Regular Maintenance Keeps Chiller Efficiency High

July 11, 2002
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Today’s centrifugal chillers offer some of the highest efficiencies ever. The technology may be more advanced, and the refrigerants are newer, but some things remain the same: the systems’ need for regular maintenance.

To help today’s high-efficiency chillers maintain those efficiencies in the field, their major components — tubes, oil, compressor, condenser, refrigerant, and starting equipment, to name a few — need to be inspected and maintained regularly.

Jeff Carpenter, marketing manager, Carrier Commercial Service (Syracuse, NY), said that the top areas that affect chiller efficiency are:

  • Tubes (“Make sure they are cleaned and maintained”);

  • Controls, which need regular inspection and calibration;

  • Refrigerant charge optimization; and

  • Regular oil changes and yearly oil analysis.

    The most overlooked work, he said, is “taking normal, everyday and weekly readings.” If there are no operating problems, staff might ignore this simple task. Taking regular readings, however, allows them to watch for trends, possibly catching problems before they result in equipment damage and unexpected downtime.

    This article provides an overview of regular chiller maintenance pointers provided by Carpenter and Carrier technical literature.

    TUBE CLEANING, WATER CIRCUITS

    The condition of local water will have a lot to do with the frequency required for tube cleaning and inspection, Carpenter said. Know your water quality. The frequency with which tube cleaning winds up being performed is determined mainly due to water conditions; it may also make you decide to upgrade your chiller’s water treatment system.

    “Higher-than-normal condenser pressures, together with the inability to reach full refrigeration load, usually indicate dirty tubes or air in the chiller,” states Carrier. This is where taking daily measurements and tracking them over time really pays off. “If the refrigeration log indicates a rise above normal condenser pressures, check the condenser refrigerant temperature against the leaving condenser water temperature.”

    If the reading is higher than design, the condenser tubes may be dirty, or water flow rates may be incorrect.

    The first step is to inspect the heat exchanger tubes and flow devices. “Inspect and clean the cooler tubes at the end of the first operating season,” the company advises. “Because these tubes have internal ridges, a rotary-type tube-cleaning system is needed to fully clean the tubes.”

    An inspection of the tubes’ condition will help you determine how often future cleaning will need to be done, and whether the system needs improved water treatment in the chilled-water/brine circuit. Look for signs of corrosion and scale. The chilled-water cooler tubes are usually part of a closed circuit and less susceptible to “dirty water.” Nevertheless, they should be inspected after the first season of operation.

    While you’re there, inspect the entering and leaving chilled-water temperature sensors and flow devices for corrosion or scale. Remove scale if possible, but replace the sensor or Schrader fittings if they’re corroded.

    “Keeping the condenser tubes clean is essential in maintaining peak performance,” Carpenter said. “On the condenser side, at a minimum we recommend yearly tube inspections.”

    Cleaning can be done as required depending on the results of the inspections. The presence of scale or other corrosion may require chemical treatment or cleaning beyond just brushing the tubes. Inspect the entering and leaving condenser water sensors and flow devices for signs of corrosion or scale. Again, replace the sensor or Schrader fitting if corroded; remove any scale.

    REFRIGERANT CHARGE

    Proper refrigerant charge in the chiller is essential for optimal performance and energy efficiency, noted Carpenter. Too much refrigerant in the unit can cause refrigerant carryover, a condition where liquid refrigerant enters the compressor and evaporates. This could lead to reduced capacity, an overloaded motor condition, excess power consumption, and possible damage to the compressor impeller.

    On the other hand, Carpenter pointed out, insufficient refrigerant charge can result in the top or uppermost layers of the cooler tube bundle not being submerged in liquid refrigerant. In this situation, the “lift” on the compressor increases, resulting in higher-than-normal power consumption. In undercharged units, add refrigerant to minimize power consumption.

    Carrier recommends that service contractors trim the refrigerant charge to obtain optimal chiller performance. If it becomes necessary to adjust the refrigerant charge, the company says, “Operate the chiller at design load and then add or remove refrigerant slowly, until the difference between the leaving chilled-water temperature and the cooler refrigerant temperature reaches design conditions or becomes a minimum. Do not overcharge.”

    The company says that refrigerant can be added through the storage tank or directly into the chiller, per the manufacturer’s procedures. Excess refrigerant should be removed by following the manufacturer’s recommended procedures.

    STARTING EQUIPMENT

    Often overlooked, but critical to precise operation, are control devices and the control system.

    “While less obvious than the need for mechanical maintenance,” explains Carpenter, “servicing controls is just as important to the overall operation and efficiency of the chiller or chiller plant. And because the mechanical and electrical components were designed to work together, servicing your system in its entirety is the only way to ensure optimal performance and prevent serious problems.”

    If the chiller is part of a central plant control or integrated into a building energy management system, Carpenter states, “It is good practice to perform regular system evaluations to ensure that performance is optimized. Trend reports can give you a complete picture of your chiller plant or entire building HVAC system. Armed with reliable information, specialists can suggest and make improvements that will enhance system operation and reduce operating costs.”

    A chiller controls test is part of this regular evaluation. It “facilitates the proper operation and test of temperature sensors, pressure transducers, compressor guide vane operation, oil pump, water pumps, cooling tower control, and other on/off outputs,” says Carpenter. “Individual sensors can be calibrated on an as-needed basis or in regular intervals.”

    Cleaning and examining the control contacts is another aspect of regular control service. During this part, it is less critical that proper safety procedures be followed. “Before working on any starter, shut off the chiller, open and tag all disconnects supplying power to the starter,” the company warns.

    The company also warns that “The disconnect on the starter front panel does not de-energize all internal circuits. Open all internal and remote disconnects before servicing the starter. Never open isolating knife switches while equipment is operating. Electrical arcing can cause serious injury.”

    First, inspect starter contact surfaces. Look for wear or pitting on mechanical-type starters. “Do not sandpaper or file silver-plated contacts,” the company states. Follow the starter manufacturer’s instructions.

    Vacuum or blow off accumulated debris on the internal parts periodically with a high-velocity, low-pressure blower.

    The company also notes, “Power connections on newly installed starters may relax and loosen after a month of operation. Turn the power off and retighten them.” Recheck them once a year; loose power connections can cause voltage spikes, overheating, malfunctioning, or failures, the company warns.

    OIL, LUBRICATION SYSTEM

    Changing the oil, of course, is critical to operation, perhaps more so than it is to efficiency; analyzing it once a year is equally important, said Carpenter. That determines the frequency of future oil changes, and may alert you to other problems in the system that can be addressed during planned maintenance rather than emergency downtime.

    “Do an oil and filter change after the first year of operation,” Carpenter said, “then do a yearly oil analysis. If that’s clear, you can go do an oil change, in some cases, up to every five years.” If the oil is dirty, of course, it needs to be changed more often. Particle size can indicate if it’s from a compressor wear problem or normal wear and tear. Regardless, “Change the oil filter on a yearly basis,” the company states, “or when the chiller is opened for repairs.” The refrigerant filter-drier should be changed “once a year or more often if its condition indicates a need for more frequent replacement,” the company says.

    The lubrication system should be checked every week, the company says. “Mark the oil level on the reservoir sight glass; observe the level each week while the chiller is shut down.”

    If the level goes below the lower sight glass, make sure the oil reclaim system is operating properly. If more oil is required, add it through the oil drain charging valve. “A pump is required when adding oil against refrigerant pressure.” Note the amount and date that any oil is added.

    Note: “Any oil that is added due to oil loss that is not related to service will eventually return to the sump,” the company points out. The oil must be removed from the sump when it reaches a high enough level.

    STILL MORE TO IT

    Chiller maintenance includes many other areas, such as checking for refrigerant leaks and inspecting power transducers. The point is to be regular, and to check the system’s vital signs (temperatures and pressures) daily.

    Maintaining the compressor, Carpenter says, is critical to proper operation and overall equipment reliability, minimizing downtime and maximizing uptime; however, it may not be directly related to the big efficiency picture.

    To inspect the bearings, the company says, a complete compressor teardown is required. Only a trained service tech should remove and examine the bearings.

    Bearings and gears should be examined on a regular, scheduled basis for signs of wear. Gear inspections require a complete compressor teardown. They, too, should only be done by trained technicians. How often? That is determined by the hours of chiller operation, load conditions during operation, and the condition of the oil and lubrication system.

    Excessive bearing wear can sometimes be detected through increased vibration or increased bearing temperature.

    One problem is, many companies don’t have enough staff available to take daily readings. Carpenter says that’s where a service like Carrier’s National Monitoring Center can help, by monitoring remotely and alerting onsite staff to potential problems. “It speeds up the whole service process,” he says.

    Companies that invest in a new, high-efficiency chiller deserve to get their money’s worth. By performing regular maintenance and taking daily readings, you can help ensure that the chiller fulfills its promise.

    For more information, contact Carrier Corp., P.O. Box 4808, Syracuse, NY; www.carrier.com (website).

    Publication date: 07/15/2002

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