DE8532157U1 - Filter press for liquid separation from sludge down to a high solids content - Google Patents

Description

HAKU Hans Küble*. GritbH .1. ...

Filter systems and mechanical engineering ^{p 85 111} Dietlinger Straße 93

7534 Birkenfeld

Filter press for liquid separation from sludge down to a high solids content.

I The invention relates to a filter press for the

j f- Liquid separation from sludges such as municipal

&iacgr; ' sewage sludge or industrial sludge up to a

I high solids content and with the other/ in the upper

t concept of patent claim 1, generic

i matching characteristics.

jj A filter press of this type is characterized by

&Idigr; DE-OS 34 02 165 known.

? The well-known filter press comprises several presses

I elements which, each considered individually, are independent

i functional chamber filter presses each with a

S press chamber that can be fed with wet sludge

5 (J. Each of the press elements comprises a

I Lower membrane mounted on a base frame

plate, a cup-shaped closing piece that can be placed on it by means of a hydraulic drive device and an upper membrane plate that is movably guided in the closing piece and can also be moved up and down by means of the hydraulic drive device, which together delimit the press chamber. The press elements are, viewed in the transport direction of an upper and a lower filter cloth, which, as is the case with sieve technology,

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known from band presses/ are designed as closed belts arranged next to each other or one behind the other/ so that these filter cloth belts can be used as inner lining for all of the existing press elements. To adapt to the shape of the press chambers, the upper filter cloth is provided with, for example, flat pyramid-shaped bulges. The sludge cake enclosed between the filter cloths in the press chambers can be pressed out to a relatively high solids content of around 50% by pressing down the movable membrane plate.

A significant disadvantage of the known filter press is that the upper filter cloth with the bulges, which is subjected to considerably greater stress when the filter cake is pressed out than the lower filter cloth and is therefore subject to increased wear, must be completely replaced or removed and repaired if it is damaged, which is not only time-consuming but also involves considerable costs. Another factor that makes wear and tear difficult is that even a relatively small, permanent stretch of the upper filter cloth that occurs after prolonged use means that the distance between the bulges, even if only slightly, no longer matches the distance between the press elements, viewed in the longitudinal direction of the belt, which can result in uneven stress on the bulges of the upper filter cloth. A filter press of the type mentioned at the beginning therefore requires extensive maintenance work at relatively short intervals.

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The object of the invention is therefore to improve a filter press * of the type mentioned at the beginning so that maintenance and repair work, which in particular affects the upper filter cloth, can be carried out much more easily and quickly and is associated with considerably lower costs. ·

This object is achieved according to the invention by the features mentioned in the characterizing part of patent claim 1.

The filter cloth is then divided into segments assigned to the individual press elements, which are coupled together by easily connectable and detachable pin seams, whereby each of the segments is provided with a bulge adapted to the shape of the press chamber of the respective press element.

A significant advantage of the filter press according to the invention is that a damaged filter cloth segment can be replaced quickly and with little effort without the filter cloth having to be removed from the filter press as a whole. It is also advantageous that filter cloth segments with different designs of bulges and/or different filtration properties can be used or replaced if required. The adaptation of the filter press to different qualities of sludge is therefore possible in a simple and cost-effective manner.

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The filter cloth segments assigned to the individual press elements can be conveniently combined to form a self-contained filter cloth by means of a band-shaped cloth section that is also connected by means of pin seams.

The features of claim 2 indicate how bulges of the filter cloth segments adapted to the most diverse designs of press chambers of the individual press elements can be created in a particularly simple manner.

The features of claims 3 to 5 advantageously provide simple measures that provide a sufficiently load-resistant anchoring for the realization of the pin seams provided for eyelets on the filter cloth segments.

The features of claim 6 specify a design of such eyelets which, in conjunction with flat bar-shaped anchoring rods which can be pushed through the eyelets of segments to be connected to one another, enables compensation for slight expansion of the filter cloth segments.

The feature of claim 7 provides the advantage of reducing wear on the filter cloth segments.

The design of the filter press elements specified by the features of claim 8 has the advantage that, in the case of a displacement of the upper membrane

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As the pressing force increases, the closing force that keeps the press chamber closed is also increased proportionally and the press chamber remains securely closed.

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Further details and features of the invention emerge from the following description of a specific embodiment based on the drawing. They show:

Fig.1 shows the basic structure of a filter press according to the invention with two press elements arranged next to each other,

Fig.2a Details of a press element of the to 2d press according to Figure 1 in different functional phases/

Fig.3 a press element in a perspective exploded view with an upper filter cloth composed of segments that are connected to one another by pin seams and a lower, continuous band-shaped filter cloth and

Fig.4 one of the pin seams of the upper filter cloth according to Figure 3 in cross section.

The filter press shown in Figure 1, to the details of which reference is expressly made, and designated as 10, is to be considered for the following description of structural and functional details without restriction of generality as a filter press for the development

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A suitable press is required for washing municipal sewage sludge. However, it is understood that such a filter press 10 can also be used to extract other liquids, e.g. oils or solvents, from industrial sludge of various kinds.

The filter press 10 comprises in the illustrated, special - embodiment two in total each with 11 or 12

signed press elements, which are based on a common I

s frame 13 are mounted. c

Each of the two press elements 11 and 12 is, in terms of its basic structure, a chamber filter press, which, as can best be seen from Figures 2a to 2c, to whose details reference is also expressly made, has only one single press chamber 14 that can be filled with wet sludge, which, according to Figures 2a to 2c, is formed by a lower membrane plate 16 with an approximately rectangular basic shape, a rectangular,

cup-shaped closing part 17, which is open towards the lower membrane plate 16, and a

movably guided, upper membrane plate 18, the base area of which, apart from a slight clearance between its circumferential end faces and the longitudinal and transverse walls 17' and 17'' of the closing part 17 running perpendicularly ZZi of the lower membrane plate, corresponds to the clear cross-sectional area of the cup-shaped closing part 17.

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"Longitudinal direction" means the direction marked by arrow 19 in Figure 1 or Figure 2a, along which an upper filter cloth, designated overall by 21, and a lower filter cloth 22 pass through the two press elements 11 and 12 and can be transported together along this direction 19 for discharging the largely dewatered sludge cake (Figure 2c) in the special embodiment shown.

The filter cloths 21 and 22 are designed as self-contained belts, the course of which is schematically indicated in Figure 1. The upper filter cloth 21 is guided over rollers 24, which are used both for guiding and deflecting as well as for driving the upper filter cloth, and the lower filter cloth 22 over corresponding rollers 26. A drive for the two filter cloths 21 and 22, which can be implemented in a known manner, can be implemented using known means | and is not shown for the sake of simplicity.

The press elements 11 and 12 are, as can be seen from Figure 1, mounted on the base frame 13 in an inclined position, whereby the membrane plates 16 and 18 and the cover plate 27 of the closing part, which is parallel to them, and an upper guide plate 28, which is acted upon by a hydraulic drive device, designated as a whole by 29, have an inclination of approximately 30°. This inclined arrangement of the filter elements 11 and 12, which is symmetrical in the illustrated embodiment according to Figure 1 with respect to the vertical transverse center plane 31 of the filter press, enables both the introduction of the wet sludge via high filling nozzles 32 and

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the outflow of the filtrate from the press chambers 14 is facilitated via low-lying outlet openings 33.

To explain further structural and functional details of the press elements 11 and 12 of the filter press 10, reference is now made to Figures 2a to 2c.

in which one of the press elements - the frenzing element 12 - is shown in a horizontal arrangement for the sake of simplicity.

The lower membrane plate 16 and the housing 29' of the

The hydraulic drive device of the press element 12, designed as a double-acting hydraulic cylinder, is mounted in a fixed position on the press frame (the relevant fastening for the cylinder housing 29' is not shown for the sake of simplicity).

By alternatively applying and releasing pressure to the working spaces 36 and 37, the closing and pressing movements of the closing part 17 and the upper membrane plate 18 or the opening strokes of the pressing element 12 for discharging the filter material are controlled.

Figure 2a shows the basic position of the press element after completion of a pressing process, in which the press chamber 14 is opened

j and the mud cake has already been carried out. The

Hydraulic cylinder piston 38, which is firmly connected to the upper guide plate 28 via a piston rod 39, the locking piece 27 and the upper membrane plate 18 are in their upper end position,

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wherein the clear distance h of the downwardly facing free end faces 41 and 42 of the longitudinal and transverse walls 17' and 17'' of the closing piece 17 from the parallel counter surfaces 43 and 44 of the lower membrane plate 16, which are slightly raised compared to the remaining inner surface of the lower membrane plate 16, is approximately 10 mm.

Two guide bolts 46 are firmly connected to the upper membrane plate 18, the longitudinal axes 48 and 49 of which run parallel to the central axis 51 of the hydraulic cylinder and are arranged symmetrically with respect to this and together with it in the vertical longitudinal or transverse center plane, in the illustrated embodiment in the longitudinal center plane, of the press element 12. These guide bolts 46 and 47 each pass through a guide sleeve 53 of the cover plate 27 of the locking piece 17, which is provided with seals 52, and a sliding bearing bush 54 of the upper guide plate 28, which is arranged in alignment with this guide sleeve 53, and are supported - in the illustrated basic position of the press element 12 - by a stop ring 56 on the top side 28' of the upper guide plate 28.

Between the upper guide plate 28 and the cover plate 27 of the closing piece 17, a ring 57 is arranged that runs in the edge areas and is closed in itself and is made of an elastic material, e.g. a polyurethane foam. The ring 57 is under a minimum pre-tension and thus forces the closing piece 17 into contact with the upper membrane plate 18. The

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The filling nozzle 32 provided on the long side has a shaped piece 32', the cross section of which is geometrically similar to the clear cross section of an opening delimited by opposite edge recesses 58 and 59 of the closing piece 17 or the lower membrane plate in the closed state of the press element 12, whereby this opening 58, 59, when the press chamber is closed, as shown in Figures 2b and 2c, is tightly closed by this shaped piece 32' and the edge sections of the filter cloths 21 and 22 arranged between it and the edge recesses 58 and 59.

In the open position of the press element 12 or the press chamber 14 shown in Figure 2a, the shaped piece 32' of the filling nozzle 32 is slightly lifted away from the lower membrane plate 16 to such an extent that the edge sections of the upper filter cloth and the lower filter cloth 22 mentioned can be pulled unhindered between the shaped piece 32' and the closing piece 17 or the shaped piece 32* and the lower membrane plate 16.

In preparation for a filtration or drying process, the press element 12 is closed, whereby the upper guide plate 28, the closing piece and the upper membrane plate 18 assume the position shown in Figure 2b relative to one another at the end of the closing stroke of the hydraulic cylinder 29. The pressure exerted by the piston 38 of the hydraulic cylinder 29 is - in a special design - approximately 25 mm larger than that required to close the press element 12.

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chamber 14, the lowering stroke h of the closing piece is required, so that the elastic ring body 57 is compressed and thus further pre-tensioned as soon as the closing piece 17 is supported on the lower membrane plate 16 via the filter cloths 21 and 22. The closing force generated by this additional pre-tension of the elastic ring body 57 and evenly distributed to the closing piece 17 is dimensioned such that the press chamber 14 remains securely closed under the hydrostatic pressure, typically 2 bar, with which the filter material is fed into the press chamber 14. The stroke H carried out to increase the preload in the elastic ring body 57, by which the clear distance between the upper guide plate 28 and the cover plate 27 of the locking piece 17 is reduced, is dimensioned such that the upper guide plate 28, after the preload stroke has been carried out, rests just against the stop rings 61 of the guide bolts 46 and 47. Due to the preload stroke of the upper guide plate 28

, until the position shown in Figure 2b changes

the position of the lower guide plate 18 in relation to the closing piece 17 does not change, since the friction between | the guide sleeves 53 and the guide bolts 46 and 47 is sufficiently great that the upper membrane plate 18, | although its upper stop ring 36 lifts off the upper guide plate 28 when it moves towards the closing piece 17, "stands still" within it, as it were. In this functional position of the press element 12, which corresponds to the maximum volume of the press chamber 14, the filling of the press chamber 14 with wet sludge is carried out. A &trade;

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typical design of the filter press 10 corresponding

The value of this volume is - with an approximately square floor plan of the press chamber - 14-80 liters. The press is fed with ^{ tons of wet sludge, which has a solids content typically around 5%, by means of a pump which, in a suitable design, has a delivery rate of 400 l/min at an operating pressure of approx. 2 bar.

In the functional position of the fracturing element corresponding to the largest volume of the press chamber 14, the clear height of the

Press chamber 90 mm. The depth of the flat depression 16* extending between the flat edge surfaces 43 and 44 of the lower membrane plate 16 and taking up by far the largest part of the base area of the press chamber 14 is approximately 10 mm.

The upper filter cloth 21 is, as can best be seen from Figure 3, to whose details reference is made in addition, divided into segments 21', which are connected to one another or to adjoining parts of the filter cloth 21 by pin seams running transversely to the longitudinal direction 19 of the filter cloth 21, designated in total by 62. These filter cloth segments 21* consist of a filtration fabric, which can also be used in conventional chamber filter presses. In a preferred design, these segments 21' are made of thermoplastic synthetic materials.

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fibers. The filter cloth segments 21' are dimensioned in terms of their base areas and arrangement relative to one another in such a way that each of these segments 21' can form the upper inner lining of the press chamber 14 of the two press elements 11 and 12 of the filter press 10 according to Figure 1, with the pin seam 62 connecting these two filter cloth segments 21' being arranged in the free space between the two press elements 11 and 12. The filter cloth segments 21' are provided with bulges 63 which are truncated pyramid-shaped in the illustration in Figure 3 and which, when the press chambers 14 are fed with wet sludge via the filling nozzles 32, lie over a large area against the upper membrane plate 18 of the respective press element 11 or 12 in the manner shown in Figure 2b, so that the liquid formed by the filter cloths 21 - the lower, continuous band-shaped filter cloth 22 is pressed against the lower membrane plate 16 - enclosed receiving space for the introduced wet sludge takes up practically the entire volume of the chamber. During the loading process, water escaping upwards and downwards from this space of the press chamber 14 enclosed by the filter cloths ²¹ and 22 through the filter cloths 21 and 22 can flow out unhindered through longitudinal grooves 64 and 66 of the lower membrane plate 16 and the upper membrane plate 18, respectively, which are in communication with the outlet opening 33 of the respective lower membrane plate 16. By loading the press chamber 14 with wet sludge at a pressure of approx. 2 bar as explained above and the filtration taking place at the same time, the part of the chamber volume enclosed by the filter cloths 21 and 22 is closed.

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An enrichment of the solids content to a value of around 20% with a very good homogeneous solids distribution is achieved. A further increase in the solids content or relative drying of the wet sludge by further addition of wet sludge is practically impossible through a filtration process alone, as explained above; accordingly, the feeding process is terminated, e.g. after a period of time corresponding to an empirical value, when the aforementioned homogeneous distribution of the solids content in the press chamber 14 is achieved.

For further "drying" of the wet sludge enclosed in the press chamber or enclosed within the filter cloths 21 and 22, a pressing process is now carried out by renewed pressure application to the hydraulic cylinder 29, during which the upper membrane plate 18 moves towards the lower membrane plate 16 in the sense of a reduction in the volume of the press chamber 14, which additionally removes liquid from the sludge cake

. pressed out and a further increase in its solid

This pressing process is terminated after a maximum value of the working pressure in the working chamber 36 of the drive cylinder 29, linked to the maximum value of the pressing force, has been maintained for a certain period of time and no significant additional dewatering of the sludge cake 23 can be achieved by further increasing the pressing force. This is generally the case when the solids content of the sludge cake 23 has reached a value of around 50 to 55%.

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In order to achieve such - relatively high - solids contents in the sludge cake 23, it is sufficient if the drive cylinder 29 is designed in such a way that the hydrostatic internal pressure resulting in the sludge cake 23 when compressed with the highest possible pressing force is approximately 12 bar.

In order to achieve this with certainty,

- the filter press 10 according to the invention is provided that

the maximum pressing stroke that the upper membrane plate can perform corresponds to the clearance d that the upper membrane plate 18 has in the basic position of the press element shown in Figure 2a from the lower boundary surfaces 41 or 42 of the closing piece 17, and that this maximum pressing stroke of the upper membrane plate 18 corresponds to a volume reduction of the press chambers 14 to about 1/8 of their maximum volume. The exploitability of the drastic reduction in the volume of the press chamber 14 that is possible here for the purpose of effective sludge dewatering is made possible by the

( Exemplary example according to Figure 3 truncated pyramid "

shaped design of the bulges 63 of the segments 21' | of the upper filter cloth 21, since even a moderate stretching of the cloth material in the area of the bulges 63 is sufficient for them to lie over a large area against the wall parts of the press chamber formed by the upper membrane plate 18 and the closing piece 17 and to nestle closely along their course, which makes it possible to use the largest possible chamber volume for the sludge feed.

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Due to the limited expansion of the filter cloth segments 21 * in the area of their - truncated pyramid-shaped - bulges 63, whose base area limited by the base lines 63 ¹ expediently corresponds to the clear cross-sectional area of the press chamber 14, as well as the reduction of this expansion during a subsequent pressing process,

>- , . the filter cloth segments 21' only slightly on

Stretching stress and are therefore subject to only a correspondingly low level of wear.

It is understood that instead of the specifically explained truncated pyramid-shaped bulges 63, those with other, e.g. flat dome-shaped, designs can also be provided, which appear to be most suitable for the stated purpose, namely an approximate adaptation to the shape of the press chamber, depending on its design. Such bulges can be easily produced if the filter cloth segments 21' are made of thermoplastic material.

(_. in a deep drawing process in practically any size

design can be produced.

The pin seams 62 intended for connecting two segments 21' of the upper filter cloth 21 can be implemented in a special design in the manner shown in Figure 4, to the details of which reference is now made:

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on the transverse edge sections 21'' to be connected to one another of two filter cloth segments 21· that follow one another in the longitudinal direction of the upper filter cloth 21, eyelets 67 are anchored at regular - small - distances from one another, which have a slot-shaped clear cross-section, with the center planes of the eyelet openings 68 running perpendicular to the filter cloth plane. The eyelets 67 are made of a steel wire that has a thickness of 0.3 to 0.5 mm, and the lateral distance f ' of the eyelets 67 of the respective filter cloth segment

:s ments 21 ' is between 4 and 8 mm, so that the

eyelets 67 of one filter cloth segment 21· can be arranged with a slight lateral offset between 2 eyelets 67 of the opposite filter cloth segment 21' and a steel rod 65 can be pushed through the free cross-sectional areas of the eyelets 67 of the two filter cloth segments 21' which overlap, as seen in the longitudinal direction of the filter cloth 21, and which has a diameter which is slightly smaller than the clear width of the eyelets 67 measured perpendicular to the filter cloth plane and has sufficient stability for the tensile connection of the filter cloth segments 21·. The eyelets 67 have L-shaped anchoring recesses.

sections 69 and 71 of different lengths, which are arranged directly next to each other and

: bent at right angles to opposite sides

« have curved anchoring legs 69' and 71*, which

&Ggr; parallel to the filter cloth plane and on the filter

cloth fabric. The anchoring sections 69 and 71 are embedded in a plastic layer 72 which is firmly connected to the filter cloth fabric of the filter cloth segments 21', e.g. by welding.

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which provides a tensile connection of the eyelets 67 with the respective filter cloth segment 21'. The different lengths of the anchoring sections 69 and 71 of the eyelets 67 are provided so that the greatest possible length of the anchoring cross legs 69' and 71' can be selected with the smallest possible lateral distance between the eyelets 67, which is important for the tensile anchoring of the eyelets 67. The plastic layer 72 is conveniently designed so that it encloses the free transverse edge 73 of the filter cloth fabric 74 in a U-shape.

The rod 65, which can be inserted through the eyelets 67 of the filter cloth segments 21' to be connected to one another, is, as only schematically indicated in Figure 4, provided with a transverse bore 76 on each of its free end sections, through which a split pin can be inserted, which secures the rod 65 against disengaging from its engagement position with the eyelets 67 of the two filter cloth segments 21'. An equivalent securing of the rod 65 against disengaging from its engagement position with the eyelets 67 of the filter cloth segments 21' to be connected to one another can also be implemented in such a way that the rod is provided with a head on one of its free ends, and has a short threaded section on its other end, onto which a nut and a lock nut can be screwed.

In the special embodiment of a filter press 10 according to the invention shown in Figure 1, it is in principle sufficient if the upper filter cloth has only two recesses 63

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provided filter cloth segments 21*, which are further connected to one another by a flat, level transport section, which is also coupled to the two filter cloth segments 21' in a tensile manner by pin seams 62 of the type explained.

The filter press 10 is also expediently provided with a suction device, by means of which a negative pressure can be generated in the press chambers 14 of the press elements 11 and 12, which promotes the escape of liquid from the sludge cake 23. The use of a liquid ring pump 77, shown schematically in Figure 1, has proven to be particularly advantageous for this purpose. This pump is connected to a collecting line 78 communicating with the outlet openings 33 of the two press elements 11 and 12 and can be conveniently accommodated below the press elements 11 and 12 in the base frame 13 of the filter press 10. A valve arrangement, by means of which the inlet nozzles 32 of the press elements 11 and 12 can be shut off during evacuation operation, is not shown for the sake of simplicity.

The liquid ring pump 77 can also be used in any operating phase of the filter press to transport the filtrate away.

Claims (8)

et· re « · m a &lgr;&bull;&Lgr; ait·· it * - &bgr; HAKÜ Hans KUBIe1T GirfBH P 85 111 Filter systems and mechanical engineering Dietlinger Straße 93 Eirkenfeld sayings 1. Filter press for liquid separation from ^r-. Sewage sludge or industrial sludge up to a high solids content, with press chambers each delimited by a fixed membrane plate and a pot-shaped closing part that can be moved towards and away from the fixed membrane plate, in the closed state of which two interrelated filter cloths, which run through the press chambers and are clamped along the edges of the closing piece pointing towards the fixed membrane plate between the latter and the fixed membrane plate, whereby the sludge is fed through the filling nozzles of the press chambers between the filter j cloths can be inserted, and with a of the closing piece on the fixed membrane plate movable towards and away from it, acting as a press stamp, through whose movement towards the fixed membrane plate the package formed by the cloths and the mud cake can be compressed, whereby the filter cloth facing the movable membrane plate is provided with arching bulges which, when /2 P85 111 - 2 - When the press chambers are filled with sludge, they can bulge and rest against the movable membrane plates on at least part of the surfaces thereof, characterized in that the filter cloth (21) having the bulges (63) is divided into segments (?1 ') which are assigned to the individual press chambers (14), and that these segments (21') are connected to one another and to the conveying parts of the filter cloth (21) by means of pin seams (52). 2. Filter press according to claim 1, characterized in that the filter cloth segments (21') consist of a fabric made of thermoplastic fibers, and that the bulges (63) are produced in a deep-drawing process. 3. Filter press according to claim 1 or claim 2, characterized in that eyelets (67) provided for forming the pin seams (62) are anchored in the transverse edges (73) of the filter cloth segments (21') reinforcing plastic strips (7?) (Figure 4). 4. Filter press according to claim 3, characterized in that the eyelets (67) are designed as steel wire brackets which have anchoring sections (69 and 71) of different lengths with anchoring legs (69* and 71) bent at right angles, which are embedded in the reinforcing plastic layer (72). /3 P85 111 5. Filter press according to claim 3 or claim 4, characterized in that the strip-shaped reinforcing plastic layers (72) surround the free transverse edges (73) of the filter fabrics (74) of the filter cloth segments (21') in a U-shape (Figure 4). 6. Filter press according to one of the preceding claims/ characterized in that eyelets (67) provided for forming the pin seams (62) have slot-shaped eyelet openings (68). 7. Filter press according to one of the preceding claims, characterized in that the end faces (41 and 42) of the longitudinal and transverse walls (17* and 17 ¹¹ ) of the closing piece (17) facing the lower membrane plate (16) adjoin the inner surfaces of these closing piece walls (17 ¹ and 17") with a smooth curvature. 8. Filter press according to one of the preceding claims, characterized in that the upper, movable membrane plate (18) of the respective press element (11 or 12) is provided with guide bolts (46 and 47) which are slidably mounted in guide sleeves (5 3) of the cover plate (27) of the closing piece (17) and in plain bearing bushes (56) of a guide plate (28) firmly connected to the piston rod (39) of a drive hydraulic cylinder (29), whereby these guide bolts (46 and 47) are provided with stop rings (56) arranged above the guide plate (28), P85 111 -A- that an elastic body (57) is arranged between the guide plate (28) and the cover plate (27) of the closing piece (17) which is under a minimum pre-tension/ which, in the open position (Figure 1) of the respective press element (11 or 12), pushes its closing piece (17) and with it the movable membrane plate (18) into its position with the greatest possible distance from the guide plate (28)/ in which position the ring-shaped stops (56) of the guide bolts (46 and 47) rest on the upper side (28 ¹ ) of the guide plate (28), and that the guide bolts (46 and 47) are provided with further stop rings (61) arranged between the guide plate (28) and the cover plate (27) of the closing piece (17), the distance of which, seen in the open position (Figure 1) of the respective press element (11 or 12), from the guide plate (28) corresponds to a compression stroke by means of which the elastic body (27) can be further displaced in order to achieve a minimum closing force, and whose distance from the cover plate (27) of the closing piece (17), also seen in the open position (Figure 1) of the respective press element (11 or 12), is somewhat greater than the maximum pressing stroke of the movable membrane plate (18). /5