Compressed air cylinder temperature during rapid depressurisation

[Airou]

Hi guys,

Here's an interesting problem for you. I'm trying to understand what happens to the temperature of air cylinders as the compressed air within is rapidly expanded to see if the surface in contact with the expanding air gets so cold it passes through the ductile to brittle transition temperature.

I can use Refprop and iteration to work out the temperatures and pressures adiabatically, but what I am struggling with is figuring out the heat flow into the rapidly cooling air from the surroundings through the wall of the cylinder.

For the external skin of the cylinder (in contact with ambient, room temperature air) I have assumed an air-to-surface thermal resistance of 0.123 m^2K/W used as a rule of thumb in the construction industy for internal wall surfaces in calculating heat losses in buildings, but I have no idea what the surface-to-air resistance would be from the steel to compressed air and how that figure changes with pressure and temperature.

Any thoughts/pointers to where I can find that info would be greatly appreciated.

 

[mawais15]

you have not mentioned the amount of compression. also the convection coefficient varies as the density of the air within the cylinder changes during expansion.

 

[Airou]

you have not mentioned the amount of compression. also the convection coefficient varies as the density of the air within the cylinder changes during expansion.

Thanks for the reply. I'm trying to make a model of the system that allows me to vary the starting pressure in the cylinder and deplete it to various end pressures as I vary the time period over which the depressurisation occurs. Typically though I will be at the 300 bar level where ideal gases can't be assumed.

Yes, it is how the convection coefficient changes with time as the thermodynamic properties of the air changes as it goes from 300 bar at ambient pressure to 40 bar at circa -100C in about 15 seconds that I am looking for. All my books, googling etc. only really look at what occurs at ambient conditions and with ideal gases.

I'm trying to ascertain if repeated blowdown of cylinders present a risk to the integrity of the pressure vessel by embrittling the surface in contact with the expanding air.

Many thanks :)

 

[mawais15]

what i got from ur question is that what will be the heat transfer coefficient between the compressed gas and the cylinder wall.

We can use another approach. let's say the convection coefficient between outside air and cylinder wall is X
now the air inside the cylinder is expanding to 40 bar which is greater than outside pressure, hence the density of the air inside the cylinder would also be greater making this clear that the inside heat transfer coefficient would be less as compared to the outside. so u can assume the overall thermal resistance as : X + Y + X
where Y is the thermal resistance of cylinder wall.

 

[mawais15]

of course repeated compression and expansion exerts a fatigue stress in the pressure vessel. within elastic limit, the vessel would face as many stress changes as there are strokes of compression expansion

 

[Airou]

what i got from ur question is that what will be the heat transfer coefficient between the compressed gas and the cylinder wall.

Yes, I want to understand how the heat transfer coefficient changes as the temperature and pressure of the air in the cylinder falls.

We can use another approach. let's say the convection coefficient between outside air and cylinder wall is X
now the air inside the cylinder is expanding to 40 bar which is greater than outside pressure, hence the density of the air inside the cylinder would also be greater making this clear that the inside heat transfer coefficient would be less as compared to the outside.

Intuitively I would have thought that a higher pressure gas has a higher heat transfer coefficient as the molecules would be closer together enabling more rapid heat transfer as the mean path between collisions would be less.

My guesses seem to be reflected in this table:

http://www.engineeringtoolbox.com/heat-transfer-coefficients-exchangers-d_450.html

so u can assume the overall thermal resistance as : X + Y + X
where Y is the thermal resistance of cylinder wall.

I appreciate the answer, but I'm not really looking for a quick fix. I'd really like to know how to calculate the R-value for an air film for a given pressure and temperature.

 

[mawais15]

I appreciate the answer, but I'm not really looking for a quick fix. I'd really like to know how to calculate the R-value for an air film for a given pressure and temperature.

well, what you want is not impossible, but you would need to involve some mathematics in your solution

i could've worked with a general solution about the variation in the thermal resistance with temperature and pressure of an ideal gas but I'm stucked in a job deeply . .

Regards