Nitrogen expansion after pressure reducer valve

nihili · Jul 13, 2023

Good morning,

I have a installation which needs Nitrogen. I am using Nitrogen gas cylinders. The issue is that I have simulated the expansion proccess in a very simple way:
- Inlet line with fixed flow and pressure (around 200 bar)
- Pressure reducer valve
- Outlet line. Same flow (mass) and pressure (around 2,3 bar)

The problem is that depending on the thermodinamic model, temperature after the pressure reduction is very low or not so low. Any recommendation of which model I have to use?

Apart from that, Some years ago I used these cylinders to feed Nitrogen to a vessel for blanketing and I cannot remember any issue with temperature, so I am doubting if I am simulating it properly.

LittleInch · Jul 13, 2023

You should get a drop of about 30C if doing this in one step.

In practice nothing is ideal so the actual temperature you get at delivery point is often higher as heat enters the system and also unless you're only using gas from the cylinder it mixes with other gases and heats up.

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GBTorpenhow · Jul 13, 2023

No thermodynamic 'model' is required. A regulator or pressure reducing valve is an isenthalpic device. Look up the specific enthalpy at the inlet pressure and temperature, then find the temperature that gives that same specific enthalpy at the outlet pressure.

Rules of thumb for Joule-Thompson temperature drop based on pressure cut exist, but the 'exact' answer depends a little bit on inlet temperature, e.g. using NIST REFPROP:

at 20°C and 200 barg inlet, h=270 kJ/kg
at 270 kJ/kg and 2.3 barg, T_out = -12.0°C

at 0°C and 200 barg inlet, h=243.1 kJ/kg
at 243.1 kJ/kg and 2.3 barg, T_out = -37.7°C

Snickster · Jul 13, 2023

I agree with GBTorpenhow in that the expansion across a throttle valve is and isenthalpic process. Attached is the pressure - enthapy chart for nitrogen. Find inlet pressure, temperature conditions and enthalpy. Then move down figure at contant enthalpy to the lower exit pressure. Then read temperature at lower pressure at constant enthalpy. This is an example of Joule-Thompson coolling.

pierreick · Jul 13, 2023

Hi,

https://en.wikipedia.org/wiki/Joule–Thomson_effect

Pierre

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Nitrogen expansion after pressure reducer valve

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