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Design Steps : Centrifugal Compressors
P M V Subbarao
Professor
Mechanical Engineering Department
A Practice of Progressive Detailing…..
1 D Design Process
• The activity starts with the preliminary design in which the required duty of the compressor is simply stated.
• The range of design choices that will satisfy the duty are studied and compared, usually on a largely empirical and 1D basis.
• The initial study focuses on the design point.• The 1D design study is then broadened to include off-
design performance.• Subsequently some revision of the initial design or
nominal point choices is usual, implying an iteration between the design and off-design activities.
Aims of the preliminary design
• The importance of achieving a reliable preliminary design cannot be overemphasized.
• The aim of the preliminary design can be stated simply to achieve the design duty on a 1D basis, within the mechanical limitations of the available materials, with best achievable efficiency.
• This can be expanded into the following, more specific aims at design mass flow to achieve:
• (a) the desired work input (stagnation enthalpy rise),• (b) the desired efficiency,• (c) the desired pressure ratio [automatically achieved
if analysis, (a) and (b) are achieved.
Work input
• The work input is often expressed as the work input factor or stage loading:
22
2
22
2
tan1 r
V
r
V
r
h fslip
Vw2
Vr2
Va2
Vf2
The second term is the slip velocity ratio and the third term includes the impeller exit velocity coefficient andthe backsweep angle.
7.02
2
cos
Zr
Vslip
Z, Number of vanes
Back Sweep in Centrifugal Impellers
• The predominant influence is backsweep and it is through this influence that backsweep exerts its powerful effect on stage stability;
• the reduced work factor at higher values of backsweep results in a more steeply rising characteristic of work versus mass flow and hence also a steeper pressure ratio characteristic.
• The work factor also determines the impeller tip speed and hence impeller stress levels; the stage pressure ratio
12
22
211
rop Mωr
Δhr
i
ωrRT
rM
2
2