GB1601432A - Plate heat exchanger - Google Patents

Description

(54) PLATE HEAT EXCHANGER (71) I, AXEL EMANUEL WIN ROTH. 37 Hiigklintavgen, S-172 38 Sundbyberg, Sweden, a Swedish subject do hereby declare the invention, for which I pray that a patent may be granted to me and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a plate heat exchanger of the type having two end plates arranged in a parallel manner and constituting fluid distribution plates and provided with inlet and outlet means for the fluids between which the heat exchange is to take place and several heat transfer plates which are arranged side by side with intermediate seals and/or packings and which are clamped between the end plates by means of clamping devices.

A plate heat exchanger of this type normally contains two different channel systems: one system intended for the heat emitting fluid and the other intended for the heat absorbing fluid.

The heat transfer plates in the plate heat exchanger delimit between them small spaces or chambers, a first set of which is arranged to form a first channel through which the heat emitting fluid flows whilst the heat absorbing fluid flows through a second set, constituting a second channel.

The heat exchange between the fluids takes place through the walls formed by the plates and separating the adjacent spaces for the two fluids. Packings and/or seals are arranged between the plates to seal the spaces. The fluids flow into and out of the respective spaces via various sets of openings in the plates.

The heat exchange between the plates in a heat exchanger of this type can take place in parallel flow, counter flow or cross flow but these various flow types can also be combined in the same heat exchanger. The fluids can be gases or liquids or one fluid can be a gas while the other is a liquid.

Previously proposed plate heat exchangers are characterized, inter alia, by the fact that the plates must be corrugated in a suitable manner so as to resist the pressure stresses which occur as a result of the different pressures which can prevail in adjacent spaces on opposite sides of the plates. The corrugation is also intended to bring about a turbulent flow in the spaces.

This is of considerable importance if the heat transfer between the partition walls dividing the spaces is to be efficient.

Another feature which characterizes previously proposed plate heat exchangers is that they embrace plates of at least two different types. Two edge packings are provided between each plate and between adjacent plates. This means that the heat exchanger has as many edge packings as it has plates. The plates are usually single plates provided with a suitable type of corrugation.

The present invention proposes a new type of plate heat exchanger which is simpler from the design point of view and which requires less space (is more compact) than previously proposed plate heat exchangers with the same heat transfer capacity.

According to the present invention there is provided a plate heat exchanger comprising two end plates which are arranged in parallel and which constitute fluid distribution plates and have inlet and outlet means for the fluids between which heat exchange is to take place and a plurality of heat transfer plates which are arranged side by side with intermediate packings and are clamped between the end plates by clamping means, said heat transfer plates being formed by substantially identical plate cassettes each of which consists of two joined, profiled cassette plates with an edge flange extending around the periphery of one side of the plate and with two or multiples of two openings one or half of said openings in each plate each constituting a first opening and being encircled by a collar extending on the same side of the plate as the edge flange and the remaining opening or openings each constituting a second opening, the plates in each cassette being disposed with their edge flanges and collars towards each other and tightly joined together along their adjacent surfaces with said collars defining a first passage or passages extending through said cassette for interconnecting the opposite exterior sides of the cassette to conduct fluid from one exterior side of the cassette to the other exterior side thereof, in use of the heat exchanger; each cassette plate having a channel extending inwardly of the edge flange for receiving one side of an endless seal, which seal has, in cross-section, a central part with a flat-sided flange and which serves as a seal and a spacer between the cassette incorporating said cassette plate and an adjacent cassette; a plurality of depressions forming projecting bosses on the same side of the plate as the edge flange and collar, and provided around the openings and in other parts of each cassette plate, said bosses being joined to opposing bosses on the other plate of the plate cassette so that they combine to form a support means between the plates in the cassette, said support means constituting turbulence-generating obstacles to fluid flow within the cassette, in use of the heat exchanger; and a packing ring disposed between adjacent plate cassettes around each said second opening for defining a second passage or passages interconnecting the interiors of adjacent cassettes to conduct fluid from within one cassette to within an adjacent cassette, in use of the heat exchanger, said packing ring having anchoring projections locating in the depressions around each said second opening.

As used herein the term "cassette" is employed to indicate a structure comprised by two cassette plates joined together at or in proximity to their edges in generally parallel relation so as to enclose a space therebetween.

Thus the present invention provides a plate heat exchanger in which the chambers for the two fluids consist respectively of the internal spaces in plate units designed as closed plate cassettes and the spaces between the outsides of these plate cassettes.

All of the plate cassettes in the heat exchanger are of an identical construction and differ only in their orientation and these identically similar plate cassettes each consist of two closely jointed (e.g. welded) identically similar cassette plates.

The following advantages, amongst others, are achieved by means of a plate heat exchanger constructed in the manner of the present invention.

The plate heat exchanger becomes extremely simple to assemble (and dismantle for cleaning, for example) due to the fact that it consists only of identically similar plate cassettes manufactured of two similar cassette plates. This means that the entire plate stack between the end plates in the heat exchanger consists of one type of standardized element only. The construction of the plate heat exchanger thus provides major advantages from the production engineering point of view.

Since no packings are required in the interior of the enclosed plate cassettes, each of which consists of a pair of cassette plates closely joined together, packings are required only for sealing the spaces between the outsides i.e. exterior sides of adjacent plate cassettes. In other words, the number of edge packings or seals required is only half as large as the number of cassette plates.

In addition, the plate cassettes do not need any complicated corrugations since the welded edge flanges, the edge channels, the pairs of joined collars and the pairs of opposing bosses in the cassettes form stiffening devices which provide the necessary strength and rigidity for the cassettes to withstand the relevant internal and external pressures or differential pressures. Since the plate cassettes are provided with such efficient stiffening devices, they can be manufactured of comparatively thin sheet metal.

This is a major advantage from both the cost point of view and ease of manufacturing point of view.

If the heat exchanging fluids consist of a "cleaner" and a "dirtier" fluid, the clelaner fluid can conveniently be directed to flow through the enclosed interior of the cassettes while the dirtier fluid flows through an "external channel system" formed by the spaces between the cassettes and the first passages extending through the cassettes. In this way, the soiled spaces in the plate heat exchanger can be extremely efficiently cleaned once the plate cassettes have been dismantled so that their outsides (including the exposed walls of said first passages) become accessible for mechanical cleaning and flushing. The "internal channel system" through the inside of the cassettes can be cleaned by means of through-flushing which can, in certain cases, be carried out without dismantling the plate cassettes.

According to a preferred embodiment of the plate head exchanger in accordance with the invention, in which the plate cassettes have a generally square shape with rounded corners, the depressions (the bosses) in each cassette plate are uniformly grouped around symmetry points which are symmetrically located in relationship to the centre of the plate, e.g. the corner points of an imaginary square, the centres of the openings in the cassette plates coinciding with respective ones of the symmetry points.

With this type of plate heat exchanger the plate cassettes can, for the sake of simplicity for example. be so located internally that each cassette (as viewed in the axial direction from one end plate of the heat exchanger towards the other end plate) is turned 90" in its plane in relationship to the immediately preceding cassette. Alternatively, stacks of cassettes which are located in an axially similar manner internally. can be turned 90O in relationship to the immediately preceding cassette or group of axially similarlv located cassettes.

Preferably the sides of the imaginary square are parallel to the sides of the plate cassette and at least four depressions (bosses) are grouped around each of the four symmetry points.

Another important advantage to be obtained from a plate heat exchanger in accordance with the invention is that the relationships between. on the one hand, the volume of the internal channel system. i.e.

the total enclosed internal volume for the plate cassettes plus the small additional volume of the first passages, and, on the other hand, the volume of the external channel system i.e. the remaining volume between the cassettes plus the small additional volume of the second passages can very easily be adapted to suit prevailing conditions such as fluid flow rates, temperature differences between the fluids, and the viscosities of the fluids. This is made possible bv the fact that several different sets of edge seals and packing rings with central portions of different thicknesses can be used together with the same set of plate cassettes.

In this way, it becomes possible to increase or reduce the thickness (height) of the spaces between the outsides of the plate cassettes to suit the prevailing requirements, thus changing the total volume of the "external channel system" and, consequently. changing the abovementioned relationship between the channel systems.

Desirably the endless seals locating in the plate cassette channels should have a symmetrical cross section with a rectangular portion providing said central part and bead potions which project in opposite directions from the aforesaid central part and which are formed so as to be received in said plate cassette channels of respective adjacent plate cassettes.

The bead portions, which can. for example, be formed with opposing trapezoidal cross-sections will thus have the advantageous property that they provide a seal which is increasingly efficient the higher the pressure in the space sealed by the endless seal. This increase in the sealing effect is due to the fact that the central part is urged outwardly by the prevailing pressure which acts on an inwardly disposed flat-sided flange of the central part turned towards said space whereupon the seal is compressed so that the bead portions attempt to expand into their respective edge channel thus resulting in the abovementioned improved sealing effect vis-a-vis the two adjacent plate cassettes.

The packing rings intended for the second openings conveniently have, in crosssection, a central portion similar to that of said endless seal with each packing ring having pairs of projections extending in opposite directions from the central portion for housing in the depressions around said second openings around which said packing rings are disposed.

Despite the fact that the four depressions (bosses) around each of the, for example four, symmetry points in each cassette plate in a plate cassette should be sufficient, i.e.

sixteen internal supports in the cassette should impart a sufficient degree of structural rigidity and strength, a considerably larger number of supports could, particularly in cassettes intended for higher pressures, be conveniently provided in the cassette.

For this reason, each plate cassette could, in certain cases, advantageously have several pairs of opposing depressions (bosses) in the cassette plates which are symmetrically located with respect to the centre of the cassette, the bottoms of the aforesaid depressions which come in contact with each other being firmly joined, for example, by means of spot welds.

As an example, it is hereby noted that one example of a plate heat exchanger of the invention includes some 90 supports (collaborating pairs of bosses) inside each cassette.

The following is also hereby noted concerning the flow path through the plate cassettes. The relevant fluid which is to flow through the enclosed interior of the cassette is led into the cassette through one of the two second openings and is led out of the cassette through the other. These openings, which constitute an inlet and an outlet respectively, of the cassette, are provided with surrounding packing rings as has been described above. In addition, the openings are so located in the two plates of the cassette so that they are not axially aligned with each other.Instead, they are located so that the fluid which enters the enclosed interior of the cassette is immediately directed laterally beyond the inlet and is then constrained to distribute itself within the cassette before finally making its way to the outlet opening which should lie at a position spaced substantially from the inlet opening. The inlet opening and the outlet opening can, for example, be located at diagonally opposed corners of the above referred to imaginary square of the cassette.

The fluid flow is also forced to undergo a large number of changes of direction on its path from the inlet to the outlet as a result of the turbulence-generating obstacles formed by the pairs of opposing bosses within the cassette. The non-linear flow of the fluid and the constant changes in direction within the cassette mean that heat exchange is efficiently achieved between the interior and exterior of the cassette plates. In this context it should also be noted that the "rear sides" of the bosses, i.e. the depressions in the outsides of the cassette walls, also give rise to turbulence in the fluid which flows through the "external" spaces between adjacent plate cassettes.

An embodiment of the invention will now be described by way of example. with reference to the accompanying drawings in which: Figure 1 shows a side view of a complete plate heat exchanger in accordance with the invention; Figure 2 is a plan view, seen from above, of a cassette plate of the type which is included in the plate cassettes in the plate heat exchanger shown in Figure 1; Figure 2a is a cross section taken on the line A-A in Figure 2; Figure 2b is a cross section taken on the line B-B in Figure 2; Figure 3 is a plan view, seen from above, of a plate cassette consisting of two welded cassette plates in accordance with Figure 2; Figure 3a is a cross section taken on the line A-A in Figure 3; Figure 4 shows the plate cassette illustrated in Figure 3 provided with a packing ring and edge seal on the top side and on the bottom side; and Figure 4a shows a cross section taken on the line A-A in Figure 4.

The plate heat exchanger which is illustrated in Figure 1 comprises two horizontal end plates, 1 and 2, between which several identical plate cassettes 3. which are parallel to the end plates, are clamped. The end plates, 1, 2 constitute fluid distributing plates for the two fluids between which heat is to be exchanged in the stack of plate cassettes 3, and the end plates are provided with inlet and outlet means 4, 5, 6, 7 for the supply and removal of the fluids to and from the upper and lower parts of the plate cassette stack. The respective flows of the heat emitting fluid and the heat absorbing fluid are led from and to the top and bottom plate cassettes, 3' and 2" to and from the end plates 1 and 2, in such a way that the fluids are kept separated and constrained with a respective channel system extending through the stack of plate cassettes 3.

Endless edge seals 8, are disposed between adjacent plate cassettes 3, to form spacers therebetween as well as constituting edge seals for the spaces or chambers, 9, formed between the adjacent plate cassettes (see Figure 4a).

The plate heat exchanger has two separate internal channel systems, one for the heat emitting fluid and the other for the heat absorbing fluid. One of the channel systems consists of the enclosed interior spaces of the plate cassettes together with the second passages enclosed within the packing rings, 10, (see Figure 4) which interconnect these interior spaces. The other channel system consists, in turn, of the slot-shaped spaces 9, between the exterior sides of the enclosed plate cassettes 3, together with the first passages 11 defined by collars 23 (further described below) and extending through the cassettes between the exterior sides thereof.

The end plates 1 and 2 are joined together by means of parallel tie rods 13, which can be tensioned and which are arranged at regular intervals around the plate cassette stack so as to hold the plate cassettes 3, and the edge seals, 8, clamped together between them to form a heat exchange fluid-tight stack of heat exchanger elements. The tie-rods 13 are fixed between bearing lugs, 14 with their lower ends articulated, on the outside of the lower end plate 2, and the rods 13 are tensioned by tightening nuts 16 against shoulders 15 on the upper end plate 1.

In order to facilitate movement of the plate heat exchanger before it has been connected (via inlet and outlet means 4, 5, 6, 7) to and after it has been disconnected from a heat exchange fluid circuit, the lower end plate 2 is provided with wheels 17 which are journalled so as to be rotatable.

Although Figure 1 shows the plate heat exchanger with horizontally arranged end plates and intermediate plate cassettes, i.e.

with a vertically extending plate cassette stack, there is, of course, nothing to prevent the end plates and plate cassettes being arraged on edge, side by side, if necessary so that the axis of the plate heat exchanger becomes horizontal. In other words, the direction of the stack of plate cassettes can be selected freely to meet requirements and to suit the available space. Plate cassette stacks inclined at an angle can, of course, also be used if necessary.

We shall now pass on to a closer study of the standardized construction elements of the plate heat exchanger, i.e. the actual plate cassettes 3 with their relevant edge seals 8 and packing rings 10.

As has been mentioned above, the plate heat exchanger comprises several identical, plate cassettes 3 which, in turn, consist of two cassette plates 18 which are tightly joined together by means of, for example.

welding or brazing, so as to enclose an interior space. A cassette plate of this type is shown in Figure 2. An endless edge flange 19 projecting from one side of the main plane of the plate extends along the straight sides and the rounded corners of the cassette plate 18. Two openings 20 and 21, located at equal distances from the centre point 22, of the plate and located at an angle of 90" from each other seen from the centre point are provided in the cassette plate af the illustrated embodiment. A first opening 20 has a collar 23 located on the same side of the plate plane as the edge flange 19. The second opening 21 on the other hand, has no collar.In addition, a packing channel 24 for the endless edge packing 8, which packing 8 has a central part with a flat-sided flange 25 (seen in cross section in Figure 4a) and bead portions 26 projecting in opposite directions from said central portion, extends along the periphery of the plate slightly inwardly of the edge flange 19. Similarly, symmetrically located depressions 27, which form projecting bosses on the same side of the plate plane as the edge flange 19, and the collar 23, are provided around each of the openings 20 and 21, and around two points, 35 and 36 located symmetrically in relationship to the centres of the openings 20, 21. As can be seen from Figure 2a, the edge flange 19, the bottom part of the channel 24. and the bottoms of the depressions 27, project an equal distance from the main plane of the cassette plate 18.

We shall now deal with the standardized plate cassette, 3. consisting of two cassette plates 18. welded together and shown in Figure 3. The plate cassette 3 is formed from two cassette plates 18, which have been joined together with the edge flanges 19, and the opening collars 23, abutting each other so that the opening 21, in one of the plates, lies opposite the symmetrical point 35 in the other plate in the manner illustrated in Figure 3. The two cassette plates 18 are tightly joined, by means of continuous weld joints 28 and 29, along the contact edges of the free edges of the edge flanges 19 and along the free edges of the collars 23, which are in contact with each other. In addition to which the bottoms of the depressions or bosses 27, which are located oposite each other, are joined by means of spot welds 30.Each pair of depressions or bosses 27. welded together in this manner forms "double conical" internal supports or studs 31 i.e. studs having two frusto-conical portions joined end to end at their smaller diameter ends, in the interior of the plate cassette. As has been mentioned above, these supports 31 constitute turbulencegenerating obstacles for the medium flow within the cassette. As can be seen from Figure 3, the plate cassette contains sixteen such supports 31.

Figure 4 and Figure 4a show a complete plate cassette 3 provided with edge seals 8 on the edge channels 24 on the top and bottom sides as well as packing rings 10, around the collarless openings 21 in the top and bottom cassette plates 18 respectively.

The bead portions 26 of the edge seals 8 are thus housed in the endless channels 24, while the central parts 25, are disposed between the outsides of adjacent cassette plates. The packing rings 10 are, in turn, anchored around each collarless opening 21 due to the fact that projections 32 from the central part of the rings locate in the depressions 27 around said openings 21.

It should be apparent from what has been said above that plate heat exchangers of the invention can be assembled to meet an extremely wide range of requirements and fields of application with the aid of a suitable number of plate cassettes 3, for example, of the type shown in Figure 3, stacked side by side. As may be seen in Figures 4 and 4a, the edge seals 8 and packing rings 10, are located between the plate cassettes 3 in a manner shown therein.

The size of the exterior spaces 9 between the plate cassettes can be easily varied by selecting seals and packings with central parts 25 of various thickness. It follows from this that it is possible, with a standardized size of plate cassette, to construct plate heat exchangers with widely varying heat exchange capacities. The variations involved here are, of course, partly with respect to the number of plate cassettes and partly with respect to the thickness of the edge seals and packing rings which are used between the cassettes.

Although the embodiment shown in the drawing shows only two holes or openings 20 and 21 in each cassette plate 18, plate cassettes with two openings 20 with collars, and two collar-less openings 21, in each cassette plate 18, can just as easily be provided. In this case, the two openings 20, are conveniently located diagonally opposite each other for a square cassette plate and the two collar-less openings 21, for the other fluid are similarly located diagonally opposite each other along the other diagonal of an imaginary square. In the case of cassette plates with a round shape in plan, the pairs of openings 20, are conveniently located diametrically opposite on a first diameter while the openings 21, are located diametrically opposite on a second diameter, at right angles to the first.

WHAT I CLAIM IS: 1. A plate heat exchanger comprising two end plates which are arranged in parallel and which constitute fluid distribution

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. two cassette plates 18 which are tightly joined together by means of, for example. welding or brazing, so as to enclose an interior space. A cassette plate of this type is shown in Figure 2. An endless edge flange 19 projecting from one side of the main plane of the plate extends along the straight sides and the rounded corners of the cassette plate 18. Two openings 20 and 21, located at equal distances from the centre point 22, of the plate and located at an angle of 90" from each other seen from the centre point are provided in the cassette plate af the illustrated embodiment. A first opening 20 has a collar 23 located on the same side of the plate plane as the edge flange 19. The second opening 21 on the other hand, has no collar.In addition, a packing channel 24 for the endless edge packing 8, which packing 8 has a central part with a flat-sided flange 25 (seen in cross section in Figure 4a) and bead portions 26 projecting in opposite directions from said central portion, extends along the periphery of the plate slightly inwardly of the edge flange 19. Similarly, symmetrically located depressions 27, which form projecting bosses on the same side of the plate plane as the edge flange 19, and the collar 23, are provided around each of the openings 20 and 21, and around two points, 35 and 36 located symmetrically in relationship to the centres of the openings 20, 21. As can be seen from Figure 2a, the edge flange 19, the bottom part of the channel 24. and the bottoms of the depressions 27, project an equal distance from the main plane of the cassette plate 18. We shall now deal with the standardized plate cassette, 3. consisting of two cassette plates 18. welded together and shown in Figure 3. The plate cassette 3 is formed from two cassette plates 18, which have been joined together with the edge flanges 19, and the opening collars 23, abutting each other so that the opening 21, in one of the plates, lies opposite the symmetrical point 35 in the other plate in the manner illustrated in Figure 3. The two cassette plates 18 are tightly joined, by means of continuous weld joints 28 and 29, along the contact edges of the free edges of the edge flanges 19 and along the free edges of the collars 23, which are in contact with each other. In addition to which the bottoms of the depressions or bosses 27, which are located oposite each other, are joined by means of spot welds 30.Each pair of depressions or bosses 27. welded together in this manner forms "double conical" internal supports or studs 31 i.e. studs having two frusto-conical portions joined end to end at their smaller diameter ends, in the interior of the plate cassette. As has been mentioned above, these supports 31 constitute turbulencegenerating obstacles for the medium flow within the cassette. As can be seen from Figure 3, the plate cassette contains sixteen such supports 31. Figure 4 and Figure 4a show a complete plate cassette 3 provided with edge seals 8 on the edge channels 24 on the top and bottom sides as well as packing rings 10, around the collarless openings 21 in the top and bottom cassette plates 18 respectively. The bead portions 26 of the edge seals 8 are thus housed in the endless channels 24, while the central parts 25, are disposed between the outsides of adjacent cassette plates. The packing rings 10 are, in turn, anchored around each collarless opening 21 due to the fact that projections 32 from the central part of the rings locate in the depressions 27 around said openings 21. It should be apparent from what has been said above that plate heat exchangers of the invention can be assembled to meet an extremely wide range of requirements and fields of application with the aid of a suitable number of plate cassettes 3, for example, of the type shown in Figure 3, stacked side by side. As may be seen in Figures 4 and 4a, the edge seals 8 and packing rings 10, are located between the plate cassettes 3 in a manner shown therein. The size of the exterior spaces 9 between the plate cassettes can be easily varied by selecting seals and packings with central parts 25 of various thickness. It follows from this that it is possible, with a standardized size of plate cassette, to construct plate heat exchangers with widely varying heat exchange capacities. The variations involved here are, of course, partly with respect to the number of plate cassettes and partly with respect to the thickness of the edge seals and packing rings which are used between the cassettes. Although the embodiment shown in the drawing shows only two holes or openings 20 and 21 in each cassette plate 18, plate cassettes with two openings 20 with collars, and two collar-less openings 21, in each cassette plate 18, can just as easily be provided. In this case, the two openings 20, are conveniently located diagonally opposite each other for a square cassette plate and the two collar-less openings 21, for the other fluid are similarly located diagonally opposite each other along the other diagonal of an imaginary square. In the case of cassette plates with a round shape in plan, the pairs of openings 20, are conveniently located diametrically opposite on a first diameter while the openings 21, are located diametrically opposite on a second diameter, at right angles to the first. WHAT I CLAIM IS:

1. A plate heat exchanger comprising two end plates which are arranged in parallel and which constitute fluid distribution

plates and have inlet and outlet means for the fluids between which heat exchange is to take place and a plurality of heat transfer plates which are arranged side by side with intermediate packings and are clamped between the end plates by clamping means, said heat transfer plates being formed by substantially identical plate cassettes each of which consists of two joined, profiled cassette plates with an edge flange extending around the periphery of one side of the plate and with two or multiples of two openings, one or half of said openings in each plate each constituting a first opening and being encircled by a collar extending on the same side of the plate as the edge flange and the remaining opening or openings each constituting a second opening, the plates in each cassette being disposed with their edge flanges and collars towards each other and tightly joined together along their adjacent surfaces with said collars defining a first passage or passages extending through said cassette for interconnecting the opposite exterior sides of the cassette to conduct fluid from one exterior side of the cassette to the other exterior side thereof, in use of the heat exchanger; each cassette plate having a channel extending inwardly of the edge flange for receiving one side of an endless seal, which seal has. in cross-section. a central part with a flat-sided flange and which serves as a seal and a spacer between the cassette incorporating said cassette plate and an adjacent cassette; a plurality of depressions forming projecting bosses on the same side of the plate as the edge flange and collar, and provided around the openings and in other parts of each cassette plate, said bosses being joined to opposing bosses on the other plate of the plate cassette so that they combine to form a support means between the plates in the cassette, said support means constituting turbulence-generating obstacles to fluid flow within the cassette, in use of the heat exchanger; and a packing ring disposed between adjacent plate cassettes around each said second opening for defining a second passage or passages inter-connecting the interiors of adjacent cassettes to conduct fluid from within one cassette to within an adjacent cassette, in use of the heat exchanger. said packing ring having anchoring projections locating in the depressions around each said second opening.

2. A plate heat exchanger as claimed in claim 1, in which the plate cassettes are generally square-shaped, but with rounded corners, in plan, and the bosses in each cassette plate are uniformly grouped around symmetry points located symmetrically in relationship to the centre of the plate, the centres of the openings in the cassette plates around which said bosses are grouped each coinciding with a respective symmetry point.

3. A plate heat exchanger as claimed in claim 2, in which the symmetry points are disposed at the corners of an imaginary square the sides of said imaginary square being parallel to the sides of the cassettes and at least four bosses being grouped around each of the four symmetry points.

4. A plate heat exchanger as claimed in any one of claims 1 to 3, in which the endless seals having their sides disposed in said plate cassette channels have a symmetrical cross section comprising a rectangular portion providing said central part with said flatsided flange and bead portions projecting at opposite sides of the central portion, said portions being formed so as to be received in said plate cassette channels of respective adjacent cassettes.

5. A plate heat exchanger as claimed in claim 4. in which the packing rings for the second openings have, in cross-section, a central portion similar to that of said endless seal and in which each packing ring has pairs of projections extending in opposite directions from the central portion for housing in depressions disposed correspondingly around the second openings around which said packing rings are disposed.

6. A plate heat exchanger as claimed in any one of the preceding claims in which each plate cassette has a plurality of pairs of opposing depressions or bosses in the cassette plate and symmetrically located with regard to the centre of the cassette, the bottoms of which depressions, in contact with each other, are firmly connected.

7. A plate heat exchanger as claimed in claim 6 wherein the bottoms of the depressions in said pairs of opposed depressions are interconnected by spot welds.

8. A plate heat exchanger as claimed in any one of the preceding claims, in which the plate cassettes have a generally square or round shape in plan.

9. A plate heat exchanger substantially as hereinbefore described with reference to the accompanying drawings.