The flushing mechanism is summarized in the following points.
- Since all of the water in the tank enters the bowl in
about three seconds, it is enough to fill and activate the siphon effect, and all of the water
and waste in the bowl is sucked out.
The figure below shows the structure of the toilet bowl (Credit: Selected Lectures in Toiletology 101).
Inside the rim is
a number of holes so that water leaves the rim and enters the bowl after flushing.
An inverted U-pipe is attached to the back of the bowl and it works like a siphon pipe.
- When flushing the toilet, all of the water in the tank (around 2 gallons, that is 7.6 liters)
is dumped into the bowl in about
three seconds. A good portion of the water flows down to a larger
hole at the bottom of the bowl. This hole is known as the siphon jet.
Water rises in the bowl
and flows over the dam, but no siphon or flushing action has yet started.
As more water enters the bowl, the volume and velocity of water flowing over the dam also increases,
creating a curtain of water through the passageway, creating a partial vacuum -- the start of siphonic
action. The curtain of water also prevents air from entering the passageway through the outlet.
- As incoming water continues to accelerate, more of the air in the down leg of the passageway is displaced, as
shown in the figure below.
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When the passageway is filled, a good flush or siphon action is created. Everything in the bowl is sucked out
through the passageway.
- As soon as the level of the water in the bowl drops to the level where air is again introduced into the
passageway, the siphon is broken.
- When deep seal of water is not restored with refill water, sewer gas will enter. The below figure shows
the foul gas coming up from the sewer.
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- What is a siphonic action?
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As mentioned before, a siphon is an inverted U-pipe that can transfers water
from a higher reservoir to a lower reservoir by lifting that water upward from the higher reservoir and
then lowering it into the lower reservoir. As a demonstration on siphonic action, please click the right movie
(Credit: Department of Physics, The Wake Forest University) to see how siphon pipe works.
- In fact, the water
is simply seeking its level, just as it
would if you connected the two reservoirs with a pipe at their bottoms. In that case, the water in the
higher reservoir would flow out of it and into the lower reservoir, propelled by the higher water pressure
at the bottom of the higher reservoir. In the case of a siphon, it's still the higher water pressure
in the higher reservoir that causes the water to flow toward the lower reservoir, but in the siphon
the water must temporarily flow above the water levels in either reservoir on its way to the lower reservoir.
The process is initiated by your first suck on the open end of the pipe to the lower reservoir. However,
the water is able to rise upward a short distance with the help of air pressure, which provides the
temporary push needed to lift the water up and over to the lower reservoir. At the top of the siphon,
there is a partial vacuum--a region of space with a pressure that's less than atmospheric pressure. The same
kind of partial vacuum exists in a drinking straw when you suck on it and is what allows atmospheric pressure
to push the beverage up toward your mouth.
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The siphoic action can be explained in terms of energy.
- We know that the higher water is, the more gravitational potential energy it has. Water, like a
ball at height, accelerates in whatever direction reduces its total potential energy.
Consider the case, you connect two containers which are at different levels, one higher than another,
with a straight pipe. As long as the water goes only downhill
through the pipe, the situation is simple. However, replacing the straight pipe by an inverted U-pipe, the
flow starts upward before it bends downward.
The mystery lies in the fact that water has more than one type of potential
energy. In addition to gravitational potential energy, water has potential energy associated with
its pressure.
- High-pressure water has more potential energy than low-pressure water, which explains
why water tends to accelerate from high pressure toward low pressure-from high potential energy to
low potential energy-even in the absence of gravity. Clearly, the pressure of water in a sealed
pipe decreases with altitude. Because of this pressure effect, the total potential energy (gravitational
plus pressure) of water in a closed pipe doesn't change, even as that water rises a short distance
upward inside the pipe! Undoubtedly, the gravitational potential energy of the water is increasing as the
water rises, but its pressure potential energy is decreasing by an equal amount.
- In a siphonic U-pipe,
the weight of water in descending portion of the pipe actually decreases the pressure inside the
rising portion of the pipe. As a result of this extra pressure drop, water in the high container
can reduce its total potential energy by accelerating toward and then through the pipe. Water
begins flowing through the pipe, even though it has to go upward for a short time during that
passage.
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Practically, the failure of the siphon effect occurs when the water is about
30 feet (10 meters) above the higher container. You can't use a siphon to lift water higher than
30 feet because above that height, an empty region will develop at the top of the pipe and stop
the siphon process.
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