Stall & Parallel Operation

Witt&Sohn AG Oct-14

Stall & Parallel Operation

1. Definition of Stall

CONTENT

1. Definition of Stall

2. Requirements for Parallel Operation

3. Operation of Parallel Fans

4. Stall-free Fans & VSD

Witt&Sohn AG Oct-142

Stall means:� status of non-uniform flows,� two air flows at one pressure value,

Definition of Stall, Stall Definition

Typical Axial Fan Curve, Single Operation:

Stall area:forbidden for operation. Operation range

Witt&Sohn AG Oct-143

Stall means, � status of non-uniform flow, � two air flows at one pressure value,

Definition of Stall, Stall Definition

Typical Axial Fan Curve (2 Fans – Parallel Operation)

Operation range

Typical Axial Fan Curve (2 Fans – Parallel Operation)

Stall area:forbidden for operation.

Witt&Sohn AG Oct-144

Typical Axial Fan Curve (4 Fans – Parallel Operation)

Stall means, � status of non-uniform flow, � two air flows at one pressure value,

Definition of Stall

Typical Axial Fan Curve (4 Fans – Parallel Operation)

Operation range

The more fans run in parallel� the larger the stall area� the smaller the operation rangeThe STALL zone for axial fans in parallel often is much larger than expected !

Stall area:forbidden for operation.

Witt&Sohn AG Oct-145

Definition of Stall

� Disturbance causes the air flow to separate from one or more of to separate from one or more of the blades.

� This blocks the air flow through the corresponding blade cell.

Witt&Sohn AG Oct-146

Curve of one fan

Summary curve of two fans

Definition of Stall

Max. allowed system resistance without any saftey margin

Several fan suppliers do not show or do not know in detail the STALL area of their fan curves

Stall area of pd-line

Witt&Sohn AG Oct-147

Always ask for fan curves starting at 0m³/s!

Stall area of one fan Stall area of

two fans (summary curve)

pd-line

Possible Consequences:- rupture of air stream lines at blade edge - high vibrations with low amplitude at blade

Definition of Stall

- high vibrations with low amplitude at blade- damage of bearings- fatigue break of impeller

Witt&Sohn AG Oct-148

Reasons:� Turbulent inlet or outlet flow at the fan� System resistance too high because of unexpected aerodynamic

behaviour (wrong system design)

Definition of Stall

behaviour (wrong system design)� Short-term pressure pulses

(due to Piston Effect of by-passing trains)

Witt&Sohn AG Oct-149

Piston Effect

Axial Fans(none stall-free)

Piston effect

Stall operation likely for none stall-free fans due to piston effect of by-passing trains (Metro/Railway application)

Witt&Sohn AG Oct-1410

stall !

Piston effect

- 610Pa

+610Pa

Piston Effect

Axial Fans(none stall-free)

Stall operation likely for none stall-free fans due to piston effect of by-passing trains (Metro/Railway application)

stall !

Piston effect

+610Pa - 610Pa

Witt&Sohn AG Oct-1411

stall !+610Pa - 610Pa

Detection of stall

Stall detection probe by WITT&SOHNWITT&SOHN

�� Stall detection is done by a “Petermann” probein combination with a pressure sensor

- +� Probe located radially in the area of blade tip

� During normal (stable) operation the probedetects a negative or no pressuredifference.

� Stall is detected if the pressure difference exceeds a predefined value

Witt&Sohn AG Oct-1412

NOTE: A stall detection probe does not eliminatethe stall area of the fan. It only detectsthat the fan is not running in a stable area. Operation of fans in such conditions shall be avoided!

Stall & Parallel Operation

1. Definition of Stall

CONTENT

1. Definition of Stall

2. Requirements for Parallel Operation

3. Operation of Parallel Fans

4. Stall-free Fans & VSD

Witt&Sohn AG Oct-1413

4. Stall-free Fans & VSD

For System:� High volume flow

(Qdesign > Qsingle fan)

Requirements for Parallel Operation

(Qdesign > Qsingle fan)� Moderate pressure drop � Restricted space� Heat resistance� Reversibility (up to 100%)� Cost-effective solution� Robust design

Witt&Sohn AG Oct-1414

For Fan Maker:� None stall-free fan curve

with shown stall area� Cumulative fan curve of parallel fans

Requirements for Parallel Operation

� Cumulative fan curve of parallel fans

INFO:

Most fan makers do not know/ show the STALL area characteristic of their fans! (stall control impossible)

Witt&Sohn AG Oct-1415

(stall control impossible)

For Fan Maker:� None stall-free fan curve

with shown stall area� Cumulative fan curve of parallel fans

Requirements for Parallel Operation

� Cumulative fan curve of parallel fans

or

� Stall-free fan curve

Witt&Sohn AG Oct-1416

Stall & Parallel Operation

1. Definition of Stall

CONTENT

2. Requirements for Parallel Operation

3. Operation of Parallel Fans

4. Stall-free Fans & VSD

Witt&Sohn AG Oct-1417

Installation:� Keep distance between fans to prevent disturbance and interference of

fans

Operation of Parallel Fans

Bad Good

Witt&Sohn AG Oct-1418

Installation:� Connect to plenum with separation walls� Ensure symmetric openings� Use shut-off dampers at one side

Operation of Parallel Fans

� Use shut-off dampers at one side

Bad Good

symmetric openings

asymmetric opening

Witt&Sohn AG Oct-1419

separation wall

openings

short circuit of flow

Installation - Samples:

Operation of Parallel Fans

Witt&Sohn AG Oct-1420

Operation of Parallel Fans

Parallel Fan OperationI. Start in cascade (DOL, weak supply net)

�synchronize fan start with opening of damper�synchronize fan start with opening of damper1. ensure all dampers are initially closed2. start-up 1st fan 3. wait for signals

“1st fan: full speed“ and “1st damper: open“4. start-up next fan (step 2. to 4.)�stop procedure with any notice of failure

� But:

Witt&Sohn AG Oct-1421

� But:

Operation of Parallel Fans

Problem:Switch on an additional fan might lead to � Asymmetry of individual air flow� Asymmetry of individual air flow� Moving of duty point of remaining fans into stall area� Uncontrollable pulsing of system � System performance problems � Fan damage

Witt&Sohn AG Oct-1422

Operation of Parallel Fans

Parallel Fan OperationII. Start in parallel (DOL, strong supply net)

�synchronize fan start with opening of damper�synchronize fan start with opening of damper1. ensure all dampers are initially closed2. start-up all fans simultaneously 3. wait for signals

“all fans: full speed“ and “all dampers: open“�stop procedure with any notice of failure

� But difficult to achieve� Requires strong supply net

Witt&Sohn AG Oct-1423

� Requires strong supply net

Operation of Parallel Fans

Parallel Fan OperationIII. Start-up in parallel (VSD)

�synchronize fan start with opening of damper�synchronize fan start with opening of damper1. ensure all dampers are initially closed2. start-up all fans simultaneously:

use 1st VSD as “master“, others as “slave“(AOmaster: actual speed � AIslave: speed set point)

3. wait for signals “all fans: full speed“ and “all dampers: open“

�stop procedure with any notice of failure

� But expensive

Witt&Sohn AG Oct-1424

� But expensive

Stall & Parallel Operation

1. Definition of Stall

CONTENT

1. Definition of Stall

2. Requirements for Parallel Operation

3. Operation of Parallel Fans

4. Stall-free Fans & VSD

Witt&Sohn AG Oct-1425

Stall-free Fans & VSD

Stall-free fans by WITT&SOHNWITT&SOHN :

1. Fans with truly STALL-free fan curve

� No limitation during parallel duty� No limitation during parallel duty(trouble free)

� Max. stability against pressure pulses

Witt&Sohn AG Oct-1426

Stall-free Fans & VSD

Stall-free fans by WITT&SOHNWITT&SOHN :2. Fans with Anti-Stall Device and

quasi STALL-free fan curve

� Still limitation during parallel duty:

Anti-Stall Device

� Still limitation during parallel duty:Anti-stall device reduces effect of stall,but continuous operation in stabilized areanot recommended

� High stability against pressure pulses

� Provides a significant higher level of reliability of the ventilation system

Parallel operation(trouble-free)

Witt&Sohn AG Oct-1427

reliability of the ventilation system

Stall-free Fans & VSD

Stall-free fans by WITTWITT&&SOHNSOHN :3. Fans with STALL-free fan curve and VSD

� Multiple duty points achievable with one blade angle

� Constantly high efficiencies

� No complex pitch-in-motion impellers required

Witt&Sohn AG Oct-1428

Summary

� Axial flow fans should not be operated in STALL or in the stabilized STALL area� Piston effects have to be taken into account� Stall can be detected by a “Petermann” probe� Stall can be detected by a “Petermann” probe� Parallel duty is restricting the operation range of fans with none stall-free

characteristics� For parallel duty it is required to know the characteristic of the stall area� The plant layout should be symmetrically designed� Fan isolation dampers have to be installed for parallel duty � With stall free fans curves parallel duty is no problem anymore

Witt&Sohn AG Oct-1429