25-06-2012, 04:24 PM
CENTRIFUGAL COMPRESSORS
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INTRODUCTION TO COMPRESSORS
Centrifugal compressors is a steady flow power absorbing turbomachine in which energy transfer occurs from a rotating member to the continuously flowing working fluid by dynamic action
They are classified into 1)fans 2)blowers 3)compressors
A fan usually consists of a single rotor with or without a stator. generally it raises the pressure upto a maximum of 0.07bar.
Blower may consists of one or more stages of of compression with its rotors mounted on a cam shaft. The overall rise may be in the range of 1.5 to 2.5 bars.
Compressors is used to produce large pressure rise ranging from 2.5 to 10bar of more.
In general the centirfugal compressors may be known as a fan, blower , supercharger etc, depending on the need to be served. Broadly speaking, fans are the low pressure comressors , blowers are the medium pressure compressors. It is therefore the analysis of one, say centirfugal compressors , will also holds good to the other machines like blower fans.
INLET CASING
The function of the inlet casing with the convergent nozzle is to accelerate the entering fluid to the impeller inlet.
The inlet nozzle accelerate the fluid from the initial condition(state 0) to the entry of the Inlet Guide Vanes(IGV) which direct the flow in the desired direction at the inlet of the impeller(state 1)
IMPELLER
The impeller convert the supplied mechanical energy whereby the fluid kinetic energy & the static pressure rises.
An impeller is made of radial blades which are brazed to the shroud.It can be made from a single piece consisting of both the inducer and a largely radial portion
The flow approaching the impeller may be with or without swirl. The inlet diameter of the impeller will be taken as the average of the hub and tip diameters of the eye.
Since the centrifugal compressors have to run at very high speeds, generally radial blades are used in the impeller.
The impeller rises the pressure of the fluid from state 1 to state 2
DIFFUSER
The diffuser recieves the flow from the impeller through a vaneless space and it rises the static pressure of the fluid further an account of conversion of exit high kinetic energy to pressure energy.
The diffuser may be of vaneless type or diffuser vane type. The diffuser changes the state of the fluid from 2 to 5.
OUTLET CASING
The flow at the outer periphery of the diffuser is collected by a spiral casing known as the volute, which discharges the flow into the delivery pipe.
PRINCIPLE OF OPERATION
Fig Shows the enthalpy-entropy diagram for a centrifugal compressors.
Air enters the impeller eye through an accelerating nozzle. As the fluid velocity increases in the nozzle, there will be a pressure drop between the nozzle exit and the impeller inlet, and is represented by the process 00-1.
The air is then enters the impeller with a static pressure and temperature p1 & T1 respectively.
Even though there is increase of entropy due losses & the pressure drop in the accelerating nozzle & the IGV, the stagnation enthalpy at the inlet of the nozzle & the impeller inlet remains same ( h00=h01) because of no work transfer during this process. The energy transfer occurs only in the impeller blades.
Process 1-2 shows the actual compression process in the impeller where the pressure of air increases from p1 to p2 due to the centrifugal effect. The process 1-2‘ is the isentopic compression.
The stagnation pressure corresponding to the exit state of the impeller is p02.