Digester screen for a continuous cellulose pulp digester
The digester screen is for a continuous digester in which cooked cellulose pulp is produced. The digester screen has a number of screen bars arranged vertically in the continuous digester that have withdrawal slots (S) between the fixed screen bars through which withdrawal slots cooking fluid can be withdrawn from the column of pulp (P) of the digester. By having every second screen bar fixedly arranged and recessed a distance (D) relative to the neighboring screen bar an increased withdrawal capacity for the screen is obtained.
1. A digester screen for a continuous digester in which cooked cellulose pulp is produced, comprising:
a vertical screen surface having a number of fixed screen bars fixed in a wall of the continuous digester and arranged vertically in the continuous digester,
vertical withdrawal slots (S) defined between the fixed screen bars, through which withdrawal slots cooking fluid is withdrawable from the column of pulp (P) of the digester, and at least one screen bar in the digester screen being fixedly arranged in the wall of the digester and recessed by a distance (D) relative to adjacent fixed screen bars.
2. The digester screen according to claim 1 , wherein several fixed screen bars in the digester screen are recessed relative to adjacent fixed screen bars.
3. The digester screen according to claim 2 , wherein the recessed screen bars are recessed by the distance (D), which is at least as large as a width of the withdrawal slot between adjacent fixed screen bars.
4. The digester screen according to claim 3 , wherein the recessed and fixed screen bars are recessed by the distance (D) along a complete vertical distance of the screen bars.
5. The digester screen according to claim 4 , wherein the recessed and fixed screen bars have a width (W) that does not exceed a width of the adjacent screen bars, the width (W) extends in a direction of a circumference of the digester.
6. The digester screen according to claim 5 , wherein all fixed screen bars have a greater flexural rigidity in a radial direction in the digester than a flexural rigidity in a circumferential direction of the digester, a dimension of the screen bars in the radial direction is at least 100% greater than a dimension of the screen bars in the circumferential direction of the digester.
7. The digester screen according to claim 1 wherein a non-recessed fixed screen bars has a T-shaped cross-section that has a first broad glide surface facing towards the column of pulp of the digester at a glide surface and a narrower radially directed web section.
8. The digester screen according to claim 1 wherein the withdrawal slot (S) has a dimension in an interval of 2-10 mm.
9. The digester screen according to claim 1 wherein recessed fixed screen bars are recessed by the distance (D) relative to adjacent fixed screen bars.
10. The digester screen according to claim 1 wherein recessed fixed screen bars are recessed by the distance (D) relative to adjacent fixed screen bars the dimension (D) is related to a dimension (S) of the withdrawal slot by D =1.0*S to 2.0*S.
11. The digester screen according to claim 7 wherein surfaces of the non-recessed and fixed screen bars that face the column of pulp (P) of the digester are located at a same distance from a center of the column of chips as the wall of the digester directly above a screen section ( 4 a , 4 b , 4 c ) with relevant screen bars.
12. The digester screen according to claim 1 wherein the digester screen is arranged at a lower part of the digester, below a half-height of the digester.
13. The digester screen according to claim 1 wherein the fixed screen bars are flush with the vertical screen surface and the at least one screen bar is recessed by the distance (D) relative to the adjacent fixed screen bars that are flush with the vertical screen surface.
14. The digester according to claim 13 wherein the fixed screen bars each has a glide part that is perpendicular to a narrower radially directed web-section.
15. The digester according to claim 14 wherein the at least one screen bar, that is recessed, has a glide part that is parallel to the glide part of the fixed screen bars.
16. The digester according to claim 13 wherein the at least one screen bar, that is recessed, has a glide part that is perpendicular to a narrower radially directed web-section.
17. The digester according to claim 16 wherein the narrower radially directed web-section of the at least one screen bar is shorter by the distance (D) than the narrower radially directed web-sections of the fixed screen bars.
18. The digester according to claim 16 wherein the glide part of the at least one screen bar has a length (W) that is identical to a length of the glide parts of the fixed screen bars.