Civil Engineering Reference
In-Depth Information
9.7 Sequence control of multiple chiller plants
9.7.1 An overview of chiller sequence control
Centralized chilling systems with multiple centrifugal chillers are commonly
used in commercial buildings for providing cooling to the occupied spaces.
Over one-third of total energy consumption is used by these chillers in most
air-conditioned commercial buildings in a climate like Hong Kong's. Large
buildings normally employ multiple numbers of chillers. There are many
advantages of using multiple chillers in a building's central chilling system.
The running chillers can remain at high efficiency in partial system load
as some chillers can be turned off and the running chillers can be heavily
loaded. Therefore, operation of chillers in extremely low load can be avoided,
where a surge of centrifugal compressors may happen or chillers may lose
temperature control. The life of equipment is lengthened as the total number
of running hours of an individual machine is reduced, resulting in lower
maintenance cost and reduced chances of failure. Some of the chillers can be
set aside for routine maintenance in order to improve the overall reliability
of the system.
When multiple chillers are employed, sequencing their operation cor-
rectly becomes an essential function, which must be provided by the control
systems for central chilling systems. Such control is normally called chiller
sequence control. The chiller sequence control provides economical loading
and unloading of chillers. The control can be as simple as manually turning
on and off a second chiller. However, the ultimate aim of the sequence control
is to make the operating chillers achieve an overall COP as high as possible
while fulfilling the demanded cooling load. A proper chiller sequence plays
an important role in the overall performance of an air-conditioning system. It
can become very complex when multiple chillers of different cooling capaci-
ties, chiller types and efficiency are installed.
There are various methods of chiller sequence control employed in vari-
ous buildings with different complexities in terms of control parameters and
equipment. Their differences mainly lie in how the instantaneous building
load is measured and the ways to determine how many and which chillers are
to be put into operation accordingly. It should be mentioned that details of
the chiller sequence control are also dependent on the part-load characteris-
tics of the chillers and their associated chilled water circulation pumps as well
as on the circuit design of the chilled water distribution system. However,
this section focuses on the sequence control methods and their performance.
Typical chiller sequence control methods include:
temperature-based sequence control;
bypass flow-based sequence control;
direct power-based sequence control;
total cooling load-based sequence control.
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