RLM series Commercial Centrifugal Plug Fans for HVAC OEM

Customer Requirement

The OEM customer submitted a technical brief for a commercial HVAC system. The project covered four sizes of unhoused centrifugal plug fans. The required static pressure was 700 Pa, while airflow varied by AHU size from 5,000 m³/h to 9,000 m³/h. The available power supply was 380V, 3-phase, 50Hz. The customer provided 2,850 RPM as the reference speed for all the fans. The proposed motor powers were 1.5 kW, 2.2 kW, 2.2 kW, and 3.0 kW.

Operating temperatures ranged from an ambient -20°C up to a demanding 85°C. This thermal range points directly to high-heat commercial or light industrial heat recovery ventilation applications. Noise control was especially important for the 9,000 m³/h model, while bearing load and long-term operation also needed to be considered. The OEM customer also requested new internal model codes for ERP system integration.

Engineering Solution

LONGWELL application engineers evaluated the customer’s initial 2850 RPM uniform speed request and quickly identified a critical aerodynamic mismatch. Running a 450 mm impeller at 2850 RPM to hit 9000 m³/h at 700 Pa generates excessive acoustic resonance. The standard 3.0 kW motor requested by the customer was too small. It would have failed thermally under those physical conditions. Instead of accepting the flawed input, the engineering team decoupled the motor speeds and resized the drives to properly match the specific impeller geometries.

For the 5000 m³/h requirement, the RLM355-R-003 model was selected. It runs at 2762 RPM and pairs with a 3-pole/2 motor drawing 2.38 kW shaft power. To achieve 6000 m³/h within the exact same 355 mm frame size, the RLM355-R-004 was engineered to run faster at 3111 RPM, drawing 3.28 kW shaft power. The 7,000 m³/h duty point required a larger impeller. LONGWELL moved to the 400 mm RLM400-R-003, which reaches the required 700 Pa at 2,608 RPM. Finally, for the maximum 9000 m³/h target, the RLM450-R-004 was deployed. By increasing the diameter to 450 mm, the rotational speed dropped drastically to 1435 RPM using a 4/4 pole configuration.

This specific design choice lowered the shaft power requirement to 3.17 kW. All final models feature ThermoFlex™ compatible materials to safely withstand the required 85°C peak operating temperature without warping.

Technical Specifications

The adjusted shaft power ratings demonstrate the physical reality of moving large volumes of air against high static resistance. Lowering the RPM on the 450 mm model maximized aerodynamic efficiency, reducing electrical draw while maintaining the required 700 Pa pressure boundary.

Technical Documentation

This image details the OEM’s original 2850 RPM requirement matrix.

LONGWELL-RLM-Series-Spec-Sheet.pdf
This comprehensive document contains the AMCA 210-16 verified performance curves, dimensional drawings, and noise spectrum data for all four specified RLM fan models.

Why LONGWELL RLM Series Plug Fans for HVAC OEMs?

• The plug fan design eliminates the traditional volute housing, making it easier to install directly into AHUs with limited internal space.
• The RLM Series covers multiple impeller sizes, allowing engineers to match different airflow requirements. This project selects three plug fan models with diameters of Ø355, Ø400, and Ø450 mm.
• RLM models are available with different impeller sizes, speeds, motor outputs, and control configurations. OEM customers can also define internal model coding based on their own product system. For controls, options include 0–10V, PWM, Modbus, and BACnet, depending on the HVAC platform.
• The series is not limited to a single-fan setup. Multiple RLM fans can be arranged as a FanWall for AHUs and CRAH units, especially when the system needs more airflow or backup capacity.

Explore our full Centrifugal Fans range.

Frequently Asked Questions

Q: Can the RLM plug fans handle an 85°C operating environment?

A: Yes. The RLM series used in this project can operate continuously at an ambient temperature of up to 85°C.

Q: Why wasn’t the original 2850 RPM used for every fan size?

A: The 2850 RPM value was the initial reference value provided by the customer. In fact, this speed does not suit the requirements of the four airflows. The increase in the diameter of the impeller results in changes to the required speed and the load on the motor.

Q: Are the performance curves verified for 380V/50Hz power supplies?

A: All performance data was lab-tested specifically on 380V/3-phase/50Hz circuits. The final selections were matched to the required airflow and 700 Pa static pressure for each AHU size.

Project Item: 7070563566
Project Date: 2026-08-10 03:38:16