28
ENG
“EVD evolution” +030222041 - rel. 1.0 - 01.06.2008
PROTECTORS7.
These are additional functions that are activated in specific situations that
are potentially dangerous for the unit being controlled. They feature an
integral action, that is, the action increases gradually when moving away
from the activation threshold. They may add to or overlap (disabling)
normal PID superheat control. By separating the management of
these functions from PID control, the parameters can be set separately,
allowing, for example, normal control that is less reactive yet much
faster in responding when exceeding the activation limits of one of the
protectors.
Protectors7.1
The protectors are 4:
LowSH, low superheat;•
LOP, low evaporation temperature;•
MOP, high evaporation temperature;•
HiTcond, high condensing temperature.•
Note: The HITCond protection requires an additional sensor (S3) to
those normally used, either installed on the driver, or connected via tLAN
or pLAN to a controller.
The protectors have the following main features:
activation threshold: depending on the operating conditions of the
•
controlled unit, this is set in Service programming mode;
integration time, which determines the intensity (if set to 0, the •
protector is disabled): set automatically based on the type of main
control;
alarm, with activation threshold (the same as the protector) and delay •
(if set to 0 disables the alarm signal).
Note: The alarm signal is independent from the effectiveness of the
protector, and only signals that the corresponding threshold has been
exceeded. If a protector is disabled (null integration time), the relative
alarm signal is also disabled.
Each protector is affected by the proportional gain parameter (K) for the
PID superheat control. The higher the value of K, the more intense the
reaction of the protector will be.
Characteristics of the protectors
Protection Reaction Reset
LowSH Intense closing Immediate
LOP Intense opening Immediate
MOP Moderate closing Controlled
HiTcond Moderate closing Controlled
Tab. 7.a
Reaction: summary description of the type of action in controlling the
valve.
Reset: summary description of the type of reset following the activation
of the protector. Reset is controlled to avoid swings around the activation
threshold or immediate reactivation of the protector.
LowSH (low superheat)
The protector is activated so as to prevent the return of liquid to the
compressor due to excessively low superheat valves from.
Parameter/description Def. Min. Max. UOM
CONTROL
LowSH protection: threshold 5 -40 (-72) set point
superheat
K (°R)
LowSH protection: integration
time
15 0 800 s
ALARM CONFIGURATION
Low superheat alarm delay
(LowSH) (0= alarm disabled)
300 0 18000 s
Tab. 7.b
When the superheat value falls below the threshold, the system enters
low superheat status, and the intensity with which the valve is closed is
increased: the more the superheat falls below the threshold, the more
intensely the valve will close. The LowSH threshold, must be less than
or equal to the superheat set point. The low superheat integration time
indicates the intensity of the action: the lower the value, the more intense
the action.
The integration time is set automatically based on the type of main
control.
t
t
t
OFF
ON
R
OFF
ON
Low_SH
Low_SH_TH
SH
D
B
Fig. 7.a
Key:
SH Superheat A Alarm
Low_SH_TH Low_SH protection threshold D Alarm delay
Low_SH Low_SH protection t Time
B Automatic alarm reset
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.
Parameter/description Def. Min. Max. UOM
CONTROL
LOP protection: threshold -50 -60 (-72) Protection MOP:
threshold
°C (°F)
LOP protection: integration time 0 0 800 s
ALARM CONFIGURATION
Low evaporation temperature
alarm delay (LOP)
(0= alarm disabled)
300 0 18000 s
Tab. 7.c
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
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