In many manufactures, self-acting filters are fed by the plumber’s common contrivance of a ball-cock in which the sinking and rising of the ball, within certain limits, serves to open or shut off the supply of liquor, as it may be required or not. Dumont has adopted this expedient for his system of filtering syrup through a stratum of granularly ground animal charcoal or bone-black. [Fig. 388.] is a front view of this apparatus with 4 filters C; and [fig. 389.] is a cross section. The framework B supports the cistern A, in which the syrup is contained. From it the liquor flows through the stop-cock b, and the connection-tube a, into the common pipe c, which communicates, by the short branch tubes e, with each of the four filters. The end of the branch tube, which is inside of the filter tub, is provided with a stopcock d f, whose opening, and thereby the efflux of the liquor from the cistern through the tube a, is regulated by means of the float-ball g. Upon the brickwork D the filter tub stands, furnished at h with a false bottom of zinc or copper pierced with fine holes; besides which, higher up at i there is another such plate of metal furnished with a strong handle k, by which it may be removed, when the bone-black needs to be changed. In the intervening space l, the granular coal is placed. o is the cover of the filter tub, with a handle also for lifting it. One portion of it may be raised by a hinge, when it is desired to inspect the progress of the filtration within. m m is a slender vertical tube, forming a communication between the bottom part h, and the upper portion of the filter, to admit of the easy escape of the air from that space, and from among the bone-black as the syrup descends; otherwise the filtration could not go on. p is the stopcock through which the fluid collected in the space under h is let off from time to time into the common pipe q, [fig. 388.] r is a trickling channel or groove lying parallel to the tube q, and in which, by means of a tube s, inserted at pleasure, the syrup is drawn off in case of its flowing in a turbid state, when it must be returned over the surface of the charcoal.
The celerity with which any fluid passes through the filter depends, 1. upon the porosity of the filtering substance; 2. upon the pressure exercised upon it; and 3. upon the extent of the filtering surface. Fine powders in a liquor somewhat glutinous, or closely compacted, admit of much slower filtration than those which are coarse and free; and the former ought, therefore, to be spread in a thinner stratum and over a more extensive surface than the latter, for equal effect; a principle well exemplified in the working of Dumont’s apparatus, just described.
In many cases filtration may be accelerated by the increase of hydrostatic or pneumatic pressure. This happens when we close the top of a filtering cylinder, and connect it by a pipe with a cistern of fluid placed upon a higher level. The pressure of the air may be rendered operative also either by withdrawing it partially from a close vessel, into which the bottom of the filter enters, or by increasing its density over the top of the liquor to be filtered. Either the air pump or steam may be employed to create a partial void in the receiver beneath the filter. In like manner, a forcing pump or steam may be employed to exert pressure upon the surface of the filtering liquor. A common syphon may, on the same principle, be made a good pressure filter, by making its upper leg trumpet-shaped, covering the orifice with filter paper or cloth, and filling the whole with liquor, the lower leg being of such length so as to create considerable pressure by the difference of hydrostatic level. This apparatus is very convenient either on the small or great scale, for filtering off a clear fluid from a light muddy sediment. The pressure of the atmosphere may be elegantly applied to common filters, by the apparatus represented in [fig. 390.], which is merely a funnel inclosed within a gasometer. The case A B bears an annular hollow vessel a b, filled with water, in which receiver the cylindrical gasometer d, e, f, i, is immersed. The filter funnel C is secured at its upper edge to the inner surface of the annular vessel a b. In consequence of the pressure of the gasometer regulated by the weight g, upon the air inclosed within it, the liquid is equally pressed, and the water in the annular space rises to a corresponding height on the outer surface of the gasometer, as shown in the figure. Were the apparatus made of sheet iron, the annular space might be charged with mercury.
In general, relatively to the application of pressure to filters, it may be remarked, that it cannot be pushed very far, without the chance of deranging the apparatus, or rendering the filtered liquor muddy. The enlargement of the surface is, generally speaking, the safest and most efficacious plan of increasing the rapidity of filtration, especially for liquids of a glutinous nature. This expedient is well illustrated in the creased bag filter now in use in most of the sugar refineries of London. See [Sugar].
In many cases it is convenient so to construct the filtering apparatus, as that the liquid shall not descend, but mount by hydrostatic pressure. This method has two advantages: 1. that without much expensive apparatus, any desired degree of hydrostatic pressure may be given, as also that the liquid may be forced up through several filtering surfaces placed alongside of each other; 2. that the object of filtering, which is to separate the particles floating in the fluid without disturbing the sediment, may be perfectly attained, and thus very foul liquids be cleared without greatly soiling the filtering surface.