Fan Selection Criteria and Efficiency by John Magill

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.

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

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.

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

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

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

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

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

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

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

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

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

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

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)

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

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

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

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

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

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!

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.

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)

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 = 0.075 lbm/cu ft

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

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.