The illustration, Fig. 245, on the opposite page represents _the first
practical steam pump ever made_; on pages 67-69 will be found an
interesting account of it. The water end is _single acting_; the steam
end is, of necessity, _double acting_ to 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 is _transformed
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 but _the 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 1782 _the double acting steam engine_ was patented by Watt. This
was a device in which the live steam acted on each side of the piston
alternately, the opposite side of the cylinder being in communication
with the condenser. The same patent covered the method of applying _the
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.
When the steam after being utilized in the cylinder makes its exit
directly to the open air, the engine is called _single expansion_ for
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 called _double
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 condenser_ which 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
action _a certain quantity of heat always produces a certain quantity
of work_.
[Illustration: FIG. 246.
Relative Volume of Steam at 200 pounds Pressure from One Cubic Inch of
Water
Relative Volume of Steam at Atmospheric Pressure from One Cubic Inch of
Water]
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 the atmosphere 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]
[Illustration: 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 have accomplished 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 A: NOTE.—This unit has been recently changed to 778.]