10.6 Fan Categories and Pressure Dynamics Across the Fan

Fan Categories and Pressure Dynamics Across the Fan

The industrial exhaust fan is the mechanical prime mover of a Local Exhaust Ventilation (LEV) system, converting electrical energy into aerodynamic fluid power. The fan creates the static pressure differential required to overcome hood entry losses, duct friction, dynamic fitting turbulence, and air cleaner pressure drops, inducing continuous contaminant capture and pneumatic transport. Selecting the correct fan type, understanding fan-system curve interactions, and mastering the Fan Laws are essential competencies for certified industrial hygienists.

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1. Aerodynamic Fan Categories and Operational Profiles

Industrial fans are divided into two major aerodynamic classes based on the flow path of air through the impeller: Centrifugal Fans and Axial Fans.

   +-------------------------------------------------------------------------+
   |                  CENTRIFUGAL VS. AXIAL FAN FLOW PATHS                   |
   +-------------------------------------------------------------------------+
   |                                                                         |
   |  CENTRIFUGAL FAN (Radial Flow):         AXIAL FAN (Axial Flow):         |
   |  Air enters parallel to the shaft,      Air enters and exits along the  |
   |  turns 90°, and is accelerated          same parallel longitudinal axis |
   |  radially outward by centrifugal force. through the propeller/impeller. |
   |                                                                         |
   |                 Outlet (Air Exits)                                      |
   |                    ^                                                    |
   |                    |                                                    |
   |               +----+----+                                               |
   |               |  Scroll |               +=======================+       |
   |               | Housing |               |      Cylindrical      |       |
   |   Inlet       |   (O)   |               |        Housing        |       |
   |   (Air Enters)+----+----+               |   (==|Propeller|==)   |       |
   |      ------>    Drive Shaft             +=======================+       |
   |                                            Air In ---> ---> Air Out     |
   +-------------------------------------------------------------------------+

Centrifugal Fan Categories

Centrifugal fans are classified by the curvature and orientation of their impeller blades:

   +-------------------------------------------------------------------------+
   |                    CENTRIFUGAL FAN BLADE GEOMETRIES                     |
   +-------------------------------------------------------------------------+
   |                                                                         |
   |  1. RADIAL BLADE (Paddle Wheel):                                        |
   |     - Straight, flat blades radiating from center.                      |
   |     - Self-cleaning, open wheel. Ideal for heavy dust & material.       |
   |     - Moderate efficiency (55 - 65%). Power curve rises steadily.       |
   |                                                                         |
   |  2. BACKWARD INCLINED / AIRFOIL:                                        |
   |     - Blades curve away from the direction of rotation.                 |
   |     - Highest aerodynamic efficiency (75 - 85%). Clean air / light dust.|
   |     - NON-OVERLOADING power curve (motor protected if resistance drops).|
   |                                                                         |
   |  3. FORWARD CURVED (Squirrel Cage):                                     |
   |     - Large number of small blades curved in direction of rotation.     |
   |     - Compact, quiet HVAC supply; low static pressure (< 3 in. w.g.).   |
   |     - OVERLOADING power curve. Unsuited for LEV (dust clogs cups).      |
   +-------------------------------------------------------------------------+

1. Radial Blade Fans ("Paddle Wheel"):
   • Design: Heavy steel blades oriented straight radially from the hub. Open-wheel construction provides large clearances between blades and housing.
   • Performance: Moderate static efficiency (55% to 65%). High static pressure capability (up to 30 in. w.g. or more).
   • Industrial Hygiene Application: The industry workhorse for material handling and heavy particulate exhaust. Ideal for conveying heavy mineral dusts, wood shavings, buffing lint, metal grindings, and sticky particulates. Self-cleaning blade geometry prevents particulate deposition and wheel unbalance.
2. Backward-Inclined / Backward-Curved / Airfoil Fans:
   • Design: Heavy backward-swept blades (or streamlined aerodynamic hollow airfoil blades) leaning away from the direction of rotation.
   • Performance: Highest aerodynamic efficiency (75% to 85%) of any centrifugal class. Operates at higher rotational speeds.
   • Power Curve Characteristic: Non-overloading power curve. Brake horsepower reaches a maximum near the point of peak efficiency and then flattens or decreases as airflow increases toward free delivery. If a duct breaks or system resistance drops, the motor will not draw excessive current or burn out.
   • Application: Clean air exhaust, gas/vapor collection, HVAC supply, and downstream of high-efficiency dust collectors (baghouses, scrubbers) where air is particulate-free.
3. Forward-Curved Fans ("Squirrel-Cage"):
   • Design: Many shallow blades curved forward in the direction of rotation.
   • Performance: Delivers high volumetric flow at low rotational speeds and low static pressures (< 2.5--3.0 in. w.g.). Low efficiency (50% to 60%).
   • Power Curve Characteristic: Severely overloading power curve. Power rises exponentially with increasing airflow.
   • Application: HVAC clean air supply and residential blowers. Strictly unsuited for industrial LEV particulate exhaust because dust rapidly accumulates inside the concave cup pockets of the blades, severely unbalancing the wheel and causing catastrophic bearing failure.

Axial Fan Categories

Axial fans move air parallel to the impeller rotational axis:

1. Propeller Fans (Panel / Wall Fans):
   • Operates against negligible static pressure (< 0.5 in. w.g.). Moves large air volumes at low cost. Used exclusively for general dilution ventilation, warehouse wall exhaust, and roof purge units.
2. Tubeaxial Fans:
   • Heavy-duty propeller encased in a cylindrical duct section. Develops medium static pressures (up to 2.0--2.5 in. w.g.). Used in ducted industrial applications without high particulate loadings, such as paint spray booths and solvent drying tunnels.
3. Vaneaxial Fans:
   • Highly engineered axial impeller housed in a precision casing equipped with downstream aerodynamic stator guide vanes. The guide vanes straighten the rotating helical discharge swirl, recovering rotational kinetic energy into usable static pressure.
   • Develops high static pressures (up to 8.0--10.0 in. w.g.) with high efficiencies (75% to 85%). Used in compact inline industrial ventilation, marine exhaust, and underground tunnel ventilation.

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2. Pressure Dynamics Across the Fan: FTP, FSP, and VP

To specify and test an industrial fan, an industrial hygienist must define total, static, and velocity pressures at the fan inlet (suction) and fan outlet (discharge).

   +-------------------------------------------------------------------------+
   |              PRESSURE PROFILE ACROSS AN LEV EXHAUST SYSTEM              |
   +-------------------------------------------------------------------------+
   |  Pressure (in. w.g.)                                                    |
   |  (+) |                                   * TP_outlet                    |
   |      |                                  /|                              |
   |      |                                 / |* SP_outlet                   |
   |  0.0 +--------------------------------/--+----------------------------> |
   |      | Hood Entry                    /   |              Stack Discharge |
   |      |  * SPh                       /    |                              |
   |      |   \   Duct Friction         /     |                              |
   |      |    \   Losses              /      |                              |
   |      |     \--------* TP_inlet   /       |                              |
   |  (-) |               \          /        |                              |
   |      |                \        /         |                              |
   |      |                 * SP_inlet        |                              |
   |      +-----------------------------------+----------------------------+ |
   |             [ HOOD ] ===> [ DUCT ] ===> [ FAN ] ===> [ STACK ]          |
   +-------------------------------------------------------------------------+

Fan Total Pressure (FTP)

Fan Total Pressure (FTP) is the total mechanical energy per unit volume imparted by the fan to the airstream, calculated as the algebraic difference between Total Pressure at the fan outlet (TPoutlet) and Total Pressure at the fan inlet (TPinlet):

$FTP = TP_{\text{outlet}} - TP_{\text{inlet}} = (SP_{\text{outlet}} + VP_{\text{outlet}}) - (SP_{\text{inlet}} + VP_{\text{inlet}})$

Fan Static Pressure (FSP)

Fan Static Pressure (FSP) is defined by AMCA (Air Movement and Control Association) as the Fan Total Pressure minus the fan outlet velocity pressure (VPoutlet):

$FSP = FTP - VP_{\text{outlet}}$

Substituting FTP = (SPoutlet + VPoutlet) - (SPinlet + VPinlet) into the FSP equation yields:

$\mathbf{FSP = SP_{\text{outlet}} - SP_{\text{inlet}} - VP_{\text{inlet}}}$

Exam Key Formulation: When a fan exhausts directly to the outdoors without an outlet discharge duct (SPoutlet = 0), the equation simplifies to: FSP = |SPinlet| - VPinlet.

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