Throttling and Joule-Thomson Expansion: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • Throttles are modeled as adiabatic.
  • Energy balance for a throttle: enthalpy in = enthalpy out (for liquid or gas feed).
  • Throttles are irreversible; the effluent entropy is higher than the inlet entropy.
  • A high-pressure liquid fed to a throttle can partially vaporize at the lower exit pressure if the exit pressure is below the saturation pressure at the feed temperature. Because a throttle is adiabatic, the heat of vaporization is obtained by lowering the exit temperature. The exiting gas-liquid mixture is at saturation.
  • An ideal gas does not exhibit a temperature change in a throttle because its enthalpy only depends on temperature, not pressure.
  • For most gases, Joule-Thomson expansion through a throttle results in a temperature drop, but the temperature can increase. That is, the Joule-Thomson coefficient can be positive or negative; it depends on temperature, pressure, and the gas.
From studying this module, you should now be able to:
  • Determine the outlet conditions for a throttle, given the inlet pressure and temperature, the outlet pressure, and an equation of state or a table of properties.
Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder