Recuperator And Method Of Making

Abstract

Recuperator having unusually high heat transfer capability is made up of casing with headers, and matrix having tubes extending between headers and fins integral with tubes. In preferred form a continuous web of metal foil, such as stainless steel, is repeatedly folded on itself to form multiple spaced corrugations extending from one side of web. Each corrugation is pinched together at its open edge and seam welded to produce tube integral with web. Web is then wrapped on itself in spiral arrangement to produce matrix of multiplicity of very small parallel tubes. The tubes of each wrap are in contact with the web of the adjacent wrap but not secured thereto. All portions brace each other, and integral fins provide maximum heat transfer. Tubes are rigidly connected only to headers, and mid portion of matrix is free to work in response to temperature gradients, greatly reducing localized stresses.

Description

BACKGROUND OF THE INVENTION

This invention lies in the field of recuperators, or heat exchangers, of the general type used in connection with gas turbines to recover waste heat from the exhaust gas and transfer it to the incoming air, although it is not limited to this type of equipment. It is directed particularly to a recuperator which is light and durable while achieving very high heat transfer efficiency in terms of unit volume or unit weight.

The exhaust gas from the turbines of aircraft jet engines must necessarily be expelled directly to atmosphere since it is the source of thrust. However, many gas turbines in small size ranges deliver their power through mechanical shaft means to operate land vehicles, electric power generators, and other devices, and it is common to recover energy from their exhaust gases by passing them through heat exchangers, commonly referred to as recuperators, in which incoming air for the turbines is passed in heat exchange relation with the exhaust gases.

The recuperators commonly used for this purpose comprise casings through which a plurality of tubes pass to serve as flow paths for the exhaust gas. Incoming air is passed through such casing and around the tubes to extract heat from the exhaust gas. Heat transfer is increased by one or more schemes such as providing multiple fins brazed on the tubes, or fin-like framework within the casing holding the tubes in place and increasing the heat transfer area, or providing tortuous air flow paths back and forth over the tubes. All of these schemes are helpful but the efficiency is still low in terms of weight and volume. The tubes are usually relatively large so that a great deal of gas is transmitted without giving up much heat, and the tubes are widely spaced with respect to their diameters so that the volume of the casing is not efficiently used. Also, if the tubes are supported by framework, the heat flow paths to the remote parts of the framework are long and relatively ineffective. If the tubes, fins, and framework are brazed together, corrosion problems arise and high temperature stresses result from uneven heat transfer.

Attempts have been made from time to time to produce a more unitary heat exchanger by uniting two sheets of metal with weld lines which define tube outlines between them, and then applying internal pressure between the weld lines to expand the areas into tube-like formations. This is a very difficult and expensive operation and does not produce very uniform results. In addition, the structure must then be curved or wrapped into some sort of annular configuration, and this is very difficult and unsatifactory because of the great stiffness of the welded structure. Moreover, the double walls use up space, add weight, and reduce the rate of heat transfer.

SUMMARY OF THE INVENTION

The present invention overcomes the difficulties mentioned above and provides a recuperator which is very compact and light and operates at an efficiency far higher than previous such devices. Moreover it is easy to manufacture and very durable in service.

Generally stated, and in presently preferred forms, the heart of the device is the core material for the recuperator matrix. This core material comprises an elongate web of metal foil, such as stainless steel, only a few thousandths of an inch thick which is repeatedly folded back and forth on itself to form a multiplicity of corrugations protruding from only one side of the general plane of the web. The corrugations are parallel to each other and to the web and are rather closely spaced, and they extend from edge to edge of the web laterally of its principal axis. The open edges of the corrugations are then pinched together and welded along lines parallel to the web to make them into a multiplicity of tubes, all of which are integral with the web. The entire web is readily bendable about its lateral axes because of the flexibility of the thin foil material.

The web is then wrapped tightly on itself in a spiral arrangement about a small core or a large hollow core depending on its intended use until a cylindrical matrix is produced in which the wraps or layers are radially overlaid and the tubes of one layer are in contact with the web of an adjacent layer although not bonded thereto. Headers are then applied to each end of the matrix by any suitable means, such as electrical discharge machining, powder metallurgy, or ceramic molding. Casing walls are connected to the header to complete the unit.

Air inlets and outlets are formed in the casing walls near the ends and radial flow paths are formed by providing apertures in some or all of the web portions between the tubes and adjacent to the ends of the matrix. A longitudinal air flow path through the matrix is defined by the longitudinal spaces between the tubes and the web.

While the spiral wrap configuration is the most preferred form at present, it is perfectly feasible to divide the elongate web into short lengths determined by the shape of the casing to be used, and to stack the web sections or layers in tube to web relation and achieve the same goal of compact and light weight construction.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other advantages and features of novelty will become apparent as the description proceeds in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic perspective view of an elongate web of foil material, part of which has been corrugated;

FIG. 2 is a view similar to FIG. 1 after a second forming step;

FIG. 3 is a view similar to FIG. 1 after a third forming step;

FIG. 4 is a view similar to FIG. 1 after a fourth forming step;

FIG. 5 is a view similar to FIG. 4 showing a modification;

FIG. 6 is a schematic end view of a matrix spirally wrapped on a minimum core;

FIG. 7 is a view similar to FIG. 6 but with a large hollow core to receive a component;

FIG. 8 is a schematic perspective view of a recuperator with the matrix of FIG. 7 with portions of the sidewalls removed;

FIG. 9 is a schematic plan view of a portion of the web used to make up the matrix of FIGS. 7 and 8;

FIG. 10 is a schematic sectional view of the matrix and headers of FIG. 8;

FIG. 11 is a view similar to FIG. 10 with the matrix wrapped on a minimum core; and

FIG. 12 is a schematic cross sectional view of a modified form of recuperator.

DESCRIPTION OF PREFERRED EMBODIMENTS

The manner of producing core material for use in making up the matrix is illustrated schematically in FIGS. 1 to 5. The relative sizes of components in these and the other figures are modified for ease of illustration and description. It will be seen in FIG. 1 that the starting material for the matrix is a very long continuous flat length or web 10 of metal foil, such as stainless steel, which is only a few thousandths of an inch thick. The first step is to process the web in any suitable machine to form a multiplicity of reversely bent portions or corrugations 12 which are generally U-shaped and project only from one face of the web. The corrugations extend in spaced parallel relation laterally of the principal axis of the web.

In the next phase, the open edges 14 of the corrugations are pinched together and welded from end to end along lines parallel to the plane of the web to produce individual sealed tubes 16 or tear drop configuration as seen in FIG. 2. A series of slots or apertures 18 are then pierced in some or all of the web portions 20 between the tubes and inward of the side edges 22 of the web, as seen in FIG. 3. Typically, the slots may be about 1/4 to 3/8 inch in length with lands of about 1/16 inch between them to maintain the web strength. For maximum flow capacity a sizing tool is then passed through each tube to give it the cylindrical formation shown in FIG. 4. The core material is now completed and ready to be wrapped around a mandrel or core in a spiral arrangement.

The full width of the web, as in FIG. 4, is retained for applying headers to the spirally wrapped matrix by means of electrical discharge machining in the well known manner. However, when other systems are used for forming the headers in situ, as by powder metallurgy or ceramic molding, the side edges 22 together with the neck members 24 which connect the tubes to the web are cut back as shown in FIG. 5 to form new side edges 26, with the tube ends protruding a predetermined distance.

In order to make up a matrix, the web may be readily bent about lateral axes because of the flexibility of the very thin foil and wrapped in spiral arrangement about a mandrel 28 provided with a cam-like formation 30 to initiate the spiral form as indicated schematically in FIG. 6. The wrapping is continued with the tubes of one wrap or layer in contact with the web of the adjacent wrap or layer until a matrix 32 of the desired capacity is attained. Headers are then applied in any suitable manner so that the tube ends pass through the headers and are permanently secured thereto, and then the casing is applied to complete the closure. Since all of the layers are in contact, they are mutually self supporting. At the same time, only the tube ends are held rigidly and the remainder of the matrix may work in response to high temperature gradients because the layers are not rigidly secured to each other, and the stresses of uneven heating are substantially eliminated.

When it is desired to form the matrix as a muff to surround some other component such as the combustor of a jet engine, the same process is used starting with a hollow mandrel 34 large enough to encompass the desired component and provided with a spiral initiating formation 36. Again the wrapping is continued until a matrix 38 of the desired size is attained, and headers and casing are applied in any suitable way.

A complete recuperator incorporating the muff-type matrix of FIG. 7 is schematically illustrated in FIG. 8, where it will be seen that headers 40 and 42 are permanently secured to the tube ends and a casing wall 4 surrounds the matrix between the headers. The end portions of the casing wall have been omitted for clarity of illustration. Air enters the casing at the end adjacent to header 42 and flows radially inward through the slots or apertures 18 to all portions of the cross section.

It then flows longitudinally through all of the passageways defined between the tubes and the web, and at the end adjacent to header 40 it flows radially inward through slots or apertures 18 to the interior of the core where it may be used as the combustion air for the turbine. The exhaust gas from the turbine flows through tubes 16 from header 40 to header 42. Thus this arrangement produces a combination of lateral flow and counter flow to achieve maximum heat transfer. The effectiveness is, of course, greatly enhanced by the unusually large number of tubes and the large area of integral fins made possible by the present construction. A typical example is a recuperator configured approximately like FIG. 8 having an inside diameter of about 10 inches and an outside diameter of about 20 inches and a length of about 13 inches, and containing upward of 22,000 tubes each having an outside diameter of about 0.078 inch.

It is desirable in most recuperator constructions in which air enters and leaves the casing radially to provide the greatest area of flow path adjacent to the inlet and outlet and to gradually decrease the area to a minimum at the wall remote from the entrance and exit in order to improve distribution through the various portions of the matrix. In the present construction, this may be accomplished in the manner indicated in FIG. 9, in which a web 10 with its integral tubes not shown is provided with apertures 18 extending laterally inward from adjacent the margins or side edges 22. At one end of the web, their extent is a predetermined minimum and at the other end their extent is a predetermined maximum, which the extent gradually increasing along the length of the web. The web shown in this figure is suitable for making up the matrix for the recuperator of FIG. 8. If it is wrapped from either end of the web the resulting matrix will have maximum extent slots at the radially inner side of one end and at the radially outer side of the opposite end as in FIG. 8. The result is more graphically shown in FIG. 10 which represents the matrix formed by spirally wrapping the web of FIG. 9.

The same scheme may be used to produce the matrix 32 having a minimum core. In such case, both the inlet and the outlet are at the radially outer side of the matrix, and therefore the slot arrangement must be modified so that the minimum slot extent at both margins is at one end of the web and the maximum slot extent is at the other end. The minimum slot end of the web must be located at the core so that the smallest flow path areas will be at the center of the matrix. When this is done, the resulting matrix will be as indicated in FIG. 11.

While the spiral wrap arrangement is the most preferred for general use, there are various special installations where some modifications is desirable. In the example shown in FIG. 12, the total web is cut into individual web sections 46 of predetermined length and the sections are then stacked in multiple layers to produce a matrix which just fills casing 48. Headers are attached in the same way as with the previous forms and the web portions are appropriately slotted to accord with the inlet and outlet locations. This type of construction may be used with cylindrical casings but its greatest utility lies in its accommodation to casings of any size and shape, whether regular or irregular, in which a spiral type cannot properly be fitted.

It will be apparent that the constructions disclosed above provide a recuperator which is superior in terms of efficiency, weight, volume, and durability because it is extremely compact and heat transfer is multiplied manyfold since it is practical to use an unusually large number of tubes in a given space with adequate finning and mutual reinforcement.

While the invention has been described in detail in its present preferred embodiment, it will be obvious to those skilled in the art, after understanding this invention, that various changes and modifications may be made therein, specifically in the fabrication of the core material, without departing from the spirit or scope thereof.

Claims

Having thus described the invention, what is claimed as new and useful and is desired to be protected by U.S. Letters Patent is:

1. A recuperator comprising: a casing having side walls and a header at each end, and a heat exchanger matrix located within the casing; the matrix comprising a plurality of layers of heat exchanging core material arranged in overlying relation with each other; the core material consisting of metal foil web means provided with a multiplicity of elongate tubes integrally formed from the web means and arranged at one side of the web means in closely spaced parallel relation with each other and parallel with the web means; the tubes of one layer being in contact with the web means of the adjacent layer while remaining free and unsecured thereto whereby the tubes and web means are free to move relative to each other when subjected to thermal stresses produced in the respective members; the ends of the tubes extending through and being secured to their associated headers; the tubes providing a flow path through the headers and the length of the casing for the flow of a first heat exchanging fluid; fluid inlet and outlet means formed in the casing; and the spaces between the tubes, the web means, and the casing walls providing a flow path through the interior of the casing between the inlet and outlet means for the flow of a second heat exchanging fluid.

2. A recuperator as claimed in claim 1; the layers of core material being arranged generally circumferentially about the longitudinal axis of the casing with the tubes being directed parallel to the longitudinal axis and spaced peripherally and radially from each other.

3. A recuperator as claimed in claim 2; the core material comprising continuous web means wrapped upon itself in a spiral arrangement.

4. A recuperator as claimed in claim 1; the core material being divided into a plurality of web sections to define discrete layers.