It is an important point to remember that when water is expanded about 1,700 times into steam, it is simply expanded water, as ice is hardened water, i.e., in expanding into steam the two constituent gases do not separate. Hence, in dealing with the impurities inside the boiler, it is to be observed that in no sense do they change the essential nature of water itself. The impurities are simply foreign bodies, which have no legitimate place in the boiler, and are to be expelled as dangerous foes. As a general principle, it may be stated that it is more profitable to soften and filter the water used in boilers than to trust to blowing out or dissolving the sediment and scale that will be otherwise formed, for observations show that “anti-incrustators” containing organic matter help rather than hinder incrustations, and are therefore to be avoided. For the remedy of foul water there are numerous contrivances to prevent it from entering the boiler, which is far better than trying to extract the sediment after it is there, though there are many ingenious methods for doing that also, some of which will be detailed hereafter.
PRELIMINARY PRECIPITATION OF WATER.
A good method of avoiding incrustations in steam boilers is evidently a preliminary purification of the feed-water, provided it can be done by means sufficiently simple. This is a problem which it is claimed has been solved by M. Dehne of Halle, by means of an arrangement which we will herewith describe. The fresh water, which is taken up by a feed pump, is sent into a heater where it is raised to a temperature that will be favorable to chemical reaction. It then passes into a mixer where it encounters certain reacting agents which have been pumped in there by a pump of special design. These reacting agents are composed of a mixture of carbonate of soda and of caustic soda, the carbonate of soda serving to precipitate the sulphate of lime contained in the feed water, while the caustic soda precipitates the carbonate of lime and the magnesia. The relative dimensions between the special pump and the feed pump are calculated in such a way that the proportions of carbonate of soda and caustic soda in the mixture have always a certain relation to the amount of lime and magnesia to be precipitated. The water of the mixture is frequently very much disturbed by the precipitations which are formed, and passes into a filter where all the matters that are held in suspension are retained. It then goes into the boiler. In cases where the feed-water is taken from a tank, the heater, the mixer, and filter are put in the suction pipe of the feed pump, but if, as often happens, the water is already under pressure and will pass directly through the three, the feed pump will take the water directly from the filter and pump it directly into the boiler.
A PRECIPITATOR FOR SEA WATER.
It is quite possible to prepare sea water in such a way as to practically prevent any serious deposit forming from it.
The process employed is to add to the sea water a known quantity of precipitator powder consisting chiefly of soda ash, and having done this in a closed vessel, to heat the mixture by blowing into it waste steam, until a pressure of from 5lbs. to 10lbs. is created; under these circumstances practically all the magnesium and calcium salts separate from the water and are easily got rid of by filtering it under pressure into the hot-well.
A precipitator 6 ft. 4 in. high and 3 ft. in diameter, holds a ton of water, and the time taken, from the first running the sea water in, to its delivery into the hot-well, need not exceed 1 hour and 15 minutes, so that in practice, giving plenty of time between the makes, it would be perfectly easy to prepare 8 to 12 tons in the 24 hours with a small precipitator of the size named. The prepared water has a density of l⁄32nd, and may with safety be evaporated until its density is 5⁄32nds, the salts present not crystalizing out until a density of from 6⁄32nds to 7⁄32nds is reached.
In preparing sea water in the way proposed, every precaution must be taken to add slightly less of the precipitant than is necessary to entirely throw down the calcium and magnesium salts, as it is manifestly impossible in practice to guard against small quantities of sea water finding way into the boiler either from leaky condensers or else being fed in by the engineer during some emergency, and if under these conditions any excess of the precipitant were present in the boiler, a bulky precipitate would be thrown down and cause trouble, although it would not bind into a solid scale.
Briefly recapitulated the means which are best adapted for preventing the formation of the dangerous organic and oily deposits considered are:
I. Filtration of condensed water through a coke column.