Questions: Throttling and the Joule-Thomson Effect

Question 1

A gas is throttled through a valve at room temperature. The gas is known to be above its Joule-Thomson inversion temperature. What will happen to the gas temperature after throttling?

• A. It will decrease, because pressure drop always causes cooling
• B. It will remain constant, because throttling conserves enthalpy
• C. It will increase, because a negative Joule-Thomson coefficient means heating on pressure drop
• D. It will decrease, because lower pressure means lower enthalpy

Answer: (C)

Above the inversion temperature, the Joule-Thomson coefficient μ_JT = (∂T/∂P)_h is negative. Since throttling reduces pressure (dP < 0), dT = μ_JT·dP is positive — the gas warms. Hydrogen and helium at room temperature behave this way, which is why they must be pre-cooled before Joule-Thomson liquefaction is possible. Throttling conserves enthalpy, not temperature.

Question 2

Why does an ideal gas show no temperature change when throttled, even though its pressure drops significantly?

• A. Because throttling is reversible for ideal gases, so entropy is conserved
• B. Because for an ideal gas, enthalpy depends only on temperature, so constant enthalpy means constant temperature
• C. Because ideal gas molecules have no intermolecular forces and therefore no internal energy
• D. Because the ideal gas law ensures pressure and temperature always change proportionally

Answer: (B)

For an ideal gas, enthalpy h = u + Pv depends only on temperature (u depends only on T, and Pv = RT also depends only on T). If enthalpy is conserved across the throttle, temperature must be unchanged. It is real gas intermolecular interactions — not absent in an ideal gas — that cause temperature to change when pressure changes at constant enthalpy.

Question 3

Throttling typically causes a gas to cool, which is why it is universally used in refrigeration.

• T. True
• F. False

Answer: False

Throttling cools a gas only when it is below its Joule-Thomson inversion temperature, where μ_JT > 0. Above the inversion temperature, throttling heats the gas. For most common gases (air, nitrogen, CO₂), the inversion temperature is well above room temperature, so throttling typically cools them. Hydrogen and helium at room temperature are above their inversion temperatures and actually warm up when throttled.

Question 4

In a throttling process, the specific enthalpy of the fluid is conserved even though both temperature and pressure change.

• T. True
• F. False

Answer: True

This is the defining feature of throttling: it is an isenthalpic process. The first law for a steady-flow device with no heat transfer and no shaft work reduces to h_in = h_out. Temperature and pressure both change, but their combined effect on enthalpy cancels out. For real fluids, the Joule-Thomson coefficient captures exactly how temperature adjusts with pressure to maintain constant enthalpy.

Question 5

If enthalpy is conserved during throttling, why does temperature change for a real gas but not for an ideal gas? Explain the role of intermolecular forces.

Model answer: For an ideal gas, enthalpy depends only on temperature, so conserving enthalpy forces temperature to remain constant. For a real gas, intermolecular forces mean that internal energy depends on molecular spacing as well as temperature. When pressure drops and molecules move farther apart, they must work against intermolecular attractions, changing internal energy. Temperature must then adjust to keep enthalpy constant.

The Joule-Thomson coefficient is zero for an ideal gas and non-zero for real gases precisely because real gas enthalpy depends on both T and P. Below the inversion temperature, attractive forces dominate and the gas cools on expansion; above it, repulsive interactions dominate and it warms. The ideal gas has no intermolecular interactions to create this effect.