Chilled Water Economized With Cooling Towers

This example is of a Chilled Water PHE system with Water Cooled Chillers and Cooling Towers. In this model a Plate Heat Exchanger (PHE) is located upstream of the Chiller and acts as an economiser which attempts to cool the return water from the Data Hall CRAC units. The diagram illustrates the mechanical operation of this class of data center with the addition of the PHE. The cabinets are in open hot and cold aisles, with 27°C return air temperature set point.

In this class of facility the power infrastructure is designed with a minimum resilience of 2N. The UPS topology consist of dual (N+1) systems. 

To reflect the increase in pipework across the chilled water system, for the Tier III models, the costs associated to capital and maintenance for the Primary, Secondary and Condenser Water pumps are adjusted accordingly.

The capacity of the PHE is demonstrated in the plot below for the target CHW return temperature of 21°C.

If the target CHW supply temperature of 15°C is achieved solely via the PHE, the Chiller is not required, and the data center is fully economised with maximum free cooling (usually during peak Winter). For this to occur a total approach (air to water) temperature of 8°C across the system of Cooling Tower and PHE must be overcome (5°C across the Cooling Tower and 3°C across the PHE). Therefore, a minimum external ambient wet bulb temperature of 8°C is required to achieve the target CHW supply condition via free cooling. If the target CHW supply temperature is not achieved by the PHE, the Chiller compressors are required to do some work. 

The Chiller Compressor power is its maximum during peak Summer conditions when the opportunity for free cooling through the PHE is reduced. In this model the PHE is shut off and deemed ineffective at 19°C. 

Note that the cooling towers are represented by two nodes, to define operational characteristics for the fans in the summer condition and the winter condition. The summer mode operation is to maintain a 20°C Glycol return temperature, whereas the winter mode operation is to maintain a 10°C Glycol return temperature. The ‘BMS Control’ upstream of the Cooling Tower nodes will revert between Summer and Winter modes at 19°C external wet bulb ambient.

In this class of facility the power infrastructure is designed with a minimum resilience of 2N. The UPS topology consist of dual (N+1) systems. 

To reflect the increase in pipework across the chilled water system, for the Tier III models, the costs associated to capital and maintenance for the Primary, Secondary and Condenser Water pumps are adjusted accordingly. 

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

  • Target IT Inlet Temperature: 24oC (max)
  • Air Supply: 21oC supply / 27oC return
  • Hot/Cold aisle configuration
  • Non airflow contained
  • 15oC CHW supply
  • 21oC CHW return
  • External conditions for economization: 8oC (Low) 19oC (high/cutoff)
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