V2.2
-00
12
CONTROL OF ELECTRONIC EXPANSION VALVE
Function
To control correct filling of the evaporator with refrigerant in order to obtain highest efficiency, whilst protecting
the compressor against slugging.
Description
The expansion valve is controlled by Propor In Derivative logic. The ideal theoretical opening
RDETAn of the expansion valve on circuit n is determined and compared with the actual opening. Depending on
the variation observed, an opening or closing order is issued to the expansion valve.
F
Calculation of RDETAn
Theopening to be achieved RDETAn depends on the following parameters :
SURCHD
ó
Superheat instruction (°C)
TASPn-TBPn
ó
Superheat measured on circuit n (°C)
RDETn
ó
Opening measured on expansion valve (°C)
I.e.
∆
et the difference at time t between the measured superheat and the instruction:
∆
e
t
= (TASPn - TBPn) - SURCHD
REDTAn = RDETn + Kp
.
∆
e
t
+ Ki
.
∑
∆
t
ni
-
t
t
e
+ Kd
.
∑
∆
∆
t
nd
-
t
1
-
t
t
)
e
-
e
(
F
Order issued to expansion valve
3 cas se présentent :
ð
If RDETAn > RDETn, the expansion valve must be heated (DETn=1) to open it to the desired opening.
ð
If RDETAn < RDETn, the expansion valve is not heated (DETn=0) so that it can close.
ð
If RDETAn = RDETn, the expansion valve is maintained at its current opening value by alternating start
and stop heating operations successively.
Note
:
Before starting up the first compressor on the circuit, the expansion valve is preheated to an opening
value calculated on the basis of the low pressure prevailing in the circuit. This procedure avoids any
risk of slugging at compressor start-up.
Derivative
Integral
action
Proportional action