Chiller performance varies greatly with operating conditions. Full-load performance is not a good indicator of overall performance because chillers rarely operate at full load. Instead, part-load performance is critical to good overall chiller performance. Refer to ASHRAE Standard 90.1-2001, which lists minimum chiller efficiency requirements. There are several variables that can affect the efficiency of a chiller.
The flow rate must be closely regulated, as too low a flow rate can reduce the efficiency of the chiller, resulting in laminar flow. The minimum flow rate is usually about 3 feet per second. However, excessive flow rates can cause vibration, noise and pipe corrosion. The recommended maximum flow rate is usually about 12 feet per second.
The amount of cooling any cooler can provide depends on the amount of refrigerant it passes through the compressor per unit time. It is important to maintain the proper level of refrigerant.
Leaks, as well as air and moisture, can reduce efficiency and system reliability. A low refrigerant charge (usually caused by leaks) will cause the compressor to work harder and achieve poorer cooling.
Modular Type Air Cooled Scroll Chiller
Heat transfer has the greatest impact on chiller performance. The condenser and evaporator tubes in large chillers can exceed 5 miles, so high heat transfer is fundamental to maintaining efficiency.
When tubes scale, chiller efficiency decreases rapidly. Contaminants such as minerals, scale, slurry, algae and other impurities can increase thermal resistance and reduce overall performance. In both closed-loop and open-loop systems, these contaminants can accumulate on the water side of the heat transfer surface. Scaling can occur gradually over time, depending on the quality and temperature of the water used.
The approach temperature of the compressor, i.e. 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 increase in approach temperature is the main indicator of a decrease in heat transfer efficiency. An accurate log sheet will show when the temperature starts to change from the effective level.
Condenser tubes should be brushed at least once a year using an automatic rotary cleaning machine, rather than chemical cleaning, to keep them free from contamination.
Air Cooled Scroll Chiller
Condenser water circuits that use open cooling sources, such as atmospheric cooling towers, require water treatment to prevent scaling. Erosive conditions, such as sand flowing through the tubes at high velocities, can dent the tubes and thus reduce their efficiency. Untreated water can damage pipes, ducts and other materials.
Cooling tower discharge or drainage is the most efficient way to remove solids and contaminants. When a sensor detects high water conductivity, an automatic valve discharges some water and its dissolved and suspended solids.
Visual inspection is also a good, though less accurate, indicator of water quality. Check the chilled water circuit once a year for general water quality and signs of corrosion.
Lowering the temperature of the water entering the condenser can improve the efficiency of the chiller. In some building systems, operators can lower the chilled water set point to overcome air handler deficiencies, such as dirty coils. Beware of this practice; it may stop the symptoms, but it will not solve the problem. It makes the chiller work harder to get the same net cooling effect.