Share on Facebook Share on Twitter Email
Answers.com

Wobbe index

 
Sci-Tech Dictionary: Wobbe index
Home > Library > Science > Sci-Tech Dictionary
(′wä·bə ′in′deks)

(thermodynamics) A measure of the amount of heat released by a gas burner with a constant orifice, equal to the gross calorific value of the gas in British thermal units per cubic foot at standard temperature and pressure divided by the square root of the specific gravity of the gas.

Search unanswered questions...

Enter a question here...
Search: All sources Community Q&A Reference topics

Measures and Units: Wobbe index
Top
Home > Library > Science > Measures and Units

engineering An indicator of the quality of a fuel gas, measured from the heat produced by burning through a defined orifice in standard temperature and pressure conditions, quantitatively defined as the ratio of Btu's per cubic foot to the square root of the specific gravity of the gas. Traditional ‘town gas’, which is mostly carbon monoxide (CO), typically has an index in the range 701 to 760 and is given the classification label G4; inferior forms can be much lower, with G number G5 for 641 to 700, G6 for 591 to 640, G7 for 531 to 590, and G8 below that. Natural gas, which is mostly methane (CH4) at the well and almost entirely methane after refining for public use, typically has an index of 1 300 or more. Most bills for gas involve a heat-value factor to correct for variations in quality; measured centrally to represent average quality fed into the distribution network, this is applied to the measured volume consumed by each customer to establish the energy charge. The factor could be the Wobbe index, but may be in common energy units or the ratio of current heat-energy content to the reference value used in setting the tariff.

Wikipedia: Wobbe index
Top
Home > Library > Miscellaneous > Wikipedia

The Wobbe Index (WI) is an indicator of the interchangeability of fuel gases such as natural gas, liquified petroleum gas (LPG), and Town Gas and is frequently defined in the specifications of gas supply and transport utilities. If VC is the higher heating value, or calorific value, and GS is the specific gravity, the Wobbe Index, IW, is defined as

I_W = \frac {V_C} {\sqrt{G_S}}.

That is, Wobbe Index = higher heating value/(square root of gas specific gravity).

The Wobbe Index is used to compare the combustion energy output of different composition fuel gases in an appliance (fire, cooker etc.). If two fuels have identical Wobbe Indices then for given pressure and valve settings the energy output will also be identical. Typically variations of up to 5% are allowed as these would not be noticeable to the consumer.

The Wobbe Index is a critical factor to minimise the impact of the changeover when analyzing the use of substitute natural gas (SNG) fuels such as propane-air mixtures.

Wobbe Index of common fuel gases

Fuel Gas         Upper Index
Kcal/Sm3		         Lower Index
Kcal/Sm3		         Upper Index
MJ/Sm3		         Lower Index
MJ/Sm3
Hydrogen 11,528 9,715 48.23 40.65
Methane 12,735 11,452 53.28 47.91
Ethane 16,298 14,931 68.19 62.47
Ethylene 15,253 14,344 63.82 60.01
Natural gas 12,837 11,597 53.71 48.52
Propane 19,376 17,817 81.07 74.54
Propylene 18,413 17,180 77.04 71.88
n-butane 22,066 20,336 92.32 85.08
Iso-butane 21,980 20,247 91.96 84.71
Butylene-1 21,142 19,728 88.46 82.54
LPG 20,755 19,106 86.84 79.94
Acetylene 14,655 14,141 61.32 59.16
Carbon monoxide 3,060 3,060 12.80 12.80


Note: 1 Joule = 2.3901×10−4 kcal.


Usage

The Wobbe index is commonly expressed in British Thermal Units (BTU) per standard cubic foot or megajoules per standard cubic meter (1000 BTU/scf = 37.3 MJ/Sm3). In the case of natural gas (molar mass 17 g/mol), the typical heating value is around 1,050 BTU per cubic foot and the specific gravity is approximately 0.59, giving a typical Wobbe index of 1,367 BTU/scf (51 MJ/m3).

There are three ranges or "families" of fuel gases that have been internationally agreed based on Wobbe Index. Family 1 covers manufactured gases, family 2 covers natural gases (with high and low ranges) and family 3 covers liquefied petroleum gas (LPG). Combustion equipment is typically designed to burn a fuel gas within a particular family: hydrogen-rich Town Gas, Natural Gas or LPG.

Family     Wobbe Index range (MJ/Sm3)         Type of gas    
1 22.5 – 30 Town gas / Syngas
2 L 39 – 45 Natural
2 H 45.5 – 55
3 73.5 – 87.5 LPG

Other flame characteristics and composition limits may determine the acceptability of the replacement gas, e.g. flame speed, "yellow tipping" due to incomplete combustion, sulfur content, oxygen content, etc.

External links


This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

 
 

 

Copyrights:

Sci-Tech Dictionary. McGraw-Hill Dictionary of Scientific and Technical Terms. Copyright © 2003, 1994, 1989, 1984, 1978, 1976, 1974 by McGraw-Hill Companies, Inc. All rights reserved.  Read more
Measures and Units. A Dictionary of Weights, Measures, and Units. Copyright © Donald Fenna 2002, 2004. All rights reserved.  Read more
Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Wobbe index".  Read more