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.
Supply air temperature control has become ever more popular in recent years, because the server inlet temperature is increasingly required as a constant control variable. With the supply air temperature or server inlet temperature as the setpoint and a constant airflow, the return air temperature then varies as heat loads fluctuate. If the supply air setpoint is set e.g. to 18 °C, and the air conditioning system is designed for a temperature difference of 15 K, under full load the return air temperature would be 33 °C. With a partial load of 40 % to 60 % and a constant airflow, return air temperatures would lie at 24 °C to 27 °C.
Diagram 2 shows the return air temperatures produced at a supply air temperature of 18 °C, a constant airflow and a temperature difference of 15 K at full load for various partial load scenarios.
In the case of controllers that also adapt the airflow in line with partial load, the temperature difference changes only a little or not at all, the return air and supply air temperatures remain roughly the same, while the airflow varies.
This is due to the fact that the cooling capacity is also the product of the airflow and the temperature difference.
The decision whether to use return air temperature control or supply air temperature control should also factor in the air conditioning system as a whole. In systems with Free Cooling, the chosen type of control – return air temperature or supply air temperature control – has an influence on the efficiency of the air conditioning system. The definitive variable here is the supply air temperature.
Efficiency is inherently poorer in systems with supply air temperature control. This is also because the (low) supply air temperature is kept constant, i.e. all measures to make use of Free Cooling take place below this temperature level.
On the other hand, systems with return air temperature control have a supply air temperature that rises at partial load. Consequently, this increased supply air temperature can also be "produced" during Free Cooling when the outside air temperature is higher. If Free Cooling can be used just 1 K "longer", for instance, that is still several hundred hours' more Free Cooling.
Each type of control has its raison d'être and you simply have to decide in each case which type is the most suitable for the data center in question.
