15
ENG
“EVD mini” +0300036EN - rel. 1.2 - 23.04.2018
5. FUNCTIONS
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 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.
5.1 Control
EVD mini is a superheat controller and can be used as an analogue
positioner. The type of refrigeration unit can be selected using the
“Operating mode” parameter.
Parameter/description
Def.
Operating mode
1
Multiplexed cabinet/cold room
2
Air-conditioner/chiller with plate heat exchanger
3
Air-conditioner/chiller with tube bundle heat exchanger
4
Air-conditioner/chiller with finned coil heat exchanger
5
Analogue positioner (0 to 10 V)
6
Superheat control with 2 temperature probes
7
Subcritical CO2 showcase/cold room
8
Hot gas bypass by pressure
9
Hot gas bypass by temperature
1 = multiplexed
cabinet/cold room
Tab. 5.a
Based on the operating mode setting , the driver automatically sets a
series of control parameters.
Operating mode
PID:
proport.
gain
PID:
integra-
tion time
Super-
heat
set
point
LowSH protec-
tion
LOP protection
MOP protection
Bypass
pres-
sione:
setpoint
(bar)
Bypass
tempe-
ratura:
setpoint
(°C)
th-
reshold
Integra-
tion time
th-
reshold
Integra-
tion time
th-
reshold
Integra-
tion time
1
Multiplexed cabinet/cold room
15
150
11
5
15
-50
0
50
20
2
Air-conditioner/chiller with plate heat exchanger
3
40
6
2
2,5
-50
4
50
10
3
Air-conditioner/chiller with tube bundle heat
exchanger
5
60
6
2
2,5
-50
4
50
10
4
Air-conditioner/chiller with finned coil heat
exchanger
10
100
6
2
10
-50
10
50
20
5
Analogue positioner (0 to 10 V)
-
-
-
-
-
-
-
-
-
6
Superheat control with 2 temperature probes
15
150
11
5
15
-50
0
50
20
7
Subcritical CO2 showcase/cold room
20
400
13
7
15
-50
0
50
20
8
Hot gas bypass by pressure
20
200
-
-
-
-
-
-
-
3
-
9
Hot gas bypass by temperature
15
150
-
-
-
-
-
-
-
-
10
Tab. 5.b
S
uperheat
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 (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.
S
uperheat 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 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
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.
