Two Basic Principles of compressed air (general information): 

class0There are two basic principles for the compression of air (or gas), the displacement principal and dynamic compression.  In the dynamic compression air is drawn into a rapidly rotating compression impeller and accelerated to a high speed.  The air (gas) is then discharged through a diffuser where the kinetic energy is transformed into static pressure.  A dynamic compressor is a flow machine where the pressure increase takes place at the same time as the gas flows.  The flowing gas accelerates to a high velocity my means of the rotating blades, after which the velocity of the gas is transformed to pressure when it is forced to decelerate under expansion.  All of these are suitable for large flow rate volumes. 

A displacement compressor on the other hand is characterized by enclosing a volume of gas or air and then increasing the pressure by reducing the area of the enclosed volume.  A bicycle pump is the simplest form of a displacement compressor. Other examples include the piston and rotary screw compressors.

When compared these two methods of compression it is important to note that the dynamic compressor (or centrifugal) is a machine with a variable capacity and constant pressure.  On the other hand a displacement compressor is a machine with a constant capacity and a variable pressure.


Air compressors – different models - explained.

Piston Compressor: 

image002The piston compressor is the oldest and most common of all compressors.  It is available as single  or double acting, oil lubricated or oil-free with a different number of cylinders in different configurations.  With the exception of really small compressors with vertical cylinders, the “V” configurations are the most common. On a piston compressor the air is drawn into a compression chamber which is closed from the inlet.  Thereafter the volume of the chamber decreases’ and the air is compressed.  When the pressure has reached the same level as the pressure in the outlet manifold, a valve is opened and air is discharged at a constant pressure, under continued reduction of the compression chamber’s volume.

Lubricated Screw Compressors:

image004The principle of a rotary displacement compressor with a screw form was developed during the 1930s, when a rotary screw compressor with high capacity and stable flow in varying conditions was required.

The screw element’s main parts are the male and female rotors, which move towards each other while the volume between them and the housing decreases. Each screw element has a fixed, integrated pressure ratio that is dependent of its length, the pitch of the screw and the form of the discharge port.  To attain the best efficiency the pressure ratio must be adapted to the required working pressure.


The screw compressor is equipped with a valve and has no mechanical forces that cause unbalance.   This means it can work with a high shaft speed and combine a large flow rate with small exterior dimensions.  An axial acting force, dependent on the pressure difference between the inlet and outlet pressures, must be taken up by the bearings.  The screw, which originally was symmetrical, has now been developed in different asymmetrical helical profiles.

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Watch an animation of a rotay lube compressor

Screw Compressors Designed for 100% Oil Free Compressed Air:

image006The first screw compressors had a symmetric profile and did not use liquid in the compression chambers. Since then they have been called OIL FREE or DRY screw compressors.  At the end of the 1960’s a high speed oil free screw compressors was introduced with an asymmetric screw profile. The new rotor profile resulted in significantly improved efficiency due to reduce internal leakage. 

An external gear is used in dry screw compressors to synchronize the counter rotating motors. As the rotors neither come into contact with each other nor with the compressor housing, no particular lubrication is required in the compression chamber.  Consequently the compressed air is completely Oil FREE.  The rotors and housing are manufactured with great precision to minimize leakage from the pressure side to the inlet. The integrated pressure ratio is limited by temperature difference between the intake and the discharge pressures.  This is why oil-free screw compressor is frequently built with several stages.

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Tooth Compressors Designed for 100% Oil Free Compressed Air

image008The compression elements in a tooth compressor consist of two rotors that rotate toward each other in a compression chamber.  The compression process consists of intake, compression and outlet. During the intake phase, air is drawn into the compressor chamber until the rotors block the inlet.  During the compression phase the drawn air in the compression chamber gets smaller and smaller as the rotors move. 

The outlet is blocked during compression by one of the rotors while at the same time the inlet is open to draw in new air into the opposite section of the compression chamber. Discharge takes place when one of the rotors opens the channels and the compressed air is forced out of the compression chamber.  Intake and outlet take place radially through the compression chamber, which allows the use of simpler bearing design and improve filling properties. Both rotors are synchronized via a gear wheel.

Watch an animation of a tooth (lobe) compressor

Scroll Compressor designed for 100% Oil Free Compressed Air

image010A scroll compressor is a type of OIL FREE rotating displacement compressor; it compresses a specific amount of air in an ever decreasing volume.  The compressor element consists of a fixed spiral in the element housing and a motor powered eccentric moveable spiral.  The spirals are mounted with 180º phase displacement to form air pockets with a varying volume.  This provides the elements with radial stability.  Leakage is minimized as the pressure difference in the air pockets is less that the pressure difference between the inlet and outlet. 

The moving spiral is driven by a short stroke crankshaft and runs eccentrically around the centre of the fixed spiral.  The intake is situated at the top of the element housing.  When the moving spiral runs anticlockwise air is drawn in, and is captured in one of the air pockets and compressed variably in towards the centre where the outlet and non-return valve are situated.  The compression cycle is a progress for 2.5 turns, which virtually give constant and pulsating free air flow.  The process is relatively silent and vibration free, as the element has hardly and torque variation compared to a piston compressor.

Watch an animation of a scroll compressor


Centrifugal Compressors Designed for 100% Oil Free Compressed Air


A centrifugal compressor (dynamic type) is characterized by the radial discharge flow.  Air is drawn into the Centre of rotating impeller with radial blades and is thrown out towards the periphery of the impeller by centrifugal force. Before the air is led to the Centre of the next impeller, it passes a diffuser and volute where the kinetic energy is converted to pressure.




The pressure ratio across each stage is determined by the compressor’s final pressure.  This also gives a suitable velocity increase for the air after each impeller.  The air temperature at the inlet of each stage has decisive significance for the compressor’s power requirement, which is why cooling between stages is needed.  Centrifugal compressor with up to six stages and pressure up to 365 PSI are not uncommon.  The impeller can have either an open or close design.  Open it the most common with air applications. The impeller is normally made of special stainless steel alloy or aluminum.  The speed of is very high compared with other type of compressors 15,000 to 100,000 RPM.


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