1. Understanding Fan Operation and Performance
Walter Hilbish
Perry Nuclear Power Plant
FirstEnergy

2. Fan Definition What is a “fan”? Defined by ASHRAE
“A fan is an air pump that creates a pressure difference and causes airflow. The impeller does work on the air, imparting to it both static and kinetic energy, which vary in proportion, depending on the fan type.”
2000 ASHRAE Systems and Equipment Handbook Chapter 18

3. Fans
A fan is a constant volume machine. For a fixed speed, the fan will move the same volume of gas irregardless of gas density.

4. Fans
Most manufacturer’s rate the fan based upon tests conducted using AMCA 210.
Testing uses ideal configurations (i.e straight duct runs, 10 duct diameters, no obstructions, etc.).

5. AMCA Installation Types
Four basic test installations are defined.
AMCA Certified Ratings require that the test installation be identified.
Example:
“Performance certified is for installation Type A: Free inlet, Free outlet. Performance ratings do not include the effects of appurtenances (accessories). Performance ratings do not include the effects of cross winds.”

6. Fan Performance Tables

7. Fan Curves
Graphical depiction of pressure-volume performance of given fan size and speed operating at various points from shutoff to free delivery. Graph may also include power, efficiency, system, and stall curves.
Unless otherwise specified the curve is at STP .
STP - standard temperature (70° F) and pressure (29.92 in Hg) with gas density of lb/ft3.

8. Fan Testing

9. Fan Laws

10. Fan Laws

11. Fan Laws

12. Types of Fan Two basic types of fan are:
Centrifugal - rotating impeller to increase velocity of an airstream.
Axial - air is pressurized by the aerodynamic lift of the fan blade.

13. Centrifugal Fan Components

14. Axial Fan Components

15. Axial Fan Components
Vaneaxial fan shown

16. Centrifugal Fans - Blade Designs

17. Centrifugal - Airfoil Blade curves away from direction of rotation
Highest efficiency
Relatively low noise
High structural strength
Power reaches max near peak efficiency, then reduces twoards free delivery (self-limiting).
For given duty, has highest speed of centrifugal design

18. Centrifugal - Backward Inclined
Efficiency slightly less than airfoil design
Pronounced region of instability where low efficiency and pulsations (stall, surge, bi-stable flow)
Power reaches max near peak efficiency, then reduces t0wards free delivery (self-limiting).
Consideration for use in applications where airfoil blade erosion concern

19. Centrifugal - Backward Curved
Power reduces towards free delivery (self-limiting).
Promote smoother flow than BI fan
Operate at higher speeds and therefore require sturdier construction than FC
Less susceptible to flow/fan instabilities due to less pronounced instability region

20. Centrifugal - Radial Rugged blade design, self-cleaning
Lowest efficiency centrifugal
Used for material handling due to blade strength
Moderate to high pressure industrial applications
Power rises continually to free delivery (overloading).

21. Centrifugal - Radial Tip
Blade is radial at outer edge of wheel, curved in direction of flow as it moves inward
More efficient than Radial blade
Used for mildly erosive environments
Higher pressure than Airfoil

22. Centrifugal - Forward Curved
Also called “Squirrel Cage”
Blade curved forward in direction of rotation
Low pressure HVAC applications (residential, etc.)
Lower speed than other centrifugals
Single thickness blade, lightweight construction

23. Axial Fans - Types
Propeller - generally operate at < 1/2 inch water gauge, inefficient, steeply rising power curve (overloading).
Tubeaxial - propeller mounted in cylindrical tube, ranges up to ~ 4 inches water gauge, overloading.
Vaneaxial - utilizes guide vanes to recover rotational energy, up to 10 inches (single stage), overloading.

24. Axial Fans - Curves
Tubeaxial
Propeller
Vaneaxial

25. Axial Fan - Guide Vanes
Function - reduce the helical flow pattern of the fan.
Inlet - vanes guide airflow into a rotational pattern opposite to fan rotation to provide neutralize deflection by blades and result in axial flow
Outlet - vanes take rotational pattern from blades and redirects it to an axial flow, converting some rotational velocity into static pressure.

26. AMCA Class
Definition AMCA 99 - An established minimum performance level for fan aerodynamic performance in terms of pressure and airflow rate.
Simply put in general terms - the higher the Class, the greater mechanical strength (and price).

27. Gaps/Clearances
Ensure gaps and clearances set per Vendor Manual

28. Gaps/ Clearances continued

29. Centrifugal Fan Volume Control
Generally 3 methods:
Variable speed – changing fan speed results in a change in fan flow.
Outlet damper – functions by changing system resistance, forcing the fan to operate against higher backpressure and reducing the flow.
Inlet damper – combination of resistance change and potential flow characterisitic change (spin, reduced turbulence) at fan inlet.

30. Centrifugal Fan Vortex
The inlet box damper (positioned properly) and both types of inlet vane dampers pre-spin the air in the direction of fan rotation. By creating spin in the direction of fan rotation, the airflow, pressure and horsepower of the fan are reduced, resulting in a new fan performance curve.

31. Recognition
Following references were used in the generation of this presentation for technical or pictorial support:
CML Northern Blower Fanfacts (1991)
Improving Fan System Performance: A Sourcebook for Industry - DOE/AMCA (2003)
Engineering Papers - Moore Fans

32. Recognition (continued)
CML Northern Blower Fanfacts (1991)
Fan Handbook: Selection, Application, and Design - Frank P. Bleier (1997)
ASHRAE Systems and Equipment Handbook Chapter 18, Fans (2000)
AMCA Fan Application Manual, Publications &
AMCA Standards Handbook Publication 99

33. Recognition (continued)
Fan Engineering, 9th Edition - Howden Buffalo
Value in the Air: Why Direct Drive Backward Curved Plenum Fans? - AAON

34. Additional Information may be found at the following websites:
For some history of fans, see the following:
Benjamin Franklin Sturtevant
History of the Vaneaxial Fan