18
ENG
“EVD evolution” +030222041 - rel. 1.0 - 01.06.2008
CONTROL5.
Superheat control5.2
The primary purpose of the electronic valve is ensure that the flow-rate
of refrigerant that flows through the nozzle corresponds to the flow-rate
required by the compressor. In this way, the evaporation process will
take place along the entire length of the evaporator and there will be
no liquid at the outlet and consequently in the branch that runs to the
compressor.
As liquid is not compressible, it may cause damage to the compressor
and even breakage if the quantity is considerable and the situation lasts
some time.
Superheat control
The parameter that the control of the electronic valve is based on is the
superheat temperature, which effectively tells whether or not there is
liquid at the end of the evaporator.
The superheat temperature is calculated as the difference 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
(*) suction
If the superheat temperature is high it means that the evaporation process
is completed well before the end of the evaporator, and therefore flow-
rate of refrigerant through the valve is insufficient. This causes a reduction
in cooling efficiency 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
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 un
undetermined risk to the compressor that must be avoided. Moreover, a
high superheat temperature as mentioned corresponds to an insufficient
flow-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
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.
S2
S1
EVD
evolution
P
E
V
S
F
L
M
T
CP
C
EEV
Fig. 5.a
Main and auxiliary control5.1
EVD evolution features two types of control
main;•
auxiliary.•
Main control is always active, while auxiliary control can be activated
by parameter. Main control defines the operating mode of the driver.
The first 10 settings refer to superheat control, the others are so-called
“special” settings and are pressure or temperature settings or depend on
a control signal from an external controller.
Parameter/description Def.
Configuration
Main control multiplexed
cabinet/
cold room
Superheat control
multiplexed cabinet/cold room
cabinet/cold room with on-board compressor
“perturbed” cabinet/cold room
cabinet/cold room with sub-critical CO
2
R404A condenser for sub-critical CO
2
air-conditioner/chiller with plate heat exchanger
air-conditioner/chiller with tube bundle heat exchanger
air-conditioner/chiller with finned coil heat exchanger
air-conditioner/chiller with variable cooling capacity
“perturbed” air-conditioner/chiller
Special control
EPR back-pressure
hot gas bypass by pressure
hot gas bypass by temperature
transcritical CO
2
gas cooler
analogue positioner (4 to 20 mA)
analogue positioner (0 to 10 V)
Tab. 5.a
Note:
R404A condensers with subcritical CO•
2
refer to superheat control for
valves installed in cascading systems where the flow of R404A (or other
refrigerant) in an exchanger acting as the CO
2
condenser needs to be
controlled;
“perturbed” cabinet/cold room or air-conditioner/chiller refer to units •
that momentarily or permanently operate with swinging condensing
or evaporation pressure.
Auxiliary control features the following settings:
Parameter/description Def.
Configuration
Auxiliary control Disabled
Disabled
High condensing temperature protection on S3
Modulating thermostat on S4
Backup sensors on S3 & S4
Tab. 5.b
Important: the “High condensing temperature protection” and
“Modulating thermostat” auxiliary settings can only be enabled if the
main control is superheat control (first 10 settings). On the other hand,
“Backup sensors on S3 & S4” can always be activated, once the related
sensors have been connected.
The following paragraphs explain all the types of control that can be set
on EVD evolution.
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