How to Choose the Right Cross Flow Fan Size: 30mm to 150mm Selection Guide
You have a chassis drawing, a target airflow, a noise ceiling, and a power supply. The first question on the drawing board is not “which brand?” or “AC or EC?” — it is what impeller diameter. Get that wrong and everything downstream (motor selection, housing design, P-Q curve matching) is wasted effort. This guide walks you through the full 30 mm to 150 mm cross flow fan diameter range with real catalog data, so you can narrow from 13 options to one in an afternoon.
We use Longwell (Ningbo Longwell Electric Technology) as the reference catalog because their LWCA / LWCD / LWCE families cover the entire 30–150 mm spectrum with consistent type coding. The same methodology applies to any manufacturer’s tangential blower line.
Why Diameter Is the First Decision
In a cross flow fan, the impeller diameter sets three things that cannot be compensated for later:
- The pressure ceiling. A 30 mm impeller will never push past ~15 Pa of static pressure, no matter how fast you spin it. A 150 mm impeller can reach 200+ Pa. If your system resistance curve demands 120 Pa at the operating point, you can eliminate everything below 90 mm before you open a datasheet.
- The maximum airflow per unit length. Airflow scales roughly with the square of the diameter at constant speed and length. A 120 mm impeller delivers about 4× the airflow per millimetre of length compared to a 60 mm impeller.
- The blade count and impeller geometry. Smaller diameters use fewer, shallower blades (20 blades at 30 mm vs. 50 blades at 150 mm in Longwell’s standard impeller line). This changes the noise spectrum and the stable operating range.
Impeller length, motor type, and housing design all matter — but they are downstream of the diameter decision. Pick diameter first.
The Diameter Spectrum at a Glance
Here is the full range from Longwell’s standard impeller catalogue, mapping each diameter to its blade count, available lengths, and typical airflow class. Numbers are from the current catalog and P-Q data sheets; always confirm against the latest revision.
| Diameter (mm) | Blade count | Available lengths (mm) | Typical airflow range | Typical pressure class | Typical noise range |
|---|---|---|---|---|---|
| 30 | 20 | 90–854 | 23–221 m³/h | 8–16 Pa | 25–31 dB(A) |
| 43 | 20 | 135–300 | 40–90 m³/h | 10–20 Pa | 28–34 dB(A) |
| 45 | 20 | 145–360 | 50–110 m³/h | 10–22 Pa | 28–35 dB(A) |
| 52 | 20 | 141–360 | 60–130 m³/h | 12–25 Pa | 30–37 dB(A) |
| 60 | 22–24 | 180–480 | 80–200 m³/h | 15–30 Pa | 32–40 dB(A) |
| 65 | 22 | 145–600 | 100–250 m³/h | 15–35 Pa | 34–42 dB(A) |
| 71 | 25 | 145–800 | 120–320 m³/h | 18–40 Pa | 35–44 dB(A) |
| 81 | 30 | 145–800 | 150–420 m³/h | 20–50 Pa | 38–48 dB(A) |
| 90 | 35 | 360–1000 | 260–500 m³/h | 21–30 Pa | 42–45 dB(A) |
| 100 | 40 | 360–1050 | 700–1500 m³/h | 95–203 Pa | 55–61 dB(A) |
| 110 | 45 | 360–1000 | 900–1750 m³/h | 128–255 Pa | 61–68 dB(A) |
| 120 | 48 | 360–1050 | 1000–2000 m³/h | 68–161 Pa | 63–72 dB(A) |
| 150 | 50 | 360–1050 | 1200–4000 m³/h | 52–135 Pa | 69–75 dB(A) |
Read that table as a funnel. If your system needs 1,500 m³/h, you are looking at 100 mm or above. If you need 45 dB(A) maximum, you are looking at 90 mm or below (or a larger diameter de-rated). The intersection of airflow and noise often narrows the field to two or three diameters.
Small Diameter (30–60 mm): Compact Appliances and Electronics
The 30–60 mm range is where cross flow fans are irreplaceable. No other fan type delivers a 100–480 mm wide air curtain from a chassis that is only 40–60 mm deep. This is the domain of electric fireplaces, microwave ovens, dishwashers, air purifiers, and small floor convectors.
Key characteristics:
- Very low noise. The LWCA-30 series operates at 25–31 dB(A) — quieter than a whisper at 1 metre. This is why electric fireplace manufacturers almost universally specify 30 mm cross flow fans.
- Low pressure, low power. Static pressure tops out around 15–30 Pa, and motor power is typically 8–15 W. These fans are designed to move air through an open or lightly-filtered path, not against system resistance.
- Voltage flexibility. The 30 mm series is available in 115 V, 230 V (AC), and 12 V / 24 V (DC), which covers the global appliance market. The model code tells you:
LWCA-30150SN-07means 30 mm diameter, 150 mm length, 230 V (S), with safety grid (N = without), series 07. - Length is the airflow lever. At 30 mm diameter, doubling the impeller length from 150 mm to 300 mm nearly doubles the airflow (38 → 59 m³/h) without changing the diameter, motor type, or housing depth. This is the fundamental scaling rule for small cross flow fans.
Where to use 30–60 mm: Electric fireplaces, microwave oven cooling, small air purifiers, compact floor convectors (trench heating under windows in residential buildings), and electronic enclosure cooling where the chassis depth is under 80 mm.
Medium Diameter (65–90 mm): HVAC Indoor Units and Trench Heaters
The 65–90 mm range is the heart of residential and light-commercial HVAC. Split air conditioner indoor units, fan coil units, trench heaters, and elevator cabin ventilation all live here.
Key characteristics:
- The “comfort noise” sweet spot. At 35–48 dB(A), these fans are in the range that building codes and comfort standards target for occupied spaces. The LWE-90 elevator series (Capacitor Induction motor, 110/220 V, ball bearing, 30,000 h service life) delivers 260–440 m³/h at 42–48 dB(A) — exactly what an elevator cabin needs.
- Moderate pressure. 15–50 Pa is enough to push air through a coarse filter, a short duct, or a heat exchanger coil. This is where cross flow fans start to compete with small centrifugal blowers, and usually win on noise and chassis depth.
- Motor variety. This range is where all three motor technologies are viable: AC shaded pole (LWCA-65 to LWCA-90), DC brushless (LWCD-60, LWCD-65), and EC (LWCE-80, LWCE-90). The motor choice is driven by control, efficiency, and regulatory requirements — see our AC vs DC vs EC guide for the decision tree.
- Length-to-airflow scaling still strong. The LWCE-80 series scales from 350 mm / 460 m³/h to 820 mm / 1,200 m³/h — a 2.3× length increase yields a 2.6× airflow increase, slightly super-linear because the end losses become a smaller fraction of total flow.
Where to use 65–90 mm: Split AC indoor units (1.0–3.5 kW class), fan coil units, trench heaters and floor convectors (1–3 m length), elevator cabin ventilation, air purifiers for rooms up to 40 m², and display case cooling in supermarkets.
Large Diameter (100–150 mm): Air Curtains, AHUs, and Industrial
The 100–150 mm range is where cross flow fans deliver serious volumetric flow. Air curtains over commercial doorways, air handling units, FFU (fan filter units) in cleanrooms, and industrial heating equipment all specify in this range.
Key characteristics:
- High airflow, higher pressure. The LWCA-150 series reaches 4,000 m³/h at 135 Pa, and the LWCE-150 double-blower configuration pushes 3,700 m³/h. This is enough to create an effective air curtain over a 1.5 m wide, 3 m tall commercial doorway.
- Single and double blower options. Above 120 mm, manufacturers offer single and double impeller configurations. A double blower roughly doubles airflow and pressure for a marginal increase in housing depth. Choose double when door height exceeds 2.5 m or when the system resistance is higher than a single impeller can overcome.
- EC dominance at this size. The LWCE-100 to LWCE-150 EC series is the recommended choice for continuous-duty commercial applications. The integrated 0–10 V / PWM control, FG signal output, and 80–92% efficiency make it the default for energy-regulated markets. Ball bearings are standard; L10 life is 40,000 h at 25 °C.
- Noise is a design constraint, not a feature. At 55–75 dB(A), these fans are not for living spaces. They belong in commercial doorways, equipment rooms, and ducted systems where ambient noise masks the fan. If you need 1,500 m³/h at 45 dB(A), you need a different approach — typically two 90 mm fans in parallel rather than one 120 mm.
Where to use 100–150 mm: Air curtains (commercial doorways 0.9–2.0 m wide), air handling units, FFU / cleanroom modules, industrial ovens and drying equipment, and large trench heaters in commercial buildings.
Impeller Length: The Scaling Lever You Control
Once you have fixed the diameter, impeller length is your primary airflow adjustment. The relationship is approximately linear: double the length, double the airflow, at the same diameter and speed. This is why cross flow fans are so popular in linear applications — you scale the fan to the width of the opening, not the other way around.
Here is the real scaling data from Longwell’s LWCA-120 AC series (Ø120 mm, 230 V, 1,350 rpm):
| Impeller length (mm) | Airflow (m³/h) | Static pressure (Pa) | Power (W) | Noise dB(A) |
|---|---|---|---|---|
| 380 | 1,000 | 68 | 109 | 63 |
| 560 | 1,380 | 100 | 120 | 65 |
| 680 | 1,560 | 122 | 135 | 67 |
| 780 | 1,780 | 140 | 150 | 71 |
| 900 | 2,000 | 161 | 170 | 72 |
Notice that pressure and power also increase with length — this is because a longer impeller moves more air, which increases the system’s mass flow and the back-pressure the fan must overcome. The scaling is not perfectly linear because of end effects (air leaks from the open ends of the impeller), but at lengths above 300 mm the non-linearity is under 10%.
Practical rule of thumb: For a door or opening of width W (in mm), specify an impeller length of 0.8W to 1.0W. For a 900 mm door, a 780–900 mm impeller is the standard choice. The 10–20% margin ensures the velocity profile covers the full door width without gaps at the edges.
How to Map Your Application to a Diameter: A 6-Step Workflow
If you are starting from a blank sheet, here is the workflow that will get you to a candidate model in under an hour.
Step 1 — Calculate your airflow requirement
Determine the volumetric flow (m³/h or CFM) your application needs. For air exchange: room volume × ACH (air changes per hour). For air curtains: door width × door height × target discharge velocity (typically 6–10 m/s). For cooling: heat load / (ΔT × 1.2 × Cp).
Step 2 — Measure your static pressure budget
Add up the pressure drops across every element the fan must push through: filters, grilles, ducts, heat exchangers, louvers. This is your system resistance curve. If it exceeds 200 Pa, cross flow fans may not be the right choice — see our fan type comparison guide.
Step 3 — Measure your chassis envelope
Record the available width, depth, and height. The depth is usually the binding constraint: a 30 mm impeller needs ~45–55 mm of housing depth; a 120 mm impeller needs ~150–180 mm.
Step 4 — Set your noise ceiling
What is the maximum acceptable noise at 1 m from the product? This eliminates large diameters in occupied spaces and small diameters in high-flow applications. Use the spectrum table above as a first filter.
Step 5 — Cross-reference the diameter spectrum
Overlay your airflow, pressure, depth, and noise constraints on the diameter spectrum table. Usually two or three diameters survive the filter. For each survivor, check whether the available impeller lengths can reach your target airflow without exceeding the noise ceiling.
Step 6 — Select motor technology and request P-Q curves
Once you have a candidate diameter and length, the motor choice (AC / DC / EC) is a separate decision driven by supply, control, and regulation. Request the P-Q curve, dimensional drawing, and STEP file for your candidate model from the manufacturer. Longwell’s engineering support responds within 24 hours with matching documentation.
Real Spec Comparison: Three Diameters, One Application
To make this concrete, here is a side-by-side of three candidate fans for a 900 mm wide air curtain over a 2.2 m tall retail door, requiring approximately 1,500 m³/h at 8 m/s discharge velocity, with a 65 dB(A) noise ceiling.
| Metric | LWCA-100820SN-06 (Ø100) | LWCA-120780SN-06 (Ø120) | LWCE-100820SN-06 (Ø100, EC) |
|---|---|---|---|
| Diameter | 100 mm | 120 mm | 100 mm |
| Length | 820 mm | 780 mm | 820 mm |
| Motor | AC shaded pole | AC shaded pole | EC external rotor |
| Supply | 230 V AC, 50 Hz | 230 V AC, 50 Hz | 230 V AC, integrated drive |
| Max airflow | 1,500 m³/h | 1,780 m³/h | 2,290 m³/h |
| Max static pressure | 203 Pa | 140 Pa | 619 Pa |
| Power | 110 W | 150 W | 229 W |
| Noise | 61 dB(A) | 71 dB(A) | 55 dB(A) |
| Speed control | None (triac optional) | None (triac optional) | 0–10 V / PWM |
| L10 life at 25 °C | 30,000 h | 30,000 h | 40,000 h |
| Best fit | Budget retrofit, no control needed | Higher flow, exceeds noise ceiling | Optimal — meets all constraints |
In this comparison, the Ø120 AC fan exceeds the noise ceiling (71 dB(A) vs. 65 dB(A) limit). The Ø100 AC fan meets airflow and noise but offers no speed control. The Ø100 EC fan is the engineering winner: it meets all four constraints, adds proportional control for demand-based operation, and extends bearing life to 40,000 h. This is the kind of trade-off the diameter-first workflow surfaces early.
Frequently Asked Questions
What is the most common cross flow fan diameter for HVAC applications?
The 90 mm and 120 mm diameters are the most common in residential and light-commercial HVAC. The 90 mm range (LWCE-90, LWCD-90, LWE-90) is the standard for split AC indoor units in the 1.0–2.5 kW class, elevator cabin ventilation, and fan coil units. The 120 mm range (LWCA-120, LWCE-120) is the standard for air curtains over 0.9–1.2 m doorways and for larger trench heaters. If you are designing a new HVAC product and unsure where to start, these two diameters cover 70–80% of residential applications.
Can I use a larger diameter fan at reduced speed to get lower noise?
Yes, and this is a common strategy. A 120 mm impeller running at 60% speed can deliver the same airflow as a 90 mm impeller at full speed, but with a lower noise floor because the blade tip speed is lower and the broadband spectrum is smoother. The trade-off is chassis depth: the 120 mm housing is 50–70 mm deeper than the 90 mm housing. This approach is most common in premium HVAC products where acoustic comfort is a selling point. EC motors make this easy because they support continuous speed control from 0–100% via 0–10 V or PWM.
How does impeller length affect noise?
Impeller length has a much smaller effect on noise than diameter or speed. Doubling the length of a 120 mm impeller from 380 mm to 780 mm increases noise by about 8–9 dB(A) (from 63 to 72 dB(A) in the LWCA-120 series), while doubling the airflow. The noise increase is roughly proportional to the logarithm of the airflow increase, not the linear value. This means a longer impeller is almost always quieter per unit of airflow than running a shorter impeller at higher speed.
What diameter do I need for an air curtain over a 1.5 m door?
For a 1.5 m wide door with a standard 2.2–2.5 m height, a 120 mm or 150 mm diameter impeller is typical. The LWCA-120900SN-06 (Ø120, 900 mm length, 2,000 m³/h, 72 dB(A)) covers a 1.2–1.5 m doorway at 8 m/s discharge. For taller doors (above 2.5 m), a 150 mm double-blower configuration (LWCE-1501200SN-06, 3,700 m³/h) provides the additional throw needed to reach the floor. Always verify the velocity profile at the discharge grille — a uniform 8 m/s across the full door width is more effective than a higher peak velocity with edge gaps.
Are smaller cross flow fans less efficient than larger ones?
Not necessarily, but the motor technology matters more than the diameter. A 30 mm AC shaded-pole fan is indeed inefficient (25–35% at full load), but a 30 mm DC brushless fan can reach 55–65% efficiency. The efficiency difference between a 60 mm and a 120 mm fan of the same motor type is typically under 5 percentage points. The real efficiency lever is motor technology, not diameter. See our AC vs DC vs EC motor guide for the full comparison.
Where to Go From Here
- If you have a specific application in mind: use the 6-step workflow above, then request P-Q curves and STEP files for your candidate models from Longwell’s engineering team.
- If you need to understand the fan type comparison first: our cross flow vs axial vs centrifugal guide covers when a cross flow fan is the right choice vs. the alternatives.
- If you are sizing an air curtain specifically: our air curtain selection and installation guide takes the diameter discussion from this article and applies it to doorway sizing, discharge velocity, and installation depth.