Table A. - Waters Obtained From Rivers And Loch

Table B. - Waters Obtained From Wells

Table C. - Waters Obtained From Pits

Table D. - Sewage-Impregnated Waters

Pit waters and surface waters are liable to contain matter in suspension, and this increases very greatly the difficulties of dealing with the water. Waters derived from coal-pits, and containing coal-dust in minute portions, or those obtained from shale pits, and having suspended sharp clean particles of shale, can be used with advantage, if not acid; but waters containing earthy impurities or suspended chalk should not be employed. The presence of coal and shale dust in water tends to give a scale or deposit much less hard in structure than would otherwise be the case, and may sometimes even cause the incrustation to become a loose mud.

Surface waters from the neighbourhood of chemical works, especially those where sulphuric, sulphurous, or hydrochloric acids are made or evolved, as in alkali, manure, and paraffin works, or in the vicinity of burning bings of ironstone or blaes, generally contain much free acid in solution, and are most injurious as feed waters. The drainings from blaes or coal slacks and from shale refuse contain acid sulphates of iron and alumina, and should not be employed. The same remark applies to burnt or unburnt sulphur ore (iron pyrites), or copper pyrites.

Moss waters, or waters derived from sources where peat abounds, are of first-class quality for boiler purposes. These waters almost invariably contain a small quantity of saline matter in solution, and what little deposit is formed is in the condition of a mud, much mixed with organic vegetable matter, and readily removed by the blow-off cock.

The composition of the incrustation left by these waters is given in Tables E and F; they will be seen to divide themselves into a carbonate class and a sulphate class, the latter being much the more common of the two.

Table E. - Boiler Incrustations. - Carbonate Class. No Anti-Incrustator Used

Table F. - Boiler Incrustations. - Sulphate Class. No Anti-Incrustator Used

Carbonate scales are usually more brittle than sulphate deposits, which tend to toughness in fracture. The carbonate deposits occasionally appear as a pulverulent mass somewhat resembling chalk in consistence. Very much depends on the method of reducing the firing before cleaning. If the fire be drawn and the water run off whilst the flues are hot, the deposit becomes dry and tough, and very difficult to remove. On the other hand, where the water is allowed to remain to cool slowly before any attempt is made to remove the deposit, the scale is more easily detached, and less chipping is necessary.

Pit waters are frequently acid in character, from the presence in solution of the sulphates of alumina and iron, derived from the oxidation of the pyrites present in the coal or shale. The scale or deposit left in the boiler always contains much iron as oxide, whilst the calcic carbonate is converted either totally or partially into sulphate. The analysis of a deposit from a boiler fed with acid pit water illustrates these changes: -

Table G. - Incrustation. - Acid Pit Water Used

In the above incrustation, the acidity of the feed water was overcome by the addition of extra fresh water containing calcium carbonate, so that the ferric oxide must have been present in the feed water. Had no extra water been used, or had such been of a permanently hard nature, much damage must have been done to the boiler plates by the employment of this water.

Sea .water, especially when heated, has much action on iron and other metals. This is due more especially to the decomposition of the chloride of magnesia contained in the water. An analysis of the incrustation formed on the salt-water pipes supplying hot water to a large hydropathic establishment will illustrate the composition of such deposits: -

Table H. - Deposit From Salt-Water Pipes

Macadam finds it impossible to consider even a large proportion of the many and various substances from time to time introduced to prevent or destroy boiler scale or incrustation. Very many of the patents are absolutely worthless, some decidedly hurtful or dangerous. He throws the more common bodies into classes, and offers the following notes on their properties, dividing the subject under two heads; The first and more crude method being the employment of an anti-incrustator placed inside the boiler; whilst the second takes in the various methods of softening and removing deleterious substances before the water is admitted into the heating chamber. Some of the anti-incrustators profess to remove scale by chemical means - in other words, to dissolve the hard calcareous deposit. These substances are not in general favour, and rightly so, as it is found that the regular employment of such bodies causes much damage to boiler plates and to the brass and gun-metal fittings of the cylinders. The larger number of anti-incrustators profess to prevent the formation of a scale rather than its removal, and although, after a time, they may cause the previously-formed scale to break up and be easily removed, yet such is traceable as much to mechanical means as to chemical action.

The probable explanation is that the iron plates being more readily expanded on the application of heat, or more readily cooled on the withdrawal of the fire, cause unequal expansion in the incrustation, and consequent Assuring. The cracks, from the presence of a preventive anti-incrustator, are not repaired, and so in course of time the old scale is broken up and removed from the boiler.

The anti-incrustators can be considered in the following classes: (1) Saline; (2) Fats and Oils; (3) Paraffin and Paraffin Products; (4) Other Organic Substances (including the gelatinous group).