CA1041081A - Heat transfer structure - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A heat transfer structure and system including a matrix of tubes and spheres bonded together to provide a conduit for a first fluid such as water and a plurality of interconnected paths for a second fluid such as hot flue gas. The paths are made up of the spaces between the spheres such that the walls of the paths are portioned of spherical surfaces. The total path length is made less than twenty times the average radius of curvature of the spherical surfaces and the spacing between adjacent tube elements is of the same order of magnitude as the average length of the paths. A water heater or boiler so constructed may be made to transfer substantially all the heat in a flue gas to water in an average path length of one inch or less.

Description

0~0~1 This application is a division of our Canadian application Serial No. 105,018 filed February 10, 1971.
This invention relates to heat exchange structures and systems `and more particularly to structures and systems useful where large tempera-ture differentials exist between the heat source and the heat sink, such as steam generators or water heaters.
; It is well known that many systems for transferring heat from one . .
fluid medium to another have been devised which have a relatively high ef-ficiency, that is a relatively large amount of thermal energy is transfer-.:,,: - ~
red with a relatively low expenditure of power for blowers, pumps, or other similar devices. Some systems have elaborate thin fin structures which are difficult to fabricate. Such fin structures are also subject to melting when exposed directly to hot gases of combustion at velocities encountered in conventional boiler designs.
Conventional boiler designs which do not use fins or other such .:
extended surfaces, are relatively costly and bulky, and the average length of the hot gas passages is at least several feet. Because of the large `surface area required for a given heat transfer in conventional boilers, a large number of header joints are generally required since the length of a water tube at elevated temperatures is limited by structural considerations.
; In accordance with the present invention, a heat exchanger and a heat exchange system are provided which are compact, rugged and highly `
efficient in the transfer of thermal energy from a gas having a temperature above 1000F, such as boiler systems in which the fuel air combustion prod-ucts impinge directly on the heat exchanger surface.
Thus, in accordance with one aspect of the present invention, there ; is provided a heat exchange system comprising: a heat exchanger matrix providing a conduit for a fluid and a plurality of interconnected passages therethrough; the major portion of the total surface area of said passages ,,, -.~ - 1 - ~

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comprising areas which are curved in two directions; and means for directing a gaseous medium through said passages at velocities producing a heat trans-fer between said mediwn and said matrix greater than 100 watts per square inch of the surface area of said conduit contacting said fluid.

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According to another aspect of the invention there is provided a heat exchange system comprising: a plurality of conduits for a fluid spaced around a central plenum with each conduit having a plurality of bodies bonded to the exterior surface thereof; the surface area of said conduits contacting said fluid being substantially less than the surface area of said lo conduits and said bodies with the spaces between said conduits and bodies providing a plurality of passages for a gaseous mediwm outwardly from said plenwm between said conduits, means rigidly interconnecting at least the ends of said conduits; burner means comprising a ported burner wall positioned in said plenwn, and blower means for supplying a gaseous fuel-air mixture to the - interior of said burner at velocities supporting combustion in said plenum between said burner wall and said heat exchanger as a continuous flame front spaced from said wall and extending across the spaces between ports in a wall of said burner through which said gaseous fuel-air mixture is directed ~- by said blower means to produce transfer of thermal energy to said heat exchanger at rates in excess of 50,000 BTU's per square foot of the surface areas of said conduits contacting said fluid.
Specifically, the invention provides for a ma~rix having a plurality of interconnected passages formed by the spaces between a plurality of solid spheres which together with tubular elements are bonded into an integral matrix. The tubular elements provide a conduit through which a fluid such as water is directed. A hot gaseous medium is directed through the inter-connected passages between the spheres. Since the average length of the passages is less than twenty times the average radius of curvature of the surfaces forming the walls of the passages, a heat exchanger may be

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fabricated in which the average length of the passages is less than two inches long and as much as 90~ of the heat generated by the complete com-bustion of a fuel air mixture will be transferred through the matrix into . , .
water in the conduit.
- The transfer of substantially all of the heat of combustion from a gas to a matrix in passages having an average length of less than two inches results in a system where the velocity of the hot gas may be directed through the passages at radically increased velocities with pressure drops along the gas paths similar to those encountered in conventional boiler systems such as for example, a pressure drop of one inch of water. Such ; radial velocity increases over those previously commercially feasible provide for a system in which the heat transfer very substantially exceeds that of - any known commercial heat transfer system. For example, in conventional small boiler designs heat transfer rates of less than 10,000 BTU's per hour per square foot of boiler tubing are usual and heat transfer rates as high as 100 watts per square inch or roughly 50,000 BTU's per hour per square foot, are rarely if ever achieved. This invention, however, provides for heat transfer rates in ;~

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small residential heat ~ ~ Q ~ ~excess of 200,000 BTUIs per hour per square foot, and in large high pressure high temperature embodiments of the inven-tion heat transfer rates on the order of 1,000,000 BTU~s per square foot per hour are commercially feasible.
In addition, the invention provides for the heat transfer to the matrix to be made more efficient, and hence the passages shorter, by provi-ding for substantial variation in the cross sectional area of the passages along the length thereof. The resultant turbulence of a hot gaseous medium flowing through the passages materially reduces the stagnant layer of the medium adjacent the matrix surfaces forming the walls of the passages and increases the hot gas heat transfer.
This invention further discloses that for any given volume filled with a plurality of spheres, the total surface area of the spheres varies inversely with their average diameter. Accordingly, a matrix structure form-ed of spheres and tubes provide a total surface area of the interconnected `
passages which, for a given tube diameter, will vary substantially inversely with the diameter of the spheres filling the space between the tubes. By the use of spheres having an average diameter substantially less than the av-erage diameter of the tubes, a heat exchange structure is provided in which n, , substantially all of the heat of the combustion gases is transferred in a path length not substantially greater than the spacing between adjacent tubes. This spacing is generally not substantially greater than the diameter of the tubes and accordingly the average path length for the hot gas through the matrix should be less than twice the diameter of the tubes or twice the average spacing between the tubes, whichever greater. Any additional path length provides substantially no additional heat transfer between the hot gas and the matrix and increases total pressure drop along the passage , thereby reducing the volume of hot gas passing through the passages for pressure drop acceptable in commercially feasible boiler designs. In com-_ 3 _ '~

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- mercially feasible boilers embodying the invention the passage length is less than twenty times the average radius of the spheres. For bodies having surfaces curved in two directions~ many other than spherical such as egg or pebble shapes, the average length of the passages is less than twenty times the average radius of curvature of those portions of the surfaces of the passage walls which are curved in two directions.
The largest transverse dimension of the region of the passage having the smallest cross-sectional area is made relatively small, that is smaller than the length of the passages so that dimensional stability and structural rigidity is maintained even under conditions of large temperature i differential between the hot gas flowing through the passages and the matrix.
In large high pressure water tube boilers, when the tubes are exposed direct-ly to the hot combustion products, high gas velocity can result in substan-tial vibrations of the tubes and the production noise which can exceed am-bient levels by over 100 decibels. Due to the structural rigidity of this invention such high velocities at elevated temperatures, and hence high per-formance can be achieved without such undesirable vibrations or noise.
An additional feature of this invention is the discovery that this .
structure is extremely stable when subjected to high thermal shock, that is rapid warm-up and cool-down cycles are possible since the multiple conductive ,. .
connections through the matrix rapidly equalize temperature gradients in the solid, and thereby reduce the uneven stresses which could otherwise cause failure in high temperature high pressure boilers.

The heat exchange structure further provides, by reason of the bonded matrix of spheres surrounding tubes, a structure in which for boiler designs the tube diameter may be made smaller for a given length and hence the tube walls thinner while still retaining the same or greater rigidity along the length of the tube structure. With thinner tube walls which, for a given pressure, can result from smaller diameter tubes, a greater heat transfer - 10410i~
rate may be achieved for a given interior wall temperature, which is determin-ed essentially by the water steam mixture passing through the tube, before the outer surface of the tube reaches a temperature at which it loses substantial strength. As a result, heat transfer rates in excess of one million BTU's per hour per square foot of water area are possible with commercially available boiler tube steels at pressures in excess of 500 psi and temperatures in excess of 500 F, whereas commercial high pressure boiler designs are currently limited to between two and three hundred thousand BTUIs per hour per square foot of surface area.
A further advantage of this invention results from the discovery that the matrix will remain substantially free of deposits of combustion products in the hot gas passages even when the size of passages is reduced to that between spheres one-sixth of an inch in diameter, whereas in conventional boiler designs substantially greater dimensions for the hot gas passages are 7, required. The generally turbulent nature of the flow through the matrix which results from the sperical wall surfaces of the gas passages reduces or pre-vents the formation of such deposits even at low velocities corresponding to idling rates of burner design. This feature is particularly advantageous in those commercial boiler applications where a varying load requires a wide range of firing rates.
The invention as well as specific embodiments thereof will now be described~ reference being directed to the accompanying drawings in which:
Figure 1 is a vertical cross sectional view taken along line 1-1 of Figure 2 of the preferred embodiment of the invention for heating a fluid with hot gas;
Figure 2 is a transverse cross sectional view of the embodiment shown in Figure 1 taken along line 2-2 of Figure l; ~ -Figure 3 is a schematic representation of a close cycle heat exchange system utili~ing the heat transfer structure illustrated in Figures ' :

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1 and 2, Figure 4 is a schematic representation of a heat transfer system for heating water utilizing the heat transfer structure illustrated in Figures 1 and 2;
Figure 5 is a schematic representation of a heat transfer system utilizing the heat transfer structure illustrated in Figures 1 and 2 for heating oil or other organic media;
: Figure 6 is a vertical cross sectional view taken along line 6-6 of Figure 7 of a heat transfer structure illustrating a further embodiment ., of the invention;
Figure 7 is a transverse cross sectional view taken along line 7-7 of Figure 6 of the embodiment of the invention illustrated in Figure 6;
. Figure 8 is a vertical cross sectional view taken along line 8-8 .~ of Figure 10 of a heat transfer structure illustrating a further embodiment ,:
of the invention;
. Figure 9 is an enlarged fragmentary view of a portion of the matrix included within the line 9-9 of Figure 8; and Figure 10 is a transverse cross sectional view of the matrix structure illustrated in Figure 8 taken along line 10-10 of Figure 8.
Referring now to Figures 1, 2 and 3, there is shown a preferred embodiment of the invention. A matrix 10 is formed of a plurality of tubes . 11 which are~ for example~ of steel~ approximately one half inch in diameter and six inches long, surrounding a central plen~m region 12. A plurality of : spheres 13 are positioned in the spaces between the tubes 11, and, as shown in this embodiment, are approximately one-sixth of an inch in diameter.
Matrix 10 extends in a direction radial to the axis of the plenum for a distance of approximately four rows of spheres such that the inner row of spheres is approximately tangent to the inner most portions of tubes 11 while the outer row of spheres is posi*ioned beyond a circle tangent to the outer ~.~4~0~3~
most portions of the tubes 11.
The tubes 11 and spheres 13, which may be of any desired thermally conductire material, are as shown herein, commercial grade steel coated with a bonding material such as copper or a copper alloy. The tubes and spheres have been bonded together by heating the elements above the melting tempera-ture of the copper or copper alloy coating in an inert atmosphere to form the unified heat conductive matrix 10. The area of contact of the spheres to each other and to the tubes are enlarged due to capillary action of the molten brazing coating.
; 10 In practice, a copper coating approximately .001 inch thick will produce between two spheres approximately one-sixth of an inch in diameter, a contact surface having a diameter of around .070 inch so that the conductive heat path between the spheres and between the spheres and the tubes is maintained at a low value of impedance to heat flow, both because of the - enlarged contact a~eas and because the copper used at the contact areas has high thermal conductivity.
The tubes 11 constitute a conduit for the flow of water or other fluid to be heated through the matrix. For this purpose an upper header member 14 is provided constituting a plate having holes through which the - 20 upper ends of tubes 11 extend and to which tubes 11 are connected by any desired means such as brazing. Plate 14 also acts to close the upper end of the plenum 12 and a cover dish 15 is sealed to header 14 in the region outside the tubes 11, for example, by brazing. A lower header member 16 consists of an annular plate through which the lower ends of tubes 11 extend and to which they are sealed in a manner similar to the attach~lent to upper header 14. Two semi-annular covers 17 each cover half of the portions of lower header 16 through which the tubes 11 extend, one of the covers having an in-put pipe 18 and the other cover having an output pipe 19 attached thereto.
Extending centrally upwardly from lower header plate 16 into .: . . . ~ , .
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plenum 12 is a burner assembly 22 made up of a cylindrical screen 23 attached, ~ for example, by welding to a lower annular support plate 24 which is removably - attached to loser header plate 16~ for example by bolts 25~ The details of the ; burner assembly are more specifically described in Canadian application 95~184 filed October 8~ 1970~ by W.H. Hapgood and D~Go Protopapas and assigned to the assignee of this application.
In order to increase the combustion capability of the plenum 12 above that normally possible for its size, a screen 26 of high temperature re-fractory metal such as that sold under the trade mark "Kanthol" is positioned ; 10 around the burner screen 23~ In the particular embodiment of the invention : .
; disclosed herein, the diameter of the burner screen is slightly less than one-.
third of the diameter of the plenum and the diameter of the refractory screen 26 is slightly greater than one-half the diameter of the plenum 12~ The screen -~ ~ 26 is attached by rods 27 to the upper header member 13 and the upper end of : .
the screen 26 is closed by a block of refractory material 28 while the lower end of the screen 26 contacts and is pressed against a lower block of refrac-~- tory material 29~
The screen 26~ during operation, becomes incandescent and radiates heat outwardly toward the matrix 10 and inwardly toward the burning flame adjacent the screen 23~ thereby accelerating the combustion process and per-mitting complete combustion of the fuel gas mixture. In the particular design ill~strated having a plenum volume between the screen 23 and the tubes 11 of .. ~
thirty to forty cubic inches, efficient and complete combustion may be achieved at rates of over 180,000 BTU's per hour, which represents a combustion rate of about ten million BTU's per hour per cubic foot of combustion volume.
The air fuel mixture is supplied by a blower illustrated in Figure 3 and removably connected by a tapered flange 47 to the annular burner support plate 24~ for example, by bolts. An ignition device such as a spark plug 40 is screwed into plate 24 and extends into the plenum 12 in the region _ 8 ~

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between screen 26 and matrix 10. The size of screen 26 is sufficiently coarse, for example, six spaces to the inch, that the gas fuel mixture pass-ing through burner screen 23 and refractory screen 26 will ignite and the flame path will travel back through refractory screen 26 to the burner screen 23. The hole diameter and spacing, for example, .027 inch in diameter spaced in an orthogonal pattern with twenty holes per inch, prevent the flame surface from traveling through the burner screen 23.
In Figure 3, the embodiment of the invention shown in Figures 1 and 2 is collectively referred to as a heat transfer structure indicated at 50. A blower 51 is connected through the fitting 47 to feed the air and gas mixture into the heat exchanger where~ upon ignition, it burns to provide a combustion gas having a temperature of several thousand degrees.
A gas from a source 53, which may be a public gas main having a pressure of several inches of water or a bottled gas supply having a pres-... .
sure of several pounds per square inch, is fed through a solenoid controlvalve 54 and regulator 55 to the inlet 52 of the blower 51. The size of blower 51 is such that it will provide a pressure in the combustion chamber of heat exchanger 50 on the order of one inch of water. The hot burned gas passes through the heat exchanger matrix 10 and out through a flue 42.
The exhaust temperature is dependent primarily upon the length of the passages through the matrix 10 and the quantity and temperature of hot gas passed therethrough. In general, the path length is made short enough to provide for an exhaust temperature above the condensation temperature of the exhaust products, for example 300-400F.
A pump 59 pumps water into the heat exchange structure 50 and the outlet which may be water or steam at any desired temperature and pressure, depending on the rate at which pump 59 pumps the water into the heat exchange structure 50 and the rate at which fuel is burned, flows to a load 57 which may be a hot water or steam radiator for commercial or residential heating _ g _ .. . . . . . .

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purposes. Alternatively, the load 57 may be a power generation system such ; as a steam turbine, and condenser.
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- The heat exchanger illustrated in Figures 1 and 2 may be operated at convective heat exchange rates exceeding one million BTU?s per hour per square foot of the liquid surface. While additional blower power is needed to achieve these heat transfer rates, it is comparable to that required in con-ventional convective boiler designs in which heat exchange rates of 10,000-- 20,000 BTUts per square foot of liquid surface are typical.
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~ Heat transfer rates over one million BTUIs per square foot of : ,-liquid surface is possible in practical designs of high pressure boiler for power generators since tube size may be made relatively small~ for example, ~ one inch diameter or less and several feet long with the necessary stiffness ' achieved by adjacent tubes being interconnected with spheres to form an integral matrix in accordance with the invention. In such a matrix, tubes made of inexpensive carbon steel or steel alloys with a wall thickness of one to two tenths of an inch will provide for heat transfer rates on the order of one million BTUts per square foot of liquid surface with the liquid under a pressure in excess of 500 psi and at a temperature in excess of 500F.
Referring now to Figure 4, there is shown a heat exchange system with a heat exchange structure 50 embodying the elements of the invention illustrated in Figures 1 and 2 in which heat is supplied by a blower 51 blow-ing a mixture of air and gas from a supply 53 through a control valve 54 and regulator 55 similar to that discussed in connection with Figure 3. Water from a main 56 is supplied through suitable metering, control and check valves not shown through an inlet tube 45 to the heat exchange structure 50 and . after passing therethrough flows through a hot water pipe 46 to a hot water spigot 58 for instant usage. A temperature and pressure relief valve 59 may be disposed in the line 46.
Referring now to Figure 5, there is shown a heat exchange system : ~o4~013~
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using a heat exchange structure 50 embodying the elements of the invention illustrated in Figures 1 and 2 in which heat is supplied by a blower 51 blow-ing a mixture of air and gas from a supply 53 through a control valve 54 and regulator 55 similar to that discussed in connection with Figure 3. Oil from a vat 61 is circulated through the heat exchange structure 50 by means of a circulating pump 59. Such a device will heat the oil or other organic liquid relatively uniformly without hot spots which would carbonize or decompose the oil. The oil in the vat may be used, for example for cooking, in which case a screen 60 is provided over the intake pipe leading to the pump 59 to prevent material other than oil from entering the heat exchange structure 50. Alternatively, the oil could be heavy oil which requires heat-ing prior to combustion or for use in industrial processes.
Referring now to Figures 6 and 7, there is shown a further embodiment of a heat exchange structure illustrating the invention. A plural-ity of tubes 11 approximately one-half inch in diameter and several inches long are arranged in a circle several inches in diameter surrounding a plenum 12. There are, for example, 24 such tubes and the spacing between the tubes is approximately one-half the diameter of the tubes. A plurality of spheres, 13, approximately one-sixth of an inch in diameter fill the spaces between the tubes. The tubes and spheres are made, for example, of steel with a coat-ing of copper or copper alloy and the entire assembly bonded together as dis-closed in connection with Figures 1 and 2 to produce a matrix 10. The ends of the tubes 11 extend through upper and lower header plates 14 and 16 respec-tively, and the tubes are connected together in two series by upper header cover members 15 and lower header cover members 17 respectively. The unconnec-ted ends of the tubes 11 form entrance and outlet connections 18 and 19 respectively for the liquid.
Burner assembly 22 consists of a burner screen 23 extending into the interior of the space defined by the matrix and supported by plate 24 ",. . ~ .

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attached to the lower header plate 16 by bolts 25, screen 23 being attached by welding or otherwise to the plate 24. The diameter of the screen 23, as shown in this embodiment, is slightly greater than one-third of the interior diameter of the space formed by the matrix and is supplied with an air fuel mixture, for example through the blower 51 illustrated in Figure 3. Refractory ; material blocks 28 and 29 are attached to the upper header plate 14 and the burner supported plate 24 respectively.
- It should be noted that in the particular embodiment illustrated herein, three layers of spheres 13 are shown, however, a greater or lesser number of spheres could be used dependent upon the total heat transfer desired.
The spheres 13 do not extend beyond the inner or outer circles defined by the walls of the tubes 11 and therefore this structure is particularly adapted for mass production of residential heating structures where heat transfer rates on the order of 100~000 BTUIs per square foot of liquid surface - are desired and the temperature of the combustion gases after passing through the heat exchanger still remain above the condensation temperature of cor-rosive constituents of the flue gas, for example in the range of 300-400 F.
Referring now to Figures 8, 9 and 10 there is illustrated a further embodiment of the invention. A helical tube 11 is embedded in, surrounded by a plurality of solid metallic spheres 13 bonded together and to tube 11, for example by brazing, to form an integral thermally conductive matrix 10.
The thermal conductivity of the materials used, the pressure drop across the foraminous matrix structure, and the thermal flux desired determine the spacing between adjacent elements of tube 11 which make up the - fluid conduit. Good performance has been achieved when the distance between adjacent elements of the tube 11 is approximately equal to the diameter of ; the tube 11 and substantially all the heat is transferred to the matrix when - the radial thickness of the matrix 10 is less than twice the spacing between adjacent conduit elements. - 12 -:' -An inlet 18 and outlet 19 are respectively provided at the ends ofthe tube 11 through which the fluid to be heated passes. The matrix surrounds a central plenum 12 which acts as a combustion chamber at the lower end of which a burner plate 34 is provided having a plurality of holes 35 for the admittance of an air gas mixture under a pressure of, for example, on the ', ' order of one inch of water, from a source coupled to inlet duct 36 and feed-ing through conical section 37 into the combustion chamber 38. Extending , . .
into one side of burner plate 34 is an ignition means 40 of any well known construction such as a spark plug to provide the necessary igni~ion of the gaseous fuel mixture.
; An outer wall member 41 surrounds the heat transfer structure and a flue 42 provides for passage of the exhaust gases out of the heat transfer structure. A top plate member 43 is secured to the heat transfer structure by a stud embedded in the matrix and extending through plate 43 together with a nut 44 threaded on the stud and engaging the upper surface of plate 43.
In the embodiment shown in Figures 8 through 10, the number of spheres may be increased or decreased dependent on the total amount of heat to be transferred from the hot gas into the conduit. For example, if the total number of spheres one-sixth of an inch in diameter is formed into a - 20 matrix having a radial thickness of approximately eight rows, with a spherical diameter of approximately one-sixth of an inch so that the total matrix thick-ness is approximately an inch and a quarter, a heat transfer rate in excess of one half million BTUts per hour per square foot of tubing surface can be achieved. If desired, combustion may occur outside of the plenum 12 and be directed into the plenum 12 to increase the combustion volume to achieve heat transfer rates up to a million BTUIs per hour per square foot of tubing surface while still retaining an exhaust temperature of around 700 F. With a half million BTU~s per hour per square foot of heat transfer, the exhaust temperature would be around 400 F. Such a structure is particularly useful ., - . ~ ~ . ~. . .

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~;~ in high pressure mobile boilers, for example, for use in steam motor vehicles.
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While the embodiments of the invention described in Figures 8, 9 and 10, as well as the other embodiment of the invention disclosed herein i. .
i are particularly useful with water as the fluid inside the conduit, it is : :.
- often desirable to use fluids such as a water steam mixture of organic ç compounds having heat transfer co-efficient less than that of water. Under ~"~''',' these conditions when it is desired to produce a gas from a liquid in the conduit and to simultaneously transfer the heat of vaporization to the fluid in what is called a two-phase condition, this invention provides for an extend-ed surface comprising spheres inside the conduit as shown, for example, in ,. . .
Figure 9 as a variation of the embodiment of the invention shown in Figure 8 which does not have spheres inside the conduit. The spheres are bonded to ~ the interior of tube 11 along a portion on the entire length thereof in a `- similar fashion to that used to bond the matrix 10. The spheres in the con-duit have been found to enhance turbulent flow of the fluid in a manner particularly useful in heat transfer to two-phase fluids.
This completes the description of the embodiment and the invention illustrated herein, however, many modifications thereof will be apparent to persons skilled in the art without departing from the spirit and scope of this invention, for example, spheres of the matrix may be of sizes and shapes other than spherical such as ovid and the matrix structure may be formed with the spheres and tubes cast as one integral piece and for certain high temperature application the spheres and other portion of the heat exchange - structure may be made of non-metallic substances such as graphite. According-ly~ it is intended that this invention not be limited to the particular e=bodiments described herein except ~a defined by the appended clai ~.

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Claims (11)

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 heat exchanger matrix provid-ing a conduit for a fluid and a plurality of interconnected passages there-through; the major portion of the total surface area of said passages compris-ing areas which are curved in two directions; and means for directing a gaseous medium through said passages at velocities producing a heat transfer between said medium and said matrix greater than 100 watts per square inch of the surface area of said conduit contacting said fluid.

2. A heat exchange system according to claim 1 wherein said gaseous medium comprises the products of combustion.

3. A heat exchange system according to claim 1 wherein the gaseous medium has been elevated in temperature by combustion.

4. A heat exchange system according to claim 1 wherein said heat exchanger passages are interconnected.

5. A heat exchange system according to claim 1 wherein said major portion of the total surface area of said passages comprises substantially spherical surface areas.

6. A heat exchange system according to claim 1 wherein said conduit comprises a plurality of tubular elements spaced from each other within said matrix.

7. A heat exchange system according to claim 6 wherein said tubular elements are formed into a continuous helical coil.

8. A heat exchange system according to claim 6 wherein said conduit surrounds a plenum.

9. A heat exchange system according to claim 1 wherein said conduit comprises a plurality of tubular elements surrounding a central plenum from which said gaseous medium is directed substantially radially outwardly through said matrix.

10. A heat exchange system according to claim 1 wherein said matrix comprises said conduit and a plurality of spheres interposed between elements of said conduits and integral therewith and the average radius of said spheres being substantially less than the average radius of said elements.

11. A heat exchange system comprising: a plurality of conduits for a fluid spaced around a central plenum with each conduit having a plurality of bodies bonded to the exterior surface thereof; the surface area of said conduits contacting said fluid being substantially less than the surface area of said conduits and said bodies with the spaces between said conduits and bodies providing a plurality of passages for a gaseous medium outwardly from said plenum between said conduits; means rigidly interconnecting at least the ends of said conduits; burner means comprising a ported burner wall positioned in said plenum; and blower means for supplying a gaseous fuel-air mixture to the interior of said burner at velocities supporting combustion in said plenum between said burner wall and said heat exchanger as a continuous flame front spaced from said wall and extending across the spaces between ports in a wall of said burner through which said gaseous fuel-air mixture is directed by said blower means to produce transfer of thermal energy to said heat exchanger at rates in excess of 50,000 BTU's per square foot of the surface areas of said conduits contacting said fluid.