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

1-D Design Process

Work input The work input is often expressed as the work input factor or stage loading: V w2 V r2 V a2 V f2 The second term is the slip velocity ratio and the third term includes the impeller exit velocity coefficient and the backsweep angle. Z, Number of vanes

Back Sweep Angle

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

Variation in tip speed with pressure ratio r 0p