Page 99 - Advanced Mine Ventilation

80 Advanced Mine Ventilation

Around 1920, axial flow fans became popular. Its design improved with better knowl-
edge of aerodynamics. Even the radial flow fans improved their efficiency by
designing the blades aerodynamically. Both types of fans are in use today, and hence
both types will be discussed in detail.

6.1 Radial Flow Fans

Radial flow fans, also known as centrifugal fans, are basically designed to contain
radial blades with arrangements to let air enter at the base, at the center, and exit at
the periphery of the rotors. Two independent actions produce pressure in a radial
flow fan: the centrifugal force due to the rotation of air and the kinetic energy imparted
to the air as it leaves the tip of the impeller blades. The magnitude of kinetic energy
depends mainly on the tangential velocity (tip speed) of the blades, V t . The centrifugal
energy imparted to air is a function of the change in the radial velocity, V r , of air
entering and leaving the impeller.
The ideal performance characteristic of a centrifugal fan can be derived from the
energy transfer and slip relationship. The theoretical head, H, is given by the following
equation:

Q Q
V t 2 V t 1
H ¼ V t 2 cot q 2 V t 1 cot q 1 (6.1)
g A 2 g A 1
where V t is the tangential velocity; Q is the air flow rate; A 1 and A 2 are areas
perpendicular to radial velocity at the inlet and outlet; q is the blade angle; subscripts
1,2 refer to the inlet and outlet of the impeller; Q/A is the radial air velocity.
The second term in Eq. (6.1) becomes zero if we assume that there is no inlet whirl.
Hence,

Q
V t 2
H ¼ V t 2 cot q (6.2)
g A
This equation will be used to calculate the theoretical head developed by different
types of centrifugal fans.
Fig. 6.1 shows the velocity diagram for the three types of centrifugal fans: (a) radial
bladed, (b) backward bladed, and (c) forward bladed fans.

6.1.1 Head Developed by Radial Bladed Fans

Because the blade angle, q, is 90 degrees in this case, and cot 90 degrees is equal to
zero, Eq. (6.2) reduces to:

V 2 t
H ¼
g