Chap. XXI. STEAM CONSIDERED AS A MOVING TOWER.
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The most certain and safe method for low-pressure boilers is to balance the pressureof the steam bv a column of water, of a diameter sufficient to allow its escape ; theheight of the tube for different measures may be found readily, by considering that acolumn of water equivalent to one pound on a circular inch is for common temperature3*1 feet; for pressure of 4 pounds on the circular inch it will be four times this, or12-4 feet, which will be equal to a little more than 5 pounds per square inch : forhigh-pressure boilers the most careful attention to security is necessary, and seriousaccidents have occurred for the want of it, and it is obvious that the aperture by which thesteam is to escape ought to be of a size that it may freely have vent, as fast as it isgenerated in the boiler: as l£ feet of fire-surface is sufficient to convert a cubic foot ofwater into steam, it is necessary for the purposes of security to calculate upon only 1 footof surface being sufficient. If the density of the steam corresponding to the pressure befound, multiplied by 7*5, and this product again multiplied by the square root ofthe quantity, if the density is greater than 1, and the feet of fire-surface divided by the
product; the quotient will be the square of the diameter of the movement part of the
valve in inches.
Fusible metal plugs have been used as the means of forming safety-vents to steam boilers,which, by melting at certain temperatures, suffer the steam to escape, and alloys which meltfrom 212° to 600° of heat have been applied, but this kind of plug cannot be relied uponin practice.
Chimneys, and their Area for Steam-engine Boilers , form our next consideration. Thefire-grate should have one superficial foot area for each horse-power ; one-fifth of the area ofthe fire-grate gradually diminishing to a chimney, which shall have one-tenth the area of
the fire-grate, is admitted to be an excellent proportion : the chimney should be of one
diameter throughout its height, and if 40 feet, and its area one-tenth that of the fire-grate,its draught will be sufficient; when the height of the chimney exceeds this, the area maybe diminished as the square root of the height is increased. For a low-pressure enginethe area of the chimney, when above 10 horse-power, should be 112 times the horse-power,divided by the square root of the height of the chimney: when less than 10 horse-powerits dimensions may be made less in proportion.
When engines are worked in the best manner, they require only 9 to 11 pounds of thebest coal per hour for each horse-power, when the power of the engine exceeds ten horse.The flue must be proportionally increased when the smoke is the result of a consumptionof a greater quantity of coal; when wood is used, there is a larger quantity of smoke, butas it is much lighter than that produced by coal, about one and half times the areanecessary for coals will be ample. When circular chimneys are used, they should not belarger than is necessary to give effect to the fuel; this will be obtained when the square ofthe diameter is equal to 90, multiplied by the horse-power, and divided by the square rootof the height in feet.
Piston Rods , their Strength _The height of a column of water at a temperature of 60°,
which presses with the force of one pound upon a square inch, is 2*31 feet, and upon acircular inch 2-94 feet; and in calculating the strength of the various parts of an engine, itis not only necessary to bear this in mind, but also the power of the steam in the boiler, andit will be perhaps safer always to take in our calculation double the pressure on the valve,for it is not always in the same degree of action: when the load on the valve is 8 poundson the circular inch, call it double, and add this to the pressure of the atmosphere, which isII-5 pounds; we then have 27'5 pounds for the strength of the steam acting on thecircular inch : in steam-boats 39 pounds pressure per circular inch on the piston maybe allowed.
The stress which acts on any portion of an engine may be ascertained by considering itinversely as the number of revolutions multiplied by the diameter of the circle, or thechord of the arc described by the point where the force acts; thus a wheel 4 feet indiameter, making three revolutions while the piston makes one stroke of 5 feet in length,will give 4x3: 5 :: pressure on the piston: the stress on the teeth of the wheel equal tothat of the pressure on the piston. The cohesive force being known, the strain the materialwill bear is taken at about one-third.
Suppose D the diameter in inches of the piston, L the length of the stroke in feet, P thedouble of the whole elastic force in pounds upon the circular inch ; l the length from thecentre of motion to the centre of stress in feet; d equal to the depth or diameter, and hequal the breadth in inches; f equal the cohesive force of a square inch at the point ofalteration; and R equal to the radius of the wheel. The force on the piston is then D s Pin pounds, or to find the diameter of the piston-rod in inches, multiply the diameter of thesteam-piston in inches by the square root of twice the elastic force of the steam in theboiler in pounds per circular inch, and divide the product by 84. This rule applieswhen the strain on the rods is tensile, but when they are alternately compressed as well asextended, the diameter of the piston in inches should he multiplied by the square root oftwice the pressure of the steam on a circular inch, and the product divided by 45, which