COLUMN DATA CENTERS relatively high dry bulb switch points between wet and dry operation. Alternatively, conventional closed-circuit cooling towers or open circuit cooling towers coupled with plate & frame heat exchangers can be paired in series with dry coolers. The dry coolers begin to handle part of the load as the outside air temperature drops until the air temperature is cold enough that the dry coolers can handle the entire load. The evaporative heat rejection units essentially function as trim coolers when needed to provide the design liquid temperature. For facilities located in moderate climates that are designed to operate with relatively warm supply fluid (113°F [45°C] or higher), dry coolers can be utilized for the heat rejection duty. The main advantage is that makeup water is not needed in a dry cooler based system, nor a backup water supply. Offsetting this advantage is much higher fan horsepower for the same thermal duty (though no open loop recirculating spray pump is required), along with a significant increase in the plan area required for the heat rejection equipment. Like closed circuit cooling towers, the use of glycol must be considered in subfreezing climates. Alternatively, freeze-protected dry coolers employing a thermosyphon refrigerant loop can operate with water in the closed cooling loop. To offset this higher fan energy and footprint while expanding the applicable climate zones of air-cooled alternatives, adiabatic fluid coolers can be considered. These units consist of a dry cooler with a wetted precooling section that adiabatically lowers the air temperature entering the finned coil section close to the wet-bulb of the entering air. The pre-cooling section typically consists of evaporative cooling pads that are wetted with water from either a small integral sump that is drained multiple times per day or the potable water system on a once through basis. The design fluid temperature off these units is lower than that achievable from dry coolers but higher than traditional closedcircuit cooling towers. Water use is also less with this J U LY 2 0 19 ashrae.org ASHRAE JOURNAL 61https://www.ashrae.org/