An isenthalpic process or isoenthalpic process is a process that proceeds without any change in
enthalpy
Enthalpy () is the sum of a thermodynamic system's internal energy and the product of its pressure and volume. It is a state function in thermodynamics used in many measurements in chemical, biological, and physical systems at a constant extern ...
, ''H''; or
specific enthalpy, ''h''.
Overview
If a steady-state, steady-flow process is analysed using a
control volume
In continuum mechanics and thermodynamics, a control volume (CV) is a mathematical abstraction employed in the process of creating mathematical models of physical processes. In an inertial frame of reference, it is a fictitious region of a given v ...
, everything outside the control volume is considered to be the ''surroundings''.
[G. J. Van Wylen and R. E. Sonntag, ''Fundamentals of Classical Thermodynamics'', Section 2.1 (3rd edition).]
Such a process will be isenthalpic if there is no transfer of heat to or from the surroundings, no work done on or by the surroundings, and no change in the kinetic energy of the fluid.
[G. J. Van Wylen and R. E. Sonntag, ''Fundamentals of Classical Thermodynamics'', Section 5.13 (3rd edition).] This is a sufficient but not necessary condition for isoenthalpy. The necessary condition for a process to be isoenthalpic is that the sum of each of the terms of the energy balance other than enthalpy (work, heat, changes in kinetic energy, etc.) cancel each other, so that the enthalpy remains unchanged. For a process in which magnetic and electric effects (among others) give negligible contributions, the associated energy balance can be written as
If
then it must be that
Where K is kinetic energy, u is internal energy, Q is heat, W is work, h is enthalpy, P is pressure, and V is volume.
The
throttling process is a good example of an isoenthalpic process in which significant changes in pressure and temperature can occur to the fluid, and yet the net sum of the associated terms in the energy balance is null, thus rendering the transformation isoenthalpic. The lifting of a relief (or safety) valve on a pressure vessel is an example of throttling process. The specific enthalpy of the fluid inside the pressure vessel is the same as the specific enthalpy of the fluid as it escapes through the valve.
With a knowledge of the
specific enthalpy of the fluid and the pressure outside the pressure vessel, it is possible to determine the temperature and speed of the escaping fluid.
In an isenthalpic process:
*
,
*
.
Isenthalpic processes on an
ideal gas
An ideal gas is a theoretical gas composed of many randomly moving point particles that are not subject to interparticle interactions. The ideal gas concept is useful because it obeys the ideal gas law, a simplified equation of state, and is ...
follow
isotherms, since
.
See also
*
Adiabatic process
An adiabatic process (''adiabatic'' ) is a type of thermodynamic process that occurs without transferring heat between the thermodynamic system and its Environment (systems), environment. Unlike an isothermal process, an adiabatic process transf ...
*
Joule–Thomson effect
In thermodynamics, the Joule–Thomson effect (also known as the Joule–Kelvin effect or Kelvin–Joule effect) describes the temperature change of a Real gas, ''real'' gas or liquid (as differentiated from an ideal gas) when it is expanding; ty ...
*
Ideal gas laws
*
Isentropic process
An isentropic process is an idealized thermodynamic process that is both Adiabatic process, adiabatic and Reversible process (thermodynamics), reversible. The work (physics), work transfers of the system are friction, frictionless, and there is ...
References
Bibliography
* G. J. Van Wylen and R. E. Sonntag (1985), ''Fundamentals of Classical Thermodynamics'', John Wiley & Sons, Inc., New York
Notes
Thermodynamic processes
Enthalpy
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