Centrifugal Chillers

Best Practices for Efficient Operation

The following best practices will 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.

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. Below is an example of a daily log that can be adapted for use with your chiller.D

Best Practices for Maintenance

Compared to a major chiller failure, a sound preventive and predictive maintenance program is a minor cost. Implementing a best-practice maintenance plan will save money over the life of the chiller and ensure longer chiller life.

Substandard operating practices frequently go unnoticed and become the accepted norm. Training personnel in both maintenance and operating practices is the best prevention. Many chiller manufacturers offer training for building operating engineers in operating and maintaining their chillers.

To effective maintain chillers, you must 1) bring the chiller to peak efficiency, and 2) maintain that peak efficiency. There are some basic steps that facilities professionals can take to make sure their chillers are being maintained properly. Below are some of the key practices.

Reduce Scale or Fouling

Failure of the heat exchanger tubes is costly and disruptive. The evaporator and condenser tube bundles collect mineral and sludge deposits from the water. Scale buildup promotes corrosion that can lead to the failure of the tube wall. Scale buildup also insulates the tubes in the heat exchanger reducing the efficiency of the chiller. There are two main preventive actions:

Checking water treatment: Checking the water treatment of the condenser-water open loop weekly will reduce the frequency of condenser tube cleaning and the possibility of a tube failure.

Checking the water treatment of the chilled-water closed loop monthly will reduce the frequency of evaporator tube cleaning and the possibility of a tube failure.

Inspecting and cleaning tubes: The tubes in the evaporator and condenser bundles should be inspected once a year, typically when the chiller is taken offline for winterizing. Alternately, for systems that operate all year to meet process loads, tube scaling and fouling can be monitored by logging pressure drop across the condenser and evaporator bundles. An increase in pressure from the inlet to the outlet of 3-4 PSI indicates a probable increase in scale or fouling requiring tube cleaning.

Inspect for Refrigerant Leaks

If possible, monitor the air-purge run timer. Excessive or increased air-purge time may indicate a refrigerant leak. If an air-purge device is not installed, bubbles in the refrigerant sight-glass may also indicate refrigerant leak. Gas analyzers can also be used to identify refrigerant leaks.

All Western Mechanical Inc technicians are certified in all major chiller manufacture products.