A second inlet roughly doubles flow at the same wheel and speed. When that's the right answer — and when it isn't.
Reviewed by Jitamitra application engineering
Share on LinkedInTwo 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.
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.
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.
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.
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.
Sources & basis. AMCA Standard 99 (distinct single- and double-width configurations, separate performance limits, and drive-side definitions); vendor engineering literature on Arrangement-1 bearings, paired inlet dampers, heavy-impeller starting checks, and hot-gas axial-overlap practice; and a 1927 public-domain fan catalogue for the single-width and "duplex" double-width wheel history. The extra capacity of a double-width wheel over a same-diameter SWSI wheel is quoted as a selection result at your stated duty point, not as a fixed multiplier.
Two constructions, written two ways. SWSI — also written 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.
A double-width wheel moves substantially more air than a single-width wheel of the same diameter at the same speed, and the page puts it as roughly doubling the flow, but the figure you design to is a selection answer, not a multiplier applied blind. The honest answer is a selection answer at your duty point, and that is exactly how it should be quoted — as a selection result, not a fixed number.
That is DWDI's home ground. High-volume, lower-pressure air handling suits the double inlet fan, 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. With the box height fixed, that lower profile frequently makes the selection work at all.
Bearings, control and inlet access. 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 from a single-width Arrangement-1 fan, which carries its bearings out of the airstream. Two inlet dampers share one control arm and shaft, and both must open and close together or the wheel sees lopsided flow.
Usually not. 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. 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.
Not on its own. A steel DWDI wheel is the heavy-impeller case, so 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. And remember both inlets need a clean approach: a starved inlet on one side is a hidden performance tax no curve on paper will warn you about.
Flow, static, gas temperature, application — or attach a spec, GA drawing or a multi-fan schedule. Engineer to engineer.
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