The supplying of the troops at Suakim and in the Soudan with water is one
of the most important items in the whole conduct of the Egyptian war.
Even in cold or temperate latitudes fresh water is a first necessity for
animal life; much more is this the case in the desert; and the wells in
the country forming the scene of our military operations form in
themselves valuable strategical points. Their supply, however, has to be
supplemented, and to do so artificial means and the aid of the engineer
have to be enlisted into this service.
Many of our readers see notices from time to time in the newspapers about
this or that ship being employed, or at least her steam fittings, in
distilling water for the use of the troops; and although most of, if not
all, our readers are engineers, still it is no disparagement to some of
them to assume that they are more or less unfamiliar with sea water
distillation on the scale on which the process is now being carried on at
Suakim; and as the subject is of general interest, we give a short
description of the process.
In a general sense, fresh water is obtained from sea water by simply
generating steam from the sea water, passing the said steam through a
surface condenser, and filtering the resulting water. The obtaining of
fresh water in this way has been in practice on board sea-going ships for
many years. It is supposed by some authorities on this subject that the
first time fresh water was thus obtained at sea was by an old captain of
a brig which ran short of water, and he cut up some pewter dishes into
strips, which he bent and soldered into a pipe. He, with the carpenter's
aid, fitted a wooden lid in one of the cooking boilers, and fixed one end
of his pipe in it. He next sawed a water cask in half, bored a hole in
the bottom of one half, and took his pipe through it, filling the space
round the pipe with sea water. Thus he extemporized a worm and still or
condenser. The distilled water, however, was scarcely drinkable. Not to
be beaten, however, the captain got some pieces of charred wood which he
put in the water, which so far improved it as to render it at all events
fit to sustain life, and our skipper brought his brig and her screw
safely to port. What suggested the use of charcoal to his mind history
does not tell. For many years past scarce any sea-going vessel leaves
port that is not fitted with a properly constructed distiller; and one
conspicuous advantage attending this practice is that each ship thus
fitted to the satisfaction of the Board of Trade inspector is allowed to
sail with only half the quantity of fresh water on board which she should
have if not provided with a distiller. The distiller and filter occupy
very much less space than that which would be occupied by the casks or
tanks of water otherwise required to be carried.
Coming now a little to detail, sea water distillers are usually fitted in
connection with the winch and its boiler, which latter supplies the steam
both for distillation and to drive the engine working its circulating
pump. Smaller distillers are worked without a pump, the cooling water
merely passing through by gravitation. These smaller affairs again are of
two kinds, the one being mounted at one end of the cooking hearth, as in
outline sketch, which shows a two oven hearth with distiller at one end.
A is the supply pipe to admit air to aerate the water; B is the cock
where fresh water is drawn off; C is a pipe conveying cooling water to
the condenser E, placed on three little feet on top of the boiler, F,
whose steam rises up a central pipe to the dome top, where it expands out
and returns downward through a number of tubes about 1 in. diameter, in
which it is condensed, collected in a bottom chamber, and drawn off
through the cock, B. A distiller of this size would make about thirty
gallons of fresh water per day. Very frequently a distiller, such as is
shown in the sketch, is mounted separately, and placed near the winch or
donkey boiler, which supplies it with steam, the lower part, F, being
then used as a filter. The diameter of E is from 15 in. to 18 in., the
outer casing being either iron or copper. Another form of distiller is
one like the above, but larger, and having a small donkey engine and
circulating pump attached thereto. As a rule these distillers are
vertical, but larger apparatus are arranged horizontally. To give our
readers some general idea of size, weight, and produce of water, we may
say that a plain cylindrical distiller, mounted on a square filter case,
measuring 3 ft. 9 in. high, weighing 4½ cwt., will distill twelve gallons
per hour. A larger size, measuring 6 ft. 2 in. high, and weighing about
23 cwt., will give 85 gallons; while a still larger one, measuring 7 ft.
high and weighing 32 cwt., yields 150 gallons. These have no pumps. When
an engine and pump are fitted, the weight is increased from about 80 per
cent. in the smaller to 50 per cent. in the larger sizes. An immense
advantage attends the use of those distillers that are combined with a
winch boiler. Of course, the chief use of the winch is while in dock;
some use is made of it at sea to do heavy pulling and hauling, to wash
decks, and in case of emergency the circulating pump is used as a fire
engine. Were it not, however, for the distiller, the winch boiler would
simply be idle lumber at sea. The distiller, however, finds useful
employment for it, and has also this excellent effect, that as steam is
pretty constantly kept up for the distiller, in the evil event of a fire
the boiler is ready to work at once. In horizontal types of distiller an
engine and pump are mounted on a cast iron casing as a bed, and in this
casing is placed a number of tubes through which the steam passes to be
condensed, the whole being simply a surface condenser with engine and
pump above. Another type is that of a small single-flued horizontal
boiler with combustion chamber and twenty or thirty return tubes--in
fact, the present high-pressure marine boiler on a small scale. A boiler
of this sort, measuring 4 ft. to 5 ft. long, 3 ft. 9 in. to 4 ft. 6 in.
diameter, would have a horizontal donkey engine on a bed at its side, and
at the end of the engine a vertical cylindrical condenser.
[Illustration]
Few have done more, perhaps none so much, as Dr. Normandy to make sea
water distillation not only a success as a source of water supply, but
also to supply it at a minimum cost for fuel. He by a peculiar
arrangement of pipes embodied something of the regenerative system in his
apparatus, using the heat taken from one lot of steam to generate more,
and again the heat from this he used over again. The defect of his older
arrangements was undue complexity and consequent trouble to keep in
order.
As can be well imagined, the distillers in use at Suakim are on a much
more colossal scale, and owing to the now almost universal use of surface
condensers in ocean steamers, no great difficulty ought to attend the
adaptation of the boilers and condensers of one of our transports. One of
these full-powered steamers will indicate, say, 5,000 horse-power, and
assuming her engines to use 25 lb. of steam per indicated horse-power, or
2¼ gallons, she could distill some 12,000 gallons of water per hour. As
no appreciable pressure of steam need be maintained, the boilers would
suffer little from deposit, especially if regularly blown out. Hard
firing need not be resorted to; indeed, it would be injudicious, as, of
course, priming must be carefully guarded against. Of course, the salt
water distilled will affect the working, not exactly of the distillers,
but of the boilers. If the water in the harbor, as is not improbable, is
muddy, some method of filtering it before pumping it into the boilers
ought, if at all practicable, to be resorted to, for the twofold reason
of preserving the boiler plates from muddy deposit, and also to prevent
priming, which would certainly ensue from the use of muddy water. No
doubt the medical staff take care that the distilled water is alike
thoroughly aerated and efficiently filtered. The most successful method
of aerating is, we believe, to cause the current of steam as it enters
the condenser to suck in air by induced current along with it. The
filtering ought not to present any difficulty, as at all events sand
enough can be had. Charcoal, however, is another affair, and all
distilled water ought to be brought into contact with this substance.
Simple, however, as such an arrangement as this appears to be, practical
difficulties, which it is _said_ are insurmountable, stand in the way of
its adoption, and the distilled water produced for Egypt is made in
special apparatus, and various forms of condenser are employed, made
under various patents. The principle involved is, however, in all cases
the same. Steam is generated in one of the ships' boilers, and condensed,
filtered, and aerated in a special apparatus. The great objection to the
use of the ordinary surface condenser is that the main engines would, in
the majority of cases, have to be kept going, in order to pump the
distilled water out of the condenser, and to supply circulating water.
But it is easy to see that if engineers thought proper, this difficulty
could be readily got over. Separate circulating pumps, usually
centrifugal, are now freely used, and the addition of a special pump for
lifting the condensed water presents no difficulty whatever. While the
main engines are running, the withdrawal of much condensed water would
no doubt risk the safety of the boiler; but in the case of so-called
"distilling" ships, there need be no trouble incurred on this
score.--_The Engineer_.
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