When touching oxygen, Acetylene, LPG gas cylinders, we can feel a cold. But why does the compressed air get cold?
The compressed air only gets cold while releasing some portion of it. In such a case, the speed of the moving atoms slows down. As a result, the kinetic energy of the gas decrease and it causes to absorb heat from the environment to keep the equilibrium.
Under normal conditions, the temperature of the compressed air is as same as the room temperature. However, we can feel the compressed air containers/ tanks are cold because they are made of metals. Since metals are highly conductive, they can absorb heat.
Let’s go a little deep into the topic.
Temperature When Compressed Air is Released
The compressed gas supplies the energy necessary to force the expanding gas from its container. When compressed air is released, the moving speed of the gas particles slows down, so the kinetic energy of the gas also decreases. As a result, the temperature of the compressed gas decreases and tries to absorb the heat energy from the environment.
We can explain this scenario using the Ideal gas law.
PV = nRT
Where, P = pressure, V = volume, n = number of moles, T = temperature, R = gas constant
Let’s solve this for the temperature,
T= PV/nR
Only the R and Volume V are constants.
T ∝ P/n
A gas needs energy to expand. When some potion is removed, it absorbs energy from the environment.
Temperature When Compressing Air
When compressing air, we need to do work against its pressure by reducing the volume of the container. In that way, we can store a large amount of gas in a smaller space. However, during the compression, the speed of the gas atoms increases. As a result, the kinetic energy of the gas particles increases, and it causes an increase in temperature.
We can explain this scenario using the Ideal gas law.
PV = nRT
Where, P = pressure, V = volume, n = number of moles, T = temperature, R = gas constant
Let’s solve this for the temperature,
T= PV/nR
The number of moles n and R are constants.
T ∝ PV
If the air is compressed at a slow speed, the temperature doesn’t increase. However, when the air is compressed quickly, the temperature increases.
Adiabatic Expansion
Adiabatic expansion is another method that is used to explain why the compressed air gets cold when it’s being released. However, adiabatic expansion is not considered a valid explanation.
Adiabatic compression is the process of reducing heat by changing air pressure due to volume expansion. And it’s only valid for an ideal system where the pressure is constant. However, when the air is compressing, the pressure inside the compressor changes. Therefore, the theory is not valid for explaining this scenario.
Canned Air Getting Cold
With compression, we can turn a gas into a liquid. And a liquid is a greater way to store gas in a container since it only takes up a small space. For different reasons, there are different types of canned air in the market.
When the gas inside a gas can is released, the phase change from a liquid into a gas. And it requires an amount of heat energy to change the phase.
Press the nozzle of an air can and then it’ll start releasing the vapor. And, when some portion of gas in the can goes out, it’ll decrease the moving speed of gas atoms. Then the speed of atoms and the internal pressure decreases. Due to low pressure, the liquid inside the can evaporates. For this prosses, the can needs energy, so it absorbs the energy from the environment.
In colder environments, the speed of the released air can be slower. Therefore, people use bottle warmers to keep the compressed air cans from getting cold.
Summary
- A closed cylinder with compressed air: Temperature is the same as the surrounding environment.
- Compressing air into a cylinder: Temperature goes up and provides heat to the environment.
- Releasing air in a cylinder: Temperature goes down and absorbs heat from the environment.
Metal cylinders are cold because of their high thermal conductivity. And it has nothing to do with the compressed air.
Conclusion
Remember, if the speed of compressed air cans is low, you can warm them up. However, high heat can cause them to explode. So, you need to be careful when warming up the compressed air cans.
This is the principle used in refrigerators as well.