Pressure differences in a leaking vacuum vessel

[some bloke]

Hello all, I'm new to this forum and I'm mucking about with some self-made physics conundrums, and I need some help!

So, What I am aiming for is a roughly cylindrical vessel (it can, and I think probably has to, be tapered to achieve the effect) with the air being pumped out of one end, and a valve offering a controlled amount of resistance at the other end to create a partial vacuum. The valve can be opened to a point where the air flows freely through the pipe and the pressure difference from outside is negligible.

What I want to achieve is for the pressure, compared with the ambient pressure around the vessel, to be different at one end of the pipe to the other.

I know that "sucking" isn't a thing, it's the pressure difference pushing the air through, and I know that if a pipe is constricted and then opened up again, there will be a pressure difference between the two sides. Will the vessel require an hourglass shape? or simply a taper from one end to the other? I assume if the hourglass shape is used then the highest pressure difference would be between the narrowest point and the air outlet (from which the air is pumped) as the constricted diameter would reduce the flow.

The aim of this vessel is for it to hold a capped rubber tube open (as per the classic balloon-lung demonstration where the balloon is connected by a pipe to the outside and the air is sucked out, causing the balloon to inflate) and then, as the valve is opened, the tube constricts onto a metal cylinder which is placed inside the rubber tube. The need for a pressure difference is that I want the rubber tube to deflate at the capped end slightly before the open end as the valve closes, to try and eliminate any air entrapment between the tube and the cylinder. If the open end deflates first, then the air inside the tube will have no means to escape.

It's all still at the "thought experiment" stage right now, so any alternative means to achieve this will also be gratefully received.

 

[Dale]

It sounds like what you want is less of a pressure vessel and more of a wind tunnel. Narrow sections of a wind tunnel will have lower pressure and higher velocity (assuming low Mach number flow throughout)

 

[Chestermiller]

Why do you feel that you need a conical or hour-glass shape to achieve the partial vacuum?

 

[some bloke]

The conical or hourglass profile was not to achieve the partial vacuum (this is achieved by the pump sucking air out of one end and a valve at the other end restricting the air allowed back into compensate), it was to achieve a different pressure at one end to the other.

A wind tunnel is probably a good comparison, actually, as it is a flow of air through the vessel. The pressures involved are less than those outside the vessel, as (for want of a better phrase) the air is being sucked out, not pumped in.

Result I'm aiming for is a consistent lower pressure at one end than the other, and all of it lower than atmospheric.

As I have little knowledge on this one, for all I know the pressures are already doing what I wanted them to do!

 

[russ_watters]

What I want to achieve is for the pressure, compared with the ambient pressure around the vessel, to be different at one end of the pipe to the other.

How much different? The only way to make it different, for an open path, is to have a high airflow over an obstruction, creating a pressure drop. The obstruction could be a valve, orifice plate or narrow piping.

Can you explain why you are trying to do this?

 

[Chestermiller]

The conical or hourglass profile was not to achieve the partial vacuum (this is achieved by the pump sucking air out of one end and a valve at the other end restricting the air allowed back into compensate), it was to achieve a different pressure at one end to the other.

A wind tunnel is probably a good comparison, actually, as it is a flow of air through the vessel. The pressures involved are less than those outside the vessel, as (for want of a better phrase) the air is being sucked out, not pumped in.

Result I'm aiming for is a consistent lower pressure at one end than the other, and all of it lower than atmospheric.

As I have little knowledge on this one, for all I know the pressures are already doing what I wanted them to do!

You can put orifice plates at intervals along a cylindrical vessel to achieve a pressure gradient (at least a pressure gradient greater than that which would naturally occur due to ordinary turbulent friction).

Oops. I see that Russ beat me to it.

 

[some bloke]

So if I had orifice plates installed (which I'm assuming would be a reduced bore to restrict the air flow), would that lead to a greater pressure difference (relative to outside, IE more vacuum) on the side of the suction or the side of the air in valve?

The end goal is to apply a rubber sleeve to an object with minimal air entrapment. Initially I am using a cylinder/rod for proof of concept, as it will easily highlight any air entrapment, but moving forwards the objects could be irregular or fragile shapes where sliding or rolling the sleeve on would be impossible. My solution is to basically inflate the sleeve, put the object inside, then deflate the sleeve. the sleeve needs to deflate from the closed end first to assist in removing any air entrapment - if the open end closes first, you still have a "balloon" around the object!

The reason - it was just a thought experiment. I'm prone to inventing things, usually reinventing the wheel a few times along the way. But I like to understand how things work, and with each problem I solve making things with no real application, I make myself more capable if a similar problem arises with real-world applications.

 

[nettleton]

in the absence of obstacles, changes in cross-section pressure differences in a lengthy vessel will depend on the difference between the rate of pumping and the diffusion of gas molecules.

 

[nsaspook]

Normally you would look at vacuum conductance to make rough calculations. That depends on the level of desired vacuum for the type of flows expected.
https://vacaero.com/information-res...25-conductance-in-vacuum-pumping-systems.html

 

[DaveC426913]

It sounds like you're doing something sort of like coating an object in a flexible sleeve, like shrink-wrapping. (I don;t suggest it's the same thing, I'm just trying to visualize it)

The only reason to have to lower the ambient air pressure is to force the air out from under the wrap layer. But if the air can only escape at very specific points (such as the two ends) then there's not really much accomplishing by having the entire object in low pressure - since air can't escape anywhere but the ends. Right?

So why not just apply suction to the open end of the shrink wrap to suck any air out?

 

[some bloke]

So why not just apply suction to the open end of the shrink wrap to suck any air out?

The tube starts off smaller than the object being wrapped. the low pressure inflates the tube, the object is put inside, then the valve will be opened to reduce the pressure difference and allow the tube to constrict around the object. The air can only escape at one end of the tube (which is open to the outside). The aim is to ensure that the tube constricts slightly quicker at the closed end than the other, so that air entrapment is kept to a minimum.

I haven't done any tests yet (I'm still designing the chamber) so it might not actually be a problem. But I started thinking about it, and now I have to know if there's a way to accomplish it, even if it's not a problem!

 

[gneill]

Would it not be simpler to just dip the object in liquid latex and let it dry?

 

[LURCH]

So if I had orifice plates installed (which I'm assuming would be a reduced bore to restrict the air flow), would that lead to a greater pressure difference (relative to outside, IE more vacuum) on the side of the suction or the side of the air in valve?

Suction side. Try thinking of the orifice plates as a series of valves set at intervals along the length of the tube. Obviously, they are not adjustable, so consider them to be valves with a very large Internal Diameter, all of which are stuck partially open.

 

[some bloke]

Suction side. Try thinking of the orifice plates as a series of valves set at intervals along the length of the tube. Obviously, they are not adjustable, so consider them to be valves with a very large Internal Diameter, all of which are stuck partially open.

Ok, that makes sense and will probably be the direction I'll take this.

Out of curiosity, what effect will a greater restriction of the bore have? I'm thinking that if the rubber sleeve expands (as I hope it will) then it will reduce the opening and so affect the pressure - will it stabilise, or just result in the sleeve expanding until its tension can overcome the suction, or it blocks the tube?

As for dipping it in liquid latex, I'm looking for a quick method of applying a fairly thick (2-3mm) rubber coat, so liquid latex would take too long.

 

[LURCH]

Out of curiosity, what effect will a greater restriction of the bore have? I'm thinking that if the rubber sleeve expands (as I hope it will) then it will reduce the opening and so affect the pressure - will it stabilise, or just result in the sleeve expanding until its tension can overcome the suction, or it blocks the tube?

Either of those is a possibility, it would depend on how narrow the Internal Diameter of the plates would be, and how stretchy the sleeve is. But I don’t know if that matters because, as I try to picture the device, I just don’t see how it could work. Is this sleeve open only at one end, like a balloon? If so, then the current design idea will give you a sleeve that is largest at the closed end, which is (if I understand correctly) exactly the opposite of what you want.

Perhaps a better solution is closer to your original idea; make the vacuum chamber tapered. However, it might be useful to make it small enough so that the sleeve expands to come into contact with the inner walls all the way along the length. With the vacuum pump attached to the narrow end, and the open end of the sleeve stretching out to cover the wide end, suction would cause the sleeve to expand and seal itself against the inner walls, rather like vacuum-packing, but in reverse. Then you could place whatever you want inside the sleeve, and pull the hose for the vacuum pump off of the narrow end (at this point, it might be necessary to have a valve at the narrow end, to control the rate at which air can rush back into the chamber).

As the air comes in, the sleeve detaches itself from the narrow end of the chamber first, which allows more air to get between the sleeve and the inner wall, and the process works its way up to the open end in a manner that should be controllable.

 

[russ_watters]

The tube starts off smaller than the object being wrapped. the low pressure inflates the tube, the object is put inside, then the valve will be opened to reduce the pressure difference and allow the tube to constrict around the object. The air can only escape at one end of the tube (which is open to the outside). The aim is to ensure that the tube constricts slightly quicker at the closed end than the other, so that air entrapment is kept to a minimum.

Is the tube not uniform in thickness/strength? Or is the open end reinforced? Why else would it inflate instead of the air just rushing out the open end?

I don't think a pressure gradient will keep the end open and I don't think the orifice plates are a real solution because they neither provide a real gradient nor allow the chamber to be open enough inside to put objects into it.

To me, it seems when you pump-down the chamber the air should force it's way out the open end. The reason it doesn't with a balloon is the open end of the balloon is reinforced for the exact purpose of not expanding, but instead remaining tight around a nozzle.

 

[Tom.G]

Taper the wall thickness of the rubber tube such that the end you want to collapse first is stronger (thicker) than the rest of it.

Cheers,
Tom

 

[some bloke]

Either of those is a possibility, it would depend on how narrow the Internal Diameter of the plates would be, and how stretchy the sleeve is. But I don’t know if that matters because, as I try to picture the device, I just don’t see how it could work. Is this sleeve open only at one end, like a balloon? If so, then the current design idea will give you a sleeve that is largest at the closed end, which is (if I understand correctly) exactly the opposite of what you want.

Perhaps a better solution is closer to your original idea; make the vacuum chamber tapered. However, it might be useful to make it small enough so that the sleeve expands to come into contact with the inner walls all the way along the length. With the vacuum pump attached to the narrow end, and the open end of the sleeve stretching out to cover the wide end, suction would cause the sleeve to expand and seal itself against the inner walls, rather like vacuum-packing, but in reverse. Then you could place whatever you want inside the sleeve, and pull the hose for the vacuum pump off of the narrow end (at this point, it might be necessary to have a valve at the narrow end, to control the rate at which air can rush back into the chamber).

As the air comes in, the sleeve detaches itself from the narrow end of the chamber first, which allows more air to get between the sleeve and the inner wall, and the process works its way up to the open end in a manner that should be controllable.

I hadn't thought about letting the sleeve meet the inner wall of the vessel, now that you say it it becomes more obvious!

Like you said, this would restrict which parts of the sleeve can constrict and should make it constrict in the way I'm after. And yes, the sleeve is only open at one end, like a balloon.

Thanks!