3.9. Compressor control - electrical slide control
3.9.1 Electrical slide control
(SAB 330 screw compressors )
These screw compressors are fitted with a capacity slide (master slide) driven by an AC motor
through magnetic transmission, gearbox and spindle and a hydraulically Vi slide (baby slide)
controlled by a solenoid valve. The digital output for the valve is always controlled automati-
cally by Unisab III. Only the capacity slide is fitted with a position transmitter.
Note that the minimum pulse when moving the capacity slide is 1 second. For a SAB 330L this
corresponds to a minimum capacity position change of 1.8%. Keep this in mind when adjust-
ing the PID regulators. Further, there is a pause of at least 1 second when changing the rota-
tion direction of the motor.
3.9.2 Part load and full load
There are two operating modes for capacity control and Vi adjustment: Part load and full load.
At full load, the capacity is by definition 100%. By activating the solenoid valve, the Vi slide is
forced in mesh with the capacity slide. Vi is adjusted by changing the capacity slide position,
which consequently moves the Vi slide. Note that in the survey pictures, full load is indicated
in the display by a solenoid valve symbol:
At part load, the Vi slide is reset to minimum position by deactivating the solenoid valve. The
capacity slide is moved according to the capacity requirement.
Unisab III changes from part load to full load when capacity exceeds the selected limit for Vi
mode included in the picture Setup/Configuration/Compressor Block. The default limit is Vi
mode = 97% but it can be adjusted within the range 70 to 97%.
Unisab III returns to part load when the capacity requirement decreases.
The changes described are automatically performed in any of the modes Manual, Auto and
Remote.
3.9.3 Calculated Vi position
When changing to or running in full load mode, Unisab III determines an optimum calculated
Vi position x
Vi
[%] according to suction pressure/discharge pressure ratio, compressor type,
refrigerant and whether there is an economiser. Furthermore, the corresponding capacity
slide position is calculated:
x
cap
[%] x
Vi
=100 [%] -x
Vi
[%] *
(Vi-slide [mm] /Cap-slide [mm])
While in full load mode, the x
cap
[%] x
Vi
is the set point of the capacity slide position regula-
tor.
3.9.4 Change to full load
When changing from part load to full load, Unisab III will run through the following steps:
1. Move capacity slide if necessary (see below).
2. Activate solenoid valve of Vi slide to force Vi slide into mesh with capacity slide.
3. Move capacity slide (and thus Vi slide) to optimal position x
cap
[%] x
Vi
.
As regards point 1, Unisab III compares the above defined x
cap
[%] x
Vi
with the actual capaci-
ty slide position. If the actual position is much higher than x
cap
[%] x
Vi
, the result would be
Compressor control and surveillance
Engineering manual - Unisab III 1.10
001930 en 2014.09
139/319
To Next Page
Loading...
Need help?
General
