This question comes up very often in HVAC design 👷‍♂️
Round pipe vs square pipe, there is no fixed answer as to what is "always better".

But you must choose one that suits your performance + space + budget + construction work.

I summarize it as an engineer actually using it.

🔵 Round Duct

✅ Advantages
 • Loss is lowest.
Best perimeter cross-sectional area → Low friction
 • Uses less Static Pressure → Smaller fan / Saves energy.
 • Leakage is low, especially Spiral Duct.
 • Low noise (even airflow)
 • Use less sheet metal to connect the same CFM.

❌ Limitations
 • Consumes a lot of ceiling height.
 • Difficult to run pipes in confined spaces.
 • Connecting some devices (VAV, FD, Volume Damper) is more messy.
 • Decoration work in some office buildings is not liked.

🎯 Suitable for the job
 • Factory / Warehouse
 • Food court / Hall / Auditorium
 • High volume air system (High CFM)
 • Work that emphasizes Energy Efficiency.

🔶 Rectangular Duct
✅ Advantages
 • Save ceiling height. (Very important in the building)
 • Easy to arrange layout to match the beams/ceiling.
 • Install equipment (VAV, FD, Damper) conveniently.
 • Familiar with general construction work. Use a lot.

❌ Limitations
 • Loss is higher than round pipe.
 • Greater noise and vibration (Stiffener required).
 • Leaks more easily if the work is not thorough.
 • More materials are used in the same CFM.

🎯 Suitable for the job
 • Office buildings
 • Hospital
 • Hotel / Condo
 • Low ceiling area

📊 Short comparison (Engineer finished reading in 30)
Topic: round pipe, square pipe
Pressure Loss ⭐⭐⭐⭐ Low ⭐⭐ Higher
Save energy ⭐⭐⭐⭐ ⭐⭐
Use ceiling space ❌ a lot ✅ little
The sound is quiet and loud.
Installing equipment is difficult and easy.
General building work, less popular

🧠 "Professional" concepts that I actually use
Do not choose one or the other for the entire system.

✔️ The best way
 • Main Duct → Round Duct (reduces Fan Power)
 • Branch / Above Ceiling → Square pipe (easy access to the area)

📌 This method is often used in
Office / Mall / Hospital / High-rise building
📐 Design Notes (Important)
 • If required large square pipe
➜ Avoid long:short side ratio > 4:1
 • Control the wind speed.
 • Main duct: 6–8 m/s
 • Branch: 4–6 m/s
 • Add Turning Vanes / Radius Elbow to help reduce loss.
 • Don't forget to consider Fan Static + Leakage Class.

🔧 Summary like Ajarn Thep
 • If you focus on saving energy → round pipe
 • If space is limited → square pipe