British thermal unit
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 In most other areas, it has been replaced by the SI unit of energy, the joule (J).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 | |
| Thermochemical | ≡ 1054.35026444 | Uses the "thermochemical calorie" of exactly 4.184 J |
In the United States, the term "BTU" is often used to describe either energy (properly) or (improperly) power. It is used correctly to describe the heat value (energy content) of fuels, and just as frequently to describe the power in BTU per hour (often confusingly and incorrectly abbreviated to "BTU") of heating and cooling systems (such as barbecue grills or air conditioners). The BTU is a unit of energy, and therefore not a unit of power, so while power can be measured in BTU-per-hour (BTU/h), it cannot be measured in BTU any more than it can be measured in milliseconds.
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Conversions
One BTU is approximately:
- 1054–1060 joules
- 252–253 cal (calories, small)
- 0.252–0.253 kcal (kilocalories)
- 778–782 ft·lbf (foot-pounds-force)
Other conversions:
- 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.
Associated units
The BTU per hour (BTU/h) is the unit of power most commonly associated with the BTU.
- 1 watt is approximately 3.4 BTU/h [1]
- 1000 BTU/h is approximately 293 W
- 1 horsepower is approximately 2540 BTU/h
- 12,000 BTU/h is referred to as a "ton of cooling" in most North American air conditioning applications. It is the amount of energy needed to melt one short ton of ice in 24 hours.
The therm is defined in the United States and European Union as 100,000 BTU – but the U.S. uses the BTU59 °F whilst the EU uses the BTUIT. A unit called the quad (short for quadrillion) is defined as 1015 BTU, which is about one exajoule (1.055×1018 J). Quads are occasionally used in the United States for representing the annual energy consumption of large economies: for example, the U.S. economy used about 94.2 quads/year in 1997. [2].
The BTU should not be confused with the Board of Trade Unit (B.O.T.U.), which is a much larger quantity of energy.
See also
External links
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "British thermal unit". |