Heat Exchanger

Abstract

A liquid-to-gas heat exchanger having an internal ribbonlike flow passage for the liquid, having means in the passage of adding extended surface and of interrupting, separating, or causing turbulence in the boundary layer to increase the liquid side film coefficient of heat transfer. The heat exchanger is of cast construction, and is provided with external fins for the passage of air or other gas thereover.

Description

BACKGROUND OF THE INVENTION

This invention relates to heat exchangers, and more particularly to heat exchangers of the type wherein a liquid circulating through a closed path is passed through a heat exchange relationship with an externally flowing gas.

Liquid-to-gas heat exchangers are well known wherein the liquid flows through a plurality of tubes, which may be finned, with gas passing over the exterior of the tubes. In order to secure good heat transfer, there must be a large number of such tubes of relatively small size, with the associated problems of providing fins for each, since large tubes would not provide a large enough surface to volume ratio.

In order to increase the heat transfer from the liquid it has been known to provide flat liquid passages, and a further increase of flow friction and heat transfer has been obtained by forming such flat passages in a rippled configuration in the direction of flow, or by providing them with internal discontinuities such as turbulators. However, it has heretofore been possible to form such heat exchangers only of sheet metal, which is a very expensive form of construction and causes considerable difficulty in the fitting and mounting of external fins. Further, such sheet metal heat exchangers cannot be relied on to contain high internal pressure without danger of leakage and changes in form and dimension. Also, thermal expansion and distortion of such accordion-like structures is considerable, whether the heat derives from the liquid or from the gas.

SUMMARY

The present invention provides a heat exchanger having a hollow body with a ribbonlike liquid passage therethrough, the exterior of the hollow body being provided with fins integral therewith for the passage of air or other gas thereover. The interior of the liquid passage is provided with extended surface and with means of disrupting, or preventing the formation of, a continuous boundary layer along the walls, which would limit the amount of heat transfer. Such disrupting means include turublators such as protuberances from the interior surface of the walls or interruptors in the flow path, or corrugated walls which provide an undulatory flow path. The terms corrugated and undulatory as used herein are intended to cover all similar forms such as pleated, rippled, wavy, serrated, sinusoidal, zigzag, and such generally crankled configurations, as will be explained in more detail in the description of a preferred embodiment. The ribbon-like lquid passage may be of a single pass through the hollow body, or it may be serpentined in a plurality of bights to secure a longer flow path within a relatively short space.

The hollow body is formed of two cast members which when joined together on mating faces define the internal ribbonlike passage, and which bear integrally cast fins on their exterior sides. This construction allows the inexpensive production of heat exchangers with internal boundary layer disrupting means, avoiding the expensive and time-consuming formation of sheet metal and the difficult fitting and mounting of fins thereon. The joined halves of the cast heat exchanger will bear much greater internal pressure without leakage than a sheet metal one, and the cast exhanger is much less subject to thermal distortion. Its production cost is approximately half that of less satisfactory sheet metal exchangers of the same capacity.

It is therefore an object of this invention to provide a liquid-to-gas heat exchanger having means for disrupturing the boundary layer of the liquid.

It is another object to provide such a heat exchanger having high flow friction for the liquid side and a high film coefficient of heat transfer on the liquid side.

A further object is to produce such a heat exchanger of cast construction, at low cost.

Other objects and advantages will become apparent on reading the following specification in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the preferred embodiment of the heat exchanger of the invention, partially broken away, and with no cover on the fins;

FIG. 2 is a cross-section taken on line 2--2 of FIG. 1;

FIG. 3 is an enlarged fragmentary cross-section similar to FIG. 2;

FIG. 4 is an enlarged cross-section taken on line 4--4 of FIG. 1;

FIG. 5 is a view taken in the direction of line 5--5 of FIG. 1;

FIG. 6 is a cross-section similar to FIG. 4 of another embodiment;

FIG. 7 is a similar cross-section of a further embodiment; and

FIG. 8 is a cross-section taken on line 8--8 of FIG. 7.

DESCRIPTION OF A PREFERRED EMBODIMENT

This invention contemplates a plurality of embodiments, all utilizing the same principle of disrupting and preventing formation of a stable boundary layer of liquid in a flat ribbonlike passage, and differing only in the specific physical means by which the objects are achieved. Some forms of the heat exchanger are slightly more efficient than others, some are slightly less expensive. On balance, the one considered preferred is an embodiment, now to be described, having a ribbonlike liquid passage with the internal walls thereof corrugated to produce a generally undulatory flow path therethrough.

In FIG. 1 there is shown a view of the exchanger 11 having the top plate 12 partially broken away to show the interior of the lower plate 13. When the two plates 12 and 13 are joined at their mating inner faces, as by soldering, brazing, or a high temperature adhesive, they mutually define a flow passage 14 through the heat exchanger, having an inlet 16 and an outlet 17 for the flow of liquid. Although the inlet and outlet are shown on the same side of the exchanger, it will be understood that they may equally well be on opposite sides, and that the direction of flow may be the reverse of that shown.

Flow passage 14 is of a flat, generally ribbonlike form, undulating in the direction of flow, and as viewed in cross-section in a plane perpendicular to the flow direction (better shown in FIG. 4) it is defined by a pair of parallel long sides 18 joined by a pair of substantially shorter sides 19, which may be curved as viewed in this plane. The long sides 18 of passage 14 are corrugated across the direction of flow, as seen in FIGS. 1 and 2, providing a flow path of approximately constant cross-section and generally undulatory in the direction of flow.

Such a configuration of the flow passage 14 is produced by corrugations in each of the plates 12 and 13 extending crosswise of the flow passage between walls 19, the corrugations of one plate being staggered in spacing with respect to those of the other plate so that the crests of the corrugations in one plate are opposite the troughs in the other. The crests of the corrugations in one plate preferably come to approximately the same plane as the crests of the corrugations in the other plate, so that there is no clear line of sight along the flow passage in the direction of flow. This is readily accomplished by having the crests in each plate come flush with the mating surfaces, although in the castings they may be allowed to come a few thousandths of an inch below the faces of the enclosing walls to allow for flat grinding of the faces if necessary. If the quality of the castings is good enough not to require such grinding, a minor sight-through of that magnitude is of no significance.

The troughs may be as deep as desired, depending on the amplitude of the undulation it is desired to produce. The shape of te corrugations as viewed in transverse cross-section, such as that shown in FIGS. 2 and 3, may be of several related forms. As shown, the cross-section is generally that of an obtuse isosceles triangle, with the crests slightly rounded and the troughs rounded on a larger radius, which maintains good parallelism of the surfaces of the two sets of corrugations and keeps the cross-section of the flow passage substantially constant. However, the altitude of the corrugations can be greater to produce a more sharply serrated form, or they may be sinusoidal or of any other simlar conveneint form.

Further, the crests of the corrugations in one or both plates need not be flush with the mating surfaces. As shown in FIG. 3, the crests on plate 13 are substantially flush with the plate surface, but those of plate 12a are on a plane lying below its mating face, resulting in a certain degree of sight-through of the passage 14a. The amount of separation between the planes of the crests may be divided between the two plates rather than produced only in one as shown, but in any case it is preferable that it should not be too great, for instance not more than about one-third of the distance between apposed crests and troughs. If the distance were greater than that there is the possibility that the liquid would simply travel through the clear sight passage without much rippling, with dead volumes of liquid in the troughs.

It is also contemplated that in certain cases the crests of the corrugations may project beyond the plane of junction of the mating surfaces, in one or both plates. This may be done where the crests on opposite sides come to approximately the same level, or even when there is a separation between opposite crests, but it is particularly useful when it is desired that the crests of one side shall be re-entrant within the troughs of he other side, in order to produce a more sharply rippled flow pattern.

The heat exchanger may be so formed as to have an undulatory flow passage traveling straight through the exchanger, with the inlet at one side and outlet at the other, but for high efficiency and economy of space it may be formed as shown in FIG. 1 with the flow passage serpentined in a plurality of bights between a plurality of walls 19. The interior walls 19 project from alternate side walls to a distance short of the opposite side wall equal to the width of passage 14, so that the liquid flows in alternate directions from side to side in its path through the exchanger. The interior walls 19 are congruent in the two plates, with surfaces flush with the joining plane so that they form part of the mating surface which is bonded together. The corrugations of passage 14 at each turn may go around the corner, radiating from the end of wall 19, so that the flow path is always transverse to the corrugations. However, for convenience of fabrication the corrguations may continue parallel to each other, as shown, out to the side walls, without substantial loss of efficiency.

The heat transfer effect of a passage of this undulatory configuration is in part due to its extended length, that is, it has a length as if the undulations were pulled out straight into a flat ribbonlike passage. However, this is a minor effect as compared with that of the corrugations in disrupting and preventing the build-up of a continuous boundary layer. At each undulation in the passage the fluid turning in its flow impinges strongly on the walls of the passage with increased contact pressure and high flow friction, resulting in disruption and separation of such boundary layer as may have formed within each half-undulation, and preventing build-up of the boundary layer to a constant thickness. The heat transfer effect due to this boundary layer disruption is many times that of increased flow length.

Each of the two plates 12 and 13 bears integral cast heat-dissipating fins 21 on the exterior thereof, over which the gas of the heat exchange relationship flows. The fins may be disposed either parallel or transverse to the direction of the internal flow path, but when the exchanger is constructed with a plurality of serpentine bights in the flow passage it is preferable that the fins should be disposed transverse to the internal walls 19. Such an exchanger has a considerable plate area subject to internal pressure. The walls 19, being bonded together on their mating surfaces, provide stiffness against such pressure in one direction, and transverse fins provide stiffness in the other direction.

When the exchanger is used in an environment where there is a strong current of air or other gas across it, the fins may be left exposed and will dissipate enough heat from the liquid to be serviceable for some uses. However, where such a free gas is not avaiable, or when the direction of heat exchange is from the gas to the liquid, the fins are covered and a gas stream ducted to them.

The fins 21 have a jacket or cover 22 of thin plastic, sheet metal, or foil bonded to their external edges, again by soldering, brazing, or a high temperature adhesive. High temperature epoxy is suitable for such bonding, or other adhesives capable of withstanding the expected temperature. The cover over the fins confines the gas flow to the spaces between the fins, and may comprise a separate cover on each side of the exchanger for two separate gas flows. The exchanger may also be formed as shown in FIG. 5, wherein the fins 21 of each plate are carried halfway around one end of the exchanger with curved outer edges, and bonded together at their mating edges at the juncture plane at that end. In such a case the cover 22 is curved around the continuous fins at that end and brought back along the other side. This forms a continuous cover with the gas passages between fins open only at one end, on opposite sides. The gas is therefore introduced at the open end on one side, passes entirely around the exchanger, and returns to discharge adjacent the entry.

Although the cover sheet 22 is shown of substantial thickness in the drawings for convenience of illustration, for cost saving reasons it should be as thin as possible consistent with convenience in fabrication and handling. In an environment wherein the exchanger is not subject to any impact or puncture, a foil of 0.005 inch thickness has been found satisfactory.

The heat exchanger is provided with appropriate lugs 23 for mounting, positioned as required. In the embodiment shown the two plates are also provided with lugs 24 having holes for self-tapping screws 26, which provide registry of the two plates and pressure during bonding. Any other convenient means of registry and pressure may be used.

The two plates of the heat exchanger described above may be conveniently and inexpensively formed by die casting, permanent mold casting, low pressure casting, or any other similar technique which is adapted to rapid production and which holds close tolerances. Each of the two plates is cast with all its fins, lugs, mounting holes, inlet and outlet apertures, etc. produced in the mold. The mating surfaces of the two plates are generally flat enough for bonding without any dressing, but when necessary a simple flatting operation on a sander may be performed. Suitable metals are those having good thermal conductivity, such as copper or aluminum or their alloys.

FIG. 6 is a cross-section, in a plan perpendicular to the flow direction, of another embodiment having a flow passage 14b employing other means of providing extended surface and disruption of the boundary layer. Protuberances 27, integral with plates 12b and 13b, project into the passage 14b. The protuberances as shown are generally cylindrical studs having their free ends rounded and their bases filleted to the passage wall. However, protuberances 27 may be of other shapes, such as rectangular finlike members, generally airfoil-shaped members disposed at such angles as to direct the flow in a tortuous path, or other members causing turbulence in the flow.

Such protuberances extend the internal surface of passage 14b without extending its length, providing additional heat transfer between the liquid and the fins exposed to gas flow. They also cause turbulence which disrupts such boundary layer as may form for short distances, and prevent the build-up of a continuous layer. The protuberances 27 are preferably staggered in the passage, both across the passage and longitudinally thereof, for maximum effect. As in the previously described embodiment, they may project approximately to the juncture plane in both plates, or beyond or below it in one or both, and they may be re-entrant into the spaces between the protuberances of the opposite plate.

FIGS. 7 and 8 show a further embodiment in which extension of surface and disruption of the boundary layer are provided by another type of turbulator member. FIG. 7 is a cross-section similar to FIG. 6, showing a passage 14c having flat, smooth walls. A strip of wire screen 28 is positioned in passage 14c in contact with both the flat walls of the passage, the screen being contoured in such a manner as to present a series of successive meshes to the flow of the liquid and to constrain the liquid to pass through the meshes. Such a contour of the screen is produced by providing it with corrugations extending across the width of the passage, giving it an undulated shape in the direction of flow. In order to insure good contact of the screen with the passage walls and consequent heat transfer, the overall height of the corrugations from crest to trough is somewhat greater than the height of the passage, as shown in FIG. 8. At assembly when the two plates 12c and 13c are bonded together along their mating faces the corrugations of the screen are squeezed so that their crests are in firm contact with the passage walls.

The secreen is formed with an open mesh so that it will not act as a filter, and extends across the width of the passage 14c and along its full length. If such an exchanger is made with a plurality of serpentine bights it is not necessary that the screen should extend around the turns, and a separate screen 28 is positioned in each bight. Such a screen requires the liquid to flow successively through a plurality of meshes, and provides the desired turbulence of flow and disruption of boundary layer, and increased surface.

The body plates of the exchangers of this invention are formed as thin as consistent with the pressure conditions expected, and their actual thickness may be quite minimal in view of the stiffness imparted by the external fins and the transverse internal walls. Heat exchangers fabricated according to any of the embodiments of this invention are far less expensive than exchangers made of sheet metal, since they avoid the time-consuming operations of forming and assembly of the prior art.

Claims

What is claimed is:

1. A liquid-to-gas heat exchanger comprising: a. A hollow body having a ribbonlike liquid passage therethrough undulating in the liquid flow direction and having a substantially constant cross-section, the cross-section as viewed in any plane perpendicular to its flow direction being of elongated form defined by a pair of relatively long parallel sides joined by a pair of substantially shorter sides; b. said hollow body having a two-part rigid die-cast construction with the two parts being bonded together at a generaly planar junction with the plane of the junction passing through the liquid passage between and generally parallel to the long sides of the passage, the undulating form of the passage being produced by corrugations normal to the general direction of flow in the passage portion of each body part, the corrugations of one body part being positioned in a staggered relation to those of the other body part with the crests of the corrugations in one part being opposed to the troughs of the corrugations in the other part to maintain the substantially constant cross-section of the flow passage, the crests of the corrugations being sufficiently high to substantially impede a clear line of sight along the flow direction of the passage;

c. each of the two hollow parts of the body having a plurality of spaced parallel fins die-cast integral therewith and projecting outwardly therefrom for coolant gas flow between said fins, the height of the fins being a plurality of times greater than the spacing therebetween; and

d. the undulations of the liquid passage directing the liquid flow alternately against the gas-cooled sides of the hollow body.

2. The combination recited in claim 1, wherein the crests and the troughs of the corrugations are rounded and the troughs are rounded on a larger radius than the crests to maintain substantial parallelism of the flow surfaces and substantially constant cross-section of the flow passage.

3. The combination recited in claim 1, wherein the crests of the corrugations of the flow passage do not project beyond the plane of the junction of the two parts of the hollow body.

4. The combination recited in claim 3, wherein the crests of the corrugations of the flow passage are approximately flush with the plane of the junction of the two parts of the hollow body.

5. The combination recited in claim 1, wherein the liquid passage is serpentined in a plurality of parallel bights in a common plane through the hollow body.

6. The combination recited in claim 5, wherein the fins extend across the exterior of the hollow body in a direction generally transverse to the flow direction of the liquid passage.

7. The combination recited in claim 5, wherein the fins of each of the two body parts are extended around one edge of the body to the junction plane, the edges of the fins of the two parts being bonded together at the junction plane, the outermost edges of the fins having a sheet cover member bonded thereto continuously along the edge of each fin to define non-communicating gas passages between the fins, the fin cover being continuous from one side of the body around the bonded portions of the fins to the other side of the body and defining coolant gas passages continuous from one side of the body to the other, the gas passages being open only at their ends and having their open ends at the same edge of the body.