Fan Capacity Diagrams

Overview

Engineering reference data for Fan Capacity Diagrams in piping systems.

Key Formulas

Continuity

$A_1 v_1 = A_2 v_2$

Mass conservation in pipe flow.

Pressure Drop

$\Delta P = f \frac{L}{D} \frac{\rho v^2}{2}$

Darcy-Weisbach equation.

Pipe Area

$A = \frac{\pi D^2}{4}$

Cross-sectional area of a pipe.

Variables

Diagram Line Interpretation

In fan capacity diagrams, the following lines are commonly used to visualize performance:

• Red lines: Represent pressure head as a function of air volume flow rate and fan speed. These curves show how static and dynamic pressures vary with flow at different rotational speeds.
• Green lines (marked with "l"): Indicate throttle lines, which are lines of constant system resistance. They help in assessing how throttle adjustments impact fan operation.
• Blue lines: Depict power consumption, illustrating the power required to drive the fan across various flow and pressure conditions.

The relationships governing these diagrams are derived from fundamental fluid dynamics principles. Key equations include:

1. The total pressure $p_t$ is the sum of static pressure $p_s$ and dynamic pressure $p_d$, often considered constant along a streamline: $p_t = p_s + p_d$

2. Dynamic pressure is given by: $p_d = \frac{1}{2} \rho v^2$ where $\rho$ is the air density (kg/m³) and $v$ is the air velocity (m/s).

3. The net pressure change or head developed across the fan, from inlet (1) to outlet (2), is: $\Delta p = p_{s2} - p_{s1} + \frac{\rho}{2} (v_2^2 - v_1^2)$

These formulas are crucial for interpreting the curves and calculating fan performance parameters.

References

• Original Source