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Size a boiler fan in plain words
A boiler breathes through two fans. The FD fan pushes fresh air into the furnace; the ID fan pulls the hot smoke out and up the chimney. Set your boiler below and every number, and the machine on the left, updates.
Site and boiler conditions
This fan
How the fan is sized
Ten small steps, each using the one before. Hover a step to light up the part it decides.
What the fan pushes against
Why it matters
Fan power is flow × pressure. Every extra 10 mmWC on this fan costs about of shaft power, around the clock. Clean ducts, a low-loss dust collector and open dampers are the cheapest energy saving in the boiler house.
Running below full load
A boiler seldom runs flat out. Throttle the fan with a damper and it keeps spinning at full speed, burning off the extra pressure. Slow it with a VFD and power falls with speed cubed.
Inside the fan
Tap a part to fly the camera to it. Bearings and shaft open the exploded view.
FD and ID side by side
Same boiler, same machine type, very different jobs.
Method: steam enthalpy from IAPWS-IF97 saturated-steam tables; theoretical air ≈ 0.333 kg per MJ of fuel heat; ideal-gas densities corrected for altitude and fan-inlet suction; impeller from the pressure coefficient ψ (Δp = ψ·ρ·u²/2) with meridional velocity ≈ ¼ tip speed; part load from a quadratic fan curve and the affinity laws. First-pass design numbers for learning and budgeting — buy against a fan maker's tested selection (IS 4894 / AMCA 210).