Overview
This lecture covers the unique HVAC cooling requirements of data centers, explores air and liquid cooling strategies, and details key data center layout and air management techniques to maximize energy efficiency.
HVAC Cooling Needs in Data Centers
- Data centers cool IT equipment (ITE), which emits much more heat than people.
- A person emits 400–450 BTUs/hour, while one rack of IT equipment emits 17,060–102,360 BTUs/hour.
- Data centers operate 24/7/365, creating continuous, high heat loads.
Air-Cooled vs. Liquid-Cooled Racks
- Air-cooled racks use cold air from the front, which absorbs heat and exits hot at the back.
- Blanking plates and covered gaps prevent cold and hot air mixing, optimizing airflow.
- Liquid cooling is more efficient for power densities between 5kW–80kW; air-cooled racks handle 1kW–5kW.
- Liquid-cooled designs include integral coils, rear door heat exchangers, liquid on-board, and immersion cooling.
- Liquid has up to 4x the heat capacity of air.
Data Center Layouts & Air Management
- Racks are set in rows with aisles designated as “hot” (receiving exhaust) or “cold” (receiving supply air).
- Raised floors create an underfloor supply air plenum to deliver cold air via perforated tiles.
Cooling Strategies: Room, Row, and Rack
- Room-based: Cold air supplied to the entire room, but mixing can reduce efficiency.
- Air management involves separating cold supply air from hot return air to improve efficiency.
- Aisle containment (hot or cold) prevents air mixing, increases efficiency, and avoids hot spots.
- Cold aisle containment: Cold air is contained, delivered directly to rack inlets; hot air is uncontained.
- Hot aisle containment: Hot air is contained and returned directly to HVAC; cold air is allowed to circulate.
In-Row and In-Rack Cooling Solutions
- In-row cooling units sit between racks, cooling hot aisle air and delivering it to the cold aisle.
- These units handle variable rack loads and can be installed overhead, on the floor, or under raised floors.
- In-rack cooling dedicates cooling to a single rack, often using heat exchangers or self-contained AC units.
- Chilled water improves performance in high-density setups.
Cooling Distribution Units (CDUs)
- CDUs separate rack liquid cooling loops from outdoor heat rejection (e.g., cooling towers).
- CDUs have redundant pumps and provide water above dew point to avoid condensation.
- CDUs avoid refrigeration equipment, using dry coolers or towers for energy-efficient operation.
- Small water volumes reduce risk in case of leaks.
Key Terms & Definitions
- BTU (British Thermal Unit) — A unit of heat energy.
- Raised Floor Plenum — Underfloor space used to distribute cold supply air.
- Blanking Plate — Panels used to block airflow through unused rack spaces.
- Aisle Containment — Physical barriers to prevent mixing of hot and cold air in aisles.
- CRAC (Computer Room Air Conditioner) — DX-cooled HVAC unit for data centers.
- CRAH (Computer Room Air Handler) — Chilled-water-based air handler.
- CDU (Cooling Distribution Unit) — Device that transfers heat from rack cooling loops to facility cooling.
Action Items / Next Steps
- Review assigned reading on air management and containment strategies in data centers.
- Study cooling system diagrams and layouts for upcoming quiz.