Units:

Equal Friction Method: Size ducts to maintain a constant pressure drop per unit length throughout the system. Recommended friction rate: 0.8 – 1.2 Pa/m for HVAC supply ducts.

Duct Parameters

Airflow Rate L/s

Friction Rate Pa/m

Duct Shape

⭕Circular

⬜Rectangular

Duct Material Roughness factor

Segment Name (optional)

Duct Schedule — Equal Friction Method

Segment	Airflow	Friction Rate	Diameter (mm)	W × H (mm)	Velocity (m/s)	ΔP (Pa/m)	Status

Velocity Method: Size ducts by setting the desired air velocity. Select your duct application type below and the recommended velocity will be pre-filled based on ASHRAE standards.

Duct Parameters

Airflow Rate L/s

Duct Application

Design Velocity m/s (ASHRAE recommended)

Duct Material

Duct Shape

⭕Circular

⬜Rectangular

Segment Name (optional)

Duct Schedule — Velocity Method

Segment	Airflow	Design Vel. (m/s)	Diameter (mm)	W × H (mm)	Actual Vel. (m/s)	ΔP (Pa/m)	Status

ASHRAE Recommended Duct Velocities

Duct Application	Recommended (m/s)	Maximum (m/s)	Notes
Main Supply Duct	5.0 – 8.0	10.0	Low velocity system
Branch Supply Duct	3.0 – 5.0	7.5	Sub-branches off main
Main Return Duct	4.0 – 6.0	8.0	Return air system
Branch Return Duct	3.0 – 4.5	6.0
Exhaust Duct	6.0 – 8.0	10.0	Kitchen/toilet exhaust
Riser / Vertical Duct	5.0 – 8.0	10.0	High-rise buildings
Outside Air Duct	2.5 – 4.0	5.0	Fresh air intake
Fan Discharge	8.0 – 12.0	15.0	At fan outlet
Hospital OT Supply	0.3 – 0.45	0.5	UCV laminar flow
Cleanroom Supply	0.3 – 0.5	0.6	ISO Class 5–6

Recommended Friction Rates — Equal Friction Method

Application	Friction Rate (Pa/m)	Notes
Standard HVAC Supply	0.8 – 1.2	Most common design value
Low Pressure System	0.4 – 0.8	Noise-sensitive areas
High Pressure System	1.2 – 2.0	Industrial / VAV systems
Return Air System	0.6 – 1.0	Slightly lower than supply
Kitchen Exhaust	1.0 – 1.5	Grease-laden air

Standard Duct Sizes (Circular) — Galvanised Steel

Nominal Diameter (mm)	Cross-Section Area (m²)	Airflow at 5 m/s (L/s)	Airflow at 8 m/s (L/s)
100	0.00785	39	63
125	0.01227	61	98
150	0.01767	88	141
200	0.03142	157	251
250	0.04909	245	393
315	0.07793	390	623
400	0.12566	628	1005
500	0.19635	982	1571
630	0.31173	1559	2494
800	0.50265	2513	4021
1000	0.78540	3927	6283

Duct Roughness Values

Material	Roughness (mm)	Typical Use
PVC / GRP / Smooth plastic	0.003	Cleanrooms, labs
Galvanised steel (smooth)	0.09	Commercial HVAC
Galvanised steel (standard)	0.15	Standard HVAC
Flexible duct (extended)	0.5 – 2.0	Final connections
Concrete	1.5	Plantrooms, shafts
Brick / Masonry	3.0	Building voids

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