Temperature & Pressure
Liquid fuels such as gasoline and Diesel hold about the same volume of fuel across a broad range of conditions. Like most gaseous molecules, CNG volume expands in higher temperature conditions and contracts in lower temperature conditions. As a result, a CNG vehicle tank that can hold 45 GGE at 3600 psi at 70 degrees F may only hold 75% (or less) of that volume on a 95 degree day. Temperature compensated dispensers make sure vehicles get the best fill possible without lifting safeties on the vehicle, balancing maximum fuel volume on the vehicle while preventing otherwise vented and wasted fuel.
On a hot day, a CNG fast-fill dispenser will stop flow at a given pressure. After a short time, CNG in the tank cools and pressure is therefore reduced. Keep in mind, the same volume of energy is contained in this sealed vehicle storage tank even though you may see a reading of 4150 psi on the vehicle storage tank’s pressure gauge right after refueling and 3500 psi after an hour of sitting still. The following chart was developed by ANGI Systems to represent the relationship between temperature and pressure in a CNG storage vessel:
|Temp F||PSI (for 3600 PSI @ 70 F)||PSI (for 3000 PSI @ 70 F)|
The Department of Energy, Clean Cities web site describes that the volume of CNG that can be stored in a vehicle’s tank varies based on the following variables:
- Fueling rate: As the rate of fueling increases, the temperature of the fuel also increases—dramatically. As the fuel warms up it expands and becomes less dense and, therefore, contains less energy by volume when the fuel system reaches the rated pressure. For this reason, you are usually able to get more CNG into a tank with a time-fill versus a fast-fill application. This is because when gas molecules are compressed they create heat. The faster they are compressed, the more they heat up and expand. So when you compress the gas rapidly, using a fast-fill station, the molecules will heat up and expand more than if filled slowly over time.
- Ambient temperature: The outside temperature affects the temperature of the CNG. At higher temperatures, CNG is less dense, and therefore does not contain as much energy per unit volume as it would at a lower temperature. When the CNG is stored in warm ambient temperatures, it expands and becomes less dense, so when the tank reaches the rated pressure, the CNG inside does not contain as much energy as it would at lower temperatures.
- Pressure rating: The typical industry standard for CNG fueling system pressure is 3,600 psi. Some systems in the U.S. and many systems overseas are rated at 3,000 psi. These fill pressures are based on a 70 degree F ambient temperature. The cylinders are actually designed to hold up to 125% of their operating pressure. So, a 3,000 psi tank can be filled to 3,750 psi and a 3,600 tank can be filled to 4,500 psi. This makes it possible to fill a tank to a higher pressure on hot days when the gas is expanding, as well as compensate for the heat of recompression. A good rule of thumb is that for every 10 degrees plus or minus 70 degrees F, the pressure will expand or contract 100 psi.
- Cylinder type: There are four types of cylinders (Type 1-4). The type designation is based on the way the cylinder is made and the material it is made out of (aluminum, steel, carbon fiber, etc.). The material used to make the cylinder also affects the temperature in the tank, and thus, affects how “full” you can fill the tank. For example, carbon fiber tanks hold heat better than steel tanks. The temperature inside the tank affects the expansion and contraction of the gas, and therefore, how much gas the tank will hold. Tanks also come in various sizes and hold different volumes of gas depending on their construction.
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