Centrifugal Compressor

Most cooling systems, from residential air conditioners to large commercial and industrial chillers, employ the refrigeration process known as the vapor compression cycle. At the heart of the vapor compression cycle is the mechanical compressor. A compressor has two main functions: 1) to pump refrigerant through the cooling system and 2) to compress gaseous refrigerant in the system so that it can be condensed to liquid and absorb heat from the air or water that is being cooled or chilled .
There are many ways to compress a gas. As such, many different types of compressors have been invented over the years. Each type utilizes a specific and sometimes downright ingenious method to pressurize refrigerant vapor. The five types of compressors used in vapor compression systems are Reciprocating, Rotary, Centrifugal, Screw and Scroll.
We can classify compression process into to types :
(a) Compression by decreasing volume:-
  • Required pressure is developed by trapping a gas in a chamber, reducing the volume of the chamber and increasing the pressure of the gas by the ratio of initial chamber to the final volume.
(b) Compression by accelerating fluid :-
  • The second method of compressing gases is based on the conversion of kinetic energy into the potential energy. Accelerating fluid to a higher velocity and then decelerating it by changing its direction of flow transforms the accumulated energy into potential energyclip_image002
We are going to talk about Centrifugal Compressor
Centrifuclip_image004gal Compressors
Centrifugal compressors use the rotating action of an impeller wheel to exert centrifugal force on refrigerant inside a round chamber (volute). Refrigerant is sucked into the impeller wheel through a large circular intake and flows between the impellers. The impellers force the refrigerant outward, exerting centrifugal force on the refrigerant. The refrigerant is pressurized as it is forced against the sides of the volute. Centrifugal compressors are well suited to compressing large volumes of refrigerant to relatively low pressures. The compressive force generated by an impeller wheel is small, so chillers that use centrifugal compressors usually employ more than one impeller wheel, arranged in series. Centrifugal compressors are desirable for their simple design and few moving parts.
  • clip_image006Centrifugal compressor works on the principle of accelerating a gas to a high velocity and converting its KINETIC ENERGY (velocity) into POTENTIAL ENERGY (pressure) by decelerating the gas. The gas enters the eye of the impeller and is accelerated to the outward edge of the impeller as it rotates. It then enters a diffuser where its direction is changed, causing deceleration. This deceleration converts the KE into the PE, pressure. If the gas is to be further compressed, then a return chamber directs it from the diffuser to the eye of the next impeller in series. The gas enters a collector or volute when it is to leave the compression stage. It is discharged to the process through a discharge nozzle.
§ The centrifugal compressor has no connecting rods, pistons and valves; so the shaft bearings are the only points subject to wear. The compressor discharge pressure is a function of gas density, impeller diameter and design, and impeller speed. Centrifugal compressor impellers rotate very rapidly:
                 Low speed                                    3,600 RPM
                 Medium speed                              9,000 RPM
                 High speed                         above 9,000 RPM
Power is supplied by an electric motor or steam turbine. Vapor enters the on the center of impeller around the shaft and is directed through the impeller blades. As the impeller accelerate the gas, by the kinetic energy of the impeller is converted to the kinetic of fast moving gas. As the gas enter the volute, it is compressed, and the kinetic energy is converted to the potential energy of compressed gas. The velocity of the gas leaving the impeller is extremely high.
In order to pass from the low pressure in impeller to the higher pressure of the diffuser, the refrigerant necessarily needs high velocity In absence of sufficient velocity, the refrigerant will stuck in the impeller and the compressor will stall (surge) .
Therefore one of the fundamental parameter of centrifugal compressor design is the “tip speed ‘
Which must be in the range of 240 to 231 m/sec. This parameter determines the clip_image010speed and the dimension of the impeller:
RPM=[TipSpeed(m/s)*1910]/ Diameter(cm.)
A centrifugal compressor must be specifically designed for the right pressure difference we want to achieve.
Part load conditions are the most critical for centrifugal compressors
Gas speed is even more critical in unloaded condition, when the gas flow decreases and therefore could not be able to overpass differential pressure to the diffuser. If this happens, the flow can go back into the impeller bringing compressor to stall .
One of the most innovative technologies which allows to keep sufficient gas speed also in strongly unloaded conditions and extends unit working range, is the movable impeller discharge technology.clip_image014
  • Based on the type of casing design,centrifugal compressors are classified into two types:
1) Horizontally split casing design(MCL/MCH type):- This design is used for low working pressure below 40ata.These casings are in two halves with horizontal parting plane.DMCL Compressor have two stages of compression in parallel in a single casing.The solution is most balanced.The other aspects of construction are the same as for the MCL Compressor. clip_image018
2) Verically Split Casing design (BCL/BCH type):-This design is made of barrel construction closed on the sides by end covers with help of studs.This type of construction is suitable for high pressure operations up to 750kg/cm2. Another type of compressor is the PCL(Pipeline compressor),which has casing in the form of a cup with a single closing flange in the vertical plane instead of two as with the BCL. clip_image020
Example of Designation of BCL Compressor:
Example :        2    BCL   40    7 /  b
2 BCL 40 7 / bclip_image022

Compressor Components
§ Casing
§ Counter Casing (If Applicable)
§ Diaphragm
§ End Cover (If Applicable)
§ Shaft
§ Impellers
§ Shaft Seals
§ Journal Bearing
§ Thrust Bearing
§ Coupling

Construction of centrifugal compressor:
§ Every centrifugal compressor consist of two parts:- An impeller which forces the gas into the rotary motion by the action of blades, and the casing which directs the gas to the eye of the impeller and then leads it away from the impeller perimeter at a higher pressure. For most multistage compressors, shaft end seals are located inboard of the bearings. The internal passages are formed by a set of diaphragms.
Rotor Assembly
Assembly of Diaphragms in Counter Casing
Assembly of Journal Bearingclip_image031
There exists some amount of thrust generated at each impeller because of the differential pressure prevailing across it. The cumulative thrust generated across all the impellers is huge and it needs to be supported by the thrust bearing. The thrust bearing size becomes huge if all the gas thrust has to act on the bearing. For this purpose a rotating element called “Balancing Drum” is incorporated on the rotor. The drum is acted upon by a pressure differential which pushes the rotor in a direction opposite to that of the gas thrust. The thrust so generated balances the thrust produced by the impellers. Amount of balancing depends upon the size of the drum and pressure differential created across it. The pressure differentiated is maintained by an efficient seal placed over the balancing drum.

Cross-section of a BCL Compressor


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