y harmless and from which they are readily removable.
[Illustration: Longitudinal section.
Section at AB--Front elevation.
FIG. 17.--Bobcock & Wilcox Water-tube Boiler (marine type).]
_Oil Separators._--When the steam from the engines is condensed and used
as feed-water, as is the case with marine boilers, much difficulty is
often experienced with the oil which passes over with the steam.
Feed-filters are employed to stop the coarser particles of the oil, but
some of the oil becomes "emulsified" or suspended in the water in such
extremely minute particles that they pass through the finest filtering
materials. On the evaporation of the water in the boiler, this oil is
left as a thin film upon the heating surfaces, and by preventing the
actual contact of water with the plates has been the cause of serious
trouble. An attempt has been made to overcome the emulsion difficulty by
uniformly mixing with the water a small quantity of solution of lime. On
the water being raised in temperature the lime is precipitated, and the
minute particles separated apparently attract the small globules of oil
and become aggregated in sufficient size to deposit themselves in quiet
parts of the boiler, whence they can be occasionally removed either by
blowing out or by cleaning. Much, however, still remains to be done
before the oil difficulty will be thoroughly removed.
_Corrosion._--When chemicals of any kind are used to soften or purify
feed-water it is essential that neither they nor the products they form
should have a corrosive effect upon the boiler-plates, &c. Much of the
corrosion which occasionally occurs has been traced to the action of the
oxygen of the air which enters the boiler in solution in the feed-water,
and the best practice now provides for the delivery of the feed into the
boiler at such positions that the air evolved from it as it becomes
heated passes direct to the steam space without having an opportunity of
becoming disengaged upon the under-water surfaces of the boiler.
Where corrosion is feared it is usual to fit zinc slabs in the water
spaces of the boiler. Experience shows that it is better to make them of
rolled rather than of cast zinc, and to secure them on studs which can
be kept bright, so as to ensure a direct metallic contact between the
zinc and the boiler-plate. The function of the zinc is to set up
galvanic action; it plays the part of the negative metal, and is
dissolved while the metal of th
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