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Technology, Oil-Free Air Compressors

Compressor Selection Basics: Positive Displacement versus Dynamic Compression


centrifugal compressor ZH Atlas Copco

The following is an excerpt from the article, Compressor selection basics: positive displacement compression vs. dynamic compression, recently published on Plant Services magazine’s website. The article is by Deepak Vetal, product marketing manager — ZH and High Pressure, U.S. Oil-Free Air Division. Look for the article in the September 2014 issue of Plant Services magazine.

There are two basic principles of air or gas compression: positive displacement compression and dynamic compression.

Positive displacement compression

In positive displacement compression, the air is drawn into one or more compression chambers, which are then closed off from the inlet. The enclosed volume of each chamber decreases through the displacement of one or more moving parts and the pressure increases, compressing the air internally. Once the pressure reaches the maximum pressure ratio, a port or valve opens; the continued reduction of volume in the compression chamber discharges the air into the outlet system.

Dynamic compression (turbocompressors)

In dynamic compression, air is drawn between the blades on a rapid rotating compression impeller and accelerates to high velocity. The air or gas is then discharged through a diffuser, where the kinetic energy is transformed into static pressure. Most dynamic compressors are turbocompressors with an axial or radial flow pattern and are often designed for large-volume flow rates. Unlike a positive displacement compressor which works with a constant flow, a dynamic compressor works at a constant pressure.

Selecting positive displacement or dynamic compression

Selection of either technology depends on the application, but there is a rule of thumb: Dynamic compression technology is best suited for base load requirements, while positive displacement compression is better suited to variable load. For larger flows and variable demand applications, a combination of both technologies often helps to reach the optimal usage of compressed air while simultaneously reducing energy consumption.

You can read the full article, which provides more detailed information on the variables affecting positive displacement and dynamic compression as well as pressure and flow rate comparisons, by visiting Plant Services magazine’s website: Compressor selection basics. If you would like to learn more about selecting a compressor, contact us or leave a comment below.

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