Oleg Tchetchel

By Author: Oleg Tchetchel
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The fan efficiency is directly contingent upon selection of fan and the operating point relevant to the system resistance.

Fan efficiency is defined as the ratio of air power (output) to shaft power requirement (input). An optimally designed fan impeller with improved aerodynamics requires less shaft input power for desired delivery of air volume with total pressure rise as per the system requirement. With reduced shaft power requirement, there is a commensurate reduction in energy consumption.

Most fans are tested using procedures specified by Air Movement and Control Association International Inc. (AMCA), Illinois, USA. The fan under test is mounted near the outlet of the test rig and air in-flow through the rig is varied by the movable gate assembly situated at the other end of the test rig.

In a typical axial fan, the effective progress of the air is straight through the impeller at a constant distance from the axis. The primary component of blade force on the air is directed axially from inlet to outlet and thus provides the pressure rise by a process that may be called direct blade action.

The ...
... blade force necessarily has an additional component in the tangential direction, providing the reaction to the driving torque : this sets the air spinning about the axis independently of its forward motion.

The air delivering capacity of axial flow fans ranges from 100 to 500,000 cubic feet per minute (3,000 to 15,000 cubic meters per minute). Impellers usually have blades with airfoil cross-sections.

As compared to curved sheet blades, aerofoils can apply greater force to the air, thereby increasing maximum pressure and can maintain better efficiency over a wider range of volumetric flow. Also by increasing the thickness & curvature of the inner sections, the blades can be made stiffer, this limits flutter and allows the impellers to be run at higher speeds.

Selection of a a right fan one should consider such factors like cost optimization, power rating & noise levels govern the selection of a fan suitable for a given application. Various combinations are possible to meet any given duty or operating requirements for best performance, lesser noise, power and cost characteristics.

Once the volume of airflow and the static pressure of the system are known, it is possible to specify a fan. The governing principle in fan selection is that any given fan can only deliver one flow at one pressure in a given system. To select a fan one must consider the following parameters:

- total airflow required

- total operating pressure

- fan installation space

The way fan impellers (blades or rotors) are designed, the amount of air they can move decreases as the pressure they are working against increases.

The airflow vs. pressure information for a particular fan is called the fan performance data. The performance of a fan depends on the size, shape & speed of the impeller. The fan performance data is essential for selecting fans as per the desired airflow and system pressure.

The airflow through the rig is determined by measurement of the pressure differential and simultaneous readings are taken of the static pressure developed by the fan, air velocity at the exit side, power consumed by the fan driving motor etc.

Fan performance curves and data show how much pressure and airflow a given fan can produce at a given speed. This data is based on the system resistance (pressure) occurring away from the fan. Fan performance curves are developed by fan manufacturers based on the testing of a prototype fan fabricated based on the theoretical design.

The axial flow fans are widely used for providing the required airflow for heat & mass transfer operations in various industrial equipment and processes. These include cooling towers for air-conditioning & ventilation, humidifiers in textile mills, air-heat exchangers for various chemical processes, ventilation & exhaust as in mining industry etc. All the major industries use large number of axial flow fans for operations, such as :

- Power generation

- Petroleum refining & petrochemicals

- Cement industry

- Chemicals & pharmaceuticals production

- Fertilizer production

- Mining manufacturing

- Humidification in textile mills

- building Ventilation, etc..

Additional information can be found at the Buffalo Fan web site http://www.buffalofan.com/inquiry.html.

Oleg Tchetchel
Industrial Process Air Specialist
Buffalo Fan Co.
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