_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 the shell is kept electro-positive. Care
must always be taken that the fragments which break off the zinc as it
wastes away cannot fall upon the heating surfaces of the boiler.
_Evaporators._--In marine boilers the waste of water which occurs from
leakages in the cycle of the evaporation in the boiler, use in the
engine, condensation in the condenser and return to the boiler as
feed-water, is made up by fresh water distilled from sea-water in
"evaporators." Of these there are many forms with different provisions
for cleaning the coils, but they are all identical in principle. They
are fed with sea-water, and means are provided for blowing out the brine
produced in them when some of the water is evaporated. The heat required
for the evaporation is obtained from live steam from the boilers, which
is admitted into coils of copper pipe. The water condensed in these
coils is returned direct to the feed-water, and the steam evaporated
from the sea-water is led either into the low-pressure receiver of the
steam-engine or into the condenser.
[Illustration: FIG. 18.--Handhole Fittings.]
_Efficiency of Boilers._--The useful work obtained from any boiler
depends upon many considerations. For a high efficiency, that is, a
large amount of steam produced in proportion to the amount of fuel
consumed, different conditions have to be fulfilled from those required
where a large output of steam from a given plant is of more importance
than economy of fuel. For a high efficiency, completeness of combustion
of fuel must be combined with sufficient heating surface to absorb so
much of the heat produced as will reduce the temperature of the funnel
gases to nearly that of steam. Completeness of combustion can only be
obtained by admitting considerably more air to the fire than is
theoretically necessary fully to oxidize the combustible portions of the
fuel, and by providing sufficient time and opportunity for a thorough
mixture of the air and furnace gases to take place before the
temperature is lowered to that critical point below which combustion
will not take place. It is generally considered that the amount of
excess air required is nearly equal to that theoretically necessary;
experience, however, tends to show that much less than this is really
required if proper means are provided for ensuring an early complete
mixture of the gases. Different means are needed to effect this with
different kinds of coal, those necessary for properly burning Welsh coal
being altogether unsuitable for use with North Country or Scottish coal.
As all the excess air has to be raised to the same temperature as that
of the really burnt gases, it follows that an excess of air passing
through the fire lowers the temperature in the fire and flues, and
therefore lessens the heat transmission; and as it leaves the boiler at
a high temperature it carries off some of the heat produced. A reduction
of the amount of air, therefore, may, by increasing the fire temperature
and lessening the chimney waste, actually increase the efficiency even
if at the same time it is accompanied by a slight incompleteness of
combustion.
[Illustration: FIG. 19.--Normand Boiler.]
_Mechanical Stoking._--Most boilers are hand-fired, a system involving
much labour and frequent openings of the furnace doors, whereby large
quantities of cold air are admitted above the fires. Many systems of
mechanical stoking have been tried, but none has been found free from
objections. That most usually employed is known as the "chain-grate"
stoker. In this system, which is illustrated in fig. 13 (Woodeson
boiler), the grate consists of a wide endless chain formed of short
cast-iron bars; this passes over suitable drums at the front and back of
the boiler, by the slow rotation of which the grate travels very slowly
from front to back. The coal, which is broken small, is fed from a
hopper over the whole width of the grate, the thickness of the fire
being regulated by a door which can be raised or lowered as desired.
Thus the volatile portions of the coal are distilled at the front of the
fire, and pass over the incandescent fuel at the back end. The speed of
travel is so regulated that by the time the remaining parts of the fuel
reach the back end the combustion is nearly complete. It will be seen
that the fire becomes thinner towards the back, and too much air is
prevented from entering the thin portion by means of vanes actuated from
the front of the boiler.
_Draught._--In most boilers the draught necessary for combustion is
"natural," i.e. produced by a chimney. For marine purposes, although
"natural" draught is the more common, many boiler installations are
fitted with "forced" draught arrangements. Two distinct systems are
used. In that known as the "closed stokehold" the stokehold compartment
of the vessel is so closed that the only exit for air from it is through
the fires. Air is driven into the stokehold by means of fans which are
made so that they can maintain an air pressure in the stokehold above
that of the outside atmosphere. This is the system almost universally
adopted in war vessels, and it is used also in some fast passenger
ships. The air pressure usually adopted in large vessels is that
corresponding to a height of from 1 to 1-1/2 in. of water, whilst so much
as 4 in. is sometimes used in torpedo-boats and similar craft. This is,
of course, in addition to the chimney-draught due to the height of the
funnel. In the closed ashpit or Howden system, the stokehold is open,
and fans drive the air round a number of tubes, situated in the uptake,
through which the products of combustion pass on their way to the
chimney. The air thus becomes heated, and part of it is then delivered
into the ashpit below the fire and part into a casing round the furnace
front from which it enters the furnace above the fire. In locomotive
boilers the draught is produced by the blast or the exhaust steam. With
natural draught a combustion of about 15 to 20 lb. of coal per sq. ft.
of grate area per hour can be obtained. With forced draught much greater
rates can be maintained, ranging from 20 lb. to 35 lb. in the larger
vessels with a moderate air pressure, to as much as 70 and even 80 lb
per sq. ft. in the express types of boiler used in torpedo boats and
similar craft.
_Performance of Boilers._--The makers of several types of boilers have
published particulars regarding the efficiency of the boilers they
construct, but naturally these results have been obtained under the most
favourable circumstances which may not always represent the conditions
of ordinary working. The following table of actual results of marine
boiler trials, made at the instance of the British admiralty, is
particularly useful because the trials were made with great care under
working conditions, the whole of the coal being weighed and the
feed-water measured throughout the trials by skilled observers. The
various trials can be compared amongst themselves as South Welsh coal of
excellent quality was used in all cases.
In experimental tests such as those above referred to, many conditions
have to be taken into account, the principal being the duration of the
trial. It is essential that the condition of the boiler at the
conclusion of the test should be precisely the same as at the
commencement, both as regards the quantity of unconsumed coals on the
fire-grate and the quantity of water and the steam-pressure in the
boiler. The longer the period over which the observations are taken the
less is the influence of errors in the estimation of these particulars.
Further, in order properly to represent working conditions, the rate of
combustion of the fuel throughout the trial must be the same as that
intended to be used in ordinary working, and the duration of the test
must be sufficient to include proportionately as much cleaning of fires
as would occur under the normal working conditions. The tests should
always be made with the kind of coal intended to be generally used, and
the records should include a test of the calorific value of a sample of
the fuel carefully selected so as fairly to represent the bulk of the
coal used during the trial. The periodic records taken are the weights
of the fuel used and of the ashes, &c., produced, the temperature and
quantity of the feed-water, the steam pressure maintained, and the
wetness of the steam produced. This last should be ascertained from
samples taken from the steam pipe at a position where the full pressure
is maintained. In order to reduce to a common standard observations
taken under different conditions of feed temperatures and steam
pressures, the results are calculated to an equivalent evaporation at
the atmospheric pressure from a feed temperature of 212 deg. F.
(J. T. Mi.)