In centrifugal fan datasheets and RFQs, two acronyms appear constantly: SISW and DIDW. Buyers who confuse them often compare fans that were never intended for the same duty. This guide defines both terms, then walks through a real case comparison using two SUNXFAN 230 VAC EC fans with almost identical airflow — but a 205 Pa gap in static pressure that comes almost entirely from the inlet configuration.
Quick Answer: SISW vs DIDW in One Paragraph
SISW (single inlet, single width) centrifugal fans take air in through one side and discharge at the top or side — compact, ideal for embedded and space-limited cooling. DIDW (double inlet, double width) fans draw air from both sides of the motor and discharge forward — with twice the inlet area, the same diameter wheel moves more air against higher static pressure, which is why DIDW dominates AHU and ducted HVAC duty.
What SISW and DIDW Actually Mean
- SISW — Single Inlet, Single Width. The centrifugal fan has one side inlet and the outlet is on the top side of the scroll housing. Air enters through a single throat, and the impeller is as wide as that single inlet needs.
- DIDW — Double Inlet, Double Width. The centrifugal fan has inlets on either side (both sides of the motor) and the outlets are on the front side of the scroll. The impeller is effectively two wheels back to back — double the width, twice the inlet area.
Everything else — backward-curved blade geometry, EC motor technology, scroll function — can be identical. The inlet count and wheel width change how much air the fan can move and how much static pressure it can build.
Case Study: Two 230 VAC EC Fans, Nearly Same Airflow
Both models below are EC external rotor motor fans running directly on 230 V AC mains — same voltage class, similar airflow. The difference is the inlet architecture.
| Specification | SF146C-E092B3-01 (DIDW) | DG200D-E092E3-01 (SISW) |
|---|---|---|
| Inlet / outlet | Double inlet / front discharge | Single inlet / top discharge |
| Motor type | EC, 230 V AC | EC, 230 V AC |
| Airflow (0 Pa) | 1250 m³/h | 1300 m³/h |
| Max static pressure | 705 Pa | 500 Pa |
| Power input | 220 W | 190 W |
| Speed | 1650 r/min | 1200 r/min |
| Noise | 65±2 dB(A) | 64 dB(A) |
| Protection | IP44 | IP54 |
| Impeller | Galvanised sheet steel | PP+GF (galvanised optional) |
| Bearings | NSK ball bearings | NSK ball bearings |
| Speed control | 0-10V/PWM, 0-5V, 0-6.5V, 4-speed | 0-10V/PWM, 0-6.5V, 4-speed |
| Temperature range | -25 °C ~ +60 °C | -25 °C ~ +50 °C |
View the full datasheets: SF146C-E092B3-01 · DG200D-E092E3-01
Why the DIDW Fan Reaches 705 Pa
The double-inlet SF146C-E092B3-01 effectively combines two single-inlet wheels on one shaft. With twice the inlet area, air enters at a lower face velocity, so the fan can sustain a much deeper vacuum in the scroll before performance collapses. The result: at duct resistance above roughly 500 Pa — filters loaded with dust, dense coil stacks, long silencer runs — the DIDW fan still delivers stable airflow while a comparable single-inlet unit would fall off its curve.
That is why double-inlet double-width fans dominate air handling units (AHU), fan coil units, FFU cleanroom modules and data center CRAC systems, where the scroll’s front discharge aligns naturally with rectangular duct connections. If you compare the two intake layouts in more detail, our dual-inlet vs single-inlet guide covers the trade-offs.
Why the SISW Fan Is the Compact Workhorse
The single-inlet DG200D-E092E3-01 gives up 205 Pa of maximum static pressure but wins on integration:
- Narrower frame depth — air enters one side only, so the fan fits equipment where there is no space for a second inlet throat.
- Top-side discharge — easy vertical duct routing in compact cabinets.
- IP54 protection and a PP+GF composite impeller — glass-fibre reinforced polypropylene resists corrosion in humid, coastal or outdoor-air applications where galvanised steel would eventually degrade.
- Lower speed (1200 r/min vs 1650 r/min) for the same airflow class, which eases bearing load and broadband noise management (64 dB(A)).
These traits make SISW fans the default in fresh-air ventilation units, telecom and outdoor cabinets, marine HVAC, EV charging cabinets and medical equipment, where the fan is embedded inside the machine rather than connected to a duct network.
Selection Rules You Can Apply in 30 Seconds
- System resistance above ~500 Pa (filters, coils, long ducts) → choose a DIDW fan like the SF146C family.
- Fan embedded inside equipment, single-sided space → choose a SISW fan like the DG200 family.
- Corrosive or humid airstream → SISW with PP+GF impeller (IP54).
- Rectangular AHU duct connection → DIDW front discharge.
- Both need 0-10V/PWM speed control on 230 V AC → both models qualify.
Key Takeaways
SISW and DIDW describe the inlet and discharge geometry of a centrifugal fan — not its motor or efficiency class. For the same airflow and voltage, the double-inlet design buys you static pressure headroom (705 Pa vs 500 Pa in our case), while the single-inlet design buys you compactness, corrosion resistance and easier top-outlet routing. Decide by measuring your system’s resistance and the space around the fan — not by the acronym alone.
Explore the full families: the SF146C series (double inlet) and the DG200 series (single inlet), both available across AC, DC and EC motor types. For the fundamentals behind inlet configurations, start with our axial vs centrifugal comparison.