Free Cooling Using Winter's Chill
An air-cooled chiller with integrated free cooling and custom-designed PLC controls can help plants achieve year-round closed-loop cooling at the lowest possible energy cost.
What if you could turn off your chiller for three or four months out of the year without compromising your year-round industrial process cooling operation? How much energy would you save?
For plants located in regions that experience cold winter climates (below 41°F [5°C]), using a chiller with a winter economizer system can substantially reduce energy consumption. These chillers are designed to take advantage of cold outside temperatures to provide “free cooling” for the manufacturing process during the winter months. At the same time, the technology extends the useful life of the chiller equipment by selectively reducing its operating hours. With advanced integrated controls, these free-cooling chillers can help plants achieve year-round cooling with the lowest possible energy and maintenance costs.
A traditional approach to year-round cooling is an indoor, water-cooled chiller, connected to an outdoor open-draft or closed-loop evaporative cooling tower. Such systems typically have valves and crossover piping that allow the chiller to be bypassed for winter operation. An open tower generally requires an intermediate plate-and-frame heat exchanger to keep the airborne contaminants entrained in the tower water from entering the process flow.
Physical locations north of a theoretical east-west line from Norfolk, Va., to San Francisco, Calif., provide the best ambient environment to optimize operation of integrated free-cooling chillers for year-round cooling.
These evaporative systems can save significant winter operating costs, but they have some drawbacks due to their higher initial equipment, installation, maintenance and power costs. All evaporative towers require makeup water equivalent to approximately 2 gal/min/million BTU/hr for evaporative losses, plus sump blowdown and drift losses. Water treatment is required for the makeup water. An indoor water-cooled chiller also occupies potentially valuable floor space. Finally, open towers cannot use glycols and have low temperature limitations to avoid winter freezing problems.
Simply installing an outdoor air-cooled chiller with low ambient controls can provide year-round cooling but creates an energy expense you would not have with other methods. Additionally, the coldest winter days are the most challenging operating conditions for air-cooled chillers.
In recent years, some of these year-round air-cooled chiller systems have been retrofitted with remote glycol coolers (radiators) to pre-cool the return chilled water in the winter months. While these units are a practical, energy-saving addition, they occupy roof or ground space that could be used for other purposes. Also, additional piping and insulation are required, along with increased fan power for the fluid cooler. Control integration with the original chiller requires careful consideration, especially when partial free-cooling is both available and desirable in the moderately cool weather found during the nighttime, spring and fall. Using partial free-cooling requires a motorized valve, additional temperature controls and an advanced PLC to permit maximum savings.
An Integrated Approach
Systems with integrated free cooling and advanced PLC controls can provide complete year-round closed loop cooling a low energy cost. The free-cooling coils typically are fully integrated inside the chiller package, so extra floor space for fans, pumps, valves, tanks or heat exchangers is not required, nor is additional piping, electrical work, evaporative spray or makeup water needed. These integrated free-cooling chillers are designed to produce 50 percent free cooling at ambient temperatures approximately 10°F (6°C) below the design chilled water temperature, or 100 percent free cooling at approximately 20°F (11°C) below the chilled water supply temperature.
A major additional benefit of a free-cooling chiller is that the mechanical refrigeration plant is not required to operate in the most difficult (winter) environment. Head pressure control becomes increasingly difficult at low ambient temperatures, and starting a refrigeration system in very low ambient temperatures can be especially troublesome. Free cooling eliminates this requirement for mechanical cooling in the winter, and therefore extends the useful life of the mechanical system by reducing the number of operating hours and eliminating the most difficult operating conditions.
Control. The free-cooling PLC determines when any free cooling can be achieved. It continuously monitors the outside ambient temperature, the return chilled water temperature, and the required chilled water supply temperature. The controller automatically directs the return chilled water through the economizer coils as soon as the ambient is cold enough to provide any useful free cooling.
The refrigeration compressors run less frequently as the free-cooling effect increases, until eventually they are no longer required and remain in standby mode. At this time, full cooling capacity is provided by condenser fan power alone. Variable-frequency drive control of the fans allows precise control of the head pressure, maximizes energy savings and controls the chilled glycol temperature at low ambient temperatures, when less airflow is required to achieve 100 percent free cooling. All aspects of the mechanical and free-cooling operation are controlled by the purpose-designed PLC, including glycol-valve operation, fan-speed control and safety controls and alarms, in addition to normal compressor and pump operation. On dual-circuit free-cooling chillers, the PLC permits one circuit to operate in free-cooling mode (with the compressor off and fans at full speed) while the other circuit operates mechanically (with the compressor on and fan speed determined by head pressure). In this way, maximum free-cooling savings can be achieved at any ambient temperature.
The change from mechanical to partial or full free-cooling is transparent to the operator and process. The LCD display indicates when the free-cooling system is operational. The “smart” PLC control ensures the temperature does not swing outside the design temperature control band. Similarly, when the system switches out of free cooling, changes are not made to the chilled water temperature control. At certain times of the year, the system might switch compressors on and off several times during the day or night if the ambient is hovering close to the free-cooling commencement temperature. The PLC prevents short cycling of the free-cooling controls and compressors during any phase of the operation. Compressor running hours are automatically logged by the PLC, so the energy savings are available for inspection or verification at any time.
A Rapid Payback
AK Steel in Toledo, Ohio, needed a chiller for its new steel-tube production line. The production line required year-round chilled water cooling for optimum steel-tube production capacity. Specifically, the cooling process was quenching the tubes at a controlled temperature of 70°F (21°C) immediately after the forming operation to maintain the designed mechanical and metallurgical tolerances. The Toledo area experiences approximately 500 hours at 50 to 55°F (10 to 13°C) (25 percent free-cooling), 1,052 hours at 41 to 50°F (5 to 10°C) (50 percent free-cooling) and 3,484 hours below 41°F (100 percent free-cooling). With four 30-ton Motivair air-cooled chillers installed inside the mill on an existing line, AK Steel felt confident about the brand and wanted to take advantage of winter free-cooling opportunities for its new production line.
In late 2006, the company installed a Motivair MLC-FC 330 free-cooling chiller. The chiller was installed outside the building and supplies 60°F (16°C) chilled glycol in a closed loop to the “cold side” of two plate-and-frame heat exchangers in the plant. The “hot side” of the exchangers feeds water at 70°F (21°C) in an open circuit to the large quenching tanks that process the finished steel tubes.
The cooling load is approximately 122 tons, cooling 324 gal/min of 40 percent glycol from 70 to 60°F (21 to 16°C) year-round. This application was especially advantageous for a free-cooling chiller because the required chilled-water supply temperature is relatively high and the Toledo area experiences a relatively cool winter. As a result, more hours are available per year for free-cooling operation, and compressor life is extended through reduced operating hours.
The free-cooling chiller installed at AK Steel includes six scroll compressors in two independent refrigeration circuits for maximum redundancy and reduced noise level. Duplex pumps with auto-change and alarm functions ensure continuous glycol circulation. The PLC monitors all chiller functions and records compressor operating hours and the most recent 100 alarm conditions in memory.
The total compressor power is 112 kW. At an average electrical cost of $0.07/kWh, it was estimated that AK Steel will save approximately $32,412 per year in energy costs compared to a standard air-cooled chiller without free cooling for year-round cooling use. The payback period was calculated at less than the first year for the capital cost of adding the free-cooling option to the air-cooled chiller. In this advantageous application, the electrical energy cost savings is sufficient to pay for the entire chiller and installation cost in approximately five years.
The AK Steel example demonstrates that packaged air-cooled chillers with integrated free-cooling can pay for the initial investment in a short amount of time. The additional advantage of extended compressor life, achieved through reduced run time, makes these chillers an attractive proposition to consider for many year-round cooling applications. For free cooling to “make sense,” the only requirements are an outside location for the packaged air-cooled chiller with integrated free-cooling, and the addition of glycol as an antifreeze agent to the cooling loop. Systems that incorporate the tank, pumps and controls in a single outdoor, weatherproof package can minimize installation costs. Then only connections to chilled glycol and electrical power are required.
The free-cooling savings can be calculated for an installation using ASHRAE temperature bins, provided the design chilled-water temperature, installation location and local electrical kW/h costs are known. Physical locations north of a theoretical east-west line from Norfolk, Va., to San Francisco, Calif., provide the best ambient environment to optimize operation of integrated free-cooling chillers for year-round cooling. In locations north of that theoretical line, the typical average investment recovery period is 1 to 2 years, depending on the geographic location and the chilled water temperature. Higher chilled water temperatures and colder winter climates produce increased savings.