Engineering: One System, Multiple Design Variables An AHU is not just a box with a fan and cooling coil. A small design mistake can increase: ✗ Fan power ✗ Pressure drop ✗ Noise ✗ Operating cost ✗ Maintenance frequency 1. Typical AHU Air Path Fresh Air → Louver → pre-Filter → Heat Recovery → Cooling/Heating Coil → Fan → Silencer → Supply Air Return/exhaust air paths must also be considered, especially where heat recovery, pressure control and IAQ are important. 2. Airflow Sizing The basic relationship is: Q = A × V Where: • Q = Airflow (m³/s) • A = Face area (m²) • V = Air velocity (m/s) Example: 20,000 m³/h = 5.56 m³/s At 2.5 m/s face velocity: Required area ≈ 2.22 m² Lower face velocity generally means a larger AHU footprint but can reduce component pressure drop and fan energy. ASHRAE notes that designers should not automatically default to 2.5 m/s; lower coil/filter velocities can provide energy benefits. (ASHRAE Handbook) 3. Pressure Drop = Hidden Energy Cost Typical AHU res...
HVAC Ducts: 10 Types Every HVAC Engineer Should Know
For an HVAC Engineer, understanding duct types is important for air distribution, pressure loss, energy efficiency, noise control, space utilization, and maintenance.
1. Rectangular Duct
- Common in commercial buildings and hospitals.
- Suitable where ceiling space is limited.
- Easy to fabricate and install in large air-distribution systems.
- Requires proper reinforcement to prevent vibration and deformation.
2. Round Duct
- Provides efficient airflow with relatively low-pressure loss.
- Requires less material for a given airflow compared with rectangular ductwork.
- Commonly used for supply, return, and exhaust systems.
3. Spiral Duct
- A type of round duct manufactured with a spiral seam.
- Strong, lightweight, and relatively easy to install.
- Frequently used in commercial and industrial HVAC systems.
- Can provide a clean architectural appearance when exposed.
4. Oval Duct
- Provides some of the airflow advantages of round ductwork while requiring less vertical space.
- Useful where ceiling or service-space height is restricted.
5. Flexible Duct
- Used mainly for short connections between rigid ductwork and diffusers/grilles.
- Quick and economical to install.
- Should be properly supported and kept as short and straight as practical because excessive bends increase pressure loss.
6. Fabric/Flexible Textile Duct
- Made from specialized textile materials.
- Distributes air through nozzles, perforations, or permeable fabric.
- Common in selected industrial, sports, food-processing, and commercial applications.
7. Double-Wall Duct
- Consists of an inner liner and outer metal casing.
- Provides improved thermal insulation and acoustic performance.
- Useful for applications where noise control and condensation prevention are important.
8. Insulated Duct
- Ductwork provided with internal or external thermal insulation.
- Helps reduce heat gain/loss and prevents condensation.
- Particularly important for chilled-air systems in humid climates.
9. Fire-Rated / Smoke-Control Duct
- Designed for applications where fire resistance or smoke containment is required.
- Used in buildings with fire-rated compartments and smoke-control systems.
- Requires appropriate fire/smoke dampers and installation according to the applicable fire code.
10. Kitchen Exhaust / Grease Duct
- Designed to carry hot, greasy exhaust air from commercial kitchens.
- Requires special construction, access for cleaning, fire protection, and appropriate clearances.
- Must comply with applicable fire and mechanical codes.
Key point for HVAC Engineers
Selecting a duct tape is not simply about shape. The engineer should consider:
Airflow → velocity → pressure drop → duct size → noise → insulation → leakage → fire/smoke requirements → space constraints → material → maintenance → energy efficiency.
For a hospital HVAC system, particular attention should be given to duct cleanliness, leakage, filtration, pressure relationships, infection-control requirements, fire/smoke protection, insulation, access doors, and proper balancing.
Selected for applications where fire resistance, hygiene or corrosion resistance is critical.
‣ ENGINEERING POINTS THAT MATTER
A good duct design should consider:
Airflow → Velocity → Pressure Drop → Fan Static Pressure → Leakage → Insulation → Noise → Fire Safety → Maintenance
A simple sizing relationship is:
A = Q / V
Where:
A = Duct cross-sectional area
Q = Airflow
V = Design air velocity
But sizing the duct is only the beginning.
The design must also consider fittings, elbows, transitions, dampers, filters, coils, diffusers and total system pressure drop.
‣ DON'T MISS PRESSURE CLASS
SMACNA duct construction standards recognize pressure classes including:
0.5, 1, 2, 3, 4, 6 and 10 in. w.g.
approximately 125, 250, 500, 750, 1000, 1500 and 2500 Pa respectively. (SMACNA)
Duct construction and sealing should be selected based on the actual operating pressure and project requirements, not simply by calling a system “low pressure” or “high pressure.”
SMACNA also provides dedicated guidance covering duct construction, inspection, leakage testing and commissioning. (SMACNA)
‣ ENGINEER'S TAKEAWAY
Good HVAC design is not about choosing the biggest duct.
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