Analysis of Compressors

P M V Subbarao

Professor

Mechanical Engineering Department

Active Devices for Infusion of Life into Fluid….

Classification of Compressors

Rotodynamic Compressors Positive Displacement of Compressors vdp pdv

CentrifugalCompressors

MixedCompressors

AxialCompressors Reciprocating

CompressorsRotoryCompressors

Centrifugal Vs Axial : Flow Area

Centrifugal Vs Axial : Level of interaction

In search of Time scale in Requirement

• Primary characteristics of a requirement.

• The effect: p

• The Capacity: Flow Rate Q (m3/s).

• Density of fluid: (kg/m3).

Machine-Fluid Matching Parameter:

:, pQf

For compressible fluid machines

kji pQ

j

i

mkg

smkg

s

m

3

23

.

j

i pQ

ji

s

m

s

m

2

23

23

2

23

s

m

s

mi

Take j = -3/2

3

33

s

m

s

mi

Take i = 1/2 & j = -3/4

43

2

22

13

s

m

s

m

4

3

2

1

p

Q

4

3

p

Q

23

23

21

23

s

m

s

m

Time Scale of a Machine to Resource

Speed: N (rpm) or n (rps)

Scale Time Machine

Scale Timet Requiremenscale timeessDimensionl

n1

scale timeDimension -Non

This is named as Specific Speed, Ns

4

3

p

mnN s

4

3

p

Q

4

3

p

mnN s

Centrifugal AxialMixed

Role of specific Speed on Design of A Compressor

Centrifugal Compressors

• The centrifugal compressor consists essentially of a stationary casing containing a rotating impeller which imparts a high velocity to the air, and

• A number of diverging passages (Diffuser) in which the air is decelerated with a consequent rise in static pressure.

• Air is sucked into the impeller eye and whirled round at high speed by the vanes on the impeller disc.

• Centripetal acceleration leads to increase in rise of some amount of static pressure.

• The remaining increase in rise of pressure occurs in diffuser.

Multi Stage Compressors

Impeller–Diffuser :Rotor-Stator

Anatomy of A Centrifugal Compressors

Vr1

Vw2= Utip

Vf2 =Vr2

Va2

b1

Gas Dynamics of A Perfect Impeller

Vf1=Va1

Ueye

Vw1=0

Vr1

Vf1=Va1

Vw2

Vw2

Vr2

Vr2

Va2

Va2

Vf2

b1

Reality of Fluid Mechanicsb2