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Gas Flow Through Pipes and Throttling

Thermodynamics Basic Theory Compressed Air Wiki Physics of Air Compressors

One very interesting branch of physics is thermodynamics, especially for getting insight in air compressors. In this article we are talking about gas flow and throttling, following up on our introduction to thermodynamics.

The Reynolds number

formula reynolds number, calculation

The Reynolds number is a dimensionless ratio between inertia and friction in a flowing medium.
It is defined as:

What are the different types of flow in a pipe?

gas flowing through pipe

In principal, there are two types of flow in a pipe. With Re <2000 the viscous forces dominate in the medium and the flow becomes laminar. This means that different layers of the medium move in relation to each other in the proper order. The velocity distribution across the laminar layers is usually parabolic shaped.

With Re≥4000 the inertia forces dominate the behavior of the flowing medium and the flow becomes turbulent, with particles moving randomly across the flow. The velocity distribution across a layer with turbulent flow becomes diffuse.

In the critical area, between Re≤2000 and Re≥4000, the flow conditions are undetermined, either laminar, turbulent or a mixture of the both. The conditions are governed by factors such as the surface smoothness of the pipe or the presence of other disturbances. To start a flow in a pipe requires a specific pressure difference to overcome the friction in the pipe and the couplings. The amount of pressure difference depends on the diameter of the pipe, its length and form as well as the surface smoothness and Reynolds number.

What is the Joule Thomson effect?

When an ideal gas flows through a restrictor with a constant pressure before and after the restrictor, the temperature remains constant. However, a pressure drop occurs across the restrictor, through the inner energy being transformed into kinetic energy. This is the reason for which the temperature falls. For real gases, this temperature change becomes permanent, even though the energy content of the gas remains constant. This is called the Joule-Thomson effect. The temperature change is equal to the pressure change across the throttling multiplied by the Joule-Thomson coefficient.


If the flowing medium has a sufficiently low temperature (≤+329°C for air), a temperature drop occurs with the throttling across the restrictor, but if the flow medium is hotter, a temperature increase occurs instead. This condition is used in several technical applications, for example, in refrigeration technology and in separation of gases.


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