Adiabatic Direct Air Economized

The direct (outside air) economised data center is popular with operators seeking to make maximum capital and operational savings in the data center but can be constrained by climate and IT equipment environmental range. Humidity is a particular concern for direct air systems and is directly represented in these models as it can make a substantial contribution to overall cost and efficiency.

This model represents an Adiabatic Direct Air economised data center with supplemented DX cooling system. The design IT load is 1MW and the data hall consists of a total of fifteen configured AHUs with economisation control based on Humidity, Enthalpy and Wet Bulb temperature. Each AHU consists of Supply and Extract fans. A back up DX system is also employed in the form of a DX coil within the AHU supply air path serviced by a DX compressor and external variable speed DX condenser (the backup systems performance is not included in the modelled results here).

The main AHU supply and extract fans are controlled independently. The supply fans are always online and respond to load. The AHU extract fans will be brought offline when the system reverts to full mechanical cooling or when the external conditions are cold enough to allow for mixing with data hall return air. The former is controlled based on the upper enthalpy limit of 25°C WB (checked by the ‘UPPER ENTHALPY’ node). The latter is controlled by the ‘COLD SPLITT’ node, whereby the fans start to ramp down at 15°C external ambient DB. 

The LV room cooling system and economiser controls are similar to that of the Data hall, but set colder due to the lack of air flow containment and UPS battery temperature limits. 

A summary of the operating temperatures within the Data Center are shown below:

  • Target IT Inlet Temperature: 25oC (max)
  • Air Supply: 25oC supply / 35oC return
  • Hot/Cold aisle configuration
  • Fully airflow contained
  • Enthalpy <0.016 (kg w/kg a) 
  • 100% Fresh Air at <22.5°C WB 
  • Blend in Mech cooling from 22.5°C WB and revert to 100% Mech cooling for >25°C WB
  • Enthalpy >0.016 (kg w/kg a) 
  • Blend in Mech cooling based on Enthalpy change, from a min. dew point of 21°C WB to a max of 24°C WB, where enthalpy change exceeds 10 kJ/kg air
  • >25°C WB - AHU extract fans offline, system diverts to full recirculation 
  • <15°C DB - AHU extract fans start to ramp down due to mixing with return air