B op eqsyschillers

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B op eqsyschillers

  1. 1. Operation and Maintenance of ChillersTopics: Introduction Types of Chillers Key Components of Mechanical Compression Chillers Safety Issues Best Practices for Efficient Operation Sample Operating Log for Chillers Best Practices for Maintenance Maintenance Schedule for Chillers ReferencesIntroductionChillers are a key component of air conditioning systems for large buildings. They produce cold water toremove heat from the air in the building. They also provide cooling for process loads such as file-serverrooms and large medical imaging equipment. As with other types of air conditioning systems, mostchillers extract heat from water by mechanically compressing a refrigerant.Chillers are complex machines that are expensive to purchase and operate. A preventive and predictivemaintenance program is the best protection for this valuable asset. Learn more about establishing a Best Practice O&M Program.Chillers commonly use more energy than any other piece of equipment in large buildings. Maintainingthem well and operating them smartly can yield significant energy savings. Produced by BetterBricks, the commercial initiative of the Northwest Energy Efficiency Alliance BOpEqSysChillers • 1
  2. 2. Chiller and associated HVAC systemsTypes of ChillersMechanical CompressionDuring the compression cycle, the refrigerant passes through four major components within the chiller:the evaporator, the compressor, the condenser, and a flow-metering device such as an expansion valve.The evaporator is the low-temperature (cooling) side of the system and the condenser is the high-temperature (heat-rejection) side of the system. Produced by BetterBricks, the commercial initiative of the Northwest Energy Efficiency Alliance BOpEqSysChillers • 2
  3. 3. The refrigeration cycleMechanical Compressor ChillersMechanical 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 condenserinstead of being exhausted out a tailpipe. The pistons have intake and exhaust valves that can be openedon 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 thesystem load. This is more efficient than using a hot-gas bypass to provide the same capacity variationwith all pistons working. Some units use both methods, unloading pistons to a minimum number, thenusing hot-gas bypass to further reduce capacity stably. Capacities range from 20 to 125 tons. Produced by BetterBricks, the commercial initiative of the Northwest Energy Efficiency Alliance BOpEqSysChillers • 3
  4. 4. Reciprocating compressorRotary Screw: The screw or helical compressor has two mating helically grooved rotors in a stationaryhousing. As the helical rotors rotate, the gas is compressed by direct volume reduction between the tworotors. Capacity is controlled by a sliding inlet valve or variable-speed drive (VSD) on the motor.Capacities range from 20 to 450 tons.Screw compressorCentrifugal: The centrifugal compressor operates much like a centrifugal water pump, with an impellercompressing 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 watercapacity control. Capacities are 150 tons and up. Produced by BetterBricks, the commercial initiative of the Northwest Energy Efficiency Alliance BOpEqSysChillers • 4
  5. 5. Centrifugal compressorFrictionless Centrifugal: This highly energy-efficient design employs magnetic bearing technology. Thecompressor requires no lubricant and has a variable-speed DC motor with direct-drive for the centrifugalcompressor. Capacities range from 60 to 300 tons.Turbocor© frictionless centrifugal compressorAbsorption ChillersAbsorption chillers use a heat source such as natural gas or district steam to create a refrigeration cyclethat does not use mechanical compression. Because there are few absorption machines in the NorthwestU.S., this document covers only mechanical-compression chillers. You can learn more about absorptionchillers at the Energy Solutions Center.Key Components of Mechanical Compression ChillersEvaporatorChillers produce chilled water in the evaporator where cold refrigerant flows over the evaporator tubebundle. The refrigerant evaporates (changes into vapor) as the heat is transferred from the water to therefrigerant. The chilled water is then pumped, via the chilled-water distribution system to the building’s air-handling units. Learn more about Operation and Maintenance of HVAC Water Distribution Systems. Produced by BetterBricks, the commercial initiative of the Northwest Energy Efficiency Alliance BOpEqSysChillers • 5
  6. 6. Learn more about Operation and Maintenance of Air Distribution Systems.The chilled water passes through coils in the air-handler to remove heat from the air used to conditionspaces throughout the building. The warm water (warmed by the heat transferred from the buildingventilation air) returns to the evaporator and the cycle starts over.CompressorVaporized refrigerant leaves the evaporator and travels to the compressor where it is mechanicallycompressed, and changed into a high-pressure, high-temperature vapor. Upon leaving the compressor,the refrigerant enters the condenser side of the chiller.CondenserInside the water-cooled condenser, hot refrigerant flows around the tubes containing the condenser-loopwater. The heat transfers to the water, causing the refrigerant to condense into liquid form. Thecondenser water is pumped from the condenser bundle to the cooling tower where heat is transferredfrom the water to the atmosphere. The liquid refrigerant then travels to the expansion valve. Learn more about Operation and Maintenance of Cooling Towers.Expansion valveThe refrigerant flows into the evaporator through the expansion valve or metering device. This valvecontrols the rate of cooling. Once through the valve, the refrigerant expands to a lower pressure and amuch lower temperature. It flows around the evaporator tubes, absorbing the heat of the chilled waterthat’s been returned from the air handlers, completing the refrigeration cycle.ControlsNewer chillers are controlled by sophisticated, on-board microprocessors. Chiller control systems includesafety and operating controls. If the equipment malfunctions, the safety control shuts the chiller down toprevent serious damage to the machine. Operating controls allow adjustments to some chiller operatingparameters. To better monitor chiller performance, the chiller control system should communicate with thefacility’s direct digital control (DDC).Safety IssuesChillers are typically located in a mechanical equipment rooms. Each type of refrigerant used in a chillercompressor has specific safety requirements for leak detection and emergency ventilation. Consult yourlocal mechanical code or the International Mechanical Code for details.The EPA has enacted regulations regarding the use and handling of refrigerants to comply with the CleanAir Act of 1990. All personnel working with refrigerants covered by this act must be appropriately licensed.Best Practices for Efficient OperationThe 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 bymatching the building loads to the most efficient combination of one or more chillers. In general, the mostefficient chiller should be first one used.Raise chilled-water temperature: An increase in the temperature of the chilled water supplied to thebuilding’s air handlers will improve its efficiency. Establish a chilled-water reset schedule. A resetschedule can typically adjust the chilled-water temperature as the outside-air temperature changes. On acentrifugal chiller, increasing the temperature of chilled water supply by 2–3°F will reduce chiller energyuse 3–5%.Reduce condenser water temperature: Reducing the temperature of the water returning from thecooling tower to the chiller condenser by 2–3°F will reduce chiller energy use 2–3%. The temperature Produced by BetterBricks, the commercial initiative of the Northwest Energy Efficiency Alliance BOpEqSysChillers • 6
  7. 7. setpoint for the water leaving the cooling tower should be as low as the chiller manufacturer will allow forwater entering the condenser. The actual leaving tower water temperature may be limited by the ambientwet bulb temperature.Purge air from refrigerant: Air trapped in the refrigerant loop increases pressure at the compressordischarge. This increases the work required from the compressor. Newer chillers have automatic airpurgers that have run-time meters. Daily or weekly tracking of run time will show if a leak has developedthat permits air to enter the system.Optimize free cooling: If your system has a chiller bypass and heat exchanger, known as a water-sideeconomizer, it should be used to serve process loads during the winter season. The water-sideeconomizer produces chilled water without running the chiller. Condenser water circulates through thecooling tower to reject heat, and then goes to a heat exchanger (bypassing the chiller) where the water iscooled sufficiently to meet the cooling loads.Verify Performance of hot-gas bypass and unloader: These are most commonly found onreciprocating compressors to control capacity. Make sure they operate properly.Maintain refrigerant level: To maintain a chillers efficiency, check the refrigerant sight-glass and thesuperheat 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 achiller’s capacity and efficiency.Maintain a daily log: Chiller O&M best practices begin with maintaining a daily log of temperatures, fluidlevels, pressures, flow rates, and motor amperage. Taken together, these readings serve as a valuablebaseline reference for operating the system and troubleshooting problems. Many newer chillersautomatically save logs of these measurements in their on-board control system, which may be able tocommunicate directly with the DDC system. Below is an example of a daily log that can be adapted foruse with your chiller. Produced by BetterBricks, the commercial initiative of the Northwest Energy Efficiency Alliance BOpEqSysChillers • 7
  8. 8. Sample Operating Log for Chillers Produced by BetterBricks, the commercial initiative of the Northwest Energy Efficiency Alliance BOpEqSysChillers • 8
  9. 9. Best Practices for MaintenanceCompared to a major chiller failure, a sound preventive and predictive maintenance program is a minorcost. Implementing a best-practice maintenance plan will save money over the life of the chiller andensure longer chiller life. For more information on this topic go to Best Practice O&M Program.Substandard operating practices frequently go unnoticed and become the accepted norm. Trainingpersonnel in both maintenance and operating practices is the best prevention. Many chiller manufacturersoffer training for building operating engineers in operating and maintaining their chillers.To effectively maintain chillers, you must 1) bring the chiller to peak efficiency, and 2) maintain that peakefficiency. There are some basic steps that facilities professionals can take to make sure their chillers arebeing maintained properly. Below are some of the key practices.Reduce Scale or FoulingFailure of the heat exchanger tubes is costly and disruptive. The evaporator and condenser tube bundlescollect mineral and sludge deposits from the water. Scale buildup promotes corrosion that can lead to thefailure of the tube wall. Scale buildup also insulates the tubes in the heat exchanger reducing theefficiency of the chiller. There are two main preventive actions:Checking water treatment: Checking the water treatment of the condenser-water open loop weekly willreduce the frequency of condenser tube cleaning and the possibility of a tube failure. Learn more about Operation and Maintenance of Cooling Towers.Checking the water treatment of the chilled-water closed loop monthly will reduce the frequency ofevaporator tube cleaning and the possibility of a tube failure. Learn more about Operation and Maintenance of HVAC Water Distribution Systems.Inspecting and cleaning tubes: The tubes in the evaporator and condenser bundles should beinspected once a year, typically when the chiller is taken offline for winterizing. Alternately, for systemsthat operate all year to meet process loads, tube scaling and fouling can be monitored by loggingpressure drop across the condenser and evaporator bundles. An increase in pressure from the inlet to theoutlet of 3–4 PSI indicates a probable increase in scale or fouling requiring tube cleaning.Inspect for Refrigerant LeaksIf possible, monitor the air-purge run timer. Excessive or increased air-purge time may indicate arefrigerant leak. If an air-purge device is not installed, bubbles in the refrigerant sight-glass may alsoindicate refrigerant leak. Gas analyzers can also be used to identify refrigerant leaks.The table below provides a checklist for maintenance tasks.Maintenance Schedule for Chillers MaintenanceDescription Comments FrequencyFill out daily log Check all setpoints for proper setting and function. Daily (4×) Make sure there are no unusual sounds and the space temperature is acceptable.Chiller use/sequencing Turn off or sequence unnecessary chillers DailyCheck chilled water reset Check settings for approved sequence of operation Annuallysettings and function at the beginning of each cooling seasonCheck chiller lockout setpoint Check settings for approved sequence of operation Annually at the beginning of each cooling seasonClean evaporator and Indicated when pressure drop across the barrel Annuallycondenser tubes (tube bundle) exceeds manufacturers recommendations, but at least annually. Produced by BetterBricks, the commercial initiative of the Northwest Energy Efficiency Alliance BOpEqSysChillers • 9
  10. 10. MaintenanceDescription Comments FrequencyVerify motor amperage load Motor amperage should not exceed manufacturers Annuallylimit specificationCompressor motor and Conduct vibration analysis: Check all alignments to Annuallyassembly specifications. Check all seals. Lubricate where necessary.Compressor oil system Perform analysis on oil and filter. Change if Annually necessary. Check oil pump and seals Check oil heater and thermostat Check all strainers, valves, etc.Electrical connections Check all electrical connections and terminals for Annually full contact and tightnessCheck refrigerant condition Add refrigerant if low. Record amounts and address Annually leakage problems.Check for condenser and Indications include: poor water quality, excessive As neededevaporator tube corrosion and fouling, and age of chiller. Eddy current testing mayclean as needed. be done to assess tube condition.ReferencesFEMP 2004. O&M Best Practices Guide 2.0.FEMP 2002. Continuous Commissioning Guidebook for Federal Energy Managers. Produced by BetterBricks, the commercial initiative of the Northwest Energy Efficiency Alliance BOpEqSysChillers • 10

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