Custom 310mm Cross-Flow Impeller for Commercial HVAC Units

What the HVAC OEM specified

An HVAC OEM building compact fan coil units needed a cross-flow impeller 310mm long. They sent a drawing for a similar part at 700mm, and asked for the same 180mm diameter in a much shorter length.

Shorter was the whole point. The new chassis is smaller, yet the airflow could not drop to match.

The brief set some firm quality limits.

• Airflow between 1000 and 1500 CFM, roughly 1700 to 2550 m3/h.
• Residual unbalance no greater than 0.3g when tested at 1200 RPM.
• No abnormal noise anywhere up to 1600 RPM.
• A standard 12mm steel shaft end, since the motor was already chosen.
• Enough consistency that the finished unit passes EN 60335 first time.

That 0.3g figure tells you where these units end up. Namely offices, hotels and shops, where a faint buzz becomes a complaint long before anyone measures it.

Re-engineering the cross-flow impeller

This was a change rather than a clean sheet, so it entered our NPI process as light development. Even so, it was never just a matter of making the part shorter.

A shorter cross-flow impeller behaves differently. Its stiffness, its natural frequencies and its balance all shift. Consequently, scaling the drawing down would not have worked.

Choosing the material

We looked at aluminum first, for stiffness. Then we rejected it. Metal rings more readily, which raises noise. Furthermore, the fine blade shape makes aluminum costly and awkward to build at volume.

A cheap plastic such as polypropylene went the other way. It is quiet and low cost, yet far too bendy. At 1600 RPM the blades would flex, and flex becomes vibration.

Here the answer was AS+GF30%, acrylonitrile styrene with 30% glass fiber. It is stiff enough to stop blade flutter, steady in shape across HVAC temperatures, and quieter than metal. The glass content supplies the stiffness.

Making the balance figure repeatable

Meeting 0.3g once is not hard. However, meeting it on every unit is a making problem rather than a design one.

The drawing therefore calls for ultrasonic welding to join blades to end rings. It gives an even bond with no added glue weight. Glue weight, in turn, is exactly the small variation that ruins a tight balance figure.

Since the shorter length changes the dynamics, the team also wrote a new multi-stage balancing routine. It allows up to three small clips per impeller to reach the target.

Keeping the tooling cost down

Meanwhile the approved design, LWPI-φ180×310-01, reuses the existing 180mm blade tooling. Only the assembly jigs are new, and those are length-specific. Consequently both the development time and the tooling charge stayed low.

Technical Specifications

The balancing line below shaped this whole cross-flow impeller project. Everything else follows from holding it repeatably.

Technical Documentation

Here the general assembly drawing, LWPI-φ180×310-01, provides mechanical engineers with critical dimensions, material specifications, and assembly tolerances for system integration.

Download the complete technical specification sheet for detailed manufacturing requirements, including runout tolerances, balancing notes, and material callouts needed by quality assurance teams.

Open Technical Specification: LWPI-φ180×310-01 (PDF)

Why the OEM chose us for the tooling

Custom impellers usually stall on lead time, tooling cost or proof of quality. In this case, four things dealt with all three.

• A modification inside 90 days. Changing an existing diameter to a new length fits our NPI process well. imported European brands often ask for 12 to 18 months and a substantial order commitment for comparable tooling work.
• Tooling reused rather than repeated. Keeping the 180mm blade profile, and buying only new jigs, kept the NRE charge small. Together with Ningbo production, the finished cross-flow impeller sits well below a European custom part.
• Every unit balanced, not a sample. Under our ISO 9001 system we balance 100% of these impellers after assembly and log the results. That is what makes a 0.3g figure mean anything.
• Airflow figures from a real lab. The 1000 to 1500 CFM reference is measured in our on-site lab to AMCA 210/211 and ISO 5801 methods, so the OEM can build a system model on it.

Browse our cross-flow impeller range, or the cross-flow fan series for complete assemblies. Balance quality grades are set out in ISO 21940.

Questions about custom impellers

What are the lead time and MOQ for a custom impeller like this?

A cross-flow impeller change of this kind gives first article samples inside 90 days from specification lock. Once approved, mass production runs 25 to 35 days. MOQ for a new custom part at this size is typically 50 to 100 pieces, and lower quantities are possible on repeat orders under a supply agreement.

Is AS+GF30% suitable for a return air plenum?

Yes, within normal HVAC conditions of roughly 0C to 40C, where it resists creep and holds its shape. However, it does not carry our ThermoFlex rating for extremes. Below -20C or above +60C we would suggest a different polymer, or an aluminum build instead.

What balance grade does 0.3g at 1200 RPM represent?

It is considerably tighter than the G6.3 grade in ISO 21940 that standard commercial fans use, and it approaches G2.5. In practice that means low transmitted vibration and low noise. Every unit is individually balanced and logged rather than sampled.

Why does a shorter impeller need a new balancing protocol?

Because unbalance acts as a moment along the length. Shortening the rotor changes how a mass error at one end feeds into the bearings. It moves the natural frequencies too. Consequently a routine written for a 700mm part will not hold 0.3g on a 310mm one.