Civil Engineering Reference
In-Depth Information
h
rtn
h
fr
100~200%
Rescale
Economizer
logic
h
rtn
: return air enthalpy
h
fr
: fresh air enthalpy
M
setDCV
: fresh air rate
set-point
by DCV control
M
fr
: actual fresh air rate
Maximum position
when
h
rtn
≥
h
fr
0~100%
{
Fresh air
damper
u
c
DCV based fresh air
damper control
activated when
h
rtn
<
h
fr
{
Wide open when
u
c
=100%
Closed when
u
c
≤
0%
Cooling coil
valve
(Heating coil valve closed )
M
fr
M
setDCV
Set-point
Maximum position
when
u
dT
=100%
Minimum position
when
u
dT
≤
0%
{
M
setDCV
< M
fr
0~100%
DCV control
logic
Feedback
control
u
Fresh air
damper
u
dT
u
dT
Supply air
temperature
(Heating and cooling
coil valves closed)
M
setDCV
≥
M
fr
-100~0%
DCV-based fresh air
damper control activated
{
Wide open when
u
h
=100%
Closed when
u
h
≤
0%
0~100%
Rescale
Heating coil valve
u
h
(Cooling coil valve closed)
Figure 8.26
Logic of AHU sequential split-range strategy combining DCV control.
cooling demand, namely partial free cooling. When the fresh air enthalpy
is larger than the return air enthalpy, DCV control is used and the fresh air
damper is regulated according to the control signal from the DCV-based fresh
air damper control,
u
dDCV
, to keep acceptable indoor air quality.
When the output from the temperature control loop is between 0 and 100
per cent, the fresh air damper is adjusted to control the amount of the fresh
air flow to ensure the supply air temperature remains at the set-point (i.e.
total free cooling is activated). In this process, the DCV control logic is used
to determine whether the fresh air flow based on temperature control is suf-
ficient or whether the DCV-based fresh air damper control should be applied.
If the set-point of the fresh air flow based on DCV control is larger than or
equal to the actual fresh air flow rate, the DCV-based fresh air damper con-
trol is activated to modulate the fresh air damper to take in sufficient fresh
air. Simultaneously, the output from the feedback temperature control is
reset to 0 per cent by resetting the PID terms of the three temperature-based
control loops.
When the output from the temperature control loop is between 0 and -100
per cent, it is scaled to 0 to 100 per cent to modulate the heating coil valve to
heat the air (i.e. the process enters heating mode). Here 0 per cent represents
the closed position of the heating coil valve (i.e. no heating demand), and
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