Protectors - Carel EVD mini User Manual

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.

6.1 Protectors

The protectors are 3:
•
LowSH, low superheat;
•
LOP, low evaporation temperature;
•
MOP, high evaporation temperature;
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
timeout (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 influenced by the proportional gain parameter (CP) of
PID superheat control. The higher is the value of CP, the more intensely
the protection will react.
Characteristics of the protectors
Protection Reaction
LowSH Intense closing
LOP Intense opening
MOP Moderate closing
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.
Note: all the alarms are generated after a fixed delay, as shown in
the table:
Protectors Delay (s)
LowSH 300
LOP 300
MOP 600
LowSH (low superheat)
The protector is activated so as to prevent the low superheat from
causing the return of liquid to the compressor.
Par. Description
C1 LowSH protection: threshold
C2 LowSH protection: integration time
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.
6. PROTECTORS
Reset
Immediate
Immediate
Controlled
Tab. 6.a
Def. Min. Max. U.M.
5(9) -5(-9) Set point K(°F)
superheat
15 0 800 s
19
The integration time is set automatically based on the type of
main control.
SH
Low_SH_TH
ON
Low_SH
OFF
ON
A
OFF
key:
SH Superheat
Low_SH_TH Low_SH protection threshold
Low_SH Low_SH protection
B Alarm automatic 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.
Par. Description
C3 LOP protection: threshold
C4 LOP protection: integration time
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.
"EVD mini" +0300036EN - rel. 1.2 - 23.04.2018
ENG
t
t
t
D
B
Fig. 6.a
A Alarm
D Alarm delay
t Time
Def. Min. Max. U.M.
-50 -85 MOP protec.: C(°F)
(-58) (-121) threshold
0 0 800 s