Cooling and Environmental Control
What the domain tests
Cooling and Environmental Control tests the airflow and environmental envelope side of the facility: ASHRAE TC 9.9 recommended and allowable ranges, airflow management, containment, delta-T, humidity control, PUE and liquid cooling. Expect calculations using heat, airflow and temperature difference, plus questions on where recirculation and bypass destroy capacity.
The environmental envelope
ASHRAE TC 9.9 defines recommended and allowable ranges for IT equipment inlet air. Recommended conditions are roughly 18-27 C and a humidity range defined by dew point and relative humidity limits; the allowable A1 envelope extends to about 15-32 C. Operating inside the allowable but outside the recommended range raises failure rates and may void warranties. Humidity control is specified by dew point and relative humidity together, because relative humidity alone does not describe condensation risk.
Airflow management
Cold aisle and hot aisle layout, blanking panels, brush seals, raised-floor tile tuning and containment (cold or hot aisle) reduce mixing. Key metrics: supply and return temperatures, delta-T across the IT equipment, bypass airflow and recirculation index. Design airflow follows Q = P / (rho x cp x deltaT). For air, a practical form is CFM = 3.16 x kW / deltaT in degrees Fahrenheit, or about 160 CFM per kW at a 20 F rise.
Efficiency
PUE = total facility power / IT power. It is a ratio measured at a specific load and ambient, so a design PUE and an annualised PUE are different claims. Partial load usually worsens PUE because fixed losses dominate. Free cooling, higher supply temperatures, VSD fans and variable-speed pumps all improve it.
Worked example
A hall draws 200 kW of IT load with a 10 K air temperature rise. Volumetric airflow = 200 / (1.2 x 1.005 x 10) = 16.6 m3/s, about 35,000 CFM. If the installed CRAH capacity delivers only 25,000 CFM, the actual rise becomes about 14 K, pushing return air and rack inlet temperatures up unless supply temperature is lowered or bypass is eliminated. Adding containment and blanking panels recovers the missing airflow more cheaply than adding units.
Traps that cost marks
Using nameplate kW rather than measured IT load; confusing relative humidity with dew point; quoting a design PUE as an annual figure; assuming containment alone fixes bypass; and forgetting altitude derating for fans and coils.