All images by © RACA Journal www. refrigerationandaircon. co. za RACA Journal I July 2025 27
Feature
A BUILDING’ S LUNGS: WHY PROPER DUCTING AND FANS MATTER
Complied by Eamonn Ryan
Air guidance systems, particularly ducting in HVAC systems, are more complex than they appear, often hidden from view.
DUCTWORK: THE UNSEEN NETWORK OF AIRFLOW Though frequently unseen, this intricate component is vital for air supply and extraction, relying on a network of trunks, branches and components like grilles and dampers.
Historically, early‘ ducting’ emerged to manage smoke from heating systems. Modern ducting has evolved since the early 1900s with advancements in fans and air conditioning, and now comes in diverse shapes, sizes and materials such as steel, aluminum and fabric.
Airflow through ducts involves technical principles like velocity, friction and pressure. Crucially, a high-spec HVAC system will underperform without properly designed and installed ductwork, leading to discomfort, high energy costs and poor air quality. Effective design considers investment, space, future expansion and operational costs.
MANAGING AIRFLOW Duct systems primarily convey air( supply or return) driven by pressure differential. Air moves from high to low pressure, adhering to three fundamental laws of physics: conservation of mass, energy, and momentum.
• Conservation of mass dictates that air entering a duct junction equals air leaving it. For practical purposes, air in ducts is usually considered incompressible, simplifying duct size calculations based on velocity
• Conservation of energy states that energy transforms, it doesn ' t disappear. This is the basis of Bernoulli ' s equation, which relates pressure loss to total pressure differences in an airflow stream
• Conservation of momentum, derived from Newton ' s law, helps understand air behaviour within a duct system as forces influence its motion
“ The choice of ducting material goes beyond simple air movement and depends heavily on the specific application and environment.”
Air distribution structures include components based on the overall HVAC system ' s function. There are five typical functions of ductwork:
• Supply air ductwork: Delivers conditioned air from an air handling unit to a space
• Return air ductwork: Removes air from a space and returns it to an air handling unit for re-conditioning; some may be exhausted outdoors
• Fresh air ductwork: Supplies outdoor air directly to a space or air handling unit for ventilation
Francois Schoombie( N. Dipl. Mech Eng. TUT), technical manager and EC product specialist for ebm papst.
• Exhaust( relief) air ductwork: Discharges air from specific areas( e. g., toilets, kitchens) to the building exterior
• Mixed air ductwork: Combines outdoor and return air
Diffusers, grilles and louvres: These are terminal devices that control airflow direction and mix conditioned air within a space. Grilles supply or return air without deflection and are generally not used for supply due to poor flow control. Louvres are similar to grilles but feature adjustable deflectors and dampers to regulate air supply, return, or exhaust.
Where all galvanized metal ducting begins – on a roll.
All images by © RACA Journal www. refrigerationandaircon. co. za RACA Journal I July 2025 27