Propane (C3H8) is a popular and efficient fuel source used in everything from backyard grills to home heating systems. It is stored and transported as a liquid under pressure. However, many users find that appliances begin sputtering or cease functioning completely when the weather turns cold, as the fuel’s performance depends heavily on temperature and pressure.
The Role of Vapor Pressure in Propane Use
To be usable by an appliance, the liquid propane must change phase into a gas, a process called vaporization. This phase change occurs continuously inside the tank, creating pressure in the vapor space above the liquid. This pressure, known as vapor pressure, is the sole driving force that pushes the gaseous fuel to the appliance.
The vapor pressure inside the tank is governed by the temperature of the liquid propane. As the temperature rises, the molecules gain energy, increasing the rate of vaporization and the internal pressure. Appliances require a consistent and sufficient flow of gas, which mandates a minimum supply pressure to the regulator. If the temperature is too low, the pressure becomes insufficient to maintain the necessary flow rate, causing the appliance to perform poorly or fail.
Defining the Critical Temperature Threshold
The absolute physical limit for propane functionality is defined by its boiling point, which is -44°F, or -42°C. Below this temperature, pure liquid propane cannot transition into a gas at atmospheric pressure. The tank pressure will drop to nearly zero, making it impossible to withdraw any usable fuel.
Appliances stop working effectively long before the temperature reaches the boiling point. Most high-demand devices, such as furnaces or large heaters, begin to struggle when the ambient temperature falls into the range of 0°F to 10°F. In this range, the vapor pressure drops too low to sustain the high flow rate required. For instance, at 0°F, the vapor pressure of propane is only about 23.5 pounds per square inch gauge (PSIG).
While this pressure may seem adequate, it cannot supply the volume of gas needed to meet a high British Thermal Unit (BTU) demand. The regulator steps the tank pressure down to the typical appliance operating pressure of 11 to 14 inches of water column (WC). The regulator cannot function correctly without a sufficient minimum supply, which is why a grill or heater may sputter and eventually shut down, even if the temperature is well above the boiling point.
Why Propane Tanks Fail Above the Boiling Point
Tanks often fail when the ambient temperature is still relatively mild, sometimes above freezing. This phenomenon results from the physics of vaporization, known as evaporative cooling. The transition from liquid to gas requires a substantial amount of energy, called the latent heat of vaporization.
When an appliance draws gas, the liquid propane inside the tank must absorb heat to vaporize and replace the withdrawn gas. This required heat is drawn directly from the liquid propane and the tank walls. This heat withdrawal causes the temperature of the liquid and the tank to drop significantly below the surrounding air temperature.
The internal temperature of the tank determines the vapor pressure, not the external air temperature. As the appliance draws more gas, the internal temperature continues to fall. Eventually, the liquid propane becomes too cold to vaporize fast enough to maintain the required pressure. This cooling effect is confirmed by the formation of frost or ice on the lower exterior portion of the tank.
The presence of ice indicates that the liquid propane inside has cooled to or below the freezing point of water, regardless of the air temperature. The tank’s ability to absorb heat from the environment relates directly to its size and liquid level. A smaller tank or one with a low liquid level has a reduced surface area exposed to the ambient air, limiting the rate of heat transfer. This causes the internal temperature to drop quickly, leading to appliance failure sooner than with a full, larger tank.