3.8. Compressor control - variable zero position (screw compressors
with hydraulic slides)
3.8.1 General
The flow of screw compressors at low slide positions seen in relation to the fully loaded "flow"
is strongly dependent on the operating conditions. It has turned out to be expedient to enter
a "floating zero point" which ensures that the compressor is operating with a correct slide po-
sition.
Unisab III ensures that the SAB 202 compressor always adapts to the variable operating con-
ditions.
Based on discharge pressure, suction pressure, compressor type and capacity, Unisab III cal-
culates a zero point below which the compressor capacity slide is not permitted to drop while
the compressor is operating.
When the compressor starts, the new zero point will be calculated and the capacity slide
moves as quickly as possible from a physical zero point (mechanical impact) to the calculated
zero. As long as the slide position is below the calculated zero point, the capacity is shown as
a negative value in the display.
If the calculated zero point cannot be set within a certain time limit depending on the calcula-
ted movement, a shutdown alarm will be issued.
When the compressor stops, the slide will drop to the calculated zero point. The motor stops
and the slide is pushed down to the physical zero position by the built-in spring.
Once the differential pressure across the compressor has been equalised (P=1), the calculated
zero point and the physical zero position will be the same.
If pressure conditions are changed during operation so that the calculated zero point of the
slide is moved (whereby the corrected capacity is changed), the capacity may be a negative
value for a certain period.
For all other screw compressor types than SAB 202 a zero point can be set manually, if re-
quired. This manually set zero point has the same function as described above in the sections
for compressor start and stop. See subsection 3.8.4. Manual setting of new zero point (all
screw compressors).
3.8.2 Corrected capacity
Unisab III will automatically correct the capacity reading in relation to both the new calculated
zero point and the Vi slide position when automatic Vi regulation is included.
With manual Vi regulation, the capacity reading is automatically corrected in relation to the
new calculated zero point.
Remember that whenever the Vi slide is moved manually, the position transmitter of the ca-
pacity slide must be recalibrated at 100%.
The corrected capacity is calculated according to the following principle:
Compressor control and surveillance
Engineering manual - Unisab III 1.10
001930 en 2014.09
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