harmful to a boiler. At Warrington some steam users have a special pumping plant to pump the Mersey water to their works, and attribute the excellence of the water to the presence in it of the sewage of Manchester and other towns, but in this connection it should be remembered that the Mersey and its tributaries flow almost wholly over rocks of the Millstone Grit or New Red Series, and that the sewage has as basis a soft water supply and is probably alkaline from the waste of bleach works. In Oldham, water is derived from the sewers as a regular thing. The effect of water containing sulphate of lime may be seen in boilers at Burtonon-Trent. These are often of a glistening white inside, and thick with scale, under which may be found the familiar black oxide of iron produced by the presence of this scale. As it will usually cost a minimum of 6d. per 1,000 gallons to purchase water, this must be set against the cost of a free supply with treatment, and usually, if the free supply is hard, the purchased water will frequently also be hard. But where a good water can be purchased it may be economised by means of surface condensation, and very little need be purchased if proper use is made of the roof area for supplementing the supply. Useful Figures. In the calculations of the steam engineer it is convenient to remember that the square of the diameter of a pipe or a pump barrel gives the weight of water in a yard length of pipe. Thus a six-inch pipe holds 36 pounds or 3.6 gallons per yard. Again 1 pound of coal should evaporate 1 gallon of water; 1 gallon of water will give steam to work in the best engine yet made at the rate of 1 h.p. hour. 2 gallons will serve an ordinary compound engine per h.p. hour and 3 gallons a good condensing engine for each h.p. hour. Approximately too, 1,000 B. Th. U. generated represents one pound of steam, so that the number of thousands of units capacity of a pound of fuel represents the theoretical evaporation in pounds of water. As the latent heat of steam is practically 1,000 B. Th. U. at a temperature of 100°F., it will be perceived that to convert water into vapour at ordinary atmospheric environment must demand 1,000 heat units. This principle is made use of in the atmospheric condenser, wherein steam in the pipes is deprived of its latent heat by means of films of water outside the pipes, which are kept cool by the absorp tive action of the wind which carries off much of the outer water, and the act of vaporization abstracts heat from that left behind. Thus the loss of one pound of, perhaps, a cheap cooling water, will save an equal weight of water inside the condenser for boiler feeding purposes. In any case a surface condenser of some sort is a means of feeding a boiler with water that contains no incrusting material, though its contained oil from the cylinders is an objection to its use, and this should be removed. Solubility of Salts. As a rule this increases with the temperature, but at a slow rate, except for sodium chloride and a few other exceptions. For the sulphates of magnesium and potassium and the chlorides of barium and of potassium, solubility is proportionate to the increase of temperature. With sulphate of soda the solubility first increases and then falls off again. The solubility of calcium sulphate decreases with temperature. The following table gives the solubility of a few salts at various temperatures in parts per 100. The solubility at 212° F. is really at a higher temperature, being the solubility at boiling point which is always raised slightly by the solution of a salt. Sea water contains 38 parts per 1000, of dissolved matter; of this from 25 to 28 parts are common salt NaCl. The Black Sea contains only 17-7 parts, the Caspian Sea 14.0, and the Baltic 6-7, owing to the large fresh water rivers which flow into them. The other salts of sea water are magnesium chloride, calcium sulphate, magnesium sulphate, potassium sulphate and chloride, bromide of soda, the carbonates of lime and magnesia, and traces of other salts and organic substances. Hardness. By this term is meant 1 grain per gallon of lime carbonate CaCO,. Temporary hardness is that which can be reduced by boiling. Permanent hardness is not reduced by boiling. Pipes. The ordinary velocity of flow of water in pipes may be taken as 72 inches per second. This velocity is to be reduced 1 inch per second for each 20 pounds pressure. Thus in feed pipes at 160 pounds pressure, the velocity will be 72-8= 64 inches. Practical considerations demand, except where several boilers are fed through one pipe, that the pipes shall be much larger than would give such a velocity in many cases. Pipes less than 1 inches are rarely advisable for feed pipes, and if pipes are liable to be scaled up they ought to be made initially larger than necessary to allow of a considerable deposit of scale without unduly diminishing their capacity. TH Chapter X WATER PURIFICATION Incrustation. HE presence of scale in a steam boiler is a cause of poor evaporative duty and therefore a waste of fuel, for all are agreed that iron is a better material for boiler plate than flagstone. If the question of eliminating the scale-forming elements were better understood, there would be more frequent cases of it being undertaken. Where water is intended for dietetic purposes there is some amount of objection to the use of certain of the materials employed in water softening when certain peculiar kinds of scale-forming substances are contained in the water. For most manufacturing purposes, however, there is no objection to the use of any substance that will not injure the boiler, and this is more especially the case where steam is sought merely for motive power purposes. There are several scaleforming materials found dissolved in water, and they cannot all be removed by the same means, and, as usually presented, the question is rather too much complicated by the endeavour to treat the whole subject at once, in place of taking the various impurities separately, as if each were the only impurity present that required dealing with, though we may very well inquire into the methods of dealing with waters containing more than one scale-forming impurity. Speaking generally, there is nothing wrong in this detailed method of treating the question, for even if a water contains both carbonate of lime and sulphate of lime in equal or any other proportion, it is easy to ignore the latter and treat for the former only, by doing which we should remove one substance and reduce the amount of scale formed by the proportion of the impurity removed. To use a mechanical simile, we may sieve out buckwheat coal from a ton of mixed coals of all sizes, and leave the pea coal in the mixture. By the use of a larger sieve, we can then sieve out the pea coal. We need not, unless we wish, use the larger sieve first, so eliminating both the small coals together. So in the water softening process we may use a single chemical process and remove one substance, or we may use a double process and remove two or more substances together. As a rule, the most usual impurity in water, and the only one of account in most chalk waters, is carbonate of lime, and, as it is easiest and cheapest to remove, it will be best to deal first with it. Strictly speaking, this carbonate of lime is nearly insoluble in water, and upon this fact depends the process of removing it from solution. This sounds contradictory and demands explanation. We must know first what it is, of what it is chemically composed. First, what is lime? Lime is the oxide of the metal calcium, and this is a yellow metal of extremely small weight, its specific gravity being only three-fifths that of aluminium. It is very oxidisable, and is not found in nature as a metal. It burns to lime. In 56 pounds of lime (CaO) there are 40 Carbon when burned yields pounds of calcium and 16 pounds of oxygen. carbonic acid gas, in 44 pounds. of which there are 12 pounds of carbon. Now, lime has a strong affinity for carbonic acid, and absorbs it greedily from the air; hence the use of lime water to sprinkle down wells in which the air is impure with carbonic acid (choke damp); 56 pounds of lime will absorb 44 pounds of this gas, becoming carbonate of lime. (Expressed in chemical symbols, to understand which it is not necessary, and they may be skipped without rendering the matter much less understandable; the combination is expressed thus— This absorption of carbonic acid is what takes place when mortar hardens. When, however, limestone is burned, the carbonic gas is driven off by heat and simple lime caustic or quick lime is produced. Carbonate of lime being practically insoluble in water, how then is it formed therein? Carbonic acid Carbonic acid gas is very soluble in cold water, so that one gallon of water at freezing point will absorb 1 gallons of the gas, or 500 pounds of water will absorb 1 pound of gas. All natural waters contain this gas, and such waters will dissolve carbonate of lime, so that, if we wish to dissolve 100 pounds of carbonate of lime, we shall want as much water as will contain 44 pounds of carbonic gas. Let us suppose that a certain sample of water contains 10 grains of carbonate of lime per gallon (of 10 pounds). Then this implies the presence of 45 grains of carbonic gas. But a gallon of water may contain as many as 140 grains of the gas, so that there is no difficulty in it dissolving the above amount of lime. Now, if we require to render the carbonate insoluble we must take away the excess of carbonic acid gas which renders it soluble. Water be freed from this gas by boiling, for, though very soluble in cold water, it is may not at all soluble in boiling water. When therefore we feed a boiler with water that contains carbonate of lime, we speedily render this lime salt insoluble, for the gas is boiled away and goes off at the steam outlet, and the lime salt is left behind in a very finely divided state, and finally it settles down at the quietest part of the boiler as mud, some of it is blown out as mud, some is scummed off the surface before it has settled, and a lot of it stays behind and forms scale, and heat is lost both by blowing off hot water and by the resistance of the scale to the passage of heat. Getting the carbonate out of solution is therefore an easy and simple matter, and if we first boiled all the feed in an open tank and let the deposit settle or filtered it out before we pumped the feed into the boiler, we should have a clean boiler. But such a course would cost money in the shape of heat, and we could not use feed heaters in the flues, and yet we ought to deposit the carbonate before it enters the boiler. There is a simple way of doing this; we can abstract the excess of carbonic acid gas by some other substance, and there are two substances will do this, and may ordinarily be employed. One of these is caustic soda, the oxide of the metal sodium; the other is caustic or quick lime, already described. Both act on similar lines. To remove 100 pounds of carbonate of lime we add 80 pounds of ordinary caustic soda (Sodium hydroxide). The process is thus set down in chemical symbols Carbonate of Lime +Carbonic Gas +Caustic Soda. CaCO3 +CO2 +Na,CO2 +2 HNaO. = + H2O. |