Control and optimization of air- conditioning systems - Intelligent Buildings and Building Automation - page 164

FC-1

M-2 Pre-set flow

difference

CO 2

F2

E.A.

T2 H2

R.A.

Damper
motor

Pre-set s tatic
pressure PC-1

Cooling
coil

Heating
coil

M-1

P1 To space

F1

O.A.

T1 CO 2

CO 2 F3 T3 H3

Supply fan

Pre-set
temp.

V-1 V-2

DC-1 TC-1

Fresh air set point

FC-2

TC-1: Supply air temperature controller
DC-1: Fresh air damper controller
PC-1: Supply air static pressure controller
FC-1: Return air flowrate controller
FC-2: DCV based fresh air set point 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 8.24 Control diagram of VAV system with DCV control.

minimize the outdoor air intake. Outdoor air economizer control optimizes
the ventilation control by concerning itself with the energy issue only. To
minimize the energy use while providing sufficient or minimum required
ventilation, the best approach is the combination of economizer control and
demand-controlled ventilation.
Figure 8.24 shows a schematic diagram of an outdoor air ventilation
control strategy of a VAV system combining economizer control and DCV
control. Temperature control, return fan control and supply fan control are
the same as that shown in Figure 8.15. The main difference is that DCV
control is introduced and combined with the economizer control. The
temperature controller TC-1 generates the control signals for modulating
the heating coil valve, cooling coil valve and the fresh air damper (in total
free cooling mode) to control the supply air temperature. DCV-based fresh
air set-point controller FC-2 generates an outdoor air flow rate set-point
(demanded minimum outdoor air flow rate) for the damper controller DC-1
to maintain acceptable indoor air quality when DCV is beneficial, which
determines the flow rate set-point according to the actual occupancy load.
The supply air temperature and fresh air control are performed by sequential
split-range control.
Figure 8.25 illustrates the sequential split-range control strategy combin-
ing DCV control. When DCV control is preferable, damper control ( DC-1 )