3.3.3. Change of Fan Size with Constant Density and Speed

The volume flow changes proportionally to the third power of the wheel diameter:

 \frac{q_{V,I}}{q_{V,II}}=\left(\frac{D_I}{D_{II}}\right)^3

The pressures (static, dynamic, total) change proportionally to the square of the wheel diameter:

 \frac{p_{V,I}}{p_{V,II}}=\left(\frac{D_I}{D_{II}}\right)^2

The power requirement at the shaft changes proportionally to the fifth power of the wheel diameter:

 \frac{P_I}{P_{II}}=\left(\frac{D_I}{D_{II}}\right)^5

 These relationships are shown in Figure 3.3.3.

Figure 3.3.3.: Change of pressure and shaft power