Air Change Rate

Overview

Engineering reference data for Air Change Rate in HVAC systems.

Key Formulas

Sensible Heat

$Q = \dot{m} c_p \Delta T$

Heat causing temperature change.

Latent Heat

$Q = \dot{m} h_{fg} \Delta\omega$

Heat causing moisture change.

COP (Cooling)

$COP = Q_c / W$

Coefficient of performance.

Variables

Symbol	Description	Unit
$Q$	Heat transfer	W
$\dot{m}$	Mass flow rate	kg/s
$c_p$	Specific heat of air	J/(kg·K)
$\Delta T$	Temperature difference	K

Typical Air Change Rates

For well-insulated spaces, recommended air changes per hour depend on the building's exposure to outdoor air:

Worked Example

Problem: Calculate the air change rate for a room with an air flow of 3 m³/s and a volume of 20,000 m³.

Solution: Using the SI unit formula:

$n = \frac{3600 \times q}{V} = \frac{3600 \times 3 \text{ m}^3\text{/s}}{20000 \text{ m}^3} = 0.54 \text{ h}^{-1}$

This means the room's air is replaced approximately once every 1.85 hours ($1/0.54 \approx 1.85$).

References

• Original Source