Johnson Controls SABROE Unisab III Control, Engineering Manual

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Engineering manual - UniSAB III 1.10.8
001930 en 2021.06
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Compressor control and surveillance
3.8 Compressor control - variable zero position (screw compressors with hy-
draulic 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 position.
UniSAB III ensures that the SAB 202 compressor always adapts to the variable operating conditions.
Based on discharge pressure, suction pressure, compressor type and capacity, UniSAB III calculates 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 calculated move-
ment, 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 required. 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 calcu-
lated zero point.
Remember that whenever the Vi slide is moved manually, the position transmitter of the capacity slide
must be recalibrated at 100%.
The corrected capacity is calculated according to the following principle:
The capacity slide signal from the position transmitter is scaled in such a way that 0% is read when the
slide position corresponds to the calculated/set zero point and 100% is read when the capacity slide
meets the Vi slide. It makes no difference whether the setting is carried out automatically or manually.
0%
1.5 12 25 4.5 3 2
100%
0% 100%
0% 0% 100%
Corrected capacity
Pressure cond.
Cap. slide
Measured slide pos.
Volume cond.:
Zero % pos.
“Zero slide”
“Vi slide”