Section 03 - Compressor Basics

Compressor basics

COMPRESSORS

DYNAMIC

STATIC

RECIPROCATING

SCREW

CENTRIFUGAL

AXIAL

Compressor basics Compressor selection

Compressors Turbo compressors  ____ 

Positive displacement machine Reciprocating compressors Compression

Rotary vane compressors

Screw compressors m

Volume

Centrifugal compressors

Axial-flow compressors

Conversion of kinetic energy

Motion

Reciprocating

Rotating

Rotating

Delivery

Intermittent

Quasi-continuous

Continuous

Volume flow

Low

Compression

High

Low

Low/medium Medium

Low/high

Very high

Medium

Low

ration Operation

Constant volume - Variable pressure

Variable volume - Quasi constant pressure

Principle

Function of a a compressor 

To render a specific volume of gas from a given initial condition into a desired final condition (pressure/temperature)

Compressor basics Flow path Diffusers

7

Discharge

Suction

1

4

2

▲

6

▲ ▲ ▲▲

5 3 Impellers

Compressor basics Flow characteristics The theoretical performance curve typically has a shape as is shown in the figure below. Friction losses 

      D       A       E       H

T   H   E   O R  S H   E   D  O  T   I   I   C   S K   L O  C  K    A L  S S E   F   R  C  H   I   C   S   A R  T   I   O N    A C   &  T   E   L E   R I   S T    A K   I   C    A G  E  L  O S  S E   S  F   R I   C  T   I   O N   L O  S S E   S  S H   O C   K   L O  S S  SPEED = CONSTANT E   S 

FLOW

Friction losses are the energy losses occurring in the various flow passages of the stage caused by gas friction (like pipe losses). Shock losses 

Shock losses occur at the impeller vanes, diffuser  vanes and return guide vanes. At high positive or  negative incidence (inlet angle of the gas) the flow separates from the vane surfaces, leading to

increased losses.

Compressor basics Centrifugal compression theory Energy transfer through impeller 

U = Circumferential velocity W = Relative velocity C = Absolute velocity 1 = Impeller inlet 2 = Impeller outlet on outlet Theoretical head without losses:

Compressor basics

Compressor basics Developing the compressor curve

Rc/

Rprocess,2 Rprocess,1

∆Pc/

Hp Rc2 Rc1

Compressor  curve for a specific speed N1

Q2

Q1

Qs / Qv / Qm

Compressor basics Developing the compressor curve process limit

Rc

adding control margins maximum speed surge limit power limit

stonewall or  choke limit Actual available operating zone minimum speed

stable zone of operation

Qs,

vol

Compressor basics Developing the surge cycle on the compressor curve (1) From A to B From C to D A-B-C-D-A

• • •

20 - 50 ms 20 - 120 ms 0.3 - 3 seconds

Drop into surge Jump out of surge Surge cycle

H B

A

Pd Pv

D

Rlosses

C

Qs,

vol

Compressor basics Anti-surge controller  VSDS Compressor 

FT 1

Suction

PsT 1

UIC 1

PdT 1

Discharge

Compressor basics Compressor performance control  Also called: Throughput control Capacity control Process control

•

 –  –  –

•

•

Matches the compressor throughput to the load

Can be based on controlling:  –

Discharge pressure Process PT 1

PIC 1

Compressor basics Compressor performance control  Also called: Throughput control Capacity control Process control

•

 –  –  –

•

•

Matches the compressor throughput to the load

Can be based on controlling:  –

Discharge pressure

 –

Suction pressure Process PT 1

PIC 1

Compressor basics Compressor performance control  Also called: Throughput control Capacity control Process control

•

 –  –  –

•

•

Matches the compressor throughput to the load

Can be based on controlling:  –

Discharge pressure

 –

Suction pressure

 –

Net flow to the user  •

Blow off 

Process PT 1

PIC 1

Compressor basics Compressor performance control  Also called: Throughput control Capacity control Process control

•

 –  –  –

•

•

Matches the compressor throughput to the load

Can be based on controlling:  –

Discharge pressure

 –

Suction pressure

 –

Net flow to the user  •

Blow off 

•

Recycle

Process PT

1

PIC 1

Compressor basics Compressor performance control  Also called: Throughput control Capacity control Process control

•

 –  –  –

•

•

Matches the compressor throughput to the load

Can be based on controlling:  –

Discharge pressure

 –

Suction pressure

 –

Net flow to the user  •

Blow off 

•

Recycle

•

Guide vanes

Process PT 1

PIC 1

Compressor basics Compressor performance control  Also called: Throughput control Capacity control Process control

•

 –  –  –

•

•

Matches the compressor throughput to the load

Can be based on controlling:  –

Discharge pressure

 –

Suction pressure

 –

Net flow to the user  •

Blow off 

•

Recycle

•

Guide vanes

•

Speed

SIC 1

Process PT 1

PIC 1

Compressor basics

P1 Throttling P2 Throttling   s   s   e   r    P  .    h   c   s    i    D

Design Point Recycle Control

Speed control