Therm

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Natural Gas is usually measured by volume in the United States and is stated in cubic feet. A cubic foot of gas is the amount of gas needed to fill a volume of one cubic foot under set conditions of pressure and temperature. To measure larger amounts of natural gas, "CCF" (centum cubic-feet) is used to denote 100 cubic feet, and "MCF" (mil cubic-feet) is used to denote 1,000 cubic feet. To provide greater accuracy in comparing fuels, energy content is measured in terms of "British Thermal Units (BTU's)". A BTU is the amount of heat required to raise one pound of water (approximately a pint), one degree Fahrenheit at or close to its point of maximum density.

One British therm (symbol thm) is a non-SI unit of heat energy equal to 100,000 British thermal units (BTU). It is approximately the energy equivalent of burning 100 cubic feet (often referred to as 1 CCF) of natural gas.

Since natural gas meters measure volume and not energy content, a therm factor is used by (Natural) gas companies to convert the volume of gas used to its heat equivalent, and thus calculate the actual energy use. The therm factor is usually in the units therms/CCF. It will vary with the mix of hydrocarbons in the natural gas. Natural gas with a higher than average concentration of ethane, propane or butane will have a higher therm factor. Impurities, such as carbon dioxide or nitrogen, lower the therm factor.

The volume of the gas is calculated as if measured at standard temperature and pressure (STP). The heat content of natural gas is solely dependent on the composition of the gas, and is independent of temperature and pressure.

One therm is equal to about 105.5 megajoules, 25,200 kilocalories or 29.3 kilowatt-hours. One therm can also be provided by about 96.7 cubic feet (2.74 m3) of natural gas. The therm sometimes has been confused with the thermie (see below). The names of both units come from the Greek word for heat.

A thermie (th) is a metric unit of heat energy, part of the meter-tonne-second system sometimes used by European engineers. The thermie is equal to the amount of energy required to raise the temperature of 1 tonne of water by 1 °C. The thermie is equivalent to 1,000 kilocalories, 4.1868 megajoules or 3968.3 BTU.

Definitions

  • Therm (EC) ≡ 100,000 BTUIT[1]
= 105,506,000 joules
≈ 29.3072222 kWh
The therm (EC) is often used by engineers in the US.
  • Therm (US) ≡ 100,000 BTU59°F[2]
= 105,480,400 joules
≈ 29.3001111 kWh.
  • Therm (UK) = 105,505,585.257 348 joules[3]
≈ 29.30710701583 kWh

Ten therms are a decatherm. Common abbreviations are as follows:[4]

Dthdecatherm
MDththousand decatherms
MMDthmillion decatherms

Usage

United Kingdom regulations were amended to replace therms with joules with effect from 1999. Despite this, natural gas is now usually retailed in kilowatt-hours, although the wholesale UK gas market trades in therms, with a typical forward transaction being for 25,000 therms/day (31 MW). In the United States, however, natural gas is commonly billed in CCFs (100 cubic feet) or therms.

Carbon footprint

According to the Pacific Gas and Electric Company emissions rate, burning natural gas produces on average 13.446 pounds (6.099 kg) of carbon dioxide per therm.[5]

See also

References

  1. ↑ Official Journal L 073, P. 0114 27 March 1972
  2. ↑ 15 USC Chapter 6
  3. ↑ The Units of Measurement Regulations 1995
  4. ↑ washingtonpost.com
  5. ↑ "Carbon Footprint Calculator Assumptions" (PDF). Pacific Gas and Electric Company. Retrieved 2009-10-13. 
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