_An air pump_ is an apparatus for, 1, the exhaustion; 2, compression or
transmission of air.
_A vacuum pump_ is an apparatus consisting of, 1, a chamber or barrel;
2, a suction pipe with a valve to prevent return flow; 3, a discharge
pipe which has a valve which is closed when the chamber is emptied and,
4, a steam induction pipe provided with a valve that is opened when the
chamber is filled with water and closed when the chamber is filled with
steam.
It is not right to call an air pump a vacuum pump, _as the latter does
not move_ air alone; it removes water, vapor and air from the condenser
to form a vacuum. An air pump is designed to pump air alone.
_A vacuum is a space entirely devoid of matter._ That is, it is a space
that contains nothing—no oxygen, no hydrogen, no air, no water, no
pressure. It is for this reason that a perfect vacuum in practice is
very difficult to obtain, especially as applied in a steam engine, as a
liquid when in the presence of a vacuum generally gives off some vapor,
owing to the fact that the surface is more or less in tension, besides
its usual evaporative quality. Among all the liquids it has been found
that mercury, on account of its very high specific gravity, can be best
used to produce a vacuum and maintain it, and it is for this reason
that the words “vacuum” and “inches of mercury” are synonymous.
NOTE.—The pressure of the atmosphere will also balance a column of
water in a vacuum the same as a column of mercury but the height of
the water column must necessarily be greater on account of the lesser
weight of the water. A cubic inch of water weighs 13.6 times less
than a cubic inch of mercury, so that the column of water which the
atmosphere must balance must be 13.6 higher or 13.6 × 30 = 408 inches
which is equivalent to 34 feet.
_A water barometer_ can be made in a similar manner to a mercury
barometer except that instead of a tube slightly over 30 inches in
length, a tube over 34 feet in height must be used. Advantage of this
fact is taken in the so-called gravity condensers which require no
air pump, the condensing apparatus being placed about 34 feet above
the level of the hot well, the discharge pipe being sealed by always
keeping its lower end below the level of the water in the hot well.
The particular feature that makes steam valuable in producing a vacuum
is the fact that when it is condensed, it decreases 1600 times in
volume and except for this small quantity of water and some vapor
which even cool water gives off in a vacuum, a perfect vacuum would be
established and it is only necessary to draw off the condensed steam
and vapor by proper apparatus to enable the vacuum to be maintained
which the condensation has created. The apparatus for doing this is
called the air pump _and the reservoir in which this condensation takes
place is called the condenser_.
The condensation of steam in the condenser is effected in two ways.
The exhaust steam either meets in direct contact the water which is to
condense it, or, the steam impinges upon cool metallic surfaces the
temperature of which is kept down by circulating cool water through
them. In the first case the condensed steam and the condensing water
meet and mingle. The condenser is an iron pot or shell into which the
steam is exhausted and the cooling water enters it in the form of a
sheet or spray. Such condensers are called _jet condensers_ for this
reason, and the cooling water is called _the injection_. All water that
is used for condensing steam is therefore called _the injection water_.
When the exhaust steam strikes cool surfaces and is condensed by
those surfaces, such condenser is called _a surface condenser_. The
cooling surface is usually a series of pipes or tubes made of brass
or copper to secure a rapid transfer of heat. These tubes are usually
tinned inside and outside to prevent corrosion and in marine practice
are made 5/8″ in diameter. In most cases, condensation is effected by
bringing the exhaust steam in contact with the outside of the tubes,
the circulating water being inside.
_In the surface condenser_, as the circulation does not mingle with the
condensed steam, the air pump has nothing to do with this water but is
only required to pump out the condensed steam and air which enter the
condenser; the pump which takes care of the circulation is called _the
circulating pump_. When large quantities of water are used and the
difference in level through which the water must be raised is slight as
on board ship, _centrifugal pumps_ are generally used.
_In the jet type of condenser_ where the water acts directly on the
steam, the injection water will cause a lower temperature with less
water and less apparatus than a surface condenser. The amount of
injection water varies from 20 to 30 times the weight of steam to be
condensed in cool seasons and from 30 to 35 times the amount in summer
season. With fresh water this can be pumped into the boiler when the
oil is extracted from it. It is for this reason that surface condensers
are universally used for sea-going vessels to avoid salt water. They
are also much used on land in places where the feed water contains
mineral salts and is injurious to the boiler.
In places where the cost of hydrant water is excessive, it is of
importance to use the same injection water over and over again,
but this cannot be done until the water is first cooled. There are
numerous methods by means of which this is done. All of these methods
utilize the principle of scattering the injection water in the way
best calculated to bring the greatest surface in contact with the
largest quantity of air so that evaporation may take place quickly and
effectively.
This is sometimes done by pumping the water through a number of spray
nozzles up into the air, allowing it to fall into a lake or cold well
below, or, as is more usually the case, the injection water is allowed
to descend in a tower in a fine state of division over tiles or wire
gauze or corrugated surfaces. A current of air, either forced by a
fan or drawn up through it, causes a vaporization of the film of warm
water pouring over the different surfaces, and the air cooling and the
evaporation combined withdraw the heat from the water so that when it
reaches the bottom it is in condition to be used again.
[Illustration: FIG. 344.]
[Illustration: FIG. 345.]
Cooling towers are used with either jet or surface condensers and can
be used either with or without a fan, depending upon the design. In
general these towers usually lower the temperature of the water from
120 degrees to 80 degrees, which is sufficient to maintain a vacuum of
about 26 inches. As they depend chiefly upon the results of evaporation
to do the cooling, they work better on a dry day than when the air is
humid.
The figures on the opposite page are designed to illustrate the use of
_an air pump in connection with a jet condenser_; this combination is
properly called a vacuum pump because it not only pumps air, but water
and vapor as well. The _steam end_ of this apparatus is described in
Part One, page 324, of this work.
_The air and vacuum end_ has a cylinder lined with composition-brass
bored smooth; the piston has square rubber and canvas packing. The
discharge—as shown in cut—is located sufficiently high, so that the
cylinder retains a large portion of water. This forms a seal and
causes the pump to work more advantageously than it would with air
alone. A small pipe leads from the discharge chamber to the piston-rod
stuffing-box. This contains a double packing and the water which flows
through this small pipe forms a continuous seal around the piston-rod
and thus prevents air from entering.
The injection water enters the elbow at the top and is drawn through an
annular opening into the condenser. This opening may be regulated by
the small hand wheel shown at the top end of the stem.
The exhaust from the steam end flows into the condenser through the
pipe as may readily be observed—or escapes into the atmosphere by
throwing the switch valve.
NOTE.—_Utilizing hot discharge water._ In manufacturing
establishments where large quantities of water are required,
advantage can be taken of the fact that in condensing apparatus of
this and similar pumps, the water, after performing useful work in
the condensing chamber, can be elevated to a tank in any portion of
the building, _and used over again for another purpose_, such as
washing, cooling metal plates, rolling-mill rolls, etc. The fact that
the temperature of this discharge water will range from 100° to 120°
will, in many cases, be advantageous, and effect a saving in the cost
of heating other water for purposes in which this discharge water
will answer equally well. _When the water is not required in the
tank, the stop-valve may be opened, and the water allowed to escape
into a drain, or any other convenient place._
A ball-float attached to an air valve is located at the right hand of
the condenser so that in case the pump should fail to operate from any
cause, the injection water will lift the ball-float, which in turn
will open the air valve and by discharging the vacuum will prevent the
flooding of the engine cylinder with water.
It is a well-known fact that the atmosphere exerts what is usually
termed “back pressure” of 14.7 pounds per square inch upon the piston
area of a steam engine, also that water converted into steam, may
be converted into its original state by condensation. Now, if this
back pressure, which is, in reality, the weight of the surrounding
atmosphere, be removed from the piston of a steam engine, the steam on
the opposite side of the piston would have that much (14.7 lbs.) less
work to do.
Applying this to steam engines means conveying the exhaust, or expanded
steam, which would otherwise be allowed to escape into the open air,
into a closed chamber, where it is met by a spray of cold water, which
so rapidly absorbs the heat contained in the steam that it ceases to
retain its gaseous form, and is again reduced to its original bulk as
water. A great change has now taken place, and the steam is reduced
to its liquid form. As this water of condensation only occupies about
1/1600 of the space filled by the steam from which it was formed, the
remainder of the space is vacant, and no pressure exists.
The difference in volume accounts for the atmospheric pressure on the
outside of the chamber, and as the vacuum extends throughout the whole
distance which the exhaust steam originally occupied, it, of course,
is made available in the cylinder of the engine in the shape of a
decreased pressure on the exhaust side of the piston; the atmospheric
pressure remains constant, therefore we have the atmospheric pressure
acting on one side of the piston, and absent on the other; the gain
being 14.7 pounds per square inch, if a perfect vacuum could be
secured. It amounts in average engineering practice to from 12 to 13
pounds, or 24 to 26 inches of mercury, as the graduations usually read
on vacuum gauges.
Jet and Surface Condensers are further described and illustrated in a
special allotted section of this work. The vacuum pump is usually of
the reciprocating order, although other methods have been employed for
emptying condensers, but not with equally satisfactory results.
_The gain to be secured by using a condensing apparatus_ may be
measured in two ways: first, by the decrease in fuel consumption over
that necessary when running non-condensing, which will represent a
constant decrease in running expenses; or, second, by the increase of
power working quite up to its economical limit, in a non-condensing
engine.
By the use of a condenser a further increase of power is realized in
raising the mean effective pressure of steam within the engine cylinder
without increasing the demand upon the boiler.
_The application of a condenser to a steam engine_ increases its
economy from 20% to 25% depending upon circumstances, while by
compounding and condensing an economy of 35% to 40% is effected.