Fan aerodynamics

Figure 3.2 Theoretical flow pattern at impeller outlet for radial blades

In fan work it is usual to know the pressure developed (p = pgH) and therefore p = p(vw2 u2 — vw1 u.,).

Under normal circumstances at the design duty, the air will en­ter the easiest way, i. e. radially and then vw1 =0.

Thus:

Figure 3.3 Theoretical flow pattern at impeller outlet for forward curved blades

Equ 3.15

P = PVW2 u2

Considering the impeller in cross-section with a width at its tip of b2, it may be said that the volume of air or gas delivered per unit timeQ= n d2 b2 v{2.

Now the impeller blades at the outlet may be either:

A) Backward inclined (straight, curved, or aerofoil) as in Figure 3. 1.

When

U2 = Vw2 + Vf2 cot P2

Or

Vw2 =U2-Vf2 COtP2

Flow rate—Q

Figure 3.4 Theoretical p-Q characteristics for different values of impeller dis­charge angle

Equ 3.16

This theoretical characteristic is a straight line with a downward slope.

B) Radial (straight shrouded, open or backplate paddle, or radial tipped) as in Figure 3.2 when vw2 = u2 and vf2 = v2

P=pu22 Equ3.17

This theoretical characteristic is a horizontal straight line.

C) Forward curved as in Figure 3.3 when vw2 — u2 = vf2 cot (180°-[32)

Tangential whirl velocity at entry to the impeller blade. Both of these factors reduce the pressure that the fan is capable of pro­ducing, but they do not affect the efficiency.

Friction losses

These are caused by gas friction and also include volute losses. (The volute is that part of the fan which converts velocity energy into pressure energy. This is normally achieved by ar­ranging the discharge channel so that the cross-sectional area gradually increases, thus reducing the flow velocity.)

Shock losses

Losses arise at entry to, and exit from, the impeller blade be­cause the blade angles are only correct for the design duty. On both sides of this shockless flow condition losses will occur.

Other losses

• Leakage: occurs from discharge to suction and through the shaft entry hole.

• Disc friction: due to the rotation of the impeller shroud and backplate within the gas.

• Mechanical losses: caused by the bearing friction and fric­tion at any shaft seal. These losses differ from those of the previous three groups in that whilst they affect the overall ef­ficiency they do not alter the basic fan characteristic.

The actual characteristic, with its losses are shown for a back­ward inclined impeller in Figure 3.5. Actual against theoretical

Equ3.18

This theoretical characteristic is a straight line with an upward slope.

It will be seen that for a given speed of rotation and a given pres­sure, the volume flow rate is dependent on the width of the im­peller and the blade angle. Reputable centrifugal fan manufac­turers will have many different width ranges with varying blade numbers and outlet blade angles to meet all duties economi­cally. All these theoretical characteristics are shown in Figure 3.4.

The theoretical pressure will be reduced by the following fac­tors, the aim of the fan engineer being to keep them to a minimum:

Relative rotation losses

In addition to the normal flow of fluid within the impeller, the iner­tia effect of the fluid causes a rotation of the fluid relative to the impeller. Also, when the impeller is mounted between bearings due to the effect of the rotating shaft, the fluid will have a definite