Designing an Elastomer Pressure Vessel for Constant Pressure Applications

[spiri]

Hello Physics Forum. I'm looking to use the vapor pressure of a liquid to apply a constant pressure to a piston. The vapor pressure of the liquid is higher than the pressure I'm looking to exert (vapor pressure is ~16psi and I need to push with a pressure of 5.5psi). I'm thinking of using an elastomer that is fairly stiff so that it will effectively reduce the pressure from 16psi down to 5.5 due to it's elasticity. Does anyone know what the governing equations would be? I'm thinking of this like an expanding balloon, but as the piston is moving, the pressure needs to remain constant and I'm not even sure that's possible because the starting volume of the liquid is 0.01 mL and the piston needs to move a total volume of 3 mL. What do you folks think, will something like this work?

 

[Baluncore]

Is the elastomer a balloon in space or on the surface of the Earth.
Atmospheric pressure is 14.5psi, you will not have enough differential pressure.

 

[spiri]

Thanks, but I'm not sure I follow. The liquid inside the balloon will generate a pressure of 16psi once it reaches its vapor pressure temperature, correct?

 

[Baluncore]

Gauge pressures are measured with reference to atmospheric pressure.
Absolute pressures are measured with reference to a vacuum.
http://en.wikipedia.org/wiki/Gauge_...e_and_differential_pressures_-_zero_reference
Vapour pressures are absolute pressures.

Take a balloon, insert only the liquid, seal the balloon. The vapour pressure of the gas in the liquid – gas equilibrium will be a function of the temperature. If the vapour pressure specified for your liquid is 16 psi (absolute) then at sea level, it will be opposed by the atmospheric pressure of 14.5 psi absolute. That will mean only 16 – 14.5 = 1.5 psi will remain to inflate the balloon against atmospheric pressure. That will not be sufficient to apply the differential pressure of 5.5 psi that you specify.

To get the 5.5 psi pressure you require, you will need to climb to an altitude where the atmospheric pressure is 14.5 – 5.5 = 9 psi. That will be at about 4km = 13000 feet above sea level.
Alternatively you could heat the balloon to a temperature where the vapour pressure was 14.5 + 5.5 = 20 psi.

 

[spiri]

Got it, thanks! As it turns out, the temperature is higher than RT so the pressure will suffice, since I can also choose from a list of propellants. Any thoughts on a container design that can stretch as much as I need it to stretch?

 

[Baluncore]

Any thoughts on a container design that can stretch as much as I need it to stretch?

How much do you need it to stretch? 0.01 mL to 3.00 mL = 300 times the volume.
But the volume comes from a moving piston, not from the vessel?
I do not know what the liquid is or what temperature and chemical compatibility is required.
You will need to present much more detailed information before PF can help you further.

 

[spiri]

No, the increased volume comes from the expansion of the propellant inside a "capsule" made out of an elastomer. The chemicals can be from the following list at 37C:diethyl ether
1-fluorobutane
2-fluorobutane
1,2-difluoroethane

What I'm looking for is a material that will expand and contain any of these materials. Does that help?

 

[Baluncore]

I would expect those compounds to escape into or through polymers. How good must the seal be ?

What volume range ?
Over what storage temperature range ?

What is the vapour pressure range over those temperatures ?

How often will the elastomer cycle through a change in volume ?
10 times per second, once per day or once per recharge ?