All natural waters contain some impurities which, when introduced into a
boiler, may appear as solids. In view of the apparent present-day
tendency toward large size boiler units and high overloads, the
importance of the use of pure water for boiler feed purposes cannot be
over-estimated.
Ordinarily, when water of sufficient purity for such use is not at hand,
the supply available may be rendered suitable by some process of
treatment. Against the cost of such treatment, there are many factors to
be considered. With water in which there is a marked tendency toward
scale formation, the interest and depreciation on the added boiler units
necessary to allow for the systematic cleaning of certain units must be
taken into consideration. Again there is a considerable loss in taking
boilers off for cleaning and replacing them on the line. On the other
hand, the decrease in capacity and efficiency accompanying an increased
incrustation of boilers in use has been too generally discussed to need
repetition here. Many experiments have been made and actual figures
reported as to this decrease, but in general, such figures apply only to
the particular set of conditions found in the plant where the boiler in
question was tested. So many factors enter into the effect of scale on
capacity and economy that it is impossible to give any accurate figures
on such decrease that will serve all cases, but that it is large has
been thoroughly proven.
While it is almost invariably true that practically any cost of
treatment will pay a return on the investment of the apparatus, the fact
must not be overlooked that there are certain waters which should never
be used for boiler feed purposes and which no treatment can render
suitable for such purpose. In such cases, the only remedy is the
securing of other feed supply or the employment of evaporators for
distilling the feed water as in marine service.