DESIGN, OPERATION AND MAINTENANCE OF DIRECT AND INDIRECT EVAPORATIVE COOLING SYSTEMS IN DATA CENTER THERMAL MANAGEMENT

Abstract

Evaporative cooling is an alternative data center cooling solution that presents significant energy cost savings in acquisition and operation when compared to conventional air conditioning and mechanical refrigeration systems. This research focuses on developing a deeper understanding in implementing, operating and maintaining evaporative cooling systems integrated with air-side economization to realize the energy savings potential in data center cooling. The goal of the research is to develop a comprehensive guide to assess and contrast different approaches of evaporative cooling being used in data center cooling and provide guidance in improving water and energy efficiency. An air handling unit (AHU) serves as a building block of any cooling equipment and houses the primary heat exchanger, air filters, air movers, a duct and damper system along with necessary electrical and control systems. Both direct and indirect evaporative cooling technologies are considered. The proposed approach systematically develops a comprehensive body of knowledge in evaporative cooling applicable to data center cooling. The territorial and climatic limitations of each evaporative cooling system integrated with air- and water-side economization for favorable ambient conditions is analyzed based on a psychrometric-based and data-driven modeling approach coupled with typical meteorological year data for various climate zones.

For direct evaporation cooling, three types of wet cooling media pads are experimentally characterized for the saturation effectiveness and system pressure drops. Analysis of the media pads based on experimentally validated CFD models establishes the criteria for media pad selection and sizing. The impact of Calcium scaling due to continuous evaporation in the media pad is studied by designing and conducting an accelerated degradation test that determines the health monitoring parameter of the media pad and the maintenance interventions in the field. To achieve incremental humidification, a vertically split and staged media pad with discrete pumps for each stage is proposed to maximize water savings. The effectiveness of the staging is demonstrated by conducting characterization testing on the air flow bench with controlled inlet air conditions. The mixing chamber in the AHU is comprehensively studied to eliminate the thermal stratification issue which can exacerbate the hot spots in the data center.

For indirect evaporative cooling, a separate AHU is designed, fabricated, and commissioned in Dallas, TX. The heat exchanger is epoxy-coated Aluminum plate heat exchanger in the crossflow configuration with a water distribution system consisting of spray nozzles/sprinklers. The primary side is the hot air return from the data center and the secondary air is the outside air. Upon wetting the heat exchanger on the secondary side, water is evaporated off of the heat exchanger plates facilitating sensible cooling of the primary air while the secondary air is humidified and exhausts at a higher temperature. Two different types of water distributors are tested and at various heights from the top surface of the heat exchanger. A water collection grid is designed to map how well the spray system effectively distributes water within the heat exchanger passages. Heat exchanger performance is tested for fully wet, partially wet and flooded wetting configurations.