Variable-speed control of a centrifugal chiller can produce energy savings as much as 30% annually when compared to a constant-speed chiller, and savings can reach 75% at lighter loads. With energy savings of this magnitude, the added cost of the variable-speed drive like can be paid off quickly, as was the case at this Conectiv Thermal Systems 17,000-ton district cooling plant in Atlantic City, NJ.

Traditionally, the goal of a carefully planned, proactive preventive maintenance program for centrifugal chillers has been to improve chiller reliability and increase equipment life. Experience showed that a well-organized schedule for routine checkups and minor repairs reduced the frequency of unscheduled, often expensive, service calls and minimized the risk of catastrophic equipment failure and the associated potential for downtime, injuries, and unbudgeted, costly equipment replacement. Planned repairs could be budgeted and chiller life extended when periodic inspections identified and addressed small problems.

All of this remains true today. A systematic schedule of inspections, tests, and repairs can delay system deterioration and prevent premature equipment failure. But with energy bills for comfort cooling and refrigeration equipment in the United States now approaching $90 billion a year, the focus of preventive maintenance has expanded to include improving energy efficiency and reducing energy consumption.

Chillers are often the largest single energy-using components in large office buildings, and, as is the case with most energy-using systems, energy is the largest component of the life-cycle cost of the chiller. It is logical, then, as energy prices increase, that building owners and facility managers are identifying chillers as potential sources for significant energy savings through improved operation and maintenance practices.

A proactive, preventive maintenance program for an electric-drive centrifugal chiller includes simple checks that can be performed daily, often accomplished by walking around the chiller room observing gauges and listening to noise levels. Other more complex tests will require shutting down the chiller and replacing parts to maintain chiller efficiency and reliability and to guarantee responsible energy consumption. A list of tools to conduct these and other regularly scheduled inspections and tests includes safety glasses, a volt/ ammeter, test thermometers, test pressure gauges, a megohmmeter, a phase meter, an electronic leak detector, and various hand tools.

Daily Logs

Regularly scheduled chiller maintenance may seem like one of the most un-glamorous jobs in a facility. Fortunately, many facilities and energy managers realize that chiller maintenance, when done properly, is likely one of the most cost-effective activities on the preventive maintenance (PM) schedule. Regular inspections on auxiliary systems, like refrigerant filters, can help identify chiller operating problems before they become problematic and catastrophic

The permanent daily log that records operating conditions at regular intervals throughout each 24-hour operating period is one of the most effective tools in the preventive maintenance program. The maintenance section of most operating manuals includes a sample log sheet that should be used to record temperature readings, fluid levels, pressure readings, and flow rates.

Taken together, these readings serve as a valuable reference for operating the system. Readings taken when a chiller is newly installed will establish baseline conditions (provided they match the manufacturer's specifications) with which to compare later readings.

Careful observation of these readings can help operators establish a history of operating conditions and pinpoint trends in the system before they become a problem. For example, dirty condenser tubes may be indicated by a higher than normal temperature difference between leaving condenser water and refrigerant leaving the condenser.

Accurate records also enable operators to identify energy-saving opportunities and measure the effect of different energy-saving strategies. Additionally, they document maintenance procedures that may simplify warranty claims.

Graphic control panels, standard equipment on many chillers since 1999, eliminate the need to manually record many of these statistics. These panels provide user-friendly logs on one large, active-matrix screen and enable operators to view multiple related parameters simultaneously on a single screen.

They also have the ability to quickly generate on-screen graphs of daily, weekly, or monthly trends for instant analysis. A printer interface with an optional local printer provides hard-copy documentation of the chiller's control parameters and operating history. In many cases, graphic control panels can be retrofitted on older chillers that presently use microprocessor control panels.

Control Centers

Leaks in the refrigerant system need to be repaired immediately and charge levels checked monthly. In high-pressure chillers, refrigerant can leak out, reducing refrigerant charge and limiting the chiller's heat-transfer capacity (thus increasing head and energy use). Too little refrigerant charge can also decrease evaporator temperature. For every 1?F the evaporator temperature can be raised, 1.5% of the full-load energy can be saved.<

An important part of preventive maintenance, then, is to regularly inspect the chiller's control center, whether it is a microprocessor panel or a graphic control center. Although panel layout and the number of indicators vary according to manufacturer, most control centers utilize two kinds of controls: safety and operating. In general, safety controls shut down the chiller system when a malfunction is sensed, preventing damage or catastrophe when a key operating condition is not met. Operating controls allow some adjustment of the chiller and/or give the status of a key function.

Although checks on safety cutouts and operating checkpoints may be performed automatically, it is still important to check them manually, evaluating the safety and operating control settings and recording the maintenance at regular intervals based on the manufacturer's recommendations. Factory settings of controls should not be changed. The manufacturer has designated safety set points for the protection of the unit. Altering them could shorten the expected life of the chiller.

It is also important to ensure the proper calibration of the control panel, transducers, and thermisters and to be certain the leaving chilled-water temperature (LCHWT) is set to the proper temperature. A 1iF increase in LCHWT can result in a 2 to 3% decrease in energy consumption.

Similarly, it is important to confirm that the condenser inlet water temperature is set to the minimum level as recommended by the manufacturer. Energy savings at full load are about 1.5% for every 1iF the entering condenser water temperature (ECWT) is reduced, so the strategy is to adjust the system to obtain the lowest ECWT consistent with the manufacturer's recommendations.

There are additional maintenance requirements for the electrical components within the chiller starter. Overload settings need to be confirmed and the tightness of electrical connections checked. Loose connections can overheat, causing damage to power wiring or components. Variable-speed drives and solid-state starters have additional requirements within their cooling circuits.

Mechanical Components

Tom Brown, holding vibration analyzer

Vibration analysis is an important tool that enables a technician to "look inside" the compressor and determine the condition of bearings, gears, and other rotating components. Worn bearings and components emit a distinct signal corresponding to the severity of the damage and the rotational speed. Sensitive vibration equipment can detect the signal and record it for diagnosis by a vibration engineer.

Vibration analysis should be scheduled during chiller commissioning to establish a baseline level for comparison to future readings, during seasonal start-up to confirm component condition, prior to winter shut down to determine whether off-season repairs need to be accomplished, and whenever a change in unit sound level is detected.

The compressor requires a host of preventive maintenance procedures, beginning with the semi-annual replacement of the oil filter. If the system is dirty, the filter may need to be replaced more frequently. An inspection of the used filter should be part of the replacement procedure. The presence of metal particles on the filter may indicate wear on the compressor's rotating components and suggest the need for a factory-trained service technician to track down the source of the particles.

Compressor oil levels are easier to check, but just as important. Frequent readings via sight glasses provide valuable information regarding oil levels and contamination. An excessive oil charge can cause problems, and a change in lubricant color generally indicates contamination.

Oil that appears dark or cloudy should be analyzed for the presence of harmful acids, corrosion-causing water, corrosion products, and metal particles indicating abnormal parts wear. An annual chemical oil analysis is a valuable aid in assessing the internal mechanical condition of a chiller and identifying potential problems so they can be remedied before they become expensive, disruptive headaches, including unexpected shutdowns and catastrophic chiller failure.

If conducted on a scheduled basis, oil analysis can keep a chiller in peak operating condition, affording reliable chiller performance throughout the life of the unit.

The compressor motor needs to be checked routinely to ensure the tightness of the motor-mounting screws. As part of a motor check, the motor alignment and coupling should also be examined for wear and to be certain bolts are tight. A megohmmeter can be employed to check the motor for moisture or deterioration of the winding insulation.

A preventive maintenance program ensures the proper operation of the oil return system by verifying the oil return flow to the compressor sump and looking for excessive levels of oil in the refrigerant charge. It is also a good idea to check the eductor for foreign particles that could obstruct the jet and to change the dehydrator and strainer semiannually.

The purge unit dehydrator should also be changed regularly, preferably every three months. Other preventive maintenance procedures that pertain to the purge unit include annual cleaning and inspecting of valves and orifices as well as draining and flushing oil and refrigerant from the purge unit shell. Where applicable, purge unit compressor maintenance should be performed as required by the manufacturer.

Refrigerant Leaks and Charge Levels

The refrigerant system of a centrifugal or screw chiller presents a number of opportunities to practice good preventive maintenance and ensure energy-efficient chiller operation. Charge levels on large chillers with flooded evaporators can be checked via sight glass levels, or, preferably, by comparing the temperature difference between the LCWT and the evaporator refrigerant. On DX chillers, a frequent check of the refrigerant sight glasses may reveal bubbles. If these bubbles do not dissipate after several minutes of stable operation, they may indicate a low refrigerant charge or a leak in the system.

Leaks in the refrigerant system need to be repaired immediately and charge levels checked monthly. In high-pressure chillers, refrigerant can leak out, reducing refrigerant charge and limiting the chiller's heat-transfer capacity (thus increasing head and energy use). Too little refrigerant charge can also decrease evaporator temperature. For every 1iF the evaporator temperature can be raised, 1.5% of the full-load energy can be saved.

In low-pressure chiller systems, air can leak into the chiller. Collecting in the condenser, it displaces refrigerant vapor and causes higher condenser pressure, again increasing energy use.

When performed as part of a regular maintenance program, laboratory analysis of refrigerant can become an essential part of maintaining efficiency and preventing downtime. Refrigerant analysis can identify the presence of rust, sludge, and harmful acids in a chiller system that carry with them the potential to reduce operating efficiency and cause unnecessary parts wear and catastrophic chiller failure.

Rust build-up on the heat-exchanger tubes decreases heat-transfer efficiency, causing the compressor to work harder and waste energy. Rust particles falling to the bottom of the evaporator shell mix with the oil and can cause improper lubrication, premature parts wear, and clogged valves and orifices. Hydrofluoric and hydrochloric acids, formed by the combination of moisture and refrigerant, are highly corrosive.

By comparing the results of the refrigerant analysis against a chiller's historical operating data, experts can reliably diagnose the condition of a chiller and make recommendations accordingly, pinpointing the causes of contamination in the process and indicating whether tear-down and visual inspection is warranted.

A preventive maintenance program also looks carefully at the water-side system of the chiller. The presence of sludge, slime, minerals, rust, and foam can decrease chiller efficiency and capacity. Addressing these problems through regularly scheduled maintenance will ensure more reliable operation from chillers, water pumps, and cooling towers.

Tube Failure

The single most costly failure in a water chiller is a tube failure. Corrosion and the formation of scale or algae in the condenser tubes make a water-treatment program essential to chiller efficiency and long life. Scale build-up can foul a chiller's condenser tubes, increasing the thermal resistance in the heat exchanger and, as a result, increasing energy consumption. If the fouling factor rises from its standard value of 0.00025 to 0.003, for example, the additional energy cost per year for a 500-ton chiller is $25,300.

Consequently, the condenser tubes should be cleaned annually or more frequently if conditions warrant. For example, if the temperature difference between the leaving condenser water and the refrigerant is more than 4iF greater than the manufacturer's recommended temperature differential, it is a good indication that the condenser tubes need to be cleaned. For every 1iF this temperature difference can be lowered within specified limits, full-load energy consumption can be reduced 1%.

To eliminate the need for traditional cleaning methods that employ caustic, abrasive cleaners, products using an electronic process have been developed to prevent scale buildup and reduce maintenance costs.

Under normal circumstances, cooler tubes do not require cleaning. If, however, the temperature between the refrigerant and the chilled-water leaving the evaporator increases slowly over the operating season, it is an indication that the cooler tubes may be fouling or that there may be a water bypass in the water box. This may require the replacement of a gasket.

The condition of heat exchanger tubes is vital to efficient, reliable chiller operation. Tube failure due to freeze damage, internal pitting, erosion, metal deposit, and support wear can result in catastrophic chiller failure and expensive, time-consuming repairs.

Unfortunately, tube condition is not readily visible. Tubes are hidden inside the shell, and the condition of the internal tube walls is also hidden from sight.

Eddy-current tube testing addresses these challenges by using a specially designed electronic probe to induce small electrical current through the tube being tested. Any variation from normal tube structure (including pitting, cracking, and wear) causes a change in the flow of electrical current, producing electrical disturbances (eddy currents) that are picked up on an oscilloscope or a chart recorder. A specially trained operator then interprets the data to identify existing or potential problems and to recommend corrective action before a leak occurs. Eddy-current tube testing should be performed on chiller heat-exchanger tubes every three years or as needed.

Additionally, water-side strainers should be cleaned annually (or more often as needed) to maintain the proper flow rate of condenser water. Reduced flow-rate increases compressor head and energy consumption. A 20% flow rate reduction can increase full-load energy use 3%, making it critical to maintain the flow rate within design limits established by the manufacturer.

Manufacturers' Recommendations

It is important to remember that preventive maintenance recommendations vary from manufacturer to manufacturer and are clearly stated in operation/maintenance manuals. Additionally, some manufacturers have developed chillers with on-board computers that aid in troubleshooting and problem identification. In most cases, these chillers tie into facility-wide computer systems to facilitate remote monitoring and diagnosis of equipment problems. With the help of computer software, information can be collected and evaluated to ensure efficient chiller operation.

Whether or not a chiller is equipped with a computerized maintenance system, an effective preventive maintenance program requires familiarity with the operating/maintenance manual and ensures that only qualified personnel or industry-certified technicians perform maintenance to eliminate the dangers of injury and legal liability.

The result is well-maintained equipment that efficiently and safely delivers comfort to building occupants, consumes less energy and lasts longer, reduces the frequency of expensive repairs, and extends the life of the system by helping to prevent premature equipment failure.

A proactive preventive maintenance program saves money over the lifetime of the chiller. It costs less to make small repairs and perform routine maintenance than it does to replace large components during costly, unscheduled downtime, and it minimizes large equipment expenditures by ensuring longer chiller life. But the single most easily identified savings are in efficiency. Well-maintained chillers use less energy, thereby sharply reducing operating costs and clearly improving return on capital investment.

Variable-Speed Drive Corrects Inefficiencies and Generates Savings

Incorporating procedures that promote efficient operation and reduce energy consumption can enhance preventive maintenance goals. How and when a chiller operates can be as important to efficiency as care of the chiller components.

Chillers typically operate at off-design conditions 95% of the time, the result of low load and/or low entering-condenser-water temperatures (ECWTs). The resulting energy inefficiencies are motivating building owners and facility managers to explore whether chiller plants can be operated more efficiently during these conditions. In the process, variable-speed drive is emerging as a solution to the energy inefficiencies associated with off-design conditions.

Conventional chillers reduce capacity at off-design conditions by maintaining a constant motor speed and restricting the flow of refrigerant by closing the compressor's inlet guide vanes. This closure induces flow losses that reduce compressor efficiency.

On a variable-speed chiller, the drive motor slows down or speeds up depending on the operating conditions. The variable-speed drive monitors several operating conditions, including chilled-water temperature, chilled-water-temperature set point, evaporator and condenser pressures, inlet vane position, and motor speed, and then determines the optimal motor speed and inlet-vane position in order to consume the least amount of energy.

Variable-speed control of a centrifugal chiller can produce energy savings as much as 30% annually when compared to a constant-speed chiller, and savings can reach 75% at lighter loads. With energy savings of this magnitude, the added cost of the variable-speed drive for the chiller can be paid off quickly. Additionally, the use of a variable-speed drive may lower maintenance costs because it soft-starts the chiller, saving wear-and-tear on the drive line and extending its life for years.

It is not unusual, then, that a preventive maintenance program that seeks to improve chiller efficiencies, reduce energy consumption and extend equipment life considers the addition of a variable-speed drive. The resulting off-design energy savings ensure a quick payback on the variable-speed drive, often in as little as one to three years.