The Physics of how a toilet flushes

[fourthindiana]

I read David Macaulay's description of how a toilet flushes in Macaulay's book The New Way Things Work, and I don't fully understand it. Macaulay says that air pressure in the tank of water for a toilet exerts a force that pushes down on the water in a tank of a toilet. I suppose the air pressure that exerts a force pushing downward on the water in the tank of a toilet is just the atmospheric pressure. I understand everything so far. But then I read Macaulay's numbered descriptions of how a toilet flushes, and I no longer fully understand it. Macaulay's description of how a toilet flushes is divided into three stages. I understand the third stage, so I won't describe the third stage. The following is a quote of Macaulay's description of the first two stages of a toilet flushing:
"1. The Tank Flushes
After the handle is pressed down, water is lifted up the siphon tube by the disk. The water reaches the bend in the siphon pipe and then travels around it. As it falls, the water in the tank follows it.

2. The Valve Opens
When the water level in the tank falls below the bottom of the bell, air enters the bell and the siphon is broken. By this time, the float has fallen far enough to open the valve, and water under pressure enters to refill the tank and the float begins to rise again."

Below are photographs I took of the pictures in Macaulay's book.

Since the air pressure exerts a force that pushes downward on the water in the tank at all times, and since the force that air pressure exerts on the water is sufficient to push all the water up the siphon tube and past the bend until the water level falls below the bottom of the bell, I don't see why the air pressure would not force the water up the siphon tube on its own without the disk lifting water up the siphon tube. Why won't the air pressure exerting a downward force move all the water in the tank up over the bend in the siphon tube until the water level falls below the bell without the disk pushing the water up the siphon tube?

 

[Andrew Mason]

The reason is that the contents going down out of the toilet exit have kinetic energy and simple atmospheric air pressure is not sufficient to stop it. The toilet is designed so that the rush of water from the tank displaces enough water to completely fill and block the lower part of the toilet drain channel (as it goes down and exits the toilet under gravity). This creates a partial vacuum within that portion of the drain channel within the toilet such that the downward pressure from incoming water and atmospheric pressure forcefully pushes the remaining contents downward and out the drain channel. The principle has some similarity to how a slinky pulls itself down stairs: part of the slinky gains enough kinetic energy so that it pulls the rest of it down with it. The descending water creates "suction" which assists in the contents above being pulled down (actually pushed down by a greater force from above than is pushing up from below, but we tend to think of suction as pulling).

AM

 

[fourthindiana]

The reason is that the contents going down out of the toilet exit have kinetic energy and simple atmospheric air pressure is not sufficient to stop it.

When you say it in the sentence above, I am assuming that "it" means the water going down out of the toilet. My interpretation of what you said: The reason is that the contents going down out of the toilet exit have kinetic energy and simple atmospheric air pressure is not sufficient to stop the water going down out of the toilet.

If I am correct about what you mean by the word "it" in the sentence above, I don't see how your response answers my question.

The toilet is designed so that the rush of water from the tank displaces enough water to completely fill and block the lower part of the toilet drain channel (as it goes down and exits the toilet under gravity). This creates a partial vacuum within that portion of the drain channel within the toilet such that the downward pressure from incoming water and atmospheric pressure forcefully pushes the remaining contents downward and out the drain channel. The principle has some similarity to how a slinky pulls itself down stairs: part of the slinky gains enough kinetic energy so that it pulls the rest of it down with it. The descending water creates "suction" which assists in the contents above being pulled down (actually pushed down by a greater force from above than is pushing up from below, but we tend to think of suction as pulling).

AM

From your description, it seems like David Macaulay did a lousy job explaining how a toilet works. Macaulay never even uses the word vacuum in his explanation in how a toilet works.

I don't really understand how the movement of water down the drain of the toilet could create a vacuum. I don't fully understand your explanation.

By the way, it appears to me that your response to me is just a general explanation for how a toilet works, rather than answering my specific question.

 

[fourthindiana]

https://en.wikibooks.org/wiki/Wikijunior:How_Things_Work/Flush_Toilet

wikibooks.org seems to do a better job of explaining how a toilet works than David Macaulay's book.

From wikibooks.org: "When you press the handle, a lever inside the tank pulls the piston up, forcing some water through the siphon. This provides suction in the siphon, and the rest of the water within the siphon 'U' tube follows."

How does forcing some water through the siphon create suction in the siphon?

 

[PeterDonis]

since the force that air pressure exerts on the water is sufficient to push all the water up the siphon tube and past the bend until the water level falls below the bottom of the bell

It isn't. When the toilet is not being flushed and the tank is full, and everything is just sitting there, the level of water in the tank will be below the top of the siphon tube. There will also be water inside the siphon tube itself--but not up to the top. The top will have air in it, and the air will push back against the water inside the siphon with the same pressure as the air inside the tank is pushing on the water in the tank outside the siphon. So everything will be in equilibrium.

How does forcing some water through the siphon create suction in the siphon?

By removing the air at the top of the siphon tube that was pushing back: as the piston pushes the water up and over the top of the siphon, it pushes the air along with it. And behind the water that was pushed up and over by the piston, there is nothing--unless more water flows in from the tank. That's where the suction comes from.

 

[fourthindiana]

It isn't. When the toilet is not being flushed and the tank is full, and everything is just sitting there, the level of water in the tank will be below the top of the siphon tube. There will also be water inside the siphon tube itself--but not up to the top. The top will have air in it, and the air will push back against the water inside the siphon with the same pressure as the air inside the tank is pushing on the water in the tank outside the siphon. So everything will be in equilibrium.

Ah---I didn't consider that the air in the top of the siphon tube would push back against the water inside the siphon.

By removing the air at the top of the siphon tube that was pushing back: as the piston pushes the water up and over the top of the siphon, it pushes the air along with it. And behind the water that was pushed up and over by the piston, there is nothing--unless more water flows in from the tank. That's where the suction comes from.

So as the disk pushes the water up the siphon tube, the air that was in the top of the siphon tube sucks the water through the siphon tube and into the toilet bowl?

 

[Andrew Mason]

When you say it in the sentence above, I am assuming that "it" means the water going down out of the toilet. My interpretation of what you said: The reason is that the contents going down out of the toilet exit have kinetic energy and simple atmospheric air pressure is not sufficient to stop the water going down out of the toilet.

If I am correct about what you mean by the word "it" in the sentence above, I don't see how your response answers my question.

From your description, it seems like David Macaulay did a lousy job explaining how a toilet works. Macaulay never even uses the word vacuum in his explanation in how a toilet works.

The "partial vacuum" is just a reduction in pressure caused by the movement of liquid within a closed space. That creates a siphon effect by drawing liquid into the space it leaves.

I don't have access to Macaulay's book and I can't see the whole toilet from what you have posted. I thought you were asking about the flush of the contents of the bowl through the S trap in the bottom of the toilet and down the drain.

I don't really understand how the movement of water down the drain of the toilet could create a vacuum. I don't fully understand your explanation.

Initially you have water filling in the bottom of the toilet. A large volume of water descends into the bowl pushing water over the lip of the S trap and filling the pipe below. That water drops downward through the drain pipe. This creates a partial vacuum in the space that the water previously occupied. This low pressure draws contents in the bowl to fill the space. That is the siphon effect that flushes the toilet.

AM

 

[fourthindiana]

I don't have access to Macaulay's book and I can't see the whole toilet from what you have posted. I thought you were asking about the flush of the contents of the bowl through the S trap in the bottom of the toilet and down the drain.

AM

I was asking about why the air pressure in the toilet tank (not the toilet bowl) does not push all the water up the siphon tube without the disk pushing water upward into the siphon tube. I don't see how anyone could possibly think I was asking about the flush of the contents of the toilet bowl through the S trap in the bottom of the toilet.

 

[Andrew Mason]

I am not sure what kind of vintage toilet Macaulay is explaining.

In any event, the principle is the same: you want to set a fluid in motion using the siphon effect. In order to do that you need to start fluid moving in a closed space. That creates lower pressure that draws fluid into the space vacated and that continues until there is no more fluid to be drawn in.

AM

 

[PeterDonis]

So as the disk pushes the water up the siphon tube, the air that was in the top of the siphon tube sucks the water through the siphon tube and into the toilet bowl?

No. The disk pushes the water in one side of the siphon tube up and over the top of the tube, where it then falls down the other side. The air that was at the top of the siphon tube is ahead of that mass of water; it goes down the other side of the siphon before the water does. What is left on the back side of that mass of water is nothing, unless water from the tank moves into follow it. That's the suction effect.

 

[PeterDonis]

I don't see how anyone could possibly think I was asking about the flush of the contents of the toilet bowl through the S trap in the bottom of the toilet.

In older toilets, the S trap is the siphon; its purpose is to suck water out of the toilet bowl and down the outflow line downstream of the S trap. See, for example, here:

http://web.mit.edu/2.972/www/reports/toilet/toilet.html

The kind of toilet in the article you link to is a newer kind where a piston pushes water up a siphon tube inside the tank. The purpose is to reduce the amount of water per flush by getting the water moving faster before it flows into the bowl (in the older type of toilet, the water just starts gravity flowing out of the tank into the bowl, and there is no suction effect until that mass of water pushes the water in the bowl out and over the top of the S trap).

 

[fourthindiana]

No. The disk pushes the water in one side of the siphon tube up and over the top of the tube, where it then falls down the other side. The air that was at the top of the siphon tube is ahead of that mass of water; it goes down the other side of the siphon before the water does. What is left on the back side of that mass of water is nothing, unless water from the tank moves into follow it. That's the suction effect.

I don't see how your description here is different than what I said. In both your description and mine, the air that is ahead of the mass of water pushed up by the siphon sucks the water through the siphon; correct?

Or are you saying that the fact that there is nothing behind the mass of water pushed up the siphon somehow pushes the water up the siphon? If so, I don't see how that would be suction. I have always thought that by definition, suction is something pulling towards itself, not pushing away from itself.

Please clarify.

 

[PeterDonis]

In both your description and mine, the air that is ahead of the mass of water pushed up by the siphon sucks the water through the siphon; correct?

No. The air ahead of the mass of water can't suck anything; its pressure is not lower than the pressure of the water behind it. The pressure behind the mass of water being lowered (because the water was pushed up by the piston leaving a void space for the water in the tank to flow into) is the suction.

are you saying that the fact that there is nothing behind the mass of water pushed up the siphon somehow pushes the water up the siphon?

No, it sucks the water from the tank up the siphon.

I have always thought that by definition, suction is something pulling towards itself, not pushing away from itself.

This is just quibbling over words. "Suction" means there is low pressure ahead of whatever is being sucked. But "low pressure" is relative--"low" compared to what? Obviously, to the pressure behind whatever is being sucked (which in this case is the pressure of the water in the tank, and ultimately the pressure of the atmosphere pushing down on the water in the tank).

 

[fourthindiana]

No, it sucks the water from the tank up the siphon.

My understanding is that in the sentence you wrote above, "it" is the mass of water left on the backside of the disk. Another way of stating it is how you did it earlier in post #13: the pressure behind the mass of water being lowered. I can see how the low pressure underneath the disk can suck water from the tank to underneath the disk. But how can the pressure underneath the disk suck water from the tank up the siphon? When the disk raises up, the low pressure area you are speaking of is underneath the disk. Low pressure areas can only suck water from the tank towards themselves, not towards somewhere else.

 

[PeterDonis]

My understanding is that in the sentence you wrote above, "it" is the mass of water left on the backside of the disk.

You need to read more carefully. I was responding to this question of yours:

are you saying that the fact that there is nothing behind the mass of water pushed up the siphon somehow pushes the water up the siphon?

And I answered:

No, it sucks the water from the tank up the siphon.

Substitute "sucks the water from the tank up the siphon" from my sentence for "somehow pushes the water up the siphon" in your sentence. What do you get? And what does that tell you about the referent of the word "it" in my sentence?

how can the pressure underneath the disk suck water from the tank up the siphon?

I assume by "disk" you mean the piston, as it is shown in Macaulay's drawing (since the word "disk" doesn't even appear in the other sources referenced in this thread). There are two answers to this:

(1) The disk doesn't stay up; it falls back. Once it falls back, water from the tank gets sucked up the siphon by the low pressure area there that was left behind the mass of water that the disk pushed up and over the siphon.

(2) As noted in the wiki site you referenced: "The piston is usually covered by a plastic membrane, which is constructed to be "sucked" out of the way by the emptying siphon tube water, allowing the rest of the water in the tank to rush past the piston".

 

[fourthindiana]

I assume by "disk" you mean the piston, as it is shown in Macaulay's drawing (since the word "disk" doesn't even appear in the other sources referenced in this thread). There are two answers to this:

(1) The disk doesn't stay up; it falls back. Once it falls back, water from the tank gets sucked up the siphon by the low pressure area there that was left behind the mass of water that the disk pushed up and over the siphon.

Now I get it. Thank you. I think I understand this completely now.

 

[Andrew Mason]

My understanding is that in the sentence you wrote above, "it" is the mass of water left on the backside of the disk. Another way of stating it is how you did it earlier in post #13: the pressure behind the mass of water being lowered. I can see how the low pressure underneath the disk can suck water from the tank to underneath the disk. But how can the pressure underneath the disk suck water from the tank up the siphon? When the disk raises up, the low pressure area you are speaking of is underneath the disk. Low pressure areas can only suck water from the tank towards themselves, not towards somewhere else.

It is a bit difficult to figure out what the entire mechanism does because you have only shown a few parts and they don't seem to show the same thing. But in the right side drawing showing the plunger or piston with two discs, the lever lifts the discs and the top disc pushes the water above it up through the siphon pipe starting the flow. The top disc goes through the hole and stops when the second larger disc gets stopped by the edge of the hole. The second disc has holes so the low pressure below the discs draws water up and through the holes continuing the flow. So the flow continues after the piston stops.

AM