Indirect Air Economized

This is an Indirect Air system with supplemental DX mechanical cooling. The design IT load is 1MW and the data hall consists of a total of 15no. cooling units. The economisation control is based on the air-to-air approach temperature across the Indirect air heat exchanger. Each cooling unit consists of Data Hall Supply fans which re-circulate the data hall airstream via the heat exchanger. The scavenger fans (External fans) draw in fresh air through the heat exchanger and cool the data hall return air. In this particular indirect system the data hall air stream is a closed system, i.e. there is only sensible heat transfer between the internal and external air streams. 

In this model the heat exchanger operates dry, without adiabatic assist. A back up DX system is employed in the form of a DX coil within the 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 model topology indicates two cooling connections from the AHU supply fan, one to the Scavenger fans and the other to the air-to-air heat exchanger. In this model the switch over from economisation to mechanical cooling is dictated by the dry bulb temperature. The scavenger fans (external fans) respond to the external dry bulb temperature and start to ramp down below 25°C. For economisation, a full load approach of 6°C (10F) must be overcome over the air-to-air heat exchanger to achieve the target supply temperature of 25°C (77F) into the data hall. Therefore, 100% economisation (no DX cooling) is achieved for external dry bulb 19°C (66F) or less. The mechanical cooling system starts to blend in as the external dry bulb exceeds 19°C (66F) and reverts to 100% DX at an external dry bulb of 35°C. 

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
  • 19oC DB low / 35oC DB high/cutoff
  • To allow for a 6°C full load approach temperature across the air-to-air heat exchanger a 19°C DB external air temperature is required to achieve full free cooling. 
  • Partial free cooling available at external temperatures below 35°C DB where the data hall return air stream can be cooled. 
  • Above 35°C DB system reverts to 100% mechanical cooling with full recirculation.