IQEG CALCULATION LAB
Fan versus system curve
IQ-CAL-004 · Rev 0 Indicative · 11/11/2026 · Not a certificate
IQ Engineering Group · Sydney · IQ-CAL-004 · Fan versus system curve · millimetres, kN, kNm, MPa unless noted
Fans + air
Tick up to three catalogue fans. Curves are at the selected frequency and air density.
ρ = 1.204 kg/m³ · p = 101.3 kPa · ν = 1.51e-5 m²/s
Normal (20 °C, 0 m): ρₙ = 1.2 kg/m³. Qₙ = Q × ρ/ρₙ.
What the curves are
- Fan curve
- Catalogue static pressure vs actual volume, scaled by speed (Q∝n, P∝n²) and by ρ/1.20. Chart Y is fitted to the fan so a steep system does not squash it.
- Design takeoff
- ΣΔP of the pipes and fittings below, evaluated at each Q. ΔP ∝ Q² (Darcy and K both go with V²). Equipment ΔP is entered at Qdes and scaled (Q/Qdes)².
- 0.8× / 1.25×
- Same takeoff × 0.8 (open damper, clean filter) or × 1.25 (restricted). Duty is the intersection of the fan with the ticked 1.0× curve, unless Q is locked.
- Y-scale
- Pinned to the fan. If a system curve runs off the top, the takeoff at that Q is above the fan — the fan cannot deliver it.
Curves
System ΔP = Σ f(L/D)ρV²/2 + Σ K ρV²/2 + equipment. 0.8× open / clean; 1.25× restricted. Chart Y is the fan — a steep system going off the top means the fan cannot make that Q.
Operating point
Catalogue fan pressure is scaled by ρ/1.20. Volume Q is actual. Mass flow is ρQ. Qₙ = Q × ρ/1.20 at 20 °C sea level.
System takeoff · add pipes and fittings
Straight uses Darcy with Haaland f from the selected surface. Knees 90° / 45° (r/D = 1.5) and mitred knees use K. Equipment ΔP is entered at Qdes and scales (Q/Qdes)².
| Component | n | D mm | L m | Surface | K / ΔPdes | |
|---|---|---|---|---|---|---|
| — | — | |||||
| f from Moody | ||||||
| — | — | |||||
| f from Moody | ||||||
| — | — | |||||
| — | — | |||||
| — | — | |||||
| — | — | |||||
| — | — |
Loss schedule at Qdes = 1.20 m³/s
Same Q as the design flow (or locked Q). Moody on the right holds f. Re and f live there to keep this short.
| Item | n | Ø | V | ΔP Pa |
|---|---|---|---|---|
| Sharp entry | 1 | 315 | 15.4 | 71 |
| Straight pipe / duct | 1 | 315 | 15.4 | 121 |
| 90° elbow r/D = 1.5 | 3 | 315 | 15.4 | 116 |
| Straight pipe / duct | 1 | 315 | 15.4 | 121 |
| 90° elbow r/D = 1.5 | 3 | 315 | 15.4 | 116 |
| 45° elbow r/D = 1.5 | 2 | 315 | 15.4 | 46 |
| Damper (open) | 1 | 315 | 15.4 | 29 |
| Equipment ΔP (at Qdes) | 1 | 315 | 15.4 | 650 |
| Discharge / exit | 1 | 315 | 15.4 | 143 |
| TOTAL | 1,412 |
Moody chart · f vs Re
Re = VD/ν = 3.22e+5 · ε/D = 0.00048 · f = 0.0179 · Galvanised duct ε 0.15 mm · 20 °C · 0 m
Calculation · one component at Qdes
- 01
Air at the stated temperature and altitude. ISA pressure. Density from p / RT. Normal condition is 20 °C, sea level, ρₙ = 1.20 kg/m³.
T = 20 °C = 293.2 K z = 0 m p = 101,325 Pa ρ = 1.204 kg/m³ ν = 1.51e-5 m²/s Qₙ = Q × ρ/ρₙ - 02
Velocity and Reynolds number in the reference diameter at Qdes (actual).
D = 315 mm A = πD²/4 = 0.0779 m² V = Q/A = 15.40 m/s Re = VD/ν = 3.22e+5 ρV²/2 = 142.8 Pa - 03
Friction factor. Laminar 64/Re below Re 2300. Turbulent Haaland. Transition blended 2300–4000.
ε = 0.15 mm ε/D = 0.00048 1/√f = −1.8 log10[ (ε/D / 3.7)^1.11 + 6.9/Re ] f = 0.0179 - 04
Straight pipe — Darcy–Weisbach. ΔP = f (L/D) (ρV²/2).
L = 15.0 m × n=1 f = 0.0179 ΔP_straight = 121 Pa - 05
Sum the takeoff. Fan catalogue pressure is scaled by n² and by ρ/1.20. Intersect with the selected fans.
ΣΔP(Qdes) = 1,412 Pa Q* = 1.291 m³/s actual Qₙ* = 1.295 m³/s ΔP* = 1,633 Pa air kW = 2.11