Control and optimization of air- conditioning systems - Intelligent Buildings and Building Automation - page 139

of zoning of the building, the air-side systems can be divided into single-zone
systems and multiple-zone systems.
The advantages of all-air systems are as follows:
￿ separation of mechanical areas and major equipment from occupants;
￿ possibility of using outdoor air for cooling and recovery;
￿ flexibility to outdoor and occupancy changes;
￿ flexibility in design and control;
￿ well suited to applications requiring unusual exhaust and makeup air
quantities (+/- pressurization).
The disadvantages of all-air systems are:
￿ they require additional duct space;
￿ the air balance required can be difficult;
￿ they require close cooperation between architectural, system and struc-
tural designers to ensure accessible terminal devices.
Fan-coil systems are the most commonly used air-water systems . A fan-coil
system is simple in design, operation and control, and it is less expensive
than all-air systems. It is flexible for zoning and requires less space for the
air ductwork. However, the system is mounted in occupied spaces, which can
cause noise problems. There is little flexibility in outdoor ventilation control,
and free cooling is not really achieved.
It is not the objective to list all different air-side systems or to discuss their
design issues in this chapter. This chapter will mainly address the operation
and control of typical air systems, while the basic characteristics of the typical
systems are discussed when presenting their control and operation issues.
8.1.2 Cascade control
Cascade control can be used when there are several measurement signals
and one control variable. It is particularly useful when there are significant
dynamics (e.g. long dead times or long time constants) between the control
variable and the process variable. Tighter control can then be achieved by
using an intermediate measured signal that responds faster to the control
signal. Cascade control can be categorized into two kinds. One is called
basic cascade control, built up by nesting the control loops as shown in the
block diagram in Figure 8.1. The other is called reconfigured cascade con-
trol, which is tailored for specific applications, such as the example shown
in Figure 8.2.
Basic cascade control has two loops. The inner loop is called the secondary
loop . The outer loop is called the primary loop . The reason for this terminol-
ogy is that the outer loop deals with the primary measured signal. It is also
possible to have a cascade control with more nested loops. The performance