US3117624A - Plate heat exchanger - Google Patents

Description

FRITZ JOHAN WENNERBERG 3,

PLATE HEAT EXCHANGE-R Jan. 14, 1964 5 Sheets-Sheet 1 Filed June 20, 1960 N 1 v 4 \M y E um J fin i G WW v H 0 Am% My:

A. FBL

Jan. 14, 1964 FRITZ JOHAN WENNERBERG 3,117,624

PLATE HEAT EXCHANGER INVENTOR. FH/z 70 7017 enn erb 81' Wf W W Jan. 14, 1964 Filed June 20, 1960 FRITZ JOHAN WENNERBERG PLATE HEAT EXCHANGER 5 Sheets-Sheet 3 INVENTOR. l'H/Z Johan Wen/verbal Jan. 14, 1964 FRITZ JOHAN WENNERB'ERG 3,

PLATE HEAT EXCHANGER Filed June 20, 1960 5 Sheets-Sheet 4 Fig /3 IN V EN TOR. 57') Z .70/70/7 Wen/7 er-b erg Jan. 14, 1964 FRITZ JOHAN WENNERBERG 7,

PLATE HEAT EXCHANGE-R 5 Sheets-Sheet 5 Filed June 20, 1960 INVENTOR.

United States Patent 3,117,624 PLATE HEAT EXCHANGER Fritz :lohan Wennerberg, Lund, Sweden, assignor to Aktiebolaget Separator, Stockholm, Sweden, a corporation of Sweden Filed June 20, 196i), Ser. No. 37,382 Claims priority, application Sweden June 22, 1959 4 Claims. (Cl. 165-167) This invention relates to heat exchangers of the type having a series of plates held in a pack so as to form flow spaces between adjacent plates. More particularly, the invention relates to an improved plate heat exchanger and to a novel plate for use in the exchanger.

In plate heat exchangers as commonly made, each plate is provided with four corner portions each having it throughfiow passage. Two of these corner passages are used for feeding and discharging one of the two heat exchange media, and the other two corner passages are used for feeding and discharging the other heat exchange medium. Such heat exchangers are used for various purposes. For example, they are used as evaporators for the concentration of milk, fruit juices, salt solutions, and other liquids. When the exchanger is used as an evaporator, the liquid to be evaporated is fed to every second interspace between the plates and is there evaporated at least in part by means of steam which is at a temperature higher than that of the liquid and which is fed to the other plate interspaces where the steam condenses while giving off heat. Plate heat exchangers may also be used to form steam, as by means of hot water, or to condense steam, as by means of cooling water.

Plate heat exchangers for the above-noted purposes, as constructed heretofore, have not given as good results as mi ht be expected in view of the size of the heat exchange surfaces and other factors. This is due primarily to the fact that the large volume taken up by the steam makes it difficult or impossible to obtain satisfactory flow through the channels of the plate heat exchanger. As a result, the steam pressure and the temperature in the spaces between the plates are increased incident to evaporation and are decreased incident to condensation, which, in turn, means a relatively small temperature differential between the spaces at opposite faces of each plate, whereby the steam formation effect or the condensation effect of the exchanger is reduced.

A principal object of the present invention is to provide a plate heat exchanger which overcomes the above-noted diliiculties.

According to the present invention, each heat exchange plate has the usual four corner passages and is also provided with at least one additional throughflow passage located at the middle portion of the plate. Packing means on one face of the plate forms a single enclosure for an area of the plate including this additional passage and only two of the corner passages, whereby a heat exchange medium is adapted to flow over this area in two parallelconnected streams between such additional passage and each of tiese two corner passages. Preferably, the passages and paclc'ng means are so arranged that at least one of the two heat exchange media flows through each corresponding plate interspace in the form of at least two parallel-connected sub-streams which are substantially symmetrical with respect to the throughflow passage at the middle portion of each plate, which passage may be referred to as a mid-passage.

VJith the new plate construction, the liquid to be evaporated may be led into a plate interspace (a space between adjacent plates) through a mid-passage and, after vaporization in the interspace, may be discharged through two of the corner passages. Also, steam used for the evaporation may be led into a plate interspace through two corner ice passages, and the condensate formed by cooling of the steam may be discharged from this space through a midpassage of the respective plates. The number of midpassages in each plate may vary, depending upon the nature of the heat exchange media and the results desired. Thus, one or more mid-passages may be used for special purposes, as for the discharge of air or other gas which has entered a plate interspace with one of the heat exchange media. By the present invention, therefore, it is possible in each individual case to adjust the number of passages in each plate to suit the intended purpose, whereby the steam which occupies a relatively large volume is led through two parallel-connected corner passages of each plate whereas the condensate, for example, which is of relatively small volume, is led away through only one mid-passage of each plate. By virtue of the present invention, the advantage is also gained that the length of the flow path through each plate interspace is reduced to practically one-half that which would otherwise be the case, and the flow velocity in plate interspaces is reduced by about one-half. In this way, the fiow resistance is reduced to about one-eighth of that which would otherwise be the case, and the temperature diiierential between the two heat exchange media is increased so that the heat transmission through a plate between the two media is increased by about 50%.

A further advantage of the invention is that the new heat exchange plate requires less metal as compared with prior plates of the same size, which is highly important in view of the fact that such plates are usually made of highgrade stainless steel. The new plates can be made by means of the same pressing tool as that used for ordinary plates, after only slight adjustment of the tool.

In the preferred plate heat exchanger, the heat exchange plates are arranged vertically with their longer edges horizontal so that the condensate outlet or outlets are located adjacent the lower edge of each plate, thereby permitting most rapid removal of condensate depositing on the heat transmission surfaces of the plates. This results in further improvement of the heat transmission through the heat exchange plates.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which FIGS. 1 and 2 are plan views of two heat exchange plates embodying one form of the invention and which are adapted for assembly into a plate pack wherein plates as shown in FIG. 1 are alternated with plates as shown in FIG. 2;

FIGS. 3 and 4 are views similar to FIGS. 1 and 2 but showing modified forms of the two co-acting plates;

FIGS. 5 and 6 are views similar to FIGS. 1 and 2 but showing further modifications of the two co-acting plates;

FIGS. 7 and S are views similar to FIGS. 1 and 2 showing still further modifications of the two different plate structures for use in a pack;

FIGS. 9 and 10 are views similar to FIGS. 1 and 2 showing additional modifications of the plates;

FIG. 11 is an end view (looking from the right in FIG. 12) of a plate heat exchanger embodying the invention;

FIG. 12 is a side elevational view of the heat exchanger shown in FIG. 11;

FIG. 13 is a schematic view of a plate heat exchanger system suitable for evaporation purposes and comprising a series of plate packs embodying the invention; and

FIG. 14 is a perspective exploded view of a plurality of the plates in the exchanger of FIGS. 11 and 12, showing the several fluid flow paths.

The heat exchange plates as illustrated are generally rectangular in form and are provided in their four corner portions with holes 1, 2, 3 and 4 forming throughfiow passages. Each plate of FIGS. 1 and 2 is provided at its middle portion, adjacent one long side, with a hole 5 forming another throughflow passage. In the plate P of FIG. 1, the holes 1 and 2 lie inside a packing cord 6 extending along the edge portion of the plate, whereas the holes 3, 4 and 5 are outside the area enclosed by this packing cord. In the plate P1 according to FIG. 2, the holes 3, 4 and 5 lie within an area enclosed by a packing cord 7, whereas the holes 1 and 2 lie outside the area enclosed by packing cord 7. The holes or passages situated outside the packing cords 6 and 7 are each enclosed by a packing ring 8 or 8a. By assembling a number of the plates shown in FIGS. 1 and 2 so that every second plate is of the one type (P) and the remaining plates are of the other type (P1), a plate pack is obtained in which each plate P has its packing engaged with the rear face of an adjacent plate P1, and each plate P1 has its packing engaged with the rear face of an adjacent plate P, the holes 3 and 4 being adjacent the long upper edges of the respective plates. In this plate pack, the plate interspaces form two groups of parallel-connected flow spaces or channels, every second interspace belonging to one group, the primary side, and the other interspaces belonging to the other group, the secondary side. The interspaces in one group are connected with each other through the passages 1 and 2, and the interspaces in the other group are interconnected through the passages 3, 4 and 5. If the passages 1 are connected to a common discharge line and the passages 2 to a common feed line, a flow is obtained in the appurtenant plate interspaces in accordance with the arrows shown in FIG. 1. The passages 3 and 4 are assumed to be connected to a common discharge line for one of the heat exchange media and the passages 5 to a feed line for this same medium, whereby the flow through the plate interspaces communicating with the passages 3, 4 and 5 takes place in accordance with the arrows indicated in FIG. 2. This embodiment is used when the exchanger is supplied with a liquid medium which, by taking up heat in these plate interspaces, is vaporized wholly or partly. In this case, during the passage through each plate interspace, this medium is thus divided into two parallel-connected partial flows, whereby the flow speed in the interspace, as Well as the length of the flow path through it, is reduced to about onehalf. As the flow resistance is directly proportional to the length of the flow path and to the square of the flow speed, the fiow resistance is reduced, through the flow division illustrated in FIG. 2, to about one-eighth of that otherwise obtained.

The plates according to FIGS. 3 and 4, as in the case of the plates of FIGS. 1 and 2, are each provided with another passage 5 adjacent the middle of one long side. However, the passage 5 as well as the passages 1 and 2 are arranged within a packing cord 6a on the plate P2 of FIG. 3; and in the plate P3 according to FIG. 4 the passages 3 and 4 lie within the corresponding packing cord 7a while the passages 1, 2 and 5 lie outside the latter cord. The passages 1 and 2 are intended for supply of a vaporous medium which by being cooled in the plate interspaces is caused to condense and is thereupon led off through the mid-passage 5, as is indicated by the arrows in FIG. 3. The cooling of the vaporous medium is effected by means of a cooling medium which is fed through the passage 3 and led off through the passage 4, as indicated by the arrows in FIG. 4.

The plates of FIGS. 14 are each provided near its upper long edge with a further mid-passage 9 which is situated inside the packing cord 6 or 6:: and outside the packing cord 7 or 7a and which can be used for leading off air or other gas, if any, from the plate interspaces for one medium. This discharge of air or gas is facilitated if the plates in the heat exchanger are disposed vertically With their long edges, adjacent the holes 9, uppermost and horizontal.

The previously described embodiments provide only 'two passages for one of the heat exchange media, which is always in a liquid state, namely, the passages 1 and 2 according to FIGS. 1 and 2, and the passages 3 and 4 according to FIGS. 3 and 4. One of these two passages in each case is in communication with a feed line and the other with a discharge line. However, by combining the plates according to FIGS. 2 and 3, both of the heat exchange media in the plate interspaces can be made to form partial or divided flows.

According to FIGS. 5 and 6, the plates P4 and P5 are each provided with two mid-passages 10 and 11. The passage 10 in plate P4 is located outside its marginal packing 12 but in the other plate P5 it is located inside the marginal packing 13; and the reverse is true of the passage 11. Through the passages 1 and 2 a vaporous or gaseous medium is supplied which, by being cooled in each plate interspace partly defined by packing cord 13, is caused to condense and thereupon discharges through the corresponding mid-passage 10, as indicated with the arrows in FIG. 6. Through the passage 11, a liquid is fed which is heated in the plate interspace partly defined by each packing cord 12 and is transformed into a vaporous or gaseous medium which discharges through the corresponding passages 3 and 4, as indicated by the arrows in FIG. 5. The plates according to FIGS. 5 and 6 may each be provided with two additional midpassages 14 and 15 which are located adjacent the upper long edge of the plate. The passage 14, like the passage 9 of the plates shown in FIGS. 14, may be used for leading oti gases separated in the plate interspaces for one of the heat exchange media. The other passage 15 may be used in conjunction with the passage 11 for supply of the other medium. With this arrangement of the plates, the partial or divided flows through the plate interspaces will not be exactly symmetrical. In practice, however, the deviations are quite insignificant and in any event are considerably smaller than is indicated in the drawings, which are not drawn to scale. In the plates according to FIGS. 5 and 6, the passages 10, 11, 14, 15 are, however, symmetrically arranged with respect to the vertical center line of each plate, so that these plates are identically alike if one of them is turned about an axis at right angles to the plane of the plate. The plates are thus uniform and may be made by means of one and the same pressing tool. This is also true of the plates according to FIGS. 1 and 2 and FIGS. 3 and 4, respectively, if they are provided with the passages 9. I

In the plates according to FIGS. 1-6, all the passages are arranged inside the ordinary contour of each rectangular plate. The mid-passages may, however, be placed outside this contour, as shown in FIGS. 7-10, so that they do not encroach upon the size of the available heat transmission surface. The plates of FIGS. 7-10 are therefore provided with parts 16 which protrude outside the ordinary contour and in which the mid-passages are arranged. Specifically, each plate P6 and P7 of FIGS. 7-8 has a single protruding part 16 provided with midpassages 10 and 11; while each plate P8 and P9 of FIGS. 9- 10 has two opposed protruding parts 16 provided, respectively, with passages 10 and 11 and with passages 14 and 15. Otherwise, the plates according to FIGS. 7-l0 are similar to the plates of FIGS. 5 and 6, except, of course, that thermarginal packings 18, 17, 19 and 20 on the plates of FIGS. 7, 8, 9 and 10, respectively, are shaped according 'to the locations of the corresponding midpassages.

Other embodiments of heat exchange plates than those described above may fall the scope of the present invention. For example, the plates need not be rectangular but may have an oval or other shape. In any case, however, a midapassage which is in open communication with a plate interspace in the plate pack of the 'heat exchanger should lie at about the same distance from the two corner passages which are in communication with the same plate interspace.

A plate heat exchanger comprising a pack of plates P6 and P7 as shown in FIGS. 7 and 8 is schematically illustrated in FIGS. ll and 12. As there shown, a series of the heat exchange plates P6 and P7, arranged in alternation with each other, is clamped in the usual way by means of bolts or the like (not shown) between two pressure plates 22 and 23, one plate 22 of which is fixed to the floor and provided with the required connecting lines, while the other pressure plate 23 is detachable in order to enable cleaning and inspection of the plates. Such a heat exchanger may be used to advantage for evaporation or concentration of liquids. Steam of relatively high temperature required for this purpose is delivered through two feed pipes 24 and 25 to the plate interspaces of the primary side by way of corner passages 1--2 of the plates, so that the hotter of the two heat exchange media passes through these interspaces. After the steam has given up heat in these interspaces of the primary side, it discharges therefrom as condensate through passages to a discharge pipe 26 leading from pressure plate 22. (In FIG. 12, the front :faces of the plates as shown in FIGS. 7-8 are facing to the left, so that in FIG. 11 the relative positions of the plate passages 10-11 are reversed as compared with FIGS. 7-8.) The heat exchange medium for the secondary side, that is, the colder medium, is supplied through a feed pipe 27 and mid-passages 11 to the plate interspaces of the secondary side, where it undergoes more or less complete vaporization. It then discharges in parallel streams through corner passages 3'4 into two outlet pipes 28-29 which open into a common collecting container 30, where the vaporous and liquid phases of the medium are separated from each other. The steam is thereupon discharged through an outlet line 3-1 from the upper part of the container 30, while the liquid is discharged through an outlet line 32 from the lower part of the container. The feed line 27 may be connected, as shown, to the liquid compartment of the container 3% through a pipe line 33, so that a circulation circuit is obtained through which the liquid is caused to circulate until the intended concentration is attained, the extent of this circulation depending upon the amount of concentrate drawn oif through the outlet line 32 and the amount of liquid fed through the inlet line 27 in relation to each other and to the evaporation capacity of the apparatus. Steam formed in the plate interspaces of the secondary side moves constantly in an upward direction and thus facilitates the circulation of liquid and steam through the apparatus.

The several fluid flow paths through the plates, as described above in connection with FIGS. 11 and 12, are illustrated in FIG. 14. As there shown, the liquid medium for the secondary side (the colder medium) is supplied to the plates by way of their mid-passages or apertures 11 and enters the flow spaces defined by gaskets 18 of the alternate plates P6, the liquid evaporating in these spaces and discharging as foam and vapor through the two apertures 3 and 4 in the upper corners of each plate P6. The steam for the primary side is supplied in parallel streams to the plates by way of their lower corner passages 1 and 2 and enters the flow spaces defined by gaskets 17 of plates P7, the steam condensing in these spaces and discharging as condensate through the midpassage or aperture :10 in the lower portion of each plate P7.

Two or more heat exchangers constructed as shown in FIGS. 11 and 12 may be combined. They may, for instance, be parallel-connected and work as a single-stage evaporator, in which case they are provided with a common collecting tank. In this way, the risk of excessive resistance in the passages of the plates or in the plate interspaces is avoided. They may also be seriesconnected, however, so that they form a multistage evaporator in which the diiferent plate packs are placed over one another and the steam is supplied at the bottom, whereby the pipelines can be relatively short and condensate formed in the pipes can flow downward and be vaporized in an underlying apparatus.

An example of such an evaporating plant is shown schematically in FIG. 13. As there shown, a number of plate packs are parallel-connected in pairs and these pairs in turn are series-connected to form a three-stage evaporator. The plate packs are indicated at 34, 35, 36, 37, 38 and 39. The plates in these packs are assumed to be shown in FIGS. 7 and 8. For the sake of clarity, however, the outlet lines from the passages 10 of the packs are not shown in FIG. 13. The two lowermost plate packs 34 and 35 are on the primary side supplied with live steam through a steam inlet line 40, and on the secondary side they are supplied with the liquid to be evaporated through a feed line 41 and its downwardly directed branches. Through outlet lines 42 and 43, a mixture of steam and liquid discharges from the plate interspaces of the secondary side to a collecting tank '44 where the two phases are separated from each other. The steam is led from the tank 44 through a steam discharge line which has two branches forming inlet lines to the primary sides of the two next plate packs -36, 37. From the liquid compartment in the tank 44 extends a liquid outlet line 46 which is connected to the inlet line 41a for the secondary sides of the plate packs 36 and 37. A branch line 47 returns some of the liquid from tank 44 to the secondary sides of the plate packs 34 and 35, by way of feed line 41. Liquid separated in the tank 44 is thus in part returned to the inlets of the secondary sides of the plate packs 34 and 35 in order to be subjected again to evaporation in these packs, and is in part transferred to the inlets of the secondary sides of the plate packs 36 and 37 for continued evaporation. How large a part of the liquid goes either way will depend upon the flow resistance and may be controlled by means of a valve (not shown) located in the line 46. The plate packs 38 and 39 are connected to the plate packs 36 and 37 by the parts 42a through 47a corresponding to the similarly numbered parts previously described. The packs 38-39 are provided with parts 41b through 4712 corresponding, respectively, to the parts 41a through 47a. The liquid finally evaporated is discharged through the outlet 46b in the third stage. The steam outlet 45b in this stage is connected to a vacuum pump or other vacuum source (not shown). The condensate formed on the primary sides in the different stages may either be discharged directly from the system or be fed, together with steam transferred from the secondary sides, to the primary sides of the next stage.

It will be apparent that in each of the illustrated ernbodiments of the heat exchange plate, the two corner portions having their throughfiow passages enclosed by a packing common to both (such as packings 6 and 7 in FIGS. 1 and 2, respectively) also have a common side edge or" the plate (such as the lower and upper edges in FIGS. 1 and 2, respectively). Thus, where the common packing also encloses an additional throughflow passage located at the middle portion of the plate, such as passage 5 in FIG. 2, the two parallel-connected streams flowing between this additional passage and each of these two corner passages Will be substantially symmetrical with respect to the additional passage.

1 claim:

1. A plate heat exchanger comprising a pack of elongated heat exchange plates each having a length at least double its width and having a pair of generally longitudinal opposed side edges partly defining four corner portions two of which are at each end of the plate, each plate having a throughflow aperture at each corner portion and an additional throughfiow aperture remote from said ends and located adjacent one of said side edges substantially midway between a pair of said corner apertures at opposite ends of the plate, packing means including first packings located between alternate pairs of the plates and defining therewith a series of first flow spaces each extending along opposed faces of the two adjacent plates over an area of each plate which includes said additional aperture and a first pair of said corner aeprtures consisting of only one corner aperture at each end of the plate, said packing means also including second packings disposed between the other pairs of plates and defining therewith a series of second flow spaces alternating with said first spaces, said second spaces each extending along opposed faces of the two adjacent plates over an area of each plate which excludes said additional aperture and said first pair of corner apertures but includes the other pair of said corner apertures, first conduit means for a first heat exchange medium leading to said first spaces and from which said packing means direct the medium through each first space by way 'of an inlet and an outlet of which one is formed by a said additional aperture, the other of said inlet and out-let being formed by a said first pair of corner apertures, and second conduit means for a second heat exchange medium leading to said second spaces and from which said packing means direct the second medium through each second space by way of'an inlet and an outlet including a said second pair of corner apertures.

2. A plate heat exchanger according to claim 1, in which each plate also has an extra aperture remote from said ends and located adjacent one of said side edges substantially midway between a pair of said corner apertures at opposite ends of the plate, said first packings excluding the extra apertures from said first spaces, the second packings including the extra apertures in said second spaces, one of said inlet and outlet of each second space being formed by a said extra aperture and the other by said second pair of corner apertures.

3. A plate heat exchanger according to claim 1, in which said inlet to each first space is formed by a said additional aperture, the exchanger comprising also a receiving container communicating with said first pair of corner apertures of the plates and forming a steam separating chamber for said first medium discharged through said first pair of corner apertures, and a pipe for conducting liquid condensate from said separating chamber to said first conduit means to recycle said condensate to said first spaces by way of said additional apertures.

4. A plate heat exchanger according to claim 1, in which the plates in said pack are generally rectangular in shape and are held substantially vertically with their long sides substantially horizontal.

References Cited in the file of this patent UNITED STATES PATENTS 2,562,739 Risberg July 31, 1951 FOREIGN PATENTS 834,829 France Dec. 2, 1938 23,393 Germany Aug. 3, 1883 57,305 Germany Aug. 3, 1883

Claims (1)

1. A PLATE HEAT EXCHANGER COMPRISING A PACK OF ELONGATED HEAT EXCHANGE PLATES EACH HAVING A LENGTH AT LEAST DOUBLE ITS WIDTH AND HAVING A PAIR OF GENERALLY LONGITUDINAL OPPOSED SIDE EDGES PARTLY DEFINING FOUR CORNER PORTIONS TWO OF WHICH ARE AT EACH END OF THE PLATE, EACH PLATE HAVING A THROUGHFLOW APERTURE AT EACH CORNER PORTION AND AN ADDITIONAL THROUGHFLOW APERTURE REMOTE FROM SAID ENDS AND LOCATED ADJACENT ONE OF SAID SIDE EDGES SUBSTANTIALLY MIDWAY BETWEEN A PAIR OF SAID CORNER APERTURES AT OPPOSITE ENDS OF THE PLATE, PACKING MEANS INCLUDING FIRST PACKINGS LOCATED BETWEEN ALTERNATE PAIRS OF THE PLATES AND DEFINING THEREWITH A SERIES OF FIRST FLOW SPACES EACH EXTENDING ALONG OPPOSED FACES OF THE TWO ADJACENT PLATES OVER AN AREA OF EACH PLATE WHICH INCLUDES SAID ADDITIONAL APERTURE AND A FIRST PAIR OF SAID CORNER APERTURES CONSISTING OF ONLY ONE CORNER APERTURE AT EACH END OF THE PLATE, SAID PACKING MEANS ALSO INCLUDING SECOND PACKINGS DISPOSED BETWEEN THE OTHER PAIRS OF PLATES AND DEFINING THEREWITH A SERIES OF SECOND FLOW SPACES ALTERNATING WITH SAID FIRST SPACES, SAID SECOND SPACES EACH EXTENDING ALONG OPPOSED FACES OF THE TWO ADJACENT PLATES OVER AN AREA OF EACH PLATE WHICH EXCLUDES SAID ADDITIONAL APERTURE AND SAID FIRST PAIR OF CORNER APERTURES BUT INCLUDES THE OTHER PAIR OF SAID CORNER APERTURES, FIRST CONDUIT MEANS FOR A FIRST HEAT EXCHANGE MEDIUM LEADING TO SAID FIRST SPACES AND FROM WHICH SAID PACKING MEANS DIRECT THE MEDIUM THROUGH EACH FIRST SPACE BY WAY OF AN INLET AND AN OUTLET OF WHICH ONE IS FORMED BY A SAID ADDITIONAL APERTURE, THE OTHER OF SAID INLET AND OUTLET BEING FORMED BY A SAID FIRST PAIR OF CORNER APERTURES, AND SECOND CONDUIT MEANS FOR A SECOND HEAT EXCHANGE MEDIUM LEADING TO SAID SECOND SPACES AND FROM WHICH SAID PACKING MEANS DIRECT THE SECOND MEDIUM THROUGH EACH SECOND SPACE BY WAY OF AN INLET AND AN OUTLET INCLUDING A SAID SECOND PAIR OF CORNER APERTURES.

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