Cooling Systems and Airflow Management
What this domain tests
Cooling is where most efficiency opportunity lives. Questions test your grasp of airflow physics, containment, setpoints, and the trade-offs between air- and liquid-based approaches.
Core concepts to know:
- Airflow management: hot-aisle/cold-aisle layout, blanking panels, brush seals, and containment (hot or cold aisle). Bypass airflow and recirculation are the two failure modes; containment addresses both.
- Temperature setpoints and ASHRAE guidance: the allowable and recommended envelopes for inlet temperature and humidity. Raising the supply setpoint within the recommended range reduces compressor lift and economizer hours.
- Economizers: air-side and water-side free cooling; effectiveness depends on climate and hours available.
- Part-load performance: chillers, CRAH fans, and pumps follow affinity laws - fan power scales roughly with the cube of speed, so variable-speed control yields large savings.
- Liquid cooling: direct-to-chip and immersion for high-density racks where air cannot economically remove heat.
- Airflow metrics: rack inlet temperature uniformity, return air temperature, and CFM per kW.
Traps to watch:
- Recommending setpoint increases without checking that all rack inlets stay within the recommended envelope - the coldest inlet is not the constraint, the hottest is.
- Forgetting that containment must be paired with blanking and grommets to be effective.
- Assuming economizer hours from outdoor temperature alone, ignoring humidity limits.
- Treating fan affinity as linear rather than cubic.
Worked example
A CRAH fan runs at 100% and draws 10 kW. Reducing speed to 80% via VFD control gives power ≈ 10 × (0.8)³ = 5.1 kW, roughly half. That is the affinity-law rule of thumb: an 80% speed cut saves about 50% of fan power.