CENTRIFUGAL COMPRESSOR

Centrifugal compressor is a constant pressure & variable volume machine as contrast to a positive displacement compressor being a constant volume & variable pressure machine .Centrifugal compressor consists of a impeller & a device to convert kinetic energy into pressure energy. Process of converting kinetic energy is known as diffusion & consequently that part is known as diffuser .Fluid is sucked into the eye of the impeller , whirled round at high speed by the vanes on the impeller disc &driven away by Centrifugal force. At any point in the flow of fluid through the impeller, the centripetal acceleration is obtained by pressure head, so that the static pressure of the fluid increases from the eye to the tip of the impeller. The remainder of the pressure rise is obtained in the diffuser, where the very high velocity with which the fluid enters the empeller eye. Modern practice is to design the compressor so that about half the pr. Rise occurs in the impeller & half in the diffuser.

Performance curve :

Q1 = WV1 = Wz1RT / P1* 144Q1= Inlet volume ,cfm

W = Total mass thruput , lb/minV1= Sp. Volume at inlet condition, ft3/lbZ1 = Inlet compressibility factorR = Universal gas constantT1 = Inlet Pr. psia

It is possible to change completely the design inlet condition conditions & still maintain design inlet cfm .For all practical purposes , the polytropic head & efficiency of a given single stage centrifugal compr. remains the same even though the design gas & gas conditions have changed, as long as design inlet cfm & speed is obtained .

Hp = Zm RT ( knp/ k-1) [ (P1/P2)k-1/npk -1

CRITICAL SPEED : For a symmetrical body or for the compressor/Turbine shaft, has a particular length, diameter, weight, volume, contour of shaft or configuration of the shaft, material of construction and mountings. For the above factors the said turbine/compressor shaft itself has particular frequency one or more than one, called the natural frequencies of the shaft.

During the construction/manufacture of the shaft one tries to match or coincide geometric axis and mass centre of the shaft. But for practical reasons mass centre of shaft never coincides with the geometric axis. As a result, when the shaft rotates by any driving force, than there will be a transverse force whose direction of rotation at the same speed as the shaft with a particular frequency. This frequency is called the rotor frequency or the shaft frequency. At a particular r.p.m. when the natural frequency of the rotor or shaft, coincides with the rotor or shaft frequency, then there will be a resonance, causing excessive vibration and hence tending to deflect the shaft.

The speed at which natural frequency of the shaft coincides with the shaft frequency and resonance & high vibration occurs to deflect the shaft, called the critical speed.

In certain cases, if the speed is increased still further a second resonant speed is encountered then the third and so on.

The normal running speed of the shaft is always for above or for below the critical speed/speeds.

AXIAL COMPRESSORS :

The axial compressor is a dynamic type of machine, identified by the use of moving and stationary blading to accomplish the velocity pressure conversion for pressure increase. In general, axial compressor design is based on the theory of 50% reaction. This means that half of the pressure rise is accomplished in the rotor blade and half in stator blade.

As air or gas flows through the rotating blades, static pressure and kinetic energy both increase. Each raw of stationary blades converts the kinetic energy to pressure, acting as a diffuser for the air or gas flowing out of the preceding row of rotating blades. Also, the stationary blades acts as nozzles to guide the air or gas into the next row of rotating blades.

RADIAL COMPRESSOR :

The gas flow through the impellers in a direction radial to the wheel. For smaller capacity and higher operating pressure radial type compressor is normally used. They are having impeller instead of blades of the moving part.

1. General description of Parts : The centrifugal compressor is of multistage type. It is horizontally split in the case of MCL type and vertically split in the case of BCL type, and consists mainly of a static group i.e.

1) casing2) diaphragms3) bearings4) seals, etc.

D

A BO

C OD: Static DeflectionDC: Dynamic deflection with respect to D

Mass Centre

Shaft Geometric Axis

and a group of rotating parts i.e. 1)shaft 2)impellers etc.

2 MCL and BCL type compressors are modifications over the above basic types and contains two phases of compression in one casing i.e. the gas will be compressed in the first phase and before entering the next phase passes through a cooler and is further compressed to the final delivery conditions in the second phase.

1.3 Rotor : The rotor comprises a shaft, impellers, balancing drum, thrust collar , spacer rings and lock-nuts.

1.3.1 Shaft : The shaft is machined from low alloy steel forgings. It is precision machined and ground at the journal portions. The shaft surfaces between any two impellers are protected by chromium steel spacers to prevent the coming in contact with the gas.1.3.2 Impellers : The impellers are made from alloy steel forgings. They are closed type, with blades curved backwards. They are of either welded or spark eroded construction depending upon the width of the gas passages. Each impeller, after manufacture, is dynamically balanced and subsequently spin tested at a speed 15% above the maximum continuous speed, before assembled on the shaft. The impellers are shrunk fitted & keyed on to the shaft. The spacers between the impellers are also shrunk fitted and they correctly position the impellers on the shaft.

1.3.3 Balancing Drum : The balancing drum is assembled on the shaft at the end adjacent to the last stage impeller. This drum reduces considerably the thrust acting towards the suction. This is achieved by connecting the space between the balancing drum and the corresponding end labyrinth steel to the

compressor suction, by means of a thrust balancing pipe so as to create a continuous differential pressure across the balancing drum.

The remaining unbalanced thrust is taken care of in the thrust bearing. The balancing drum is made from forged chromium steel and is shrunk fitted and keyed to the shaft.

1.4 Journal Bearings : The journal bearings are normally of sleeve type and are force feed lubricated. They are housed in such a way that it is possible to inspect and disassemble these without depressurising the casing. The bearings are made from steel forgings and are lined with white metal of patented composition. The bearings are horizontally split.

1.4 Thrust Bearings : The thrust bearing which is installed at the suction end of the shaft is of Kingsbury make and is provided with an oil control ring to reduce power losses due to viscous friction. It is designed to take the remainder of the axial thrust that is not fully balanced in the balancing drum.1.8 Capacity Control Device : For compressors which are used for recycle gas compression in synthesis trains, a capacity control device with adjustable inlet guide vanes is mounted on the end cover of the recycle suction side. This device consists mainly of a crown gear wheel to which a number of pinions built on the control vanes shafts are geared. One of the vanes is manipulated from the outside and thus becomes the driving element. The spindle of the above vane protrudes out of the casing and is directly coupled to the drive shaft outside, on which a gear (driven by external control system) is installed. The movement of the adjustable inlet guide vanes is accomplished by a pneumatic cylinder through a rack and pinion of the shaft. On the outer end of the shaft which carries the gear coupled to the rack, a graduated drum is installed from which the angle of the guide vanes can be ascertained. Zero position indicates that the guide vans are in radial position.