Ductwork Equations

Reference data and engineering information about ductwork equations for hvac systems applications.

ductworkequations

Overview

Engineering reference data for Ductwork Equations 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

A velocity diagram can be a valuable tool for quickly estimating air velocity based on duct size and airflow rate. The diagram below is configured with default values for an air flow of 400 cfm (680 m³/h) and a duct size of 8 in (200 mm), showing a corresponding velocity of 1150 fpm (5.8 m/s).

xychart-beta
    title "Air Duct Velocity Estimation"
    x-axis "Air Flow (cfm)" [200, 400, 600, 800, 1000]
    y-axis "Velocity (fpm)" [600, 1200, 1800, 2400, 3000]
    line [575, 1150, 1725, 2300, 2875]

Note: This is a simplified illustration. For precise calculations, use the formulas provided or download a detailed diagram from engineering resources.

For offline calculations and quick reference, the following mobile applications are available:

Air Duct Flow App
Air Duct Velocity App
Air Duct Head Loss App

Additional Considerations

While not detailed in the source text, Air Ducts Friction Loss is a critical factor in system design. Head loss due to friction in ductwork must be calculated to properly size fans and ensure system efficiency. This typically involves using friction loss charts or equations that consider duct material, air velocity, and duct dimensions.

References

Original Source

Source: engineeringtoolbox.com