This document discusses different types of fans and their characteristics. It describes axial fans like propeller, tube-axial, and vane-axial fans which move air perpendicular to blade rotation. It also describes centrifugal fans which move air in the direction of impeller rotation and come in airfoil, backward inclined, radial, and forward curved types. The document discusses fan laws for predicting performance at different speeds and includes an example problem. It also covers system curves, catalog fan curves, variable flow control devices, and their performance characteristics.

1. ME 425
Fans
Keith Elder, P.E.

2. Fan Types
Fans are classified by the direction of air
flow relative to the impeller
Axial Fans
Air flow is perpendicular to blade rotation
Centrifugal Fans
Air flow is in the same direction as impeller
rotation

3. Axial Fans
Propeller
Designed to move
air from one
enclosed space to
another in a wide
range of volumes at
low pressures.
Tube-axial
An axial flow wheel in a cylinder which moves a wide range of air
at medium pressures.
Vane-axial
Vane-axial fans have a set of air guide vanes mounted in the
cylinder before or behind an airfoil-type wheel. They move air over
a wide range of volumes and pressures.

4. Axial Fan Characteristics

5. Centrifugal Fans
Designed to move air over a wide volume
range. Static pressures can go up to 25 inches.
Centrifugal fan wheels come in the following
types:
Airfoil
Backward Inclined
Radial
Forward Curved

6. Centrifugal Fan Characteristics

7. Fan Horsepower
fan6359 ηx
CFMxTP
BHP =
6359
CFMxTP
AHP =
Where:
ηfan= Fan Efficiency
AHP = Air Horsepower
BHP = Brake Horsepower
TP = Total Pressure, inches H2O

8. Fan Laws
Fan Performance at different speeds from the
manufacturer’s fan curve can be predicted
using the fan laws.
C F M
C F M
R P M
R P M
e s s u r e
e s s u r e
R P M
R P M
B H P
B H P
R P M
R P M
2
1
2
1
2
1
2
1
2
2
1
2
1
3





 =











 =











 =






P r
P r

9. Fan Law Problem
A fan has the following characteristics:
5000 CFM
1.25 inches static pressure
782 RPM
1.98 BHP
What RPM is necessary to increase the flow to 6,000
CFM?
What BHP is required?

10. System Curve
Pressure
(in. w.c.)
CFM

11. Fan Curve
Pressure
(in. w.c.)
CFM

12. System Operating Point
Pressure
(in. w.c.)
CFM

13. Catalog Fan Curve

14. Method of Obtaining Fan
Curves

15. System Effect
Fan data is based upon discharge conditions
at the time of testing, including certain
minimum. For low velocity systems, the
effective duct length is 2 - 1/2 equivalent duct
diameters

16. System Effect

17. System Surge
Occurs when
system resistance
and fan
performance
curves do not
intersect at a
distinct point but
rather over a range
of volumes and
pressures. - Trane

18. Fan Surge
Occurs near
“block-tight”
conditions when
blade rotation is
insufficient to
overcome pressure
difference between
wheel center &
discharge.

19. Variable Flow Control Devices
Bypass circuits
Discharge Damper
Variable inlet vanes
Vari-Cone
Econo-Cone
Variable Frequency Drive

20. Inlet/Discharge Control
Schemes

21. VAV Control Performance

22. Variable Flow Device
Performance
100
80
60
40
20
0
0 20 40 60 80 100
Variable Frequency Motor Control
Variable Speed Drive (Mechanical)
Inlet Vane Control
Discharge Damper
Bypass Circuit
%
BHP
% CFM

23. Discharge System Effect