Scientific American Suppl. No. 299
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Various >> Scientific American Suppl. No. 299
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In being applied to the slubber a breakage either at the front or back
can be arranged for. Upon intermediates the breakage of either one of
the strands, if the machine was running two into one, from the creel to
the roller, would cause the stoppage of the machine, or the breaking or
tangling of ends between the front roll and the nose of the flier.
There are many other places where this motion can be applied. With
mechanical means we require motion; with electricity we require simple
contact of two differently arranged surfaces, and this can always be
had by letting the cotton drop out from between the rollers; no radical
changes are necessary, and we are glad to find that this electrical
attachment is meeting with a very good success in England, France,
and, so far, in the United States, and, undoubtedly, further and more
extended opportunity will be found for this application.--_Textile
Record_.
* * * * *
ON THE PROGRESS AND DEVELOPMENT OF THE MARINE ENGINE.
[Footnote: A paper recently read before the Society of Mechanical
Engineers by F.C.Marshall.]
The author began by referring to a paper read at the Liverpool meeting
in 1872, by Mr. F. J. Bramwell, F.R.S., on "The Progress effected in
Economy of Fuel in Steam Navigation, considered in Relation to Compound
Cylinder Engines and High-pressure Steam;" then proceeded to continue
the subject from the date of that meeting, to trace out whether any, and
if so what, progress had been made; further, to consider whether or no
we have reached the finality so strongly deprecated by Sir Frederick
Bramwell in the discussion referred to, and, if not, then in what
direction we are to look for further development.
From a table it would seem that the steam pressures are now much higher,
the boilers have less heating surface, and the cylinders are much
smaller for the indicated horsepower developed than in 1872; and at the
same time the average consumption of fuel is reduced from 2.11 lb. to
1.828 lb., or by 13.38 per cent.
MARINE ENGINES.
The author then briefly described the modern marine engine and boiler.
The three great types of compound engines may be placed as follows in
the order of their general acceptance by the shipowning community: (1)
The two-cylinder intermediate-receiver compound engine, having cranks at
right angles. (2) The Woolf engine in the tandem form, having generally
the high-pressure and low-pressure cylinders in line with each other,
but occasionally alongside, and always communicating their power to one
crank. Such a pair of engines is used sometimes singly, oftener two
pairs together, working side by side to cranks at right angles; recently
three pairs together, working to cranks placed 120 deg. apart. The
system affords the opportunity of adding yet more engines to the
same propeller to an indefinite extent. (3) The three cylinder
intermediate-receiver compound engine, with one high and two
low-pressure cylinders, the steam passing from the high-pressure
cylinder into the receiver, and thence into the two low-pressure
cylinders respectively. The cranks are placed at equal angles apart
round the crank shaft, so as to balance the forces exerted upon the
shaft.
These three types may be said to embrace all the engines now being
manufactured in this country for the propulsion of steam vessels by the
screw propeller. In their leading principles they also embrace
nearly all paddle engines now being built, whether the cylinders be
oscillating, fixed vertically, or inclined to the shaft.
The compound engine, in fact, in one of these three forms, may now be
said to be universally adopted in this country; and the question of the
relative value of simple expansion in one cylinder, and of compound
expansion in two or more cylinders, which agitated the minds of some of
our leading engineers ten years ago, is now practically solved in favor
of the latter.
THE MARINE BOILER.
The marine boiler of to-day is in all its main features the same as it
was ten years ago. The single-ended boiler, made with two, three, and
sometimes four furnaces, is the simplest form, and for all powers
under 500 indicated horse power is the most generally adopted. The
double-ended form is largely used. It has been found more economically
efficient than the single-ended form, by as much as ten per cent, in the
writer's own experience. It is generally adopted for engines of large
power, but for small power is inconvenient, owing to its occupying more
room lengthwise in the vessel, and also involving two stokeholds and
therefore more supervision. At one time great difficulty was found
in keeping the bottoms of boilers of this kind tight. Owing to their
length, the unequal expansion due to different temperatures at the
top and bottom caused severe racking strains on the bottom seams and
riveting--so severe in some cases as to rend the plating for a large
part of the bottom circumference of the shell. This difficulty has now
been to a large extent got over, in consequence of the greater attention
given to the form and direction of the water spaces in the boiler
itself, so as to induce circulation of water; the introduction of the
feed-water at the top instead of near the bottom; the more careful
management now usual on the part of engineers; and lastly, the use of
larger plates, welded horizontal seams, drilled rivet holes, and more
perfect workmanship throughout. A modification of double-ended boiler is
that introduced by Mr. Alfred Holt. It has many decided advantages,
but is costly to make. The formation of the two ends into separate
fire-boxes leaves the bottom of the boiler free to adapt itself to the
variations of temperature to which it is exposed. The separation of the
furnaces from the combustion chamber, excepting through the opening
afforded by a connecting tube, is an advantage in the same direction,
and avoids almost entirely the racking strains due to irregular furnace
action. The weight of water carried is less, and that of the boiler
may also be made less; while the elliptical form of the two ends gives
greater steam space.
A type of boiler largely used in her Majesty's Navy, somewhat like a
locomotive boiler, is highly efficient in regard to weight and power
developed. Many examples have yielded one indicated horse-power in the
cylinders for every three square feet of heating surface, under natural
draught and with a very moderate height of funnel; and this with a
consumption of fuel not exceeding 21/2 lb. per indicated horse-power per
hour under a working pressure of 60 lb. With the aid of a steam jet in
the funnel, the heating surface per indicated horse-power has fallen
below 21/2 square feet. The large water surface afforded for escape
of steam secures almost entire freedom from priming, without the
incumbrance of steam domes; and the large combustion chamber allows of
the thorough combustion of the gases before their passage through the
tubes. The locomotive type of boiler has lately occupied the writer's
attention, with a view to its more definite introduction into marine
work. The difficulties, however, which lie in the way of applying it to
steamers going long voyages are very great. The principal difficulty
lies in the necessity of burning a large quantity of fuel in a very
limited space and time. This can only be done either by direct pressure
or exhaust action applied at the furnace. In other words, we must either
exhaust the funnel, which will absorb a large amount of power, but would
be comparatively easy of application; or our stokers, as is the case
with our miners, must work under a pressure of air.
STEEL BOILERS.
The writer stated that his experience in the manufacture and working of
steel boilers was satisfactory. Many steel boilers of sizes varying from
six feet diameter to fourteen feet six inches diameter have left the
works at St. Peter's since 1877, when the first was made; and in no
case has there been a failure of a plate after being put into a boiler,
either in the process of manufacture or in working at sea. The mode
of working is as follows: For shell plates, from five-eighths inch
to seven-eighths inch thick, to warm each to a dark red heat before
rolling, having previously drilled a few holes to template for bolting
the strakes together; the longitudinal seams are usually lap joints
treble riveted, requiring the corners to be thinned, which is done after
rolling. The furnace plates are generally welded two plates in length,
and flanged to form Adamson rings, and at the back end to meet the tube
plate; the back flame-box plates are flanged, also the tube plates and
front and back plates; and wherever work is put on to the plate it
is annealed before going into the place. The rivet holes are drilled
throughout. In the putting together the longitudinal seams of the
thicker plates of the shells, great care is always taken to set the
upper and under plates for the lap to their proper angle before they
are bolted together, a point generally overlooked by the practical
boilersmith.
CORROSION OF BOILERS.
The question of corrosion is one which is gradually being answered as
time goes on; and so far very satisfactorily for steel. Some steel
boilers were examined a few weeks ago which were among the first made;
and the superintending engineer reports: "There is no sign of pitting
or corrosion in any part of the boiler; the boilers are washed out very
carefully every voyage, and very carefully examined, and I cannot trace
anything either leaking or eating away. No zinc is used, only care in
washing out, drying out, and managing the water." This is the evidence
of an engineer with a large number of vessels in his charge. On the
other hand, some of the most prominent Liverpool engineers always use
zinc, and take care to apply it most strictly. The evidence of one
of them is as follows: "We always fix slabs of zinc to most boilers,
exposing not less than a surface of one square foot for every twenty
indicated horse-power, and distributed throughout the boiler. This zinc
we find to be in a state of oxide and crumbling away in about three
months. We then renew the whole, and find this will last twelve months
or more, when it is renewed again. Meanwhile we have no pitting and no
corrosion; but on the contrary, the interior surfaces appear to have
taken a coating of oxide of zinc all over, and we have no trouble with
them."
HOW THE MARINE ENGINE MAY BE IMPROVED.
Then the writer considered our present marine engine as to its
efficiency and capability of further improvement. The weight of
machinery, water, and fuel carried for propelling ships has not had due
attention in the general practice of engineers. By the best shipping
authorities the writer is assured that every ton of dead weight capacity
is worth on an average L10 per annum as earning freight. Assuming,
therefore, the weight of the machinery and water of any ordinary vessel
to be 300 tons, and that, by careful design and judicious use of
materials, the engineer can reduce it by 100 tons, without increasing
the cost of working, he makes the vessel worth L1,000 per annum more to
her owners. That there is much room for improvement in this direction is
shown by the following statement, giving, for various classes of ships,
the average weight of machinery, including engines, boilers, water, and
all fittings ready for sea, in pounds, per indicated horse power:
Lb. per I. H. P.
Merchant steamers.......................... 480
Royal Navy................................. 300
Engines specially designed for light draught
vessels...................................280
Royal Navy, Polyphemus class (given by Mr.
Wright).................................. 180
Modern locomotive.......................... 140
Torpedo vessels............................. 60
Ordinary marine boilers, including water... 196
Locomotive boilers, including water......... 60
The ordinary marine boiler, encumbered as it is by the regulations of
the Board of Trade and of Lloyds' Committee, does not admit of much
reduction in the weight of material or of water carried when working.
The introduction of steel has reduced the weight by about one-tenth; but
it will be the alteration of form to the locomotive, tubulous, or some
other type, combined with some method of forced draught, to which we
must look for such reductions in weight of material and water as will be
of any great commercial value. The engine may be reduced in weight by
reducing its size, and this can only be done by increasing the number of
revolutions per minute.
It has hitherto been the practice to treat the propeller as dependent
upon the size of engines, draught of water, and speed required. This
process should be reversed. The propeller's diameter depends on the
column of water behind necessary to overcome the resistance in front of
it due to the properties of the vessel. This fixed, the speed will then
fix the number of revolutions, which will be found much greater than is
usual in practice, and from this the size of the engines and boilers
will be determined. Great saving in weight can be effected by careful
design and judicious selection and adaptation of materials, also by the
substitution of trussed framing and a proper mode of securing the engine
to the structure of the vessel, as worked out in H.M.S. Nelson, by Mr.
A. C. Kirk, of Glasgow, and in the beautifully designed engines by Mr.
Thornycroft, in place of the massive cast-iron bedplates and columns of
the ordinary engines of commerce. The same may be said of the moving
parts. In fine, the hull and engines should be as much as possible one
structure; rigidity in one place and elasticity in others are the
cause of most of the accidents so costly to the ship-owner; under such
conditions mass and solidity cease to be virtues, and the sooner their
place is taken by careful design, and the use of the smallest weight
of material--of the very best kind for the purpose--consistent with
thorough efficiency, the better for all concerned.
CONSUMPTION OF FUEL IN MARINE ENGINES.
Coming to the question of the consumption of fuel, a considerable saving
has been effected in nine years, as shown in the following table:
Item. 1872. 1881.
Working pressure, lb. per sq. in......... 52.5 77.4
Heating surface per I. H. P., sq. ft.... 4.64 3.919
Piston speed, feet per min.............. 376 467
Coal burnt per I. H. P., lb.............. 2.11 1.828
This shows a saving equal to 13.38 per cent, in quantity of fuel
consumed. Mr. Marshall then read a letter from Mr. Alfred Holt, of
Liverpool, bearing on this subject, in which Mr. Holt spoke favorably of
the single-crank engine, and stated his belief that the compound system
would ere long be abandoned for the simple engine. He is endeavoring to
feel his way to using the steam in one cylinder only, and so far the
results have been encouraging, and he is now fitting a 2,200-ton vessel
on that system. He is also endeavoring to do without a crank shaft, the
forward end of the screw shaft carrying an ordinary crank with overhung
pin. This experiment also promises satisfactorily. In his opinion the
great improvement of the immediate future is to increase the steam
production of our boilers. A ton weight of a locomotive boiler produces
as much steam as six tons of an ordinary steamboat boiler.
Mr. Holt speaks of the coal account as one of the minor disbursements
of a steamer. He does not give the ratio which coals bear to the total
disbursements, but from other reliable sources Mr. Marshall found that,
according to the direction of the voyage, it varies from 16 to
20 percent.--or, say, an average of 18 per cent.--of the total
disbursements, in a vessel carrying a cargo of 2,500 tons. This will
represent to-day about L3,000 per annum, and in 1872, at equal prices,
the cost would have been L3,750--showing a saving of L750, equal to a
dividend of, say, 3 per cent. on the value of the ship. Again, the cost
of coal per mile run for such a vessel, in 1872, would have been at
least 161/2d.; to-day it does not exceed 13d.
EVAPORATIVE EFFICIENCY OF MARINE BOILERS.
The marine boiler as now made is very efficient, but if the quantity of
steam used be considered in relation to the increased pressure, it will
be seen that the boiler of to-day is little if anymore efficient than
that of ten years ago. The present boiler has an evaporative efficiency
of about 75 per cent., and cannot be much improved so long as air
is supplied to the furnace by the natural draught. To increase the
efficiency from 75 to 82.5 per cent. would require about double the
heating surface, the weight of boiler and water being also doubled,
while the gain would be only 10 per cent. Mr. Blechynden's formula, used
in Mr. Marshall's works for weights of cylindrical marine boilers of the
ordinary type, and for pressures varying from 50 lb. to 150 lb., is as
follows:
W = (P + 15) (S + D squared L) / C
or W = 2S (P + 15) / C
when S = D squared L, which is a common proportion.
Here W = weight in tons.
P = working pressure as on gauge.
S = heating surface, in square feet.
D = diameter, in feet.
L = length, in feet.
C = a constant divisor, depending on the class of
riveting, etc. For boilers to Lloyds' rules,
and with iron shells having 75 per cent.
strength of solid plate, C = 13,200.
This formula, if correct--and it is almost strictly so--would give the
relative weight of boilers per sq. ft. of heating surface, for 105 lb.
and 150 lb. total pressure, assuming we wish to increase the efficiency
10 per cent, as follows:
Weight at 105 lb. = 105 x 1 / C
Weight at 150 lb. = 150 x 1.75 / C = 263 / C
Hence the ratio of weight = 263 / 105 = 2.5
In other words, the boiler with the higher efficiency would weigh two
and a half times that with the lower efficiency. In the case of a vessel
of 3,000 tons, with engines and boilers of 1,500 indicated horse power,
the introduction of locomotive boilers with forced draught would place
at the disposal of the owner 150 tons of cargo space, representing
L1,500 per annum in addition to the present earnings of such a vessel.
MARINE LOCOMOTIVE BOILERS.
Mr. Thornycroft has for some years used the locomotive form of boiler
for his steam launches, working them under an air pressure--produced
by a fan discharging into a close stokehold--of from 1 in. to 6 in. of
water, as may be required. The experiments made gave an evaporation of
7.61 lb. of water from 1 lb. of coal at 212 deg. Fahr., with 2 in. of
water pressure, and 6.41 lb. with 6 in. of pressure. These results are
low, but it is to be remembered that the heating surface is necessarily
small, in order to save weight, and the temperature of the funnel
consequently high, ranging from 1,073 deg. at the first pressure, and
1,444 deg. at the 6 in. With the ordinary proportions of locomotive
practice the efficiency can be made equal to the best marine boiler
when working under the water pressure usual in locomotives, say from
3 in. to 4 in., including funnel draught.
It has fallen to the lot of the writer to fit three vessels recently
with boilers worked under pressure in closed stokeholds. The results,
even under unfavorable conditions, were very satisfactory. The pressure
of air would be represented by 2 in. of water, and the indicated horse
power given out by the engines was 2,800, as against 1,875 when working
by natural draught, or exactly 50 per cent. gain in power developed.
Mr. Marshall then proceeded to refute the arguments which may be urged
against the use of the locomotive boiler at sea, and which we need not
reproduce. Coming to the engines, Mr. Marshall said that the total
working pressure of to-day may be accepted as 105 lb., or equal to seven
atmospheres. If it were boldly accepted that eleven atmospheres, or 165
lb., were to be the standard working pressure, the result would be a
gain of 14.55 per cent., provided no counteracting influence came into
play. Of course, there are forces which war against the attainment of
the full extent of this advantage, viz., the greater condensation in the
cylinders and loss in the receiver or passages.
In regard to the former, it may be questioned whether by steamjacketing
the high pressure cylinder, correctly proportioning the steam passages,
and giving a due amount of compression in both cylinders, this may not
be reduced far below the generally received notion; and the latter cause
of loss may be considerably reduced in its effect by a more carefully
chosen cylinder ratio. The ratio usually adopted, between 3.5 and 4 to
1, whether the pressure be 70 lb. or 90 lb., may well be questioned.
With a cylinder ratio of 2.95 to 1, the economic performance is very
good, and equal to any with the higher ratio. A lower cylinder ratio has
another advantage of considerable value, viz., that the working pressure
can be much reduced as the boilers get older, while by giving a greater
amount of steam the power may be maintained--at an extra cost of steam,
of course, but not so great a cost as with higher ratios. The cut-off
in the high-pressure cylinder usually takes place at about 0.6, and
the ratio of expansion has decided the ratio of cylinders. The use of
separate starting valves in both cylinders obviates that necessity.
The difficulties in the way of taking advantage of the higher economic
properties of greater pressures than hitherto used on board ship, are,
it is submitted, not insuperable, and it would be to the interest of all
that they should be firmly and determinedly met. It may be accepted as
an average result that the Woolf engine, as usually arranged, will use
10 per cent. more steam than the receiver engine for the same power.
Of the three-cylinder receiver type the data are insufficient to form a
definite opinion upon; but so far the general working of the Arizona is
stated to be as good, economically, as any of the two-cylinder receiver
class. The surface condenser remains as it was ten years ago, with
scarcely a detail altered. In most engines it remains a portion of the
framing, and as such adds greatly to the weight of the engine.
It is a question seriously worth consideration whether or no the surface
of tubes can be reduced. The practice at present is to make the surface
one-half the boiler surface as a minimum, that is, equal to about 2
square feet per indicated horse power. In practice, the writer has found
1.4 square feet per indicated horse power to maintain a steady vacuum of
271/2 inches.
Mr. Marshall has just completed six pairs of engines for three twin
screw ships, having steel shafts of 10 inches diameter, and has in each
case run the engines at 120 revolutions per minute, while indicating
1,380 horse power from each pair for ten to fifteen hours without
stopping; and in no case has a single bearing or crank pin warmed or
had water applied, the surfaces on examination being perfect. In these
engines all working bolts, pins, and rods, except the piston and
connecting rods, are of steel, all rods in tension being loaded to 8,000
lb. per square inch. The boilers are of the Navy type, made throughout
of Siemens-Martin steel plates, riveted with steel rivets, all holes
drilled. Furnaces are welded and flanged; the tubes are of brass. In
comparison with an ordinary merchant steamer's iron boilers of the
double ended type, they weigh, including water and all appurtenances, as
follows:
Double ended Type. Navy Type.
Weight, tons............ 135 ........... 146
I. H. P................. 1,400 .......... 2,760
Draught................ Natural ......... Forced.
SCREW PROPELLERS.
The screw propeller is still to a great extent an unsolved problem. We
have no definite rule by which we can fix the most important factor of
the whole, namely, the diameter. Mr. Froude has pointed out that by
reducing the diameter, and thus the peripheral friction, we can increase
the efficiency; and this is confirmed by cases--of Iris reduced 2 feet
3 inches, and the Arizona reduced 2 feet. This must, of course, be
qualified by other considerations. The ship has by her form a definite
resistance, and a certain speed is required; if the propeller be made
too small in diameter, the ship will not be driven at the required
speed, except at serious loss in other directions. This question was too
large and complicated to be dealt with here, and should, in the first
instance, be made the subject of careful and extended experiment, on
which a separate paper should be written.
To sum up the whole. Progress has been made during the past nine years,
and in the following particulars:
1. The power of the engines made and making show a great increase. 2.
Speeds hitherto unattainable are now seen to be possible in vessels of
all the various classes. 3. The consumption of fuel is reduced by 13.38
per cent. on the average; and numbers of vessels are now working on much
less coal than that average, while the quality of the coal is in nearly
all cases very inferior, so that it is not unfair to take credit for
20 per cent. reduction. 4. The working pressures of steam are much
increased on the average, and are still increasing; many steamers now
being built for 120 lb. per square inch, while 90 lb. is the standard
pressure now required.
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