Therm

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The therm (symbol, thm) is a non-SI unit of heat energy equal to 100,000 British thermal units (BTU).[1] It is approximately the energy content of 100 cubic feet (2.83 cubic metres) – often referred to as 1 CCF – of natural gas at standard temperature and pressure. As there are differing definitions of the BTU in use around the world, there are various slightly differing values for the energy content of the therm.

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 expressed in units of therms per 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. Non- flammable Impurities, such as carbon dioxide or nitrogen, lower the therm factor. The therm factor is an alternative to the Wobbe Index of a fuel gas, which also allows comparison of heat per unit volume burnt.

The volume of the gas is calculated as if measured at standard temperature and pressure (STP).

One therm is equal to about 105.5 megajoules, 25200 kilocalories, 25.2 thermies or 29.3 kilowatt-hours.

The therm sometimes has been confused with the thermie. The names of both units come from the Greek word for heat.

Definitions[edit]

  • Therm (EC) ≡ 100000 BTUISO[2]
    = 105506000 joules
    29.3072 kWh
    The therm (EC) is often used by engineers in the US.
  • Therm (US) ≡ 100000 BTU59 °F[3]
    = 105480400 joules
    29.3001111111111 kWh.
  • Therm (UK) ≡ 105505585.257348 joules[4]
    29.3071070159300 kWh

10 therms are known as a decatherm (sometimes, dekatherm;[5] commonly abbreviated Dth), which is 1000000 Btu (of whichever type). Further common abbreviations are MDth for a 1000 decatherms, and MMDth for 1000000 decatherms.[5][failed verification]

Usage[edit]

United Kingdom regulations were amended to replace therms with joules with effect from 1999, with natural gas usually retailed in the derived unit, kilowatt-hours. Despite this, the wholesale UK gas market trades in therms. In the United States, natural gas is commonly billed in CCFs (hundreds of cubic feet) or therms.

Carbon footprint[edit]

According to the EPA burning one therm of natural gas produces on average 5.3 kg (11.7 lb) of carbon dioxide.[6]

See also[edit]

References[edit]

  1. ^ IEEE Std 260.1-2004
  2. ^ Official Journal L 073, P. 0114 27 March 1972
  3. ^ 15 USC Chapter 6 Archived 2006-01-25 at the Wayback Machine
  4. ^ The Units of Measurement Regulations 1995
  5. ^ a b Jerry Knight (22 January 1978). "Gas Utilities Stepping Up Efforts to Add Customers". The Washington Post. Retrieved 1 June 2016.
  6. ^ epa.gov "Greenhouse Gases Equivalencies Calculator - Calculations and References