The Wobbe Index measures the interchangeability of fuels like natural gas and propane, with different fuels requiring adjustments for safe use. The index is named after Goffredo Wobbe, who determined the relationship between heat output, specific gravity, and calorific value. Fuel gases are divided into three families, with LPG fuels having the highest index numbers. Appliances are generally equipped to handle gas from a single family, with a five percent range in the Wobbe index being acceptable for replacement fuel. The index is calculated by dividing the highest heating index by the square root of the specific gravity of the gas. The lower the Wobbe index, the less energy is released per volume burned, which can result in underburning and failure to ignite.
The Wobbe Index is a measure of the degree of interchange between fuels such as natural gas and propane. It’s an important measure for appliances that use these fuels, since a gas line to your home can carry gas that comes from a variety of different sources and suppliers. The index provides an easy way to compare which particular fuels are directly compatible and which may require adjustments to be used safely.
The index is named after the Italian scientist Goffredo Wobbe. In the late 1920s, he first determined the relationship between the heat output of a burner, the specific gravity of the gas used and its highest calorific value. His work is critical given the important assumption that not all fuel gases are created equal.
For the purposes of the Wobbe index, fuel gases are divided into three families, which are manufactured or “city” gas, natural gas, and liquefied petroleum gas (LPG). Pressure, specific gravity and calorific value constitute the potential energy of a fuel and vary between families and even between gases of the same type, depending on the supplier. LPG fuels typically have the highest index numbers, with natural gas in the middle and town gas on the low end.
The appliances are generally equipped to handle gas from a single family with a limited degree of variation in composition. Generally, a five percent range in the Wobbe index is acceptable for replacement fuel. Any greater differential requires at least the adjustment or replacement of a burner component.
A fuel’s Wobbe index number is calculated by dividing its highest heating index, the amount of energy released as heat during combustion, by the square root of the specific gravity of the gas. The result is a number that can be used as a basis for comparing different gases. British thermal units per standard cubic foot (BTU/scf) or megajoule per standard cubic meter (MJ/Sm3) are the common units of measurement for the index, but to eliminate confusion it is common practice to simply deal with the Wobbe metric calculation numbers and do not list the type of unit.
The lower the Wobbe index of a fuel, the less energy it releases per volume burned. A hob designed for gas with a Wobbe rating of 50, for example, will deliver too much air when fired with gas with a Wobbe rating of 35. This can result in underburning and even failure to ignite, which can be extremely dangerous.
Protect your devices with Threat Protection by NordVPN
