This invention relates to heat exchangers of the kind comprising a stack of plates which are in sealed engagement one with another to provide spaces between the plates for the flow of two separate fluid streams which are to undergo heat exchange.

A known arrangement in widespread use has each plate provided with four ports, one at each corner of a rectangular shape. Two ports are used as inlets for the respective fluids and the other two are as outlets for the same. Each fluid divides into a plurality of streams equal in number to half of the total interplate spaces and each part of the stream flows individually through a different one of the interplate spaces on its way from the one inlet port to the corresponding outlet port. Such an arrangement may be considered as involving parallel streams flow.

A rather different flow pattern, which may be called serial streams flow results from use of the arrange"ment described in U.K. Patent 1430491, although as far is known to the applicants this is a paper proposal which has not been manufactured. In this, a series of identical circular plates each have two axially short stub tubes opening from the plate in opposite directions. The plates are assembled together by engaging the upwardly directed tube on the one plate to the downwardly directed tube on the next, which automatically ensures that each upwardly extending one is angularly shifted, relative to the axis of the circular plate, and two interleaved sets of spaces ensue. Fluid is led (countercurrent according to the patent) through the two sets to effect heat exchange. The first fluid flows through the odd numbered spaces and the second one through the even numbered spaces.

The heat exchanger of the mentioned prior patent is assembled by making the upwardly extending part of different diameter to the downwardly extending one,

telescoping each two connected parts and flaring the smaller one outwardly. This is to fix the parts together both axially and angularly. The assembly is built one plate at a time and a flaring operation to complete the joint is necessary before adding another plate, and so on; moreover (after the first operation) only one end of the part to be flared is available for access by a forming tool, which may dictate the use of ductile material and even then places technical limitations on the design.

The object of the present invention is to provide an improved heat exchanger of this kind.

According to the invention we make the plates . polygonal with equal angles and equal sides. This ensures perfect angular alignment between one stub tube and the next without needing to secure them together by means of a flaring operation. This also enables the stub tubes to make end-to- end contact if desired being later sealingly connected together by furnace brazing; although in order to have faces in contact rather than edges, it is preferred that the one is of a smaller diameter than the other so that they are telescoped together.

The location of the stub tubes depends upon the shape of the plates and is such that when one plate is turned relative to the next so that the edges still coincide, then one e.g. upwardly extending stub tube of one plate is aligned with the downwardly extending tube of the next plate and so on.

Conveniently and preferably the polygon is a square.

Preferably the individual plates have a continuous peripheral flange which is substantially but not quite normal to the plane of the plate and the angle of the flange has some effect on the degree of nesting of one plate within the next.

According to an important, albeit optional feature of the invention, the flange has an outward extension and

a downward terminal portion: or it is outwardly rolled. The significance of this is explained hereinafter.

Preferably also the plates have dimples formed on their faces extending in opposite directions and arranged so that in the required angular relationship, the upwardly extending dimples of one plate are aligned with the downwardly extending dimples of the next plate. When these features are combined, the assembly may be made, and then the stack of plates compressed to take the dimples into face to face contact thus controlling the extent of the axial compression and ensuring that the flanges are in tight contact one with another along the lengths of the respective flanges, prior to furnace brazing or another sealing operation. The telescoping connection of the stub tubes may be particularly convenient to permit this.

The dimples may be conical or hemi-spherical , but truncated cones are preferred.

Preferably all of the plates except the end plates (i.e. all of the intermediate plates) are identical to one another and the number of plates (including those end plates) is equal to xn + 1 where x is equal to the sides of the polygon and n is any desired whole number. This brings the inlet and outlet of one series stream co¬ axial, and the inlet and outlet of the other likewise. Hence the heat exchanger of the invention can be inserted into a pair of parallel pipes carrying the heat exchange fluid pair merely by cutting and connecting the ends of the respective parts without needing any additional pipework. However, a three-plate, two-space heat exchanger will be useful for some purposes, and this will not have co-axial inlets and outlets.

According to another optional but important feature, each of the end plates is reinforced by a thicker planar cap plate, and the adjacent end plate is identical to the intermediate plates except that it has dimples only on the face adjacent the next intermediate plate and not on the face adjacent the cap plate. The significance of

this is described later herein.

An embodiment of the invention is now more particularly described with reference to the accompanying drawings wherein:-

Figure 1 is an underneath plan view of an intermediate plate;

Figure 2 is a side elevation of the same;

Figure 3 is a section on the line A-A of Figure 1;

Figure 4 is an exploded diagrammatic perspective view of a heat exchanger including the plates of Figures 1-3 to show the angular relationship;

Figure 5 is a section through part of the length of a heat exchanger on the line B-B of Figures 1, 6 and 7;

Figure 6 is a view similar to Figure 1 showing one end plate;

Figure 7 is a view similar to Figure 1 showing the other end plate; and

Figure 8 is a fragmentary sectional view showing a modification.

Turning now to the drawing a square intermediate plate 10 is shown in Figure 1 therein having, in this case, nineteen rows of dimple-like projections arranged with projections extending to one or other of the opposite faces of the plate. The dimples are pitched apart at equal intervals in regular pattern, and the rows of dimples are staggered at half pitch intervals. The plate is provided with a pair of apertures 50,52 symmetrically related to a perpendicular bisecting the length dimension and at a like spacing from a second perpendicular at right angles to the first and bisecting the transverse dimension. Two areas of the plate have dimples which are all upwardly directed (the area around point 6) or downwardly directed (around point 8) and these are arranged according to the direction through which the plates are turned in making the assembly, so that dimples are directed away from a port 50, 52 aligned therewith. As seen in Figure 3, one of the projections is in the form of a stub tube or collar 54 projecting in one direction and the second 56 in the opposite

direction. The stub tubes are designed so that they can be sealably joined to the next, for example by furnace brazing and for this purpose the stub tubes may be telescopically engaged, or abutted end-to-end as mentioned. The use of non-circular plates is particularly convenient since the edges of the plates hold the tubes in exact angular alignment with no risk of the stub tubes becoming non-coaxial which would at least open up gaps preventing their sealing by furnace brazing and which could make the heat exchanger useless by allowing mixing of the fluids involved in heat exchange if such a heat exchanger were put into use.

Each plate has an upstanding peripheral edge 30 which is slightly inclined so that the next plate may be partially nested.

The arrangement of these intermediate plates according to Figure 1 in a typical heat exchanger is shown in Figure 4. In order to have inlet and outlets aligned axially so that the heat exchanger can be installed in a pair of parallel pipelines the number of spaces will always be a multiple of 4, and the number of plates will be a like multiple plus 1 so as to provide e.g. 5, 9, 13 ... plates. Figure 4 shows five plates, and hence four inter-plate spaces.

It should be understood that with square plates it is not necessary to provide a number of plates which is a multiple of 4 plus 1, providing non-aligned inlet and outlet tubes can be accepted. One possibility which is particularly useful where a small number of plates is required for example has four plates providing three spaces of which the first and third will form a series connected flow path for one fluid while the second is for flow of the other fluid . In another similar arrangement, six plates may be used to provide five spaces of which the first third and fifth are serially connected for flow of a first fluid and the second and fourth for a second fluid. It will be understood by those skilled in the art that flow through any space results in a pressure drop, and the greater the thermal

length (all other things being equal) the greater the pressure drop. Hence, in the first mentioned instance here, using three spaces, the fluid flowing through two of the spaces will undergo a substantially greater pressure drop than that flowing through the single space: the heat exchanger design according to the invention may be tailored to utilise these factors according to the surrounding parameters.

In any case in the preferred embodiments the stack of plates includes both intermediate plates and two end plates 70,72. Plate 70 is shown in Figure 6. This plate as illustrated has dimples 42 extending only in a downward direction as seen in Figure 4, and these dimples are located in exactly the same positions as the downwardly directed dimples in Figure 1, when the plate is in the same angular position as in Figure 1. Likewise, plate 72 has dimples 42 extending only in a upwards direction but located in exactly the same positions as the upward dimples in Figure 1 subject to the same angular position. These plates are simple to manufacture since one and the same set of pressing tools can be used to make all of the intermediate plates and all the end plates. The forming dies, upper and lower ones respectively, are provided with inserted pins to form the dimples, and the pins are simply omitted where dimples are not required and included where they are required.

The arrangement of dimple-like projections in each row is such that the face 40 of each dimple 42 contacts the similar face of the aligned dimple on the next plate 10, 70, or 72 as the case may be so as to assist in controlling the spacing of the plates when the latter are subject to compression in assembly prior to brazing or otherwise joining together, and the dimples are intended to become brazed together to provide great structural strength. However, the effect of the dimples is also to provide a plurality of flow paths across the plates leading to turbulence and mixing, and provide additional surface for heat exchange from fluid to plate and plate to fluid. The brazing essentially joins the edges of the

plates together and the stub tubes together and desirably the dimples together.

In Figure 1 upward projections (convex) are shown as plain circles, and downward projections (concave) are marked with a cross, which is purely diagrammatic and for illustration only - that is to say the cross appears in the drawing but not on the dimples themselves.

The arrangement is completed with reinforcement cap plates 20 which are relatively thick and solid plates, apertured to allow connection tubes 14, 16 to be added the assembly and located immediately next to the end plates 70, 72. Again, the cap plates are substantially identical to one another and are dimensioned to fit within the peripheral edge 30. The absence of dimples projecting towards the cap plate from the adjacent end plates 70, 72 enables the cap to become brazed in face- to-face contact over substantially its entire area with the adjacent end plate. This contributes greatly to the pressure capability of the complete heat exchanger.

Figure 5 shows a fragmentary section through a nine plate assembly according to the invention, this comprising seven intermediate plates 10 ¹ , 10 , 10 ...10 . The sectional drawing is made through three of the downwardly directed dimples in one row of plate 10 ¹ (Figure 1) on the lefthand side of Figure 5, and then through three upwardly directed dimples in the adjacent row, on the righthand side of Figure 5. This illustrates the alternating dimension axially of the stack of the inter-plate spaces which improves flow therein, reduces film effects and thus improves heat exchange. In use, one fluid of the heat exchange pair flows through spaces 90, 92, 94, 96 in series (or in the reverse order), whilst the other flows through spaces 98, 100, 102, 104 in series (or in the reverse order).

The assembly is made by arranging the plates in a stack with each turned 90°, always in the same direction, relative to the next plate. The arrangement of the dimples is such that each downwardly directed dimple then

becomes aligned with an upwardly directed dimple (and vice versa). With the end plates 70, 72 the dimples are directed inwardly of the stack and essentially planar faces are disposed towards the reinforcing cap plates.

A modification in Figure 8 also contributes to the pressure capability. In this modification the edge 30 is rolled at 80. The rolling can be through less than 180° or more than 180°. Alternatively, the equivalent can be formed by a series of angularly related planar portions namely an outwardly projecting flange and then at least a downwardly projecting flange. The result is to stiffen each plate and there may also be some advantage in providing a natural groove between each two plates when assembled together, extending about their periphery, which will accommodate a fillet of the brazing material used in completing the assembly.

Short lengths of tube may be fixed to the stub tubes 50,52 which project from opposite ends of the complete assembly and may be connected by the furnace brazing or like operation which seals all of the parts of the assembly. These tubes may for example be internally screw threaded so as to connect existing pipes to the heat exchanger. It will be appreciated that one of the tubes at one end is to open to the interplate space between the first pair of plates 10 and the other at the same end to the second interplate space between the second and third plates 10, and similary at the other end .

It will be understood that Figure 5 shows clearances between the parts, prior to pressure clamping and brazing or other sealing to complete the manufacturing process.