29
ENG
“EVD evolution” +030222041 - rel. 1.0 - 01.06.2008
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. 7.b
Key:
T_EVAP
Evaporation temperature
D
Alarm delay
LOP_TH
Low evaporation temperature
protection threshold
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 units with compressor on board 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.
Parameter/description
Def.
Min.
Max.
UOM
CONTROL
MOP protection: threshold
50
Protection LOP:
threshold
200
(392)
°C (°F)
MOP protection: integration time 20
0
800
s
ALARM CONFIGURATION
High evaporation temperature
alarm delay (MOP)
(0= alarm disabled)
600
0
18000 s
Tab. 7.d
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. 7.c
Key:
T_EVAP Evaporation temperature
MOP_TH MOP threshold
PID
PID superheat control
ALARM
Alarm
MOP
MOP protection
t
Time
D
Alarm delay
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.
Important:
if the closing of the valve also causes an excessive
increase in the suction temperature (S2), the valve will be stopped to
prevent overheating the compressor windings, awaiting a reduction in
the refrigerant charge.
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.
HiTcond (high condensing temperature)
To activate the high condensing temperature protector (HiTcond), a
pressure sensor must be connected to input S3.
The protector is activated so as to prevent too high evaporation
temperatures from stopping the compressor due to the activation of the
high pressure switch.
Parameter/description
Def.
Min.
Max.
UOM
SPECIAL
HiTcond: threshold
80
-60
(-76)
200
(392)
°C (°F)
HiTcond: integration time
20
0
800
s
ALARM CONFIGURATION
High condensing temperature alarm delay
(HiTcond)
(0= alarm disabled)
600
0
18000 s
Tab. 7.e
The integration time is set automatically based on the type of main
control.
Note:
the protector is very useful in units with compressors on board if the
•
air-cooled condenser is undersized or dirty/malfunctioning in the
more critical operating conditions (high outside temperature);
the protector has no purpose in multiplexed systems (showcases),
•
where the condensing pressure is maintained constant and the status
of the individual electronic valves does not affect the pressure value.
