Steam pressure

“ When steam passes through an orifice from a higher to a lower pressure, as is the case with the throttling calorimeter, no external work has to be done in overcoming a resistance. Hence, if there is no loss from radiation, the quantity of heat in the steam will be exactly the same after passing the orifice as before passing. If the higher steam pressure is 160 pounds gauge and the lower pressure that of the atmosphere, the total heat in a pound of dry steam at the former pressure is 1195.9 B. t. ”

“ If such a pressure were not exerted by the atmospheric air, there would be instantly produced a quantity of steam sufficient to give rise to that tension, and it would be necessary always to overcome this pressure in order to throw out the steam from the engines into the new atmosphere. ”

“ By setting down these pressures in a table, and taking their mean, he can determine the effect, with tolerable accuracy, of any particular measure of expansion. It is necessary to remark, that it is the total pressure of the steam that he must take; not the pressure above the atmosphere, but the pressure above a perfect vacuum. ”

“ The earliest clays of screw propulsion witnessed the use of steam of ten or fifteen pounds' pressure, in a geared engine using jet condensation, and giving a horse-power at an expense of, perhaps, seven or eight pounds of coal per hour.
A little later came direct-acting engines with jet condensation, and steam at twenty pounds' pressure, costing about five or six pounds per horse-power per hour. The steam-pressure rose a little higher with the use of greater expansion, and the economy of fuel was further increased. ”

“ One pound of steam at atmospheric pressure occupies 1,642 times as much room as it does in the state of water. If, therefore, when the stroke has been completed, and we are ready for the piston to come back, we inject a little cold water into the spent steam, it will condense to about one 1600th of its volume, and leave a vacuum into which the piston can return without having to force back the atmosphere. ”

“ Fourthly, I intend in many cases to employ the expansive force of steam to press on the pistons, or whatever may be used instead of them, in the same manner in which the pressure of the atmosphere is now employed in common fire-engines. In cases where cold water can not be had in plenty, the engines may be wrought by this force of steam only, by discharging the steam into the air after it has done its office. ”

“ The expansive action of high-pressure steam may be accompanied with condensation, so as considerably to increase the mechanical effect produced; for, after the weights with which the piston is loaded have been successively raised to the extent permitted by the elastic force of the steam, and are removed from the piston, the steam will expand until it balances the atmospheric pressure. ”

“ When the expansion of steam takes place without the loss of heat by radiation or conduction, the relation between the pressure and volume is practically constant; that is, if a given quantity of steam expands to twice its volume in a closed chamber of the kind above described, its final pressure will be one-half that of the initial pressure before expansion took place. A pound of steam at an absolute pressure of 20 pounds per square inch has a volume of practically 20 cubic feet, and a temperature of 228 degrees. ”

“ The steam is expanded in these nozzles from high pressure all the way down to the normal pressure of the first stage, and in such expansion acquires a high velocity and consequently contains a great deal of energy—much more than does an equal quantity of low pressure steam. In consequence of this, high pressure steam is used with a far lower water rate than is obtained with low pressure steam, or with high pressure steam reduced to low pressure in a reducing valve. ”

“ But under greater pressure more heat is required to make steam. The water never leaps and bubbles in an engine boiler. The awful pressure compels it to be quiet. A cubic inch of water will make a cubic foot--one thousand seven hundred and twenty-eight times as much--of steam under the pressure of one atmosphere. But under the pressure of a column of water one thousand feet high, giving a pressure of four hundred and thirty-two pounds to the square inch at the bottom, water becomes steam, if at all, only by great heat. ”

“ There is a further difficulty in getting the energy of the steam into a suitable kinetic form, namely, to get the stream of issuing particles to take a single direction, without undue dispersion, when steam is allowed to expand through an orifice from a chamber at high pressure into a space where the pressure is greatly less. ”

“ They employ the steam under a very high pressure, sometimes eight to ten atmospheres, but they have no condenser. The steam, after it has been introduced into the cylinder, undergoes therein a certain increase of volume, but preserves always a pressure higher than atmospheric. When it has fulfilled its office it is thrown out into the atmosphere. It is evident that this mode of working is fully equivalent, in respect to the motive power produced, to condensing the steam at 100°, and that a portion of the useful effect is lost. But the engines working thus dispense with condenser and air-pump. ”

“ In their endeavors to escape in all directions the molecules of the gases continually bombard the walls of the vessels which contain them. They break them if they are weak enough, or else they exercise upon them a pressure which is nothing but the sum of all energies of the molecules which strike a unit of surface in a unit of time. In our steam-engines the molecules (or rather particles) of steam bombard the walls of the cylinder; they push the piston by their aggregate energies, and, setting it in motion, make it move the huge masses it has to move. This is, of course, but a hypothesis ”

“ In high pressure engines the pressure is used ad libitum from 50 to 150 pounds, and, in former times, to such an extent, that no mortal was left to measure its height, the boiler as well as the boat and its contents, animate and inanimate, having too frequently been blown into the air. In condensing engines, when moving at full speed, the steam is never “wire-drawn,” as the engineers term it, the passages being made large enough, and the valves fully opened: the same, in high pressure engines: but, when not moving at full speed, the steam can be “wire-drawn” as the engineer thinks necessary. ”

“ Seeing that the tube or pipe is capable of withstanding all the pressure that the steam can give, it is difficult to account for the tremendous repelling force, which is, undoubtedly, brought into operation in explosions or ruptures of steam pipes carrying what are now comparatively low pressures. ”

“ It would be well when the feed water is too warm to permit the ejector to work right, to reduce the pressure, and consequently the temperature of the steam supply, as low pressure steam condenses quickly, and therefore can be employed with better results than high pressure steam. ”

“ But to obtain from high-pressure engines really advantageous results, it is necessary that the fall of caloric should be most profitably utilized. It is not enough that the steam be produced at a high temperature: it is also necessary that by the expansion of its volume its temperature should become sufficiently low. A good steam-engine, therefore, should not only employ steam under heavy pressure, but under successive and very variable pressures, differing greatly from one another, and progressively decreasing. [28] ”

“ The point at which the steam is cut off should therefore be so regulated that it shall acquire a pressure equal to the resistance on the piston by its expansion, just at such a distance from the end of the stroke as the piston may be able to move through by its inertia. ”

“ If steam be used to raise a piston against the atmospheric pressure only, although a definite physical force will be exerted by it, and a mechanical effect produced, yet under such circumstances it will exert no directly useful efficiency; but after the piston has been raised, and the tube beneath it filled with steam balancing the atmosphere above it, a useful effect to the same amount may be obtained by cooling the tube, and thereby reconverting the steam into water. ”

“ To understand this it must be recalled that when using the expansive property of steam, the piston thrust could not possibly be uniform, since the greatest pressure exerted by the steam would be exerted at the moment before it was shut off from the boiler, and its pressure must then decrease progressively, as it exerts more and more work upon the piston and becomes more expanded, thus obviously retaining less elastic energy. The operation of the fly-wheel largely compensates this difference of pressure in practise, but it would be obviously advantageous could the pressure be equalized ”