1. Fan Selection Criteria and Efficiencyby
John Magill

2. The Air Movement and Control Association International (AMCA), has met the standards and requirements of the Registered Continuing Education Providers Program. Credit earned on completion of this program will be reported to the RCEPP. A certificate of completion will be issued to each participant. As such, it does not include content that may be deemed or construed to be an approval or endorsement by NCEES or RCEPP.

3. Learning Objectives List available fan types
Know fan characteristics that are required
Understand tradeoffs when selecting a fan
Define fan efficiency

4. Outline Fan Types Basic Fan Curve Applications
Performance Characteristics
Fan Selection
Efficiency, low noise, size, space and cost considerations
Mechanical considerations for a given application including balancing and vibration levels, construction, arrangements, ruggedness, spark resistance, corrosion resistance, high temperature resistance, bearings, motors, drives etc.

6. Basic Fan Types Centrifugal Axial Special Designs
Backward Inclined Airfoil-blade
Backward Inclined Flat-blade
Forward Curved Blade
Radial Blade
Radial Tip
Axial
Propeller / Panel Fan
Tubeaxial
Vaneaxial
Special Designs
Power Roof Ventilators
Tubular Inline Centrifugal
Mixed Flow
Plenum/ Plug

7. Centrifugal:Backward Inclined Airfoil-Blade
Name is derived from the “airfoil” shape of blades
Developed to provide high efficiency
Used on large HVAC and clean air industrial systems where energy savings are of prime importance

8. Centrifugal:Backward Inclined or Curved Flat-Blade
Backward inclined or curved blades are single thickness or “flat”
Efficiency is only slightly less than airfoil blade
Similar characteristics as airfoil blade
Same HVAC applications as airfoil blade
Also for industrial applications where airfoil blade is not acceptable because of corrosive or erosive environment

9. Backward Inclined or Curved Flat & Airfoil-Blade
High volume at moderate pressure
Non-overloading power characteristic
Stable performance characteristic
Low noise

10. Centrifugal: Forward Curved Blade
Blades are curved forward in the direction of rotation
Must be properly applied to avoid unstable operation
Less efficient than Airfoil and Backward Inclined
Requires the lowest speed of any centrifugal to move a given amount of air
Used for low pressure HVAC systems
Clean air and high temperature applications
Typically smallest size selection
Rising power overloading characteristic

11. Centrifugal: Radial Blade
The blades are ‘radial’ to the fan shaft
Generally the least efficient of the centrifugal fans
For material handling and moderate to high pressure industrial applications, rugged construction
Low volume at high pressure
Large wheel diameter for a given volume- higher cost
Material handling, self cleaning
Easy to maintain
Rising Power overloading characteristic
Suitable for dirty airstream, high pressure, high temperature and corrosive applications

12. Centrifugal: Radial Tip
The blades are radial to the fan shaft at the outer extremity of the impeller, but gradually slope towards the direction of wheel rotation
More efficient than the radial blade but less than backward inclined
Offers wear resistance in mildly erosive air streams

13. Axial: Propeller or Panel Fan
One of the most basic fan designs
For low pressure, high volume applications
Often used for ventilation through a wall
Available in square panel or round ring fan
Maximum efficiency is reached near free delivery
Reversible blade for reversible flow applications like jet tunnel fans
Many axial fans can overload at shutoff

14. Tubeaxial Fan More efficient than the panel fan
Cylindrical housing fits closely to outside diameter of blade tips
For low to medium pressure ducted HVAC systems
Also used in some low pressure industrial applications
Performance curve sometimes includes a dip to the left of peak pressure which should be avoided

15. Vaneaxial Fan Highest efficiency axial fan
Cylindrical housing fits closely to outside diameter of blade tips
The straightening vanes allow for greater efficiency and pressure capabilities
For medium to high pressure HVAC systems. More compact than centrifugal fans of same duty
Aerodynamic stall causes the performance curve to dip to the left of peak pressure which should be avoided. However anti-stall options available for both unidirectional and reversible axials

16. Power Roof Ventilators
A variety of backward inclined centrifugal wheels or axial impeller designs
Also available in upblast damper design to discharge air away from the building
For low pressure exhaust systems of all building types (roof mounted)

17. Inline Centrifugal Fan
Cylindrical housing is similar to a vaneaxial fan
Wheel is generally an airfoil or backward inclined type
Housing does not fit close to outer diameter of wheel
For low and medium pressure HVAC systems or industrial applications when an inline housing is geometrically more convenient than a centrifugal configuration

18. Mixed Flow Fan Specific Speed between a centrifugal and axial fan
Cylindrical housing is similar to a vaneaxial fan
High volume advantages of axial fans
Low sound, high efficiency advantages of tubular centrifugal fans

19. PLENUM / PLUG FAN Housed vs plenum fan
This is basically a centrifugal wheel and inlet in a frame without a scroll or housing. The ‘housing’ is the AHU box.
Offers tremendous flexibility for inlet and discharge in a AHU application
More efficient than a scroll centrifugal for high flows and low SP. All SP rise occurs in the blade passage
Wall clearance rules must be followed to avoid significant system effect losses
Housed vs plenum fan

20. SO YOU HAVE ALL THESE CHOICES OF FANS TYPES AVAILABLE…WHAT SHOULD YOU DO TO PICK THE RIGHT FAN FOR YOUR APPLICATION?
Let’s consider a couple of examples to illustrate the selection process from an efficiency, sound, cost and available space perspective
All Air tests based on AMCA std 210, and Sound tests based on AMCA std 300

23. All fans selected at peak SE (Static Efficiency) for Airflow=10,000 cfm, Static Pressure (SP)~2 iwc
Type
Dia (in)
Spd (rpm)
BHP
SE % (Static Efficiency)
LwiA (Inlet Sound Power ‘A’) 1
Forward Curved- SW (Centrifugal) 30
476
5.09
61.7 89 2
Backward Airfoil – SW (Centrifugal)
36.5
650
3.82
80.0 77 3
Plenum 33
800
4.25
74.0 80 4
Tubular Mixed Flow 27
1074
4.48
70.2 81 5
Tubular Vane Axial 28
1438
4.77
65.9 86 6
Propeller (Axial)
1998
4.92
54.4
103

24. Narrowing in after main Fan Type Selection..........
LESS EFFICIENT
LESS COST
MORE NOISY
In general, for all fan types, as first cost goes down, operating costs (BHP) and noise go up…trade off!

25. Tone at Blade Pass Frequency (Blade Tone)
Blade Pass Frequency, bpf= #blades * rpm / 60
Sound Power level, Lw, at bpf is a distinct audible tone. This aerodynamic tone can be very annoying and is usually the worst for radial bladed fans, followed by plenums and housed centrifugals.
Axial fans have a high pitched tone which is not as annoying.
The bpf tone is a spike in Lw over the surrounding broadband noise spectra.
Blade Tone Prominence is defined as the dominant energy level of the blade tone integrated over a narrowband region of the sound spectrum surrounding the blade tone.

26. FT-2
FT-2
Blade Tone prominence
Acoustic Engineers do not like blade tone prominence to exceed 6dB in addition to low Sound Power Levels (Lw)

27. Fan Selection based on Specific Speed
Dimensional Specific Speed, is the fan speed required to raise the SP by 1 iwc with 1 cfm airflow.
Ns = N * (Q)^0.5/(SP)^0.75
Where, N = Speed (rpm)
Q = Airflow (cfm)
SP = Static pressure (iwc)
Density = lbm/cu ft

29. All fans selected at peak SE (Static Efficiency) for Specific Speed, Ns
Type
Specific Speed, Ns
Max Static Efficiency (SE%) 1
Forward Curved-SW (Centrifugal)
26,300 61 2
Backward Airfoil-SW (Centrifugal)
40,000 80 3
Plenum
50,000 75 4
Tubular Mixed Flow
65,800 70 5
Tubular Vane Axial
90,000 65 6
Propeller (Axial)
126,000 59

30. Summary
Fan selection is not a trivial process for a given application.
Example shown applies to one design operating point. The selections will change for other operating points.
There is no magic fan that will result in least cost, best efficiency and low noise for a wide range of operating points.
Compromises should be well understood upfront.
Direct Drive (DD) selection speeds may further limit selections. Varying width options can optimize DD selections.
Mechanical design requirements like balancing and vibration levels, spark and high temp resistance, corrosion resistance, arrangements, motors, bearings, drives can further challenge the selection process.