British thermal unit

From Free net encyclopedia

The British thermal unit (BTU or Btu) is a unit of energy used in the United States. It is also still occasionally encountered in the UK, in the context of older heating and cooling systems. In most other areas, it has been replaced by the SI unit of energy, the joule (J).

A Btu is defined as the amount of heat required to raise the temperature of one pound avoirdupois of water by one degree Fahrenheit. 143 Btu is required to melt a pound of ice. As is the case with the calorie, several different definitions of the Btu exist, which are based on different water temperatures and therefore vary by about 0.5%:

Name	Value (J)	Notes
39 °F	≈ 1059.67	Uses the calorie value of water at its maximum density (4 °C)
Mean	≈ 1055.87	Uses a calorie averaged over water temperatures 0 °C to 100 °C
IT	≡ 1055.05585262	The most widespread Btu, uses the International [Steam] Table (IT) calorie, which was defined by the Fifth International Conference on the Properties of Steam (London, July 1956) to be exactly 4.1868 J
ISO	≡ 1055.056	International standard ISO 31-4 on Quantities and units – Part 4: Heat, Appendix A. This value uses the IT calorie and is rounded to a realistic accuracy
59 °F	≡ 1054.804	Chiefly American. Uses the 15 °C calorie, itself defined as exactly 4.1855 J (Comité international 1950; PV, 1950, 22, 79-80)
60 °F	≈ 1054.68	Chiefly Canadian
63 °F	≈ 1054.6	Possibly apocryphal
Thermochemical	≡ 1054.35026444	Uses the "thermochemical calorie" of exactly 4.184 J

In the United States, the BTU is often used to describe the heat value of fuels, and the BTU per hour (often confusingly abbreviated to BTU) measures the heating and cooling power of a system (such as a barbecue grill).

1 Conversions
2 Associated units
3 See also
4 External links

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Conversions

One BTU is approximately:

252–253 cal (calories, small)
0.252–0.253 kcal (kilocalories)
778–782 ft·lbf (foot-pounds-force)
1054–1060 joules
In natural gas, by convention 1 MM Btu (1 million Btu, sometimes written "mm BTU") = 1.054615 GJ. Conversely, 1 gigajoule is equivalent to 26.8 m³ of natural gas at defined temperature and pressure.

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Associated units

The BTU per hour (BTU/h) is the unit of power most commonly associated with the BTU.

1 horsepower is approximately 2540 BTU/h
1 watt is approximately 3.4 BTU/h
1000 BTU/h is approximately 293 W

A unit called the quad (short for quadrillion) is defined as 10¹⁵ BTU, which is about 1.055×10¹⁸ joules, and the therm is defined in the United States and European Union as 100,000 BTU –but the U.S. uses the BTU_{59 °F} whilst the EU uses the BTU_IT.

The BTU should not be confused with the Board of Trade Unit (B.O.T.U.), which is a much larger quantity of energy.

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