Carel Hecu CO2, Руководство пользователя

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32
ENG
Hecu
C O
2
+0300085EN rel. 2.2 - 07.05.2019
Evaporator requests
To roughly synchronise the condenser unit with evaporators managed by
third-party controllers (for example, when defrosting),
a digital input can be confi gured (on screen Baadh). This means that the
compressor does not start until the DI is activated, even if the suction
pressure is higher than the set point.
The DI is confi gured on the following screen:
Screen Baadh
6.7 HPV valve management
Management of the HPV valves, which separates the high pressure part of
the system from the medium pressure part, determines the transcritical
and subcritical operating mode of the unit. In transcritical mode, the
valve is controlled so as to obtain maximum effi
ciency, while in subcritical
mode subcooling is controlled.
The HPV valve has propor integral (PI) control, with the set point
based on an optimal gas cooler pressure value calculated using the gas
cooler pressure and temperature, as described below. Enabling HPV
valve management means also enabling system transcritical mode.
The HPV valve can be managed directly by Hecu CO2 via the driver
integrated into the controller. The algorithm for calculating the HPV valve
control set point can be optimised or customised by the user, using the
parameter settings.
Optimised set point calculation
The optimised set point is calculated as shown in the fi gure.
T₂₃
A
B
C
D
HPV C L O S ING ZONE
HPV OPENING Z
ONE
S UBCRITICA L ZONE
INTERMEDIATE ZONE
UPPER PROPORTIONA L ZONE
L
O
W
ER PROPOR
TIONA
L
Z
ONE
TRAN S CRITICA L ZONE
P₁₀₀
%
P
max
T₁₂
T
₁₀₀
%
T
min
Enthalpy (KJ/Kg)
P
ressur
e (Bar)
Fig. 6.u
The HPV valve is managed according to the area identifi ed, based on the
gas cooler outlet temperature and pressure values. To defi ne the zones,
the two pressure values P
100%
and P
max
, the two temperatures T
12
, T
23
relative to points B and C in the fi gure, and the two temperatures T
min
and
T
100%
need to be set. In the following diagram, T
gc
and P
gc
refer to the gas
cooler temperature and pressure.
The behaviour of the HPV valve in the diff erent zones is as follows:
• Transcritical zone , identifi ed by T
gc
≥ T
12
and P
gc
≤ P
max
: the valve
works with propor integral (PI) control so as to maintain the
maximum COP, determined by the optimal pressure P
opt
calculated as
a function of the gas cooler outlet temperature T
ogc
.
• Subcritical zone , identifi ed by T
min
≤ T
gc
≤ T
23
: the valve works with PI
control so as to keep subcooling constant.
• Transition zone , identifi ed by T
23
≤ T
gc
≤ T
12
: the valve works with PI
control, with a pressure set point identifi ed as the intersection of the
two points B and C in the fi gure, obtained by calculating the optimal
pressures at the limit of the transcritical and subcritical zones. This area
has the purpose of avoiding discontinuity in the transition between
the two zones.
• Upper proportional zone , defi ned by P
max
< P
gc
< P
100%
: the valve
works with proportional control only between the opening reached
at pressure P
max
and the maximum opening at pressure P
100%
. If the
pressure decreases, the opening of the HPV valve remains constant
until it returns into the transcritical zone, where control resumes as
described previously.
• Lower proportional zone , defi ned by T
100%
< T
gc
< T
min
: the valve works
with proportional control only between the opening reached at
temperature T
min
and the maximum opening at temperature T
100%
. If
the pressure increases, the opening of the HPV valve remains constant
until it returns into the subcritical zone, where control resumes as
described above. This operating mode can be disabled by parameter.
Custom set point calculation
The custom set point diff ers from optimised control in that the curve
in the subcritical phase is a straight line and is defi ned by the user, and
therefore the bands and the set point can be customised by the user.
The behaviour in the remaining bands is as described for the optimised
algorithm.
HPV valve auxiliary functions
HPV valve management includes some auxiliary functions:
• Pre-positioning : when switching the unit ON, the HPV valve remains
in a fi xed position set by parameter for a set time, so as to be able
to quickly increase the pressure in the receiver. This procedure is
reactivated whenever the unit switches OFF or the HPV valve is reset to
the minimum position when all of the compressors stop (opt.).
• Close valve with compressors off : if all of the medium temperature
compressors are switched off , the HPV valve can move to the
minimum opening when OFF, which can be set by parameter. When
a compressor restarts, valve control resumes, with the pre-positioning
procedure described in the previous point.
• Minimum and maximum opening values : the minimum opening
when OFF (from keypad, digital input or supervisor) and when ON can
be diff erentiated, while there is only one maximum opening value.
• Maximum percentage variation : the valve movement cannot exceed
the set maximum percentage variation per second.
• Set point fi lter : the HPV valve control set point is calculated taking into
account the average of the last n samples (maximum 99), so as to avoid
sudden variations due to the high variability of the gas cooler outlet
temperature.
• Minimum set point : a minimum value can be set for the HPV valve set
point; the set point can never fall below this value, regardless of the
parameters entered, so as to protect the compressors.
• Set point deviation alarm : if the gas cooler pressure remains too far
away from the calculated set point for too long (settable threshold and
delay), an alarm can be signalled.
Receiver pressure control using the HP
V
valve
If the receiver pressure falls below the set minimum operating pressure
threshold, the dynamic set point calculated for the HPV valve can be
adjusted, so as to increase the pressure inside the receiver. An off set
proportional to the deviation from the minimum threshold is subtracted
from the calculated set point, so that opening the HPV valve further
increases the pressure in the receiver. The off set is directly proportional
to the deviation from the minimum operating threshold, as shown in the
fi gure:
OFF
. M
AX
Setpoint HPV
Pmin Prec min
Fig. 6.y