Data centers with air conditioning by means of precision air conditioning units basically have two main types of control: the Control of air conditioning units based on the air inlet temperature (so-called return air temperature control) and the Control of air conditioning units based on the air outlet temperature (so-called supply air temperature control).
Return air temperature control is the best known and also the most widespread type of control. The CRAC or CRAH are equipped with temperature sensors (combined temperature and humidity sensors, as a rule) in the vicinity of the air inlet. A setpoint is set for the return air temperature and the unit controller keeps this setpoint stable. If the airflow is constant, fluctuations in the data center heat load influence the supply air temperature.
The supply air temperature is the temperature of the air as it leaves the air conditioning unit. It is approximately the same as the server inlet temperature. If the return air setpoint is set to 33 °C, for example, and the data center air conditioning system is designed for a temperature difference of 15 K, under full load the supply air temperature would be 18 °C. Since full load is seldom reached in a data center, but instead a partial load of 40 % to 60 % is common, the supply air temperature – at a constant airflow – would be 24 °C to 27 °C.
Diagram 1 shows the supply air temperatures produced at a return air temperature of 33 °C, a constant airflow and a temperature difference of 15 K at full load for various partial load scenarios.