British thermal unit
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The British thermal unit (BTU or Btu) is a unit of energy used in North America. It is also still occasionally encountered in the United Kingdom, 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 In most other areas, it has been replaced by the SI unit of energy, the joule (J).
In the United States, the term "BTU" is used to describe the heat value (energy content) of fuels, and also to describe the power of heating and cooling systems, such as furnaces, stoves, barbecue grills, and air conditioners. When used as a unit of power, BTU per hour is understood, though this is often confusingly abbreviated to just "BTU".
The unit MBTU was defined as one thousand BTU presumably from the Roman numeral system where "M" stands for one thousand (1,000). There is currently a social push to redefine MBTU as one million (1,000,000) BTU, thus making the unit more intuitive with metric system that uses "M" to mean mega, or 106. To avoid confusion many companies and engineers use MMBTU to represent one million (1,000,000) BTU.
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[edit] Definitions
A BTU is defined as the amount of heat required to raise the temperature of one pound 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 up to 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 |
[edit] Conversions
One BTU is approximately:
- 1,054-1,060 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 MMBtu (1 million Btu, sometimes written "mmBTU") = 1.054615 GJ. Conversely, 1 gigajoule is equivalent to 26.8 m³ of natural gas at defined temperature and pressure.
- 1 MMBtu = 1,000 cubic feet (Mcf) natural gas
[edit] Associated units
The BTU per hour (BTU/h) is the unit of power most commonly associated with the BTU.
- 1 watt is approximately 3.41 BTU/h
- 1000 BTU/h is approximately 293 W
- 1 horsepower is approximately 2540 BTU/h
- 1 "ton of cooling", a common unit in North American refrigeration and air conditioning applications, is 12,000 BTU/h. It is the amount of power needed to melt one short ton of ice in 24 hours.
- 1 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.
- 1 quad (energy) (short for quadrillion BTU) 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 99.75 quads/year in 2005. [1].
The BTU should not be confused with the Board of Trade Unit (B.O.T.U.), which is a much larger quantity of energy (1 kWh, or about 3412 BTU).
