by the bracket 88. This configura~ion ~hen provides for high power density applications.
An example of the application of the previously enumerated equation ~1) in ~he transfer of heat from an intense heat source ~ay be ~oted in the high power density embodiment of Figures 10 and 11. Utili~ing a direct flame source we assume that a power density of 10,000 watts per square inch of flame area is obtained and yet silver brazed copper members may be employed. Addi~ionally, it is assumed that a desired temperature drop of 100 degrees Fahrenheit - is specified. Copper has a thermal conductivity value of about 200 BTU/hour/ft/F. In the final structure we assume that a lower conductiYity will be realized due to the brazed joints and optical tensity of th thermal paths. A conductiYity factor of 50 percent then will provide a reliable design factor. Utilizing the other known values the heat path (L3 is calculated as follows:
L = 100 X lO0 = 2 X lo-2 feet = 1/4 inch ,5 X lO6 Tha characteristic dimension will then be twice the hea~ pa~h value or 1/2 inch. A thermally conductiYe bo~y 2verage size of between ; one-quarter and one-twel~th of an inch therefore is indicated for the required optimum optical density. A one-third value or one-sixth of an inch for thermal body size is preferred in most applica-tions, - 14 _ .

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Xn FlGure 12 anothcr embo(liment iB illu3tratcd. A hclical conduit hnvin~ a plurallty Or turns 50 is c~bedded within ~ matrix 91 Or thc externa]
type. Ir we ~rovide the proper desi~n critcria for the matrix membcrs surround-lne the conduit thc interior pa~sa~es thereor ~y be ~illed with other conduc-tive member~ which need not meet these 52me critical requirements. Hence7 in appl~cations where ~te~m is ~ener~tcd and in condcnsation devices particles such as mesh, wires, shavings, cuttings and the like can be ~mployed, as indi-cated collectively by numeral ~2. Such a confieuration for the obstructions within the conduit will provide ror even wider application of the invention in industry As can be seen ~rom any fieure o~ the drawings, e.g. ~ig. 1, the gas passage length through the sintered matrix of spheres 31 pre~erably does not substan~ially exceed seven layers of balls, or approximately 15 times the - aYerage radius o~ curvature of the passage surfaces, and the average radius oL
curvature of the spheres 31 is substantially less than the radius of curvature o~ the in+.erior sur~ace of conduit 30.
~he advantages of compactness and efficiency of the disclosed.
thermal trans~er device in the provision o~ vastly improved power dens~ties through the optically dense ~atrix structure will now be apparent to those skilled in the art from this description. The deslgn criteria of the number of bonded ~oints alon~ the heat path and the average size of the thermally conductive bodies in relation to th_ characteristic dimension to provide the desired optical density have been carefully enumerated. The foregoin~ dis-cussion and exe~plary application o~ the equation will also assist in the practice of the invention. In addition to the exe~plary embodiments n~nerous other con~i~urations will be evident ~or othcr applications. For example, the thcrmally conductive body members in contact with the outermost wall surfaces o~ circular conduits sh~rn in Figs. 1 and ô may be eliminated, thereb~ exposing thi6 portion o~ th~ boundary interrace conduit walls. The heat trans~er paths w~thin thc ~trix bet~/cen ~he s~aced conduit rnembers would still be defincd in ~ccordance ~ h this invention by the thermally conducti~re bodies in the ~luid flow path.

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A heat exchange system, comprising a thermally conductive structure in structural and heat exchange association with a conduit for a first fluid and a plurality of passages for directing the flow of a second and hotter fluid through the thermally conductive structure, thereby to heat said first fluid, the thermally conductive structure surrounding a central plenum, the conduit comprising a plurality of elongated conduit portions spaced around the central plenum and the thermally conductive structure rigidly interconnecting at least some of the conduit portions, the length of said passages being less than twice the average spacing between said conduit portions, a burner support-ed outside said plenum for producing combustion within said plenum, a blower connected to the burner and arranged for supplying a mixture of fuel and air thereto, and a pressure regulator for supplying said fuel in gaseous form to the input of said blower.

2. A heat exchange system according to claim 1, wherein the thermally conductive structure and said conduit are formed from a plurality of elements which are rigidly bonded together to form a unitary heat conductive structure.

3. A heat exchange system according to claim 2, wherein the said elements consist of a matrix of convexly curved bodies such that the major portion of the total wall area of the said passages is made up of surface areas which are convexly curved in all directions.

4. A heat exchange system according to claim 3, wherein the said surface areas are substantially spherical.

5. A heat exchange system according to claim 3 or 4, wherein the aver-age length of the said passages does not substantially exceed fifteen times the average radius of curvature of the surface areas.

6. A heat exchange system according to claim 2 or 3, wherein the plurality of elements are formed of a first material coated with a second material which melts at a lower temperature than the first material and wherein bonding between adjacent portions of the elements is by fusion of the coating of the second material.

7. A heat exchange system according to claim 3 wherein the points of bonding form areas whose diameters are of the same order of magnitude as the radius curvature of the curved surface areas.

8. A heat exchange system according to claim 2, wherein the said conduit portions consist of parallel tubes extending through the thermally conductive structure parallel to a central axis of the plenum and inter-connected by headers at the ends of the plenum.

9. A heat exchange system, comprising a thermally conductive structure in structural and heat exchange association with a conduit for a first fluid and a plurality of passages for directing the flow of a second and hotter fluid through the thermally conductive structure, thereby to heat said first fluid, the thermally conductive structure surrounding a central plenum, the conduit comprising a plurality of elongated conduit portions spaced around the central plenum and the thermally conductive structure rigidly inter-connecting at least at the ends some of the conduit portions, the length of said passages being less than twice the average spacing between said conduit portions, a burner rigidly supported with respect to said thermally conductive structure and said plenum for producing combustion within said plenum, a blower connected to the burner and arranged for supplying a mixture of fuel and air thereto, and a pressure regulator for supplying said fuel in gaseous form to the input of said blower.