MAIN FUNCTIONAL REQUIREMENT:
Remove human waste
DESIGN PARAMETER: A
GEOMETRY/STRUCTURE AND PARTS:
|Components of A Common Toilet
EXPLANATION OF HOW IT WORKS/ IS USED:
To flush the toilet:
- When the handle is depressed, the flush valve chain opens the flush valve
long enough for all the water in the tank (approximately 2 gallons) to rush into the
- All the water is then siphoned through a tube connected to the sewer and
out of the bowl (the concept of siphoning will be explained later). In flushing the
toilet, the float ball drops due to the lack of water in the tank, opening the inlet valve
to let more water into the tank. The flush valve closes after all the water has been
emptied from the tank and the presence of the new water helps keep the valve closed.
- The water level increases (from the supply line, through the inlet tube
and valve, passing through the bowl refill tube) until the float ball is floating on the
water. When the water reaches a certain level, it causes the inlet valve to close and
stops the tank from filling further.
- If the inlet valve did not shut off, and the water kept flowing into the
tank, the overflow tube would prevent it from doing so.
How a siphon works:
|Example of A Simple Siphon
- Siphoning is used in transporting a fluid from a reservoir of higher
elevation to one of lower elevation. Bernoullis equation (shown below) can help
explain this type of fluid transport.
- The siphon for a toilet is an upside down "U" shaped tube that
connects the bowl (higher reservoir) to the sewer/cespool (lower reservoir).
- If fluid (anything under approximately 2 gallons) is added to the toilet
bowl itself, notice that the water level does not change. The extra fluid will cause the
water level in the bowl to rise slightly for a moment, but then that extra water overflows
into the siphon tube, passes through the tube, and out of the system (but this does not
cause the toilet to flush).
- When the toilet is flushed, 2 gallons of water rush quickly through the
bowl, causing the siphon tube to fill up completely, resulting in
a change in pressure (lower pressure inside the tube ahead of the flowing water, and
higher pressure in the water (at the inlet of the siphon) in
the bowl. Once all the water has passed through the tube, air enters and interrupts the
siphoning process (causing the gurgle you hear after you flush). Then water in the bowl is
replenished when the inlet valve in the tank allows more water to come in through the
See also siphoning above
||Pressure at inlet
||Pressure at outlet
||Velocity of fluid at inlet
||Velocity of fluid at outlet
||Height of the inlet
||Height of the outlet
||Density of the fluid
||Head loss (losses due to friction)
With regard to a toilet, we can assume that the density (p) of water is
constant, gravity (g) is constant, h loss and v in are both negligible compared
to the other terms, and p in is atmospheric pressure. I timed the toilet in my
house and it took approximately 8 seconds. Since we know that it takes 8 seconds for 2
gallons of water to flow through the cross sectional area of the pipe, we can determine a
rough estimate for the flow rate (Q). Knowing Q and assuming the cross sectional Area, we
can estimate the average
velocities at specific points.
Q = A*v
Since the flow rate (Q) is constant throughout the system, according to
the law of conservation of mass () and assuming that the density
(p) remains constant, we can state that:
A*v in = A* v out
Factors that may limit the performance of the toilet would include the
cross sectional area of the bowl, the amount of water used to flush the toilet, and the
time it takes for all the water to pass through. The mechanical efficiency is not that
important in the case of the toilet.
The friction between water and the siphon pipe is relatively low and can
be determined using the Reynolds number.
Where v is the velocity of the fluid, d is the diameter of the pipe the
fluid is flowing through, p is the density of the fluid (water=1000 kg/m^3), and m is the
viscosity of the fluid (water=1x10-3 kg/m*s). When the Reynolds number is less than 2300 (as in a toilet), it is considered laminar flow and friction forces
are assumed to be negligible.
WHERE TO FIND TOILETS:
Hopefully you will allow yourself enough time to locate one. Suggestions
would include the W.C., behind a tree, next to the food court in the mall, and in worst
case scenario, you can always ask.