Carel MPXone ACS00CB000010, Руководство пользователя

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The valve must therefore be opened further. Vice-versa, if superheat is low it means that the evaporation
process has not concluded before the end of the evaporator and a certain quantity of liquid will still be
present at the compressor suction port. The valve must therefore be closed further. The superheat
working range is limited at the lower end: if the flow-rate through the valve is excessive the superheat
measured will be near 0 K.
This indicates the presence of liquid, even if the percentage of this relative to the gas cannot be
quantified. There is therefore a risk to the compressor. On the other hand, a high superheat value, as
mentioned, corresponds to an insufficient flow-rate of refrigerant. Superheat must therefore always be
greater than 0 K and have a minimum stable value allowed by the valve-unit system.
A low superheat value in fact corresponds to a situation of probable instability due to the turbulent
evaporation process approaching the measurement point of the probes. The expansion valve must
therefore be controlled with extreme precision and a reaction capacity around the superheat set point,
which will almost always vary from 3 to 14 K. Set point values outside of this range are quite infrequent
and relate to special applications. Parameters SH, tGS, tEu and PPU (valve opening percentage) are
display only variables, used to monitor the refrigeration process.
Fig.5.aa
Ref. Description
T Superheated gas temperature
E Forced-air evaporator
Ref. Description
EEV Electronic expansion valve
P Evaporation pressure
MPXone, with PID control, tends to maintain the actual superheat, calculated based on the probe
readings, around the value set for this parameter. This is done by gradually varying the opening of the
valve based on the difference between the actual superheat and the set point.
Important: the measured superheat value depends on the quality of the installation, the positioning
of the probes and other factors. Depending on the specific installation, the superheat set point may
differ from the actual value. Set point values that are too low (2 to 3 K), albeit ideally usable, may cause
problems involving the return of liquid refrigerant to the compressor.
Initial valve position when control starts (parameter cP1)
This is used to set the position of the valve as a percentage when control starts. High values ensure
intense and immediate cooling of the evaporator when each request is sent, however may cause
problems if the valve is oversized with reference to the unit’s cooling capacity. Low values, on the other
hand, allow a more gradual and slower action.
Code Description Def Min Max UOM User
User
terminal
cP1 Initial valve position when control starts 30 0 100 % M NO
Initial valve position maintenance time after defrost (parameter Pdd)
At the end of a defrost, during the dripping phase, the expansion valve can be forced open to the initial
value set for cP1 for a time equal to Pdd. This means greater immunity of the unit to return of liquid due
to an excessively high evaporator temperature.
ENG
0300086EN rel. 1.0 – 04.09.2018
5. Functions | 75