Carel MPXPRO User Manual page 50

ENG
MPXPRO can measure the saturated evaporation temperature using a
normal NTC/PTC/PT1000/NTCL243 temperature probe (see price list). This
solution, even if economically convenient, requires careful installation
and in any case does not off er the same precision as a ratiometric
probe. CAREL recommends the use of ratiometric probes for reading the
evaporation pressure, which is automatically converted to the saturated
temperature using the specifi c tables for the type of refrigerant used.
Description of operation
The values read by the probes described above are called:
• tGS = superheated gas temperature;
• tEu = saturated evaporation temperature converted from pressure
These values are used to calculate the superheat:
• SH = tGS - tEu
MPXPRO manages the proportional opening of the electronic expansion
valve, adjusting the fl ow of refrigerant in the evaporator, so as to maintain
the superheat around the value set for advanced parameter P3 (superheat
set point). The opening of the valve is controlled simultaneously yet
independently from normal temperature control. When there is a
refrigeration call (the compressor / solenoid valve relay is activated),
control of the electronic valve is also activated and then managed
independently. If the superheat value read by the probes is greater
than the set point, the valve is opened proportionally to the diff erence
between the values. The speed of variation and the percentage of
opening depend on the PID parameters set. The opening is continuously
modulated based on the superheat value, with PID control.
Note: all the references relating to control of the electronic valve
are based on the use of a CAREL E
The descriptions are then made considering the steps of the stepper
motor used for this type of valve, for example, the maximum number of
opening steps is 480. All the functions are also then described for PWM
valves. In particular, instead of the maximum opening expressed as the
number of steps, the maximum ON/OFF time of the PWM valve is
considered (default 6 seconds). The absolute openings expressed as
steps must then be suitably converted by the user and referred to the
maximum fi xed period, expressed in seconds.
Apertura valvola/Valve opening
Surriscaldamento/Superheat
Type of refrigerant (parameter PH)
This is used to set the type of gas refrigerant used in the system. The table
below shows the types of gas possible and the associated PH values.
2
For compatibility with the E V valve see paragraph 4.3. Contact CAREL if
2
installing E V valves in systems that use refrigerants not listed in the table.
MPXPRO - + 0300055EN rel. 1.0 30/08/10
2 V electronic expansion valve.
Fig. 6.n
Par. Description
PH Type of refrigerant
1 = R22 8 = R600
2 = R134a 9 = R600a
3 = R404A 10 = R717
4 = R407C 11 = R744
5 = R410A 12 = R728
6 = R507A 13 = R1270
7 = R290 14 = R417A
Important : if the type of refrigerant is not correct, there may be
return of liquid to the compressor.
Electronic valve (parameter P1)
MPXPRO can control two diff erent models of electronic expansion valve,
each with the specifi c type of optional expansion board. Parameter P1 is
used to set the model installed:
Par. Description
P1 Electronic valve
0 = not used, 1 = PWM valve
2 = CAREL E 2 V valve
Superheat set point (parameter P3)
This is used to set the reference superheat value for the control of the
electronic valve. It does not determine the actual superheat value, but
rather the desired value. MPXPRO, 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 diff erence between the actual superheat and
the set point.
Important : the set point value calculated depends on the quality
of the installation, the position of the probes and other factors.
Consequently, depending on the installation the set point read
may deviate from the actual value. Set point values that are too low (2 to
4 K), albeit ideally usable, may cause problems involving the return of
liquid refrigerant to the compressor rack.
Par. Description
P3 Superheat set point
Initial valve position when control starts (par. 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 call is sent, however may cause problems if the
valve is oversized with reference to the cooling capacity of the controller.
Low values, on the other hand, allow a more gradual and slower action.
Par. Description
cP1 Initial valve position when control starts
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 to the
compressor rack due to an excessively high evaporator temperature.
Par. Description
Pdd Initial valve position maintenance time
after defrost
50
Def Min Max UoM
3 1 14 -
Tab. 6.i.b
Def Min Max UoM
0 0 2 -
Tab. 6.i.c
Def Min Max UoM
10.0 0.0 25.0 K
Tab. 6.i.d
Def Min Max UoM
30 0 100 %
Tab. 6.i.e
Def Min Max UoM
10 0 30 min
Tab. 6.i.f