Understanding the effects that heat and pressure have on liquid propane is critical to understanding how propane behaves. Propane is affected by heat and pressure in much the same way as water. At atmospheric pressure, the boiling point of propane is -44Fo. At any temperature below that, a pool of propane will remain in liquid form, because its vapor pressure is less than atmospheric. At temperatures above -44Fo, the vapor pressure of propane is greater than atmospheric pressure, therefore the liquid will vaporize.
Keeping this in mind, when the valve of a propane appliance is open, propane will flow to the burner, and the demand for gas vapor will immediately cause a slight drop in pressure inside the container. This upsets the heat/pressure balance and causes the propane to begin boiling off vapor to replace the vapor going into the burner.
As long as the demand for propane remains, the propane will continue to boil, supplying fuel to the burner.
When the valve on the appliance is closed, the propane will stop flowing and return to its balance point. The boiling will eventually stop as the balance is reached.
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For more information about CETP and CETP E-Learning, get in touch with the Propane Education & Research Council's CETP Program Manager, Courtney Gendron, firstname.lastname@example.org.