Optimum Sequence of Operation for Energy Efficient Chiller Plants

In order to achieve energy efficient operation of chiller plant, a computer based supervisory controller is utilized. This controller receives input Data from sensors located in pipe lines and outside. Supply and Return Temperatures, pipe line pressures and chilled water flow rates are available. This data is utilized by the controller for calculating actual cooling demand in the building, and for sequencing of the plant. Plant includes all chillers, boilers, chilled/hot water pumps, condenser water pumps, cooling tower fans and variable frequency drives.

Following sequence may be used for highest energy efficiency.

1) Set point temperature for chilled water is entered to the controller. When temperature is higher than set point, chiller plant is started.

2) Integrated Part Load Values for equipment are determined and data of energy consumption at part loads for each machine is fed to the controller. Building actual cooling demand at any part load condition is calculated by the controller by taking flow reading and chilled water delta t. Controller decides which set of equipment will provide best performance at any load condition. These equipment are brought in to the circuit

3) Leaving chilled water temperature at the chiller is very critical parameter as for as energy consumption is concerned. When chiller is required to produce chilled water at lower temperatures, it consumes more energy. At higher chilled water out temperatures, chiller energy consumption is substantially low. Why temperature is set at higher level or lower level depends upon cooling conditions required. At any stage controller analyses required cooling capacity with optimum chilled water out temperature which is most energy efficient. Chiller is operated at this temperature out condition.

4) Chiller operating efficiency depends on condenser water inlet temperature. Condenser water is cooled at cooling tower. This temperature may be controlled by,
a) Modulating speed of cooling tower fan
b) Flow of water to the towers.
Ambient wet bulb temperature determines up to what condition cooling water may be cooled. Cooling tower water temperature is adjusted by the controller to get maximum efficiency by the chiller. Inlet conditions are varied in different seasons to get better operating conditions.

5) Chilled water is circulated throughout the building by pumps. These pumps consume lot of energy. As cooling demand is not constant in the building, water flow rate to the building is varied accordingly. Variable Frequency Drives on the pump motor modulates its speed. Consequently energy consumed at part loads is lower as compared to peak demand. This step saves energy wastage.