Centrifugal Compressors. Classes and comparisons between compressors. Centrifugal Compressors. Principle of Operation Centrifugal compressors consist of stationary casing containing Rotating impeller (imparts a high velocity of air),
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(b) to find the inlet angle it is necessary to determine the inlet velocity which in this case is axial;
Since the density 1 depends upon C1and both are unknown, a trial and error process is required.
Based on stagnation conditions:
, the equivalent dynamic temperature is
equivalent dynamics temperature is
and at eye root radius =136.5 m/s,
(c) the shape of the impeller channel between eye and tip is very much a matter of trial and error.
The aim is to obtain as uniform a change of flow velocity up the channel as possible, avoiding local decelerations up the trailing face of the vane.
To estimate the density at the impeller tip, the static pressure and temperature are found by calculating the absolute velocity at this and using it in conjunction with the stagnation pressure which is calculated from the assumed loss up to this point.
To calculate density at exit
thus get 2.
The required area of cross-section of flow in the radial direction at the impeller tip is
Example 4.2Consider the design of a diffuser for the compressor dealt with in the previous example. The following additional data will be assumed:Radial width of vaneless space wd = 5 cmApproximate mean radius of diffuser throat, rm =0.033m
Depth of diffuser passages dd 1.76Number of diffuser vanes nv 12Required are (a) the inlet angle of the diffuser vanes and (b) the throat width of the diffuser passages which are assumed to be of constant depth
The radial component of velocity can be found by trial and error. The iterationmay be started by assuming that the temperature equivalent of the resultant velocity is that corresponding to the whirl velocity, but only the final trial is given here.
Try Cr2= 83 m/s
No problem at sea level conditions, however at high altitude ( aircraft engine), speed of sound decreases and we might have supersonic flow.
For example at 11000 m, T=217 K
For this design, the flow is subsonic at altitude.
for details see text book