Engineering a 1585mm Replacement Impeller for a Dry Cooler

Matching Specs for a Diesel Generator Radiator

A customer asked us for a replacement dry cooler fan impeller. The unit cools a large diesel generator. First, we looked at the system design. It uses four 1600 mm nozzle rings side by side.

Together, they reject 1810 kW of heat. Consequently, the working conditions are very tough. The gear sits outside all year. It faces direct sun, heavy rain, and big temperature swings.

Furthermore, the air inside gets up to 80°C. Standard plastic parts would warp or fail fast here. Therefore, the specs called for a tough 1585 mm axial rotor. This part sits on an 11 kW, 400V motor.

It spins all day at 965 RPM. Next, the site has strict noise rules. The sound must stay under 81 dB(A) at one meter. Any new part had to match the old blades perfectly.

Otherwise, the noise would be too loud. Meanwhile, the motor needs a special aluminum hub. It holds nine blades at a 24-degree angle. First, we read the operating manual.

Next, we checked the field photos. We saw the 7.5 mm blade root. We also noted the strong PAG plastic material. Ultimately, the new part must test to AMCA 210-16 methods. It must also deliver the same 65,225 m3/h airflow as the old one.

Engineering the dry cooler fan impeller

We looked at two ways to fix this dry cooler fan impeller problem. First, we thought about sending a whole new fan unit. This would group the blades with a new 11 kW motor. However, we rejected that idea. The customer’s old 400V motors still worked fine. A full assembly would add materially higher cost. It would also make the site work much harder. Instead, we chose to replace just the rotor. We matched the exact Multi-Wing 1585/4-8/24°/PAG/9WL/TP 7.5mm spec from the drawings.

Choosing Material and Pitch

Next, we had to pick the blade plastic and hub shape. We checked the heat load first. It hits 80°C when the generator runs hard. Consequently, we rejected basic plastic. Instead, we picked Polyamide Glass-reinforced (PAG) plastic. We used this for all nine blades. PAG stays stiff at 80°C. It also fights off sun damage well. Furthermore, the system needed a 24-degree blade pitch. A steeper pitch moves more air. However, it also pulls more than the 8.59 kW power limit. It would break the 81 dB(A) noise rule too. Therefore, we kept the exact 24-degree angle.

Design Trade-offs and Final Specs

Every design choice has a physical cost. We picked a heavy cast aluminum hub and nine PAG blades. Thus, the new rotor is quite heavy. Consequently, the shipping weight goes up. The motor also works harder during startup to spin the heavy mass. Even so, this trade-off makes sense. It gives the long life needed for diesel cooling. Finally, we built a 1585 mm rotor with nine PAG blades. We balanced it perfectly for 965 RPM. In short, this new part dropped right onto the old motor shaft. It kept the exact same air flow and strict noise limits.

Technical Specifications

Here, the exact blade pitch and materials decide if the unit works. Specifically, the 24-degree angle and PAG plastic ensure success. They move the right amount of air without crossing the 8.59 kW power limit.

Reference: AMCA 210 covers the test method behind these figures.

Technical Documentation

Dry Cooler Operating Manual This manual lists the heat loads and power limits for the generator.
This photo shows the old aluminum hub and blade root setup.
Field workers use this check to confirm the 620 mm blade length.

How to Specify a dry cooler fan impeller

First, replacing a huge fan rotor requires exact parts. You must match the old air flow to protect the motor. Therefore, take these practical steps before you ask for a quote:

• Measure the hub bore and keyway with a tight tool. Consequently, the new metal hub will slide right onto your motor shaft. This prevents extra machine work on site.
• Write down the exact blade count, plastic type, and pitch angle. By contrast, guessing the pitch angle causes big problems. It often draws too much power or drops the air pressure.
• Check your top heat limits. For outdoor generators hitting 80°C, always ask for PAG plastic or metal. Standard plastics will melt or bend at high heat.

In short, we build these PAG and metal rotor parts directly in our own plant. This makes our lead times much faster than standard supply chains.
Related: Commercial Fan Blowers.

Common Questions About the dry cooler fan impeller

Browse our axial fans range, the AC axial fans section, or EC axial fans.