THE STEAM PUMP.
The illustration,Fig. 245, on the opposite page representsthe first practical steam pump ever made; onpages 67-69will be found an interesting account of it. The water end issingle acting; the steam end is, of necessity,double actingto produce the reciprocating motion. From this original design has been evolved the piston valve as well as many other designs of valve motion for pumps.
Having already taken up in some detail the construction of “the parts of the pump” and the necessary appliances connected with its use, it now remains to consider the means by which the steam power generated is made available and the mechanism by which the energy istransformed from pressure into pumping power.
It may be well to consider at some length the “Steam end” of the pump. This consists primarily of cylinders, together with their connections;these constitute the muscular system of the pump. The muscular or operating end is separate and distinct from the water end so far as construction is concerned but in operation the two are closely allied. It is therefore necessary as well as convenient to unite them in one equipment and thus enable the propelling mechanism to furnish a constant source of power.
So far as the elastic force of steam itself is concerned its history dates back to a period two hundred years B. C., when, as described by Hero, the force generated by steam was utilized for actuating certain devices constructed rather for curiosity than for any benefit which might be derived from their use. Very little advance was made in the construction of practical devices until the latter part of the eighteenth century when James Watt by his improvements placed the stationary engine on an operative basis and gave to the world what has proved to be the greatest invention of all time.
The first stationary steam engines were used for pumping water and were of the single acting type, in which the steam was admitted at one end of the cylinder, the opposite end being open to the atmosphere. The steam acting on the piston forced it to the limit of its stroke when the supply was cut off. The steam then condensed in the cylinder, forming a partial vacuum, and the force of the atmosphere upon the opposite side of the piston forced it back, causing it to complete its stroke before another supply of steam was admitted. This was a slow process, wasteful of steam and attended with many other inconveniences.
An improvement on this device was made in an engine built by Watt in 1774. This was a single acting engine butthe condenser was separated from the cylinder. The valves were so arranged as to admit live steam into the upper end of the cylinder on the top of the piston and at the same time open the lower end of the cylinder to the condenser. The steam followed the piston in its downward stroke in which action it was aided by the partial vacuum formed in the condenser. At the completion of the downward stroke the valves were changed so as to close the ports to the steam supply and the condenser, and at the same time open a communication between the two ends of the cylinder equalizing the pressure above and below the piston. The weight of the pump rod on the beam or lever connection overbalanced the weight of the piston and caused it to complete the return stroke.
In 1782the double acting steam enginewas patented by Watt. This was a device in which the live steam acted on each side of the piston alternately, the opposite side of thecylinder being in communication with the condenser. The same patent covered the method of applyingthe principle of expansion of steam in the cylinder; a non-condensing engine was also described.
Note.—This invention was of great historical importance as it covered all the essential detail of modern practice in steam engine building and constituted the fundamental principle of all steam engines.Improvements have been made in form and construction, necessitated by new adaptations which have been constantly developed. The requirements for higher speed, increased pressure which implies greater power, and the constant desire for greater economy in fuel have produced a variety of changes in detail but have not altered the fundamental idea.
Note.—This invention was of great historical importance as it covered all the essential detail of modern practice in steam engine building and constituted the fundamental principle of all steam engines.
Improvements have been made in form and construction, necessitated by new adaptations which have been constantly developed. The requirements for higher speed, increased pressure which implies greater power, and the constant desire for greater economy in fuel have produced a variety of changes in detail but have not altered the fundamental idea.
When the steam after being utilized in the cylinder makes its exit directly to the open air, the engine is calledsingle expansionfor the reason that the action of the steam takes place in one cylinder during a single stroke, and what expansion takes place must be during one half of a revolution. When the steam from one cylinder instead of exhausting into the open air, is passed to a second cylinder, of larger area, and by expanding exerts a pressure on a second piston to aid in the completion of the revolution, the engine is calleddouble expansion or compound, because the steam instead of completing its work in a single operation is afforded a double opportunity for expansion and an increased range of action.In the single cylinder the temperature of the walls is reduced in each revolution to correspond with that of the steam at the exhaust pressure.
This temperature must be restored by incoming steam at the beginning of a new stroke which means a reduction of power. With a double cylinder owing to the greater range of expansion, a higher temperature can be maintained in the first cylinder and a large amount of initial condensation is prevented.A still greater use of expansion may be obtained by the introduction of a condenserwhich allows the final exhaust to be carried below the atmospheric pressure to the extent of the vacuum formed. In stationary and marine practice triple and quadruple expansion engines are common. These are used in large units to give the greatest possible economy in fuel.
Properties of Steam.—Before taking up in detail the valve and other mechanism of the steam pump it may not be out of place to consider briefly the action of steam and its expansive properties.Heat is identical with mechanical force and the one can be converted into the other.Aside from the means used in converting or developing the actiona certain quantity of heat always produces a certain quantity of work.
relative volumes of steam at 200 pounds and atmospheric pressureFig. 246.
Fig. 246.
The temperature of steam at atmospheric pressure (14.7 lbs. absolute) is 212° Fahr. As the pressure increases the temperature rises, but is always the same for a given pressure. The sensible heat required to raise the temperature of water from 32° to 212° is 180° and the heat absorbed by the water or latent heat at 212° is 996° making the total amount of heat expended 1176°. As the temperature rises the latent heat decreases in nearly the same proportion as the sensible heat increases. This number may therefore be taken as a constant to express the unit of heat in one pound of steam from 32° up to the temperature at which evaporation takes place. Then 1176 × 772 = 907,872 pounds raised one foot which represents the mechanical equivalent or maximum theoretical duty of the quantity of heat contained in one pound of steam.
One cubic inch of water if converted into steam at the pressure of the atmosphere (14.7 pounds) will occupy the space of 1642 cubic inches or nearly one cubic foot. As the pressure increases the volume is relatively diminished and if the same quantity of water is converted into steam say at 200 pounds pressure it will occupy a space of only 133 cubic inches. Assuming that no loss occurred by condensation, if released at this pressure it would expand and again occupy its relative volume at atmospheric pressure.
This is illustrated by the accompanying diagram,Fig. 246, showing a cylinder with an internal capacity of 1642 cubic inches provided with a movable piston. A quantity of steam representing that formed from one cubic inch of water is forced into it, supposing the weight on the piston to be 200 lbs. per square inch, this weight will be raised until the space under the piston occupied by the steam will be 133 cubic inches. If the supply is now cut off (assuming that no condensation takes place) the piston will remain at this place supporting its load. If the load on the piston is diminished the volume of steam will expand and the piston will be correspondingly raised in the cylinder. This action will be continued until all the load is removed and only the weight of theatmosphere remains. The volume of steam under the piston will then be 1642 cubic inches. It will therefore be seen that the same quantity of steam has exerted a lifting pressure upon the piston due to its relative volume commencing at 200 pounds per square inch and gradually decreasing until the pressure of the atmosphere is reached.
The English unit of heat is that which is required to raise the temperature of one pound of water one degree Fahrenheit and is known as the British Thermal Unit, or B. T. U. Dr. Joule demonstrated by an ingenious device,Fig. 247, in which a weight operated a paddle wheel agitating water in a closed vessel, that it required 772 foot pounds to raise the temperature of one cubic foot of water one degree, or, on the other hand, it was deduced that one unit of heat was capable of raising 772 pounds one foot high. The mechanical equivalent of heat is therefore accepted as 772 foot pounds for one B. T. U. based on Joule’s experiment.A
Fig. 247.
Fig. 247.
The theoretical efficiency of the use of steam by expansion can never be realized, owing to losses occasioned by condensation, caused by contact with the cooler walls of the cylinder, the unavoidable friction of the working parts, and from the fact that a certain portion of the pressure must be utilized to create a draft for the fire. All these losses must be taken into consideration in calculating the work actually done.
From the foregoing it will be readily understood that if the steam is allowed to exhaust from the cylinders at or near the pressure at which it is admitted the work which it might haveaccomplished by expansion will be lost. This means not only a loss of the steam but of a part of the fuel used to generate it.
It is therefore advisable to get all the work out of the steam that is possible and the nearer to atmospheric pressure the exhaust can be brought the greater will be the economy.
FOOTNOTE:ANote.—This unit has been recently changed to 778.
ANote.—This unit has been recently changed to 778.
ANote.—This unit has been recently changed to 778.
Steamis water in a gaseous state; the gas or vapor of water; it liquifies under a pressure of 14·7 and temperature of 212° F.
Steamis a joint production of the intermingling of water and heat. Water is composed of two gases which have neither color nor taste, and steam is made up of the same two gases with the addition only of that mysterious property called heat by which the water becomes greatly expanded and is rendered invisible. The French have a term for steam which seems appropriate when they call it water-dust.
This is what takes place in the formation of steam in a vessel containing water in free communication with the atmosphere. At first, a vapor is seen to rise that seems to come from the surface of the liquid, getting more and more dense as the water becomes hotter. Then a tremor of the surface is produced, accompanied by a peculiar noise which has been calledthe singingof the liquid; and, finally, bubbles, similar to air bubbles, form in that part of the vessel which is nearest to the fire, then rise to the surface where they burst, giving forth fresh vapor.
The curious fact must be here noted that if water be introduced into a space entirely void of air, like a vacuum, it vaporizes instantaneously, no matter how hot or cold, so that of an apparent and fluid body there only remains an invisible gas like air.
That steam isdryat high pressure is proved by an experiment which is very interesting. If a common match head is held in the invisible portion of the steam jet close to thenozzle, it at once lights, and the fact seems convincing as to complete dryness, as the faintest moisture would prevent ignition even at the highest temperature. This experiment proves dryness of the steam at the point of contact, but if throttling exists behind the jet, the steam supplied by the boiler may be in itself wet and dried by wire drawing.
Dead steamis the same as exhaust steam.
Live steamis steam which has done no work.
Dry steamis saturated steam without any admixture of mechanically suspended water.
High-pressure steamis commonly understood to be steam used in high pressure engines.
Low-pressure steamis that used at low pressure in condensing engines, heating apparatus, etc., at 15 lbs. to the inch or under.
Saturated steamis that in contact with water at the same temperature; saturated steam is always at its condensing point, which is always the boiling point of the water, with which it is in contact; in this it differs from superheated steam.
Superheated steam, also called steam-gas, is steam dried with heat applied after it has left the boiler.
Total heat of steamis the same as steam heat.
Wet steam, steam holding water mechanically suspended, the water being in the form of spray.
Specific gravity of steam is ·625 as compared to air under the same pressure.
The properties which make it so valuable are:
1. The ease with which we can condense it.
2. Its great expansive power.
3. The small space in which it shrinks when it is condensed either in a vacuum chamber or the air.
A cubic inch of water turned into steam at the pressure of the atmosphere will expand into 1,669 cubic inches.