Unsteady-State Energy Balances on a Tank: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • When a valve is opened on a pressurized, adiabatic tank, the gas in the tank expands and it does work as it pushes the gas out of the tank. The gas remaining in the tank cools because internal energy was converted to work.
  • When a valve is opened on a pressurized, adiabatic tank, expansion of the gas that remains in the tank can be treated as a reversible, adiabatic expansion.
  • When gas is added to an adiabatic tank, work is done to compress the gas so its temperature increases.
  • For two insulated tanks that are connected by a valve, expanding a gas from the first tank into the second tank lowers the temperature in the first tank and raises the temperature in the second tank. However, for an ideal gas with a constant heat capacity, the average temperature does not change.
  • When gas is released from an adiabatic tank that contains a liquid-vapor mixture, liquid evaporates to maintain vapor-liquid equilibrium, and the tank contents cool because the energy required to evaporate liquid comes from sensible heat.
  • When liquid evaporates into a vacuum in an adiabatic tank, the temperature decreases until the final system is in vapor-liquid equilibrium.
From studying this module, you should now be able to:
  • Calculate the temperature in an adiabatic tank when a value is opened so gas or vapor leaves the tank.
  • Calculate temperature in an adiabatic tank when gas is added to a tank.
  • Apply the first law, mass balances, and phase equilibrium to solve problems in which gas is vented from a tank or a liquid evaporates into a vacuum within a tank.
Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder