Data Center Cooling Explained
Why Servers Need Cooling
Data center cooling is the set of systems that removes the heat produced by IT equipment so servers stay within their rated operating temperature. Nearly all the electrical power that servers consume is converted into heat, so a facility that cannot remove that heat will see equipment slow down, shut off or fail.
Cooling also controls humidity. Air that is too dry can allow static discharge, and air that is too humid can cause condensation on cold surfaces. At very high densities, air alone may not be enough, as explained in data center liquid cooling.
CRAC and CRAH Units and Airflow
Most air-cooled rooms use computer room air conditioners (CRAC units) or computer room air handlers (CRAH units). A CRAC unit contains its own refrigeration cycle, similar in principle to a home air conditioner but designed for precise control and continuous operation. A CRAH unit instead circulates room air over coils cooled by chilled water, supplied by a central plant of chillers (large refrigeration machines).
In either case, cool air is delivered toward the front of the servers, which draw it in, and warm exhaust air returns to the cooling units to be cooled again. Many facilities use raised floors or overhead ducts to move air. Cooling units are usually deployed with spares so one can be serviced without losing capacity, a principle explained in N+1 redundancy.
Hot Aisles, Cold Aisles and Containment
Racks are usually arranged in rows with the fronts of the servers facing a cold aisle and the rears facing a hot aisle. Because servers take in air from the front and exhaust it from the back, this layout keeps cold supply air and hot return air apart.
Containment takes that separation further. Physical barriers such as doors, end panels and roofs enclose either the hot aisle or the cold aisle, so air is forced through the equipment rather than around it. Mixing hot and cold air wastes energy and creates hot spots, where some servers receive air that is too warm. Open gaps in racks also let hot air recirculate, which is why unused rack spaces are typically closed with simple covers called blanking panels.
Free Cooling in Practice
Free cooling uses outside air, or cool water from the environment, instead of running mechanical refrigeration whenever conditions allow. A facility in a cool climate may rely on outside air for much of the year, switching to chillers only on hot days or when humidity is out of range. In a warm climate, free cooling covers a smaller share of the year, so both the cooling design and the local climate affect how much energy is needed. The energy result is measured with power usage effectiveness (PUE).
Frequently asked questions
What is a CRAC unit in a data center?
CRAC stands for computer room air conditioner. It is a precision air conditioning unit that controls temperature and humidity around IT equipment, usually with its own refrigerant circuit. The related CRAH (computer room air handler) uses chilled water from a central plant instead.
Why do data centers use hot and cold aisles?
Arranging racks so server fronts face a cold aisle and rears face a hot aisle keeps supply air and exhaust air from mixing. This lets cooling units work more efficiently and helps every server receive air within its temperature range. Containment strengthens the effect by sealing the aisles.