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Model-Based Approach for Optimizing a Data Center Centralized Cooling System

Beitelmal, Monem H.; Patel, Chandrakant D.


Keyword(s): data center; cooling; chiller; cooling tower; COP; heat

Abstract: A steady-state model for a centralized cooling system is developed to enable proper provisioning of cooling resources in capital intensive projects such as data centers. The model resolves the energy equations for a cooling tower and a centralized water-cooled chiller simultaneously. It requires inputs that are readily available to the design engineer. The user inputs are the ambient conditions, the cooling tower air flow rate, the condenser water flow rate, the evaporator water flow rate, the superheat and subcooling associated with the refrigeration cycle and the full load design conditions. The model utilizes an empirical relationship for the compressor power as a function of load and temperature and gives the user an option to select a constant speed chiller or a variable speed chiller. The outputs include the chiller coefficient of performance, compressor input power and compressor isentropic efficiency. The model results are validated with a manufacturer performance data [8] and compared to an experimental data collected at Hewlett- Packard Laboratories site for a 2110 kW (600 Ton) variable speed chiller and a 2286 kW (650 Ton) constant speed chiller. The model results are found to match the experimental data to within an acceptable deviation. For the constant speed chiller, the chiller efficiency increases with increasing heat load and peaks at full load. For the variable speed chiller, the chiller efficiency peaks between 50% and 70% loading depending on the ambient conditions. Notes:

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