British thermal unit

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 (Physical analogue: one four-inch wooden kitchen match consumed completely generates approximately 1 BTU). In science, 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 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 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 notation MMBtu or mmBtu is sometimes used to represent one million BTU.(see definitions below)

Definitions

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 BTU based on different water temperatures can therefore vary by up to 0.5%.

The unit MBtu or mBtu was defined as one thousand BTU, 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

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

• 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
• 1 ft³ of natural gas yields ≈ 1000 BTU
• For natural gas price conversion 1000 m³ ≈ 36.906 MMBTU and 1 MMBTU ≈ 27.096 m³

As a unit of power

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

• 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 BTU_59 °F while the EU uses the BTU_IT. The therm is used to price natural gas.^[10]
• 1 quad (short for quadrillion BTU) is 10¹⁵ BTU, which is about one exajoule (7018105500000000000♠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 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 3,412 BTU).

The BTU 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 BTU 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

• Conversion of units
• Latent heat
• Metrication
• Ton of refrigeration

References

External links

• British thermal unit (BTU) (unit of measurement) at Encyclopædia Britannica
• The Units of Measurement Regulations 1995 HMSO
• Natural Gas: A Primer Natural Resources Canada 2015-06-17