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Spring Check-up For Centrifugal Chillers

By Tom Brown, York International Corp.

The overall goals of planned preventive maintenance (PM) for centrifugal chillers are to improve chiller reliability, increase equipment life, and maintain peak energy efficiency. A well-organized schedule of routine checkups and minor repairs reduces the frequency of unscheduled, often expensive service calls and minimizes the risk of catastrophic equipment failure and the associated potential for downtime, injuries, and unbudgeted, costly equipment replacement.

A thorough inspection and service of the chiller system now will keep it "humming" all summer long, while helping achieve longer term operating goals. A basic spring maintenance program includes a number of key components.

Daily Logs Identify Problems

Perhaps one of the most effective tools in the PM program is the permanent daily log that records operating conditions at regular intervals throughout a 24-hour operating period. When starting a chiller in the spring, it is important to record temperature readings, fluid levels, pressure readings, and flow rates. Comparing them to earlier readings enables building owners and facility managers to pinpoint problems and identify energy saving opportunities.

Graphic control panels-standard equipment on many chillers since 1999-eliminate the need to record many of these statistics manually. These panels provide user friendly logs on one large, active matrix screen. They also enable operators to view multiple related parameters simultaneously on a single screen and quickly generate on screen graphs of daily, weekly, or monthly trends for analysis.

Regular Inspection Of Settings And Calibration

An important part of PM, then, is to inspect the chiller's control center regularly. Most control centers employ two kinds of controls: safety and operating. Although checks on safety cutouts and operating checkpoints may be performed automatically, it is still important to check them manually prior to the beginning of the cooling season.

At the same time, it's a good idea to ensure the proper calibration of the control panel, transducers, and thermisters and to be certain the leaving chilled water temperature (LCWT) is set to the proper temperature. A 1¡F increase in LCWT can result in a 2% to 3% decrease in energy consumption.

Similarly, it is important to confirm condenser inlet water temperature is set to the minimum level recommended by the manufacturer. Energy savings at full load are about 1.5% for every 1¡F the entering condenser water temperature (ECWT) is reduced.

Mechanical Components Also Require Attention

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 will 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 seasonal start-up to confirm component conditions. These readings should then be compared to baseline readings established at system commissioning. The compressor presents a host of springtime PM procedures, beginning with replacement of the oil filter. An inspection of the used filter for metal particles should be part of the replacement procedure.

A check of compressor oil levels is just as important. Frequent readings via sight glasses provide valuable information regarding oil levels and contamination.

Oil analysis is also critical to system efficiency. 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 wear of parts.

The compressor motor needs to be checked to ensure the tightness of the motor mounting screws. As part of the motor check, examine the motor alignment and coupling for wear and make sure bolts are tight. A megohmmeter can be used to check the motor for moisture or deterioration of the winding insulation.

A spring PM 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.

The purge unit dehydrator should also be changed. In addition, the purge unit valves and orifices should be cleaned and inspected, and oil and refrigerant should be drained and flushed from the purge unit shell.

Repair Refrigerant Leaks And Check Charge Levels

The refrigerant system presents a number of opportunities to practice good PM and ensure energy efficient chiller operation. As part of a spring check-up, verify refrigerant charge amounts by comparing the temperature difference between the LCWT and the evaporator refrigerant.

This is a good time to repair leaks in the refrigerant system. In high pressure chillers, refrigerant can leak out, reducing refrigerant charge and limiting the unit's heat transfer capacity. In low pressure chiller systems, air can leak into the unit, displacing refrigerant vapor, causing higher condenser pressure, and increasing energy use.

Laboratory analysis of refrigerant can identify the presence of rust, sludge, or harmful acids in a chiller system as well as their potential to reduce operating efficiency and cause chiller failure.

Avoid Costly Tube Failure

Tube failure is the single most costly failure in a water chiller. 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, increase the thermal resistance in the heat exchanger, and increase energy consumption as a result. Consequently, the condenser tubes should be cleaned annually before seasonal start-up.

Follow Manufacturers' Recommendations

It is important to consult and follow the seasonal recommendations of the chiller manufacturer. Additionally, some chillers are equipped with on board computers that aid in identifying and solving problems. In most cases, these chillers tie into facility wide computer systems to facilitate remote monitoring and diagnosis of equipment problems.

Whether or not a chiller is equipped with a CMMS (computerized maintenance management system), an effective PM program requires familiarity with the operating/maintenance manual and ensures that only qualified personnel work on the equipment.

This results in equipment that efficiently and safely delivers comfort to building occupants, consumes less energy, and creates fewer headaches for facility managers.

A planned seasonal PM program saves money throughout the cooling season. It costs less to make small repairs and perform routine maintenance during the off season than it does to replace large components during costly, unscheduled downtime in the middle of the summer. And because well-maintained chillers use less energy, the seasonal check-up can help perpetuate energy efficiency as well.

Sidebar:

Variable Speed Drives Can Correct Inefficiencies And Generate Savings The pre-cooling season is a good time to consider retrofitting a chiller with equipment that promotes efficient chiller operation and reduces energy consumption. At this time of year, for example, leading HVAC manufacturers guarantee savings with the installation of proven variable speed drive (VSD) technology.

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, VSDs are 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 VSD monitors several operating conditions, including chilled water temperature, chilled water temperature setpoint, evaporator and condenser pressures, inlet vane position, and motor speed. It 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 VSD for the chiller can be paid off quickly. Additionally, the use of a VSD may lower maintenance costs because it soft starts the chiller, saving wear and tear on the driveline and extending its life for years.

It is not unusual, then, that a seasonal PM program that seeks to improve chiller efficiencies, reduce energy consumption, and extend equipment life includes the addition of a VSD. The off design energy savings that result can ensure a quick payback on the VSD, often in as little as one to three years.