CDU Fan Selection: The 92mm vs 120mm Decision
Coolant Distribution Units are the heart of AI liquid cooling infrastructure. They circulate chilled water or dielectric fluid to server cold plates and include internal heat exchangers that must reject heat to room air or building chilled water. The fans inside the CDU cabinet drive airflow across these heat exchangers, and choosing the wrong size fan leads to either inadequate cooling or unnecessary noise and energy consumption.
How CDU Heat Rejection Works
A typical CDU for AI GPU clusters handles primary loop coolant returning from server cold plates at 35–45°C. The CDU heat exchanger transfers this heat to:
- Building chilled water (if using water-side economizer): Fans may be minimal or not needed
- Room air (in adiabatic or air-cooled CDU): Fans are the primary heat rejection mechanism
For air-cooled CDU configurations — common in edge AI deployments and smaller installations — fan selection directly determines maximum heat rejection capacity and CDU operating temperature limits.
92mm Fan Characteristics for CDU Applications
Advantages of 92mm CDU fans:
- High rotational speed (up to 8,000 RPM) enables aggressive PWM speed range
- Compact footprint allows dense fan arrays in limited CDU cabinet space
- Lower cost per unit — beneficial for redundant N+2 configurations
- Faster spin-up response for sudden heat load changes
Limitations:
- Higher noise at maximum speed (54–58 dB(A) at 8,000 RPM)
- Lower maximum airflow per unit (80 CFM) requires more fans for high-capacity CDUs
- Higher vibration at maximum speed can affect bearing life in continuous operation
Best for: CDU stations handling 15–30 kW, compact CDU chassis, edge AI deployments
120mm Fan Characteristics for CDU Applications
Advantages of 120mm CDU fans:
- Higher airflow per unit (110–140 CFM) means fewer fans required for equivalent cooling
- Lower operating RPM (4,500–5,500 RPM) at equivalent airflow reduces noise by 4–6 dB
- Lower vibration extends bearing life in 24/7 continuous operation
- Available in 120×25mm (thin) and 120×38mm (thick) variants for different depth constraints
Limitations:
- Larger footprint limits density in compact CDU cabinets
- Higher per-unit cost
Best for: CDU stations handling 40–100 kW, primary loop CDU installations, data center deployments with strict acoustic limits
Performance Comparison Table
| Specification | 92×25mm | 120×25mm | 120×38mm |
|---|---|---|---|
| Max RPM | 8,000 | 5,500 | 4,500 |
| Max Airflow | 80 CFM | 110 CFM | 140 CFM |
| Static Pressure | 350 Pa | 380 Pa | 420 Pa |
| Noise (max) | 54 dB(A) | 48 dB(A) | 45 dB(A) |
| Voltage | 48V DC | 48V DC | 48V DC |
| Control | 4-pin PWM | 4-pin PWM | 4-pin PWM |
| MTBF | ≥50,000 h | ≥50,000 h | ≥50,000 h |
| Best CDU Size | 15–30 kW | 40–60 kW | 60–100 kW |
CDU Fan Configuration for AI GPU Clusters
AI GPU clusters (NVIDIA DGX SuperPOD, Meta RSC) typically use CDU stations sized at 100–200 kW per unit. For a 100 kW air-cooled CDU:
- Using 120×38mm fans (140 CFM each): 6 fans + 2 spare = 8-fan array
- Using 120×25mm fans (110 CFM each): 8 fans + 2 spare = 10-fan array
- Using 92×25mm fans (80 CFM each): 12 fans + 4 spare = 16-fan array
For large installations, 172×51mm fans (280–350 CFM each) provide even higher airflow per unit and are recommended for CDU stations above 100 kW — see the 48V DC cooling fans guide for larger format options.
CDU Fan Samples from Fengheng Technology
92mm and 120mm CDU fans with 48V DC input, PWM control, and MTBF ≥50,000h. Custom configurations available. Sample lead time 2 weeks. Contact [email protected] or view full portfolio →