Liquid propane (LP) is a highly concentrated energy source that powers everything from backyard grills to home heating systems. Propane is a hydrocarbon molecule (C3H8) stored under pressure to maximize density and transportability. The fuel’s journey begins with its transformation from a liquid to a gas inside the tank. This gas then passes through a precise pressure control mechanism before mixing with air at the burner tip to create a controlled, clean flame.
Storing Propane: The Liquid-to-Gas Transformation
Propane is stored as a liquid because this state is efficient for transport and storage. When compressed, liquid propane occupies about 270 times less volume than it does as a gas. The tank holds liquid propane at the bottom and a layer of propane vapor above it.
The liquid must convert to gas—a process called vaporization—before it can be used. Propane has an extremely low boiling point, about -44°F (-42°C), allowing it to readily boil into a gas even in freezing temperatures. This phase change requires heat, which the liquid propane absorbs from the ambient air surrounding the tank.
As the liquid boils, the resulting vapor generates significant pressure within the sealed tank, typically ranging from 100 to 200 pounds per square inch (PSI). This vapor pressure pushes the fuel toward the appliance. For safety, tanks are only filled to about 80% capacity to ensure adequate space for the liquid to expand without over-pressurizing the container.
Controlling the Flow: The Role of the Pressure Regulator
The high vapor pressure exiting the tank is too high for direct use by household or grill appliances. This pressure, often exceeding 100 PSI, must be reduced to a safe and functional level. A mechanical device called a pressure regulator is installed downstream of the tank connection to perform this function.
The regulator converts the high, fluctuating tank pressure into a low, steady output, typically around 0.4 PSI (11 inches of water column). This low pressure is necessary to maintain a consistent flame and prevent an uncontrolled surge of fuel. Inside the regulator, a flexible diaphragm separates the high-pressure inlet from the low-pressure outlet chamber.
The diaphragm is connected to a valve and opposed by a spring, which is factory-set for the desired low-pressure output. When gas flows out, the pressure in the low-pressure chamber drops, allowing the spring to open the valve. If the pressure gets too high, the gas pushes the diaphragm against the spring, closing the valve and restricting the flow. This mechanism ensures the appliance receives a consistent stream of fuel despite fluctuations in the storage tank pressure.
Ignition: Mixing Air and Fuel for the Flame
The regulated propane gas travels through the supply line to the burner assembly. Here, the gas must be combined with the correct amount of oxygen from the surrounding air for combustion. This mixing often utilizes the Venturi effect.
The gas is forced through a small nozzle, increasing its velocity and creating a localized drop in pressure. This pressure difference draws in the required volume of air through sized openings near the burner port. For propane, the ideal air-to-fuel ratio, known as the stoichiometric ratio, is approximately 15.67 parts of air to 1 part of propane by mass.
A proper mix ensures complete combustion, resulting in a clean, hot blue flame. If the mixture is too rich (too much fuel), the flame will be yellow and smoky, indicating incomplete combustion and wasted energy. Once the correct mixture is achieved, a spark or pilot light initiates the chemical reaction. This reaction combines propane (C3H8) and oxygen (O2), releasing heat, light, carbon dioxide, and water vapor.