AC SYSTEM CONFIGURATION- CENTRAL CHILLER PLANT

AC SYSTEM CONFIGURATION- CENTRAL CHILLER PLANT

Central Chiller Plant (with Cooling Tower and Chilled Water distribution) The other AC configuration is called a Chilled Water or Larger Cooler system. It uses the V- C cycle, but in large systems, it s impractical to run refrigerant piping all through the system. So the DX is replaced by a chiller plant, and heat exchangers are added. In larger facilities, the DX system is often replaced by a central chiller plant. Chilled water is produced and distributed throughout the facility. There are usually multiple chillers in the central plant. Each chiller employs the V-C cycle in which the evaporator and condenser coils are replaced with tube-and-shell heat exchangers, refrigerant on one side, water on the other side.

Chiller Water Temperatures and Energy Used The amount of energy the compressor uses is based on the amount of temperature lift the difference between chilled water (CHW) and condenser water (CW) temperatures. The greater the difference, the more energy used. CW At part-loads when it is less than peak temperature and humidity CHW temperature can be raised and the cooling tower can be reset to lower the CW temperature. Energy Used CHW Such temperature resets can save significant energy generally 2 3% of energy per degree reduction in lift.

Chiller Delta T and Efficiency Chillers are typically designed for a temperature change ( delta T ) across the evaporator of 10 12 degrees F. Low delta T at part-load usually indicates excess CHW flow and reduces the efficiency and capacity of a chiller. If your system runs with low delta T, adjustments or retrofits can save significant amounts of chiller and pumping energy.

Chiller and Loads Many newer chillers perform better at part-loads as you can see from this manufacturer s integrated part-load curve. You should check the curve from your manufacturer to see how your chiller will perform at part-load. Older chillers are designed for full (design) load. Compressor capacity must be controlled to match part-loads.

Chiller Compressor Capacity Control AC equipment often runs at part-load. In small DX equipment, the compressor is simply turned off and on. This would be undesirable with the large motors of chillers. There are various ways, however, to enable chillers to operate at part-load without on-off cycling. Hot Gas By-pass A hot gas bypass allows (some of the) hot gas leaving the compressor to bypass the condenser, going directly to the evaporator. Evaporator temperature is elevated, matching the part-load. But efficiency declines as some of the compressor work does no cooling. Unloaders Compressor unloaders offer chiller energy savings by using a solenoid to hold open a valve inside the compressor so the refrigerant is not compressed in that particular cylinder where the chiller compressor unloader is open. Of course, the compressor must have multiple cylinders before this feature becomes practical so you will only find unloaders on chilled water compressors with multiple cylinders on larger chiller systems.

Chiller Compressor Capacity Control Multiple Compressor Staging Compressors that are not needed are turned off. Staging is accomplished by controls that indicate the loading of the system. Inlet Vanes (centrifugal compressors) Inlet vanes are a type of diffuser used to regulate the flow of a fluid. Closing the inlet vanes restricts the flow of the refrigerant.

Chiller Compressor Capacity Control Variable Speed Drive Control (Variable Flow Refrigerant) This includes variable refrigerant flow. A VSD will slow the motor and the refrigerant flow so that enough cooling can be produced to meet the load without excess compressor work. Most efficient method, used in newest equipment. More info: All of these techniques are done to allow the compressor to match the cooling load at any given time. Chillers work best when they have a 10-12 degree ΔT (change in temperature) of the chilled water between when it enters and leaves the chiller.

Components of Chiller Condenser Water Pump(s): Pumps condenser water to cooling tower. Requires sufficient head for lift to open loop cooling tower Condenser Tube Bundle: transfers heat to a cooling tower for heat rejection. Cooling Tower: Rejects system heat to atmosphere Condenser Water (CW) Pump Condenser Tube Bundle Cooling Tower V-C Cycle: as previously discussed, usually centrifugal or screw compressor, with shelland-tube heat exchangers ( tube bundles ) as evaporator and condenser. Controls: A controls package from the chiller manufacturer, often connected to the Building Automation System (BAS) for integrated control. Controls Evaporator Tube Bundle Chilled Water Chilled Water (CHW) Pump Evaporator Tube Bundle: Produces chilled water for distribution for space cooling. Chilled Water: Output temperatures range from 42 o F to 52 o F. Chilled Water Pump(s): May be dedicated per chiller, constant or variable flow, primary or primary-secondary arrangements. By looking at the diagram, you can see the two heat exchangers which are not present on the smaller DX configuration.

Chiller Maintenance Heat exchanger tubes should be checked for integrity (Eddy Current test) and cleaned on at least a bi-annual basis. Regularly clean water strainers, especially on the condenser loop. Keeping the cooling tower clean will also greatly help in maintenance of the condenser heat exchanger. Unless properly managed, oxygen dissolved in water will lead to scaling, corrosion and premature failure of the heat exchanger tubes. A chemical water treatment program is an important part of maintenance. Follow a scheduled routine for maintenance of compressors and pumps. ASHRAE standard 180: Standard Practice for Inspection and Maintenance of Commercial Building HVAC Systems.