One inlet or two? SWSI vs DWDI, and when each wins

Two fans can carry the same wheel diameter and be entirely different machines. One breathes through a single inlet; the other draws through both sides of the housing. That single choice decides your bearings, your dampers, your motor start — and whether the fan fits the room you have.

The vocabulary first

SWSI (you will also see SISW) is single width, single inlet: one impeller, air entering one side. DWDI (also written DIDW) is double width, double inlet: a double-width wheel taking air from both sides of the housing — equivalently, two impellers working in parallel on one shaft. Both acronym orders are industry-standard and mean the same construction.

This distinction is not cosmetic. AMCA Standard 99 treats the two widths as genuinely different configurations, with separate minimum-performance limits for single-width and double-width housed fans, and it even defines the drive side differently: on a single-inlet fan the drive side is opposite the inlet; on a double-inlet fan it is the driver's side. The idea is old, too — fan makers were cataloguing both single-width and “duplex” double-width wheels a century ago.

Where DWDI wins

High-volume, lower-pressure air handling is DWDI's home ground. The double inlet fan is the air-handling-unit builder's staple, and the parallel two-impeller arrangement fits within a lower height than a single-inlet fan of the same duty — often the deciding constraint inside a plenum or AHU casing. If you are choosing an AHU supply and return fan and the box height is fixed, that lower profile is frequently what makes the selection work at all.

How much more air does a double-width wheel move? Substantially more than a single-width wheel of the same diameter at the same speed — but we will not print a folklore multiplier here. The honest answer is a selection answer at your duty point, and that is exactly how it should be quoted. At the heavy-industrial end the double-inlet form reappears as inlet-box machines with the impeller carried between bearings — the arrangement class of large power-plant ID fans.

What you trade for the second inlet

• Bearings and drive. A single-width Arrangement-1 fan carries its bearings out of the airstream. On a double-inlet unit, rotation is read from the fixed-bearing end and the drive sits on one of the two inlet sides — a different maintenance and alignment picture.
• Control. A double-width fan pairs two inlet dampers on one control arm and shaft. Both must open and close together, or the wheel sees lopsided flow. If you throttle at the inlet — see our note on damper, IGV and VFD control — that shared linkage matters.
• Starting. A steel DWDI wheel is the heavy-impeller case. Check motor starting time against wheel inertia, not just running power; a larger motor than the duty bhp may be needed to bring it up to speed.
• Inlets. A DWDI breathes from both sides, so both sides need a clean approach. A starved inlet on one side is a hidden performance tax that no curve on paper will warn you about.

When single-inlet is the right answer

Hot and dirty duty pulls the other way. Above roughly 300 °F, double-width practice sets the axial overlap asymmetrically — about twice as much on the drive side as the floating side — because the rotor grows toward the free end. Heavy hot or abrasive service generally stays with single-inlet builds (or inlet-box double-inlet machines), which is why the dirty-duty workhorses in cement, steel and power are overwhelmingly single-inlet. If your fan runs hot from cold iron, the constraints in hot-start furnace fan design dominate the width question.

Three questions for your next high-volume fan

• Is the constraint flow in a tight space (DWDI territory) or dirty/hot gas at pressure (single-inlet territory)?
• If DWDI: how are both inlet approaches kept clean, and are the paired dampers driven by one linkage?
• Has the motor been checked for starting time against wheel inertia, not just for running kW?

Answer those honestly and the width chooses itself. Get the duty right first, though — the width question only makes sense once you have specified the duty point and settled on the wheel.

We build both configurations and select between them at your stated duty, space and gas conditions — not by habit. State the installation honestly and the width question gets answered by engineering.

Talk to us about single vs double inlet →

Jitamitra Electro Engineering · Fan-engineering notes, written for the engineer.