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Dr-Fix-It Defines the Term British Termal Unit ( BTU )
The Definition of British Thermal Unit ( BTU ), A Brief History, Practical Examples as well as Rules of Thumb..
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British Thermal Unit ( BTU )

Several hundred years ago, it was believed that heat was some sort of fluid that could not be seen or heard, only felt. Scientists of the time imagined this fluid flowing from places of high temperature to places of low temperature in the same way water flows from higher to lower elevations. At that time, the process of heat flow was not understood to be a transfer of energy, so the scientists of the day simply avoided any definition that would require this understanding! Their elegant solution was to define the thermal flow as a measurement of the effects of this thermal fluid. And so, the British Thermal Unit was proposed.
           The British Thermal Unit ( BTU ) was defined as the amount of heat necessary to raise the temperature of one pound of water one degree Fahrenheit from 58.5 to 59.5 degrees Fahrenheit.
           It wasn't until the late 1700's and early 1800's that heat was defined as a form of energy which could be measured and related to other forms of energy. However, by that time, the BTU was ingrained in common use and is still in common use to this day.

The British Thermal Unit ( BTU ) is directly related to work energy (See Power). It is equivalent to 778 Foot-Pounds or 1055 Joules. As such, the BTU divided by time relates directly to Horsepower and Wattage:

1 Horsepower = 2545 BTU / Hour
1 KiloWatt = 3413 BTU / Hour
1 Watt = 3.413 BTU / Hour
(See Power)

With the developement of steam engines, it became important to equate a common known power source, the horse, to the output of a steam engine. As an example, an engine that could do the same task as fifty horses was a 50 Horsepower engine. On paper, a 50 Horsepower engine would require at least a 50 Horsepower boiler or:

50 Horsepower x 2545 = 127250 BTU's per hour.

( The reality was that steam engines were very inefficient as were fuel burning processes. That 50 Horsepower steam engine would most likely be fed by a boiler rated at several hundred horsepower).

It is common to still see boilers rated in Horsepower as well as BTU/H. Gas furnaces, water heaters, ranges, grills, clothes dryers and appliances show the input and output BTU/H. The ratio of the output divided by the input will give an approximate figure for the efficiency. For instance, a dryer has an input of 100,000 BTU/H and a rated output of 84,000 BTU/H. The efficiency of the appliance is about 84%. One could expect about 16,000 BTU/H is going up the flue!

Air Conditioners are usually rated by the Refrigeration Ton:

1 Refrigeration Ton = 12000 BTU/H
(See Refrigeration Ton)

In the Metric System, the equivalent Thermal Unit is the KiloCalorie.

1 BTU = 252 Calories = 0.252 KiloCalories

People on diets are familiar with the term Calorie as in "counting Calories". What they are really counting is KiloCalories (Calories x 1000) which is the amount of heat generated in a laboratory when the food is burned in pure oxygen within a closed container. "Calories" of food are relative measurements of the amount of energy that the body would gain from the metabolism of that food.

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