British thermal unit
The British thermal unit (BTU or Btu) is a unit of heat energy traditionally used in engineering, particularly in the United States, defined as the amount of heat required to raise the temperature of one avoirdupois pound (approximately 453.6 grams) of liquid water by one degree Fahrenheit at the temperature at which water has its greatest density (approximately 39 °F or 4 °C) and at a specified pressure.[1][2] This definition stems from early 19th-century efforts to quantify the heating value of fuels, making it a key measure in industries like power generation, steam systems, and heating, ventilation, and air conditioning (HVAC).[3] Several variations of the BTU exist due to differences in the reference temperature for the water's specific heat capacity, leading to slight discrepancies in value.[4] The most common modern standard is the international table BTU (IT BTU), equivalent to exactly 1 055.056 joules (J).[5] Other variants include the thermochemical BTU (1,054.35 J), the 59°F BTU (1,054.80 J), and the mean BTU, which averages the heat capacity over the range from 32°F to 212°F and equals 1,055.87 J.[4] In practice, the IT BTU is the one adopted for most international engineering standards and energy calculations, while the mean BTU appears in some historical or specific U.S. contexts.[5] The BTU remains integral to energy policy and industry, especially for expressing the heat content of fossil fuels, where, for example, one cubic foot of natural gas typically contains around 1,000–1,050 BTUs.[1] It also rates the output of appliances, such as furnaces in BTU per hour (BTU/h), and features prominently in U.S. energy statistics from agencies like the Energy Information Administration, which report national consumption in quadrillion BTUs (quads).[1] Although the International System of Units (SI) joule has largely supplanted it globally, the BTU persists in North American HVAC design, fuel pricing, and building codes for its practical familiarity in imperial measurements.[5]Fundamentals
Definition
The British thermal unit (BTU or Btu) is a unit of heat energy defined in the imperial and United States customary measurement systems as the quantity of heat required to raise the temperature of one pound (avoirdupois) of liquid water by one degree Fahrenheit at a constant pressure of one atmosphere and a specified reference temperature.[6] Alternatively, the mean BTU can be expressed as one-one hundred eightieth of the total heat needed to raise one pound of water from 32°F to 212°F at standard pressure.[7] In thermodynamics, the BTU specifically measures thermal energy in the form of heat, which involves the transfer of energy due to temperature differences, distinguishing it from mechanical work units like the foot-pound that quantify energy from force applied over distance.[8] Although both represent energy, the BTU's focus on heat makes it suitable for contexts involving temperature changes in substances, particularly water as the reference medium.[6] As an imperial unit, the BTU is primarily used in the United States and, to a lesser extent, the United Kingdom for heating, ventilation, air conditioning systems, and assessing the energy content of fuels.[1] The "British" in its name reflects its origins in the Anglo-Saxon engineering traditions of the 19th century, despite its predominant modern application in American industries.[9]Variations
The variations in the definition of the British thermal unit (BTU) stem from differences in the specific heat capacity of water, which is not constant but varies slightly with temperature and pressure conditions, leading to distinct energy values for raising one pound of water by one degree Fahrenheit at different reference points.[10] These discrepancies arose historically as engineers and scientists refined measurements based on experimental data for specific applications, such as steam tables or chemical processes. The International Table BTU (IT BTU), established through international agreements in the 1960s, is defined as exactly 1055.05585262 joules and serves as a standardized value for general engineering and scientific calculations.[10] In contrast, the thermochemical BTU, valued at 1054.350 joules, derives from the thermochemical calorie and is primarily employed in chemical engineering contexts where precise thermodynamic reactions are analyzed.[5] The mean BTU represents an average value over the temperature range from 32°F to 212°F, equating to approximately 1055.87 joules, and was used in early steam engineering tables to approximate heat transfer across boiling and freezing points.[5] Other variants include the 59°F BTU, defined at 1054.80 joules, which appears in older engineering references for room-temperature applications like HVAC design, reflecting the specific heat at that ambient condition.[5] In modern usage, the IT BTU is preferred as the standard by the National Institute of Standards and Technology (NIST) for consistency with international metric conversions and contemporary energy measurements.[10]Historical Development
Origins
The conceptual origins of the British thermal unit (BTU) lie in the 1840s experiments on the mechanical equivalent of heat conducted by James Prescott Joule and contemporaries, which demonstrated the convertibility of mechanical work into heat and laid the groundwork for quantifying heat as energy in engineering contexts.[11] The earliest known definition of a unit akin to the BTU was provided by British engineer Thomas Tredgold in 1824, as the heat required to raise the temperature of one cubic foot of water by one degree Fahrenheit.[12] The term "British thermal unit" appeared in 1859 in the work of Scottish engineer William John Macquorn Rankine, A Manual of the Steam Engine and Other Prime Movers, where he defined it as the quantity of heat required to raise the temperature of one pound of water by one degree Fahrenheit for use in steam engine efficiency calculations.[13] The BTU initially found application in British engineering for evaluating coal combustion and steam power generation, reflecting the era's focus on industrial machinery, well before its broader uptake in American practices.[13] Early implementations revealed ambiguities in the definition, notably the absence of a precise reference temperature for the water, which caused variations across 19th-century engineering tables and computations.[5]Standardization
In the early 20th century, efforts to standardize the British thermal unit (BTU) focused on selecting a specific temperature reference to minimize variations in heat capacity of water. The American Society of Refrigerating Engineers (ASRE) adopted the 59°F BTU in 1929 for refrigeration and cooling calculations, defining it as the heat required to raise one pound of water from 59°F to 60°F at standard atmospheric pressure. This choice aligned with practical engineering needs in temperature-controlled environments. The merger of ASRE and the American Society of Heating and Air-Conditioning Engineers (ASHAE) to form the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) in 1959 accelerated unification. In 1961, ASHRAE incorporated the International Table BTU (IT BTU) into its data books and standards, defining it as the mean heat to raise one pound of water by one degree Fahrenheit over the range from 32°F to 212°F, equivalent to the energy for a 1°F rise at 39.37°F (4°C). The International Organization for Standardization (ISO) supported this in 1978 through ISO 31-4, which standardized related thermal units and conversions, promoting the IT BTU for international consistency in engineering tables.[14] The National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards (NBS), played a pivotal role in calibrating and fixing the IT BTU value during the 1940s and 1960s. Through precise thermochemical measurements and steam table validations, NBS established the IT BTU as exactly 1055.05585262 joules, providing a definitive reference for calibrations in energy and HVAC applications. During the 1970s U.S. metrication debates, initiated by the Metric Conversion Act of 1975 and overseen by the U.S. Metric Board, the BTU resisted full replacement by SI units like the joule due to its deep integration in the fossil fuel, HVAC, and building sectors. Despite recommendations for SI adoption, industries lobbied for BTU retention, citing cost and familiarity. NIST refined the BTU-to-joule conversion in the 1990s, confirming the exact factor in Special Publication 811 (1995 edition) to support hybrid metric-imperial workflows.[15][16] Post-2000 updates have been minimal, with no redefinition of the BTU, but the 2019 SI revision reaffirmed legacy units' role via fixed exact conversions in the SI Brochure, ensuring ongoing compatibility without altering values. NIST continues digital standardization efforts, including updated references for computational tools in the 2020s. Standardization faced challenges from regional preferences pre-1980s, such as the UK's reliance on the mean BTU from imperial steam tables (approximately 1055.06 J), differing slightly from the U.S. IT BTU, which complicated transatlantic trade and engineering collaborations until ISO harmonization reduced discrepancies.[17]Conversions and Equivalents
To SI Units
The International Table British thermal unit (BTU), the most commonly used variant, is defined as exactly equal to 1055.05585262 joules (J).[18] This exact value stems from international agreements on energy unit conversions to facilitate precise scientific and engineering calculations.[18] The general conversion equation for energy from BTU to joules is: E_\text{J} = E_\text{BTU} \times 1055.05585262 where E_\text{J} is the energy in joules and E_\text{BTU} is the energy in BTU.[18] For the thermochemical BTU, which is based on the thermochemical calorie definition, the conversion factor is 1 thermochemical BTU = 1054.3503 J.[18] Common equivalents between BTU and other energy units are summarized in the following table, using the International Table BTU for consistency:| BTU | Joules (J) | Calories (cal, international) | Watt-hours (W·h) |
|---|---|---|---|
| 1 | 1055.05585262 (exact) | ≈ 251.9958 | ≈ 0.293071 |