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The water automatic works by the
expansion of a brass tube that is connected between two steel rods.
One end of the expansion tube is connected to the bottom of the boiler while
the opposite end is attached to the top of the boiler. By being
connected in this manner the water level in the expansion tube and the
tubing connected to either end of the expansion tube for a closed loop with
the boiler. Whatever the water level is in the boiler will be
equivalent to the water level in the expansion tube and associated tubing.
The temperature of water being turned
into steam will always remain below the temperature at which enough thermal
energy has been absorbed by the water to change it from a liquid state into
a gaseous state or steam (called the saturation temperature). The
steam that is generated from the water however is just above the saturation
temperature and as a gas can absorb additional heat as a gas. In a
closed container such as a boiler the steam thus becomes much hotter than
the water that generated it since the steam can absorb additional heat.
It is this thermal difference that allows the water automatic to operate.
One end of the expansion tube is fixed
relative to the two steel rods of the water automatic. The other end
of the expansion tube is attached to a block which can slide on the steel
rods. The block also has a valve stem attached to it opposite the
expansion tube. The valve stem is part of a valve assembly fixed to
the ends of steel rods. The valve is plumbed into the discharge line
from the water pumps. When the valve is open water is allowed to flow
back to the water supply tank. When the valve is closed water is not
allowed to flow back to the water supply tank and if you follow the piping
diagram you'll see that the only place it can go is into the boiler.
Water is forced to leave the pumps, travel through the feed water heater and
then to the boiler. The water automatic is mounted nearly horizontal
at the side of the boiler so that it reacts rather quickly to changes in the
boiler water level. The valve body is fixed to the end of the steel
rods with springs. This allows the expansion tube to expand to any
length and not apply undo forces to the valve stem, ball valve, or valve
seat. The valve body can slide within the adjustment limits of the
mechanism on the steel rods to accommodate the expansion and contraction of
the expansion tube.
If the boiler's water level is above
the water automatic mounted to the side of the boiler the expansion tube
will be full of water. This doesn't put pressure on the valve stem and
ball valve and thus all the water that is pumped is returned to the water
supply tank. As steam is used from the boiler the water level of the
boiler drops. The water level in the expansion tube and associated
piping seeks the same level as the water in the boiler. Once the
boiler water level drops below the level of the expansion tube the expansion
tube will be full of steam. As this steam is significantly hotter than
the water the expansion tube will increase in length. The expanding
motion of the expansion tube slides the block along and as it does so it
pushes the valve stem into the valve body eventually forcing the ball valve
against the valve seat. This blocks off water from exiting the valve
(the exit for the valve is on the backside of the drawing) and returning to
the water supply tank.
When the boiler is sufficiently full
of water such that the water level in the boiler is above the height of the
water automatic the expansion tube is again full of water. The
expansion tube cools to the temperature of the water and returns to the
length it started at. This action relieves pressure on the valve stem
and ball valve and opens the pathway for water to again return to the water
supply tank. |
