US2217732A - External circulating system for pulp digesters - Google Patents

Description

Oct. 15 1940. K, E

EXTERNAL CIRCULATING SYSTEM FOR PULP DIGESTERS Filed Dec. 51

INVENTOR ATTORNEY .BY I

Patented Oct. 15, 1940 UNITED STATES PATENT OFFICE EXTERNAL CIRCULATING SYSTEM FOR PULP DIGESTERS poration of New York Application December 31, 1937, Serial No. 182,693 6 Claims. (C 92-7) My invention relates to improvements in external circulating systems for pulp digesters such as are employed in the paper making industry. Such systems are an essential part of an indirect heating system wherein the heat required for the cooking of the sulphate, or soda pulp and liquor is obtained by withdrawing liquor by suction from the pulp being treated, passing the withdrawn liquor in indirect contact with a heatmg medium such as steam in suitable heat exchange apparatus, and then returning the heated liquor to the body of pulp, through which it circulates and gives up heat. This cycle is carried on during the cooking process, and it has been found that the real effectiveness of indirect heating systems of this kind depends upon a uniform liquor circulation. However, external circulation may also be used with pulp digesters employing direct heating.

My invention particularly pertains to an improved form of suction strainer through which the liquor that is withdrawn from the pulp in the digester shell passes, and to other means hereinafter to be described, whereby the circulating liquor iskmore uniformly brought in contact with the mass of pulp in the digester, which is thereby heated more uniformly than has heretofore been the case, to the desired temperature, this being true whether or not the pulp and 30 liquor are initially exposed to indirect, or to direct contact with a heating medium. I

The usual form of suction strainer consists essentially of a hollow ring so apertured as to admit liquor only thereinto, and which is suitably supported in a horizontal position on the inner wall of the digester shell containing the pulp and liquor mixture. A strainer of this general type is disclosed in my copending U- S.

application Serial No. 138,333, filed April 22, 1937.

One or more suction lines connect the annular space inside the strainer with a circulating pump for the passage of the liquor that is drawn through the perforations in the wall of the strainer. It has previously been customary to 45 arrange these perforations on a uniform pitch over the entire length of the ring. For simplifying piping it is frequently desirable to have a single suction connection to the circulating pump, and since the cross-sectional area of the strainer 50 must necessarily be rather limited in order not to occupy too much space within the shell, the velocity of the liquor as it passes through the wall of the strainer is relatively high close to the suction connection, while it is progressively lower- 55 as the distance from the suctionconnection increases. The velocity of liquor within the ring progressively increases as the suction connection is approached. The efiect of this increase in velocity upon approaching the point of suction, with the addition of the frictional loss within 5 the ring, is to bring about a relatively steep hydraulic gradient between the inside and outside of the strainer at a point close to the suction connection with a progressively decreasing gradient as the distance from the suction con- 10 nection increases. This causes a non-uniform flow through the several parts of the strainer with the result that the portions of the digester away from the suction connection do not receive a proper amount of recirculated liquor and the 15 pulp in these portions is under cooked".

In accordance with my invention the suction strainer is designed with perforations or holes spaced on a relatively large pitch adjacent the suction connection and on a progressively de- 20 creasing pitch as the distance from the suction connection increases, the spacing of the perforations being such that with the existing pressure difierence at any point in the strainer the flow of liquor through the wall of the strainer per 'lineal foot of its circumference will be substantially uniform. Also special means may be provided whereby the heated liquor returned to the shell will be distributed more uniformly over the entire cross-sectional areas of the digester.

For a better understanding however of all features of my invention, reference may be had to the following description considered in connection with the accompanying drawing forming a part thereof, and in which:

Fig. l is a more or less diagrammatic view in elevation of a pulp digester and indirect heating system therefor embodying the features of my invention;

, Fig. 2 is a horizontal sectional view taken on line 22 of Fig. 1 illustrating the suction strainer 40 in plan;

Fig. 3 is a vertical sectional view showing the strainer outlet, taken on line 3-3 of Fig 2; and

Fig. 4 is a horizontal sectional view taken on line 4-4 of Fig. 1 showing a return flow fitting of volute or spiral form, which is another feature of my invention.

Like characters of reference referto' the same parts throughout the several 'views.

Referring to the drawings, reference character Ill designates a cylindrical digester shell adapted to contain the pulp and (liquor. During operation, liquor is withdrawn through draw-ofl suction pipe H, which is connected to an outlet vided with suitable flanges to which illustrated therein, is annular nozzle l2 of a suction strainer indicated generally at I3, which prevents the withdrawal of pulp or chips from the shell. From the draw-01f pipe II the liquor flows through line IE to the suction of pump l by which it is then forced through line is to the lower end of a two-pass heater I! of the shell and tube type. Steam or other heating medium is introduced into the heater at its upper end through line it, and is discharged from the lower end of the heater through line is. After passing through the heater the liquor flows through line 2d, and is returned to the upper and lower portions of the digester through lines 28 and 23 respectively which are controlled by valves 36 and 3? respectively. A perforated spray ring 22 which is connected to line it serves to evenly distribute the heated liquor flowing therethrough over the pulp or chips in the upper portion of the shell, while a return fitting 2A, in the form of a volute or spiral, as clearly illustrated in Fig. 4 of the drawing, is so designed that the returning heated liquor from line 23 will be forced circumferentially around the inner wall of shell inlet opening 25, thus giving the liquor a whirl, the centrifugal force of which will spread the stream of liquor more eifectively across the interior of the shell than would be possible if the liquor were introduced in a stream with all elements of flow parallel.

Referring to Fig. 2, the suction strainer in form, conforms to the contour of the inner wall of the digester shell and consists of a plurality of arcuate sections Eli and Ziia, each of which has a transverse cross-sectional configuration in the form of a right angled triangle clearly shown in Fig. 3 of the drawing, providing a channel 35. Except for the section 260, which is provided with the chan nel outlet opening formed by a sleeve il that passes through the wad of the shell iii, and is received by the suction nozzle id in sliding rela tionship therewith, the outer wall 28 of each of the sections 26 is imperforate, whereas the inner vertically inclined wall 29 and the lower horizontal wall 39 are perforated by a plurality of holes 39. The several walls are welded together as indicated and the sections are secured in position on the inner wall of the digester shell ill by supporting members, each of which includes a straplike clamp 32 and a backing plate indicated at 33. The clamps 32 are shaped to conform to the general contour of the walls 29 and 3d and the back ing plates 33 are welded to the shell and are prothe clamps are secured as by bolts 3 51. Adequate spacing is provided between the adjacent ends of the strainer sections to permit free expansion and contraction of the strainer sections: relative to each other and to the digester shell. A plurality of spaced stiffener members 38 are secured as by welding, to the outer face of wall 29.

As will be seen from an inspection of the drawing, the perforations or holes 3| in the strainer walls are spaced on a relatively large pitch nearest the point in the strainer of greatest suction, i. e., the suction outlet 21, and are spaced on a progressively decreasing pitch as the distance from the suction connection increases. The spacing of the holes is such that for the existing pressure difi'erence at any point in the strainer, the

flow of liquor from the digester through the holes in the walls of the strainer will be substantially uniform throughout its circumference, and hence there will be a uniform circulation, or substantially uniform circulation, of liquor through the body of pulp in the digester, and all portions of the pulp will be evenly heated or cooked.

Additionally, by means of the spiral return fitting 24, that portion of the liquor returned to the lower portion of digester, will be distributed uniformly over the contents contained therein. However, it is to be understood that the invention does not necessarily consist of the combination of both my improved form of strainer and the volute or spiral return fitting, as either of these features may be employed separately if desired.

Although I have described and illustrated a preferred form of my invention in connection with an indirect heating system, it will be understood that changes or modifications may be made in the form or parts of the apparatus, and in the heating system disclosed without departing from the principles of the invention. For example, instead of the difierential spacing of the holes being spread "ormly over the strainer, satisfactory liquor circulation may be obtained by providing the strainer with a plurality of sep arate groups of perforations, the perforations in each group being on the same pitch with different pitches in different groups and with the groups so arranged that substantially the same result heretofore described is obtained. Also if desired, there may be two or more suction connections to the strainer, with the differential spacing of the perforations so applied as to give a uniform flow of liquor throughout the length of the strainer. Or further, the suction strainer might be of tubular construction having an oval or circular cross-section instead of triangular, as shown. 1

Accordingly, no intention is entertained to limit ply-invention except by the scope of the appended claims.

What I claim is:

1. Apparatus for pulp digestion comprising a digester, an annular suction strainer of triangular crcss-section, means supporting said suction strainer on the inner wall of said digester, a circulating pump, a draw-oil conduit connecting the interior of said suction strainer to the suction of said pump for withdrawing liquor from said digester, means for heating the liquor so withdrawn, means for returning a part of said heated liquor to the upper portion of said digester, means for introducing another part of said liquor into the lower portion of said digester with a whirling motion, the perforations in said suction strainer being arranged so that the eifective straining area is progressively greater per unit of strainer area as the distance from said drawoiT conduit increases.

2. Apparatus for pulp digestion comprising a digester and a tubular strainer in the digester having an outlet arranged for connection to a source of suction, the apertures of the strainer being of substantially the same size and being arranged on a certain pitch immediately adjacent said outlet and on a pitch which progressively decreases from said certain pitch as the distance from said outlet increases to provide substantially uniform flow through the strainer.

3. Apparatus for pulp digestion comprising a digester, an annular suction strainer disposed in said digester, suction means connected to said strainer for withdrawing liquor from said digester, means for heating the liquor so withgreater per unit of strainer area as the distance er area adjacent said outlet and a progressively greater number of apertures per unit of strainer area as the distance from said outlet increases to provide substantially uniform flow through the strainer.

5. Apparatus for pulp digestion comprising a digester and a tubular strainer adjacent an upright wall of the digester and conforming substantially to the contour of said wall, the strainer having an outletvarranged for connection to a source of suction, the apertures of the strainer being of substantially the same size, there being a certain number of apertures per unit of strainer area in a portion of the strainer adjacent said outlet and a greater number of apertures per unit of strainer area in other portions of said strainer to provide substantially uniform flow through the strainer.

6. Apparatus for pulp digestion comprising a digester and a strainer adjacent an upright wall of the digester and conforming substantially to the contour of said wall and having a substantially triangular transverse cross-sectional configuration, the strainer having an outlet arranged for connection to a source of suction, the apertures of the strainer being of substantially the same size, there being a certain number of apertures per unit of strainer area adjacent said outlet and a progressively greater number of apertures per unit of strainer area as the distance from said outlet increases to provide substantially uniform flow through the strainer. I

DION K. DEAN.

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