British thermal unit (Btu)

The British thermal unit (BTU or Btu) is a traditional unit of work equal to about 1055 joules. It is the amount of work needed to raise the temperature of one pound of water by one degree Fahrenheit. One four-inch wooden kitchen match consumed completely generates approximately 1 BTU. In science and engineering, the joule, the SI unit of energy, has largely replaced the BTU.

The BTU/h is most often used as a measure of power in the electric power, steam generation, heating, and air conditioning industries. It is still used in some metric English-speaking countries (such as Canada, but notably not the United Kingdom). In North America, the heat value (energy content) of fuels is often expressed in BTUs.

The notation kBtu or KBTU is often used for thousand BTU, in sizing of heating systems and in the Energy Use Index (EUI) expressed as thousand BTU annual energy use per square foot of building. MBTU represents one million Btu, although the atypical notationMMBtu or mmBtu is sometimes used to represent one million BTU.(see definitions below)

British thermal unit

Definitions[edit]

A BTU is the amount of heat required to raise the temperature of 1 avoirdupois pound of liquid water by 1 degree Fahrenheit at a constant pressure of one atmosphere.[1] As with the calorie, several definitions of the BTU exist, because the temperature response of water to heat energy is non-linear. This means that the change in temperature of a water mass caused by adding a certain amount of heat to it will be a function of the water’s initial temperature. Definitions of the British thermal unit (BTU) based on different water temperatures can therefore vary by up to 0.5%.

Nominal temperature BTU equivalent in joules Notes
39 °F (3.9 °C) ≈ 1059.67 Uses the calorie value of water at its maximum density (4 °C or 39.2 °F)
Mean ≈ 1055.87 Uses a calorie averaged over water temperatures 0 to 100 °C (32.0 to 212.0 °F)
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,[2] Appendix A. This value uses the IT calorie and is rounded to a realistic accuracy
59 °F (15.0 °C) ≡ 1054.804 Chiefly American. Uses the 15 °C calorie, itself now defined as exactly 4.1855 J (Comité international 1950; PV, 1950, 22, 79–80)
60 °F (15.6 °C) ≈ 1054.68 Chiefly Canadian
63 °F (17.2 °C) ≈ 1054.68
Thermochemical (“Th”[3]) ≡ 1054.35026444 Uses the “thermochemical calorie” of exactly 4.184 J

The unit MBtu or mBtu was defined as one thousand British thermal unit, presumably from the Roman numeral system where “M” or “m” stands for one thousand (1,000). This notation is easily confused with the SI mega- (M) prefix, which denotes multiplication by a factor of one million (×106), or with the SI milli- (m) prefix, which denotes division by a factor of one thousand (×10−3).

To avoid confusion, some companies and engineers use the notation “MMBtu” or “mmBtu” to represent one million BTU (although, confusingly, MM in Roman numerals would traditionally represent 2,000) and in many contexts this form of notation is deprecated and discouraged in favour of the more modern SI prefixes. Alternatively, the term therm may be used to represent 100,000 (or 105) BTU, and quad for 1015 BTU. Some companies also use ‘BtuE6’ in order to reduce confusion between 103 BTU and 106 BTU.[4]

Conversions[edit]

One BTU is approximately:

  • 1.054 to 1.060 kJ (kilojoules)
  • 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)
  • 5.40395 (lbf/in2)·ft3

A BTU can be approximated as the heat produced by burning a single wooden kitchen match[5] or as the amount of energy it takes to lift a one-pound weight 778 feet (237 m).[6]

For natural gas[edit]

  • In natural gas, by convention 1 MMBtu (1 million BTU) = 1.054615 GJ.[7]
  • The energy content (high or low heating value) of a volume of natural gas varies with the composition of the natural gas, which means there is no universal conversion factor for the number of BTU to volume. 1 standard cubic foot of average natural gas yields ≈ 1030 BTU (between 1010 BTU and 1070 BTU, depending on quality, when burned)
  • As a coarse approximation, 1000 ft3 of natural gas yields ≈ 1 MMBTU ≈ 1 GJ
  • For natural gas price conversion 1000 m3 ≈ 36.906 MMBTU and 1 MMBTU ≈ 27.096 m3

As a unit of power[edit]

When used as a unit of power for heating and cooling systems, BTU per hour (BTU/h) is the correct unit, though this is often abbreviated to just “BTU”.[8]

  • 1 watt is approximately 3.41214 BTU/h[9]
  • 1000 BTU/h is approximately 293.071 W
  • 1 horsepower is approximately 2544 BTU/h

Associated units[edit]

  • 1 ton of cooling, a common unit in North American refrigeration and air conditioning applications, is 12,000 BTU/h (3.52 kW). It is the amount of power needed to freeze 1 short ton (0.893 long tons; 0.907 t) of water into 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 while the EU uses the BTUIT. The therm is used to price natural gas.[10]
  • 1 quad (short for quadrillion BTU) is 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 in 2005.[11]One quad/year is about 33.43 gigawatts.

The British thermal unit 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 3,412 BTU).

The British thermal unit is often used to express the conversion-efficiency of heat into electrical energy in power plants. Figures are quoted in terms of the quantity of heat in British thermal unit required to generate 1 kW·h of electrical energy. A typical coal-fired power plant works at 10,500 BTU/kW·h, an efficiency of 32–33%.[12]

See also[edit]

References[edit]

  1. Jump up^ “What is British thermal unit (Btu)? definition and meaning”. Businessdictionary.com. Retrieved 2011-11-11.
  2. Jump up^ International standard ISO 31-4:1992 Quantities and units—Part 4: Heat
  3. Jump up^ Btu – Kilocalories units conversion
  4. Jump up^ EIA (2012-09-17). “What are Mcf, BTU, and therms? How do I convert prices in Mcf to BTUs and therms?”. US Government.
  5. Jump up^ Energy and the Environment. Ristinen, Robert A. c. 2006, pg 13
  6. Jump up^ Energy and the Environment. Ristinen, Robert A. c. 2006, pg 14
  7. Jump up^ “Energy Measurements”. Energy.gov.ab.ca. Retrieved 5 April2014.
  8. Jump up^ Ken Matesz (2010). Masonry Heaters: Designing, Building, and Living with a Piece of the Sun. Chelsea Green Publishing. p. 148.
  9. Jump up^ 2009 ASHRAE Handbook – Fundamentals (I-P Edition). (pp: 38.2). American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc
  10. Jump up^ “The GB gas wholesale market”. OFGEM. Retrieved Jan 13,2013. The wholesale gas market in Britain has one price for gas irrespective of where the gas comes from. This is called the National Balancing Point (NBP) price of gas and is usually quoted in price per therm of gas.
  11. Jump up^ Husher, John Durbin. Crises of the 21st Century: Start Drilling-The Year 2020 Is Coming Fast, iUniverse, 2009. Page 376.
  12. Jump up^ Electric Generation Efficiency, NPC Global Oil & Gas Study, 18 July 2007