Fan Type
Fan system characteristics
Fan system curve
system resistance is used when referring to the static pressure.
The system resistance is the sum of static pressure losses in the
system. The system resistance is a function of the configuration
of ducts, pickups, elbows and the pressure drops across equipment.
The system resistance varies with the square of the volume of air
flowing through the system.
Types of Fan Pressures.
1. Static Pressure
2. Velocity / dynamic pressure
3. Total Pressure
Assessment of fan system
Step 1 Air density calculation
To calculate the velocity and volume from the velocity pressure
measurements it is necessary to know the density of the air.
The density is dependent on altitude and temperature.
Assessment of fan system
Step 2 Calculate Fan efficiency
Two types of fan efficiencies
1. Mechanical efficiency
2. Static efficiency
Energy conservation opportunities o fan system
1. Eliminate over sized fans
2. Reduce system resistance
3. Operate close to BEP
4. Periodic maintenance .
5. And most importantly! Control Fan air flow
Energy conservation opportunities o fan system
1. Eliminate over sized fans
An over sized Fan exhibits following problems
Do not operate at BEP
Risk of unstable operation
Excess energy use
High air flow noise
Stress on system
Energy conservation opportunities o fan system
2. Reduce system resistance
• Check periodically
• Check after system modification
• Reduce where possible
Energy conservation opportunities o fan system
3. Operate close to BEP
• Best efficiency point.
• Normally close to rated fan capacity.
• Deviation from BEP results in inefficiency and energy
loss.
Energy conservation opportunities o fan system
4. Periodic Maintenance
• Periodic inspection of all system components
• Proper lubrication
• Belt tightening and= replacement
• Motor repair and replacement
• Fan cleaning
Energy conservation opportunities o fan system
5. Control Air Flow
There are number of ways to control the flow
A. Pulley change
Advantages
a) Permanent speed decrease
b) Real energy saving
Disadvantage
Fan must handle capacity change
Energy conservation opportunities o fan system
5. Control Air Flow
B. Dampers
Reduces flow and increase the upstream system
pressure
Advantages
Inexpensive
Easy to install
Disadvantages
Limited adjustment
Reduced flow but not energy efficient
Fan System curve in Dampering
At fan speed N1, the fan will operate along the N1 performance curve .The
fan's actual operating point at "A" is flow (Q1) against pressure (P1).
Two methods can be used to reduce air flow from Q1 to Q2:
First method is to restrict the air flow by partially closing a damper in the
system. This action causes a new system performance curve (SC2) where
the required pressure is greater for any given air flow.
The fan will now operate at "B" to provide
the reduced air flow Q2
against higher pressure P2.
Energy conservation opportunities o fan system
5. Control Air Flow
C. Inlet Guide vans
Creates whirls in fan direction
Reduces angle at which air enters the fan blades
Advantages
Improve efficiency: reduce load and air flow
Cost effective at 80 -100 % of full air flow
Disadvantages
Less efficient at <80% of full air flow
Energy conservation opportunities o fan system
5. Control Air Flow
D. Variable speed derives
Changes motor rotation speed by adjusting frequency of power
Advantages
• most effective and easy flow control
• Improved efficiency over as wide range of operation
• Can be retrofitted to existing motors
• Compact
• Reduces energy losses and maintenance
• cost
• Disadvantages
• High investment cost, in most
• of the cases pay back > 2 years
Fan system characteristics
Fan system curve
By reducing the speed from N1 to N2, keeping the damper fully open, the
fan would operate at "C" to provide the same Q2 air flow, but at a lower
pressure P3.
Thus, reducing the fan speed is a much more efficient method to decrease
airflow since less power is required and less energy is consumed.
Fan laws (laws of Affinity
Ventilation Problems
Short Circuiting of air
1. Minimizing excess air level in systems
2. Minimizing air in-leaks in
3. The findings of performance assessment trials will automatically indicate potential areas for
improvement, which could be one or a more of the following:
1. Change of impeller by a high efficiency impeller along with cone.
2. Change of fan assembly as a whole, by a higher efficiency fan
3. Impeller de-rating (by a smaller dia impeller)
4. Change with more efficient impellers.
5. Fan speed reduction by pulley dia modifications for derating
6. Option of variable speed drives for variable duty conditions
7. Option of energy efficient flat belts, or, cogged raw edged V belts, in place of conventional V belt
systems, for reducing transmission losses.
9. Minimizing system resistance and pressure drops by improvements in duct system
