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INCRUSTATIONS.—By the use of hard water for steam boilers, an incrustation or scale is liable to be formed on their interior surfaces which materially injures their efficiency in evaporating steam, and also destroys the metal. So much has been said on this subject, however, in former volumes and in preceding numbers of this volume of the SCIENTIFIC AMERICAN, that we have but little to add new, yet that which we now present will be found very useful and generally new. In hard water, the sulphate and the carbonate of lime are the principal matters held in solution that cause incrustations in steam boilers, but the sulphate is the prime scale-former. The reason of this is, that when the water in the boiler becomes saturated with the lime in consequence of the evaporation of pure water which passes off as steam, the sulphate oflime then separates from the water in which it was formerly held in solution, and attaches itself to the whole surface of the metal. The carbonate of lime, on the contrary, although it separates from the saturated water, does not attach itself to the whole surface of the boiler ; it is precipitated, but while the water is hot, it has little or no disposition to adhere to the metal unless by cementation with the sulphate. Mr. J. Graham, whose experiments we have described in a former article, was able to prevent the formation of incrustations in boilers when using hard fresh water, by blowing off the saturated water regularly when it has attained to the "salting point," in the same manner that the concentrated brine is run off in the boilers of ocean steamships. In doing this about four per cent of the amount of water fed in is sacrificed, but this is a very small loss in comparison with the good results thereby obtained. In boilers using sea water, the sulphate of lime is really the only scale which is formed; in those using hard fresh water the scale is chiefly composed of carbonate of lime. Recent experiments regarding both kinds of these boiler incrustations have been described by James Napier, of Glasgow, an excellent practical chemist, in the London Engineer. The following is his analysis of scale taken from a boiler in which river water had been used :— Carbonate of lime..................79.0 Sulphate of lime.....................6.3 Peroxyd of iron......................3.5 Silica....................................2.2 Carbonaceous matter................4.0 Water...................................5.0 100. The next analysis is that of scale taken from the boiler of a steamer running between Glasgow and Liverpool, in which no attention was paid to " blowing off." This scale was composed of two layers; the one (that next the metal) was hard and crystalline, the other (or outer coat) was softer and granular. The thickness of the whole crust was about three-eighths of an inch : — Sulphate oflime....................81.6 Magnesia..............................4.2 Silica.........................,.........2.8 Peroxyd of iron...................... 2.4 Salt.....................................0.7 Water of crystallization...........7.7 Carbonic acid........................ 0.6 100. The next analysis was that of scale taken from the same boiler, which was worked for the same length of time, as in the former experiment, but care was taken to "blow off" regularly. The scale in this case was only one-sixteenth of an inch thick—only one-sixth the thickness of that formed when " blowing off "was neglected : — Sulphate of lime.....................94.5 Magnesia..............................1.5 Peroxyd of iron.....................0.5 Salt.................................... 1.1 t\ Water..................................2.4 n — !A 10- pS These analysis show that the sulphate of lime is the main ingredient of the scale deposited by sea water. They also afford very satisfactory evidence regarding the way to prevent incrustations by care in blowing off the saturated water regularly. The following is the method proposed by Mr. Napier for the prevention of incrustations in all boilers. He analyzes the water to be used, and if found to contain only the bi-car-bonate of lime in suspension, there is no difficulty in preventing it from forming scale. The carbonate of lime separates from the water at a high heat, and is kept suspended in the boiler while the water is hot, but when the boiler is stopped, it falls to the bottom in cooling, and when cold it hardens, adheres to the metal, and forms a crust. A boiler using hard fresh water containing carbonate of lime has thus a thin layer of scale formed every night, and at last it accumulates to a thick stony crust, which almost prevents the passage of the heat from the fire to the water. To prevent such scale, the plan to be adopted is simple. In about an hour after the engine is stopped every evening, and when the fire is cooled down, the engineer should blow off the water freely. This will discharge all the sediment which has been precipitated, and prevent it hardening and adhering to the metal. Although this method of working boilers will prevent scale, if the water only contains carbonate of lime, it will not entirely suffice to prevent incrustations when the sulphate of lime is the principal ingredient in the water, because it does not precipitate like the carbonate. Having by analysis discovered the' quantity of the sulphate of lime in each gallon of the water to be used as feed, a sufficient quantity of the carbonate of soda is to be employed to neutralize the sulphate and convert it into the carbonate. The carbonate of soda dissolved is to be fed regularly into the boiler by a pipe connected with the water feed pipe. On land boilers, the carbonate of lime thus formed should be blown off every evening when the water has cooled down ; in marine boilers, the carbonate will float near the surface when the boiler is working, and it can be blown off by the surface water cock. Any alkali will neutralize the sulphate of lime in a steam boiler, but the common carbonate of soda is the cheapest which can be used. Care, however, must be exercised not to employ it or any other alkali in excess for such a purpose, as it has a tendency to volatilize with the steam.
