section 03 - compressor basics

7/28/2019 Section 03 - Compressor Basics

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Compressor basics

Siemens AG 2007. All rights reserved.Page 1

COMPRESSORS

SCREWRECIPROCATING CENTRIFUGAL AXIAL

STATIC DYNAMIC


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Compressor selection Compressors

Reciprocating

compressors

Rotary vane

compressors

Screw compressors

m

Centrifugal

compressors

Axial-flow

compressors

Positive displacement

machine

Turbo compressors

____

Compression Volume Conversion of kinetic energy

Motion Reciprocating Rotating Rotating

Delivery Intermittent Quasi-continuous Continuous

Volume flow Low Low Low/medium Low/high Very high

Compression

ration

High Medium Medium Low

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)


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Flow path

Discharge

Diffusers

Suction

Impellers

1

2

3

5

4

7

6


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The theoretical performance curve typically has a shape as

is shown in the figure below.

THEORETICALCHARACTERISTIC

DISKFRICTION&LEAKAGELOSSESFRICTIONLOSSES

SHOCKLOSSES

SHOCKLOSSES

FLOW

HEAD

SPEED = CONSTANT

Friction losses

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 guidevanes. At high positive or

negative incidence (inlet

angle of the gas) the flow

separates from the vane

surfaces, leading to

increased losses.

Flow characteristics


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Centrifugal compression theoryEnergy transfer through impeller

U = Circumferential velocity

W = Relative velocity

C = Absolute velocity

1 = Impeller inlet

2 = Impeller outlet on outlet

Theoretical head without losses:


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Developing the compressor curve

Rc/

Pc/

Hp

Qs /Qv /Qm

Compressor

curve for a

specific speed

N1

Rprocess,1

Q1

Rc1

Rprocess,2

Q2

Rc2


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Qs, vol

Rc

maximum speed

power limit

adding control

margins

Actual available

operating zone

Developing the compressor curve

stonewall or

choke limit

minimum speed

surge limit

process limit

stable zone

of operation


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Developing the surge cycle on the compressor curve (1)

Qs, vol

HAB

C

D

Pd

Pv

Rlosses

From A to B 20 - 50 ms Drop into surge

From C to D 20 - 120 ms Jump out of surge

A-B-C-D-A 0.3 - 3 seconds Surge cycle


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Compressor basics


Anti-surge controller

1

UIC

VSDS

Compressor

1

FT

1

PsT

1

PdT

DischargeSuction


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Compressor performance control

PT

1

PIC

1

Process

Also called:

Throughput control

Capacity control

Process control

Matches the compressor throughput to the load

Can be based on controlling:

Discharge pressure


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Compressor basics



PT

1

PIC

1

Process

Also called:

Throughput control

Capacity control

Process control



Discharge pressure

Suction pressure


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Compressor basics



PT

1

PIC

1

Process

Also called:

Throughput control

Capacity control

Process control



Discharge pressure

Suction pressure Net flow to the user

Blow off


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Compressor basics



PT

1

PIC

1

Process

Also called:

Throughput control

Capacity control

Process control



Discharge pressure


Blow off

Recycle


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Compressor basics



PT

1

PIC

1

Process

Also called:

Throughput control

Capacity control

Process control



Discharge pressure


Blow off

Recycle

Guide vanes


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Compressor basics



PT

1

PIC

1

Process

SIC

1

Also called:

Throughput control

Capacity control

Process control



Discharge pressure


Blow off

Recycle

Guide vanes

Speed

section 03 - compressor basics

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