Regarding the temperature changes before and after the pressure relief valve?
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Recently, I conducted experiments using gas cylinders (nitrogen and oxygen). When the gas flow rate was high (around 100 Nm3/H), severe frosting occurred in the pipeline downstream of the pressure regulator connected to the cylinder. The pressure in the cylinder was 15 MPa, while the pressure downstream of the regulator was 1.5 MPa; the throat diameter of the regulator is unknown. I would like to ask how to calculate the temperature after the pressure relief valve? Another source provides the following description: The high temperature generated by adiabatic compression causes combustible materials to burn. For example, if the pressure before the valve is 15 MPa and the temperature is 20°C, while the pressure after the valve is at atmospheric level of 0.1 MPa, opening the valve quickly can result in an oxygen temperature of 553°C according to the adiabatic compression formula; this temperature is equal to or exceeds the ignition point of certain substances. Why does the increased oxygen outlet temperature here actually rise? What is the difference between the two questions? Thank you for clarifying!
The temperature drops after pressure reduction; relevant thermodynamics textbooks can be consulted
“For example, if the pressure before the valve is 15 MPa and the temperature is 20°C, while the pressure after the valve is at atmospheric level of 0.1 MPa, opening the valve quickly can result in the oxygen temperature after the valve reaching 553°C according to the adiabatic compression formula – a value that is equal to or exceeds the ignition point of certain substances. This issue is hard to understand; normally, the throttling effect should cause a temperature drop. I’ve never heard of a situation where the temperature rises instead – perhaps it’s just because I’m not well-informed enough. The temperature after pressure regulation can be calculated using formulas; it is necessary to obtain data such as the specific heat at constant pressure and density of the gas
It’s unlikely. Otherwise, synthetic ammonia can only produce gaseous ammonia
Liquid nitrogen or liquid oxygen is liquefied under high pressure; a pressure reducing valve only reduces the pressure without decreasing the flow rate, so the liquid nitrogen or oxygen behind the valve vaporizes, absorbing heat and causing frosting.
According to the ideal gas law, when the volume at the outlet expands by 423 times, the temperature will reach 533 degrees.
Due to the large pressure difference before and after decompression, a vacuum is formed locally during the transition from high pressure to low pressure, and lower air pressure corresponds to lower temperature. The most straightforward example can be seen in steam vacuum ejectors; the casing of such ejectors usually develops dew, and the principle is the same. :lol
That calculation doesn’t seem right. Under adiabatic conditions, if the temperature remains constant, pressure and volume are inversely proportional. But it is not adiabatic; volume expansion requires heat absorption, and the heat is drawn from the outside, which inevitably causes the external temperature to drop. Only then will the moisture in the air condense on the walls of the tube, resulting in frost formation. Laboratory gas cylinders are all equipped with pressure regulators; I have never encountered the situation you described. It’s either that the pressure regulator is faulty or there is an issue with the way you are operating it.
When high-pressure gas expands rapidly after depressurization, the system does not have time to absorb heat from the surroundings and can only draw on its internal energy; as a result, the temperature of the gas drops. The greater the pressure difference before and after expansion, the greater the temperature drop. It should be calculated using the adiabatic expansion method.
It should be correct that the high temperature generated by adiabatic compression causes the combustible material to burn. The example given later is one of expansion, and I don’t think it’s very appropriate. Throttling expansion can produce a cooling effect at the conversion temperature; the conversion temperature for oxygen is as high as 771 K, so expansion at room temperature should result in a cooling effect.