Servers and the equipment related to them generate a considerable amount of heat. The amount varies from setup to setup, however cooling is one of the core elements of any server setup.
Central installations are a necessity because the generation of heat is uneven, since more or less servers are working at a certain point in time in different physical locations around the datacenter. The amount of heat is also directly connected to the watts that the machines consume, and the power is dissipated as heat.
Previously determining what kind of cooling is required has been done with measurements of watts per square foot, kilowatts per rack, and cubic feet per minute (CFM). At some point a standard measurement system was introduced - Power Usage Effectiveness (PUE), which is derived by dividing the total power level requirement for the datacenter to the power of the IT equipment load.
In datacenters where different types of server machines are used, RLU's are used to determine the varying cooling requirements. Rack Location Units measure the amount of power the particular units in the datacenter consume in watts or Btus.
When this information is gained for the particular datacenter, one of the standard cooling systems is implemented. The commonly used ones are the following:
Computer Room Air Conditioners (CRAC) - it is based on refridgerant and is installed in the datacenter floor, moving air via a fan system - cool air comes in, and delivers the exhaust air to the outside units.
Comupter Room Air Handler (CRAH) - Again moves air through the datacenter via a fan system, however the cooling is based on cold water.
Humidifier - It is usually installed with CRAC and CRAH units, however it also exists as a standalone system. The main goal is to maintain a humidity level that complies to the standards set by ASHRAE (The American Society of Heating, Refrigerating and Air-Conditioning Engineers).
The main principles explained above are fairly simple however implementing them is a complex task. Airflow management is one of the most important aspects of datacenter cooling systems. The cold and hot airflow paths need to be completely separated so the different climates won't mix up, therefore reducing efficiency.