British thermal unit

Updated on Oct 03, 2024

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The British thermal unit (Btu or BTU) is a traditional unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is part of the British Imperial system of units. Its counterpart in the metric (SI) system is the calorie, which is defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius. Heat is now known to be equivalent to energy, for which the metric unit is the joule; one BTU is about 1055 joules. While units of heat are often supplanted by energy units in scientific work, they are still important in many fields. As examples, in the United States the price of natural gas is quoted in dollars per million BTUs. Chemical bond energies are often given in calories per mole of substance.

Definitions

A BTU was originally defined as 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. There are several different definitions of the BTU that are now known to differ slightly. This reflects the fact that the temperature change of a mass of water due to the addition of a specific amount of heat (calculated in energy units, usually joules) depends slightly upon the water's initial temperature. As seen in the table below, definitions of the BTU based on different water temperatures vary by up to 0.5%. In the table, thermochemical and steam table (IT) values, which are now defined in terms of exact values in joules, have been rounded to four decimal places.

Prefixes

The units MBtu and MMBtu are used in the natural gas and other industries to indicate 1,000 and 1,000,000 BTUs, respectively. This is presumably from the Roman numeral system, where "M" stands for one thousand (1,000).

There is an ambiguity in that the metric system uses the prefix "M" to indicate one million (1,000,000), and "MBtu" is also used to indicate one million BTUs. Because of this ambiguity, some authors have deprecated the use of the unit MBtu.

The unit therm is used to represent 100,000 (or 10⁵) BTUs. The unit quad is commonly used to represent one quadrillion (10¹⁵) BTUs.

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 or as the amount of energy it takes to lift a one-pound (0.45 kg) weight 778 feet (237 m).

For natural gas

In natural gas pricing, the Canadian definition is that 1 MMBtu (1 million Btu) = 1.054615 GJ.

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

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".

1 watt is approximately 3.41214 Btu/h

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 rate of heat transfer needed to freeze 1 short ton (907 kg) of water into ice in 24 hours.

In the US, the R-value that describes the performance of thermal insulation is typically quoted in ft²·°F·hr/BTU. For one square foot of the insulation, one BTU per hour of heat flows across the insulator for each degree of temperature difference across it.

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 in natural gas pricing in the United Kingdom.

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. 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%.

Centigrade heat unit

The centigrade heat unit (CHU) is the amount of heat required to raise the temperature of one pound of water by one degree Celsius. It is equal to 1.8 BTU or 1899 joules. This unit was sometimes used in the United Kingdom as an alternative to BTU but is now obsolete.

References

British thermal unit Wikipedia

(Text) CC BY-SA

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