Fan Design

MSYS4480
Fan Definition

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

“A fan is used to move air through ducts and to induce air motion in the space.”

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Fan Performance Tables

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Fan Curves

STP - standard temperature (70° F) and pressure (29.92 in Hg) with gas
density of 0.075 lb/ft3.

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Fan Laws
CFM varies directly with RPM
CFM1/CFM2 = RPM1/RPM2

SP varies with the square of the

RPM
SP1/SP2 = (RPM1/RPM2)2

HP varies with the cube of the RPM

HP1/HP2 = (RPM1/RPM2)3

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Types of Fan
Two basic types of fan are:

▪ Centrifugal: (Low Pressure / High Volume)

▪ Discharge air perpendicular to the axis of the impeller rotation.
▪ Preferred for higher pressure ducted systems.
▪ Backward Inclined, Airfoil, Forward Curved, & Radial Tip.

▪ Axial: (High Pressure / Low Volume)

▪ Discharges air parallel to the axis of the impeller rotation.
▪ Preferred for high volume, low pressure, and non-ducted systems.
▪ TYPES: Propeller, Tube Axial and Vane Axial.

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Centrifugal Fan Components

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Axial Fan Components

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Van Axial Fan Components

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Axial Fan Components

Vane axial fan shown

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Centrifugal Fans - Blade Designs

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Centrifugal – Forward Inclined

▪ 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

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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
towards free delivery (self-
limiting).
▪ Consideration for use in
applications where airfoil
blade erosion concern

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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

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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).

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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

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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

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Axial Fans - Types

Propeller - generally
operate at < 1/2 inch water
gauge, inefficient, steeply
rising power curve
(overloading).

Tube axial - propeller

mounted in cylindrical tube,
ranges up to ~ 4 inches water
gauge, overloading.

Vane axial - utilizes guide

vanes to recover rotational
energy, up to 10 inches
(single stage), overloading.

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Axial Fans - Curves

Tube axial

Propeller

Vane axial

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Fan Curves

Source: ASHRAE Handbook

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Generalized Fan Curves
These kinds of curves can be used
to help choose a fan.

Source: Burmeister

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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.

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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).

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Gaps/Clearances

Ensure gaps and clearances

set per Vendor Manual

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Gaps/ Clearances continued

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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.

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Fan Laws

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Example-1

A centrifugal fan is operating as shown below at point 1.

Estimate the capacity, total pressure, and power requirement
when the speed is increased to 1050 rpm. The initial power
requirement is 2 hp.

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Fan – Selection Criteria
• Air volume required - CFM
• System resistance - SP
• Air density (Altitude and Temperature)
• Type of service
• Environment type
• Materials/vapors to be exhausted
• Operation temperature
• Space limitations
• Fan type
• Drive type (Direct or Belt)
• Noise criteria
• Number of fans
• Discharge
• Rotation
• Motor position
• Expected fan life in years

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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 characteristic change
(spin, reduced turbulence) at fan inlet.

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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.

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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.).

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Fan Testing

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AMCA Installation Types

Installation Type A: Installation Type C:

Free Inlet, Free Outlet Ducted Inlet, Free Outlet

Installation Type B: Installation Type D:

Free Inlet, Ducted Outlet Ducted Inlet, Ducted Outlet

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