An isenthalpic process or isoenthalpic process is a process that proceeds without any change in
enthalpy
Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...
, ''H''; or
specific enthalpy
Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant p ...
, ''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
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 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
Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant p ...
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 a ...
follow
isotherms, since
.
See also
*
Adiabatic process
In thermodynamics, an adiabatic process (Greek: ''adiábatos'', "impassable") is a type of thermodynamic process that occurs without transferring heat or mass between the thermodynamic system and its environment. Unlike an isothermal proces ...
*
Joule–Thomson effect
*
Ideal gas laws
The ideal gas law, also called the general gas equation, is the equation of state of a hypothetical ideal gas. It is a good approximation of the behavior of many gases under many conditions, although it has several limitations. It was first stat ...
*
Isentropic process
References
* 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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