period four
Chiller-Plant Control
notes
Chiller Sequencing
▲Turning on an additional chiller
▲Turning off a chiller
▲Which chiller to turn on or off?
Figure 90
Chiller Sequencing
Chiller sequencing refers to making decisions about when to turn chillers on and off, and in what order. Typically, turning chillers on and off is performed with the goal of matching the capacity of the chiller plant to the system cooling load. In order to do this successfully, the design of the chilled-water system must provide the control system with variables that are good indicators of system load.
The hydraulic design and size of the chilled-water system will determine the possible method(s) for effectively monitoring system load. Typical methods for load monitoring include:
nIn series- or parallel-piped systems, the supply- and return-water temperatures, and sometimes chiller current draw, are monitored.
nIn a primary-secondary system, the system supply and chiller return-water temperatures and/or the direction and quantity of flow in the bypass pipe are typically measured.
nIn a variable-primary-flow system, the system supply-water temperature and the system flow rate may be monitored.
nDirect measurement of the system load (in tons, kW, or amperes) has also been used in some systems.
Other methods are also possible. It is imperative that the chilled-water system be designed with the control variables in mind; otherwise, the result may be a system that is impossible to efficiently control.
