Reactive control for discrete chiller units

#This invention relates to the simultaneous control of several individual chiller units within a system that is processing chilled coolant via common supply and return lines. In particular, this invention relates to the control of a number of different chiller units each having their own respective dedicated control units.

#Multiple chillers on a common coolant loop are subjected to the same entering coolant temperature at approximately the same time. The chillers are typically required to maintain the leaving coolant at the same temperature. This causes the individual units to simultaneously start or stop their compressors at the same time. This leads to unnecessary and at times excessive demands on electrical power consumption and excessive compressor cycling in order to provide the chilling of the coolant perceived to be necessary. In this regard, each individual chiller unit having its own particular compressor stages and control unit is trying to activate the perceived number of compressor stages necessary to achieve the desired chilling without regard to what may be happening elsewhere. This often produces an over reaction. This over action may include a significant drop in coolant temperature when increased chilling capacity has been unnecessarily added. What is needed is a control system which allows individually controlled units to provide normal control functions while at the same time supervising the reaction by the individual chiller units when certain coolant temperature conditions occur.

#It is an object of the invention to provide a control that synchronizes the individual control units of each chiller in order to maintain the desired cooling.

#Other objects and advantages of the present invention will be apparent from the following description in conjunction with the accompanying drawings in which:

#FIG. 1 illustrates a plurality of individually controlled chiller units each having a plurality of compressor stages located therein that may be activated by a supervisory control system which interfaces with the individually controlled chiller units;

#FIG. 2 is a diagram of an overall software control process residing in the supervisory control system of FIG. 1;

#Referring to FIG. 1, a series of chiller units 10, 12 and 14 are illustrated relative to a coolant return line 16 and a coolant supply line 18. The coolant will hereinafter be referred to as chilled water although it should be understood that the system would work equally well with other coolants. Each of the chiller units includes individual compressor stages such as 20, 22, and 24 for chiller unit 10 that can be individually activated by a unit controller such as 26 for chiller unit 10. In a similar manner, compressor stages can be activated by a unit controller 28 for chiller unit 12 and a unit controller 30 for the chiller unit 14. Each unit controller monitors the temperature of the chilled water in a line 18a, 18b, or 18c in order to exercise appropriate local control over the respective chiller. This local control may be in accordance with any number of well known controller methods for activating or deactivating compressors in order to achieve the desired water supply temperature. In addition to controlling the respective chiller unit, each unit controller communicates with a system controller 32 via a communication bus 34. The system controller 32 receives local unit controller information from each controller unit over the bus 34 such as chiller status, unit capacity and available capacity. The system controller also receives certain information from a user interface 36 relating to the overall system of chillers. This information includes the number of chiller units in the system, the coolant or chilled water set point, the overall system demand limit and the minimum leaving chilled water control point temperature that are to apply to each chiller control unit. The system controller also receives information about each controller unit from the user interface 36 including the particular address for each unit controller, the effective capacity and the number of stages of compression control. The system controller finally receives a chilled water supply temperature reading from a chilled water supply sensor 38 and a chilled water return temperature reading from a chilled water return sensor 40.

#Referring to FIG. 2, the programmed control within control system 32 that is responsive to the aforementioned inputs is generally illustrated. It is to be understood that this programmed control resides in a computer having sufficient memory and processing rate to execute the programmed control. The program control begins with a subroutine 44 which reads the data entered at the user interface 36 and also initializes the value of certain variables that will be used later. The normal program flow is to proceed to a subroutine 46 and extract the operating data from the unit controllers 26, 28, and 30 and calculate the number of chillers actually running. An initial demand limit and leaving chilled water control point are thereafter transmitted to each of the control units. The initial demand limit which each control unit is given may not allow the activation of a sufficient number of compressor stages to meet the chilled water control point. The chillers will nonetheless attempt to maintain their respective leaving chilled water temperatures at the control point while not exceeding their respective demand limits. A step 50 simply determines whether there is more than one chiller unit operating. In the event that there is not, the control program loops around to the subroutine 46. This allows the individual operating chiller to operate under the control of its unit controller in maintaining the desired chilled water temperature.

#Referring again to step 50, in the event that more than one chiller unit is active, the program proceeds in a subroutine 52 to read the chilled water supply temperature from sensor 38 and the chilled water return temperature from sensor 40 and calculate certain system parameters. The program thereafter in a step 54 determines whether the chillers have responded to any previous commanded change in compression capacity. If change is still pending, the program so notes in step 56 and returns to subroutine 46.