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Fans

A fan is a mechanical device for moving air or other gases. It uses the kinetic energy of the impellers to increase the volume and pressure of the air/gas stream which in turn moves them against the resistance caused by ducts, dampers and other components in a system.

Types of Fans

Fan and blower selection depends on the volume flow rate, pressure, type of material handled, space limitations, and efficiency. Fan efficiencies differ from design to design and also by types. Typical ranges of fan efficiencies are given in the table below.

 

Types of Fans Peak Efficiency Range
Centrifugal Fans
Airfoil, backward curved/inclined 79-83
Modified radial 72-79
Radial 69-75
Pressure blower 58-68
Forward curved 60-65
Axial fan
Vanaxial 78-85
Tubeaxial 67-72
Propeller 45-50

Table 1: Types of fans

Source: Bureau of Energy Efficiency, 2010 Guidebook for National Certification Examination for Energy Managers and Energy Auditors: Book 3

Fans fall into two general categories: centrifugal flow and axial flow. In centrifugal flow, airflow changes direction twice - once when entering and second when leaving (forward curved, backward curved or inclined, radial). In axial flow, air enters and leaves the fan with no change in direction (propeller, tubeaxial, vaneaxial).

 

Fan efficiency Calculation

Generally fan manufacturers use two ways to mention fan efficiency: mechanical efficiency and static efficiency. Both measures indicate how well the fan converts horsepower into flow and pressure.

The equation for determining mechanical efficiency is:

 

The static efficiency equation is the same except that the outlet velocity pressure is not added to the fan static pressure

Drive motor kW may be measured by a load analyzer. This kW multiplied by motor efficiency gives the shaft power to the fan.

 

Payback of energy saving options

Experience from the past has shown that implementing energy saving options in fans are highly profitable with payback of investment of less than 3 years.

Table 2: Payback of investment of energy saving options for fans

Options Estimated payback period
Operate the fans at best efficiency points <1 year
Adopt VFD for capacity control in place of other options 1 - 2 years
Adopt FRP blades in place of metallic blades < 1 years
Avoid in leakages and out leakages in fan systems < 1 year
Adopt proper duct sizes for pressure drop reductions < 1 years
Adopt high efficiency fans 2 - 3 years

Source: NEEP, 2012-2016, IGEAs

 

Potential areas of Energy Efficiency improvements in Fans

  • Minimizing excess air level in combustion systems to reduce FD fan and ID fan load.
  • Minimizing air in-leaks in hot flue gas path to reduce ID fan load, especially in case of kilns, boiler, plants, furnaces, etc. Cold air in-leaks increase ID fan load tremendously. Du to density increase of flue gases and in-fact choke up the capacity of fan, resulting as a bottleneck for boiler/furnace itself.
  • In-leaks/out-leaks in air conditioning systems also have a major impact on energy efficiency and fan power consumption and need to be minimized.
  • Change of impeller by high efficiency impeller along with cone
  • Change of fan assembly as a whole, by a higher efficiency fan
  • Impeller derating (by a smaller dia impeller)
  • Change of metallic/glass reinforced plastic (GRP) impeller by the more energy efficient hollow FRP impeller with aerofoil design, in case of axial flow fans, where significant savings have been reported
  • Fan speed reduction by pulley dia modifications for derating
  • Option of two speed motors or variable speed drives for variable duty conditions
  • Options of energy efficient flat belts, or, cogged raw edged V belts, in place of conventional V belt systems, for reducing
  • transmission losses.
  • Adopting inlet guide vanes in place of discharge damper control
  • Minimizing system resistance and pressure drops by improvements in duct system

References

Bureau of Energy Efficiency, 2010 Guidebook for National Certification Examination for Energy Managers and Energy Auditors: Book 3

Asian Productivity Organization, 2010 Training Manual on Energy Efficiency for Small and Medium Enterprises

Nepal Energy Efficiency Programme (NEEP), 2012-2016, Investment Grade Energy Audits