FIG. 50.
The thickness of the paper is regulated by altering the supply of pulp to the wire-cloth, and by the speed at which the machine is working. This speed may vary from about 60 feet to as high as 270 feet per minute.
The “wire” is an endless cloth made of very fine wire, the fineness depending much on the quality of the paper required. The mesh varies from 60 to 70 and even more threads to the inch. It is not woven endless, but is joined by very careful sewing with wire. Its width varies considerably, some {154} being made as wide as 126 inches; the length is generally 35–40 ft. It is carried by the breast-roll F, the lower couch-roll G, and the small rolls f ′, and by a large number of small rolls f ″. The latter and the breast-roll are supported by the frame g, while the small rolls f ′ are supported by brackets attached to it. The course of the wire is indicated by the arrows. The frame g works on two pivots g′, and receives a shaking motion from side to side from the rod j, in connection with a crank worked by two conical drums H. The supports g″ are also pivoted at their lower ends to allow for the shaking motion. This shaking motion is given for the purpose of weaving or intertwining the fibres. One or more of the rolls f ′ can be moved up or down on the support which carries it, for the purpose of stretching the wire. There is usually a large number of the small rolls f ″, as it has been found by experience that, probably owing to capillary attraction, they, cause the water to leave the pulp. Though a large quantity of water thus passes through the wire-cloth, it is necessary to assist it by artificial means. This is done by means of the suction-boxes I connected by pipes with the vacuum-pumps I′.
This part of the machine, which is called the “wet-end,” is placed at a slight slope of about 1 in. in its entire length, the lowest end being nearest the strainers.
Underneath the wire-cloth is placed a box called the “save-all” K, connected with the box E′. The water that passes through the wire-cloth contains a considerable quantity of very fine fibres, together with size, alum, clay, and colouring materials, that have passed through the wire, and which would be lost but for the arrangement now universally adopted. It flows into the box E′, and is pumped, together with the pulp that has passed through the knotter E (see p. [151]), into the high box B, whence the mixed stuff flows on the sand-tables, to be again used to dilute fresh pulp from the stuff-chests. The following numbers will give some idea of the nature and amount of fibre, &c., which passes through. {155} The paper was made from esparto and straw, sized with rosin and starch. It contained 12 per cent. of clay.
| Grains per Gallon of Waste Water. | |
|---|---|
| Fibre | 34·37 |
| Clay | 37·10 |
| Starch | 1·40 |
| FIG. 51. | FIG. 52. |
It is almost impossible to utilise the whole of the back-water passing through the wire-cloth in the way described. In some mills a portion of this water is made to pass through a “pulp-saver,” such as is shown in Figs. 51 and 52. It consists of a conical drum A, the circumference of which is covered with wire-cloth, and it is caused to revolve slowly by suitable gearing. The water enters by the pipe B, and passes through the meshes of the wire-cloth, the pulp gradually finding its way to the wider end, where it is discharged into the box C. It can then be returned to the beaters.
This pulp-saver can also be used for recovering the fibre from other waste water, such as the wash water from the washing and beating-engines; or it can be used for freeing bleached pulp from water in cases where drainage or hydraulic pressure is not resorted to.