M&M RWB series User Manual

Standard Screw Compressor - RWB Series
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M&M R
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Variable Vi Slide Stop Control
Vi refers to the volumetric ratio of the screw compressor. The Vi is computed by taking the
ratio of the volume of gas when compression begins to the volume of gas when discharge
begins. In a fixed Vi screw compressor the Vi is determined by the size of the discharge port
and must be matched to the operating conditions of the system. If the operating conditions
change the compressor will operate at a reduced efficiency due to over compression or under
compression of the gas. The RWB screw compressors contains an additional hydraulic piston
which adjusts the size of the discharge port. The Vi slide stop is adjusted in response to oper-
ating condition changes to increase compressor efficiency and reduce operating cost. The
RWB Vi slide stop ranges from a setting of 2.2 to 5.0.
Vi Adjustments
The Vi slide stop is adjusted by the CCS based on the ratio between discharge pres-
sure and suction pressure. The Vi slide stop adjustments are performed periodically
based on a Vi update timer or when requested by the operator. The duration of the Vi
update time is user programmable. Once the timer expires an adjustment is made.
During Vi adjustments, no capacity control operations are performed. While the com-
pressor is starting and for 3 minutes after it is running Vi adjustments are disabled.
Manual Vi Adjustment
In every mode except manual, the CCS adjust the Vi automatically every time the Vi
update timer expires. The operator can force a Vi adjustment in every mode from the
Vi status screen. When either of the arrow keys is pressed from the Vi status screen
a Vi adjustment is started immediately.
Vi Slide Stop and Capacity Slide Valve Interference
The adjustments of the Vi slide stop limits the travel of the capacity slide valve. If high Vi
is required, the capacity slide valve will only be able to load to a 60% indicated position.
When the capacity slide valve is at 60% and the Vi slide stop is at the high Vi position the
compressor is at full-load high Vi. If low Vi is required the capacity slide valve is able to
load to 100% indicated position. When the capacity slide valve is at 100% and the Vi
slide valve is at the low Vi position the system is at full-load low Vi. Intermediate Vi posi-
tions will restrict the movement of the capacity slide valve between 60% and 100%. The
capacity of the compressor is the same at full-load high Vi and full-load low Vi. The inter-
ference of the Vi and capacity slide valve requires that the capacity slide valve be moved
along with the Vi slide valve when above 60% indicated.
VI Position Correction
The RWB compressor Vi slide stop indicator measures the linear position of the Vi slide
stop along its range of travel. This sensor value is then corrected based on the Vi non lin-
ear characteristics to produce a corrected Vi position. The corrected Vi position is what is
displayed on status screens and recorded in the logs. The actual sensor readings can be
viewed and on the View analog screen.
 NOTE: Calibration of the Vi position should only be done using the endpoints (Minimum Vi and Max-
imum Vi). It should never be calibrated at some mid position.

M&M RWB series Specifications

General

Brand	M&M
Model	RWB series
Category	Controller
Language	English