Control and optimization of central chilling systems - Intelligent Buildings and Building Automation - page 175

9 Control and optimization of

central chilling systems

Central chilling systems consume about 25 to 50 per cent of annual energy
budgets in most air-conditioned commercial buildings. Well-monitored and
controlled central chilling systems have great potential to improve operation
reliability and reduce overall energy consumption. This chapter presents
the energy characteristics and basic control of chillers, typical chilling sys-
tem configurations, the general approaches for optimal control of chilling
systems, the optimal control of heat-rejection systems and the systematic
optimization of heat-rejection systems, the optimal set-point reset of chilled
water temperature, the sequence control of multiple chiller plants and the
optimal control of chilled water distribution systems.

9.1 Basic knowledge of chillers

9.1.1 Basic components and typical types

Generally, a central chilling system consists of three subsystems:

￿ Chillers , whose function in a central chilling system is to produce an
adequate quantity of chilled water at the required temperature.
￿ Heat-rejection system , whose function is to reject the heat from the refrig-
erant to the environment.
￿ Chilled water distribution system , whose function is to distribute the
chilled water to the user terminals.

In a central chilling system, the heat (cooling load) is carried by the chilled
water to the evaporator, where the heat is transferred to the refrigerant. The
refrigerant takes the heat to the condenser where it is passed on to the cool-
ing water.
A typical mechanical chilling system involves at least one compressor
(reciprocating, rotary-screw or centrifugal) and two heat exchangers (the
evaporator and the condenser) as well as a heat-rejection system such as a
cooling tower. The system operating cost is the cost of circulating the fluids
(refrigerants, water, etc.) in the heat exchangers and the compressor.