Decompression of dry air in relation to temperature

In summary, the conversation discusses a problem with a cylinder filled with dry air and a fixed volume of 8.5 liters undergoing depressurization through a series of valves. The final chamber is a mask where air flows out with every exhale, and there is a cooling effect observed. The question is asked about the maximum possible change in temperature at the reduction valve from 300 bar to 50 bar. The answer involves considering adiabatic expansion and non-ideal gas effects.
  • #1
CyanPhysics
1
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I have a problem that i cannot solve despite my best efforts. I do realize not all information here is relevant, but i still included it so you could get the full picture.

Imagining we have a cylinder that is filled with dry air (normal air without water vapor) that has a fixed volume of 8,5 liters. The initial pressure is 300 bar (30000 kPa). After use the final pressure is 50 bar (5000 kPa), therefore the change in pressure is 250 bar (25000 kPa). This happens in a span between 20 to 30 minutes. The cylinder is depressurizing into a tube thru a valve that reduces the maximum pressure inside that valve to 50 bar (5000 kPa) and then another reduction valve, furthermore reducing to 5 bar (500 kPa) and then again to 1.25 bar (125 kPa) the final chamber is of a volume approximately 1 liter, the sizes of the chambers in between are unknown. The final chamber is a mask and the air flows out with every exhale. From observation we have seen that there is some cooling effect, that is not measured.

My question is what is the theoretically maximum possible change in temperature at the reduction valve 300 bar -> 50 bar, if we disregard other factors from the inside. I will conduct an experiment on this and record the results, but i am still interested in the theory behind it as it will help me in the future.

Gas is normal air without water vapor.

P1 = 500 bar
P2 = 50 bar
ΔP = 250 bar
V = 8.5 Liters
t ≈ 1500 second
____________________________________________
ΔT = ? Kelvin
 
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  • #2
If heat transfer between the air within the cylinder and the cylinder itself is neglected, then the air remaining inside the cylinder at any point during the process has experienced an adiabatic reversible expansion in doing work to force the gas ahead of it out toward the exit valve. This adiabatic expansion is accompanied by cooling of the gas within the cylinder. This can be quantified.
 
  • #3
In an adiabatic process, there is no transfer of heat, then time does not matter. It should be easy looking at the classical equations of Adiabatic process and Ideal gas law.
 
  • #4
Anand Sivaram said:
In an adiabatic process, there is no transfer of heat, then time does not matter. It should be easy looking at the classical equations of Adiabatic process and Ideal gas law.
The ideal gas law is a poor approximation to the behavior of air in the 50 - 300 bar pressure range. Non-ideal gas effects need to be included. So it's not as easy as you suggest.
 
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FAQ: Decompression of dry air in relation to temperature

1. What is the purpose of decompressing dry air?

The purpose of decompressing dry air is to reduce its pressure and volume in order to decrease its temperature. This is often done in industrial and scientific settings to create a controlled environment for experiments or processes.

2. How does temperature affect the decompression of dry air?

Temperature plays a crucial role in the decompression of dry air. As air is decompressed, it expands and cools down. The rate of temperature change during decompression is directly proportional to the initial temperature of the air. This means that the higher the starting temperature, the greater the decrease in temperature during decompression.

3. What is the relationship between pressure and temperature during the decompression of dry air?

According to the ideal gas law, the pressure and temperature of a gas are inversely proportional. This means that as the pressure of dry air decreases during decompression, its temperature will also decrease. This relationship is important to consider when calculating the final temperature of decompressed air.

4. Can decompression of dry air cause condensation?

Yes, decompression of dry air can cause condensation under certain conditions. As the air cools down during decompression, its relative humidity may increase. If the relative humidity reaches 100%, water vapor in the air will condense into liquid water. This can be prevented by controlling the temperature and pressure of the air during decompression.

5. What are the potential hazards of decompressing dry air?

There are several hazards associated with decompressing dry air, including the risk of explosion or implosion if the pressure is not carefully controlled. In addition, decompression can cause a rapid drop in temperature, which can lead to frostbite or other cold-related injuries. It is important to follow proper safety protocols and use appropriate equipment when decompressing dry air.

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