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With condenser-inlet-water temperatures of 60°F (prior to free cooling), water-regulating valves would maintain a minimum acceptable head pressure of approximately 240 psig (110°F). In typical summer conditions, when tower water is 85°F, water-cooled chillers will have slightly lower operating costs than air-cooled chillers operating in 95°F ambient temperatures. However, these savings are offset by the added expenses of cooling-tower fans and pumps.

A closed-loop evaporative tower (with a backup tower) offers a cleaner solution for water-cooled chillers because the circulated process fluid is a glycol solution that flows in a completely closed loop. Spray-water treatment still is required, as are a sump heater and winter fan controls. A plate exchanger installed similarly to an open tower can provide partial or 100-percent free cooling. The closed-loop glycol solution, however, also can be diverted automatically from the chillers' water-cooled condenser to the room units without the need for an intermediate exchanger whenever conditions are right for 100-percent free cooling. Although a closed-loop system is simpler than an open-loop system and eliminates plate-and-frame exchangers, it excludes the possibility of partial free cooling. Also, the initial cost of a closed-loop evaporative tower is significantly higher than that of an open-draft tower.

In both closed- and open-loop systems, water-cooled chillers can operate in full mechanical mode until tower water is cold enough to satisfy the chilled water required by room units. They also can operate in partial-free-cooling mode if an intermediate exchanger is employed. Tower water needs to be approximately 40 to 45°F — assuming a 5- to 10°F approach in a plate-and-frame exchanger — before 100-percent free cooling is available and 50°F chilled water can be supplied to room units without compressor operation.

Air-cooled chillers

An air-cooled chiller with integrated free cooling provides energy savings whenever ambient dry-bulb temperature is below 60°F. If two 250-ton air-cooled free-cooling chillers were installed in the same 500-ton application, the absorbed compressor power (assuming the same two R-407C screw compressors per chiller) would be approximately 188 kw per chiller, or 376 kw total, in 60°F outside ambient temperature, delivering 50°F chilled water/glycol to the room units. The fan controls would limit winter head pressure to a minimum of approximately 240 psig (110°F) to maintain suction pressure and effective compressor lubrication. A 50-percent free-cooling design is practical for integrated free-cooling coils when the ambient temperature is 10°F below the required chilled-water temperature, while a 100-percent free-cooling design is practical when the ambient temperature is 20°F below the received chilled-water temperature.

Using temperature-bin data for New York City, estimated power savings for our example data center with air-cooled free-cooling chillers would be virtually identical to that of the water-cooled chillers. Again, the total savings would equal about $68,402 more per year (conservatively rounded into 25-, 50-, and 100-percent free-cooling temperature bins) than identical winterized air-cooled chillers without free cooling. Redundancy design considerations likely would require a third parallel redundant chiller for data-center operation.

Utilizing an outside packaged air-cooled chiller with dry coolers (radiators) installed in series in a chiller's return line also can save a considerable amount of free-cooling energy. This design can be used as a practical retrofit for any existing air- or water-cooled-chiller installation. A three-way diverting valve directs return chilled water/glycol through dry coolers whenever the outside ambient temperature is cold enough for partial free cooling. The dry coolers' fans are enabled automatically, with fan cycling or variable-frequency-drive fan controls minimizing fan power while also preventing overcooling of the chilled water/glycol amid extreme winter temperatures. The three-way valve can be used to partially bypass the dry coolers if necessary.

This system can be designed to achieve an even closer approach to winter ambient temperatures to maximize energy savings. However, the tradeoff requires significantly increased dry-cooler surface, fan power, and outdoor footprint because the low initial temperature differential and close approach temperature require an exponential increase in surface area.

A packaged air-cooled chiller with integrated free cooling requires much less outside space than an air-cooled chiller with a separate dry cooler for free cooling. (A water-cooled chiller requires valuable indoor space, plus outside space for a cooling tower.) Free-cooling coils are contained completely inside the chiller footprint. Moreover, the condenser fans serve a double purpose for the free-cooling coils and condenser, so no additional fan power is wasted. Air-cooled condensers and matching free-cooling coils require extended face area to maximize thermal performance while minimizing additional fan resistance.

Because the chilled glycol in an integrated free-cooling chiller is in a completely closed circuit, there is no requirement for a plate heat exchanger, crossover piping, or valves. No water treatment, evaporation, or water makeup is required. A three-way valve is incorporated inside the chiller to initiate and modulate free cooling, and a programmable logic controller automatically controls all of the system's operating and alarm functions.

Packaged air-cooled free-cooling chillers typically are available up to 300 tons. They also usually are available with an integrated chilled-glycol storage tank and one or two pumps. It is highly recommended to split the cooling load into two or more chillers for data-center operation so that redundant capacity always is available in the event of maintenance or an equipment malfunction.