| Parameter | Value |
|---|---|
| Complete the process data to populate the summary. | |
Calculation Details & Engineering Reference
| Step | Equation | Substitution | Result |
|---|---|---|---|
| Enter valid process data to see the worked calculation. | |||
| Quantity | Equation | Variables |
|---|---|---|
| JT coefficient (definition) | μJT = (∂T/∂P)h | Temperature change per unit pressure at constant enthalpy |
| Temperature change | ΔT = μJT · ΔP | μJT in °C/bar, ΔP = Pin − Pout |
| Outlet temperature | Tout = Tin − ΔT | Cooling when μ > 0, heating when μ < 0 |
| Preheat duty | Q = ṁ · cp · ΔT | ṁ mass flow (kg/s), cp kJ/kg·K → Q in kW |
| Real-gas density | ρ = P·MW / (Z·R·T) | P in Pa, T in K, R = 8.314 J/mol·K, Z compressibility |
| Mixture properties | MWmix = Σ yᵢ·MWᵢ, μmix = Σ yᵢ·μᵢ | yᵢ mole fractions — Kay's rule linear mixing |
| Hydrate temperature | Thyd(°F) = 13.47·lnP + 34.27·lnSG − 1.675·lnP·lnSG − 20.35 | Towler–Mokhatab; P in psia, SG relative to air (0.55–1.0) |
Assumptions: isenthalpic expansion (no heat exchange, no shaft work — correct for valves, regulators, orifices and chokes); constant μJT over the cut; single Z at both density evaluations. The architecture is ready for AGA8 / GERG-2008 / CoolProp property backends when higher accuracy is needed — this constant-coefficient method is the standard hand check, typically within 1–3 °C for moderate natural-gas cuts.
What is happening physically
Force a real gas through a restriction and it expands with constant enthalpy — no heat in, no work out. Expansion pulls the molecules apart against their mutual attraction; that energy comes out of their kinetic energy, so the gas cools. An ideal gas has no intermolecular forces, so it shows no JT effect at all — the effect is pure real-gas behaviour.
Why hydrogen is different
Every gas has an inversion temperature. Below it, expansion cools; above it, expansion warms. For natural gas the inversion point is several hundred °C — ambient operation always cools. For hydrogen it is about −73 °C (helium −222 °C), so at ambient temperature hydrogen warms slightly on expansion. That removes the freezing problem but adds another: check every wetted component for hydrogen embrittlement.
Why PRS stations install heaters first
A city-gate cut of 40 → 4 bar cools natural gas ~18 °C. From a winter inlet of 5 °C the gas lands near −13 °C — below the hydrate temperature at almost any line pressure. Hydrates (ice-like methane-water crystals) plug regulator internals within minutes. So the station preheats the gas before the cut, usually with an indirect water bath heater — the water bath decouples the burner flame from the high-pressure gas coil. Size that heater with the Water Bath Heater Designer using the preheat duty from this page.
Where you meet this effect
Natural gas PRS and city gates, CNG station pressure cuts (250 → 4 bar can cool >50 °C without staging and heating), wellhead chokes, LNG letdown, expansion valves in refrigeration, and cryogenic liquefiers — the Linde-Hampson cycle is nothing but a recuperated JT loop.
Common mistakes
Using gauge instead of absolute pressure in the hydrate correlation; forgetting that ΔT applies to the total cut across multiple regulator stages; assuming dry gas — commissioning gas and off-spec feed often carry water; checking only the summer case (hydrate risk is a winter-inlet problem); and treating the constant-μ answer as final design instead of a screening number.
Absolute (bar a). For the ΔT itself only the difference matters, but the hydrate correlation and the density calculation need absolute pressure — at low outlet pressures the distinction is significant.
0.4–0.6 °C/bar covers most pipeline-quality gas at moderate pressure; 0.5 is the standard screening value. Richer gas (more C2+/CO₂) sits at the high end — or enter the composition and let the mixture rule estimate it.
Not necessarily. Hydrates form well above 0 °C at pressure — at 60 bar a typical natural gas forms hydrates around 12–15 °C. That is why the tool checks against the hydrate temperature, not the freezing point.
Three standard options: preheat before the cut (water bath heater — the duty is computed on this page), split the cut across stages with reheating between them, or inject a hydrate inhibitor (methanol/MEG) when heating is impractical. Drying the gas removes hydrate risk but not external icing.
Reference only. JT expansion is isenthalpic, not isentropic — γ governs reversible expansion (turboexpanders, nozzle flow), which cools far more than JT. If someone quotes you a huge ΔT for a valve, they probably used the isentropic formula by mistake.