Control; Main And Auxiliary Control; Superheat Control - Carel EVD evolution User Manual

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5.1 Main and auxiliary control

EVD evolution features two types of control
•
main;
•
auxiliary.
Main control is always active, while auxiliary control can be activated
by parameter. Main control defi nes the operating mode of the driver.
The fi rst 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.
The two last advanced functions also relate to superheat control.
Parameter/description
CONFIGURATION
Main control

Superheat control

1= multiplexed cabinet/cold room
2= cabinet/cold room with on-board compressor
3= "perturbed" cabinet/cold room
4= cabinet/cold room with subcritical
5= R404A condenser for subcritical
6= air-conditioner/chiller with plate heat exchanger
7= air-conditioner/chiller with tube bundle heat exchanger
8= air-conditioner/chiller with fi nned coil heat exchanger
9= air-conditioner/chiller with variable cooling capacity
10= "perturbed" air-conditioner/chiller
Advanced control
11= EPR back pressure
12= hot gas bypass by pressure
13= hot gas bypass by temperature
CO
14= gas cooler transcritical
2
15= analogue positioner (4 to 20 mA)
16= analogue positioner (0 to 10 V)
17= air-conditioner/chiller or cabinet/ cold room with adapti-
ve control
18= air-conditioner/chiller with digital scroll compressor
19= AC/chiller with SIAM ANB scroll compressor(*)
20= superheat regulation with 2 temperature probes
21= I/O expander for pCO
(*) only for CAREL valve drivers
Note:
•
R404A condensers with subcritical CO
valves installed in cascading systems where the fl ow of R404A (or other
refrigerant) in an exchanger acting as the CO
controlled;
•
perturbated 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
CONFIGURATION
Auxiliary control
Disabled
High condensing temperature protection on S3 probe
Modulating thermostat on S4 probe
Backup probes on S3 & S4
Important: the "High condensing temperature protection" and
"Modulating thermostat" auxiliary settings can only be enabled if the
main control is also superheat control with settings 1 to 10 and 17, 18. On
the other hand, the "Backup probes on S3 and S4" auxiliary control can be
activated, once the corresponding probes have been connected, only for
settings from 1 to 18
.
The following paragraphs explain all the types of control that can be set
on EVD evolution.
"EVD evolution" +0300005EN - rel. 3.1 - 25.07.2011

5. CONTROL

Def.
multiplexed
cabinet/
cold room
CO
2
CO
2
Tab. 5.a
refer to superheat control for
2
condenser needs to be
2
Def.
Disabled
Tab. 5.b
5.2 Superheat control
The primary purpose of the electronic valve is ensure that the fl ow-rate
of refrigerant that fl ows through the nozzle corresponds to the fl ow-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 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 probe 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
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 un
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 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.
L
F
S
M
EEV
V
18
C
EVD
evolution
CP
E
P T
Fig. 5.a