These are all boiled at a pressure of from 20 to 25 lbs. for 10 hours, in stationary boilers without vomit, and also in boilers revolving horizontally. In some mills, where the best qualities of paper are made, iron boilers are objected to, as small particles of oxide of iron are apt to become dislodged from the interior of the boiler, and produce discolouration of the paper. In such cases wooden vats, with mechanical stirrers, are employed; sometimes a jacketed boiler is used.
Figs. 11 and 12.
Washing and Breaking.—The removal of the dirty water resulting from the boiling is effected in the washing and breaking engine, or "rag engine," as it is commonly called, which is constructed on the same principle as the beating engine, but is provided with an extra drum, called the drum-washer, which, being covered with wire gauze, allows the washing waters to escape without permitting the fibrous stuff to pass through. The rag engine, having been invented by a Dutchman, acquired, and still retains, the name of the Hollander, and although it has been considerably improved upon, its principle is still retained in the modern engines, of which there are many different forms. The ordinary rag engine, Figs. 11 and 12, consists of a cast-iron trough A, about 10 feet long, 4½ feet wide, and 2½ feet deep, and rounded at the ends, and is firmly bolted to a wooden foundation. It is provided with a partition termed the midfeather B, of such a length as to have the trough of uniform width round it. A cylinder, or roll, C, furnished with a series of steel knives, rotates in one of the divisions formed by the midfeather, and the floor of the trough in this division is inclined in such a manner as to cause the pulp, as it travels, to pass under the roll. Beneath the roll is the bed-plate, which is fitted with a series of steel knives c c similar to those on the exterior of the roll. The distance between the knives of the roll and the bed-plate is regulated by levelling screws, which are so adjusted that both ends of the roll are raised at the same time, which is a great improvement upon the older types of breaking engines in which only one end of the roll was raised, whereby the knives became unequally worn. By the present method of regulating the distance between the respective sets of knives, any required degree of fineness can be given to the fibrous substances treated. The roll is generally caused to rotate at a speed of about 230 revolutions per minute, causing the water and rags to circulate in the engine and to be constantly under the action of the knives. In the other division F F of the trough is the drum-washer H, which, being covered with fine gauze wire, allows the water to enter, but keeps back the fibrous material. The ends of the drum are formed of two discs of wood, generally mahogany, upon which the coarse gauze is fastened as a backing, and this is covered with the fine wire gauze. The interior of the drum is sometimes furnished with a series of buckets, which conduct the water to a trough in the axis of the drum, by which it is led away. This is also accomplished by dividing the interior of the cylinder into compartments by means of a partition. The drum-washer is so arranged that it can be wholly raised out of the trough, which is necessary in certain parts of the operation, when the removal of the liquid is not required; or it can be partially raised, or otherwise, according to requirement. The floor of the compartment containing the roll C is inclined at D, so as to cause the pulp to pass directly under the roll, and at D′ is the backfall, over which the pulp travels to the opposite side of the midfeather.
In working the rag engine, it is first partly filled with water, and then set in motion; the boiled stuff is then gradually put in, and a constant supply of clean water is run in from a cistern provided with means of preventing sand or other impurities from finding their way into the engine. It is of the utmost importance that the water should be abundant and of good quality, more especially as the material (rags) is mostly required for making the finer qualities of paper. In this respect the county of Kent and a few other localities on the chalk formation are considered specially suitable for this particular manufacture.
With respect to the driving of the engines, this was formerly effected by what is called toothed gearing, but cog-wheels were afterwards replaced by iron spur-wheel gearing, which enabled manufacturers to drive four or more engines from one source of power, by continuing the line of shafting and spur-wheels; but even with small rolls the wear and tear on this system was considerable, while it was quite inadequate to the driving of a number of large rolls of 30 inches in diameter, such as are now used. The introduction of belt-gearing, by Messrs. G. and W. Bertram, proved to be a great improvement on the older system, and it is found that the rags are broken not only more uniformly, but in less time, as the rolls work more steadily on the plates than with any system of wheel-gearing, while the various working parts of the engine last longer than when subjected to the vibrating action of wheel-gearing.
Fig. 13.
Bertrams' Rag Engine.—This engine, of which a drawing is shown in Fig. 13, may be used either as a washing and breaking engine, potcher, or beater. It is provided with double lifting gear, and has "all sweeps, curves, and angles" of the most improved design to save lodgments and ensure steady and thorough travelling of the pulp. The drum-washer is shown lifted by rack and pinion and worm gear, and empties down the midfeather direct to mouthpiece. The emptying can be done by spout and pipe, or by a chamber cast on the engine, down back or front side, as well as through the midfeather; but it is not advisable that it should be emptied down the midfeather if the rag engine is to be used as a beater.