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Compressors Unit 3 Rotary Compressors




Rotary compressors are positive displacement compressors; they increase the pressure by reducing the volume.

Screw compressors

Screw compressors are also known as helical-lobe compressors.  The application range of the helical lobe compressor is unique in that it bridges the application gap between the centrifugal compressor and the reciprocating compressor.

Components of the screw compressor

Within the compressor body, there are two screws with matting profile: a female screw having concave inlets and a male screw with convex helical inlets.  The screws rotate in opposite directions with the female screw receiving the driving power and transmitting this power to the male screw through a set of synchronisation gears.  In the figure below, the bottom screw is the female attached to the bearing which delivers driving power from the driver and the top screw is the male.

Figure 18: A helical lobe compressor

Figure 19: Picture showing the two screws, notice the close contact areas

Operating principles of the screw compressor

The gas enters on the right side of both the screws and exits at high pressures on the left side of both screws.

As the screws rotate, the air is drawn into the inlet port and fills up the space between the screws.  The compression of the gas starts when the end of the male thread blocks the end of the female thread.  The volume available between the compressor body and these two threads progressively decrease during rotation.  The gas is then released on the delivery outlet of the compressor.  Throughout this process, there is no contact between the screws.  Therefore lubrication is not required within the compressor space.

Advantages and disadvantages of the screw compressor


  • They are completely packaged.
  • The air is free of oil.
  • There are no special foundations.
  • Compact size.
  • Smooth delivery of cool air.
  • Easy to install.


  • The screw compressor is more sensitive to dirty inlet air.
  • The overall efficiency is low.
  • The screw compressor is difficult to repair, repair work requires specialized training.

Applications of the screw compressor

The screw compressors are used for commercial and industrial applications; they are also used in automobile engines as super chargers.

Oil-flooded screw compressors

Figure 20: The oil-flooded screw compressor

Components of oil-flooded screw compressors

Oil-injected rotary screw compressors use two intermeshing rotors in a stator housing.  The stator housing has an inlet port at one end and a discharge port at the other. The male rotor has lobes formed helically along its length; the female rotor has corresponding helical grooves or flutes.

Figure 21: Picture showing two helical rotors

Operating principles of oil-flooded screw compressors

Figure 22:  Picture showing the operating process of an oil flooded screw compressor

Air flowing in through the inlet port fills the spaces between the lobes on each rotor as shown in the above figure A.  Rotation then causes the air to be trapped between the lobes and the stator as the inter-lobe spaces pass beyond the inlet port.

As rotation continues a lobe on one rotor rolls into a groove on the other rotor, reducing the space occupied by the air.  This increases the pressure and forces the air to move towards the discharge port (figure B).  Compression continues until the inter-lobe spaces are exposed to the discharge port and the compressed air is discharged (figure C).

Oil injected into the compression chamber lubricates the intermeshing rotors and bearings. It takes away most of the heat caused by compression.  This oil also acts as a seal between the meshing rotors and between rotors and stator.

A mixture of compressed air and injected oil leaves the discharge port and is passed to a sump-separator where the liquid oil is removed from the compressed air.

Advantages and disadvantages of oil-flooded screw compressors


  • High compression ratio
  • Quick absorption of heat by oil
  • Pressure and flow are controlled by slide valves.
  • Vibration free operation therefore no special foundation required.


  • Moderately high cost
  • Oil carry-over requires proper maintenance of air-oil separator and the oil itself.

Applications of oil-flooded screw compressors

  • Refinery service
  • Fuel gas boosting in gas turbines.

Straight-lobe compressor

A straight-lobe compressor is similar to the helical-lobe compressor, but much less complex.  The straight lobe compressor is also known as a roots blower.

Figure 23: The straight lobe compressor

Components of straight-lobe compressors

The straight lobe compressor consists of two or three straight lobes/impellers that intermesh as they rotate, as shown in the figure below.  These impellers are situated in a casing that has an input and a discharge port.

Figure 24: Picture showing the lobes of the compressor

Operating principles of straight-lobe compressors

The following figure shows a simple two-lobe configuration, although a three-lobe version is also available.

Figure 25: A four-stage cycle of a straight-lobe compressor

The rotation of the impellars draws the gas from the input port into one side of the impellar (position A).  The gas is trapped between the impellar and the side of the cylinder (position B).  as rotation continues the gas is pushed around the cylinder into the discharge port (position C and D).  There is no compression as gas is moved to the discharge port.  It is compressed only by the backflow from the discharge port.

Advantages and disadvantages of straight-lobe compressors


  • The power demand in the partial load range is lower.
  • The straight-lobe compressor quickly attains the full number of revolutions.


  • A control device must be installed, in order not to exceed the operating pressures.
  • Expensive noise dampening equipment in necessary.
  • The straight-lobe compressor is sensitive to foreign matter.


Rotary vane compressor

Figure 26: The rotary vane compressor

Components of a rotary vane compressor

The rotary vane compressor consists of a moving core element, the rotor [6] with sliding blades [4], which turns on an eccentric axis within a static cylinder [3]. Due to centrifugal forces caused by the rotation, the blades slide and extend out of the rotor’s grooves [5]. In their extended state, these blades function like vanes and adjust to the cylinder’s shape as they are driven by the rotor.

Figure 27: Picture showing the rotary vane compressor

Operating principles of a rotary vane compressor

Incoming air [1] enters the compressor; this is caused by the pressure in the air circuit forces the regulating piston [2].  The air is drawn into the cylinder by the rotating vanes, which, due to centrifugal forces they are thrown off against the housing wall.  The air is compressed as the volumes between the vanes are reduced.  After maximum compression [7], the air exits through the outlet [8].  Intensive injection of oil [9] reduces the overall temperature of the compression cycle and ensures abundant lubrication. The injected oil also functions as sealant to keep the vanes airtight.

Advantages and disadvantages of rotary vane compressors


  • Slow rotational speed results in low wearing of parts and efficient production.
  • Moving parts are rotor and vanes only
  • It can be directly driven
  • Gas discharge which is devoid of pulse.
  • Vibration free operation, therefore no additional foundation is required
  • Valveless operation
  • Requires less maintenance
  • Lightweight – yet continuous rating,
  • Integrated and compact configuration
  • Extremely quiet operation
  • Coolest possible discharge of air.


  • Close contact between the compression gas and lubricating oil.
  • Poor quantity of lubrication oil needs disposal.
  • Difficulty with high pressures
  • Oil-free designs unavailable.

Applications of rotary vane compressors

  • Landfill gas gathering as well as boosting.
  • Digester mixing.
  • Fuel gas boosting.
  • Flare gas recovery.
  • Wellhead gas compression.

Scroll compressor

It resembles the rotary screw device. The gas is compressed by the use of spiral scrolls.

Components of a scroll compressor

  • The scroll compressor consists of two identical, concentric machined pieces of iron.
  • These Iron pieces (also called scrolls) are spiral in shape and fit into each other.
  • One scroll is stationary and the other rotates around it, the stationary scroll contains a discharge port to enable compressed gas to escape.

Operating principles of a scroll compressor

Figure 28: Picture showing the operation of a scroll compressor

  • Gas enters an outer opening as one scroll orbits the other as shown in Figure 7 (A).
  • The gas is drawn into the compression chamber, the opening passage is sealed (B).
  • As one scroll continues orbiting, the gas is compressed into smaller pockets (C), until it reaches the centre of the compressor where it is discharged, through precisely machined ports and returned to the system (D).
  • During the operation all passages are in operation at all times, making it a continuous process (E).


Advantages and disadvantages of scroll compressors


  • Scroll compressors operate more efficiently and quieter than older reciprocating (piston-type) compressors.
  • Compresses with fewer moving parts
  • Compared to reciprocating compressors, scroll compressors are more reliable.


The scroll compressor vibrates, which minimizes its usage in industries.

Applications of scroll compressors

  • Used as a supercharger in automotive operations
  • Used in air conditioning and refrigeration

Liquid piston compressor

These are oil-free, positive-displacement compressors with a built-in pressure ratio.

Components of liquid piston compressors

They have a single rotor with a series of blades mounted in a casing.  The blades are arranged in such a way that the space between the blade tip and the casing varies cyclically during each revolution of the rotor.

Operating principles of liquid piston compressors

The casing is partly filled with a liquid which is carried around the casing by the action of the rotor blades.  Due to the centrifugal force, the liquid forms a solid ring inside the casing, the inner wall of which varies in its distance from the rotor, as does the casing wall.

The liquid is carried at the tips of the blades and moves in and out as the rotor turns, forming a liquid piston.  The port openings are located to allow gas to enter when the liquid piston is moving away from the centre.  The port is then closed as rotation progresses and compression takes place, with the discharge port opening as the liquid piston approaches the innermost part of its travel (i.e. the closest to the centre of the rotor as shown in the figure).

Figure 29: A liquid piston compressor

The cooling of the liquid-ring compressors is direct.  Because of the direct contact of the gas and liquid, the final discharge temperature can be held close to the inlet temperature.  However, the discharge gas is saturated at the discharge temperature with the compressing liquid.

In principle the compression is isothermal, but due to the friction of the liquid against the casing, and other effects, there is an additional loss in energy.  The net energy required is much higher than that of reciprocating compressors for the same work.

Applications of the liquid piston compressor

These compressors are used in processes where only a small temperature rise throughout the compression cycle is acceptable, approximating isothermal compression.



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