Civil Engineering Reference
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different modes, which will be discussed further in Section 8.4.4.When the
strategy is more complicated in terms of programming, it is easier to pro-
gram in a high-level-language programming environment. In a symbolic
programming environment, programming is limited by the function boxes
provided.
8.3 Control of VAV systems
8.3.1 Control of VAV air-handling units
Variable air volume systems represent one major type of HVAC system in use
today. VAV systems are the results of developments of fans, motor drivers,
air diffusers and control in response to global energy crises. VAV systems are
divided into single-duct systems and dual-duct systems. In single-duct VAV
systems, the supply air temperature is controlled to be constant or is reset
by optimized supervisory control while the supply air volume changes to
satisfy the cooling load. As the volume of the supply air to the zones through
the terminal units (VAV boxes) changes, the air volume delivered by the fan
must also be adjusted. The control methods employ mainly motor-actuated
variable inlet vanes on the fan, variable-speed fan and variable pitch angle,
which are described in detail in Section 8.1.7.
Figure 8.15 presents a control diagram of a typical single-duct VAV sys-
tem with return and supply air fans equipped with variable speed control,
although there are many variations in applications. Supply air temperature
FC-1
Pre-set flow
difference
M-2
E. A.
F2
T
3
H3
R.A.
Damper
motor
Pre-set static
pressure
PC
-
1
Cooling
coil
Heating
coil
M-1
P1
F1
T
1
To space
T
4
H
4
O.A.
Supply fan
V-2
V-1
Pre-set
temperature
TC-1
DC-1
TC-1:
Supply air temperature controller
DC-1:
Damper controller
PC-1:
Supply air static pressure controller
FC-1:
Return air flow rate controller
V-1:
Cooling coil valve
V-2:
Heating coil valve
M-1:
Supply fan inlet vane damper motor
M-2:
Return fan inlet vane damper motor
Figure 8.15
Control diagram of a single-duct VAV system.
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