High-Density Racks After a Supply-Air Temperature Reset
A 200 kW IT hall has 22 racks on a raised floor: 18 racks average 8 kW and four racks at 14 kW. Cooling is provided by four perimeter CRAH units with constant-speed fans and no containment. The BMS shows 20°C supply air, 30°C return air, and rack inlet temperatures from 22°C at the bottom to 29°C at the top U of the high-density racks. The site lowered the CRAH supply-air setpoint to 16°C. Two days later the top-U temperatures are still 29–30°C, the chilled-water valves are 95% open, CRAH drain pans have standing water, and chilled-water valve bodies are condensing. Room dew point is 14°C. The operator wants to lower the setpoint to 14°C and add a portable fan. The IT load is stable at 200 kW. No airflow measurements have been taken.
1. Why did lowering the supply-air temperature fail to remove the top-U hot spots, and which measurements should be taken before changing the setpoint again?
Read the worked answer
The setpoint change treated a temperature symptom, not the airflow cause. A 14 kW rack needs roughly Q/(ρ cp ΔT) = 14,000/(1.2×1005×12) ≈ 0.97 m³/s, or about 3,500 m³/h, for a 12 K rise. Constant-speed CRAH fans deliver essentially the same rack airflow after the setpoint change, so the top-U recirculation loop persists. Lower supply air also reduces the coil ΔT and can increase valve opening without adding cold-air mass flow to the rack.
Before another setpoint change, measure rack inlet at top, middle and bottom; CRAH supply/return temperatures and airflow; static pressure under the floor and in the room; containment/blanking leakage; and fan curves. If top-U inlet is 29–30°C while supply is 16°C, the problem is recirculation or bypass, not supply temperature.
2. Assess the condensation risk at 16°C supply with a 14°C room dew point, and recommend a safe operating target and immediate actions.
Read the worked answer
At 16°C supply and 14°C room dew point, the margin is only 2 K. The cooling coil and chilled-water valves operate below the dew point, so condensation is expected; standing water in drain pans confirms it. Dropping to 14°C would put supply air at the dew point and make condensation uncontrolled. Set a supply-air target that maintains a practical margin above the measured dew point-typically at least 2–3 K at the coil leaving condition and with duct and valve surfaces above dew point-and restore humidity control.
Immediate actions: stop lowering the setpoint, verify dew-point sensors, clear and trap drains, insulate cold surfaces, repair valve stem seals, and fix airflow/containment so 18–20°C supply can satisfy the racks.