Water-cooled HVAC and process load chillers, commonly referred to as chillers, are found in commercial buildings, industrial energy plants, and data centers. Chillers that are well maintained and operating at peak performance save energy, while those operating with fouled condenser or evaporator tubes, leaking refrigerant, poor water treatment, or other issues can result in increased operational costs and often lead to unplanned chiller outages for corrective maintenance and repair.
Chiller preventive maintenance has advanced significantly due to recent developments in magnetic bearings, real-time vibration analysis, and remote monitoring technologies. As a result of these innovations, the chiller industry has been moving toward demand maintenance programs as opposed to more traditional scheduled maintenance programs. Whichever program your energy plant uses, assure that your program meets the minimum maintenance requirements specified by the chiller manufacturer. Below are some preventive maintenance best practices that VERTEX has found to be instrumental in maintaining chiller reliability and efficiency.
Chiller Maintenance Best Practices
Maintain a daily chiller log
The daily chiller log is the first step toward maintaining and ensuring maximum chiller efficiency. The daily log helps build a history of chiller operating conditions including temperatures, pressures, fluid levels, and flow rates. Real-time vibration analysis and remote monitoring allow continuous chiller evaluation, and the daily log also allows easy generation of trend reports that can be used to troubleshoot operational issues requiring preventive maintenance before an unplanned chiller outage occurs.
Clean condenser and evaporator tubes for efficient heat transfer
Heat transfer effectiveness directly impacts chiller performance, so clean condenser and evaporator tubes are fundamental to maintaining maximum chiller efficiency. Contaminants such as minerals, scale, mud, algae, and other impurities increase thermal resistance and reduce overall heat transfer performance. The condenser approach temperature, defined as the difference between the condenser liquid refrigerant temperature and the outlet condenser water temperature is an excellent indicator of heat transfer efficiency as an increase in condenser approach temperature is an indication that heat transfer efficiency is decreasing. Condenser water open loops with cooling towers require proper water treatment to minimize scale, corrosion, and biological growth within the condenser tube bundle. Condenser tube fouling impedes heat transfer and results in decreased chiller performance, so both condenser and evaporator tubes should be brushed at least annually, and preferably before the cooling season begins to keep them free of unwanted contaminants.
Keep condenser flow velocity between 3 and 11 feet/second (FPS)
Proper condenser water flow velocity directly impacts chiller performance and can help reduce condenser tube fouling. Too low a flow velocity reduces chiller efficiency and ultimately leads to laminar flow. Too high a flow velocity leads to vibration, noise, and condenser tube erosion. The 2020 ASHRAE Handbook, HVAC Systems and Equipment recommends a minimum condenser flow velocity of approximately 3 FPS, and a maximum condenser flow velocity of approximately 11 FPS. Maintain condenser flow velocity between 3 and 11 FPS for optimal chiller performance.
Maintain proper refrigerant charge
The amount of cooling a chiller provides depends on several factors including how much refrigerant moves through the compressor. It is important to maintain proper chiller refrigerant charge and to eliminate refrigerant leaks and infiltration of air and moisture into the system as these decrease chiller efficiency and reliability. A low refrigerant charge causes the compressor to work harder for less cooling effect.
Analyze compressor oil
Compressor oil samples should be collected at least annually and sent to a laboratory for chemical analysis. Like any hermetically sealed refrigeration system, the compressor oil should only be replaced if the analysis indicates it’s needed. High moisture in the compressor oil can indicate a problem with the purge unit. Chillers that operate below atmospheric pressure should have their compressor oil sampled more frequently based on purge unit operational hours. Check compressor oil filters for pressure drop and replace them whenever the oil charge is replaced. Newer, magnetic bearing frictionless chillers require distinctly different maintenance and operations from traditional chillers. Oil has been eliminated in the design of these chillers which further reduces maintenance costs.
Maintain variable frequency drive (VFD) starters and motors
For efficient operation of VFD starters and motors, check the safety and sensor calibrations on microprocessor controls at least annually. Electrical connections, wiring, and switchgear related to the chiller should be thermally scanned for “hot spots” or worn contacts. To prevent insulation faults, test the VFD motor windings for insulation resistance to ground and winding-to-winding. Check the shaft seal of open drive VFD motors for possible refrigerant leaks, and clean motor cooling air vents to ensure maximum motor cooling effect. With proper maintenance, a VFD motor will extend the chiller’s useful operating life by reducing the mechanical stresses of rapidly starting and stopping large horsepower motors.
How can VERTEX help?
VERTEX’s team of mechanical engineers has expertise with all aspects of chiller preventive maintenance, chiller plant operation & design, and chiller installation. We are available to analyze your energy plant issues and help you identify potential installation, operational, maintenance, or design challenges which could lead to component failures.