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: ρ(kg/m3).

Machine-Fluid Matching Parameter:

( )ρ:, pQf ∆=Π

For compressible fluid machines

( ) kji pQ ρ×∆×=Π

j

i

mkg

smkg

s

m

×

=Π

3

23

.

j

i pQ

∆×=Πρ

ji

s

m

s

m

×

=Π

2

23

23

2

23 −

×

=Π

s

m

s

mi

Take j = -3/2

×

=Π −

−

3

33

s

m

s

mi

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

43

2

22

13 −

×

=Π

s

m

s

m

( )4

3

2

1−

∆×=Πρp

Q

4

3

∆=Π

ρp

Q

23

23

21

23

−

−

×=Πs

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 -NonΠ=

This is named as Specific Speed, Ns

4

3

∆=

ρ

ρ

p

mnN s

4

3

∆=Π

ρp

Q

4

3

∆

=

ρp

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