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Chapter: Mechanical and Electrical - Thermal Engineering - Air Compressors

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Classification of Air Compressors

Positive displacement compressor, Dynamic compressors

CLASSIFICATION OF AIR COMPRESSORS

Types of compressors


 

Positive displacement compressor

 

Ø In the positive-displacement type, a given quantity of air or gas is trapped in a compression chamber and the volume it occupies is mechanically reduced, causing a corresponding rise in pressure prior to discharge.

 

Ø At constant speed, the air flow remains essentially constant with variations in discharge pressure.

 

Ø Ex: Reciprocating compressors,  vane compressors & so on.

 

Dynamic compressors:

 

Ø Dynamic compressors impart velocity energy to continuously flowing air or gas by means of impellers rotating at very high speeds.

 

Ø The velocity energy is changed into pressure energy both by the impellers and the discharge volutes or diffusers.

 

Ø In the centrifugal-type dynamic compressors, the shape of the impeller blades determines the relationship between air flow and the pressure (or head) generate.

 

Ø Ex: centrifugal compressors, axial compressors.

 

Reciprocating compressors

 

Ø In a reciprocating compressor, a volume of gas is drawn into a cylinder; it is trapped and compressed by piston, then discharged into the discharge line.

 

Ø The cylinder valves control the flow of gas through the cylinder; these valves act as check valves.

 


Principle of Operation

 

Ø The piston is driven by a crank shaft via a connecting rod.

 

Ø At the top of the cylinder are a suction valve and a discharge valve.

 

Ø A reciprocating compres sor usually has two, three, four, or six cylinders in it.


 

Ø The suction valve opens at point 4.

 

Ø As the piston travels toward the bottom dead center, the volume of the c ylinder increases and the vapor flows into the cylinder.

 

Ø The pressure inside the cylinder is slightly less than suction line pressu re. The pressure difference pushes the val ve open on during the suction stroke.

 

 

Ø At point 2, the pressure inside the cylinder has become slightly greater than discharge line pressure.

 

Ø This causes the valve opening allowing the gas to flow out of the cylinder.

 

Ø The volume continues to decrease toward point 3, maintaining a sufficient pressure difference across the discharge valve to hold it open.

 

Ø At point 3, the piston reaches the top dead center and reverses direction.

 

Ø At top dead center, as the piston comes to a complete stop prior to reversing direction, the pressure across the valve is equal.

 

Ø So, the discharge valve is closed.

 

Ø As the piston moves towards point 4, the volume increases and the pressure decreases in the cylinder.

 

Ø The gas trapped in the cylinder expands as the volume increases until to point 4.

 

Ø At point 4, the gas pressure inside the cylinder becomes less than the suction line pressure, so the suction valve opens again.

 

Ø The cycle then starts over again.

 

Ø The shape of the re-expansion line (Line 3-4) is dependent on the same compression exponent that determines the shape of the compression line.

 


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