Bertrams' Large Paper Machine.—The principal aim in the construction of the paper-making machine has been to imitate, and in some particulars to improve, the operations involved in the art of making paper by hand, but apart from the greater width and length of paper which can be produced by the machine, the increased rapidity of its powers of production are so great that one machine can turn out as much paper in three minutes as could be accomplished by the older system in as many weeks. The drawing represents the modern paper-machine as manufactured by Bertrams, Limited, who supplied one of these machines to Mr. Edward Lloyd, for the Daily Chronicle Mill, at Sittingbourne, which runs a wire 40 feet long by 126 inches wide, this being, we believe, the largest and widest paper-machine in the world. It is provided with 20 cylinders, chilled calenders, double-drum reeling motion, with slitting appliance for preparing webs to go direct to the printer's office without the assistance of a re-reeling machine, and is driven by a pair of coupled condensing steam-engines. On our recent visit to Mr. Lloyd's mill we were much struck with the excellent working of this splendid machine.
In the illustration, as will be seen, there are two sets of drying cylinders, while small cylinders, or felt drying-rolls, from 16 to 24 inches in diameter, are introduced to the felts of the cylinders, before the smoothing-rolls, which discharge the moisture with which the felts are impregnated from the damp paper, whereby a considerable saving in felts is effected. Messrs. Bertram state that the highest speed yet attained has been by their own machinery, and is 270 feet of paper per minute.
Fig. 25.
The progress of the pulp after it leaves the beating-engines for conversion into paper may be described as follows:—The valve at the bottom of the beating-engine is opened, when the pulp flows through a pipe into the stuff-chests, which are generally situated below the level of the engines. The beaters are then rinsed with clean water to remove any pulp that may still cling to them, the rinsing water passing also into the stuff-chests.
Stuff-chests.—These are large vessels of a cylindrical form, so that the pulp may have no corners to lodge in, and are generally made of wood, though sometimes they are made of cast-iron plates bolted together. The chests are of various dimensions, according to the requirements of the mill, being usually about 12 feet in diameter and 6 feet deep, having a capacity for 1,000 to 1,200 lbs. of stuff. To keep the pulp well mixed in the stuff-chest, of which two are usually employed for each machine, a vertical shaft, carrying two horizontal arms, each extending nearly across the interior of the chest, are provided, which are only allowed to revolve at a moderate speed, that is, about two or three revolutions per minute, otherwise the pulp would be liable to work up into knots, and thus form a defective paper. Motion being given to the shaft, the rotating arms keep the pulp and water uniformly mixed, at the same time preventing the pulp from sinking to the bottom of the stuff-chest.
The pulp is next transferred to a regulating box, or "supply box," by means of a pump called the stuff-pump. The regulating-box, which has the effect of keeping a regular supply of pulp in the machine, is provided with two overflow pipes, which carry back to the stuff-chests any superfluous pulp that may have entered them, by which the stuff in the regulating-box is kept at a uniform level, while the machine is supplied with a regular and uniform quantity of the diluted pulp. The stuff-pump conveys the pulp through a valve in the bottom of the regulating-box in a greater quantity than is actually required, the superfluity returning to the stuff-chests by the overflow pipes; thus the supply-box, being always kept full, furnishes a regular and uniform supply of pulp to the sand-tables, or sand-traps as they are sometimes called. Sand-tables are large wooden troughs, varying in size at different mills, but Mr. Dunbar gives the following proportions for a first-class sand-trap; namely, 14 feet long by 8 feet wide, and 8 inches deep. The bottom of the trap is covered with felt, sometimes old first-press felt being used, and is divided into several compartments by thin bars of lead or iron, or strips of wood, which keep the felt in position, and also retain any particles of sand or other heavy solid matter that may be accidentally present in the pulp. For the purpose of diluting the pulp for the machine, there is, attached to the inlet of the sand-traps, a box with two supply-taps, one for the delivery of pulp, and the other for water; and these being turned on, the pulp and water flow over the sand-traps, and the diluted pulp then falls into the strainers, which, while allowing the fine pulp to pass freely, keep back all lumps of twisted fibre, and particles of unboiled fibre, which latter, if not removed, would appear as specks on the surface of the finished paper.
The Strainers are formed of brass or bronze plates, in which are cut a very large number of narrow slits, which gradually widen downward, so as to prevent the pulp from lodging. Each plate has about 510 slits, and several plates, connected together by bolts, constitutes the complete strainer. When in use, the strainer receives a jogging motion, which is communicated to it by means of small ratchet wheels keyed on shafts passing beneath the machine; this causes the fibres to pass more freely through the slits. There are many different forms of strainers, which have been the subject of numerous patents. It will be sufficient, however, to give one or two examples of improved strainers which have been more recently adopted by manufacturers.
