HVAC System Components and Configurations
HVAC System Components and Configurations tests whether you can name the equipment in a data-centre cooling chain, explain how each configuration moves heat from the rack to the atmosphere, and select a sensible one for a given site.
The two air-side families
A CRAC is a self-contained direct-expansion unit: compressor, coil and either an air-cooled condenser or a glycol loop. A CRAH has no compressor - just a coil, fans and valves - and depends on a central chilled-water plant. CRAHs scale well, support waterside free cooling and centralise maintenance; DX units suit small rooms and edge sites where no plant exists.
The central plant chain
Chillers (scroll, screw or centrifugal; air-cooled or water-cooled) reject heat through an air-cooled condenser or an evaporative cooling tower, with primary and secondary pumps, a condenser-water loop, CRAH coils and controls. The heat-rejection choice drives both efficiency and water consumption: towers are more efficient per kW but need water treatment and blowdown; air-cooled condensers avoid water but raise condensing temperature and compressor lift.
Row, rack and chip-level options
In-row coolers interleave with racks; rear-door heat exchangers put a water coil on the rack exhaust (typically 25–35 kW per rack); direct-to-chip cold plates and immersion handle densities air cannot reach. All water-cooled options move the loop closer to the silicon and demand leak detection and water-quality management.
Worked example
A 600 kW coil load with a chiller COP of 5.6 draws 600 / 5.6 = 107 kW at the compressor. Condenser-water pumps and tower fans at roughly 0.15 kW per kW of cooling add 90 kW. Plant total 197 kW over 170.6 tons = 1.16 kW/ton. Note what is missing: CRAH fan power, which both consumes energy and adds heat to the airstream, so the coil load is larger than the IT load.
Traps
- Quoting chiller kW/ton as if it were plant kW/ton.
- Forgetting that fan heat belongs in the coil load.
- Oversizing for future growth so chillers run at 20 % load, where COP collapses.
- Assuming rear-door heat exchangers solve density without any water infrastructure.