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The HPV valve is managed according to the zone identifi ed based on the
output temperature and gas cooler pressure.
In order to defi ne the zones, it is necessary to set the two pressure values
P
and P
, the two temperatures T
100%
max
the fi gure and the two temperatures T
In the following, with T
and P
, the temperature and pressure of the gas
gc
gc
cooler will be indicated.
The behaviour of the HPV valve in the various zones is as follows:
• Transcritical zone, identifi ed by T
with proportional + integral (PI) type integration in order to maintain
the maximum COP given by the optimal pressure P
function of the output temperature from the gas cooler T
• Subcritical zone, identifi ed by T
regulation in order to maintain constant subcooling.
• Transition zone, identifi ed by T
regulation with a pressure setpoint identifi ed as the conjunction of
points B and C in the fi gure, obtained by calculating the optimal pressure
at the limit of the transcritical and subcritical zones. The purpose of this
zone is to avoid discontinuity in passing between the two zones.
• Upper proportional zone, defi ned by P
with only proportional regulation between the opening value reached
at pressure P
and the maximum opening value at pressure P
max
the pressure decreases, the opening value of the HPV valve remains
constant until it enters the transcritical zone, in which the regulation
restarts as previously described.
• Lower proportional zone, defi ned by T
with only proportional regulation between the opening value reached
at temperature T
and the maximum opening value at temperature
min
T
. If the pressure increases, the opening value of the HPV valve
100%
remains constant until it enters the subcritical zone, in which the
regulation restarts as previously described. It is possible to disable
operation according to this mode by parameter.
Calculation of the customized setpoint (custom)
The customized calculation diff ers from the optimized control due to
the fact that the curve in the subcritical phase is rectilinear and defi ned
by the user, therefore the defi nition of the bands and the calculation of
the setpoint can be customized by the user. Behaviour in the remaining
bands is as described for the optimized algorithm.
HPV valve accessory functions
HPV valve management includes some accessory functions:
• Pre-positioning: entering the unit ON status, the HPV valve remains at
a fi xed position that can be set by a parameter for a fi xed time, which
is also settable by a parameter, in order to be able to quickly raise the
pressure in the tank. This procedure is reactivated whenever the unit
goes into the OFF status or the HPV valve moves into the minimum
position due to all of the compressors being turned off (optional).
• Valve closure with compressors off : if all compressors in the medium
temperature unit are turned off , the HPV valve can be positioned
at the minimum opening value in the OFF status, which can be set
by a parameter. When a compressor is restarted, the valve restarts
the regulation with the pre-positioning procedure described in the
previous point.
• Minimum and maximum opening values: the minimum opening
value in Off status and in ON status can be diff erentiated (by keypad,
digital input or supervisor) which the maximum opening value is
unique.
• Maximum percentage variation: the movement of the valve cannot
exceed the maximum set percentage variation per second.
• Filter on setpoint: the calculation of the regulation setpoint of the
HPV valve can be done by taking into account the averages of the
last n samples (maximum 99) to avoid sudden variations due to high
variability of the output temperature of the gas cooler.
• Minimum setpoint: a minimum value can be set for the HPV valve
setpoint, below which the setpoint can never go regardless of
the parameters entered, in order to preserve the operation of the
compressors.
• Setpoint distance alarm: if the gas cooler pressure is too far from the
calculated setpoint for too long (threshold and delay can be set), an
alarm can be triggered.
Copyright HPL March 2021
, T
related to points B and C in
12
23
and T
.
min
100%
≥ T
and P
≤ P
: the valve works
gc
12
gc
max
calculated as a
opt
.
ogc
≤ T
≤ T
: the valve works with PI
min
gc
23
≤ T
≤ T
: the valve works with PI
23
gc
12
< P
< P
: the valve works
max
gc
100%
100%
< T
< T
: the valve works
100%
gc
min
6.6.8
Control of the receiver pressure through the
HPV valve
If the pressure in the receiver goes below the minimum work pressure
set, the dynamic calculated setpoint for the HPV valve can be changed in
order to increase the pressure in the receiver.
An off set in proportion to the distance from the minimum threshold is
subtracted from the calculated setpoint so that the greater opening of
the HPV valve contributes to increasing the pressure in the receiver.
The off set is directly proportional to the distance from the minimum
work threshold, as illustrated in the fi gure:
. If
Pmin
On the other hand, if the pressure in the receiver goes above the
maximum work pressure set, the dynamic calculated setpoint for the HPV
valve can be changed in order to decrease the pressure in the receiver.
An off set in proportion to the distance from the maximum threshold is
added to the calculated setpoint so that the lesser opening of the HPV
valve contributes to decreasing the pressure in the receiver.
The off set is directly proportional to the distance from the maximum
work threshold, as illustrated in the fi gure:
Setpoint HPV
P
41
Page 90 of 169
Setpoint HPV
Prec min
Fig. 6.ad
max
P
REC
REC
Fig. 6.ae
P4367 Rev 01
pRack PR300T +0300018EN rel. 1.5 - 02.05.2017
ENG
OFFS
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