Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

Compression and Expansion of Gases

Isothermal and isentropic gas compression and expansion processes.

Sponsored Links

The relationship between pressure and density when compressing - or expanding - a gas depends on the nature of the process. The process can be

  • isothermal,
  • isentropic (adiabatic)
  • polytropic

Isothermal Compression/Expansion Processes

If compression or expansion of gas takes place under constant temperature conditions - the process is said to be isothermal. The isothermal process can be expressed with the Ideal Gas Law as

p / ρ = constant                         (1)

where

p = absolute pressure (Pa, N/m2)

ρ = density (kg/m3)

The isothermal process can also be expressed as

pV = constant                    (1a)

or

p1V1 = p2V2                           (1b)

where

V = gas volume (m3, ft3...)

suffix1 denotes initial conditions and suffix2 denotes final conditions

Isentropic (or adiabatic) Compression/Expansion Processes

If compression or expansion of gas takes place with no flow of heat energy either into or out of the gas - the process is said to be isentropic or adiabatic. The isentropic (adiabatic) process can be expressed with the Ideal Gas Law as

p / ρk = constant                   (2)

where

k = cp / cv - the ratio of specific heats - the ratio of specific heat at constant pressure - cp - to the specific heat at constant volume - cv

The isentropic or adiabatic process can also be expressed as

pVk = constant                       (2a)

or

p1V1k  = p2V2k                        (2b)

Polytropic Compression/Expansion Process

An ideal isothermal process must occur very slowly to keep the gas temperature constant. An ideal adiabatic process must occur very rapidly without any flow of energy in or out of the system. In practice most expansion and compression processes are somewhere in between, or said to be polytropic.

The polytropic process can be expressed as

pVn = constant                      (3a)

or

p1V1n  = p2V2n                       (3b)

where

n = polytropic index or exponent (ranging 1 to 1.4)

Sponsored Links

Related Topics

  • Fluid Mechanics

    The study of fluids - liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time.

Related Documents

Sponsored Links

Search

Search is the most efficient way to navigate the Engineering ToolBox.

Engineering ToolBox - SketchUp Extension - Online 3D modeling!

3D Engineering ToolBox Extension to SketchUp - add parametric components to your SketchUp model

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with older versions of the amazing SketchUp Make and the newer "up to date" SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp Make/Pro from the Extension Warehouse !

Translate this Page

Translate this page to Your Own Language .

About the Engineering ToolBox!

Privacy Policy

We don't collect information from our users. More about

Citation

This page can be cited as

  • The Engineering ToolBox (2003). Compression and Expansion of Gases. [online] Available at: https://www.engineeringtoolbox.com/compression-expansion-gases-d_605.html [Accessed Day Month Year].

Modify the access date according your visit.

3D Engineering ToolBox - draw and model technical applications! 2D Engineering ToolBox - create and share online diagram drawing templates! Engineering ToolBox Apps - mobile online and offline engineering applications!

Unit Converter

















































2.19.10

Sponsored Links
.