Data Centre Cooling Fundamentals
Data Centre Cooling Fundamentals covers the physics every later domain assumes: how IT heat is produced and moved, what temperature and humidity the hardware is entitled to see, and how load, airflow and temperature rise are tied together arithmetically. Exam items here are often short calculations or judgement calls - for example, deciding whether a hot spot is a capacity shortfall or an airflow defect.
Heat in a white space is almost purely sensible
IT equipment converts essentially all electrical input into heat and adds no moisture. Cooling coils therefore run dry, the sensible heat ratio is close to 1.0, and any latent load (ventilation, people, door infiltration, a leaking humidifier) is an exception that must be accounted for explicitly. The practical consequence is that refrigerating the air matters far less than delivering the right mass of cool air to the right place.
The ASHRAE envelope sets the target
ASHRAE TC 9.9 publishes a recommended envelope - about 18–27 °C rack inlet dry-bulb, dew point 5.5–15 °C, 60 % RH maximum - inside a wider allowable envelope for Class A1 hardware of roughly 15–32 °C with dew point between −12 °C and 17 °C. Design to the recommended envelope; treat the allowable range as short excursions or documented trade-offs, not as setpoints.
Load, airflow and ΔT are locked together
Metric: Q (kW) = 1.21 × V̇ (m³/s) × ΔT (K). Imperial: Btu/h = 1.08 × CFM × ΔT (°F). Typical designs run an 11–14 K air temperature rise, roughly 100–160 CFM per kW of IT load. Mass flow is what removes heat; volume flow is only how you buy it.
Worked example
A 150 kW row is supplied at 20 °C and returns at 32 °C. V̇ = 150 / (1.21 × 12) = 10.3 m³/s, about 37,200 m³/h or 21,900 CFM. Now suppose containment defects let cold air bypass the racks, so the temperature rise actually across the IT falls to 8 K. The same 150 kW then requires 15.5 m³/s - half as much air again - or the racks run hot. This is why airflow repairs usually beat added capacity.
Traps that cost marks
- Sizing like comfort cooling, with latent capacity and a low room setpoint.
- Quoting allowable-envelope values as normal operating conditions.
- Confusing CRAH supply temperature with rack inlet temperature.
- Dropping supply temperature when the real fault is recirculation.