Carel EVD Evolution Twin - Transcritical CO₂ gas cooler; Analogue positioner (4 to 20 mA)

ENG
“EVD Evolution TWIN” +0300006EN - rel. 2.6 - 31.01.2019
25
S
CP
C
TB
S1
EVD evolution
twin
S4 S2
EVA_2
EVB_1
EVA_1
E
V1 V2
M T
EVB_2
S3
4...20 mA
regulator
V3
H%
Fig. 5.j
Key:
CP Compressor EVA_1, 2
EVB_1, 2
Electronic valves connected in
complementary mode
C Condenser H% Relative humidity probe
V1 Solenoid valve TB Temperature probe
V3 Non-return valve E Evaporator
S Heat exchanger
(reheating)
V2 Thermostatic expansion valve
For the wiring, see paragraph “General connection diagram”.
Transcritical CO
2
gas cooler
This solution for the use of CO
2
in refrigerating systems with a transcritical
cycle involves using a gas cooler, that is a refrigerant/air heat exchanger
resistant to high pressures, in place of the condenser.
In transcritical operating conditions, for a certain gas cooler outlet temperature,
there is pressure that optimises the eciency of the system:
Set= pressure set point in a gas cooler with transcritical CO
2
T= gas cooler outlet temperature
Default value: A=3.3, B= -22.7.
In the simplied diagram shown below control is performed by driver A and
the simplest solution in conceptual terms is shown. The complications in the
systems arise due to the high pressure and the need to optimise eciency.
Driver B is used for superheat control.
E
V1
M
CP
GC
EVA
IHE
PA TA
S2
S1
EVD evolution
twin
S3 S4
EEVB
PB TB
Fig. 5.k
Key:
CP Compressor EVA Electronic valve A
GC Gas cooler EEVB Electronic expansion valve B
E Evaporator IHE Inside heat exchanger
V1 Solenoid valve
For the wiring, see paragraph “General connection diagram”.
This involves PID control without any protectors (LowSH, LOP, MOP, see the
chapter on Protectors), without any valve unblock procedure. Control is
performed on the gas cooler pressure probe value read by input S1, with
a set point depending on the gas cooler temperature read by input S2;
consequently there is not a set point parameter, but rather a formula: “CO
2
gas
cooler pressure set point” = Coecient A * Tgas cooler (S2) + Coecient B. The
set point calculated will be a variable that is visible in display mode. Control is
direct, as the pressure increases, the valve opens.
Parameter/Description Def. Min. Max. UOM
SPECIAL
Transcritical
CO
2
: coecient A
3.3 -100 800 -
Transcritical
CO
2
: coecient B
-22.7 -100 800 -
CONTROL
PID : proportional gain 15 0 800
PID : integral time 150 0 1000 s
PID : derivative time 5 0 800 s
Tab. 5.k
Analogue positioner (4 to 20 mA)
This control function is available for driver A and driver B. Valve A will be
positioned linearly depending on the value of the “4 to 20 mA input for
analogue valve positioning” read by input S1.
Valve B will be positioned linearly depending on the value of the “4 to 20 mA
input for analogue valve positioning” read by input S3.
There is no PID control nor any protection (LowSH, LOP, MOP, see the chapter
on Protectors), and no valve unblock procedure.
Forced closing will only occur when digital input DI1 opens for driver A or
DI2 for driver B, thus switching between control status and standby. The
pre-positioning and repositioning procedures are not performed. Manual
positioning can be enabled when control is active or in standby.