BTU (THERMAL UNIT): EQUIVALENCES, USES, EXAMPLES - PHYSICAL - 2024

BTU is the acronym for British Thermal Unit, which translates to English Thermal Unit. It is a unit used in heat transfer that began to appear in engineering texts towards the end of the 19th century.

The following is one of the definitions for this unit: 1 BTU is the amount of heat required to change the temperature of one pound of water at 60ºF at atmospheric pressure by one degree Fahrenheit. Another definition used in the oil industry establishes this temperature at 59ºF.

Now, energy - of any kind - is a quantity that has many possibilities in terms of the units to measure it. Although in the International System of Units SI it is the joule or joule, in many areas, like all those related to electricity, heating and cooling, preference is given to others.

BTU-other units conversions

Below are the top conversions from BTU to other frequently used units of energy:

Table 1. BTU conversion factors

To convert a given amount from BTU to any of the displayed units, simply multiply the amount by the listed conversion factor. And if what you want is to transform from any unit to BTU, divide by that factor.

Applications

BTU is a unit widely used in air conditioners and heaters. Source: Pixabay.

Energy in its many forms is what ultimately drives humanity. So it is not surprising how many units were created as knowledge grew more and more.

On the one hand, it happens that the joule is a fairly small unit for energy or heat, and on the other, there are units of measurement that are deeply rooted in the area of ​​heat transfer, such as the calorie and the BTU itself.

-Currently the BTU is still used in everything that refers to heating systems, stoves, ovens, refrigeration and air conditioning. It frequently appears in the operation and maintenance manuals of such equipment.

-Also in the United States the BTU is used to express the price of natural gas, electricity, kerosene and other fuels (in dollars per million BTUs). For example, in the United States the price of electricity ranges around $ 39 per million BTU, according to sources from the US Energy Information Administration.

-In power plants, the efficiency of conversion of heat into electrical energy is expressed in BTU.

BTU-hour and RT as a measure of power

Even more frequent is to express the consumption of BTUs per unit of time, which is power. In this case it appears as BTU / h or with the initials BTUH. This unit is used in a variety of industries and even consumers in countries where the metric system is a standard, recognize these acronyms as distinctive in air conditioning equipment.

Table 2. BTU / h conversion factors

* RT stands for refrigeration ton or refrigeration ton, a unit of power used to specify power for refrigerators and refrigerators. It is not related to the metric ton used to measure mass and is equal to 1000 kg.

One RT is equivalent to the latent heat absorbed in the melting of a short ton of pure ice in one day, which is 12,000 BTU / hr. The short ton or short ton is worth something more than the metric ton: 1,10231 times.

Central air conditioning systems are expressed in RT. For example, one with a 2 RT compressor can remove 24,000 BTUs from the air in 1 hour.

Examples

The following examples and exercises make use of the BTU.

Heating or cooling a room

When buying a stove (heater) or an air conditioner, there are many alternatives on the market. The following procedure helps to choose the most suitable equipment for air conditioning a space, according to its dimensions:

-Measure width a, length l and height h of the room.

-Calculate the volume using the formula V = a × l × h

-Divide the result by 2 and then multiply it by 1000

-The result indicates the most suitable BTU / h for the size of that space, therefore the closest device with BTU / h should be chosen.

The above procedure is a good estimate assuming there are 1 or 2 people in the room.

Figure 2. To optimize costs, it is necessary to choose the most suitable heater according to the size of the space to be heated. Source: Pixabay.

Solved exercises

Exercise 1

Find the BTU / h required to air-condition a room whose dimensions are:

-4.0 meters wide

-3.0 meters high

-10 meters long

Solution

The volume of this space is V = 4.0 x 3.0 x 10 m ³ = 120 m ³. Half of this value is 60 and multiplying it by 1000 gives 60000. Next, you have to find a team that specifies the value closest to 60,000 BTU / h.

The higher the BTU / hr, the greater the size and cost of the equipment, as well as its electrical energy consumption. So buying the biggest team doesn't always give the best result.

Another detail to consider is efficiency: all equipment needs energy to function (input) and convert it into work (output). The efficiency is the quotient between the output (input) and the input (output), which is never equal to 1, since losses always occur.

General air conditioners are typically in the 3,000 to 1,800,000 BTU / hr range. But household equipment is almost always classified like this:

-Laptops: 8,000 - 12,000 BTU

-Split system: 9,000 - 36,000 BTU

-Window air conditioner: 3,000 - 25,000 BTU

Exercise 2

A window air conditioner has 5000 BTU / h and you want to calculate its power in kilowatts. If the computer is on for 6 hours, what will its power consumption be in joules?

Solution

Table 2 shows the conversion factor: 1 BTU / h = 0.00029307107 kilowatts, the operation to be performed is:

5000 x 0.00029307107 kilowatts = 1.470 kilowatts.

Regarding the energy consumption in joules, first the previous amount is transformed into watts by multiplying by 1000:

1,470 kilowatts = 1470000 watts.

And since power is energy per unit of time, multiply this result by 6 hours, expressed in seconds:

6 hours = 6 x 3600 seconds = 21600 seconds

Consumed energy = 1470000 watts x 21600 seconds = 3.18 x 10 ¹⁰ joules.

It is a fairly large number: no less than 318 followed by 8 zeros. As previously mentioned, the joule is a fairly small unit and in this regard, employing the BTU means working with smaller, more manageable figures.

References

1. Compact Appliance. Air Conditioning BTU's: What Are They & What Do They Mean? Recovered from: learn.compactappliance.com.
2. Essential Home and Garden. What is BTU ?. Recovered from: essentialhomeandgarden.com.
3. What is a BTU in an air conditioner and how to calculate the correct size? Recovered from: samsung.com.
4. US Energy Information Administration. FAQS. Recovered from: eia.gov.
5. Wikipedia. British Thermal Unit. Recovered from: en.wikipedia.org.