USEFUL NOTES ON THE MARINE STEAM ENGINE.

From the principle that Heat and Work are mutually convertible.

The construction of a perfect Engine, by which we mean one which can reconvert the whole of the heat developed by mechanical energy into work, is impossible.

It is estimated that a pound of ordinary coal is capable of producing nearly 10,000,000 foot pounds or units of work. Whereas, the best performance of a pumping engine on shore does not exceed 1,000,000 foot pounds or units of work per pound of coal. From careful experiment it has been shown that with an engine of good design and workmanship, and working with a low consumption of fuel, very little over ten per cent. of the total amount of heat parted with by the steam is converted into useful work; over 76 per cent is imparted to the condensing water, and the remainder diffused by radiation, &c.

As a rule we may assume that the more distant the extremes of temperature between the boiler and the condenser the larger will be the proportion of heat turned into work. In the steam engine certain quantity of power of doing work is expended, in order that a certain amount of work may be done, and the proportion which the useful work done, bears to that expended, is called the efficiency.

Causes of Waste Heat:—

1st.—The heat lost by the imperfect combustion of the fuel supplied to the furnace, loss by conduction and radiation, and by the high temperature at which the furnace gas escapes by the funnel. This latter loss is estimated at from from 700 to 800° Fahrenheit.

* From 300 to 600° Fahr. preferable.

A pound of carbon burnt into carbonic oxide gas evolves but about ths of the heat which is produced, if it is burnt into carbonic acid; therefore, for each pound of carbon allowed to escape in the form of carbonic oxide, ths of the heating effect which might have been obtained by its complete combustion in the furnace is wasted.

2nd. The heat carried away by the steam when it leaves the cylinder.

3rd. The heat power necessary to overcome the friction of the engine, &c., and that wasted in agitating the water in which the propeller works.

To, in any way, diminish this waste is the object of improvement in the economy of the marine steam engine.

The work done by the steam driving the piston may be called the indicated work, being registered by a self-acting instrument called the indicator.

All mechanical work is done by the exertion of a force through space, and is calculated and expressed as a quantity by multiplying the mean amount of the force into the space through which it acts.

P.V.

FORMULA FOR HORSE POWER = 33000

Where P equals the mean effective force of the steam on the area of the piston in square inches, the mean effective pressure being obtained from a diagram taken by the indicator; and V equals the velocity of the piston in feet per minute, sweeping through the cylinder; and 33000 the number adopted by James Watt to represent in foot pounds the work of one horse in a minute of time, i.e., 33000 pounds raised through a space of one foot per minute equivalent to one horse-power.

The force acting upon the piston of a steam engine is the excess of the forward pressure above the back pressure exerted by the steam behind the piston as it comes through the regulating valves from the boiler. There are two stages in the action of the steam-the admission and expansion. During the admission, the steam is coming from the boiler into the cylinder, and it exerts a pressure less than

that in the boiler, only by the amount to overcome the friction of the pipes, passages, and valve ports. The admission is terminated by the cut off, that is, by the closing of the valve which admits the steam into the cylinder. Then follows the expansion of the steam which is confined in the cylinder, as it drives the piston before it and exerting a continually diminishing pressure. For ordinary practical calculation we assume that the pressure varies inversely as the volume P∞ It is clear that work continues to be done by the steam driving the piston so long as the pressure behind the piston, or forward pressure, continues to be greater than the pressure in front, or the back pressure, exerted by the steam which has already done its work, and which the piston is expelling from the cylinder. Hence, the higher the forward pressure and the lower the back pressure, the greater is the efficiency of the steam in an engine. The greatest useful work is obtained by making the expansion cease when the forward pressure is just equal to the back pressure, added to the pressure equivalent to the friction to be overcome.

The pressure at which the steam should be used in the promotion of the economy of fuel is limited only by the strength of the boiler. Compound engines-by which is meant one in which the mechanical action of the steam commences in a smaller cylinder, and is completed in a larger cylinder before passing to the condenser, giving a wider range for expansion are therefore more efficient for long voyages at a high speed, the efficiency of the steam being sometimes represented by an expansive force acting upon the piston from about 60lbs. per square inch above the atmospheric pressure, to from 7 to 8 lbs. below the atmospheric pressure, before being expelled to the condenser, effecting a large economy of fuel in proportion to the power used.

Heavier cylinders and a larger space are required for compound engines, but the forces acting upon the piston, and therefore throughout the working machinery, may be more equally distributed throughout the stroke, than with a simple cylinder engine, and at the same time to compensate for such increased weight in proportion to power, the boilers and surface condensers may be considerably reduced in size.

The average amount of coal burnt in the different kinds of engines under favourable circumstances would be for

Compound, or double cylinder, 2lbs.

expansive engines

Single cylinders, with surface

per indicated horse

power per hour.

superheaters 3 to 3 lbs.

and improvements

condensers,

The increased pressure used with compound engines requires the boiler to be of a circular form to reduce the number of stays for strengthening within the shell and to render the interior accessible. Boilers are made of the circular form, with tubes returning over the furnace, dry uptakes, and working when new from 50 to 60lbs. pressure per square inch above that of the atmosphere.

er

of brass tubes

Heating surface, per Nom. H.P.

Fire grate surface

Weight of water

Boilers proved with water pressure
at per square inch

0 full.

18 sq. ft.

.7

2 cwt.

}

1401 230

lbs.

Surface condensers, cooling surface in
small tubes, per N.H.P.

Aggregate length for every 1000 from 10 to

The weight of the boiler-about one ton for every 16 I.H.P. The weight of the water-about one ton for every 40 I.H. P. The ordinary square form of boiler, with the tubes returning over the furnaces, working (when new) at a pressure of about 30 lbs. per square inch. The thickness of plates would be about—

Real power of the engine, from 6 to 7 times the nominal. proved at 60 lbs. by water pressure.

BOILER TUBES-Diameter

Boilers

Length
Thickness

from 2 to 3 inches.

6 to 6 feet.

inch.

IN CONDENSERS communicating with the boiler for the purpose of providing drinking water, on the average one ton of coals will generate enough steam to make a little more than 7 tons of water.

The Indian troop-ships while running allow one ton of coals for seven tons of water condensed.

When condensing only, the opportunity should be taken to burn up the very small coal, &c.

Working Boilers with Sea Water.

When sea water is used it is not advisable in practice to allow the ater in the boiler to contain more than a double charge of salt, or lbs. of salt to 32 lbs of water. To maintain this, then it would be