When the water contains only carbonates of lime, it suffices to heat to a temperature of about 212° F. (100° C), which may easily be obtained from the exhaust steam of the engine, and with ordinary air-pressure. When the water contains sulphates of lime or chlorides of sodium or of magnesium, as sea-water, it must be raised to a higher temperature, which has to be obtained by heating under pressure. In this case, the steam must be taken from the boiler - a fact which proves obstructive to the general use of such apparatus.
In the Alsace Society of Engineers it was mentioned, not long ago, that the gases proceeding from a furnace which was furnished with heating-pipes had a temperature of 194° F. (90° C). These pipes received water at a temperature of 68° F. (20° C), and this water was heated by the difference of temperature of the external gases and the water, viz., 70° C. It is evident that if, in this work, a dejector apparatus had been employed to heat water to 302° F. (150° C), there would have been a considerable excess in expenditure of fuel; it would no longer have been the gases which heated the heating-pipes, but the water in the heating-pipes which heated the gases.
This constitutes the principal objection to such apparatus. Wherever boilers are heated directly without return of flame, such as the Field and Belleville boilers, it is not disadvantageous to employ them. It might happen that the economy of fuel arising from the absence of calcareous deposits in the interior of boilers (as in tubular boilers) would compensate, or more than compensate, for the defects just pointed out. Careful experiments could alone determine this. A few special forms of dejector apparatus will now be described.
This apparatus is based on the principle that in a liquid which is in general ebullition, all the substances in suspension in the mass are, while the ebullition lasts, carried to the surface and remain there. The mode in which Dumery removes these foreign substances will be understood from Figs. 6 and 7. The dejector consists of an iron cylinder, having at its upper part vertical ribs, in serpentine form, against which the water circulates. These terminate in a horizontal disc, which forms, with them, closed passages. The water enters the apparatus by the tubulure A, and circulates about the partitions a, 6, and c, passing out by the tubulure B. The water taken from the boiler at its upper part by means of a diaphragm, which causes it to be inflected horizontally, is restored to the boiler at its lower part; in passing among the partitions in the dejector it is freed of the foreign matters held in suspension. These matters, immediately on leaving the liquid vein, fall to the bottom of the apparatus, from which they can be easily removed by the stopcock d.
This simple apparatus (Fig. 8) consists of a cast-iron vessel, with a cover, which forms, with the sides of the vessel, an annular space. The injected water arrives by a tubulure, and traverses a plate pierced with small holes, which divide the liquid into threads, and these, in their fall, are acted on by full-pressure steam, which comes from a boiler, and fills all the space not occupied with water. The water is thus raised to a temperature of more than 212° F. (100° C). The calcareous salts pass to the granular state, and are deposited at the bottom, whence they can be removed when necessary, and the purified water is decanted into the annular space, from which it passes out by a tubulure to the lower part of the boiler. To avoid loss of heat, the apparatus is surrounded by some non-conducting substance.
This consists of a vertical column, A (Fig. 9), containing a series of dividing discs, B; the water is injected at the upper part, and passes successively through all the plates before reaching the vessel C, where it is decanted. .In the passage through the plates it is divided into a great number of streamlets, which are heated by steam coming from the boiler through D. The water, heated and decanted, leaves the apparatus by E. The chief difference between this apparatus and the preceding consists in the increased number of plates.
This is shown in Figs. 10, 11, and 12, and, like that of Farinaux, contains a column A, with a number of perforated discs d. In the interior of the column is a vertical pipe s, which crosses the discs, and serves for the escape of the water. Steam enters the apparatus by the orifice r, and meets the water which arrives at the base of the column by the orifice e. This water passing upward through the series of discs, traverses a filter, and descends by the central pipe s, while a quantity of steam collects in the chamber at the top. The extraction of deposits from the apparatus is effected by the orifice D. Potez sometimes places his apparatus in the interior of the cylindrical from various authorities may convey some information.
The minister of public works in France recently ordered an investigation into Lesueur's method of preventing boiler incrustations. This consists ill introducing into the boiler some small ingots or clippings of sine; it is then observed that the usual earthy substances, instead of forming a hard and adherent deposit, form a noncoherent crust, which can he readily removed. If the water be very strongly impregnated with lime salts, the deposit, even if coherent and solid, does not adhere firmly to the boiler plate.