Chilled water systems remove heat from buildings by transferring heat from the air into chilled water forced through piping, which circulates through chilled water loops, returning the heated water to the chiller, where refrigerant removes heat from the water. Rarely seen in residential applications, chilled water systems are common in commercial and industrial process settings.
Chiller water system design and operation offer specialized challenges to maintenance staff, especially as the system can weigh several hundred tons and include miles of piping and heat transfer tubing. Chilled water systems also place high demands on electrical load, and even a small problem can reduce system efficiency and increase operating costs.
Remote Monitoring and Daily Logs
Remote monitoring technology allows maintenance staff to monitor large equipment daily, instead of manually checking systems every week or month. Records from remote monitoring allow you to track chiller temperature, pressure, fluid levels, and flow rates consistently. Comparing this daily log to the manufacturer’s chilled water system design guide allows for the early detection and correction of equipment issues.
Contaminants such as minerals, scale, algae, and mud can build up in chilled water system tubing, reducing heat transfer efficiency. Approach temperatures may indicate contaminated tubing, as higher approach temperatures indicate a drop in heat transfer efficiency.
Tubes should be cleaned every three years in closed systems. For open systems, periodic inspection is required. Follow the guidelines in the chilled water system design and operation manufacturer’s guide.
Tube cleaning includes mechanical and chemical cleaning processes. Mechanical cleaning requires physically brushing the interior of the tubing to remove mud, sludge, and algae, and then flushing the tubes. Chemical cleaning removes scale, and must be tailored to match your local water conditions. Mechanical cleaning is necessary after chemical cleaning to remove loosened scales.
Newer chiller models have taken much of the work out of cleaning tubing. Such systems include automatic tub brushing, where small, nylon brushes flow through the tubes. Older systems can be retrofitted to include integrated cleaning systems, a cost that typically pays for itself within two years.
Condenser Water Issues
Condenser water loops transfer water to open cooling towers or similar systems, allowing heat to escape into the atmosphere. Exposure to open air increases the risk of contaminants capable of causing scale, corrosion, and biological growth. All such contaminants can impair heat transfer and decrease system efficiency.
Loops should be inspected regularly for water quality issues and evidence of corrosion, and condenser water should be treated as needed to prevent the accumulation of contaminants.
Air and moisture can leak into chillers, causing a decrease in system efficiency and the risk of corrosion and rust. As little as 1 psi of air in a condenser reduces system efficiency by three percent. Moisture can create corrosive acids that damage motor windings and rust the inside of the chiller shell.
Regular purging controls non-condensate contamination, and all system strainers should be cleaned every three months to maximize their ability to maintain clean water.
Once a year, maintenance staff should take a sample of the system’s lubrication oil while the chiller is operating. This sample should be sent to the lab for spectrometric analysis. Low pressure systems will require more frequent sampling.
Compressor oil should only be replaced if the lab results indicate the need. Oil filters should be checked for pressure drop, and replaced if the oil is replaced.
New magnetic-bearing frictionless chillers have eliminated oil from their systems.
Maintaining Proper Refrigerant Charge
A water chilling system’s ability to cool its surrounding environment depends on how much refrigerant moves through the compressor, so maintaining the level of refrigerant charge recommended by the manufacturer is important. When levels drop, the compressor works harder to compensate.
Leaks, air, moisture, and oil can all adversely affect refrigerant. It’s not unheard of for refrigerant to contain ten percent oil in old model chillers, resulting in a twenty percent decrease in efficiency.
Water Temperature and Flow Rate
The temperature of entering condenser water affects system efficiency, with lower temperatures improving system operations. Chilled water flow rates are also an issue. Most chilled water system design and operation manuals recommend a flow rate of three to twelve feet per second. Any less, and system efficiency drops. Any higher, and you increase noise, vibrations, and erosion risks.
Starter and Motor Maintenance
All starters and motors associated with the system should be checked regularly for the following:
Inaccurate safety and sensor calibrations on microprocessor controls
Hot spots and worn contacts on electrical connections, wiring, and switchgear
Inadequate ground and wiring-to-wiring insulation resistance on electrical motor wiring
Refrigerant leaks around open drive motor shafts
Dirty or clogged motor cooling air vents.
Installing variable frequency drives on chiller motors can reduce the system’s electrical load by matching the motor efficiency to the load. VFDs also act as soft starters and reduce the mechanical shock associated with starting large motors, increasing the life of the chiller system.