Auto Variable Pitch Axial Flow Fans

Variable pitch axial flow fans are special type of axial fans with adjustable impeller blades that are built to handle broad operating ranges in critical industrial applications. These fans include an internal mechanism that can automatically adjust blade angle while the rotor is in motion. Due to their compact design and high efficiency, most of them find applications in the thermal power stations where fresh air is needed (forced draft) or process involving high temperatures such as in boilers (induced draft). These fan types are proven to have negligible loss in power at the output and hence they are also used in pulverizer air fans and flue gas desulphurization (FGD) applications.

Among the single and multi-stage designs, the two stage axial fans with variable blade pitch angle find wide usage in many industrial processes. It is also simple to maintain the fan system as most of the critical parts such as rotor - consisting of the impellers, the main bearing assembly and the propeller blades are isolated as separate units for easy accessibility.

Types of variable pitch axial fans:

Depending on number of propellers present on the shaft the axial fans with variable pitch are mainly divided into two types.

• Single Stage Type

  • Single stage type consists of a single propeller on the shaft.

  • This system is capable of delivering high volumes of about 1000 m3/s at pressures up to 15 kPa.

• Two Stage Type

  • Two stage fan type consists of two propellers mounted on a single shaft and the propellers are separated by guide vanes.

  • This can deliver high volumes relatively at high pressure when compared to single stage system. (1000 m3/s at 30kPa.)

  • As this system consists of two propellers in a single unit, it has reduced shaft length with minimum overall dimension resulting in compact system. This rigidity in spacing adds as an advantage in eliminating the problems of resonance and vibration at the work place.

Working mechanism:

The main parts that are used in varying the pitch of axial fan are hub spring, diaphragm, blade shafts, rotary air/gas joint and valve positioner.

When air/gas stream falls on propeller blades, it tries to rotate the blades. The hub spring creates a movement which helps to rotate the blade in opposing direction. The rotation of blades due to air/gas stream, exerts pressure on diaphragm to oppose the hub spring movement and this creates a variation in blade pitch. The amount of pressure exerted will determine either the amount of air/gas flow or a complete change in the direction of air/gas flow.

The basic design and operation is similar for both single and double stage types except the later one contains two propellers.

Comparison - Axial Fan vs Variable Axial Fan

• Output volume

  • Variable pitch fan types with varied blade angles can deliver higher volume rates when compared to fixed blades of normal axial fans.

• Energy consumed

  • The air/gas pressure is controlled automatically by the fan by varying the pitch of propeller, and hence there is no need for a throttling device which consumes extra energy for its working. As the blades have less stress acting on them, the total load acting on motor remains constant. In case of fixed axial blades a throttling device is to be supplied to control flow pressure, which consumes energy and the blades of the propeller are subjected to high stress enforcing extra load on motor.

Variable pitch axial flow fans are specially designed to serve specific requirements of industrial applications because of their instantaneous blade adjustment capability, higher efficiency and sustainability for higher temperatures.

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