Centrifugal Compressor And Reciprocating Compressor | Parts And Working

Centrifugal Compressor

• A centrifugal compressor is a radial flow rotodynamic fluid machine that uses mostly air as the working fluid and utilizes the mechanical energy imparted to the machine from outside to increase the total internal energy of the fluid mainly in the form of increased static pressure head.

A centrifugal compressor have three components.

• stationary casing

• Rotating impeller : it which imparts a high velocity to the air. The impeller may be single or double sided, but the fundamental theory is same for both.

• Diffuser: it consisting of a number of fixed diverging passages in which the air is decelerated with a consequent rise in static pressure.

Principle of operation:

• Air is sucked into the impeller eye and whirled outwards at high speed by the impeller disk. 

• At any point in the flow of air through the impeller the centripetal acceleration is obtained by a pressure head so that the static pressure of the air increases from the eye to the tip of the impeller.

• The remainder of the static pressure rise is obtained in the diffuser, where the very high velocity of air leaving the impeller tip is reduced to almost the velocity with which the air enters the impeller eye.

• Usually, about half of the total pressure rise occurs in the impeller and the other half in the diffuser. Owing to the action of the vanes in carrying the air around with the impeller, there is a slightly higher static pressure on the forward side of the vane than on the trailing face. The air will thus tend to flow around the edge of the vanes in the clearing space between the impeller and the casing. This results in a loss of efficiency and the clearance must be kept as small as possible. Sometimes, a shroud attached to the blades and may eliminate such a loss, but it is avoided because of increased disc friction loss and of manufacturing difficulties.

• The straight and radial blades are usually employed to avoid any undesirable bending stress to be set up in the blades. The choice of radial blades also determines that the total pressure rise is divided equally between impeller and diffuser.

ADVANTAGES AND DISADVANTAGES:

ADVANTAGES:

a.) When compared to other compressors, it is relatively agile and easy to manufacturer.

b.) As this compressor does not require any special foundation it is highly energy efficient and reliable.

c.) They consist of a small number of rubbing parts and are absolutely oil free in nature.

d.) It generates a higher pressure ratio per stage than the axial flow compressor.

DISADVANTAGES:

a.) They produce a limited amount of pressure and are not suitable for very high compression.

b.) As they work at relatively high speed an enlightened or worldly mounting is required.

c.) They are very sensitive towards problems such as stalling and choking.

RECIPROCATING COMPRESSORS

WHAT IS RECIPROCATING COMPRESSOR AND TYPES:

• Reciprocating compressors generally, employ piston-cylinder arrangement where displacement of piston in cylinder causes rise in pressure.

• Reciprocating compressors are capable of giving large pressure ratios but the mass handling capacity is limited or small. 

• Reciprocating compressors may also be single acting compressor or double acting compressor. Single acting compressor has one delivery stroke per revolution while in double acting there are two delivery strokes per revolution of crank shaft.

WORKING OF RECIPROCATING COMPRESSOR:

• Reciprocating Compressor has piston, cylinder, inlet valve, exit valve, connecting rod, crank, piston pin, crank pin and crank shaft.

• Inlet valve and exit valves may be of spring loaded type which get opened and closed due to pressure differential across them. Let us consider piston to be at top dead centre (TDC) and move towards bottom dead centre (BDC).

• Due to this piston movement from TDC to BDC suction pressure is created causing opening of inlet valve. With this opening of inlet valve and suction pressure the atmospheric air enters the cylinder.

• Air gets into cylinder during this stroke and is subsequently compressed in next stroke with both inlet valve and exit valve closed. Both inlet valve and exit valves are of plate type and spring loaded so as to operate automatically as and when sufficient pressure difference is available to cause deflection in spring of valve plates to open them. 

• After piston reaching BDC it reverses its motion and compresses the air inducted in previous stroke. Compression is continued till the pressure of air inside becomes sufficient to cause deflection in exit valve. 

• At the moment when exit valve plate gets lifted the exhaust of compressed air takes place. 

• This piston again reaches TDC from where downward piston movement is again accompanied by suction. This is how reciprocating compressor. Keeps on working as flow device. 

• In order to counter for the heating of piston-cylinder arrangement during compression the provision of cooling the cylinder is there in the form of cooling jackets in the body.

• Reciprocating Compressor described above has suction, compression and discharge as three prominent processes getting completed in two strokes of piston or one revolution of crank shaft.