In: Other
In using my propane powered grill the other day (nice day, 65°F) I noticed the metal cylinder holding the propane had frost forming on it. Why?
This process is known as auto refrigeration and it happens when the temperature of liquid propane falls below its boiling point.
When the liquid propane vaporizes, it cools the liquid and the pressure inside the cylinder holding the propane decreases as propane is released. As the pressure in the cylinder drops and liquid propane vaporizes, it cools with in the container. Moisture or frost will be collected on the cylinder till the level of the liquid if the temperature drops below the dew point of surrounding air.
When the vapors are removed very fast from the cylinder, the liquid inside the cylinder boils more vigorously which cause the increase in cooling rate of the liquid more than it can absorbs heat from the surrounding and when the temperature of the liquid inside the cylinder falls well below the dew point of surrounding air, we can observe frost forming on the cylinder. The frost acts as an insulator resulting in less heat transfer from outside to the inside of cylinder hence increase more frost.