Analysis of Axial & Centrifugal Compressors To be Selected as per Specific Speed of Applications…. P M V Subbarao Professor Mechanical Engineering Department

Multi Stage Hybrid Compressor

Static & Stagnation Scalars

Irreversible Adiabatic Stage of an Axial Flow Compressor T 03 =T 02 s T Total Losses Rotor Losses T 01 T 02s T 03s p 01 p 03s =p 02s p 02 p 03

Gas dynamics of Irreversible Compressor p 02s

Stage Efficiency The stage efficiency of an adiabatic compressor stage is: For calorically perfect gas Actual temperature rise of a fluid when compressed in an irreversible stage for a pressure ratio of p 03 /p 01 is:

System of Equations for Stage Design

Stage load coefficient Stage flow coefficient Stage reaction Selection of Global Stage Variables

The stage load distribution throughout the compressor

Selection of Design Parameters A high pressure rise per stage will decrease the number of stages for a given overall pressure rise. A high pressure rise per stage is obtained using: High blade speed. High inlet flow velocity. High fluid deflection in rotor blades.

Inlet Velocity Triangle & Flow Velocity  

V a1 V r1 Selection of Inlet Angle

Blade Speed For a given rotor speed the velocity of the blade at the tip will be maximum. The centrifugal stress in the rotor blades depends on the rotational speed, the blade material and length of the blade. The maximum centrifugal stress is given by, b, hub-tip diameter ratio. K varies in the range 0.55 – 0.65.

Fluid Deflection

Performance of Aerofoil Camber angle,  Naca 65 :  in  Circula rarc :  in 

Loss coefficient Outlet flow Angle Deflection Clues to Invent an Aerofoil

Current Design Practice ParameterRange Pressure ratio for single stage 1.5 – 2.0 Pressure ratio for two stages 2.0 – 3.5 Pressure ratio for three stages 3.5 – 4.5 Inlet mass flow rates195 – 205 kg/m 2.s Tip speed427 – 457 m/s Diffusion factor0.5 – 0.55 Fan or low pressure Compressor

Current Design Practice ParameterRange Stage loading coefficient 0.3 – 0.35 Flow coefficient0.45 – 0.55 Hub/tip ratio0.6 – 0.75 Inlet mass flow rates175 – 185 kg/m 2.s Tip speed386 – 457 m/s Diffusion factor0.5 – 0.55 High pressure Compressor

Compressor Maps

Multi Stage Compression Loss in capacity due to variation of velocity is defined as work done factor. Work done factor,, decrease with number of stages.

Multi Stage Axial-flow Compressor

V r1 V w2 < U V r2 V a2 V f2 V f1 Gas Dynamics of An Impeller V a1 V w1

1 2 3 s T T 01 p 01 T 03 =T 02 p 03 =p 02 Thermodynamic View of an isentropic Compressor p2p2 p3p3 Only Impeller can consume Power !!! p1p1

Irreversible Diffuser p 03s =p 02s p 02a p 03a T 03 =T 02 s T Overall Losses Impeller Losses T 01 p 01

Work consumed by A compressor = Increase in Stagnation Enthalpy of gas For an irreversible compression, the actual pressure rise is less than isentropic pressure rise due to (T 03 -T 01 ). Define, Adiabatic Efficiency of A Compressor:

Erection of Pump psps HsHs HdHd H pump

Internals – Pump Vs Compressor Pump Impeller Compressor Impeller Fan Impeller