GB2057083A - Tubular heat exchanger - Google Patents

Abstract

A heat exchanger of modular construction comprises a nest of tubes as 33, bonded in bores, in end plates as 30 by means of an elastic bonding composition 32 which remains permanently resilient. The modules (not shown) are displaceably insertable in a frame-like housing in drawer-like manner to provide ready removal for cleaning or replacement. <IMAGE>

Description

SPECIFICATION Tubular heat exchanger and method of its production The invention relates to a tubular heat exchanger comprised of one or more nests of tubes in a modular construction, with basic units which are located in a cube-shaped heat exchanger housing.

The invention takes in the first place the particular operational technological conditions into consideration during the energy recovery from gaseous fluids, more especially contaminated exhaust air, which may also contain corrosive components.

Tubular heat exchangers for obtaining energy or transmitting energy from gaseous fluids, e.g.

exhaust air, through heat exchangers with another fluid, e.g. water or unpolluted air, are known. Also the use of tubes from industrial silicates and different elastic fusions of these tubes in the tube bottoms appertain to the prior art.

Known tubular heat exchangers and their methods of production, e.g. in accordance with German OS Nos. 2758592, 2511033, 2659185, 2603562, use an apparatus having two parallel opposed tube bottoms which are provided with bores for insertion of the tubes. Locating and sealing the tubes therein is effected by prefabricated sealing gaiters of elastic material, which are deformed when inserting the tubes.

The energy expenditure necessary for these and the risk of fracture arising during the insertion of the tubes and the remaining sealing problems are considerable disadvantages of these known heat exchangers which, moreover, imply a lack of resistance to corrosion at the apparatus. The present invention aims to avoid these disadvantages and proposes a tubular heat exchanger without sealing gaiters or the like, which ensures a satisfactory, sealed, acidresistant, elastic fastening and retention of the tubes and is, moreover, considerably simpler to produce.

According to the present invention there is provided a heat exchanger comprised of one or more nests of tubes built in modularform with basic units which may be located in a cubeshaped heat exchanger frame housing, in which the basic units are developed as heat exchanger tubes, preferably of industrial silicates, the ends of the tubes are pre-treated to improve adhesion and sealed and permanently elastically bonded in bores in two opposite tube base plates having corrosion resistant pre-treated surfaces of glasslike structure and nature, using elastically setting grouting materials firmly adhering to the tube ends.

Preferably, each basic unit is displaceable, so that it can be replaced during the operation of the heat exchanger without decisively influencing the efficiency of the heat exchanger.

A particular preferred embodiment of the invention provides for each basic unit of the heat exchanger to be adapted as a modular block in the form of a nest of tubes gathered in rectangular tube base plates of generally parallelopiped dimensions and to form them like a drawer element displaceable optionally insertable in a heat exchanger frame with drawers of corresponding cube-like external dimensions, whereby it remains displaceably inserted, but sealed in the housing construction.

Each basic unit for the purpose of easier cleaning and convenient adaptation of the heat exchanger output may be adapted preferably as a relatively flat parallelopiped block having edge ratios H = f ~ K; K = L. Herein, H is the height, K the width, L the length of the basic unit and fa factor which accrues as inverse value of the number of modular basic units. Even when using a single basic unit a cube-shaped housing should be used to unify the dimensions of the oncoming flow passages. In each basic unit, independently of overall size and output class, the lengths, diameters and indexing numbers are conveniently adjusted with the dimensions and spacings of the tube base plates so that the same hydrodynamic characteristic factor values are attained with regard to the throughflow of the tubes on the one hand and the flow of the tubes on the other hand.

The pre-treated tube ends in accordance with a further development of the invention are moulded into the pretreated tube base plates of metal or other suitable materials whilst using acid-resistant resin compounds on a silicon base, which remain resilient on setting. Phenyl silicon resins with polar carbonyl-, amide or ester groups may be used, which besides the suitable number of polar groups also have a sufficient number of free OH groups, whereby especially the chemical affinity - i.e. the bonding with the glass surface is improved by dipole alternating effect. These resins may also contain a suitable balanced ratio between the polar and elastic groups, such as CH2 groups or Isophorondiiaganate groups having a high degree of cross-linking.

They may also be formed of systems of silicon resin mixed with methyl esters, ethyl esters and phenyl esters and those which are built up on the basis of butyl, hexyl and higher esters.

Resins of the kind described for improving the resistance against acids and solvents by mix condensation as polysiloxane of the lastmentioned kind may additionally be improved with epoxide resins.

Centering of the tubes occurs automatically by the adjusted viscosity and the surface tensions between the resin bond and the pre-treated surface of the tube. Besides a satisfactory sealing there is additionally also attained a good security against breakage during operation and transportation, because shocks and vibrations are absorbed by the elastic composition.

Other known tubular heat exchangers with glass tubes and method of producing such aehieve the retention and sealing of the tube ends by temporary adjustment of the tubes in the tube base plate with the air or the like and subsequent casting of a second tube base plate from plastics material compositions bonded with the first one which in most cases is comprised of a thin metal sheet, more especially with the aid of a mould or casting crucible, whereby the cast tube base plate adheres either to the inside surface or the outside surface of the first tube base plate. The actual supporting parts thus are the plastics material compositions cast on the ends of the tubes, which, on setting, form walls in which the end portions of the tubes are adhesively embedded.Special mention is made herein of the German Offenlegungsschrift Nos. 2610817, 2509717, 1551522,2129096,2502291,2265349, 2828412 and the US Patents Nos. 3447603 and 3633660.

The obvious disadvantages, such as the cumbersome use of casting troughs, casting gauges, the problems of detachment of the set casting composition therefrom, the high material consumption in costly resin compounds or the like casting or grouting compositions, and the inadequate acid resistance of these tube bases which has to be improved by additional pouring in of foils of "Teflon" (RTM), are avoided in the present invention.

In a further preferred embodiment, two tube base plates of metal or any other suitable materials, e.g. asbestos, are spaced in parallel opposite each other as supporting elements and are so developed that they may be used simultaneously as a mould during the elastic bonding-in of the tube ends.

The consumption of bonding-in material is minimised in that by differently tempering the grouting material, the tube base plates and the tube ends, grouting begins at low viscosities, whereby the thin liquid composition is rapidly and evenly spread over the bore holes of the tube base plate where it increases its viscosity by cooling off at the tube ends to the required values. The capillary forces thus increasing in the gap between the base plate bore hole and tube end centre the tube in the borehole of the plane. The surface properties of the tube base plate and tube material and individually adjusted bonding agents have optimum, rheological behaviour and suitable types of surface tension may be used.

For the purpose of increasing the corrosion resistance and improvement of the adhesion of the elastic bonding of the composition of the material of the tube base plate and the tube ends, the surfaces of these parts are preferably subjected to a different preliminary treatment. To improve the surface finish, polysilanes such as vinylsilane, chromium silane or the like compounds may be used, in which, in the free end groups of the silanes, fundamental refining agents in the form of organic or inorganic acid residues are incorporated, through which the bond of synthetic resins and glass surface is improved.

The tubular heat exchanger therefore has surfaces of the tube base plates, which after thorough cleaning of dirt, oil, grease and wax residues are processed with chemical compounds which enter a secure chemical bond with the surface of the material of the tube base plate, are corrosion resistant and themselves develop surface properties which are close to or attain those industrial silicates used in the tubes.

The preliminary treatment of the tube base plates is insofar differently pre-treated from the non-metallic bottoms (such as e.g. asbestos or the like). The tube ends of the glass tubes on the other hand are roughened by pre-treatment or with a diluted acid or smoothed with organic or inorganic adhesion bridges, so that adaptation to the structure of the tube surface occurs and identical adhesion values in the bond with the elastic embedding composition is attained.

Despite multiple measures and minimum consumption of bonding-in agents a satisfactorily sealing and durable corrosion resistance of the components of the tubular heat exchanger which come into contact with corrosive material flows is attained in accordance with the invention.

It is also an aim of the invention to provide a manufacturing method for such a tubular heat exchanger. This method is developed in such a manner that the tubes to be formed into a nest are positioned upright in the bores of a horizontally positioned tube base plate and are then bonded in the bore with a grouting/casting material. The tubes are thus supported, initially upright only temporarily by the bores of the tube base plate.

The bores of the tube base plate are so dimensioned relative with the outer diameter of the tubes that by suitable adjustment of the viscosity of the bonding medium and by the different tempering of tube base plate and tubes, and due to the material-conditioned surface tension compulsory centering of the tubes in the respective bores occurs by the increase in viscosity during casting of the grouting material.

Compared with other methods which can use a similar mode of operation only with relatively large tube spacings, the new method is used exclusively and independently of the spacing of the tubes with the result that only the intermediate spaces between tube and tube base bores or tube base projections are grouted with an elastic adhesive composition. Slight residues of grouting material remaining on the tube base plate have no adverse effect.

During operation of such heat exchangers with exhaust air or other energy rich gas flows from workrooms or production processes it has been found that, dependent upon the contaminatións in these media, in a relativelyshorttime considerable contamination of the heat exchanger surfaces occur which considerably impair heat exchange and reduce the efficiency of the heat exchanger. In fixedly mounted heat exchangers it is necessary to dismount them for cleaning which means interruptions of their operation.

To avoid the disadvantages connected therewith the heat exchanger in accordance with the invention is built up from basic units which may be inserted and replaced in optional frequency in a corresponding housing structure in the manner of drawers.

The manufacturing method in accordance with the invention therefore provides a modular structure of a heat exchanger using at least one displaceable basic unit and a housing frame with drawers.

The cube-shaped heat exchanger housing is manufactured with substantially equal edge lengths K' = L' = H' of various materials, mainly of zinc-coated steel plates, and to improve the surface protection is subsequently treated to be "acid-resistant".

This form of construction provides fundamentally the manufacture of housings of optionally large dimensions K' = L' = H', whereby these are primarily determined from the quantities of usable heat carrier and the optimal conditions of the heat exchanger.

The structural embodiment of the modular basic units in accordance with the invention primarily deals with the demands for more convenient and more rapid cleaning and handiness, and ease of replacement, and selfsealing connections relative to the frame structure of the housing, so that depending upon overall size of the housing the number of basic modular units to be accommodated therein varies, and may vary between two and any optional number of units.

Accordingly, higher number of drawers may be arranged in the housing, which are formed in accordance with the invention, by the insertion of suitable profile ledges of corrosion-resistant materials. For special purposes, profile ledges according to inherent designs are used which after insertion of the basic unit together with the edges of the tubes and the side plates permit adequate sealing of the interior space of the heat exchanger.

Compared to other methods of construction, the operation of the heat exchanger does not have to be interrupted when replacing individual basic units and also the hydrodynamic operating conditions are maintained when a side covering of the housing is removed. In accordance with the invention for these reasons various standardised overall sizes have been proved expedient.

By way of examples it is readily shown that the heat exchanger in accordance with the invention, by suitable variation of the dimensions H; a, b, d (see the accompanying drawings described in detail hereinafter) in the modular element and by suitable adaptation of the dimension H relative to the length of edge K can be adapted not only in a simple manner according to the energy gain problem, but also additionally presents basic modular units, exchanger units, which due to the significant limiting of the height H both by dimensions and the limited weight remain manageable.

The basic unit of the heat exchanger in accordance with the invention is hence a modular block and the device is a multi-tube nest with tube base plates in accordance with the invention which overall has parallelopiped dimensions, so that the number of tube layers is less than the number of tubes located in one layer.

The basic modular unit is hence built up from at least two pre-treated tube base plates and the tubes bonded therein at the ends. The dimensions are directed in accordance with the ratio H =f.K which for the various types of housings is determined by the factor f which reduces with increasing length K and may assume values between f= 0.5 to f= optionally small.

Bracing of the whole modular basic unit is attained on the one hand by the incorporation of at least four metal tubes of stainless materials, one at each corner or on the other hand by alternative or additional facing of the lateral flanks with components of profiled materials which are made of materials with acid resistant surfaces.

The adaptation to the dimension of the drawers in the housing is attained on the one hand by the special development of the edge ledges of the tube bottom plate, on the other hand by suitable shaping of the side plates.

It has been found expedient to fix the number of tubes nested per basic unit on account of definite, predetermined heat exchanger surfaces for various classes which in turn are directed by the gas quantities to be processed.

Owing to this, the fundamentally constant hydrodynamic conditions in accordance with the invention are the basis for calculating number, arrangement and dimension of the tubes. The manufacturing process takes this into account and is so aligned that all sizes of tube may be bondedin according to the same above-described manner, which besides the method of grouting and the selection of suitable resins is principally attained by the special development of the bores in the tube base plates.

The external dimensions of the heat exchanger housing are cube-shaped for this purpose to obtain uniform dimensions for the connecting cross-sections of the supply and discharge flow sides of both media flows. This results in uniform inflow passages.

When using only one modular basic unit a cube-shaped housing may be used, whereby the manufacturing method is based on a variation of the approach flow cross-section corresponding to the approach flow surface of the narrow side of the basic unit by guide baffles.

For flow velocities in the laminar region the length of tube is defined by the cube-shaped method of construction of the housing on the one hand and the layout of the internal diameter of the tube, dependent upon the demand of thermal compensation during internal heat transfer, and on the other hand so defined that vibration of the tubes up to break limit is avoided.

At higher approach flow velocities in the turbulent region further tube base plates per basic unit are drawn in, and by adjustment of the dimensions and accurate embodiment of the tube plate edges, so that the interior space of the heat exchanger housing is divided into two or more passages.

These additional tube base plates adhesively bond the tubes and are of identical structure as the tube base plates at the ends of the tubes, but the edges are adapted as guide elements (e.g.

tongue and groove or the like).

The bonding-in of the tubes occurs according to the same method in accordance with the invention. The modular structure in accordance with the invention having displaceable insertion units as modular blocks for certain housing frames permits adaptation with regard to size and capacity in all occurring operating states and requirements, whereby the drawer-like development of the basic units and their displaceable insertion in a housing results in a heat exchanger which permits a more readily and speedy cleaning of the installed components without interruption of operation.

The present invention will be described further, by way of example, with reference to the accompanying drawings, in which~ Fig. 1 shows the schematic structure of a basic modular unit of a heat exchanger; Fig. 2 shows the bracing and facing of the basic modular unit at the tube plate ends; Fig. 3 shows an example of profiling the edges of the tube base plate; Fig. 4 shows a further example of profiling the edges of the tube base plate; Fig. 5 shows a third example of profiling the edges of the tube base plate; Fig. 6 shows an example of sealing the basic unit of the heat exchanger relative to the housing frame by a lateral facing; Fig. 7 shows the fitting of the displaceable basic modular units into a cube-shaped housing and the unification of the onflow and off-flow cross-sections;; Fig. 8 shows the guide plates and cover plates for partitioning individual basic modular units; Fig. 9 shows a basic modular unit for higher onflow velocities with additionally inserted tube base plates; Fig. 10 shows a portion of the embodiment according to Fig. 9 on a larger scale with machined tube plate edges; Fig. 11 is a heat exchanger with readily displaceable basic modular units and illustrating their sliding direction; Fig. 12 shows the schematic structure of a cube-shaped frame housing; and Figs. 13 and 14 show the bonding-in of tube ends into a tube base plate in different embodiments.

In the basic unit shown in Fig.1, tubes 1 made from silicate are retained at their ends in bores of base plates 2 and 3. A resiliently setting grouting material is used which fills the gap between the tube plate bore and the tube end located therein and ensures a sealed and permanently resilient bonding. The tubes 1 and the base plates 2 and 3 are pre-treated as described in the introductory portion of the specification. Heat transfer occurs in such a manner that the tubes 1 are flowed around externally by a flow medium and another flow medium flows through the tubes 1. The longitudinal sides of the basic unit are covered by cover plates 4.The horizontal tube spacing is denoted in the drawing by a and the vertical tube spacing by b, whilst the tube diameter is denoted by d, h represents the edge chamfers, whereas K is the width, L is the length and H the height of the basic unit.

Fig. 2 shows a similar basic unit as Fig. 1.

Herein the lateral cover plates 4' are shown detached. The cover plates 4' are provided with beads 5 for bracing, and the tube bottom plates 2 and 3 in this embodiment, moreover, are braced by tubes 6 made of a stainless material.

The profiling of the edge 7 of the tube base plate 2 shown in Fig. 3 due to the resilient support on the profile ledge 8 of the frame housing receives resilient metal sealing. The tubes 1 are retained in the tube bottom plate 2 by an elastic bonding 9.

In the embodiment shown in Fig. 4 an elastic sealing 10 relative to the profile ledge 11 of the housing is obtained by the sealed connection between the tube base plate 2 with the tube ends bonded elastically at 12.

In the embodiment of Fig. 5 the resilient edge material 13 of the tube base plate 2 with the elastically bonded-in tube ends provides a tightly sealed connection with the correspondingly shaped profile edge 11 of the housing.

Fig. 6 shows an example of sealing the basic unit of the heat exchanger relative the housing frame by corresponding side plates 14. The edge 15 of the tube base plate 2 and a thrust bar 16 ends in a correspondingly formed sealing ledge 17. The end of the side plate 14 is adapted to the shape of the sealing ledge 1 7. The side plates 14 hence seal off the basic unit. A double arrow 18 indicates the shear direction of the basic modular unit.

Fig. 7, moreover, shows the fitting of the displaceable, basic modular units into a cubeshaped housing having the dimensions K', L', H' and the unification of the onflow and off-flow cross-sections to the dimensions of the housing.

In the frame housing 1 9 the basic units 20 are inserted.

Fig. 8 shows the principle of adaptation of the heat exchanger to smaller air capacities. Two modulated basic units 20 are accommodated in a frame housing 21. Side plates 22 of the frame housing and guide plates 23 in the passages provide the adaptation to the reduced air output.

Fig. 9 shows a specific embodiment of the structure of a basic modular unit. By suitable adaptation of the height H in relation to the length of edge K and L, by suitable tube spacings a and b and for optional tube diameter d heat exchanger units may be formed which remain manageable.

The number of tube layers is lower than the number of tubes located in the layer. For higher onflow velocities and/or with excessively long tubes additional tube base plates 24 having corresponding tube plate edges 25 are inserted.

As already shown in Fig.9, Fig. 10 shows that the inserted tube plate bottoms 24 with their tube plate edges 25 are tightly sealed and form closed air conduction passages and have an additional bracing effect.

Fig. 11 shows in a perspective view an example of air conduction through a heat exchanger. The hot discharge air is conducted downwardly through the heat exchanger having five basic modular units 26. The fresh air is conducted through the interior of the tubes.

Fig. 12 shows a schematic structure of a cubeshaped heat exchanger housing having the edges K', L', H'. It is composed of the frame 27, two cover plates 28 and guide rails 29 for the drawers.

Only one of the cover plates 28 is shown.

Fig. 13 shows another possibility for bonding the tube ends in the tube plates. A surface-treated tube base plate 30 with a bore hole 31 deepdrawn, in accordance with different methods, and with the grouting material 32 retains the surfacetreated tube end 33.

In Fig. 14, as already described with regard to Fig. 13, four tube rows are arranged in the tube base plate. As shown in Fig. 14, the surfacetreated tube ends 34 having the tube diameter d and the tube spacing b are shown in a basic unit having the height H and, as also shown in Fig. 1, having a profile bend h of the tube bottom plate.

Claims (24)

1. A heat exchanger comprised of one or more nests of tubes built in modular form with basic units which may be located in a cube-shaped heat exchanger frame housing, in which the basic units are developed as heat exchanger tubes, preferably of industrial silicates, the ends of the tubes are pre-treated to improve adhesion, and sealed and permanently elastically bonded in bores in two opposite tube base plates having corrosion resistant pre-treated surfaces of glasslike structure and nature, using elastically setting grouting materials firmly adhering to the tube ends.

2. A heat exchanger according to claim 1, in which each basic unit is displaceably formed and adapted to be replaceable during operation of the heat exchanger without decisively affecting the efficiency of the heat exchanger.

3. A heat exchanger according to claims 1 and 2, in which each basic unit is adapted as a modular block in the form of a nest of tubes gathered in rectangular tube bases of overall parallelopiped dimensions and displaceably insertable in the manner of a drawer in a heat exchanger housing frame having cube-like dimensions and a plurality of drawers corresponding to the dimensions of the basic units, optionably insertable and remaining displaceable, but when inserted forms a tight seal with the housing structure.

4. A heat exchanger according to claims 1 to 3, in which each basic unit for the purpose of ready cleaning and convenient adaptation to the heat exchanger output is formed as a relatively flat parallelopiped having edge ratios H = f ~ K; K = and when using even a single basic unit a cubeshaped housing is used to standardise the dimensions of the onflow passages.

5. A heat exchanger according to claims 1 to 4, in which in each basic unit, regardless of overall size and output class, the lengths, diameters and pitches of the tubes are so adapted with the dimensions and spacings of the tube plate bottoms that the same hydrodynamic characteristic factor values are attained with respect of throughflow of a flow medium in the tubes and another flow medium flowing about the tubes.

6. A heat exchanger according to one or more of claims 1 to 5, in which each tube base plate is so pre-shaped and pre-aligned that it may also be used as mould for bonding-in the tube ends and renders casting troughs, gauges and the like unnecessary.

7. A heat exchanger according to one or more of claims 1 to 6, in which the tube base plate of each basic unit is braced by struts or tube made from stainless materials and/or secured by profile ledges or profile plates of stainless materials, which tightly seal the individual unit with the frame housing.

8. A heat exchanger according to one or more of claims 1 to 7, in which all frame housings are cube-shaped and are provided with drawers, the number and dimensions of which are determined in accordance with the dimensions of different basic units, arranged in output categories.

9. A heat exchanger according to one or more of claims 1 to 8, in which in heat exchanger housings not completely fitted with basic units the inflow passage is reduced by partition plates to the inflow cross-section of the narrow side of the basic units and drawers not used are sealed off by plates.

10. A heat exchanger according to one or more of claims 1 to 8, in which the basic units are bonded in by further tube base plates according to the same method as the first base plates and thereby secured adhesively, elastically at least threefold against vibration fractures.

11. A heat exchanger according to claim 10, in which when using basic units having more than two tube base plates the arrangement of the central tube base plates is such that the latter divide the housing of the heat exchanger into passages of identical hydrodynamic properties, whereby special shapes of tube base plates with profile edges are used for guidance and sealing.

12. A method of production of a heat exchanger according to claims 1 to 5, in which only in the gap between tube plate bore or deepdrawn bore hole and the tube ends a resiliently setting grouting material is poured, for tubes of any diameter and independently of the spacings of the tubes.

13. A method according to claim 12, in which the tubes to be nested are arranged in an upright position vertically penetrating the bores of the tube base plate and are grouted around horizontally.

14. A method of production of a tubular heat exchanger according to claims 12 or 13, in which each tube base plate is pre-treated over its whole surface by chemical reactive treatment with corrosion-resistant materials based upon zinc, silicon and silicate compounds, so that in the final layer a glass-like structure with glass-like properties is formed.

15. A method according to claim 14, in which the surface of the tube bottom plate is treated alternately with zinc-based compounds for siliconbased and/or epoxide based acid-resistant bonding-in materials or such mixed systems with other organic compounds.

16. A method of production of a tubular heat exchanger according to one or more of claims 12 to 1 5, in which by different tempering of the grouting material, the tube base plate and the tube ends, a forced centering of the tubes in the bore holes of the base plates is attained by the temperature-dependent influencing of the viscosity and surface tension of the bonding-in medium.

17. A method according to claim 16, in which, according to the surface properties of the tube base plate and tube material, individually adjusted bonding-in media are used having an optimated rheological behaviour and surface tension values.

18. A method according to claim 16 or 17, in which for improving the surface finish polysilanes such as vinyl silane, chromium silane or the like compounds are used, in which in the free end groups of the silanes have refining agents in the form of organic or inorganic acid residues through which the bond of synthetic resins and glass surfaces are improved.

19. A method of production of a tubular heat exchanger according to one or more of claims 12 to 18, in which as grouting materials for the tubes and tube bottom plates such compounds as phenyl-silicon resin with polar carbonyl-, amide or ester groups, which besides a suitable number of polar groups also have an adequate number of free -OH groups, are used in order to improve the chemical affinity, that is the bonding with the glass surface by alternating dipole effects.

20. A method according to claim 19, in which as grouting material such compounds are additionally used which, moreover, have a suitable balanced ratio between the polar and elastic groups, such as CH2 groups or isophorondiisocyanate groups, with a high degree of cross-linking, to improve the elasticity.

21. A method according to claim 19 or 20, in which as grouting material additionally such compositions are used which are assembled as mixture systems of silicon resin systems with methyl esters, ethyl esters and phenyl esters and such resin systems which are built up on the basis of butyl, hexyl and higher order esters.

22. A method according to one or more of claims 19 to 24, in which as grouting materials additionally such compositions are used which to improve the resistance to acid and solvent residues are improved by mixed condensation as polysiloxanes are additionally improved with epoxy resins.

23. A heat exchanger substantially as herein described with reference to and as illustrated in the accompanying drawings.

24. A method of manufacturing a heat exchanger substantially as herein described.