Best Practices for Efficient Operation
The following best practices can improve chiller performance and reduce operating costs:
Operate multiple chillers for peak efficiency: Plants with two or more chillers can save energy by matching the building loads to the most efficient combination of one or more chillers. In general, the most efficient chiller should be first one used.
Raise chilled-water temperature: An increase in the temperature of the chilled water supplied to the building’s air handlers will improve its efficiency. Establish a chilled-water reset schedule. A reset schedule can typically adjust the chilled-water temperature as the outside-air temperature changes. On a centrifugal chiller, increasing the temperature of chilled water supply by 2–3°F will reduce chiller energy use 3–5%.
Reduce condenser water temperature: Reducing the temperature of the water returning from the cooling tower to the chiller condenser by 2–3°F will reduce chiller energy use 2–3%. The temperature setpoint for the water leaving the cooling tower should be as low as the chiller manufacturer will allow for water entering the condenser. The actual leaving tower water temperature may be limited by the ambient wet bulb temperature.
Purge air from refrigerant: Air trapped in the refrigerant loop increases pressure at the compressor discharge. This increases the work required from the compressor. Newer chillers have automatic air purgers that have run-time meters. Daily or weekly tracking of run time will show if a leak has developed that permits air to enter the system.
Optimize free cooling: If your system has a chiller bypass and heat exchanger, known as a water-side economizer, it should be used to serve process loads during the winter season. The water-side economizer produces chilled water without running the chiller. Condenser water circulates through the cooling tower to reject heat, and then goes to a heat exchanger (bypassing the chiller) where the water is cooled sufficiently to meet the cooling loads.
Verify Performance of hot-gas bypass and unloader: These are most commonly found on reciprocating compressors to control capacity. Make sure they operate properly.
Maintain refrigerant level: To maintain a chiller’s efficiency, check the refrigerant sight-glass and the superheat and subcooling temperature readings, and compare them to the manufacturer’s requirements. Both low-level and high-level refrigerant conditions can be detected this way. Either condition reduces a chiller’s capacity and efficiency.
Maintain a daily log: Chiller O&M best practices begin with maintaining a daily log of temperatures, fluid levels, pressures, flow rates, and motor amperage. Taken together, these readings serve as a valuable baseline reference for operating the system and troubleshooting problems. Many newer chillers automatically save logs of these measurements in their on-board control system, which may be able to communicate directly with the DDC system. Below is an example of a daily log that can be adapted for use with your chiller.