Chillers are key components of air conditioning systems for large buildings. They produce cold water to remove heat from the air in the building. They also provide cooling for process loads such as file-server rooms and large medical imaging equipment. As with other types of air conditioning systems, most chillers extract heat from water by mechanically compressing a refrigerant.
Chillers are complex machines that are expensive to purchase and operate. A preventive and predictive maintenance program is the best protection for this valuable asset.
Mechanical Compression Chillers
Mechanical compression chillers are classified by compressor type: reciprocating, rotary screw, centrifugal, and frictionless centrifugal.
Reciprocating: Similar to a car engine with multiple pistons, a crankshaft is turned by an electric motor,the pistons compress the gas, heating it in the process. The hot gas is discharged to the condenser instead of being exhausted out a tailpipe. The pistons have intake and exhaust valves that can be opened on demand to allow the piston to idle, which reduces the chiller capacity as the demand for chilled wateris reduced. This unloading allows a single compressor to provide a range of capacities to better match the system load. This is more efficient than using a hot-gas bypass to provide the same capacity variation with all pistons working. Some units use both methods, unloading pistons to a minimum number, then using hot-gas bypass to further reduce capacity stably. Capacities range from 20 to 125 tons.
Rotary screw: The screw or helical compressor has two mating helically grooved rotors in a stationary housing. As the helical rotors rotate, the gas is compressed by direct volume reduction between the two rotors. Capacity is controlled by a sliding inlet valve or variable-speed drive (VSD) on the motor. Capacities range from 20 to 450 tons.
Centrifugal: The centrifugal compressor operates much like a centrifugal water pump, with an impeller compressing the refrigerant. Centrifugal chillers provide high cooling capacity with a compact design. They can be equipped with both inlet vanes and variable-speed drives to regulate control chilled water capacity control. Capacities are 150 tons and up.
Frictionless centrifugal: This highly energy-efficient design employs magnetic bearing technology. The compressor requires no lubricant and has a variable-speed DC motor with direct-drive for the centrifugal compressor. Capacities range from 60 to 300 tons.
Absorption chillers use a heat source such as natural gas or district steam to create a refrigeration cycle that does not use mechanical compression.
Best Practices for Efficient Operation
The following best practices can improve chiller performance and reduce operating costs:
Operate multiple chillers for peak efficiency: Plants with two or more chillers can save energy by matching the building loads to the most efficient combination of one or more chillers. In general, the most efficient chiller should be first one used.
Raise chilled-water temperature: An increase in the temperature of the chilled water supplied to the building’s air handlers will improve its efficiency. Establish a chilled-water reset schedule. A reset schedule can typically adjust the chilled-water temperature as the outside-air temperature changes. On a centrifugal chiller, increasing the temperature of chilled water supply by 2 to 3F will reduce chiller energy use by 3 to 5%.
Reduce condenser water temperature: Reducing the temperature of the water returning from the cooling tower to the chiller condenser by 2 to 3F will reduce chiller energy use 2 to 3%. The temperature setpoint for the water leaving the cooling tower should be as low as the chiller manufacturer will allow for water entering the condenser. The actual leaving tower water temperature may be limited by the ambient wet bulb temperature.
Purge air from refrigerant: Air trapped in the refrigerant loop increases pressure at the compressor discharge. This increases the work required from the compressor. Newer chillers have automatic air purgers that have run-time meters. Daily or weekly tracking of run time will show if a leak has developed that permits air to enter the system.
Optimize free cooling: If your system has a chiller bypass and heat exchanger, known as a water-side economizer, it should be used to serve process loads during the winter season. The water-side economizer produces chilled water without running the chiller. Condenser water circulates through the cooling tower to reject heat, and then goes to a heat exchanger (bypassing the chiller) where the water is cooled sufficiently to meet the cooling loads.
Verify performance of hot-gas bypass and unloader: These are most commonly found on reciprocating compressors to control capacity. Make sure they operate properly.
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Maintain refrigerant level: To maintain a chiller’s efficiency, check the refrigerant sight-glass and the superheat and subcooling temperature readings, and compare them to the manufacturer's requirements. Both low-level and high-level refrigerant conditions can be detected this way. Either condition reduces a chiller's capacity and efficiency.
Maintain a daily log: Chiller O&M best practices begin with maintaining a daily log of temperatures, fluid levels, pressures, flow rates, and motor amperage. Taken together, these readings serve as a valuable baseline reference for operating the system and troubleshooting problems. Many newer chillers automatically save logs of these measurements in their on-board control system, which may be able to communicate directly with the DDC system.
Best Maintenance Practices
Compared to a major chiller failure, a sound preventive and predictive maintenance program is a minor cost. Implementing a best-practice maintenance plan will save money over the life of the chiller and ensure longer chiller life.
To effectively maintain chillers, you must 1) bring the chiller to peak efficiency, and 2) maintain that peak efficiency. There are some basic steps that facilities professionals can take to make sure their chillers are being maintained properly. Below are some key practices.
Reduce scale or fouling. Failure of the heat exchanger tubes is costly and disruptive. The evaporator and condenser tube bundles collect mineral and sludge deposits from the water. Scale buildup promotes corrosion that can lead to the failure of the tube wall. Scale buildup also insulates the tubes in the heat exchanger reducing the efficiency of the chiller.
There are two main preventive actions to reduce scale or fouling:
- Checking water treatment. Checking the water treatment of the condenser-water open loop weekly will reduce the frequency of condenser tube cleaning and the possibility of a tube failure.
- Inspecting and cleaning tubes. The tubes in the evaporator and condenser bundles should be inspected once a year, typically when the chiller is taken offline for winterizing. Alternately, for systems that operate all year to meet process loads, tube scaling and fouling can be monitored by logging pressure drop across the condenser and evaporator bundles. An increase in pressure from the inlet to the outlet of 3–4 PSI indicates a probable increase in scale or fouling requiring tube cleaning.
Inspect for Refrigerant Leaks
If possible, monitor the air-purge run timer. Excessive or increased air-purge time may indicate a refrigerant leak. If an air-purge device is not installed, bubbles in the refrigerant sight-glass may also indicate refrigerant leak. Gas analyzers can also be used to identify refrigerant leaks.
Janice Peterson is manager of building operations, the the NEEA's BetterBricks Initiative. BetterBricks is an initiative of the Northwest Energy Efficiency Alliance, Portland, OR, which is supported by Northwest electric utilities. Through BetterBricks, NEEA advances ideas to accelerate energy savings in new and existing commercial buildings.