Prior art pressure screens typically include a cylindric screen plate (also referred to as"a screen") disposed within a cylindric, pressurized housing. The housing includes an inlet or feed chamber adapted to feed the unscreened stock to a reject retaining surface of the cylindric screen plate. The cylindric screen plate is typically operatively associated with hydrofoils or rotor located close to the feed surface of the cylindric screen plate. The hydrofoils or rotor surface move along the surface generating hydraulic pulses which result in maintaining the surface of the cylindric screen plate generally clean by preventing the clogging of the screening openings by the solids contained in the suspension. The accepts fraction of the suspension, (e. g. clean fibres) which has passed through the cylindric screen plate is then collected in an accept chamber at the downstream end of the cylindric screen plate. The reject fraction of the suspension which did not pass through the cylindric screen plate, is collected in and eventually discharged from a reject chamber of the cylindric screen plate.

The reference to the"accepts"and"rejects"is not intended to refer to the fractions which are or are not suitable for further processing. The reference is made for convenience, to distinguish the fraction which has passed through the cylindric screen plate from that which was retained.

It is known that the present practice of pressure screening in pulp and paper industry is focused on providing cleaner pulp and/or paper stock to further processing, e. g. to a paper making machine. A common requirement exists that the accepts contain as clean and uniform a suspension as possible.

This task can only be solved by reducing the size of the passages or openings through the cylindric screen plates of the pressure screen. However, the reduced passage area results in a reduced capacity of the screen.

Attempts have been made to increase the capacity by'simply increasing the number of pressure screens or using a larger cylindric screen plate. Both solutions require more space which often is not readily available in existing plants. Besides, the increase in the size of the screening cylinder is not directly proportional to the increase in capacity. A 100% increase in the screen cylinder size may only yield about a 50% to 70% increase in capacity. Furthermore, the large screening baskets are extremely expensive. They have to be exchanged or replaced after a given period of operation. The cost of a replacement cylindric screen or basket is often in many tens of thousands of dollars. The large diameter cylindric screen plates tend to lose the cylindric shape with serious consequences, as the hydrofoils or rotor surface must move relatively close to the surface of the cylindric screen plate.

US Patent 4,302,327 (Martin) utilizes the known principe of two pairs of concentric screens. The pairs of screens are disposed coaxially one above the other, with each screen having a separate rotary impeller mechanism.

There is a common drive for the impellers, a common central feed of the stock to be screened and a common accepts chamber. Viewed from the standpoint of the present invention, the device may increase the capacity of a machine with a single pair of concentric screens, but this is at the expense of the space requirement which equals the requirement for two separate screens, one on top of the other.

US Patent 5,255,788 (Rienecker et al.) presents a pressure sorter wherein the screening area of the cylindric screens is doubled but here the space required for the housing is generally equal to that of two distinct cylindric chambers, each surrounding the respective screen.

Summary of the Invention It is an object of the present invention to further advance the art of the above type of screening devices. In particular, the present invention is directed towards providing an increased screening area which would not require additional exterior space within a given plant or would require a substantially less outer space and head room compared with an existing screen having comparable capacity.

It is another object of the present invention, to provide a pressurized screen having a relatively large capacity but operating with relatively small diameter screen cylinders which are sturdier under the pressurized conditions, easier to produce and are thus less expensive to replace in an existing screen.

According to the invention, a pressure screening device is provided for screening liquid suspension of solid material particles, comprising, in combination: (a) a housing which includes (i) a generally cylindric inner wall having an axial length and a diameter and defining a generally cylindric interior of said housing; (ii) a pressurized inlet for unscreened stock suspension, (iii) an accepts outlet; (iv) a rejects outlet; (b) at least three cylindric screen plates disposed within said cylindric interior and having axes radially spaced from each other and from the axis of said cylindric interior; (c) each said cylindric screen plate including (i) a reject retaining surface hydraulically connected to said inlet, (ii) an accepts side surface opposite to said retaining surface, and

(iii) a plurality of screening openings defining a screening passage area of the respective cylindric screen plate.

It is preferred, but not absolutely necessary, that there be three or more cylindric screen plates in a single housing arranged in a side-by-side fashion.

The number of cylindric screen plates is optional but those skilled in the art will readily appreciate that, typically, the upper limit of the number of the cylindric screen plates is given by a reasonable minimum diameter of the screening baskets. The screening baskets are preferably of the same type and size so that one cylindric screen plate fits any location of the cylindric screen plate thus reducing the required number of spare cylindric screen plates. The arrangement may be different when the invention is used in sorting fibrous suspensions.

Brief Description of the Drawings The invention will now be described by way of two embodiments of a proposed prototype, with reference to the accompanying diagrammatic, not-to- scale drawings, in which Figure 1 is a simplified, diagrammatic, not-to-scale vertical cross-sectional view taken on section line l-l of Fig. 2, presenting a first exemplary embodiment of a pressure screen of the present invention, with certain parts omitted for clarity; Figure 2 is a simplified, diagrammatic horizontal sectional view taken on section line ll-il of Figure 1; Figure 3 is detail III of Fig. 1 on enlarged scale; Figure 4 is detail IV of Fig. 1 on enlarged scale; Figure 5 is a view similar to that of Fig. 1 but showing a second exemplary embodiment of a pressure screen of the present invention on section line V-V of Figure 6; Figure 6 is a simplified, diagrammatic horizontal sectional view of the representation taken on section line VI-VI of Figure 5; and

Figure 7 is a simplified partial side view showing the outlet arrangements for accepts and rejects.

Description of Preferred Exemplary Embodiments I Turning firstly to the embodiment of Figs. 1-4, the pressure screen or screening device 10 includes a cylindric housing 11 defining a generally cylindric inner wall 1 2. The upper end of the housing 11 is provided with a sealed cover 13. The cover 13 combines with the housing 11 to define a pressurized interior of the housing 11. The upper portion of the pressurized interior defines a feed chamber 14 for unscreened stock suspension typically comprising water and cellulose or other fibers and particulate impurities such as fibre knots. The chamber 14 hydraulically communicates with a pressurized stock inlet 15. The stock inlet 15 is shown in phantom lines of Fig. 2 to provide the general idea of its tangential inlet into the feed chamber 14. The inlet 15 is normally connected to a feed system including a pump for maintaining the incoming stock in the housing 11 under pressure.

Disposed within the generally cylindric interior of the housing 11 are four cyiindric screen plates having their longitudinal axes such as axes 20,21 parallel with and radially spaced from each other. The axes 20,21 are also radially spaced, preferably equidistantly, from the axis 22 of the housing 11.

The cylindric screen plates or baskets 16-19 are also spaced radially inwardly from the cylindric inner wall 12.

As is well known from the art of pressure screens, each cylindric screen plate 16-19 is provided with a number of screening openings, only diagrammatically shown in the drawings. The openings may take many different shapes, for instance narrow slots or tapered openings extending between the reject retaining surface and the accept side surface. In the embodiment shown, the reject retaining surface is the inner surface of each cylindric screen plate 16-19 and the accept side surface is the exterior

surface of each cylindric screen plate. The present invention is not limited to the particular disposition of the reject and accept sides as shown.

As best seen from Fig. 3, a flat plate 23 is secured, for instance, by welding, to the surface of the inner wall 1 2 of the housing'11. The plate 23 separates the feed chamber 14 from the accepts compartment or chamber 29.

In the embodiment shown, the plate 23 defines four circular openings such as opening 64 machined into the plate to allow the screen cylinders 16, 17,18, 19 to drop through the openings 64, as best seen from Figs. 3 and 4. Tapered rings 65,66 are attached to the top and bottom of each screen cylinder 16- 19.

Four short solid cylinders such as cylinder 24, (Fig. 4) devoid of any screening passages are attached to a base plate 26 of the housing 11, one below each screen cylinder 16-19. Secured to the top of each cylinder 24 is a tapered ring 67 having a taper to match and seal the bottom tapered ring 66 of the respective screen cylinder 16. The tapered rings 67 not only seal the feed from the accepted stock but also help to center the screen cylinders 16- 19 to be concentric with the associated rotating hydrofoils or rotors.

Each clamp ring such as the tapered clamp ring 25 (Fig. 3) centers and seals the top end of the respective screen cylinder 16-19. The clamp ring 25 is held in down with bolts such as bolt 68, which thread into the plate 23.

Normally the machined fits between the clamp rings 25 and the machined surfaces of the plate 23 are such that no sealing or gasket is needed to prevent feed stock from passing to the accepted stock.

With the arrangement described, two separate chambers are provided within the housing 11: first, the reject chambers 28 at the lower end of the cylindric screen plates 16-19, each having its periphery limited by the respective solid annular section 24; and, second, the accept chamber 29 limited by the inner wall 12 and outer walls of the cylindric screen plates 16-19

and by the plate 23. A reject discharge conduit 30 hydraulically communicates through reject branches 31, with the four reject chambers 28.

As is usual in pressure screens, the reject conduit 30 is located below the housing 11 to discharge the stock which has not passed through the cylindric screen plates 16-19.

The accepts chamber 29 is connected to an outwardly projecting accept discharge conduit 32 having an accepts discharge port 33 at a downstream end thereof. The discharge conduit 32 and the port 33 are preferably disposed at an elevation above the reject discharge stock conduit 30, even though the actual location of the accepts discharge relative to the rejects discharge may be optional. In the embodiment shown, the discharge port 33 is located above the rejects discharge conduit 30 as shown, e. g., in Fig. 7.

It can thus be seen that the pressurized flow of stock through the screen of the embodiments shown is from the stock inlet 15 to the feed chamber 14 and then inside the cylindric screen plates 16-19. The flow is then split by the perforations of the cylindric screen plates into accepts flowing through the discharge conduit 32 and port 33 out of the cylindric screen plate, and rejects removed through the rejects chambers 28 of the cylindric screen plates 16- 19, the branches 31 and rejects discharge conduit 30.

The cylindric screen plates 16-19 of the pressurized screen of the present invention are each operatively associated with suitable hydraulic pulse generators for preventing at least a majority of the screening openings from becoming plugged by the rejected or unwanted solid particles of the screened stock. The particular type of the pulse generators is optional. A great variety of screen-pulse generators is known, ranging from hydrofoils rotating within a stationary cylindric screen plate or a generally cylindric rotor with pulse generating projections, to an arrangement where the cylindric screen plate rotates about stationary pulse generators. The mechanism of the pulse generators is well known in the art and does not in itself form a part of the

present invention. it will therefore suffice to briefly describe only two embodiments, one shown in Figs. 1-4, the other in Figs. 5 and 6. The pulse generating means or pulse generators are indicated only diagrammatically. in virtually all applications, the pulse generators are of the same type same for ail cylindric screen plates 1 6-1 9 within the housing 11.

With reference to Figs. 1-4, each cylindric screen plate 16-19 has pulse generating means of the type of a foil assembly which includes four hydrofoils 34 mounted in close proximity to the inner surface of the cylindric screen plate 16. Each foil 34 is fixedly secured at radially outer free ends of two axially spaced apart support arms 35,36, the root ends of the arms being fixedly secured to a cylindric rotor or hub 37. The hub 37 is provided with two radially inwardly projecting rings 38,39. The minor diameter portions of the rings are fixed to an axle 40. The lower end of axle 40 has a downwardly projecting shaft 41. The shaft 41 passes through a suitable packing assembly and through a pair of bearings 43,44 supported by a bracket 45 which is fixed to a mounting plate 46 bolted to the base plate 26 of the housing 11. A pulley 47 is secured to the shaft 44 at the lower end thereof.

Reference may now be had to the second embodiment, shown in Figs.

5 and 6 and differing from the first embodiment only in the number of the cylindric screens and in the type of the hydraulic pulse generators. The remaining parts of the second embodiment being identical to those of Figs. 1-4 and 7, they are referred to with the same numbers or the reference numbers are omitted for ciarity.

One part differing from the first embodiment, of course, is the flat plate 23a which only has three, not four openings 64. Thus, there are only three cylindric screens 16,17 and 18.

Another difference from the first embodiment is in the use of different pulse generating means known per se. Each of the three pulse generating means of this embodiment is of the type of a rotor assembly which comprises

a hollow, cylindric rotor 48 provided with a cover 69 sealingly enclosing the hollow interior of the rotor 48. The radially outer surface of the rotor is provided with a number of hydrofoil-like projections 49 (omitted from Fig. 5) disposed in close proximity to the respective cylindric screen plate 16,17 or 18. Contrary to the hydrofoils 34 of the first embodiment, the projections 49 are relatively short and present each only a small fraction of the overall axial length of the respective cylindric screen plate. They are disposed in a uniform pattern over the entire axial length of the cylindric screen plate 16,1 7 or 18.

The radially outer cylindric surface of the rotor 48 defines, with the radially inner surface of the respective cylindric screen plate 16,17 or 18, an annular space which is open at the top to admit stock coming from the feed chamber 14. The top cover 69 of the rotor 48 directs the stock to flow from chamber 14 through the annular spaces. Each rotor 48 is secured to. the upper end of an axle 51 by a bolt 50. The lower end of the axle 51 has a downwardly projecting shaft 52 passing through a packing assembly 53 and bearings 54, 55 arranged in an identical fashion to that of the first embodiment. A pulley 56 is mounted at the lower end of the shaft 52.

Figures 2 and 6 show, in a diagrammatic fashion, exemplary drive means of the pulse generating means of the two embodiments. The drive means are omitted from the remaining drawings. Like most of the parts of the two embodiments, the drive means are generally identical for both embodiments, the corresponding parts of Fig. 2 are referred to with the same reference numbers or are omitted from Fig. 6.

A drive motor 57 (omitted from Figs. 1 or 5) is secured to the housing 11 as is well known. Through the pulley 58, and belt 59, it drives a main drive pulley 60 which is coaxial with an axially spaced from transmission pulley 61.

The pulley 61 is engaged by a transmission belt 62 engaging pulleys 47 (Fig.

1) and 56 (Fig. 6) and maintained in operative tension by idlers 63.

The belt transmission system described is to be understood as an exemplary embodiment which may be modified to a greater or lesser degree

without departing from the present invention. For instance, individual hydraulic motors could be used to drive each rotor or foil assembly.

As already mentioned, the pulse generating means and their drive means per se are not a part of the present invention, They are therefore described in a diagrammatic way with many obvious details of the hydrofoil or rotor assembly being omitted for clarity. It is also emphasized again that it is much preferred that all four cylindric screen plates 16-19 or all three cylindric screen plates 16-18 of the second embodiment, be provided with the same pulse generating means, whether one or the other of those just described or a any other known embodiment. Those skilled in the art will no doubt have little difficulty deciding whether one type of the pulse generating means shown is to be used or whether some other pulse generators not shown but known in the art are preferred. The same applies to the number of cylindric screen plates even though in most application three or four cylindric screen plates will be practical.

In operation, the stock to be treated is pressure fed via inlet 15 into the feed chamber 14 and from there to the inner surface of the cylindric screen plates 16-19, the accepts pass through the cylindric screen plates (while, the pulse generators maintain the passages of the cylindric screen plates clean) into the accepts chamber 29, while the rejects eventually reach the bottom part of the respective cylindric screen plate and are removed through rejects branches 31 and the rejects discharge conduit 30. The accepted stock exits through the accepts discharge conduit 32 and the port 33.

The invention provides a significant improvement over known pressure screens in that it substantially increases the screening area while not requiring enlargement of space required for the cylindric housing. The high capacity is provided by smaller size cylindric screen plates which are easier to manufacture, have better strength properties compared to large cylindric screen plates and are less expensive to replace. The increased area of screening can be utilized by providing smaller apertures in the cylindric screen plates while

maintaining the same overall output of the screen despite the lower capacity per area unit caused by smaller passages. This results in a significant improvement of the quality of the accepts.

Those skilled in the art will readily appreciate that a vast number of embodiments differing from those described can be readily made which utilize the inventive idea. As already mentioned above, the invention can be utilized not only in screening fibrous material but also in sorting an incoming fibrous suspension to fractionate same into fractions with different physical properties, e. g. length or diameter. As is known from prior art, the sorting or fractionation is effected by pressure sorters similar to those of pressure screens, with modified rotary members secured to the rotor of the screen. Such known rotary members are shown, for instance, in the above Patent to Rienecker et al. An arrangement where the cylindric screens would be arranged in series for fractionating the fibrous suspension can also be made.

The actual number of the cylindric screen plates in the housing, the drive of the rotating members such as the pulse generators and their type are just a few examples of such modifications which may differ from what has been disclosed but still fall within the scope of the invention as set forth in the accompanying claims.