Electronic Valve - Carel MPXPRO User Manual

ENG
F5+1
F5
F1
F1-Frd
F6
FAN
F7
0%
F5+1
F5
F1
F1-Frd
F6
FAN
F7
0%
Key
Sd Evaporator probe
Sv Virtual probe
F5 Fan cut-off temperature

5.8 Electronic valve

Superheat set point (parameter P3)
Par. Description
P3 Superheat set point
SH Superheat
tGS Superheated gas temperature
tEu Saturated evaporation temperature
PPU Valve opening percentage
The parameter that the control of the electronic valve is based on is
the superheat, which eff ectively tells whether or not there is liquid at
the end of the evaporator. The superheat temperature is calculated as
the diff erence between: superheated gas temperature (measured by
a temperature sensor located at the end of the evaporator) and the
saturated evaporation temperature (calculated based on the reading of
a pressure transducer located at the end of the evaporator and using the
Tsat(P) conversion curve for each refrigerant)
Superheat = Superheated gas temperature – Saturated evaporation
temperature
If the superheat temperature is high it means that the evaporation
process is completed well before the end of the evaporator, and therefore
fl ow-rate of refrigerant through the valve is insuffi cient. This causes a
reduction in cooling effi ciency due to the failure to exploit part of the
evaporator. The valve must therefore be opened further. Vice-versa, if
the superheat temperature is low it means that the evaporation process
has not concluded at the end of the evaporator and a certain quantity
of liquid will still be present at the inlet to the compressor. The valve
must therefore be closed further. The operating range of the superheat
temperature is limited at the lower end: if the fl ow-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 quantifi ed. There is therefore an undetermined risk to the compressor
that must be avoided. Moreover, a high superheat temperature as
mentioned corresponds to an insuffi cient fl ow-rate of refrigerant. The
superheat temperature must therefore always be greater than 0 K and
have a minimum stable value allowed by the valve-unit system. A low
superheat temperature in fact corresponds to a situation of probable
instability due to the turbulent evaporation process approaching the
measurement point of the sensors. The expansion valve must therefore
MPXPRO - + 0300055EN rel. 1.0 30/08/10
Sd-Sv
F0 = 1
t
t
Sd
F0 = 2
t
t
Fig. 5.q
F1 Evaporator activation threshold
Frd Fan activation diff erential
t Time
Def Min Max
10.0 0.0 25.0
- - -
- - -
- - -
- - -
Tab. 5.y
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 are display only
variables, used to monitor the refrigeration cycle.
EEV
Key
T Superheated gas temperature
E Fan-forced evaporator
LowSH: low superheat threshold (param. P7)
The protector is activated so as to prevent the return of liquid to the
compressor due to excessively low superheat values. 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
UoM
the superheat falls below the threshold, the more intensely the valve will
K
close. The LowSH threshold must be less than or equal to the superheat
K
set point. The low superheat integration time indicates the intensity
°C/°F
°C/°F of the action: the lower the value, the more intense the action. See
%
paragraph 6.10.
Par. Description
P7 LowSH: low superheat threshold
40
MPXPRO
CAREL
!
MPXPRO
P T
E
Fig. 5.r
EEV Electronic expansion valve
P Evaporation pressure
Def Min Max
7.0 -10.0 P3
UoM
K
Tab. 5.z