Containment Retrofit on a 12 kW/Rack Row
Two rows of 15 racks each (30 racks, 12 kW average, 360 kW total) use hot-aisle containment. Seven of eight CRAH units run; each delivers 10,000 m³/h at 100% fan speed. Design supply air is 20°C and the CRAH units are controlled to a 19°C supply-air setpoint. Rack inlet temperatures range from 27°C at the bottom to 31°C at the top U. Return air in the hot aisle is 34°C, containment doors are propped open, several blanking panels are missing, and cable cutouts are unsealed. Hot-aisle static pressure is +6 Pa relative to the room, and CRAH fan speeds are at 100%. The eighth CRAH is available but the electrical panel has only 30 kW spare capacity; its fan draws 7.5 kW. The site wants to start the spare CRAH and raise fan speed. No airflow traverse has been performed.
1. Calculate the total required airflow for 360 kW at a 12 K rise, compare it with the current 70,000 m³/h, and estimate the actual rise.
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Required airflow at 360 kW and 12 K is 360,000/(1.2×1005×12) = 24.9 m³/s ≈ 89,500 m³/h. Seven CRAHs at 10,000 m³/h provide 70,000 m³/h; eight provide 80,000 m³/h, still below 89,500. At 70,000 m³/h the actual rise is 360,000/(1.2×1005×(70,000/3600)) ≈ 15.3 K, consistent with 19°C supply and about 34°C return. The top-U 31°C inlets are therefore a combination of insufficient total airflow and recirculation or bypass from the hot aisle. Starting the eighth CRAH helps but does not close the gap by itself; sealing containment and blanking is required to make the existing airflow effective.
2. Diagnose the containment leakage symptoms and prioritize corrective actions.
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The propped doors, missing blanking panels, and open cable cutouts are direct leakage paths. With hot aisle at +6 Pa relative to the room, hot air escapes and is drawn back to rack fronts, raising top-U inlet temperatures. Prioritize sealing: close and alarm doors, install blanking panels, seal cable openings, repair door gaskets, and verify pressure direction. Measure hot-aisle pressure, rack inlet profiles, CRAH return temperatures, and airflow after each fix. Do not chase fan speed until leakage is corrected; more fan energy into a leaky containment may just increase bypass.
3. Decide whether to start the spare CRAH, raise fan speed, or fix containment first, considering the 30 kW spare capacity.
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The spare CRAH draws 7.5 kW, well within the 30 kW spare, so starting it is electrically feasible and adds 10,000 m³/h. But total capacity would still be about 80,000 m³/h against 89,500 m³/h required, and containment leakage would waste much of it. First seal containment and blanking, then start the spare CRAH and re-measure. Raise fan speed only if rack inlets remain above the target and the fans have headroom. If the load is expected to grow, plan additional CRAH capacity or higher-density cooling rather than relying on 100% fan speed.