Dynamic Displacement Compressors

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Dynamic displacement compressors work by increasing the velocity of the incoming gas and then restricting the air flow.  This restriction in the airflow causes the pressure to increase.

Dynamic displacement compressors are also known as oil free or oil-less compressors.  There are no seals and valves in dynamic displacement compressors.

Centrifugal compressors are the most common type of dynamic displacement compressors.  The operation is similar to that of a centrifugal pump.

Air is drawn into the centre of an impeller and then pushed out of the perimeter at high velocity.  Centrifugal compressors operate at high speeds of the order of 75000 rpm.  They are driven by gears.
They have higher efficiencies as compared to dynamic displacement compressors.  Changes in temperature and humidity will have a significant impact on  the operating characteristics of dynamic displacement compressors.

Pneumatic Tools - Advantages and Disadvantages

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Pneumatic tools are tools which are driven by air.  In a Pneumatic tool, compressed air from a cylinder is made to pass through a control valve which is operated by the user.  The air is then passed through a small turbine which generates rotary motion.  This rotary motion is used to drive a shaft.

This rotary motion can be used for drilling or grinding purposes.  The air is then drained through an outlet.

Advantages
Compressed air is easily available
Tools can be smaller than electric tools.
Pneumatic tools develop high torque
They can be used in environments where there is a risk of fire
They are not affected by temperature
They are clean and do not leave any residue
Compressed air can be guided easily through small tubes wherever required.

Disadvantages
They are noisy
Leakages occur frequently
Air should be dried properly otherwise there can be condensation.

Uses of Compressed Air

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Compressed air has many uses in the industry.  Compressing air is an effective means of storing energy and transporting it over large distances.

Some of the applications of Compressed Air are


  1. As a source of energy to drive machines and pneumatic tools.  Pneumatic tools can be smaller than electrically driven tools.
  2. In Shot blasting applications
  3. In Spray painting
  4. In control systems. Compressed air is widely used in the petrochemical industry where electric power cannot be used due to the risk of fire.
  5. In Braking systems in trains.
  6. In Refrigeration
  7. In spray cans such as those used in perfumes and other sprays.
  8. In cleaning

Single acting and Double acting Compressors

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In single acting compressors, air is drawn and compressed on only one side of the piston.  When the piston moves down, the suction valve opens and air enters the cylinder.  When the piston moves up, at the end of the compression, the discharge valve opens and the compressed gas is released.

The other side of the piston is connected to the crankshaft. 

In a double acting compressor, air is drawn and compressed on both sides of the piston.  The piston is operated by a connecting rod through an airtight seal.  There are compression chambers on both sides of the piston and a set of suction and discharge valves.  .  

Double acting compressors have higher efficiency.  However, they are expensive to manufacture. They are usually water cooled.  


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