Why Does an LPG Tank Freeze as Gas is Used? Explained

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SUMMARY

The phenomenon of an LPG tank appearing to freeze during gas usage is due to the cooling effect associated with the gas expansion process. As liquefied petroleum gas (LPG) is drawn from the tank, the pressure decreases, causing the liquid to vaporize. This phase change absorbs energy from the tank's surroundings, resulting in a drop in temperature. Ice forms on the tank's exterior, not because the tank itself freezes, but due to the gas doing work on the surrounding atmosphere as it expands.

PREREQUISITES
  • Understanding of thermodynamics, specifically phase changes.
  • Familiarity with the properties of liquefied petroleum gas (LPG).
  • Knowledge of gas laws and pressure-temperature relationships.
  • Basic principles of energy transfer and work in physical systems.
NEXT STEPS
  • Research the thermodynamic principles of phase changes in gases and liquids.
  • Study the properties and behavior of liquefied petroleum gas (LPG) under varying pressure conditions.
  • Learn about the Joule-Thomson effect and its implications in gas expansion.
  • Explore the relationship between pressure, temperature, and energy transfer in gases.
USEFUL FOR

This discussion is beneficial for students and professionals in fields such as chemical engineering, thermodynamics, and anyone involved in the handling or utilization of liquefied petroleum gas (LPG).

Ask1122
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Hi all, I am having trouble grasping the phenomenon on why the LPG tank freezes as you use up more and more of the gas inside.

My understanding is this. Initially, the tank is full of liquefied gas, and as you use up the gas, the pressure inside the tank drops, and since the temperature inside the tank did not change, the liquid should flashes into gas. But I don't understand how this can make the tank itself become cold? Is it maybe because the liquid inside the tank used the surrounding energy to change into gas?

Thanks.
 
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the liquid inside the tank used the surrounding energy to change into gas

Well done.
The energy to change state has to come from somewhere and the tank is not a closed system.

BTW: strictly speaking the LPG tank does not freeze - what happens is that ice forms on the outside of the tank.
 
This also happens with a compressed air tank where there is no phase change. It turns out that the cooling happens because the gas "does work" on the surrounding atmosphere as it expands - it is not exactly like pushing on a solid boundary but somewhere in between that and free expansion.
 

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