[Illustration: 3000 Horse-power Installation of Cross Drum Babcock & Wilcox Boilers and Superheaters Equipped with Babcock & Wilcox Chain Grate Stokers at the Washington Terminal Co., Washington, D. C.]
Boiler Compounds--The method of treatment of feed water by far the most generally used is by the use of some of the so-called boiler compounds. There are many reliable concerns handling such compounds who unquestionably secure the promised results, but there is a great tendency toward looking on the compound as a "cure all" for any water difficulties and care should be taken to deal only with reputable concerns.
The composition of these compounds is almost invariably based on soda with certain tannic substances and in some instances a gelatinous substance which is presumed to encircle scale particles and prevent their adhering to the boiler surfaces. The action of these compounds is ordinarily to reduce the calcium sulphate in the water by means of carbonate of soda and to precipitate it as a muddy form of calcium carbonate which may be blown off. The tannic compounds are used in connection with the soda with the idea of introducing organic matter into any scale already formed. When it has penetrated to the boiler metal, decomposition of the scale sets in, causing a disruptive effect which breaks the scale from the metal sometimes in large slabs. It is this effect of boiler compounds that is to be most carefully guarded against or inevitable trouble will result from the presence of loose scale with the consequent danger of tube losses through burning.
When proper care is taken to suit the compound to the water in use, the results secured are fairly effective. In general, however, the use of compounds may only be recommended for the prevention of scale rather than with the view to removing scale which has already formed, that is, the compounds should be introduced with the feed water only when the boiler has been thoroughly cleaned.
FEED WATER HEATING AND METHODS OF FEEDING
Before water fed into a boiler can be converted into steam, it must be first heated to a temperature corresponding to the pressure within the boiler. Steam at 160 pounds gauge pressure has a temperature of approximately 371 degrees Fahrenheit. If water is fed to the boiler at 60 degrees Fahrenheit, each pound must have 311 B. t. u. added to it to increase its temperature 371 degrees, which increase must take place before the water can be converted into steam. As it requires 1167.8 B. t. u. to raise one pound of water from 60 to 371 degrees and to convert it into steam at 160 pounds gauge pressure, the 311 degrees required simply to raise the temperature of the water from 60 to 371 degrees will be approximately 27 per cent of the total. If, therefore, the temperature of the water can be increased from 60 to 371 degrees before it is introduced into a boiler by the utilization of heat from some source that would otherwise be wasted, there will be a saving in the fuel required of 311 ÷ 1167.8 = 27 per cent, and there will be a net saving, provided the cost of maintaining and operating the apparatus for securing this saving is less than the value of the heat thus saved.