Two Basic Principles of compressed air (general information):
There 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.
The 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:
The 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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Screw Compressors Designed for 100% Oil Free Compressed Air:
The 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.
Tooth Compressors Designed for 100% Oil Free Compressed Air
The 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.
Scroll Compressor designed for 100% Oil Free Compressed Air
A 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.
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.