16
ENG
“EVD ice” +0300038EN - rel. 1.1 - 23.04.2018
LOP (low evaporation pressure)
LOP= Low Operating Pressure
The LOP protection threshold is applied as a saturated evaporation
temperature value so that it can be easily compared against the technical
specifications supplied by the manufacturers of the compressors. The
protector is activated so as to prevent too low evaporation temperatures
from stopping the compressor due to the activation of the low pressure
switch. The protector is very useful in units with compressors on board
(especially multi-stage), where when starting or increasing capacity the
evaporation temperature tends to drop suddenly. When the evaporation
temperature falls below the low evaporation temperature threshold,
the system enters LOP status and is the intensity with which the valve is
opened is increased. The further the temperature falls below the threshold,
the more intensely the valve will open. The integration time indicates the
intensity of the action: the lower the value, the more intense the action.
Par. Description
Def. Min. Max.
U.M.
C3 LOP protection: threshold
-50
(-58)
-85
(-121)
MOP protec.:
threshold
C(°F)
C4 LOP protection: integration time
0
0
800
s
The integration time is set automatically based on the type of main
control.
Note:
•
the LOP threshold must be lower then the rated evaporation
temperature of the unit, otherwise it would be activated unnecessarily,
and greater than the calibration of the low pressure switch, otherwise
it would be useless. As an initial approximation it can be set to a value
exactly half-way between the two limits indicated;
•
the protector has no purpose in multiplexed systems (showcases)
where the evaporation is kept constant and the status of the individual
electronic valve does not affect the pressure value;
•
the LOP alarm can be used as an alarm to highlight refrigerant leaks by
the circuit. A refrigerant leak in fact causes an abnormal lowering of the
evaporation temperature that is proportional, in terms of speed and
extent, to the amount of refrigerant dispersed.
t
t
t
OFF
ON
ALARM
OFF
ON
LOP
LOP_TH
T_EVAP
D
B
Fig. 6.b
Key:
T_EVAP Evaporation temperature
D
Alarm timeout
LOP_TH Low evaporation temperature protection ALARM Alarm
LOP
LOP protection
t
Time
B
Automatic alarm reset
MOP (high evaporation pressure)
MOP= Maximum Operating Pressure.
The MOP protection threshold is applied as a saturated evaporation
temperature value so that it can be easily compared against the technical
specifications supplied by the manufacturers of the compressors. The
protector is activated so as to prevent too high evaporation temperatures
from causing an excessive workload for the compressor, with consequent
overheating of the motor and possible activation of the thermal protector.
The protector is very useful in self-contained units if starting with a high
refrigerant charge or when there are sudden variations in the load. The
protector is also useful in multiplexed systems (showcases), as allows all
the utilities to be enabled at the same time without causing problems
of high pressure for the compressors. To reduce the evaporation
temperature, the output of the refrigeration unit needs to be decreased.
This can be done by controlled closing of the electronic valve, implying
superheat is no longer controlled, and an increase in the superheat
temperature. The protector will thus have a moderate reaction that tends
to limit the increase in the evaporation temperature, keeping it below the
activation threshold while trying to stop the superheat from increasing
as much as possible. Normal operating conditions will not resume based
on the activation of the protector, but rather on the reduction in the
refrigerant charge that caused the increase in temperature. The system
will therefore remain in the best operating conditions (a little below the
threshold) until the load conditions change.
Par. Description
Def. Min.
Max. U.M.
C5 MOP protection threshold
50
(122)
Protection LOP:
threshold
200
(392)
C(°F)
C6 MOP protection integration time 20
0
800 s
The integration time is set automatically based on the type of main
control.
When the evaporation temperature rises above the MOP threshold, the
system enters MOP status, superheat control is interrupted to allow the
pressure to be controlled, and the valve closes slowly, trying to limit the
evaporation temperature. As the action is integral, it depends directly on
the difference between the evaporation temperature and the activation
threshold. The more the evaporation temperature increases with
reference to the MOP threshold, the more intensely the valve will close.
The integration time indicates the intensity of the action: the lower the
value, the more intense the action.
t
t
t
t
OFF
ON
ALARM
OFF
ON
PID
OFF
ON
MOP
MOP_TH - 1
MOP_TH
T_EVAP
D
Fig. 6.c
Key:
T_EVAP Evaporation temperature
MOP_TH MOP threshold
PID
PID superheat control
ALARM
Alarm
MOP
MOP protection
t
Time
D
Alarm timeout
Important:
the MOP threshold must be greater than the rated
evaporation temperature of the unit, otherwise it would be activated
unnecessarily. The MOP threshold is often supplied by the manufacturer
of the compressor. It is usually between 10 °C and 15 °C.
If the closing of the valve also causes an excessive increase in the suction
temperature (S2) above the set threshold – set via parameter (C7), not
on the display - the valve will be stopped to prevent overheating the
compressor windings, awaiting a reduction in the refrigerant charge. If
the MOP protection function is disabled by setting the integral time to
zero, the maximum suction temperature control is also deactivated.
Par. Description
Def.
Min.
Max. U.M.
C7 MOP protection: disabling threshold
30
(86)
-85
(-121)
200
(392)
°C (°F)
At the end of the MOP protection function, superheat control restarts
in a controlled manner to prevent the evaporation temperature from
exceeding the threshold again.