Parametric Calculations

The mass flow rate through the impeller is given by

	(39.1)

The areas of cross sections normal to the radial velocity components and are and

	(39.2)

The radial component of velocities at the impeller entry and exit depend on its width at these sections. For small pressure rise through the impeller stage, the density change in the flow is negligible and the flow can be assumed to be almost incompressible. For constant radial velocity

	(39.3)

Eqs. (39.2) and (39.3) give

	(39.4)

Work

The work done is given by Euler's Equation (refer to Module-1) as

	(39.5)

It is reasonable to assume zero whirl at the entry. This condition gives

and hence,

Therefore we can write,

	(39.6)

Equation (39.5) gives

	(39.7)

For any of the exit velocity triangles  (Figure 39.3)

	(39.8)

Eq. (39.7) and (39.8)

	(39.9)

where is known as flow coefficient

Head developed in meters of air =		(39.10)

Equivalent head in meters of water =		(39.11)

where and are the densities of air and water respectively.

Assuming that the flow fully obeys the geometry of the impeller blades, the specific work done in an isentropic process is given by

	(39.12)

The power required to drive the fan is

	(39.13)