US3825063A

US3825063A - Heat exchanger and method for making the same

Info

Publication number: US3825063A
Application number: US00234129A
Authority: US
Inventors: K Cowans
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-01-16
Filing date: 1972-03-13
Publication date: 1974-07-23
Anticipated expiration: 1991-07-23

Abstract

A high effectiveness compact heat exchanger having internal passageways containing substantially transverse foraminous elements of high heat conductivity includes heat-flowed passageway walls of synthetic material that are interspersed into the interstices of the foraminous elements. Metal screens of fine mesh are contiguously stacked to provide transverse heat conduction relative to longitudinal flow paths. Internal plastic barriers transverse to and at least partially penetrating the screens define composite passageway walls providing leak free construction capable of sustaining substantial pressure differentials between adjacent passageways. The passageways are varied in cross-sectional area along their lengths, as by incremental displacements of successive barriers, to provide flow equalization for maximum efficiency.

Description

United States Patent 1191 Cowans July 23, 1974 HEAT EXCHANGER AND METHOD FOR 3,409,075 11/1968 Long res/154 MAKING THE SAME 3,477,504 11/1969 Colyer et al.... 165/164 3,491,184 l/l97O Rleldljk 165/165 X [76] Inventor: Kenneth W. Cowans, 3118 Patricia L05 Angeles Cahf' Primary Examiner-Albert W. Davis, Jr. 22 Filed; 13, 1972 Attorney, Agent, or Firm-Fraser and Bogucki [21 Appl. No.. 234,129 ABSTRACT Related Apphcamn Data A high effectiveness compact heat exchanger having [62] Division-of Ser. No.3,395,Jan..16', 1970, abandoned. internal passageways containing substantially trans verse foraminous elements of high heat conductivity [52] US. Cl 165/146, 165/141, 165/165, includes h at-flowed assageway walls of synthetic j 165/179 material that are interspersed? into the interstices of [51] Int. Cl F281 13/06 the foraminous Metal screens of fine h [58] Field of Search 165/179, 164, 165, 1 are contiguously stacked to providetransverse heat 1 1 14. 147, 141 conduction relative to longitudinal flow paths. Internal plastic barriers transverse to and at least partially pen- [56] References g w etrating the screens define composite passageway UNITED STATES PATENTS walls providing leak free construction capable of sus- 1,734,274 11/1929 Schubertm' 165,166 x taining substantial pressure differentials between adja- 1,825,321 9/1931 La Mont et a1 .5165/147 cent passageways. The passageways are varied in 2,062,321 12/1936 Levin 165/140 X cross-sectional area along theirlengths, as by incre 2,460,359 2/1949 Tfumplerm 165/14l X mental displacements of successive barriers, to pro- 2,466,684 4/1949 Case 165/147 X i fl equalization for maximum ffi 1 2,716,333 8/1955 Collins... 165/141 X 3,228,460 111966 3 Claims, 14 Drawing Figures Garwin 165/179 X councuous 'PAIEIIIEIIIIIIzaIIII SHEET 1 0F 7 I I I I I I I I I I I I I HELIUM SOURCE Umm Sm; 57

OUTLET SYSTEM 24 I DESICCANT CHAMBER Fl G I PATENTED I 3. 825.063

sum 2 or 7 PROCESSED GASES l0 CONTIGUOUS SCREEN ELEMENTS 53 2 M0 00 PRECIPITAfION zone LENGTH FROM HOT END FIG.5

PATENTEnamzsxsm SHEET 5 0F 7 FORN PLASTIC ELEMENTS PRECLEAN PLASTIC FORM SCREEN ELEMENTS PRECLEAN SCREENS FIG F. N N N m mm m wm 0 m Mm Em r... n- S 0 HL P R RI: Rlns A In I. AL m w m m n wmwm nu N N A c m w M Hm n I PmEmsmmz 3.825.083

sum- 6 OF 7 HIGH PRESSURE HELIUN SOURCE CRYOGENIC REFRIGERATOR cormcuous SCREEN ELEMENTS HELIUM HELIUH PATENTEDJULZBIBM 3,825,063 SHEET 7 BF 7 councuous SCREEN ELEMENTS 1 HEAT EXCHANGER AND METHOD FOR MAKING THE SAME This is a division of application Ser. No. 03,395, filed Jan. 16, I970.

BACKGROUND OF THE INVENTION This invention relates to heat exchangers and systems using heat exchange relationships, and particularly to metal and composite heat exchangers of high heat transfer effectiveness and methods of making the same.

In many modern applications of heat exchangers particularly stringent operative design requirements must be met in terms of heat transfer effectiveness, compactness, structural properties and similar factors. The extreme demands that may be imposed on heat exchangers are exemplified by life support systems such as those disclosed in a co-pending patent applicgtion, as-

same time imposes substantial thermal gradients and stresses.

It is known to construct heat exchangers having transverse elements for lateral heat distribution between passageways, as shown by US. Pat. No. 3,228,460. As discussed in that patent, transverse perforated plates may be separated byapertured spacers in a laminate, with the spacers being configured todefine passageways for counterflowing fluids, and the plates facilitating heat interchange. An alternative construction is shown in an article entitled A New Type of Compact HeatExchanger with a High Thermal Efficiency," by G. Vonk, pp. 582-589 of Advances in Cryogenie Engineering, Vol. 13 (Plenum Press, New York 1967). In this unit, the transverse conductive elements are copper screens and the insulative elements are resin-impregnatedpaper having punched holes which define the passageways. The aforementioned heat exchanger constructions are primarily for the particular applications described, whereas greater versatility is often desired. For example, where particular thermodynamic or chemical characteristics are to be achieved during heat exchange, it may be desirable to have different temperature gradients along the length of the exchanger. Selection and control of the temperature gradient profileshould not, of course, adversely affect the other desirable characteristics.

In addition, needs exist for improved methods of fabricating exchangers, and for improved exchangers themselves, in order to reduce costs of material, labor and equipment without sacrificing reliability or. performance. The term heat exchanger, as used herein, is intended to include regenerators, as well as counterflow. concurrent flow, and other conventional types of exchangers.

SUMMARY OF THE INVENTION The purposes and objects of the present invention are achieved by heat exchanger systems comprising a plurality of transverse heat conductive, filamentary elements that are contiguously stacked along thelongitudinal axis of the heat exchanger and internal heatflowedwalls of thermoplastic material that penetrate the interstices of the filamentary elements to join them longitudinally and seal them internally. The passageway walls are varied in cross-sectional area along their length to equalize flows within the passageways.

A specific example of a compact heat exchanger system in accordance with the invention comprises a cy-.

lindrical body defined by a plurality of contiguous metal meshelernents, disposed about and transverse to a central axis and internally divided into separate passageways by non-conductive heat-flowed plastic barriers that impregnate and seal the meshes while forming composite walls. Although thermal interchange at each incremental region along the length of the exchanger is highly efficient, longitudinal conduction is minimal and high heat transfer effectiveness is achieved along with an extremely sharp temperature gradient. The unit can withstand significant pressure differentials, permits ready headering of fluids, and may have arbitrarily determined internal configurations. Such constructions may be used to handle two or more fluids in counterflow or concurrent flow relation.

The dimensions of the passageways are varied along their lengths for equalization of flows and maximizing efficiency. In such arrangements, successive plastic wall segments have slight relative displacements, but nonetheless overlap, prior to unification. After unification the walls are continuous but the passageway crosssections are non-uniform. In a specific example the passageways of one set are concave along their lengths and the passageways of another are convex along their lengths.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the invention may be had by reference to thefollowing description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a combined perspective and schematic view, partially broken away, of one example of a heat'exchanger in accordance with the invention as used in a life support system;

FIG. 2 is a side sectional view of the heat exchanger of FIG. 1;

FIG. 3 is an end sectional view, taken along the line 33.of FIG. 2, and looking in the direction of the appended arrows;

FIG. 4 is an enlarged fragmentary view of a portion of the heat exchanger of FIGS. 2 and 3;

FIG. 5 is a graph showing a temperature gradient profile along a heat'exchanger constructed as shown in FIGS. l-4;

FIG. 6 is an exploded view of a fragment of a heat exchanger in accordance with the invention showing a number of elements as they are employed in the assembly of a unit;

FIG. 7 is a block diagram representation of an example of a process for making heat exchangers in accordance with the invention;

FIG. 8 is a combined perspective and block diagram view, partially brokenaway, of a regenerator in accortion system;

FIG. 9 is a side sectional view of the regenerator of FIG. 8;

FIG. 10 is an end sectional view of the regenerator of FIGS. Sand 9; v

FIG. 11 is a cross-sectional end view of a fragment of an ultra low temperature regenerator of different configuration;

FIG. 12 is a side sectional fragmentary view of an alternative wall construction in heat exchangers in accordance with the invention;

FIG. 13 is a front sectional view of a heat exchanger construction utilizing passageway area variations along the passageway length; and

FIG. 14 is an enlarged front sectional view, somewhat idealized, of a fragment of the arrangement of FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION A heat exchanger 10 in accordance with the invention is illustrated as employed in conjunction with a life support system of the type disclosed in general terms in patent application Ser. No. 623,616 referred to above. The heat exchanger 10 and the various aspects and features it provides are utilized in combination with a processor system 12 which includes liquid oxygen 14 within a liquid oxygen vessel 16 encompassed by liquid nitrogen l8'within a temperature control cryogen vessel 20. As described in the above-identified patent application, an expired mixture from one or more users at an inlet-outlet system 22 is passed through a desiccant chamber 24, then through the heat exchanger 10, in which the expired gases are lowered to the near cryogenic level. In the processor system 12, the liquid oxygen 14 is maintained in a selected pressure range, and thermodynamic equilibrium between the expired gas mixture and the liquid oxygen 14 is utilized to return a gas mixture having a'precisely controlled partial pressure of oxygen, constituting a safe breatheable mixture. The gases after processing are passed through the heat exchanger 10 in counterflow relation to the expired gases, for reheating to near the ambient level. A helium source 26 under pressure operates through an ambient pressure responsive device 28, to make up the remainder of thebreatheable mixture.

The system shown is arranged to freeze carbon dioxide internally within the heat exchanger 10, although a separate CO removal device, such as a chemical container, may be used alternatively or in addition.

Further details as to the arrangement and operation of the life support system generally represented in FIG. 1 have been omitted for brevity, inasmuch as they are included in the copending application previously mentioned.

The heat exchanger construction shown in FIGS. 1-4 not only has the desired compactness and effectiveness for this application, but is an extremely strong and stable unit capable of withstanding extremely high pressure loadings and pressure differentials.

The exchanger 10, in general terms, comprises a plurality of circumferential passageway segments disposed substantially concentrically about a central axis of the exchanger, and includes a first pair of

headers

30, 31 at one longitudinal end of the exchanger and a second pair of

headers

33, 34 disposed at the opposite longitudinal end of the exchanger. As will be evident to those skilled in the art, other header arrangements may be utilized, and the headers at each end need not be the same. For convenience of reference the first end will be referred to as the hot end of the-exchanger 10, this being the end in which the incoming gases from the inlet-outlet 22'are at or near the ambient level, along with the counterflowing gases being returned to the inletoutlet system after passage through the heat exchanger. The cold end of the exchanger, 10 is that end at which the inflowing expired gases which have passed through the heat exchanger and the gases from the processor system 12 which are being directed into the heat exchanger are near the cryogenic level.

. Inasmuch as the header-pairs are alike in the present example, only the

first pair

30, 31 is described in detail herein. One header of the pair comprises a cylindrical plenum 36 spaced apart from the end of the heat exchanger and interconnected with the interior of appropriate passageways within the exchanger by hollow tubes 38. A chamber 40 about the tubes 38 comprises the second header, and is interconnected with end apertures 42 which are in communication with other passageways within the heat exchanger 10. A side outlet fitting 44 on the chamber 40 couples the processed gases to the inlet-outlet system 22, and an end outlet fitting 46 couples the expired gases into the plenum 36.

l The central body of the heat exchanger 10 is encompassed by a metal cylinder housing 50 that forms the outer wall of the exchanger. A refrigeration source for the exchanger 10 comprises a hollow metal tube 52 wound about a part of the cylindrical housing 50, the thermal contact being materially enhanced by solder 53. Boil off or liquid nitrogen 18 or other temperature control cryogen in the metal tube 52 therefore is in good thermal contact with the interior of the housing 50, and serves as a heat sink to the. interior gases.

The counterfiow passageways in the interior of the housing 50 are defined by internal heat-flowed plastic barriers which comprise both circumferential segments 55 lying at different radii, and radial walls 57 lying at different circumferential positions. In the present example, the expired gas mixture passes from the first end to the second end of the heat exchanger 10 in alternate radially spaced ones of the passageways, and the processed gas mixture flows in the opposite direction in the remaining alternate passageways. The particular configuration shown (best seen in the end sectional view of FIG. 3), is arranged such that the volume of gas flowing in each passageway is essentially equal to the counterflowing gases in the adjacent passageway or passageways. As will be evident from the approximation of FIG. 3, which is not drawn to scale, a regular progression of sizes does not exist when flow equality is to be maintained in a concentric passageway configuration.

The foraminous structure in this example comprises a plurality of metal mesh or screen elements 59, each of whichlies in a plane transverse to the central axis of the heat exchanger 10 and has a cross-sectional area the metal filaments of the mesh elements 59 are disposed within a matrix of heat-flowed material of relatively low thermal conductivity. A thermoplastic, specifically polystyrene, is employed in the present example for use in a cryogenic-type system. In accordance with the invention, the mesh elements 59 are substantially contiguous and in the present example are actually in abutment. Despite this contact, the longitudinal thermal path is so disrupted and discontinuous relative to the transverse conductive paths that longitudinal conduction is negligible. The plasticfills the interstices of each mesh, along the

interior walls

55, 57. The wall thickness is not shown to precise scale but may be substantial, i.e. a considerable fraction of the passageway width, or thin if desired. In the practical example being discussed of an exchanger for a life support system, the screen elements 59 are both mesh and 100 mesh aluminum and the structure is approximately 9" long and 5" in diameter. The filling factor, for this configuration, can be extremely high. In this example approximately 110 mesh elements per inch were utilized, providing open flow paths comprising approximately 58 percent of the total area.

As best seen in. the fragmentary view of FIG. 4, the screen elements 59 are adjacently disposed along the length of the exchanger 10. The cross-sectional flow area within the passageways, defined by the interstices in the elements 59, is sufficiently large to-permit life support operation with a reasonably low pressure gradient between the opposite ends of the exchanger. The filaments interspersed in the flow path create some turbulent flow, but more importantly conduct heat laterally, so as to integrate or equalize the temperature in a lateral plane within the exchanger 10. The circumferential and radial

plastic walls

55, 57 respectively inter- I pose virtually no lateral heat transfer barrier, inasmuch as the plastic is fused within the interstices of the meshes, as well as interconnecting the screens 59 so as to seal off each passageway from the adjacent passageways. i

The arrangement of composite metal-plastic passageway walls in accordance with the invention frees such structures from requirements that the laminate beuniform throughout. For example, in the heat exchanger 10 of FIGS. l-4 the mesh sizes of the screen elements 59 vary along the longitudinal axis. Specifically, about 40 percent of the total length of the structure interior of the ends comprises coarser No. 10 mesh screen elements 59 than the No. 100 mesh otherwise used. In the portion along which the No. 10 mesh is disposed, heat interchanges between the elements 59 and the flowing gases are substantially diminished by comparison to the remaining portions, and the temperature gradient is considerably lowered. Incoming hot gas containing carbon dioxide is therefore lowered in temperature to the region, 1 30C. to 140C., at which most CO freezing occurs. This shaping of the temperature profile is illustrated in FIG. 5, which is a plot of position along the length of the exchanger 10 as the abscissa, against temperature in degrees Centigrade as the ordinate. It may be seen that the opposite ends of the exchanger 10 have a temperature differential of approximately 200C, which is in excess of 10C. temperature change per inch. In the interior, relatively low temperature gradient region, however, the approximately 10C. range in which carbon dioxide solid precipitation occurs is greatly extended, so that the gradient is approximately 2C. to 3C. per inch. Thus the'carbon dioxide freezes ments 65 and 66 are assembled into a laminate, which is thereafter unified. As best seen in FIG. 6, foraminous elements 65 of high heat conductivity material, namely aluminum mesh, are formed to the configuration and size desired. These conductive elements 65 are stacked in alternating fashion with internally apertured plastic sheets 66 which correspond in external size and configuration to the foraminous elements 65. In the present example, the plastic sheets 66 are of polystyrene, and approximately 0.010" inthickness. Appropriate alignment during lamination may be assured by stacking the

elements

65, 66 in a fixture if desired. To insure that the plastic-elements 66 are added correctly if the assembly is being fabricated by hand, the elements 66 may have unequally spaced radial barriers which misalign if an element is stacked upside down.

The

elements

65, 66 may be cut, punched or molded or prepared in other conventional fashion. When the laminate has been stacked to a desired length, referring now to FIG. 7, it is thereafter unified by a time, temperature and pressurecycle sufficient to cause the nonconductive elements 66 uniformly to reach the plastic fiow state and to become activated for final cure. In this state, the material flows or impregnates into the interstices in the conductive elements 65 filling and sealing the interstices and defining the internal composite barriers or passageway walls..The flow is, however, largely limited to the adjacent mesh areas.

The following examples are illustrative of different heat exchanger fabrication methods.

Example I Aluminum alloy screen woven of 0.0045" diameter wire was utilized in conjunction with 0.0075" sheet clear polystyrene, each being cut with steel rule dies to form the appropriate configurations. The appropriate number of patterned parts were then cleaned as set out below:

A. The aluminum screens were degreased in M-l7 solvent, then immersed for 3 minutes in a solution of deionized water, sulphuric acid (specific gravity 1.84) and sodium dichromate at room temperature, then rinsed in cold water by a water spray, then cleaned with ultrasonic agitation in isopropyl alcohol for two minutes, and finally air dried with filtered air.

B. The polystyrene parts were immersed in isopropyl alcohol with ultrasonic agitation, followed by air drying with filtered air. The cleaning of both the aluminum screens and the polystyrene was effected in clean room facilities, which were thereafter also utilized during assembly.

Subsequent to the pre-cleaning of the formed ele-- ments, they were assembled into a laminate prior to bonding and concomitant unification. In the present example, the assembly comprised alternating aluminum screen elements and apertured polystyrene discs, to provide a finished cylinder approximately 5" in di- 7 ameter by 9 long. The assembly was compressed and bonded in the following sequence:

A. Uniform pressure was applied to both ends of the assembled cylinder. The cylinder was placed in a matching sleeve, and pistons slidable inwardly within the sleeve were disposed adjacent each end of the cylinder. A hard vacuum was pulled on the specimen; once a stable vacuum level was reached the pistons were clamped in place after being fully seated. Sintered brass discs permeable to helium were inserted between the ends of the stack and the pistons in the assembly.

B. Preheated helium at approximately 280F. was then flowed through the stack at 0.5 CFM. The preheated helium purged the system of other gases and brought the assembly closer to the plastic flow temperature.

C. The entire fixture was then immersed in oil maintained at 350F., for thirty minutes to cause plastic v flow of the polystyrene. At the end of this time, the helium flow was stopped and a hard vacuum was pulled on the assembly until a stable vacuum level was again reached. The pistons were released to be drawn inwardly,further compressing the assembly.

The second compression under these uniform temperature conditions assured flow of the plastic material into the interstices of the screen elements and maintenance of the densified form.

D. After the second compression, the entire assembly was maintained at 35 F for approximately 20 minutes for stabilization.

E. After stabilization, the temperature of the assembly andfixture was reduced and the assembly was allowed to cool at a slow rate, while maintaining flow of helium. After reaching the approximate temperature level of the helium, the assembly was allowed to air cool to ambient conditions.

The assembly was then removed from the fixture for affixation of the header assemblies.

Example II For the assembly of a structure utilizing copper screens, the copper screens were woven of 0.010 diameter wire, and precleaned as follows:

The copper screens were degreased in M-l7 solvent,

immersed for 10 minutes in a solution of ferric sulphate and sulphuric acid (specific gravity 1.84) at 150F., rinsed by a cold water spray, immersed at room temperature in a solution of sodium dichromate and sulphuric acid (specific gravity 1.84) until clean and bright, rinsed again in a cold water spray, cleaned for 2 minutes in isopropyl alcohol with ultrasonic agitation, and then air dried with filtered air.

As in Example I, the pre-cleaning, assembly and bonding and compression steps were undertaken in clean room facilities. The remaining assembly steps corresponded to those set out in Example I.

The passageway walls that are formed are substantially the thickness of the original wall segments in the non-conductive elements, although a minor amount of lateral spreading may occur. In the given examples, the wall thicknesses were 0.090" for the interior walls and 0.070 for the exterior walls. Those skilled in the art will recognize that the time, temperature and pressure relationships that are to be maintained will vary primarily dependent upon the nature of the thermoplastic or other material that is employed, as well as the type of construction that is desired. Higher temperatures are generally required for epoxies than for polystyrene, for example. I

The views of FIGS. 8, 9 and 10 illustrate a regenerator, specifically a unit suitable for employment with a cryogenic refrigerator system. The cryogenic refrigerator may be of any well known type using a thermodynamic cycle in which a regenerator function is essential, such as a Vuilleumier machine. Successive stages of regenerators are often employed, to achieve the final very low temperatures that are desired in some systems. At final temperature levels in the range from 5K. to 10K., for example, thelowest temperature regenerator stage must meet critical requirements as to thermal mass and heat transfer efficiency. That is, sufficient material of appropriate specific heat must be utilized, and the heat transfer properties of the unit must transfer heat to and from the stream of flowing refrigerant gas with a very low temperature differential (e.g. less than approximately 0. 1K.). The heat transfer must also be achieved with an acceptable ratio of heat transfer efficiency'to pressure drop, for the given final temperature desired. The low temperature regenerator 72 shown in FIGS. 8-10 fulfills these requirements while at the same time being economically and readily fabricated. Other-regenerator stages have not been shown for simplicity and are here considered to be part of the refrigerator system 70.

The regenerator 72 comprises a metal-plastic exchanger within an outer cylindrical housing 73, and including a stack of copper screens 75 with internal heatflowed longitudinal epoxy barriers 77, defining longitudinal passageways 78 for the refrigerant gas. In addition, however, the barriers 77 define internal chambers 79 confining a thermal massmaterial,specifically pressurized helium gas, in stagnant fashion. The conduit system for the flow of refrigerant through the passageways 78 has been shown only schematically. The helium gas, typically pressurized to the approximate range of 20-30 atmosphere, is supplied from a source through conduits 82. The number of flow passageways 78 and stagnant chambers 79 is variable at the choice of the designer, as will be evident to those skilled in the art. Thus, each refrigerant passageway 78 may be arranged with individually adjacent chambers 79 for best heat transfer properties in a specific case. In the present example, three refrigerant passageways 78 are disposed along a diameter of the cylinder and two chambers 79 are disposed on each side of the diameter.

The operative characteristics of the regenerator 72 shown by way of example adequately satisfy the requirements of ultra low temperature systems. The pressurized helium has an appropriate specific heat for these low temperatures and further provides a thermal mass which has a high ratio to that of the flowing refrigerant gas. The ratio of the total thermal mass of the regenerator to the total thermal mass of the refrigerant is in the range of 10 to 20, which 'is much more than adequate. These'factors are made possible by the leaktight, high strength composite wall constructions which can contain the pressurized helium,'together with the highly efficient thermal interchange effected by the laterally continuous heat conductive paths provided by the copper screens. The various sources of loss and inefficiency, including parasitic losses, temperature difference between the refrigerant gas and associated screen, and total fin losses, can be shown to have only a minor effect on efficiency. v

In constructions in accordance with the invention it is feasible to provide small passageways and thin walls, so that excellent interspersion of flow passageways can be achieved with respect to the thermal mass. An example of a fragment of the cross-section of one such regenerator is shown in FIG. 11, it being understood that the passageways and stagnant chambers receive gates in the fashion shown generally in FIGS. 8-10. The refrigerant flows in round passageways 83 uniformly dispersed throughout a filament-reinforced composite wall structure 84. These passageways 83 in this example are approximately 0.080 or less in diameter, and may of course have other cross-sectional shapes and be distributed differently. The helium storage chambers comprise a number of pairs of curved apertures85, each pair being concentric with a different passageway 83. The helium storage apertures 85 each have are lengths of slightly less than that of a semicircle, so that .each pair almost completely encompasses its associated passageway 83. The wall thickness between a passageway 83 and its paired helium storage apertures 85 is the same as the radial separation, and here is approximately 0.060" or less. The spacing between the facing ends of the apertures 85 is also the same as the wall thickness. The width of the apertures 85 (again expressed as a radial distance) is approximately 0.030" or less.

All of the stated dimensions can be reduced substantially by one-half or more, when regenerators are constructed in accordance with the invention. The composite walls still have adequately high strength to with stand the pressure differentials, and the melted plastic does not migrate laterally to an extent which blocks or even substantially deforms the small passageways and apertures.

Therefore, regenerators having the configuration of FIG. 11 remain leak free while providing a high ratio of helium storage volume to refrigerant flow volume. The helium is pressurized as previously described, so that the mass ratios are many times greater than the volume ratios. Moreover, because of the small passageways 83 and the small wall thicknesses, each small mass of the refrigerant interchanges thermal energy through short path lengths with a much greater thermal storage mass in the adjacent apertures 85. There is substantially insignificant thermal gradient within each passageway 83, velocity-gradients are extremely low, and the filling factor is high. For such reasons, the low losses and high efficiencies inherent in structures in accordance with the invention are further improved. For ultra low temperature regenerators of maximum efficiency it is preferred to have passageway, wall and aperture dimensions of no greater than 0.1 because of the short path lengths and high degree of interspersion this provides.

The transverse mesh elements of these heat exchangers need not be in abutment, but can be separated by composite elements which insure retention of strength. A fragmentary view of a'section of aheat exchanger employing one such configuration is illustrated in FIG. 12. A completely reinforced wall construction is achieved through the interspersion of narrow width screen wall elements 89 between the principal transverse screen elements 59 with the screen wall elements being built up along a selected length to space the principal elements 59. A plastic matrix 90 fills the interstices of the screen wall elements 89 to again form a composite filament-reinforced barrier or passageway wall. It will be appreciated that the fragment shown in FIG. 12 relates only to a fragment of one wall in a passageway system. This-wall construction can also withstand a substantial pressure differential, inasmuch as the composite material has substantial circumferential strength. Conduction across the thickness of the wall by the presence of the apertured mesh 89 serves to enhance the heat transfer characteristics, although the wall elements can be insulated if desired.

The arrangement of FIG. 12 illustrates a different way of shaping the longitudinal temperature gradient to a particular profile. The greater spacing of the principal transverse elements 59 (in comparison to the example of FIGS. 1-4) controls the transfer between two counterflowing fluids, permitting achievement of both extremely shallow and steep temperature gradients within a single heat exchanger construction.

As shown in the side sectional view of FIG. 13 and the enlarged fragmentary view of FIG. 14, in somewhat idealized form, the cross-sectional areas of separate passageways may also be varied along the length of a counterflow exchanger 10' having uniformly interspersed passageways for two fluids. In the formation and assembly of the laminate, successive nonconductive elements have successive wall segments that are slightly displaced relative to each other in a preselected pattern. For a single passageway, each successive non-conductive element defining a passageway wall overlaps the immediately adjacent non-conductive elements along a selected distance relative to the wall thickness. Thus a series of plastic elements may have the segments defining one passageway vary progressively in the size of their perimeters, but with each overlapping the adjacent elements. When the plastic is heated, melts and subsequently migrates into the mesh, the overlapping relationship is preserved and the passageway wall is unified. The wall is continuous and although there is a minute incremental displacement of the plastic at one screen relative to the adjacent screen, the overall result is the introduction of an essentially continuous curvature into the longitudinal axis.

In FIG. 13 the curvature provides alternate pairs of

curved walls

92, 93 having oppositely bowed characteristics, providing cross-sectional areas of greatest size in the end regions, and in the center region, respectively. Such an arrangement is useful in compensating flows between respective passageways in a counterflow exchanger, in order to achieve better heat transfer effectiveness by equalizing the separate flows, as described in a co-pending application for patent, owned by the assignee of the present application, entitled Flow Compensator For Exchanger Apparatus, Ser. No. 795,922, filed Feb. 3, 1969, now US. Pat. No. 3,608,629, issued Sept; 28, 1971.

Although there have been described above a number of alternative forms and modifications of structures and methods in accordance with the invention, it will be appreciated that the invention is not limited thereto, but encompasses all structures and methods in accordance with the appended claims.

What is claimed is:

1. A heat exchanger matrix for counterflowing fluids comprising:

v 12 stantially parallel to a selcted flow path axis, said a plurality of substantially contiguous thermally conductive screen elements disposed substantially transversely across desired flow paths; and

heat-flowed plastic barrier means defining walls passageways defining at least two interspersed sets vidual passageways of each set desirably maintainof passageways for counterfiowing fluids, the indidefining passageways varying in cross-sectional area along their lengths to equalize flows in said flow paths, the variations in area providing pasalong said flow paths, said barrier means impreging substantially equal flows, said body including a nating and sealing said screen elements, said barplurality of fine mesh screens substantially transrier means defining, with the filaments of said verse to the flow path axis, and interior heat-flowed screen elements, composite walls defining the deplastic means defining said passageways and filling sired flow paths, said screen elements thermally inthe interstices of said screens to provide filamenterconnecting said flow paths, said composite walls treinforced composite passageway walls, said passageways of one set being concave along their lengths and said passageways of the other set being convex along their lengths.

3. The invention as set forth in claim 2 above,

15 wherein said passageway walls have essentially continuous curvatures defined by minute incremental displacements of the plastic means'between successive screen pairs.

sageways for the different counterflowing fluids that are oppositely bowed along their lengths. 2. A high efficiency heat exchanger for counterflowing fluids comprising:

a longitudinally compressed composite body having selected plurality of interior passageways lying sub-

Claims

1. A heat exchanger matrix for counterflowing fluids comprising: a plurality of substantially contiguous thermally conductive screen elements disposed substantially transversely across desired flow paths; and heat-flowed plastic barrier means defining walls along said flow paths, said barrier means impregnating and sealing said screen elemEnts, said barrier means defining, with the filaments of said screen elements, composite walls defining the desired flow paths, said screen elements thermally interconnecting said flow paths, said composite walls defining passageways varying in cross-sectional area along their lengths to equalize flows in said flow paths, the variations in area providing passageways for the different counterflowing fluids that are oppositely bowed along their lengths.

2. A high efficiency heat exchanger for counterflowing fluids comprising: a longitudinally compressed composite body having selected plurality of interior passageways lying substantially parallel to a selcted flow path axis, said passageways defining at least two interspersed sets of passageways for counterflowing fluids, the individual passageways of each set desirably maintaining substantially equal flows, said body including a plurality of fine mesh screens substantially transverse to the flow path axis, and interior heat-flowed plastic means defining said passageways and filling the interstices of said screens to provide filamentreinforced composite passageway walls, said passageways of one set being concave along their lengths and said passageways of the other set being convex along their lengths.

3. The invention as set forth in claim 2 above, wherein said passageway walls have essentially continuous curvatures defined by minute incremental displacements of the plastic means between successive screen pairs.