heating an aluminum can

[CodeQuandary]

I'm curious, but ill-equipped to analyze: if I have an aluminium can, let's say 12 oz volume. I fill it with water and nitrogen, to create a 2 Atm internal pressure so it is structurally robust (i.e. can be stacked). My understanding is that the can is capable of withstanding 6 Atm of pressure. I need to be able to heat the can to nearly boiling. The question is, how much water can I put in it to enable heating to what temperature before I hit the 6 Atm point?
My guess is that there's not much variation in the water-to-ultimate-temperature ratio: the pressure rises so abruptly that there's not much adjustability.
But I don't know what that temperature is, nor what is the minimal nitrogen amount to work.
How much do things change if I swap out the nitrogen for air?
Thanks in advance for helping me think about this!

 

[marty007]

This sounds like a school assignment...

These forums are intended for working engineers, not for school assignments. If this question is school work related, please find another discussion group for support.

Cheers,

 

[JStephen]

For the initial case, assume the masses of water and nitrogen. Temperature and pressure are known. Assume the partial pressure of the water vapor will be the saturation pressure corresponding to that temperature, and partial pressure of the nitrogen is the remaining pressure, which is then known. Calculate the volume of the nitrogen at that partial pressure, assume the same volume for water vapor at its partial pressure, and you can calculate the mass of the water vapor. Remaining water is liquid at known specific volume. From this, you can calculate total volume, and adjust the masses of each component until the calculated volume matches the actual volume of the container.

After heating, total volume is known and the masses are the same. Assume water vapor is saturated vapor with partial pressure equal to saturation pressure, total volume is known, so volume of water vapor and liquid can be determined. Assume nitrogen volume is same as water vapor volume, calculate partial pressure for it, add it to partial pressure of water vapor, and that is total pressure at that condition.

When the can is heated and pressurized, the can volume changes due to thermal expansion and due to pressure changes and due to change of modulus of elasticity as it heats. Those may all be negligible, but you'd need to make some estimate of the difference to confirm that. Using partial pressures to calculate volume of the mixed vapors may not always be accurate. Variations in the volume of the liquid may turn out to be negligible.

 

So, if I understand it, the structural integrity of you stacked cans depends on keeping it pressurized and heated.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[CodeQuandary]

Hi, thanks for the thoughts.
Yes, the structural integrity or at least structural capacity of the cans is developed in conjunction with the internal pressure.
No, this is not a school assignment, it's a personal project. The actual intention is to use stacked water-filled cans for thermal storage, within a temperature range of about 55-180F probably. I don't have much free time to think about this, so when I get a minute I'll try to run through JStephen's thinking more carefully.

 

[LittleInch]

COde quandary,

If your aim is to store heat in these cans up to 180F, yes the water would be near boiling point at 0 barg, but at an initial pressure of 2 barg in the can, the boiling point of water is around 260-270 Deg F

Whilst there is some thermal expansion of the water which is higher than that of the can, these are very small amounts so I would guess you're looking at 90-95% water content.


not much will change in N2 for air as 79% is N2...

However your issue will be how to get heat transfer in and out and why you're bothering with a bunch of cans - just use a big tank

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