British thermal unit

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The British thermal unit (BTU or Btu) is a traditional unit of energy. It is approximately the amount of energy needed to heat one pound of water one degree Fahrenheit. One Btu is equal to about 1.06 kilojoules. It is used in the power, steam generation, heating and air conditioning industries. In scientific contexts the BTU has largely been replaced by the SI unit of energy, the joule (J), though it may be used as a measure of agricultural energy production (BTU/kg). It is still used 'unofficially' in metric English-speaking countries (such as Canada, the United Kingdom), and remains the standard unit of classification for air conditioning units manufactured and sold in many non-English-speaking metric countries.

In North America, 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' (BTU/h) is understood, though this is often 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). This is easily confused with the SI mega (M) prefix, which multiplies by a factor of one million (1,000,000). To avoid confusion many companies and engineers use MMBTU to represent one million BTU. Alternatively a therm is used representing 100,000 or 10⁵ BTU, and a quad as 10¹⁵ BTU.

Definitions

A BTU is defined as amount of heat required to raise the temperature of one pound of liquid water by one degree from 60 to 61 degrees Fahrenheit at a constant pressure of one atmosphere. 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%: A BTU can be approximated as the heat burned in a single wooden match^[1] or as the amount of energy it would take to lift a one-pound weight to a height of 778 feet.^[2]

Conversions

One BTU is approximately:

• 1 054 to 1 060 J (joules)
• 0.293071 W·h (watt hours)
• 252 to 253 cal (calories, or "little calories")
• 0.25 kcal (kilocalories, "large calories", or "food calories")
• 25 031 to 25 160 ft·pdl (foot-poundal)
• 778 to 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. So, 1 MMBtu = 28.263682 m³ of natural gas at defined temperature and pressure.
• 1 standard cubic foot of natural gas yields ≈ 1030 BTU (between 1010 BTU and 1070 BTU, depending on quality, when burned)

Associated units

The BTU per hour (BTU/h) is the unit of power most commonly associated with the BTU. The term is sometimes shortened to BTU hour (BTU.h) but both have the same meaning.

• 1 watt is approximately 3.41214 BTU/h^[4]
• 1000 BTU/h is approximately 293.071 W
• 1 horsepower is approximately 2,544 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, and is approximately 3.51 kW.
• 1 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.
• 1 quad (energy) (short for quadrillion BTU) is defined as 10¹⁵ BTU, which is about one exajoule (1.055 × 10¹⁸ J). Quads are 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. One quad/year is about 33.43 gigawatts.

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 kW·h, or about 3412 BTU).

See also

• Conversion of units
• Latent heat
• Metrication

References

1. ^ Energy and the Environment. Ristinen, Robert A. c.2006, pg 13
2. ^ Energy and the Environment. Ristinen, Robert A. c.2006, pg14
3. ^ International standard ISO 31-4:1992 Quantities and units—Part 4: Heat
4. ^ 2009 ASHRAE Handbook - Fundamentals (I-P Edition). (pp: 38.2). American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc

External links

• The Units of Measurement Regulations 1995
• Natural Gas: A Primer