This is an attachment to a steam boiler, designed to return water of
condensation. It invariably consists of three parts, viz.: the “riser,”
the “horizontal” and the “drop leg,” and usually of pipes varying in
size from three-fourth inch to two inches. Each part has its special
and well-defined duties to perform, and their proportions and immediate
relations decide and make up the capacity and strength of the system.
It is, in fact, nothing but a simple return pipe leading from the
source of condensation to the boiler, and, beyond this mere statement,
it is hardly possible to explain it; it has, like the injector and the
pulsometer pump, been called a paradox.
The range of application of the steam loop practically covers every
requirement for the return of water of condensation. If used in
connection with a steam engine, pump, etc., a separator of any simple
form is connected in the steam pipe as close as possible to the
throttle. From the bottom of the separator the loop is led back to the
boiler, and the circulation maintained by it will dry the steam before
it is admitted to the cylinder.
There is necessary to its operation a slight fall in temperature at
the head of the loop, which is accompanied by a corresponding fall in
pressure. The water accumulating in the lower end of the loop next to
the separator, as soon as it fills the diameter of pipe, is suddenly
drawn or forced to the horizontal by that difference in pressure. It is
immaterial how far the water has to be taken back, or how high it is
to be lifted. There is one system now in daily operation lifting the
condensed water over thirty-nine feet, and another lifting it over
sixty-three feet. The strength of the system is increased by length
and height, the only limit to its operation being the practicability
of erecting the necessary drop leg, the height of which depends on
difference in pressures.
[Illustration: Fig. 155.]
Fig. 155 is an illustration of its application to a radiating coil.
To understand the philosophy of its action, and referring to the
illustration, let us assume that all the valves are open, and full
boiler pressure is freely admitted throughout the steam pipe, coil and
loop. Now, if the pressure were exactly uniform throughout the whole
system, the water in the loop would stand at _a_ on the same level
as the water in the boiler. But, as a matter of fact, the pressure
is not uniform throughout the system, but steadily reduces from the
moment of leaving the dome. This reduction in pressure is due in part
to condensation and in part to friction, and although generally small
is always present in some degree. The pressure may be intentionally
reduced at the valve on the coil, and reduction necessarily results
from condensation within the coil itself. A still further reduction
takes place through the loop, so that the lowest pressure in the whole
system will be found at _a_, the point in the loop furthest from the
boiler, reckoned by the flow of steam.
Now it is known that water of condensation invariably works towards,
and accumulates in, a “dead end.” This is due to the fact that, as
already shown, the pressure is lower at the “dead end” than at any
other point in the system, and, as a consequence, there is a constant
flow, or sweep, of steam towards the point of least pressure, which
flow continues as long as condensation goes on. This sweep of steam
carries along with it all the water formed by condensation or contained
in the steam, at first in the form of a thin film, swept along the
inner surface of the loop, and afterwards, when the accumulation of
water is sufficient, in the form of small slugs or pistons of water,
which completely fill the pipe at intervals, traveling rapidly towards
the dead end. The action of the steam sweep is vastly more powerful
than is usually supposed, and, of course, operates continuously and
infallibly to deposit the water in the dead end as fast as accumulated.
In practice, water will speedily be carried over by the loop and
accumulate in the drop leg until it rises to the level _b_, which would
balance the difference in pressure. As the loop will still continue
to bring over water, it follows that as fast as a slug or piston of
water is deposited by the steam on the top of the column at _b, it
overbalances the equilibrium and an equal amount of water is discharged
from the bottom of the column through the check valve into the boiler_.
The result of the practical operation of many systems of this ingenious
device show advantages as follows:
1. Return of pure water to the boiler and saving the heat contained in
said water.
2. Preserving more uniform temperatures, thus avoiding the dangers due
to expansion and contraction.