Tutorial: designing a 400 mm duct fan with Heliciel

A craftsman has to extract the air from his finishing workshop: an air renewal computed at 3,000 m³/h, through a short duct of 400 mm diameter whose pressure losses (grille, duct, outlet) have been estimated at 30 Pa at the rated flow. He wants to carve the fan impeller himself, driven by a variable-speed motor.

Organisation and design phases:

The fan model and the “penstock” mode
From flow to air speed, from pressure to thrust
Search for the rpm… and understand the limit of the duct fan
The fan characteristic curve
Summary, noise and model to download

1: The fan model: when the model opens, note Heliciel's message: “PENSTOCK mode active: the flow section is imposed by the duct… axial induction is taken as zero and tip vortex losses are disabled”. A ducted impeller does not work like a free propeller: the tube imposes the air stream, and the tip clearance (ideally) removes the tip losses.

fan model loaded in Heliciel in penstock mode

2: Translate the need: two conversions underlie every duct fan sizing:

flow → axial velocity: Heliciel derives the flow section from the duct diameter (400 mm) and the hub (128 mm): S = 0.1128 m², so 3,000 m³/h = 0.833 m³/s → 7.39 m/s in the duct;
pressure → thrust: the pressure supplied by the impeller is its thrust spread over the section: ΔP = T / S. Our 30 Pa therefore call for T = 30 × 0.1128 = 3.4 N of thrust. A modest pressure only needs a few newtons — but every pascal has to be earned.

3: Search for the rpm… and understand: at 960 rpm (the rpm inherited from the model), the 400 mm 5-blade impeller only supplies 16 Pa at the rated flow: not enough. We then asked for much more rpm, and discovered a lesson in air handling:

at 1,850 rpm: 28.5 Pa; at 3,200 rpm: only 32 Pa, for 329 W absorbed — the extra power goes into swirl (the air leaves as a vortex with 7.4 m/s of tangential velocity) instead of becoming pressure;
this is the physical limit of the duct fan without stator: past a certain point, turning faster no longer compresses, it screws the air. For high pressures (100 Pa and more), you need straightening vanes downstream, a contra-rotating stage… or a larger diameter;
our 30 Pa target, however, remains reachable: at 1,950 rpm, the rebuilt impeller supplies 32.9 Pa at the rated flow. We finally smooth the angles of attack (“smooth angles of attack” function: the whole blade at the angle of attack of the 0.75 R zone, here 10°) for a regular, easy-to-make blade: ΔP = 34.6 Pa, 238 W absorbed, torque 1.16 N·m. 15 % margin on the need: adopted.

400 mm fan design at 1,950 rpm in Heliciel

4: The characteristic curve: a fan never lives alone: it settles in equilibrium with its network. The multi-point analysis sweeps the air speed in the duct (hence the flow) at 1,950 rpm, geometry frozen:

fan characteristic curve flow pressure in Heliciel

Flow (m³/h)	Axial V (m/s)	ΔP (Pa)	Shaft P (W)
1624	4.0	-15.6 (windmilling!)	177
2166	5.3	13.1	186
2436	6.0	22.0	188
2708	6.7	28.6	193
2977	7.3	33.7	210
3248	8.0	37.4	189
3790	9.3	42.6	189

Reading: below about 2,000 m³/h, our setting (10° angle of attack, deliberately loaded) stalls and the “pressure” becomes negative: if the network is more resistive than planned (clogged filter, obstructed grille), the flow drops and the fan leaves its working zone. The margin taken at step 3 is therefore not a luxury. This is the whole point of confronting the fan curve with the network curve — the approach is detailed on the fan and ventilation page.

5: Summary, noise and model

5-blade duct impeller Ø 400 mm (hub 128 mm), angle of attack smoothed to 10°, pitch angle 27° at 0.7 R;
rated point: 3,000 m³/h at 34.6 Pa at 1,950 rpm, 238 W at the shaft — a 4-pole motor with a variable-speed drive, or a brushless motor, will do;
noise: blade passing frequency 162.5 Hz (harmonics 325 / 487.5 Hz), blade tip at 41 m/s;
air-handling lesson: without a stator, the duct fan reaches a pressure ceiling — size the network BEFORE the fan.

The complete project can be downloaded here: ventilateur_gaine_400mm_3000m3h.hlc (open it with Heliciel). It is also included in the models supplied with the software.

I remain at your disposal for any remarks or further information at contact@heliciel.com

JF Iglesias, Heliciel developer.