DC HVAC key terms

The fraction of total cooling load that is sensible heat rather than latent heat. Data halls are dominated by sensible load, so equipment selected with a high SHR avoids wasting capacity on dehumidification.

Energy absorbed or released when moisture changes state without a temperature change, such as water vapour condensing on a cooling coil. In data centres latent load comes mainly from infiltration and humidification, not from IT equipment.

The temperature at which air becomes saturated and moisture begins to condense. Supply air below the space dew point causes condensation on cold surfaces and is avoided by raising chilled water temperatures.

A graphical plot of air properties including dry-bulb, wet-bulb, humidity ratio and enthalpy. Engineers use it to plot coil processes and confirm that supply air conditions stay within the ASHRAE recommended envelope.

The proportion of air leaving a cooling coil that has not contacted the coil surface and remains at entering conditions. A high bypass factor means warmer, wetter supply air for the same coil temperature.

A vapour-compression or absorption machine that removes heat from a chilled water loop and rejects it to a condenser water loop or directly to outdoor air. It is the primary source of cooling in most large data centres.

A precision cooling unit with a direct-expansion refrigerant circuit that cools and circulates air in a data hall. It typically controls temperature tightly but has limited humidity control compared with a chilled water unit.

A precision air handler that uses chilled water rather than refrigerant to cool data hall air. Because it has no compressor, it is quieter and often more efficient when paired with a central chiller plant.

A cooling system in which refrigerant expands directly in a coil located in the conditioned space or its air handler, with no intermediate chilled water loop. Common in small and edge data centres.

A heat rejection device that uses fans to move outdoor air across finned coils, condensing refrigerant without water. It avoids water consumption but its capacity and efficiency fall as outdoor temperature rises.

A heat rejection device that evaporates water to cool condenser water, allowing compressor work to be reduced. It consumes water and needs chemical treatment and blowdown to control scale and biological growth.

The difference between the leaving water temperature of a cooling tower or dry cooler and the entering ambient wet-bulb or dry-bulb temperature. A smaller approach means a larger, costlier unit but better plant efficiency.

The temperature of water leaving the chiller and entering cooling coils, often 12 to 18 degrees Celsius in modern data centres. Raising it improves chiller efficiency and extends hours of free cooling.

Operation in which outdoor air or outdoor-air-cooled water provides cooling without running mechanical refrigeration. Available hours depend on the ambient design conditions and the chilled water temperature selected.

An arrangement in which server racks face each other in rows so that cold supply air enters rack fronts and heated exhaust air leaves rack rears into a separate aisle. It is the basis for all containment strategies.

The physical enclosure of hot or cold aisles with panels, doors and roof sections to stop supply and exhaust air mixing. Containment raises return air temperature and improves cooling capacity without adding equipment.

Cooled supply air that reaches the return path without passing through IT equipment, for example through cable openings or unsealed rack gaps. It wastes cooling energy and lowers return air temperature.

The practice of directing cool air to equipment intakes and returning hot exhaust efficiently, using blanking panels, grommets, floor tiles and containment. Good management can recover substantial stranded capacity.

A void beneath a raised floor used to distribute conditioned air to perforated tiles in the cold aisle. Its static pressure and leakage characteristics strongly affect how evenly racks receive supply air.

A floor tile with openings that delivers supply air from the plenum into the cold aisle. Tile placement and open area should match rack heat load, otherwise some racks are starved and others over-supplied.

The temperature of air leaving the data hall and returning to cooling units. With good containment it closely tracks rack exhaust temperature, allowing higher chilled water temperatures and better plant efficiency.

The difference between supply and return air temperatures across the data hall, or between entering and leaving water temperatures across a coil. A small delta T at high flow signals excess air or water volume.

The rate at which a cooling system removes heat, usually stated in kilowatts of total and sensible cooling at defined entering conditions. Capacity ratings are meaningless unless the reference conditions are known.

The electrical power drawn by servers, storage and network equipment, essentially all of which becomes heat inside the data hall. It is the starting point for sizing cooling and for efficiency calculations.

Total facility energy divided by IT equipment energy over the same period. A PUE of 1.4 means 0.4 units of overhead energy are consumed for every unit delivered to IT equipment.

PUE calculated for a defined boundary such as a single hall, module or cooling loop rather than the whole facility. It allows comparison of subsystems and isolates the effect of a specific efficiency change.

Cooling output divided by electrical input for a chiller or cooling system at stated conditions. Higher values indicate better efficiency; part-load COP is more representative than full-load COP for most operation.

A weighted efficiency rating that combines performance at several part-load points, reflecting realistic operation rather than full-load design. It is used to compare chillers that will rarely run fully loaded.

Cooling or power capacity that exists but cannot be used because of airflow problems, thermal constraints or configuration limits. Fixing bypass and hot spots often releases capacity more cheaply than adding plant.

A location where equipment intake temperature exceeds the allowable limit, usually caused by recirculation, poor tile placement or high-density racks. Persistent hot spots force conservative set points and reduce usable capacity.

The target temperature of air leaving cooling units, commonly 18 to 24 degrees Celsius in modern halls. Raising it within equipment limits increases free cooling hours and reduces compressor energy.

The range of temperature and humidity conditions considered suitable for reliable long-term IT operation, defined in the ASHRAE thermal guidelines. Operating within it balances equipment reliability against cooling energy.

Adding moisture to keep relative humidity above the minimum that prevents electrostatic discharge. Over-humidification wastes energy, so modern designs often rely on the IT equipment's wider allowable range.

A planned calendar of tasks such as filter changes, coil cleaning, belt inspection and refrigerant checks performed at defined intervals. It prevents efficiency drift and unplanned failures in cooling equipment.

The accumulation of dust in air filters that increases pressure drop and reduces airflow. Rising pressure drop raises fan energy and can reduce cooling capacity, so filters are changed on measured pressure, not only on time.

Removal of dust, biofilm and debris from cooling coil fins and tubes to restore heat transfer and reduce air-side pressure drop. Neglected coils lose capacity and can harbour biological growth.

An electronic controller that varies the speed of a fan or pump motor to match load instead of cycling on and off. Affinity laws mean reduced speed cuts power far more than proportionally.

Relationships showing that fan airflow varies with speed, pressure with speed squared and power with speed cubed. They explain why slowing a fan by twenty percent can nearly halve its energy use.