GENERAL CONSIDERATIONS AND INSTRUCTIONS.

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surface will boil off a cubic foot of water in the hour, and this in the older class of engines was considered the equivalent of a horse power. At the atmospheric pressure, or with no load on the safety valve, a cubic inch of water makes about a cubic foot of steam; and at twice the atmospheric pressure, or with 15 lbs. per square inch on the safety valve, a cubic inch of water will make about half a cubic foot of steam. For every half cubic foot of such steam therefore abstracted from the boiler there must be a cubic inch of water forced into it. So if we take the latent heat of steam in round numbers at 1,000 degrees, and if the condensing water enters at 60°, and escapes at 100°, the condensing water has obviously received 40 degrees of heat, and it has received this from the steam having 1,000° of heat, and the 112° which the steam if condensed into boiling water would exceed the waste-water in temperature. It follows that in order to reduce the heat of the steam to 100° there must be 1,112° of heat extracted, and if the condensing water was only to be heated 1 degree, there would require to be 1,112 times the quantity of condensing water that there is water in the steam. Since, however, the water is to be heated 40°, there will only require to be one-fortieth of this, or about th the quantity of injection water that there is water in the steam. These rough determinations will enable the principle to be understood on which such proportions are determined. The proportions of the condenser and of the air-pump were determined by Mr. Watt at one-eighth of the capacity of the cylinder. In more modern engines, and especially in marine engines where there are irregularities of motion, the air-pump is generally made a little larger than this proportion, and with advantage. The condenser is also generally made larger, and many engineers appear to consider that the larger the condenser is the better. Mr. Watt, however, found that when the condenser was made larger than one-eighth of the capacity of the engine the efficiency of the engine was diminished. The fly-wheel employed in land engines to control the irregularities of motion that would otherwise exist, is constructed on the principle that there shall be a revolving mass of such weight, and moving with such a velocity, as to

constitute an adequate reservoir of power to redress irregularities. It is found that in those cases where the most equable motion is required, it is proper to have as much power treasured up in the fly-wheel as is generated in 6 half-strokes, though in many cases the proportion is not more than half this. It is quite easy to tell what the weight and velocity of the fly-wheel must be to possess this power. When we know the area of the piston and the unbalanced pressure per sq. inch, we easily find the pressure urging it, and this pressure multiplied by the length of 6 half-strokes represents the amount of power which, in the most equable engines, the fly-wheel must possess. Thus, suppose that the pressure on the piston were a ton, and that the length of the cylinder were 5 feet, then in 6 half-strokes the space described by the piston would be 30 feet. The measure of the power therefore is 1 ton descending through 30 feet, and if there were any circumstance which limited the weight of the flywheel to 1 ton, then the velocity of the rim-or more correctly of the centre of gyration-must be equal to that which any heavy body would have at the end of the descent by falling from a height of 30 feet, and which velocity may easily be determined by the rule already given for ascertaining the velocity of falling bodies. If the weight of the fly-wheel can be 2 tons, then the velocity of the rim need only be equal to that of a body falling through 15 feet, and so in all other proportions, so that the weight and velocity can easily be so adjusted as to represent most conveniently the prescribed store of power.

With these preliminary remarks it will now be proper to proceed to recapitulate the rules for proportioning all the parts of steam engines illustrated by examples :—

STEAM PORTS.

The area of steam port commonly given in the best engines working at a moderate speed is about 1 square inch per nominal horse-power, or th of the area of the cylinder, and the area of the steam pipe leading into the cylinder is less than this, or '66 square inch per nominal horse power. Since however engines

PROPER AREAS OF CYLINDER PORTS.

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are now worked at various rates of speed it will be proper to adopt a rule in which the speed of the piston is made an element of the computation. This is done in the rules which follow both for the steam port and branch steam pipe.

TO FIND THE PROPER AREA OF THE STEAM OR EDUCTION PORT OF THE CYLINDER.

RULE.-Multiply the square of the diameter of the cylinder in inches by the speed of the piston in feet per minute and by the decimal 032, and divide the product by 140. The quotient is the proper area of the cylinder port in square inches. Example.-What is the proper area of each cylinder port in an engine with 64-inch cylinder, and with the piston travelling 220 feet per minute?

Here 64 × 644,096, which multiplied by 220 = 901,120, and this multiplied by ⚫032 = 28,835.8, which divided by 140, gives 206 inches as the area of each cylinder port in square inches.

This is a somewhat larger proportion than is given in some excellent engines in practice. But inasmuch as the application of lap to the valve virtually contracts the area of the cylinder ports, and as the application of such lap is now a common practice, it is desirable that the area of the ports should be on the large side. In the engines of the 'Clyde,' 'Tweed,' 'Tay,' and 'Teviot,' by Messrs. Caird and Co., the diameter of the cylinder was 74 inches, and the length of the stroke 73 feet, so that the nominal power of each engine was about 234 horses. The cylinder ports were 33 inches long and 6 inches broad, so that the area of each port was 224-4 square inches, being somewhat less than the proportion of 1 square inch per nominal horse power, but somewhat more than the proportion of th of the area of the cylinder. As the areas of circles are in the proportion of the square roots of their respective diameters, the area of a circle of one-fifth of the diameter of the piston will have one-twenty-fifth of the area of the piston. One-fifth of 748ths is 15 nearly, and the area of a circle 15 inches in diameter is 176-7 square inches, which is considerably less than the actual

area of the port. By the rule we have given the area of the ports of this engine would, at a speed of 220 feet per minute, be about 277 square inches, which is somewhat greater than the actual dimensions. At a speed of the piston of 440 feet per minute the area of the port would be double the foregoing.

STEAM PIPE.

In the engines already referred to, the internal diameter of each steam pipe leading to the cylinder is 13 inches, which gives an area of 145.8 square inches. It is not desirable to make the steam pipe larger than is absolutely necessary, as an increased external surface causes increased loss of heat from radiation. The following rule will give the proper area of the steam pipe for all speeds of piston:

TO FIND THE AREA OF THE STEAM PIPE LEADING TO EACH CYLINDER.

RULE.-Multiply the square of the diameter of the cylinder in inches by the speed of the piston in feet per minute and by the decimal 02, and divide the product by 170. The quotient is the proper area of the steam pipe leading to the cylinder in inches.

Example.-What is the proper area of the branch steam pipe leading to each cylinder in an engine with a cylinder 744 inches diameter, and with the piston moving at a speed of 220 feet per minute?

Here 74.5 x 74.5 = 5,550.25, which multiplied by 220 = 1,221,055. and this multiplied by '02 = 24,421·1, which divided by 170 144 square inches nearly. The diameter of a circle of 144 square inches area is a little over 134 inches, so that 131 inches would be the proper internal diameter of each branch steam pipe in such an engine. The main steam pipe employed in steamers usually transmits the steam for both the engines to the end of the engine-house, where it divides into two branches-one extending to each cylinder. The main steam pipe will require to have nearly, but not quite, double the area of each of the branch steam pipes. It would require to

PROPER AREA OF SAFETY VALVES.

219 have exactly double the area, only that the friction in a large pipe is relatively less than in a small; and as, moreover, the engines work at right angles, so that one piston is at the end of its stroke when the other is at the beginning, and therefore moving slowly, it will follow that when one engine is making the greatest demand for steam the other is making very little, so that the area of the main steam pipe will not require to be as large as if the two engines were making their greatest demand at the same time.

SAFETY VALVES.

It is easy to determine what the size of an orifice should be in a boiler to allow any volume of steam to escape through it in a given time. For if we take the pressure of the atmosphere at 15 lbs., and if the pressure of the steam in the boiler be 10 lbs. more than this, then the velocity with which the steam will flow out will be equal to that which a heavy body would acquire in falling from the top of a column of the denser fluid that is high enough to produce the greater pressure to the top of a column of the same fluid high enough to produce the less pressure, and this velocity can easily be ascertained by a reference to the law of falling bodies. In practice, however, the area of safety valves is made larger than what answers to this theoretical deduction, partly in consequence of the liability of the valves to stick round the rim, and because the rim or circumference becomes relatively less in the case of large valves. One approximate rule for safety valves is to allow one square inch of area for each inch in the diameter of the cylinder, so that an engine with a 64-inch cylinder would require a safety valve on the boiler of 64 square inches area, which answers to a diameter of about 9 inches. The rule should also have reference, however, to the velocity of the piston, and this condition is observed in the following rule:

TO FIND THE PROPER DIAMETER OF A SAFETY VALVE THAT WILL LET OFF ALL THE STEAM FROM A LOW PRESSURE BOILER.

ROLE.-Multiply the square of the diameter of the cylinder in inches by the speed of the piston in feet per minute, and