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ARTICLE V.
OF THE CONSTRUCTION OF BOILERS.
As we mean to work with steam of great elastic power, say 120 pounds to the inch, above the atmosphere, it is necessary, in the first place, to discover true principles, on which we may calculate the power exerted to burst our boilers, by any given power cf the steam; that we know how to construct them with a proportionate strength, to enable us to work with perfect safety.
A circular form is the strongest possible, and the less
the diameter of the circle, the greater elastic power it will contain. Therefore
we make cylindric boilers not exceeding 3 feet diameter, and to increase
their capacity we extend their length to 20 or 30 feet, or more, or increase
their number. They must be set nearly in a horizontal position, with the
furnace under one end confining the flue to the underside to the other end;
giving the fire a large surface to act on. This is the most simple form
and suits well where fuel is cheap. But to save fuel we construct boilers
consisting of two cylinders, one ]nside of the other; the inner a little
below the centre of the outer one, when laid in a horizontal position, to
give room for steam, in the upper side above the surface of the water. They
are of equal length, both made list to the same heads or end-plates. The
space between them contains the water, and the inner one contains the fire,
which is surrounded by the water. This boiler is enclosed in brick work,
and the flue returned along the under side of the outer cylinder which gives
the fire a larger surface to act on, than the other plan, and will not re-
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quire more than two-thirds of the fuel; but it is much more expensive to make.
These boilers are made of the best iron, rolled in large sheets and strongly riveted together. The ends may be made of soft cast iron, provided the fire or flue be kept from immediate contact with them. As cast iron is liable to crack with the heat, it is not to be trusted immediately in contact with the fire.*
To ascertain the power exerted by the steam, to burst one of these boilers, and the thickness of iron necessary to hold it; let us premise, that it is known by experiments made with care, (see art. 11) that a bar of good, sound, wrought iron, 1 inch square, will bear from 68 to 84,000 pounds, (but let us say from 64 to 75,000 pounds) hung to the end of it, to pull endwise, in a fair straight direction; consequently a bar one-tenth of an inch thick, and one inch wide, will hear at least 6400 pounds.
RULE.
Multiply the diameter of the boiler in inches, by the power of the steam in the boiler, in pounds, shown by the weight on the inch area of the safety valve, and the product is the power the steam exerts to break each
*It has been said that using a great degree of heat will burn the stuffing of the piston of the engine. But I have boiled linseed oil in a wooden boiler, with a furnace inside of it, without burning the wood, which will not bear a greater degree of heat then the hempen stuffing, and by the scale of heat(art.11) linseed oil boils at 600 degrees of heat. If the scale(art.3) be continued or extended to that degree of heat in the water, the elastic power of steam would be 122,880 pounds to the inch, which shows the futility of such objections, even supposing the scale to be incorrect, and that linseed oil will boil at a much lower degree.
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ring of one inch wide, in any two opposite places. Take
half of that product for the power to break it in any one place, and divide
by 6400, and the quotient will be the thickness in decimal parts of an inch,
that the iron must be to hold that power.
EXAMPLE.
What is the power exerted to burst a boiler 36 inches diameter, when the steam is ready to lift the safety valve loaded with 1500 pounds to the inch? and what thickness must the iron be to hold that power?
Then by the rule, 36 multiplied by 1500, the product is
54,000 pounds, the power to break every ring of 1 inch wide in any two opposite
sides: and 54,000 divided by 2 quotes 27,000 pounds, the power exerted to
break each ring of 1 inch wide in any one place; and 27,000 divided by 6400
quotes 42/100 parts of an inch, the thickness of the sheet iron that will
hold that great power, of 1500 pounds to the inch. Few will believe this
until they clearly understand the pnnciples. I proceed therefore to demonstrate
the rule to be true.
DEMONSTRATION.
Suppose the circle, 36 inches diameter, be inscribed in
a square, whose sides are 36 inches in length; draw diameters to the circle,
parallel to the sides of the square; and, suppose steam to exert a power
in the square, equal to 1500 pounds to each inch; it is evident that there
will be 1500 pounds on every inch of the length of any two opposite sides,
exerted in opposite directions balancing each other, to separate the sides,
which are held together by the other two sides; add that to find
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the power exerted to break any two opposite sides, we must multiply the length of one of the sides, 36 inches, by 1500, the power of the steam, and the product is 54,000 pounds, to break the two sides; half of which is
27,000, to break one side in any one place.
Again, suppose the circle to intercept the steam from acting on the square, then it is evident that each semicircle intercepts the steam which acted against its corresponding side of the square, and that the power to break the circle in any two opposite places is just equal to the power to break the two sides of the square; which was to be demonstrated.
This may be demonstrated by the proportions of the circle, and the known laws of mechanical powers. Let us suppose the circumference of the circle, which is 113 143/1000 inches in length, to be a cord with one end made fast, and a power be applied to draw the other end of it in a straight line, so as to draw the whole l13 143/1000 powers of the steam, 1500 pounds each, amounting to 169,714 5/10, up to the centre; then these powers multiplied into their distance moved, which is 18 inches, will produce 3,054,865; and the power at the end of the cord, 27,000, multiplied into its distance moved, viz. the whole length of the circle, 1l3 143/1000 inches, produces the same sum, 3,054,865, agreeing with the known law of mechanics, viz. that the power inultiplied into its distance moved, is equal to the weight multiplied into its distance moved. The power at the end of the cord, 27,000 pounds, representing the strength of the hoop necessary to hold a power of 1500 pounds to the inch,
exerted inside of it: which was to he demonstrated.
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I have never found a solution of this so useful problem that so often occurs in practice, in arranging steam engines, water-works, pipes of conduit, &c. And, no doubt, but the simple rule, here laid down, will meet with opposition; but it is nevertheless true, and will stand the test of time and experiment. I rejoice at having discovered, that a circular vessel will hold a far greater power of steam than I at first conceived it would.
In order that we may work with a power of steam equal to
120 pounds to the inch, with peffect safety, I have, by the rule already
demonstrated to he founded on true principles, calculated the following
table, shewing the power exerted to burst each ring of 1 inch wide of the
boilers of different diameters, and the thickness of iron necessary to hold
steam of power equal to 1500 pounds to the inch area.
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A TABLE
OF THE DIAMETERS AND STRENGTH OF BOILERS.
| Diameter of the boiler ortube in inches | Power to break everyring of one inch ofthe boiler in anyplace, in pounds weight,when the steam is1500 pounds to the inchon the safety valve. | Thickness of thesheets of goodiron necessary tohold the power,in decimal partsof an inch | Power exertedon the headsto burst themout, in poundsweight |
| 42 | 31,000 | .48 | 2,077,500 |
| 40 | 30,000 | .46 | 1,884,000 |
| 36 | 27,000 | .42 | 1,525,500 |
| 30 | 22,500 | .35 | 1,069,000 |
| 25 | 18,750 | .29 | 735,000 |
| 20 | 15,000 | .23 | 471,000 |
| 15 | 11,250 | .17 | |
| 12 | 9,000 | .14 | |
| 10 | 7,500 | .12 | |
| 8 | 6,400 | .094 | |
| 7 | 5,250 | .082 | |
| 6 | 4,500 | .07 | |
| 5 | 3,750 | .058 | |
| 4 | 3,200 | .047 | |
| 3 | 2,250 | .035 | |
| 2 | 1,500 | .023 | |
| 1 | 750 | .012 |
| Diameter ofthe boiler ortube in inches | Strength of boilerto hold the head on,in punds weight | Number of inchscrew boltsnecessary to have strengthsufficient to hold on the heads | Thickness of the cast iron head in the middle in inches |
| 42 | 4,052,400 | 32 | 5 |
| 40 | 29 | 4,5 | |
| 36 | 2,037,440 | 24 | 4 |
| 30 | 16 | 3.5 | |
| 25 | 11 | 3 | |
| 20 | 918,777 | 8 | 2.5 |
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A boiler constructed from this table will hold steam with
power equal to 1500 pounds to the inch; a power almost beyond conception,
and which we will never need to work any engine. To find the numher of inch
screw bolts necessary to hold on the head, divide the force to burst the
head off; by 64,000, the strength of one bolt.
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to the end, where it should just balance the atmosphere when another valve opens to let in a similar puff of steam to drive the piston up again; while other valves open to let the steam escape from before the piston. Thus the piston is driven by strong puffs of steam, the same as an air-gun drives its bullets; with this difference, the air-gun is soon exhausted, but the fire keeps up the power of the steam; the whole power of the steam is expended on the piston, before it leaves the cylinder, except what is necessary to resist the atmosphere. This is supposing the engine to work without a condenser.
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ARTICLE VI.
OF THE MEANS OF APPLYING STEAM.
Supposing that no doubt can now remain in the mind of the intelligent reader, of our being able to work with steam of power equal to 120 pounds to the inch, with great advantage and safety, we will proceed to consider of the most economical means of using or applying this power, so that it may produce the greatest effects.
The engine may be constructed similar to that
of Bolton and Watts, except the gears for working the valves, which should
be so arranged as to open the valve, when the piston is up, to let in a
puff of the strong steam to drive it down, but to shut again as soon as
enough has got in, which, when suffered to expand, will fill the cylinder
with atmospheric power only; the steam entering the cylinder with a power
of 120 pounds to the inch, drives the piston with great force; but the valve
being shut at 1/8 part of the stroke, the steam expands and decreases in
power, all the rest of the stroke,
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A TABLE
Showing the proper time to shut off the steam, according
to the power in the boiler; and how the power and load it will carry at
every part of the stroke diminishes, in order that the steam may spend all
its power; supposing the lengths of the stroke divided into eight equal
parts, and working without a condenser.
| Power of steam in the boiler, 120 pounds to the inch | Load, deducting 15 pounds for the resistance of the atmosphere. | Power of steam in the boiler 60 pounds to the inch. | Load, deducting 15 pounds for the resistance of the atmosphere. | Power of steam in the boiler 30 pounds to the inch. | Load, deducting 15 pounds for the resistance of the atmosphere | Power of steam in the boiler 15 pounds to the inch. | Load | |
| 1 | 120 | 105 | 60 | 45 | 30 | 15 | 15 | 0 |
| 2 | 60 | 45 | 60 | 45 | 30 | 15 | 15 | 0 |
| 3 | 45 | 30 | 45 | 30 | 30 | 15 | 15 | 0 |
| 4 | 30 | 15 | 30 | 15 | 30 | 15 | 15 | 0 |
| 5 | 26.25 | 11.25 | 26.25 | 11.25 | 26.25 | 11.25 | 15 | 0 |
| 6 | 22.5 | 7.5 | 22.5 | 7.5 | 22.5 | 7.5 | 15 | 0 |
| 7 | 18.25 | 3.25 | 18.25 | 3.25 | 18.25 | 3.25 | 15 | 0 |
| 8 | 15 | 0 | 15 | 0 | 15 | 0 | 15 | 0 |
| 52.5 | 22.5 | 7.5 | 0 | |||||
| 269.5/8 | 179.5/8 | 89.5/8 | 0 | |||||
| 33.7 | 22.4 | 11.2 | 0* | |||||
| 15 | 15 | 15 | 15 | |||||
| 48.7 | 37.4 | 26.2 | 15** |
*Average load without a condenser.
**Average load with a condenser
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The foregoing table is founded on the supposition that the elastic power of steam is governed by the same laws, which govern the elastic power of permanent elastic fluids, viz. That their elasticity is in the inverse pro-portion with the space they occupy; or, as their density, called the Boylean law, (article 4.) If compressed into half the space their power is doubled, and if expanded into double space, their power is reduced to one half. But this is not strictly true with steam, because it is not a permanent elastic fluid. There will not as much heat enter into 1/8 part of the cylinder, with steam of 120 pounds to the inch elastic power, as will he sufficient to cause it to expand to fill the whole cylinder with elastic power, equal to 15 pounds to the inch, to resist the atmosphere (see article 4); it will not bear, therefore, to he shut off so soon. If a sufficient quantity of steam be admitted, to contain heat to expand it to fill the cylinder with power equal to the resistance of the atmosphere, the average load of the stroke will be greater than is shown by the table.
When the steam is 120 pounds to the inch, as in No.1 of the table, by the law it requires to be shut off at 1/8 part of the stroke, to give the steam time and room to spend all its power in driving the piston to the end, and to fill the cylinder with steam, equal to 15 pounds to the inch, just sufficient to resist the atmosphere the effective load being always 15 pounds less than the power of the steam, diminishes from 105 pounds to the inch, the load when the valve shuts, to 0 at the end of the stroke.
To find the average load or the load the steam will carry the whole stroke, and resist the atmosphere, add