Pressure builds on its own inside this water tank?

[saeppi]

Hey guys,
It‘s been a few years since I had my last physics class so please be kind to me. I came here to ask you about a phenomenon I recently discovered in a design of mine.
The object in question is a stand with 3 arms, holding a water tank (see pic below).
All openings are pretty tightly closed and for a few hours there‘s no water leaking. But after a while, some pressure is building which pushes water out via the bottle screw (encircled in black). Another evidence of increased pressure is that when I let water out via one of the arms it would gush out if I wait for several hours. But soon after it would calm down and water flows smoothly again. Any ideas what‘s happening here?

 

[Baluncore]

Welcome to PF.

How do you fill and close the reservoir?
Please give us a link to the product on the web.
If you fill it with cold water, that will warm over time, and the air will expand, building up pressure.

 

[saeppi]

If you fill it with cold water, that will warm over time, and the air will expand, building up pressure.

Hi Baluncore,
Thanks, I think you‘re on point. The pressure build up happens overnight when we turn our AC off and the room warms up. How could I potentially counter that? Small hole at the top of the glass bottle?

 

[Baluncore]

How could I potentially counter that? Small hole at the top of the glass bottle?

I have never seen one before, so I do not know how it is built or operated.

How can water drain out if air cannot get back in?
As water is removed, a reverse flow of air would need to enter and bubble up to the surface, or a partial vacuum will form, that will prevent it dispensing.

A 1 mm hole in the top of the reservoir might be drilled, and could have an air permeable plug, to prevent dust entering.

 

[sophiecentaur]

How could I potentially counter that? Small hole at the top of the glass bottle?

How can water drain out if air cannot get back in?

Exactly. All the office water coolers you see work because air is allowed in. That air need not come from the 'top' but is allowed in through wht must be a non-return valve that's at a level above the water outlet. They all make that characteristic bubbling sound when you fill your cup. ~They must all have some pressure relief and so should @saeppi 's dispenser. The problem could be a design fault (form over function? - it looks nice but) in his 'interesting' model. I guess the commercial ones have a drain (for the cooler unit ?)

Don't try for a hole in the top /bottom of the bottle. You will have problems between filling it, carrying it and turning it upside down.

There is one possible solution, I just thought of, and that is to have a narrow vent tube leading down from inside the top of the bottle (just above the water level when ' full), down into the metal collar and out from side of the collar ( above the three outlets. The vent should be very narrow so that the entertaining bubbling takes place whilst still allowing a very slow rate of air to equalise the pressure at the top as the device warms up.
That would call for some DIY skills and it would depend on what's actually in the collar which the bottle sits in. OR you could just fashion a ring of sponge or felt to sit on the base and catch the drips.

 

[saeppi]

Thanks @sophiecentaur for chiming in. The pressure relief happens indeed when water is let out (and air in).
The hole at the top isn't that big of an issue as it can easily be plugged temporarily when refilling and turned upside down.
Your vent tube, @sophiecentaur , might be effective but not blending in aesthetically, I‘m afraid.
I probably just have to live with increased pressure and let the user take action.
Does anyone know of an analog pressure sensor I could add somewhere?

 

[berkeman]

Does anyone know of an analog pressure sensor I could add somewhere?

 

[Baluncore]

Does anyone know of an analog pressure sensor I could add somewhere?

The obvious short term experiment would be to replace one outlet with a manometer tube. Maybe with a thread to prevent that outlet valve from closing.
A thin manometer tube would give you pressure proportional to height.
A thick manometer tube would show the change in volume.

 

[saeppi]

thanks @Baluncore , valid suggestions.
I think it will complicate the design too much, though. I will just have to give the customer the warning of potential thermal pressure causing leaks if not used.

 

[sophiecentaur]

The obvious short term experiment would be to replace one outlet with a manometer tube.

Is it possible that the seal in the collar is not adequate? It should be able to handle excess pressure which is roughly what the gas laws say. The pressure excess would be along the lines of the proportional change of temperature of the enclosed gas. You wouldn't contemplate putting the device in sunlight so the temperature increase would only be perhaps 10C. I think that would imply an increase in pressure in the order of 10/300 = 0.03Bar, which any simple push fit seal should handle. That figure would change with water from the fridge but it's still small.

Experiment: Take a bottle of water, half full , from the fridge, remove and replace the cap and then leave it out in the room, to reach room temperature. Remove the cap and listen for a "pffft" sound as the excess pressure in the bottle vents. Does that compare in any way to the pfft sound you get when removing the cap from a half full bottle of carbonated drink? I don't think you'd hear anything. That implies the pressure we're dealing with is low.

That said, we may be chasing the problem from the wrong direction as the seal may be faulty. Whaddya think? Could it be distorted or have some grit / dust in it?

 

[Baluncore]

Whaddya think?

Maybe it only leaks when it is over-filled, and there is insufficient air to compress against the expanding water.

 

[Lnewqban]

All openings are pretty tightly closed and for a few hours there‘s no water leaking.

Welcome, @saeppi !

The device should have at least one opening freely venting to the atmosphere.
If the problem was not happening previously, it seems that clogging of that opening could be the cause of the current lack of pressure equalization.

 

[sophiecentaur]

The device should have at least one opening freely venting to the atmosphere.

Why? It must surely depend on the actual value of any excess pressure. Do you believe that the gas laws should apply (approx / ideally)?

Is that a diagram of the actual device? I think that hole at the bottopm would be a source of leakage every time there's any 'breathing' as the temperature varies.

 

[Lnewqban]

Why? It must surely depend on the actual value of any excess pressure. Do you believe that the gas laws should apply (approx / ideally)?

Is that a diagram of the actual device? I think that hole at the bottopm would be a source of leakage every time there's any 'breathing' as the temperature varies.

I don't know how this particular device works, but most water dispensers that use an inverted bottle work on the principle shown in the following diagram.

A certain amount of air replaces the volume of the limited amount of water that can flow out of the upper bottle when the level in the lower (open to atmosphere) recipient drops due to consumption.

 

[sophiecentaur]

I don't know how this particular device works, but most water dispensers that use an inverted bottle work on the principle shown in the following diagram.

A certain amount of air replaces the volume of the limited amount of water that can flow out of the upper bottle when the level in the lower (open to atmosphere) recipient drops due to consumption.

View attachment 348495

It looks to me that the diagram has been drawn wrongly. The lower container should surely be open topped so the diagram should show it. I looked at it for a while and it didn't make sense as it is. A clever mechanism, though but a bit of an extreme mod for the OP to contemplate.
The labelling is a bit confusing too. Why would they want 'Hot Water" (do they mean 'non-chilled'?) and what does the electrothermal membrane do? Do you have a link to that particular system?

 

[Lnewqban]

Do you have a link to that particular system?

Please, see:
https://www.scirp.org/pdf/JPEE_2015051916541816.pdf

http://eng-shady-mohsen.blogspot.com/2018/07/how-automatic-water-feeder-works.html

 

[sophiecentaur]

@Lnewqban Thanks for the links. The basic system with a vent at the bottom is straightforward enough. Unlike the OP's version of drink dispenser, where there seems to be no capacity to balance the pressure so it appears to be pressurised. However, as I pointed out, the temperature variations it has to deal with mean that the pressure in there will be very low, even in a warm room. The joint between the bottle and the main unit should have no problem with those pressures. I feel sure that there's a fully working prototype somewhere that doesn't leak. Either that or the designer never tested the idea and forgot about excess pressure.

The fancy version with heating / cooling included is a bit suspect, despite the nice water system. Using a heat pump will improve energy efficiency but can it do the job in the home? The heat pump is only about 50W. For an 'instant' cool or hot drink, you'd need to wait a while (despite the hoped-for COP) . Instant hot water taps are rated in kW and desirable as they may be, they cost a disgustingly high amount and, as my (well off ) son says, they are fault prone and repairs are hard. That's not a fundamental objection to the idea but, in between the designers and the users, there are a host of marketing and sales people who are milking the idea. Hot water pipes all over the house are a really daft idea but most of us have no affordable alternative.

 

[saeppi]

Thanks for all your suggestions @sophiecentaur @Lnewqban @Baluncore
I think it‘s a faulty GL32 cap which lets the water through. Below are some more pictures to illustrate the design.

 

[sophiecentaur]

I'm a lazy devil, basically and I looked for the simplest solution. After all, an upside down soda bottle doesn't leak, does it? And the thread material should be better quality than a single use pop bottle.

 

[Lnewqban]

Isn't the gap between the top metal retainer and the red cap, as well as the 40~45 mm annular gap between the red cap and the machined metal housing, the path for air to get in each time water is let out?

If so, calcium deposits may have clogged that and using some vinegar may be the easiest solution.

May I ask about the function of that small threaded hole (circled in red)?

 

[saeppi]

Its a M3 threaded hole where the pin goes that’s needed to let water from the bottle into the arms.

 

[berkeman]

vinegar may be the easiest solution

I see what you did there.