1. Fans & Heaters technical seminar
Variable Pitch Axial Fans
Henrik Bagger

2. Variable Pitch Axial Fans
Seal air
Stalling 2
Stall warning 3
Stall probes
Corrosion protection 4

3. Variable Pitch Axial Fans
Seal air fans
All flue gas fans are fitted with seal air units to prevent the flue gas from entering the inner tube of the diffuser and intermediate piece.
The sealing air units can be supplied as stand-alone units or with back-up.
Forskellen mellem standardløsningen før 2011 og efter 2011 er, at den nye løsning er billigere, og at indsugningen (air in-take) er gået fra at være rund til at være firkantet.
Standard seal air, 2011 onwards

4. Variable Pitch Axial Fans
Seal air fans
Trykket fra spærreluft skal være 200 Pa højere end i ventilatoren
Spærreluft i diffusor og mellemstykke er standard
Om der skal spærreluft i sugekasse er afhængig af beregningerne

5. Variable Pitch Axial Fans
Stalling
The fan can be operated safely inside the performance envelope
Above the stall line the fan cannot operate for a long time

6. Variable Pitch Axial Fans
Stalling
Description of stall cells
One or more stall cells from B to D
Stall cells rotate at a lower frequency
From B to D blades are in and out of cells
At D stall cells cover the full circumference
From D to E the layer of cells gets thicker
Stall cell

7. Variable Pitch Axial Fans
Stalling
Stresses in blades
Stress amplitude 1 10
103
106
104
102
105
107
108
Number of cycles
SN Curve σg Rm
Infinite lifetime
Stress levels in blades
Normal operation – shaded area
Stall situation – non-shaded area
Example:
Natural frequency of blade: 35 Hz
No. of cycles for 15 min.:
15x60x35 = 31,500

8. Variable Pitch Axial Fans
Stall warning
A stall situation mostly occurs when a system is operated incorrectly creating a situation where the resistance in the system is too large in relation to the volume flow the fan must deliver.
Howden offers two solutions at which the risk of stall can be minimized or eliminated.
Stall Probe (Green line)
The stall detector is connected to a manometer which registers
the pressure difference in the airflow. The stall detector gives a
signal when the fan works in an unstable area before the fan
stalls.
Stall Elimination system (Red line)
The anti-stall ring is placed on the suction side of the fan where
it absorbs and turns the recirculated air coming from the
blades when a fan is working in a stall area. The anti-stall ring
is increasing the stable working area of the fan
Howden Variax has a patent on this solution.
Stall Avoidance system (Blue lines)
The actual operating fan load is calculated by means of a differential pressure measured across the side tappings of the inlet box and inlet cone installed on the fan from the inlet of the inlet box to the inlet cone (volume flow) and across the fan from inlet box to diffuser end (pressure rise) respectively. The actual operating point is specified in a graph characterised by a combination of pressure and volume.
An alarm line and a trip line are drawn on the fan graph. If the operating point (combination of volume flow and pressure) is within the stall alarm area, an alarm is released. If the operating point (combination of volume flow and pressure) moves outside the stall alarm area and into the trip area, the blade angle is automatically regulated and the operating point moves outside the trip area. Thus the risk of stall is avoided.
Red lines: Stall Warning System
Green line: Stall Probe

9. Variable Pitch Axial Fans
Stall warning
Fan equipped with stall warning measurement rings and pressure transmitters
All tubes are designed to have gravity drainage
– First generation difficult to purge and clean

10. Variable Pitch Axial Fans
Stall warning
Calculation of fan operating point
Performance curve with stall warning line

11. Variable Pitch Axial Fans
New constant purge air system

12. Variable Pitch Axial Fans
New constant purge air system
Design rules for new constant purge air system:
Single ports only – no rings connecting several ports
Two identical measuring systems are applied – averaged in the DCS
Static ports are still designed for gravity drainage
½” OD stainless steel pipes are used
Calibration of purge air system:
Fan is not running
Purge air is switched on
Absolute transmitter adjusted to show barometric pressure
The two differential transmitters adjusted to show zero

13. Variable Pitch Axial Fans
Stall probes
Pictures of plugged ports from site
Stall probe is plugged
Stall probe is plugged
and cleaned with wire brush

14. Variable Pitch Axial Fans
Stall probes
Two different sizes of stall probes
Stall probe detects stalling
Mounted upstream next to first stage
Standard probe has 5mm holes
Large probe has 10mm holes
Probes are calibrated at site

15. Variable Pitch Axial Fans
Stall probe signal for axial fans
Fan characteristic
 = Angle of attack

16. Variable Pitch Axial Fans
Stall probes
Location and design of stall probe
Stall cell

17. Variable Pitch Axial Fans
Constant purge air system
Stall probe from European site with high dust load

18. Variable Pitch Axial Fans
Calibration of purge air system
Stall probe transmitters
Fan is not running
Purge air is switched on
Adjust so differential pressure across probe is zero
To ensure correct functionality, the calibration procedure for stall probe transmitters should be made annually
Note:
The calibration of the stall probe itself (turning) must be made when the fan is running and in accordance with the manual

19. Variable Pitch Axial Fans
Corrosion protection
Bearing Housing : Normal
Intermediate casing : Corrosion on the guide vane root
Inlet box : Ash and inner tube corrosion
Diffuser : Normal
Hub surface (DE): Corrosion
Hub (NDE): Surface normal Blade (NDE): Normal
Blade root(DE): Corrosion
Hub side plate (DE/NDE): Corrosion 3

20. Variable Pitch Axial Fans
Corrosion protection
Corrosion on inlet box inner tube surface
Enhanced condensation of sulfuric acid (H2SO4) on the surface of inner tube caused by cooling effect of sealing air entering from the atmosphere and heated to 70~80℃ ( deg.F) and a high acid dew point temperature
160 C (320 F)
70~80 C (~170 F)
Heater
[ Countermeasure ]
- Blocking and reducing air entering from outside 4

21. Variable Pitch Axial Fans
Corrosion protection
Corrosion of blade root disc and hub caused by cooling effect of sealing air which passes through the hub side seal and blade root gap from inner tube of inlet box
Blade root thinning
Sealing air
: 70~80 ℃
(170 F)
Corrosion around blade root
1st side (DE) Hub
[ Countermeasure ]
- Apply special seal for seal air blocking
- Block and reduce air entering from outside
Corrosion on hub surface 5

22. Variable Pitch Axial Fans
Corrosion protection
Condensation of sulfuric acid (H2SO4 ) on the surface of hub caused by cooling effect of sealing air and high acid dew point temperature
Enhanced corrosion of drive/non-drive hub side plate caused by condensation of sulfuric acid (H2SO4) from flue gas which unexpectedly penetrated through seal air exit
Corrodsion hole
Sealing Air
Flue gas
Hub side plate : Corrosion
< Hub side plate >
Labyrinth Seal
2nd side (NDE) Hub 6

23. Variable Pitch Axial Fans
Corrosion protection
Countermeasure - Improvement of sealing air system (reduced seal air)
Minimizing seal air flow for main bearing assembly and inlet box inner tube which
prevents condensation by minimizing the cooling effect of relatively cold seal air (30 ~ 40℃ )
(85-105degF)
Contact type Labyrinth seal
Suction Box Heater
Damper and heater off
Add valve on seal air pipe
Blocking the intake air to inner tube 13

24. Variable Pitch Axial Fans
Corrosion protection
Countermeasure - Improvement of sealing air system (additional seals)
Minimizing cooling effect by reducing the amount of seal air flow by an additional labyrinth seal
Minimizes the temperature difference between hub surface and flue gas
Prevents condensation of sulfuric acid on the hub surface
Preventing unexpected penetration of flue gas to hub side plate by labyrinth seal and shield plate
Ensure no penetration of flue gas – no condensation on side plate of hub
Shield plate
Hub Surface
Special
Special Seal
Labyrinth Seal
(Contact type)
Hub
Side Plate
Blade root hole of the hub surface
Hub side plate & casing tube 14

25. Variable Pitch Axial Fans
Corrosion protection
Countermeasure – Applying special coating on hub surface and blade
When non-metallic coating frequently is in contact with sulfuric acid, it degrades chemically (blistering and delamination). Because of potential risk to accelerate local corrosion beneath blister, it cannot be a permanent solution w/o seal air improvements
After improving sealing air system condensation of sulfuric acid is reduced and non- metallic coating can suppress corrosion rate
Mild environment (no immersion ,limited H2SO4 condensation)
Severe
environment (frequently Immersed in H2SO4)
Non-met.
Non-met.
Blistering
Before corrosion test and after 6 cycles
Acid dew point corrosion simulation cycle 1h
Immersion 20% H2SO4
15 min Air drying
16-17h Oven 150C/300F
5-6 h Cooling
The test for C-276 showed a similar result – no protection

26. Coated blade with PFTE after 6 month operation

27. Coated Blade with PTFE (NEW)

28. Thank You Any Questions?