Fan Inlet Pressure

Overview

Engineering reference data for Fan Inlet Pressure in fluid mechanics.

Key Formulas

Reynolds Number

$Re = \frac{\rho v D}{\mu}$

Ratio of inertial to viscous forces — determines flow regime.

Bernoulli's Equation

$P + \frac{1}{2}\rho v^2 + \rho g h = \text{const}$

Conservation of energy for steady, inviscid, incompressible flow.

Continuity Equation

$A_1 v_1 = A_2 v_2$

Conservation of mass for incompressible flow.

Darcy-Weisbach

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

Pressure drop due to friction in a pipe.

Variables

Symbol	Description	Unit
$Re$	Reynolds number	—
$\rho$	Fluid density	kg/m³
$v$	Flow velocity	m/s
$D$	Characteristic dimension	m
$\mu$	Dynamic viscosity	Pa·s
$P$	Pressure	Pa
$f$	Darcy friction factor	—

Correction Factor Table

When inlet suction pressure exceeds 270 mm W.G (10 in W.G), reduced air density requires compensation using a correction factor:

:::note For suction pressures below 270 mm W.G (10 in W.G), the correction factor is normally neglected and $f = 1.0$. :::

Corrected Suction Pressure

The corrected suction pressure accounts for reduced air density at high vacuum conditions:

$P_{sc} = P_s \cdot f$

Where:

• $P_{sc}$ = corrected suction pressure (mm W.G. or in W.G.)
• $P_s$ = design (and measured) suction pressure (mm W.G. or in W.G.)
• $f$ = pressure correction factor from table above

Example

Given: Design suction pressure $P_s = -380$ mm W.G. (−14 in W.G.)

Solution: From the table, the correction factor $f = 1.04$

$P_{sc} = (-380 \text{ mm W.G.}) \times 1.04 = -395 \text{ mm W.G.}$

The corrected suction pressure of −395 mm W.G. should be used for proper fan selection.

Unit Conversions

Unit	Equivalent
1 in water	248.8 N/m²
1 in water	0.0361 lb/in²
1 in water	25.4 kg/m²
1 in water	0.0739 in mercury

Interactive Charts

References

• Original Source