Ten tips for improving chiller efficiency

Achieving performance is a matter of common sense

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Chillers often represent a plant's single largest electric load. A 600-ton chiller can draw 0.547 kW/ton, which, at a rate of $0.08 per kWh, adds up to more than $200,000 per year in operating costs. Factor in fouled tubes, leaking refrigerant, changes in water temperature and a myriad of other variables and operating costs can escalate by 8 to 10 percent an additional $20,000 per year. Operating a chiller at its peak performance saves energy as well as maintenance costs.

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Chiller efficiencies have improved significantly during the past 10 years, mostly as a result of new refrigerants and microprocessor controls, as well as improved compressor and motor design. However, high-tech chillers have narrower tolerances, so that service and upkeep are more crucial than ever for keeping them operating at peak performance.

Chiller performance varies significantly with operating conditions. Full-load performance is not a good indicator of overall performance, because chiller plants rarely operate at full load. Instead, part load performance is critical to good overall chiller plant performance. Consult ASHRAE Standard 90.1-2001, which lists minimum chiller efficiency requirements. Several variables affect chiller efficiency. Following are 10 tips that can help keep it riding high.

Keep a daily log

The daily log is still the first step in maintaining an efficiently run chiller plant. A log builds a history of operating conditions, including temperature, pressure, fluid level and flow rate. Although microprocessor controls are recording more of these statistics, there is no substitute for a written log detailing findings from daily inspection of controls in the chiller room.

Keep tubes clean

Heat transfer has the greatest single effect on chiller performance. Large chillers can have more than five miles of condenser and evaporator tubes, so high heat transfer is fundamental to maintaining efficiency.

Chiller efficiency declines rapidly when tubes become fouled. Contaminants, such as minerals, scale, mud, algae and other impurities, increase thermal resistance and reduce overall performance. These contaminants accumulate on the water side of heat transfer surfaces in both closed- and open-loop systems. Fouling occurs gradually over time, depending on the quality and temperature of the water used.

The compressor's approach temperature, the difference between the temperature of the fluid leaving the heat exchanger and the saturation temperature of the refrigerant being cooled or heated, is a good indicator of heat transfer efficiency. An increasing approach temperature is a prime indicator that heat transfer efficiency is decreasing. An accurate log sheet will reveal when temperatures start to change from efficient levels.

Condenser tubes should be brush cleaned, rather than chemically cleaned, at least annually with an automatic rotary-cleaning machine to keep them free of contaminants.

Treat condenser water

Condenser water loops using open cooling sources, such as atmospheric cooling towers, require water treatment to prevent fouling. Erosive conditions, for example, sand flowing through the tubes at high velocity, may pit tubes, decreasing tube effectiveness. Untreated water can damage tubes, piping and other materials.

A cooling tower bleed or blowdown is the most effective way to remove solids and contaminants. When a sensor detects high water conductivity, an automatic valve dumps some water and its load of dissolved and suspended solids.

A visual inspection also is a good, although less accurate, indicator of water quality. Inspect chilled water loops once a year for general water quality and evidence of corrosion.

Reduce entering water temperature

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