5.1 Heat, ASHRAE Envelopes & Cooling Units

The Data Centre Heat-Load Problem

Almost every watt of electrical power delivered to IT equipment is converted into heat — a server does not store energy, so its power draw and its heat output are effectively equal. A rack drawing 10 kW rejects roughly 10 kW of heat into the room, and modern AI/HPC cabinets can exceed 30-100 kW. The cooling system's job is to remove that heat at the same rate it is produced and reject it outdoors. If removal falls behind generation for even a few minutes, inlet temperatures climb, servers throttle, and thermal-shutdown protection trips.

For the CDCP exam, remember the core chain: heat is generated at the chip → carried away by air or liquid → transferred to a refrigerant or chilled-water loop → rejected to the outside environment. Cooling is normally the largest slice of non-IT energy, which is why it dominates a facility's Power Usage Effectiveness (PUE).

Sensible vs Latent Heat

The exam expects you to distinguish two kinds of heat. Sensible heat changes the dry-bulb temperature of the air — what a thermometer reads. In a data centre the load is overwhelmingly sensible (hot server exhaust), so cooling units are rated on a high Sensible Heat Ratio (SHR), often 0.9-1.0. Latent heat is the energy involved in a phase change — chiefly moisture added to or removed from air (humidification/dehumidification). Over-cooling a coil below the air's dew point condenses water, wasting energy on unwanted latent work and drying the room, which then forces re-humidification. A well-tuned data-centre unit therefore does almost pure sensible cooling and keeps the coil above dew point.

ASHRAE TC 9.9 Thermal Guidelines

The authority on IT inlet conditions is ASHRAE Technical Committee 9.9 (TC 9.9), publisher of the Thermal Guidelines for Data Processing Environments. It defines two kinds of envelope:

• The recommended envelope: server inlet air of 18-27 °C (64.4-80.6 °F) and roughly 20-80% relative humidity. This is the target for normal continuous operation and is identical across all classes A1-A4.
• The allowable envelopes: wider ranges the equipment can tolerate for limited periods, which differ by class.

A very common exam question asks which class has the widest allowable range — the answer is A4 (5-45 °C) — and whether the recommended envelope changes by class — it does not. Designing to the recommended band maximises reliability; briefly using the allowable band unlocks more free-cooling hours. Raising the cold-aisle set point toward the top of the recommended range (24-27 °C) is one of the cheapest efficiency wins because it enables more economizer operation and lets chillers run more efficiently.

CRAC vs CRAH: The Two Room Units

Two acronyms dominate this domain. A CRAC (Computer Room Air Conditioner) is a self-contained refrigeration unit: it contains a compressor and a direct-expansion (DX) refrigerant coil. Warm room air passes over the cold DX coil, refrigerant absorbs the heat, and the compressor pumps it to a condenser outside. A CRAH (Computer Room Air Handler) has no compressor; instead it uses a chilled-water coil fed from a central chiller plant. The exam trap is the direction of the difference: CRAC = DX/refrigerant/compressor; CRAH = chilled water/no compressor. CRAH units are generally more efficient at large scale because one central chiller plant serves many air handlers, whereas each CRAC carries its own compressor.

Chilled Water vs DX, and the Rejection Chain

DX (direct expansion) systems are simpler and self-contained, ideal for smaller rooms or where no chiller plant exists, but efficiency drops as you scale. Chilled-water systems centralise the refrigeration in chillers (air-cooled or water-cooled), distribute cold water to CRAHs, and are the norm for large facilities. The full heat-rejection chain for a water-cooled plant is worth memorising:

1. CRAH transfers room heat into the chilled-water loop (supply ~7-12 °C).
2. The chiller uses a vapour-compression cycle to move that heat into the condenser-water loop.
3. A cooling tower rejects the condenser heat to the atmosphere by evaporation, or an air-cooled condenser blows ambient air across coils.

Chiller efficiency is measured by Coefficient of Performance (COP) — cooling delivered divided by electrical input. A water-cooled centrifugal chiller can reach a COP of 6-7, meaning it moves 6-7 kW of heat per kW of electricity. Water-cooled plants with towers are usually more efficient than air-cooled DX but consume make-up water.

Cooling Redundancy and Common Traps

Cooling capacity is provisioned with the same redundancy schemes as power — N, N+1, 2N, 2(N+1) — so a Tier III/IV facility can lose a CRAH, chiller, or pump without losing the IT load. A common trap: N+1 for cooling means one spare unit beyond those needed for full load, not a doubling. Two more traps: (1) the recommended envelope does not change between classes A1-A4 — only the allowable band widens; and (2) do not invert CRAC and CRAH — CRAC carries the compressor (DX), CRAH uses chilled water. Finally, running the room warmer within the recommended band (up to ~27 °C) is beneficial, not risky, because it enables more free cooling and higher chiller efficiency.