Based on discharge pressure, suction pressure,compressor type and capacity, UNISAB II calculates a "zero point" below which the compressor capacity slide is not permittted to drop while thecompressor 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.In case the calculated zero point cannot be set within a certain time limit, which is dependent on the calculated movement, an alarm will be issued.When the compressor stops, the slide will drop to the calculated zero point. The motor will stop, and hereafter the slide will be pushed down to the physical zero position by the built-in spring.Once the differential pressure across the compressor has been equalized (Π=1), the calculatedzero point and the physical zero position will be the same.If the 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.Corrected capacityUNISAB II will automatically correct the capacity reading in relation to both the new calculated zero point and to the Vi slide position when automatic Vi regulation is included
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