Civil Engineering Reference
In-Depth Information
DCV control
Total free
cooling
Partial free
cooling
DCV control
Feedback transition
region (h
rtn
=h
fr
)
100%
B
h
rtn
>h
fr
Outdoor air damper
Cooling valve
Heating valve
A
h
rtn
<h
fr
C
M
setDCV
=M
fr
M
setDCV
=M
fr
0%
200%
u
0
(0%)
100%
-100%
Output of the temperature controller
h
rtn
: return air enthalpy
h
fr
: fresh air enthalpy
M
fr
: actual fresh air rate
M
setDCV
: fresh air rate set-point by DCV control
u
0
: output of the temperature controller when M
setDCV
=M
fr
Figure 8.25
Sequential split-control strategy of VAV system with DCV control.
exploits the actual fresh air flow rate (
F3
), with respect to the set-point fresh
air flow rate from the fresh air set-point controller (
FC-2
), to generate the
signal for the fresh air damper control. In total free cooling mode, the fresh
air damper controller (
DC-1
) receives a control signal from the supply air
temperature controller (
TC-1
) to control damper position to maintain a
desired supply air temperature.
Figure 8.26 presents a detailed description of the relationship between the
combined PI(D) feedback control output
u
and the control signals
u
c
,
u
h
and
u
dT
(for the cooling coil valve, the heating coil valve and the fresh air damper
respectively), and the relationship between the temperature control loop and
the DCV based fresh air damper control loop. The temperature control out-
put scale ranges from -100 to 200 per cent. When DCV control is preferable,
fresh air damper control output is between 0 and 100 per cent.
When the output for the temperature control loop is between 100 and 200
per cent, it will be rescaled to 0 to 100 per cent to actuate the cooling coil
valve to cool the air (where 100 per cent corresponds to the closed position
of the cooling coil valve, and 200 per cent corresponds to the fully open posi-
tion). In the meantime, according to the return air enthalpy and the fresh air
enthalpy, economizer logic is utilized to determine whether the DCV-based
fresh air damper control is activated. When the return air enthalpy is larger
than or equal to the fresh air enthalpy, economizer cycle control (the fresh air
damper is fully open) is exploited simultaneously to reduce the mechanical
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