US3543844A

US3543844A - Multiple-pass heat exchanger for cryogenic systems

Info

Publication number: US3543844A
Application number: US730031A
Authority: US
Inventors: Michael Jordan; Bradley S Kirk
Original assignee: Air Reduction Co Inc
Current assignee: Airco Inc
Priority date: 1968-05-17
Filing date: 1968-05-17
Publication date: 1970-12-01
Anticipated expiration: 1987-12-01

Description

United States Patent Inventors Michael Jordan Bellemead; Bradley S. Kirk, Plainfield, New Jersey Appl. N 0. 730,031 Filed May 17, 1968 Patented Dec. 1, 1970 Assignee Air Reduction Company, Incorporated New York, New York a corporation of New York MULTIPLE-PASS HEAT EXCHANGER FOR CRYOGENIC SYSTEMS 20 Claims, 16 Drawing Figs.

US. Cl 165/155, 165/ 184 Int. Cl F28d 7/00 Field ofSearch 165/154, l56,164,166,140,141,157,184,155,169

References Cited UNITED STATES PATENTS 2,532,288 12/1950 Buschow 165/141 6/1953 Buschow 165/154 SIVOZ UJJS/VVUJ Primary ExaminerRobert A, OLeary Assistant Examiner-Theophil W. Streule Att0rneysFrancis B. Henry, Edmund W. Bopp and H. Hume Mathews ABSTRACT: A plurality of thin-wall metal cylinders are concentrically nested in radially spaced relation to form respectively, annular fluid passages mutually related for heat transfer. The passages connect at their opposite ends respectively, with fluid distributing headers to make up a multiplepass heat exchanger. The concentric passages are spanned throughout by heat-conducting extensions consisting of coiled metal strip. Each coil turn is essentially rectangular and at opposite ends of the rectangle is integrally joined to the passage walls respectively, for heat transfer between contiguous passages and for imparting mechanical strength and rigidity to the thin-wall cylinders. The coiled strip is arranged for defining fluid dividing ducts that direct fluid flow through each annular passage along a generally helical path.

mgtea Dec. 1, 1910 Sheet l of 4 HEAT- ZONE R] 1-/-/EA DER TRANSFER INVENTOR$ MICHAEL JORDAN BRADLEY S. KlRK BY 5 NW ATTORNEY Patntd Y Deg. 1, 1970 3,543,844

Sheet 2 of 4 STRIP COIL HEADER C O/L cow WARM fil/GH DENS (Low osA/smy H WARM COLD F I G. 2 INVENTORS MICHAEL JORDAN BRADLEY S. KIRK ATTORNEY Patented Dec. 1, 1970 Sheet FIG. 5

I $64 I. a l!!! I I n 5 a i N K MR m w M s N w N 6 T A T H A NCA /||R MB VI B 3 STREAM MULTIPLE-PASS HEAT EXCHANGER FOR CRYOGENIC SYSTEMS This invention relates to heat exchangers, especially those of the multiple-pass type wherein a plurality of fluid passages having mutual heat transfer relationship, are arranged for counterflow of fluids with different heat content and pressure levels, respectively. In particular, the invention primarily relates to heat exchangers for use in the general field of cryogenics involving extremely low temperatures.

The most effective and practical use of heat exchangers in cryogenics is complicated by certain inseparable problems, heretofore unsolved, in the design and construction of the integrated heat exchange passages and related fluid contacting structure. The ideal characteristics of such heat exchangers include essentially:

a. highly efficient and rapid heat transfer between the stream of fluid in a respective passage and the fluid contacting structure therein, i.e., large heat transfer coefficient;

b. negligible frictional pressure drop for the desired rate of fluid flow through the exchanger;

c. high rate heat exchange between the respective fluids in contiguous counter flow passages per unit of passage length;

d. overall compactness of the heat exchanger unit for minimizing the extent of exterior surface area, thereby minimizing so-called heat-leak" from environment (especially important in cryogenic equipment); and

e. rugged mechanical strength and rigidity of the internal,

multiple-pass structure of the heat exchanger unit sufficient to withstand relative explosive and implosive forces (incident to pressure differentials between contiguous passages) tending to rupture or collapse as the case may be, the respective passage wall.

In practice, a balanced relationship or compromise, is required between the linear fluid velocity and fluid pressure drop in a given passage for obtaining optimum heat transfer coefficient, namely, the ability to transmit the heat between the fluid stream and the contact surfaces of the heat exchanger passage; also, the last factor, i.e. mechanical strength, has been considered diametrically opposed to efficient, high rate heat transfer, because relatively frail and thin heat conducting structures having very high ratio of surface area to mass are needed for the most efficient exchange of heat from one fluid passage to another.

In brief, an ideal heat exchanger should be a compact, sufficiently rugged and inexpensive device providing perfect heat transfer and having no frictional pressure drop in the two heat exchanging fluid streams. However, near perfect heat transfer requires a combination of large heat transfer area (which establishes frictional pressure drop and increases the size and cost of the exchanger), and large heat transfer coefficients per unit area. For given fluid properties and geometry of extended fluid contact areas in the passage, heat transfer coefficients can be increased only by increasing the linear velocity of the fluid, that in turn, also increases the frictional pressure drop. Therefore, in all practical heat exchangers, a linear fluid velocity must judiciously be selected that balances pressure drop against heat transfer coefficients. For a given total flow through the exchanger, the selected velocity determines the cross sectional area of the flow path within a respective passage. In conventional exchangers with straight through flow, the physical length of the exchanger is fixed according to the required total fluid contact area for heat transfer between the two streams, so that the exchanger tends to be elongated rather than compact.

In general quantitative terms, the heat transfer requirements for a simple two-pass cylindrical heat exchanger without extended surface areas, can be expressed according to known principles and empirical methods, as:

TDL

Accordingly, if it be assumed that for a given heat transfer operation the passage cross section is fixed to achieve a desired heat transfer coefficient, the parameter L is the only remaining independent variable. In other words, the length of the heat exchanger cannot very well be held constant if any of the other factors are changed, without changing the effectiveness of heat transfer.

The principal object of the present invention therefore, is an improved multiple-pass heat exchanger wherein highly efficient heat transfer and exchange characteristics, and mechanical strength and ruggedness are integrated within a compact structural unit.

A further object is to provide throughout the fluid passages of the heat exchanger, thin gage heat transfer extension structure having a very large surface area as compared with that of the corresponding passage wall, and having the triple function of lengthening the flow path for a given length of the heat exchanger, thermally interconnecting the passages for high rate heat exchange, and of mechanically trussing the respective passage walls for ensuring uniform strength and stability of the heat exchanger unit.

In accordance with the invention in a practical embodiment thereof, a multiple-pass heat exchanger is made up of a plurality of thin wall cylinders of ranging diameters respectively, nested in concentric relation to form corresponding annular passages for fluid counterflow. Each passage throughout contains a multiplicity of heat conducting extensions having in sum, a very large surface area, and consisting of thin metal strip arranged to form ducts for diving and guiding fluid flow along a helical path about the longitudinal axis of the heat exchanger, the longer helical flow path providing for decrease in the overall length of the exchanger. The strip and opposing cylinder walls of the passage are united for thermally interconnecting the fluid streams in the contiguous passages, respectively. Portions of the metal strip interconnecting the thin cylinder walls are approximately normal thereto for mechanically bracing the walls against high differential pressures in the passages. Uniform fluid diffusion at the inlet and outlet openings of the respective passages is provided by a flow distribution header at each end of the heat exchanger.

The invention will be more fully set forth in the following description referring to the accompanying drawings, and the features of novelty will be pointed out with particularity in the claims annexed to and forming a part of this specification.

Referring to the drawings:

FIG. 1 is a system diagram illustrating a fluid compressionexpansion cycle wherein heat exchangers of the invention can be advantageously used;

FIG. 2 is a simplified diagram illustrating fluid counterflow in parallel branches of the heat exchanger;

FIG. 3 is a partially exposed view in perspective of a terminal portion of a multiple-pass heat exchanger embodying the invention;

FIG. 4 illustrates in simplified form the respective inlet and outlet terminal openings of the heat exchanger passages at each end of the heat exchanger proper;

FIG. 5 is a detail view in perspective of the header and heat transfer coil structure generally indicated in FIG. 3;

FIG. 6 is a partial detail view in transverse section showing the union of the heat transfer coil to opposing cylinder walls of a fluid passage;

FIG. 7 is a partly schematic illustration in developed plan of helical flow of fluid through a passage of the heat exchanger, induced by the heat transfer coils of FIGS. 5 and 6;

FIG. 8 is a detail view in plan of heat transfer coils generally indicated in FIG. 7, showing the coil angle of flow deflectings in relation to the helix angle;

FIG. 9 is a partial view in perspective with exposed sections, of a flow distributing header for the heat exchanger;

FIG. is a fragmentary enlarged view in perspective of an alternate coil arrangement for inducing helical flow through the heat exchanger;

FIG. 11 is a detail perspective view of modified coil used in FIG. 10;

FIG. 12 is a partial view in perspective of an alternative header arrangement for the heat exchanger;

FIG. 13 is a detail view in perspective of the header coils of FIG. 12 in relation to the main coils of the heat exchanger;

FIG. 14 is a developed view illustrating diagrammatically the peripheral flow distribution in a header of the type shown by FIG. 12;

FIG. is an end view, partly broken away, of a multiplepass header, using the single-pass arrangement of FIGS. l2- ---l4, and

FIG. 16 is a sectional view illustrating multiple heat exchanger units of the invention representing, respectively, those of FIG. 1 integrated as a single heat exchanger assembly.

Referring first to FIG. 1 for a brief description of a typical cryogenic process in which the invention is especially useful, the system generally outlined uses a compression-expansion cycle for obtaining refrigeration at very low temperatures. A COMPRESSOR with fluid inlet and outlet passages at 1 and 2 respectively, communicates with the series-connected heat exchangers I-I-l, I-I-2 and I-I-3, a makeup GAS SUPPLY for the system fluid medium is connected at 3 to the compressor inlet passage 1. By way of example, the fluid medium can be helium. The gas in the compressor inlet passage is at low pressure (approximately 1 atmosphere) and is raised to comparatively high pressure such as 10 atmospheres at the compressor outlet. The resulting warm (high-pressure) gas stream in passage 2 leading to the corresponding pass 4 of exchanger H- I is of low density, thereby requiring a comparatively large passage for uniform system flow. An EXPANSION ENGINE takes about one-half the gas flow from the outlet 5 of the highpressure pass 4 and deliversthe cooled, lower density gas at the inlet 6 of the counterflow low-pressure pass 7 of intermediate exchanger H-2. The reduced flow from pass 8 of exchanger H-2, now greatly cooled and more dense, is fed to a smaller low-pressure pass 9 of the'last stage exchanger H-3 that in turn, leads to an EXPANSION VALVE, such as a Joule-Thomson valve, for discharge of the comparatively dense, refrigerated fluid now in the vapor liquid phases, into a storage tank 10. The cold, low-pressure fluid vapor at phase VAPOR, is connected to the counterflow cold pass 11 of exchanger H-3, from which (after being augmented by the EXPANSION ENGINE output to normal flow at 6) the cold" (low-pressure) stream flows in return to the COM- PRESSOR through the respective passes of exchangers I-l-2 and H-1. The refrigeration load is supplied through a control valve as indicated, from the LIQUID phase in the tank.

It will be seen from the operation of the system, that in each heat exchanger, the warm" high-pressure and cold" lowpressure streams in counterflow are of materially different pressures and densities. This involves high differential pressures and widely different gas volumes, that are properly handied in the multiple-pass heat exchanger of the invention in a parallel as indicated, between the warm"

passages

25 and 27,

and the cold"

passages

26 and 28 respectively, so that the resulting four streams are in counterflow through the exchanger. As mentioned above, the cross section of an individual passage can be varied according to the desired heat transfer coefficient and other factors.

It will be seen that the high-pressure, low density warm" streams of comparatively large volume are directed through the larger (mean diameter)

passages

25 and 27, whereas the low-pressure, high density cold streams are directed through the smaller (mean diameter)

passages

26 and 28 for reasons explained above in connection with FIG. 1. The fluid streams are distributed to and collected from each end of the exchanger by a distributing header (not shown) as hereinafter described.

FIG. 3 shows in broken section one end of an operational heat exchanger I-I (without header) embodying the invention, and conforming basically to the design of FIG. 2. The concentrically spaced metal cylinders 20 to 24 making up the heat exchanger proper consist of thin aluminum tubing having thickness for example of 0.020 inches, and define a plurality of annular passages or passes" 25 to 28 for counterflow of the warm and cold fluid streams as in FIG. 2. The passes contain throughout a multiplicity of heat conducting extensions with very large aggregate surface area that are united to the respective cylinder walls at opposing sides of the respec tive pass for diffusing and guiding fluid flow, and for thermally interconnecting and bracing the walls as will hereinafter be described.

The mechanical assembly of the heat exchanger may comprise an end retaining ring 30 suitably mounted on the outer cylinder at each end, and internal supporting structure including a stiffening ring 31 at each end of the exchanger connected by tie rods 32. Corrugated metal sheet 33 or equivalent structure is positioned between the rings and the adjacent wall of the inner cylinder 24 for supporting the wall throughout its length. The exterior wall of the outer cylinder may be jacketed by an insulating casing (not shown) in any preferred manner.

The heat conducting extensions in the respective passes constitute, in combination with the corresponding cylinders, an important aspect of the invention. As generally shown in FIG. 3, and in more detail in FIGS. 5 to 8, a zone of so-called header coil 34 extends a suitable distance from each end of a respective pass into the exchanger for further diffusing and distributing the fluid flow to and from the adjoining header. The header coil 34 preferably consists of metal wire such as aluminum, wound in spaced turn, helical fashion with approximately square cross section, FIG. 5. The coil is wrapped as a tight helix around the respective cylinder, FIG. 3, and thermally interconnects the opposing walls of the passage as presently described.

The main heat conducting extensions consist of metal strip 35 looped as coil that compactly occupies the intermediate main zone between the header coil zones. In a preferred form, the strip consists of thin metal ribbon and is wound flat in helical fashion with cross section (generally along a plane defining a coil turn) approximately rectangular and elongated as shown in FIG. 5. The coil is composed of good heat conducting material such as aluminum in thin, narrow ribbonlike strip approximately 0.010 inches in thickness in the example disclosed. At the shorter or end sides of the rectangle, the coil portions 35a and 3512 respectively, FIGS. 5 and 6, make substantially planar contact with the opposing cylinder walls of the respective exchanger passage, the longer or lateral substantially

parallel sides

35c and 35d being normal to the passage walls, i.e. extending in spanning relation transversely across the passage.

As better shown in FIG. 6, the cylinder walls indicated in part at 21 and 22 that form the annular passage 26 for example, are both mechanically and thermally interconnected by the strip coil 35. In this instance, the coil extends across the passage (and fluid stream) with the longitudinal axis of the coil helix substantially transverse to the longitudinal axis LA common to the passage and heat exchanger. Each turn of the coil at its flat end portions 350 and 35b is suitably united as by dip brazing, to the respective inner walls of the passage as generally indicated at 35a and 35b.

Accordingly, the multiplicity of thin aluminum heat conducting strips forming the lateral side portions of the coils, provide throughout the main heat transfer section of the exchanger a high capacity, low resistance (and therefore highly efficient) thermal connection between the respective thin wall cylinders, FIG. 3.

The fluid stream as it moves transversely through the succeeding rows of coils, FIGS. 3 and 7, which in effect form cascaded fluid ducts, is divided into a multiplicity of minute streams as hereinafter described, each of which makes fluid contact with closely adjacent conducting strip material. Thus, there is achieved by this highly highly divided fluid flow in combination with the exceedingly large aggregate surface area of the heat conducting strips, exceptionally high rate of heat transfer between the fluid stream and the heat conducting extensions within the passage. Since the heat conducting extensions of the contiguous annular passage or passages are each similarly thermally and mechanically connected to the thin, aluminum common wall separating the respective passages, the fluid streams in counterflow are as a result, in highly efficient and practically ideal mutual heat exchange relationship.

As mentioned above, the heat transfer coil 35 serves the ad-' ditional purpose of mechanically strengthening the thin wall cylinders, especially where adjoining passages have a cylinder wall in common. Referring again to FIGS. 5 and 6, each turn of the coil generally simulates the four peripheral walls of an open box, and serves individually as trussing between the cylinders. The lateral walls, i.e.,

strip portions

35c and 35d,which through the brazed end connections at 35a and 35b are integrally united with and substantially normal to the opposing

cylinder walls

21 and 22 respectively, FIG. 6, functions as individual compression and tensions struts. These multiple struts, notwithstanding the comparatively frail structure of an individual strut, have, together with the cylindrical walls of the heat exchanger, combined strength and rigidity sufficient to give the multiple cylinder heat exchange unit adequate strength and rigidity for withstanding the differential pressures and temperatures in the counterflow passes of the exchanger.

For example, considering heat exchanger l-I-l of FIG. 1, the high-pressure stream from the compressor at atmospheres entering the passage 25, FIG. 2, ordinarily would explode or rupture a thin wall such as 21, common to the low-pressure (1 atmosphere) counterpass 26. This pressure differential is resisted both by the tension struts in passage 25 (and outer jacket) and by the compression struts in passage 26 that resist the implosive force tending to collapse the same common wall, 21. Similarly, the high pressures imposed by the parallel branch of the high-pressure stream in passage 27 on the walls 22 and 23 (common to the low-

pressure passages

26 and 28 respectively) is resisted by the tension struts of passage 27 as regards explosive forces, and by the compression struts in

passages

26 and 28 as regards the implosive effect of the same forces tending to collapse the walls thereof.

The cylindrical configuration of the cylinders also is a material factor in supplementing the resistance of the coil struts to the differential forces, especially as regards explosive effect. This is an important consideration where the heat exchanger operates at high pressures. Whereas heat exchangers of known type using flat plates for the passage walls depend primarily on tensile and compressive support afforded by interwall structure in the passages, the present exchanger can rely on the inherent support of the cylinder walls for providing strength over and above that derived solely from the coil struts.

The mechanical trussing of the thin wall heat exchanger cylinders by the heat transfer structure in the passages therefore achieves an efficient and integrated design without the use of thick wall cylinders and/or auxiliary and comparatively bulky bracing structure in the passages that would materially reduce the heat exchange capability of the unit.

An important additional function of the heat transfer coil structure is in lengthening the flow path by deflecting the fluid into helical course through the exchanger. The length of the path for a given length of the heat exchanger depends on the helix angle, measured from the axial direction. An increase for example, of the helix angle increases the length of the flow path materially beyond the exchanger length, while decreasing the cross section of the path to an area less than the cross section of the annular passage. Also, the physical length of the exchanger can be decreased for greater compactness (even while maintaining constant length and cross section of flow path) by jointly increasing the helix angle and the exchanger diameter. This is an important feature in cryogenic refrigerators, as a short, compact unit has less heat-leak respecting environment than a comparatively long, thin unit.

Furthermore, the fluids in the two passes of a heat exchanger often have different properties, and pressure drop may be more critical in one stream than in the other. With the cylindrical exchanger, it is quite feasible to use a different helix angle in the high-pressure and the low-pressure passes, thereby obtaining the optimum length to cross section ratio for the fluid in each pass.

Finally,in cylindrical heat exchangers incorporating multiple parallel passes and in which the diameter of the innermost pass is considerably less than that of the outermost pass, the annular cross section of the parallel flow paths is greater for the outermost passes, but the physical length of the passes must be the same. However, by properly varying the helix angle in each of the parallel passes, making it greater in the inner passes, the flow path length to cross section ratio can be made the same in each of the parallel flow paths, thereby insuring a proper distribution of the fluid flow through the several parallel passes.

In brief, the overall helix angle in the cylindrical heat exchanger can be varied for varying the overall physical dimensions of the exchanger; also, the helix angle can be varied between the highand low-pressure passes to take into account the different physical properties of the two streams; and further, within the highor low-pressure passes, the helix angle in multiple parallel passes can be varied to insure proper flow distribution among the parallel passes.

Where the passage height, i.e. radial distance between cylinder walls, varies as between passes, the heat transfer coil structure (and helix angle) can be modified accordingly. Assuming materially increased height of a given passage, the coil strip can be made more rugged for its mechanical function, and the helix angle can be increased for lengthening the flow path to compensate the increased coil mass.

For establishing the helix angle and increasing the effective travel of fluid flow through the heat exchanger, and hence the contact time between the fluid and total area of the strip coil 35, each coil turn is positioned for deflecting in predetermined manner the respective stream into a helical path along the longitudinal axis of the exchanger. To this end, the lateral strip portions 350 and 35d in a given pass, FIGS. 5, 7, and 8, are uniformly inclined through a selected angle 00 with respect to the longitudinal axis of the coil, i.e. for practical purposes, the transverse axis of the passage. As will be seen from FIGS. 7 and 8, the mean plane of each strip 350 and 35d that ordinarily is substantially normal to the longitudinal axis of the exchanger, is now inclined from the normal by an angle 00, thereby opening" the coil, FIG. 7, by creating a gap between respective edges of adjacent turns.

This angle establishes the helix angle which, measured with respect to the longitudinal'axis LA, is -0c). Hence, the fluid stream in passing through the coil 35 on its way through the exchanger passage divides into a multiplicity of small streams, each of which is deflected by the lateral sides of the coil away from straight flow along the longitudinal axis LA, so that the resultant flow takes a generally helical path through the respective annular passage as diagrammatically indicated in FIG. 7. Accordingly, since angle as can be varied in forming the coil for a given pass, each branch respectively, of the counterflow streams can be guided according to desired exchanger characteristics, along a helical path of predetermined length for optimum heat exchange between contiguous streams.

Referring again to FIG. 3, the four exchanger passages 25 to 28 contain at each end thereof a limited zone of HEADER COIL, wherein the coil 34, FIGS. 3 and 5, is wound as a tight helix around the respective cylinder at each end to make up a band of desired width for diffusing the respective fluid streams that flow to and from the intermediate heat transfer zone STRIP COIL. Accordingly, both branches of each stream in addition to passing through the main heat transfer strip coil 35, pass through a diffusing zone upon entering and upon leaving the exchanger. The header coil is brazed at opposite ends to the respective cylinder walls in the manner of the strip coil described above, and is more open and rugged as regards individual turns than the strip coil. Accordingly, the header coil 34 also functions as trussing for the cylinders, and also in more limited extent, as heat transfer structure.

As mentioned above, each end of the heat exchanger has fluid diffusion means such as a distribution header connected to the passage inlet and outlet openings thereat. One form of header suitable for the exchanger of FIG. 3 is shown by FIG. 9 wherein an outer cylinder forming the exchanger casing 40 may be extended at opposite ends for housing identical headers, the left end header being shown in FIG. 9. The exchanger proper preferably corresponds to that of FIG. 3, wherein the passages 25 to 28 contain at the respective end zones thereof header coil 34. As in FIG. 4, the passage terminal area is divided into sectors, and segments of the passage openings are alternately blocked off, as by solder 41, for properly directing the counterflow streams to and from the headers.

Referring briefly back to FIGS. 2 and 4, it will be assumed that the high pressure war m" stream entering the RIGHT END of the exchanger is admitted to the 180 displaced INLET quadrants (or sectors) and divided between the

passages

25 and 27. Since the

passages

26 and 28 carry the low-pressure cold counterstream, the unused outlets of these passages in the INLET quadrants are blocked off at 41. The outlets of the warm streams at the LEFT END are at diametrically opposed OUTLET quadrants wherein the

cold passages

26 and 28 are also blocked off at 42 as described above. Similarly, in the INLET and OUTLET quadrants for the cold stream at the LEFT END and RIGHT END respectively, the cold"

passages

26 and 28 are open, and the

warm passages

25 and 27 are blocked. There is no need for peripheral alinement of the lNLETand OUT- LET quadrants, as each stream in passing through the exchanger follows a helical path that, depending on the preferred pitch of the helix, may complete more or less than one revolution about the exchanger axis LA.

The header arrangement of FIG. 9 distributes the warm and cold streams between the respective inlet and outlet sectors, generally as described above. In this instance, a larger number of sectors are indicated for more even peripheral distribution of flow. In FIG. 9 (representing the LEFT END of r the exchanger) the low-pressure cold" inlet line and the high-pressure warm" outlet line 51 are connected respectively, to concentric

annular header chambers

52 and 53.

The header chambers at the outer ends are closed by an end cover 54; the concentric chamber side walls include the outer casing 40 and spaced

circular walls

55 and 56, the wall 56 being common to both chambers. The. low-pressure inner chamber 52 has for reasons mentioned above, materially less volume than high-pressure chamber 53. Each annular chamber at the heat exchanger side opens into certain plenum chambers that are positioned in alternate order around the annular exchanger passage terminal area. Specifically, the terminal area is selectively connected at respective sectors to the

header chambers

52 and 53 through a ring of separate flow dividing plenum chambers described below, and partly indicated at 60 to 67 in FIG. 9. Each plenum chamberspans an appropriate sector wherein the exchanger passage areas are open or blocked, according to the direction of the respective stream. The plenum chambers are of generally boxlike form with common sides 68 extending radially from the inner cylinder wall 69, across the exchanger passages to join with the outer casing wall 40. That part of each plenum chamber immediately adjacent to a header chamber, 52 for example, is closed by a wall 70 at alternate sectors and connects with the header chamber through openings 52a, 52b, 520, etc. at the other sectors, respectively. Similarly, the parts of the plenum chambers immediately adjacent to the outer chamber 53 are closed by walls 71 located in staggered relation to walls 70, and connect with chamber 53 through openings at 53a, 53b, 53c, etc. at alternate sectors respectively. Thus, each individual INLET and OUTLET sector connects with the respective inlet and outlet fluid linein essentially the same manner described for FIGS. 2 and 4.

The stream distribution is indicated in FIG. 9 by inlet and outlet flow lines for the low-pressure and high-pressure streams, and is believed to be apparent, following the descriptions above. In brief, the incoming low-pressure "cold" stream from annular chamber 52 enters the

corresponding plenum chambers

60, 62,64, etc. and flows into the header coils 34 of the exchanger

cold passages

26 and 28, the high-pressure

warm passages

25 and 27 beingblocked off at 41. These plenum chambers accordingly feed evenly into the exchanger low-pressure passages at points spaced equally around the exchanger.

Similarly, the outgoing high-pressure stream from

exchanger passages

25 and 27 opening into the

corresponding plenum chambers

61, 63, 65, etc. flows into the header chamber 53 through

openings

53a, 53b, 53c, etc. the exchanger low-

pressure passages

26 and 28 being blocked off as indicated. The header arrangementv at the opposite or RIGHT END of the exchanger, FIGS. 2 and 4, wherein the high-pressure stream enters and the low-pressure stream leaves the exchanger passages, is the counterpart of that described above. I

The distributing header described in FIG. 9 above, in connection with helically wrapped coil arrangement of FIG. 3, is equally applicable to a modified form of heat transfer coil such as shown in FIGS. 10 and .11. Helical fluid flow through the exchanger in this instance is induced by spiral bands of strip coil, a single band being indicated at 75, FIG. 10. The bands are fitted side by side to fill each exchanger passage throughout, and constitute structural units that thermally and mechanically interconnect the passage cylinders essentially in the manner described above. Each unit 75 consists of a flat, thin metal trough 76 having flanged edges 77 of flat or corrugated shape as desired, that are joined to the upper cylinder wall 20 for example, and retain fluid flow essentially within the spiral band limits. The trough is filled with parallel rows of strip coil 35', FIG. 11, that extend in the same direction as the trough and therefore form a multiplicity of parallel ducts for stream dividing and effective heat transfer. The coil 35' is regularly wound in contrast with the strip coil of F 10$. 5 and 8 wherein the coil sides are angled for inducing spiral flow. Illustration of a single passage of the exchanger such as at 25, is sufficient for a clear understanding of this aspect of the invention, it being understood of course that the terminal span of each band can if preferred, define a separate sector for combination with the header as described above. The thin trough walls are of good heat conducting material as aluminum, and are sufficiently flexible for conforming to the lower cylinder wall 21. The trough base and edges are united to the

cylinders

20 and 21 by dip brazing, the coil 35' being similarly united to the trough base and cylinder 20.

FIGS. 12-15 illustrate an alternate header arrangement wherein specially formed header coil replaces the header chambers and plenum chambers of FIG. 9. Each stream passing through the exchanger is peripherally distributed for even flow within the terminal zones of header coil itself.

Referring for example, to the single passage arrangement shown by FIG. 12, segments of special header coil 85 (FIG. 13) are wrapped around the cylinder at the inlet end of the low-pressure cold passage 26 between the limits of the zone, HEADER, in successive circular paths around the cylinder. Coil discontinuities, or gaps between segments, define a plurality of separate patterns constituting

open areas

80, 81, 82, 83, 84, etc. in the HEADER zone. These areas are located in offset relation and are laterally interconnected through the header coil segments as described below.

Referring briefly to FIG. 13, there is shown at 85 header coil for this purpose. The adjoining strip coil is that of FIGS. 5- 8 above and is shown only to indicate the coil positions at the zone division. The header coil is of aluminum strip closely wound for generally rectangular cross section, thereby forming a longitudinal duct through which the

open header areas

80, 81, etc. above are interconnected. The strip at one side of the coil has alined offset loops that together constitute a lateral retaining flange 86. Each segment of coil is firmly held in place on the cylinder 22 by a tensioned wire 87 overlying the flange and encircling the cylinder. The header coil within the passage 26 is subsequently permanently united to the

cylinders

21 and 22 by dip brazing as described above, for fixing the boundaries of the open areas.

Referring again to FIG. 12, an open wedge-shaped area 80 defined by lateral gaps in the header coil 85, connects directly at the wedge base with the incoming fluid stream. A similar opening (not shown) is diametrically opposite for the same purpose. The segmented header coil sections form individual lateral ducts that terminate at the area 80 for conducting the fluid in opposite directions therefrom as indicated by the flow lines, into diametrically spaced diamond-shaped (partly shown)

areas

81 and 82 that also receive at the opposite sides (not shown) fluid from the divided stream. The resulting diffused streams from

areas

81 and 82 then flow through the coil ducts laterally as indicated, toward and into the staggered lower

triangular areas

83 and 84 respectively, which directly open at their base portions into the heat transfer STRIP COIL zone. Corresponding triangular areas are at the opposite side of the header zone. Thus, the stream is evenly distributed peripherally to the heat transfer zone where it is directed along a helical course through the exchanger as described above. The header coil as described is duplicated at the outlet end of the exchanger.

FIG. 14 illustrates by a development of the cylindrical area of FIG. 12, the diffusion and distribution of fluid flow through a single pass of the header, taking for example an incoming stream that is to be peripherally distributed at the heat transfer zone of the exchanger. The stream inlet for the header pass herein disclosed, consists of the two diametrically disposed chambers, 80 and 80, each being of inverted-pyramid shape as viewed, and the base 800 being open for connection with the inlet line as indicated by flow arrows. The sides of each chamber (which converge to close the apex) open into the lateral ducts 85 as described above, thereby confining flow laterally from the chamber in either direction through the corresponding connecting ducts.

The diamond-shaped

difiusing chambers

81 and 82 constitute the next succeeding row, and are offset as indicated, or staggered peripheral, positions with respect to the inlet cham bers 80. Lateral overlapping, as at the lower and upper parts of the

chambers

80 and 81 respectively, provides fluid inter- FIG. 15 illustrates a simple stream dividing arrangement for the exterior counterflow streams of a multiple-pass heat exchanger, using the headers of FIGS. 12 and 13.

Plenum chambers

90, 91, 92 and 93 spaced 90 apart, are mounted over alined areas 80 of the respective passages defining a distribution sector, and the appropriate pass segments are blocked as described above. All pass terminal openings between the plenum chambers are closed as shown so that flow is confined solely to the plenum chambers.

The incoming line for the low-pressure cold" stream is connected by conduits 96 and 97 to the

cold plenum chambers

90 and 92 respectively, and the outgoing high-pressure warm" line 51 is similarly connected at 98 and 99 to the

warm plenum chambers

91 and 93. Counterpart connections for the outgoing cold line and incoming warm line are provided at the opposite end of the exchanger.

As shown in a preferred form, the heat transfer strip coil is in the form of closed angular loops in the general pattern of a helix; however, the use of strip coil in extended open-loop angular form is also within the scope of the invention. Accordingly, as used herein the term strip coil considered in combination. with the opposing walls of an annular fluid passage, is meant to include thin metal strip, or ribbon, bent flatwise angularly in the form of loops, either closed or open, and wherein each loop has spaced, generally parallel sides that are normal to both passage walls.

In another aspect of the invention, a plurality of heat exchanger units that are interrelated for example as in FIG. 1, are combined in a single, compact assembly to make up a selfcontained, thermally efficient heat exchanger having the advantages of reentrant flow. This is illustrated by FIG. 16 wherein the heat exchanger units I-I-1, I-I-2, and lI-3 are nested in generally concentric manner with header connections according to the invention, to form in effect, a single, compact heat exchanger. Each exchanger unit l-I-l, etc., is represented as a two-pass counterflow exchanger with extended heat transfer coil (not shown) in the respective passages, for establishing helical flow as described above.

In cryogenic refrigerating apparatus of this type, the pressure drop through the exchanger must, for reasons of efficiency, be quite small. In all three exchanger units the pressures in the respective high-pressure passes and in the low-pressure passes are substantially constant. However, the gas temperature in respective passes does not change which, at substantially constant pressure, changes the fluid density. For example, the average temperature in exchanger unit H-l is greater I than in unit I-I-2, thus the gas is less dense in H-l. Furthermore, the temperature change from entrance to exit is generally greater in I-I-l than in I-I-2, thus H-l requires more heat transfer area than does I-I-2. For these two reasons, the

exchanger unit H-l is physically larger than is I-I-2. Only about unusually compact refrigerator assembly, with accruing adconnection between chambers through corresponding lateral ducts as indicated by flow arrows.

The lower row of

outlet chambers

83, 83', 84, and 84 directly adjoins the coils 35 of the heat transfer zone. Here, each chamber is of pyramid shape with the base 83a, 840, etc. opening into the coil area at 35. The outlet chambers are in staggered relation to, and partially overlap laterally, the

intermediate chambers

81 and 82 as described above, for distribution of the incoming stream at four peripherally spaced areas along the inlet of the heat transfer zone. It will be apparent that the number of chambers, both as regards successive rows and peripheral positions may be increased or varied as desired for finer stream division. 1

vantages in insulating the exchanger units from the comparatively warm environment and minimizing heat-leak. The coldest exchanger unit I-I-3 is shielded by unit I-I-2 which is, in turn, shielded by the warmest exchanger unit H-l where the temperature differential with respect to environment is comparatively low thereby providing an optimum arrangement, especially as regards heat-leak at the cryogenic temperature level.

The structural aspect of the assembly, FIG. 16, is shown but generally, it being understood for example, that different interconnecting and header arrangements can be used for the reentrant connections between the units H-l, l-I-2, and H-3. A simplified header system, providing for the tapoff connections at the EXPANDER, may consist of an annular plenum viewed) of units H-1 and H-2, and another annular plenum chamber 102 serving as header for the lower ends of units 1-1-2 and H-3. The compressor stream at the outlet of the high-pressure pass of H-l, flows into the annular plenum chamber 100 from which it divides as two streams, one in reentrant flow into unit H-2, and the other to the EXPANDER. The H-2 high-pressure stream again reverses for reentrant flow at 101 into the corresponding pass of cold unit H-3 that in turn feeds into the liquid vapor phase tank as previously described.

The other divided high-pressure stream is now reduced in pressure and cooled by the EXPANDERJrom which it can smoothly diffuse radially through the circular chamber 103 into the annular plenum chamber 102. The low-pressure stream from unit H-3 and the expander output merge in the plenum chamber 102 and flow therefrom in reentrant manner into the low-pressure pass-of 1-1-2, and again, through reentrant passage at 104, into the low-pressure pass of H-l, to the COMPRESSOR.

A practical aspect of the arrangement described, is the use of simple headers without diffusing partitions, etc., that is made possible by the comparatively large available etc. space, and also by division of flow, i .e. reduced gas volume, from plenum chamber 100. The same basic reentrant flow path can also be used without any stream taps in order to achieve great length of flow path in an unusually compact heat exchanger unit, that otherwise would have excessive length.

Having set forth the invention in what is considered to be the best embodiment thereof, it will be understood that changes may be made in apparatus as above set forth without departing from the spirit of the invention or exceeding the scope thereof as defined in the following claims.

We claim:

1. A multiple-pass heat exchanger comprising:

a. a plurality of thin-wall tubes of heat conducting material in telescopic relation and radially spaced for defining a plurality of annular fluid passages extending longitudinally of the heat exchanger for counterflow of fluid at differential pressures respectively;

b. each annular passage having flow dividing and deflecting coils of heat conducting material extending around and in adjoining rows along the passage to constitute with a contiguous annular passage a heat transfer zone;

c. the coils having helical loops of thin, flat strip metal extending generally transversely of fluid flow with individual loops spanning the passage and affording passage therebetween and therethrough of the fluid stream;

d. the loop ends at the flat surfaces thereof being joined respectively, to the opposing tube walls of the passage for high thermal conductivity and mechanically trussing the tubes for strength and rigidity; and

e. the flat lateral sides of the loops being disposed at an angle with respect to the longitudinal axis of the coil row for deflecting the stream through the contiguous rows in helical flow along the passage.

2. A multiple-pass heat exchanger as specified in claim 1 wherein the coil loops define a multiplicity of fluid ducts therebetween for peripherally dividing within the passage the fluid stream.

3. A multiple-pass heat exchanger as specified in claim 2 wherein the ducts conduct the fluid into diffused helical flow through the passage.

4. A multiple-pass heat exchanger as specified in claim I wherein the looped strip is of thin metal ribbon, and the ribbon at the loop ends is united at planar surfaces thereof to the passage walls respectively.

5. A multiple-pass heat exchanger as specified in claim 4 wherein the looped ribbonprogressively encircles as a helix the tube forming the inner wall of the'respective passage, and the planar surface of the ribbonat the lateral sides of each loop is inclined with respect to the longitudinal axis of the passage for deflecting fluid flow obliquely through successive rows of loops into a helical path through the passage.

6. A multiple-pass heat exchanger as specified in claim 4 wherein the loops are formed as closely wound coils and are grouped as a plurality of contiguous spiral bands extending through the passage, each band consisting of a single layer of coil sections closely disposed slide by side and each coil section generally defining fluid ducts extending in the direction of the respective spiral band for imparting helical direction to the stream.

7. A multiple-pass heat exchanger as specified in claim 6 wherein the coil sections of each spiral band are disposed in parallel arrangement within, and united to, a flexible trough of heat conducting material having lateral retaining edges, the band troughs for a respective passage being united in conforming contact to, and covering the outer surface of, the tube constituting the inner wall of the passage, and the trough edges and coil sections being united to the tube forming the other wall of the passage.

8. A multiple-pass heat exchanger having a plurality of concentrically disposed thin wall cylinders forming annular heat transfer passages respectively, for fluid flow through the exchanger, the passages containing at least in part a plurality of contiguous rows of coiled wire of good conducting material extending around the passage, the coils having a generally rectangular cross section spanning the passage with the end sides of the rectangle united to the opposing passage walls respectively, and the parallel lateral sides thereof extending normal to the passage walls.

9. A multiple-pass heat exchanger as specified in claim 1 wherein each end of the heat transfer zone is connected to a diffusing zone consisting of contiguous rows of loosely coiled wire extending transversely across the annular pasage.

10. A multiple-pass heat exchanger as specified in claim 9 wherein each row of the coiled wire has an approximately square cross section, and the rows constitute a helix wrapped around the tube forming the inner wall of the passage.

11. A multiple-pass counterflow heat exchanger having a heat transfer zone comprising a plurality of concentrically disposed annular fluid passages with heat conducting extensions therein, the opposite ends of the heat transfer zone being divided into sectors, the passages for one stream opening into a respective sector and the passages for the counterflow stream being closed at said sector, and a flow distribution header at each end of the heat transfer zone opposite the sectors thereof, comprising a header housing with partitions forming respectively, annular inlet and outlet chambers, and a plurality of circumferentially disposed plenum chambers opposite respective sectors, each plenum chamber at one end opening into a respective sector, and communicating at its opposite end with one of the annular chambers respectively, in alternate order.

12. A heat exchanger having a plurality of concentrically disposed cylinders forming therebetween a plurality of annular passages, each having heat conducting extensions, for defining a heat transfer zone for fluid flow through the exchanger, the annular passages being extended to form a combined fluid diffusion and distribution header zone at an end of and adjoining the heat transfer zone, each header zone passage containing successive rows of discontinuous coils forming fluid ducts spaced as open end segments encircling the passage, segments of contiguous rows terminating at peripheral regions respectively, for'malting with the passage walls a plurality of chambers connected by lateral ducts for progressively dividing fluid flow through the header zone.

13. A multiple-pass heat exchanger as specified in claim 12 wherein the chambers are disposed generally in succeeding rows transversely of the longitudinal axis of the exchanger, and the respective chambers of one row are in offset peripheral relation to and have fluid connection with those in the next adjoining row. l4.A heat exchanger as specified in claim 12, wherein the respective duct segments are composed of thin metal ribbon closely wound as a coil of generally rectangular cross section, and contiguous segment terminals jointly form the lateral sides of respective chambers.

15. A heat exchanger as specified in claim 13, wherein the chambers of one row are peripherally spaced in adjoining relation to the heat transfer zone and have, at the side in common, open communication therewith.

16. A multiple-pass heat exchanger as specified in claim 1 wherein the fluid is a gas and the individual loops of a respective passage deflect the gas stream obliquely with respect to the longitudinal axis of the passage and into a helical path through the exchanger, the annular passages providing for helical counterflow of comparatively low density and high density gas streams respectively, and the helix angle as established by the loops of a respective passage determine the length of the heat transfer path of the corresponding stream.

17. A multiple-pass heat exchanger as specified in claim 16 wherein the helix angle for the low density stream is different than that for the high density stream.

18. A multiple-pass heat exchanger as specified in claim 16 wherein the cross-sectional area of the low density stream passage is greater than that of the high density stream passage.

19. A multiple-pass heat exchanger as specified in claim 1 wherein the fluid is a gas for refrigerating at cryogenic temperatures and the annular passages constitute a plurality of concentrically nested counterflow heat exchanger units, and wherein the exterior unit has large passage volume for low density, comparatively warm gas streams in counterflow, and an intermediate unit has lesser volume for higher density and cooler gas streams in counterflow, inner unit has lowest volume for comparatively high density and cold gas streams in counterflow, and the inner and intermediate units are thermally shielded in turn by the respective surrounding units whereby minimum heat-leak with respect to environment is achieved.

20. A multiple-pass heat exchanger as specified in claim 19 wherein adjoining units are interconnected for reentrant flow, and the interconnections include respectively, annular plenum chambers common to such adjoining units,

Patent No. 3, 5433M; Dated December 1, 1970 Inventor) Michael Jordan and Bradley 5, Kirk It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

(301.1, line 61, "sectional" should read --section-- line 62, after "with" insert --practically--.

(301.2, line #0, "diving" should read --dividing-.

C01.5, lines B t-35, "functions" should read -function--.

001.9, line 17, "alined" should read -a1igned.

line 2 L, "shaped" should read shape. line 31, "shaped" should read -shape-. line 58, "shaped" should read --shape--. line 67, "pyramid" should read --pyramidal-.

001.10, line 5 "alined" should read -a1igned-.

line IA, the word "not" should be deleted. line 74, "tapoff" was written in the spec. -tapp 001.11, line 23, "etc' should read -radial-.

001.12, line 5, "slide" should read -side--.

001.1 1, line 8, after "counterflow," insert --and the--.

Signed and sealed this 6th day of July 1971.

(SEAL) Attest:

' FLETCHER, JR.

WILLIAM E. SCHUYLER J Attesting Officer Commissioner of Pat nt