Design Steps : Centrifugal Compressors

P M V Subbarao

Professor

Mechanical Engineering Department

A Practice of Progressive Detailing…..

Overview of the design process

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.

1D Design Stations

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.

Overview of the design process

Overview of the design process

1-D Design Process

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 Angle

18&2.02

2 Zr

V f

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

Variation in tip speed with pressure ratio

r0p