M&M
R
EFRIGERATION
I
NC
. 3-7
Chapter 3: Capacity Control
Sequenced Mode
When the STOP command is received from the master sequencer, the CCS will stop the
screw compressor automatically and leave it on-line for sequenced operation. While
stopped on-line, the compressor is available for sequencing and in standby waiting for
commands.
Capacity Control
Capacity Control on the screw compressor is performed by adjusting the position of the slide
valve. Loading the slide valve increases the compressor capacity. Unloading the slide valve
decreases the compressor capacity. Capacity Control can be performed manually or automat-
ically depending on the mode of operation.
Computed Slide Valve Position
The RWB compressor utilizes a variable Vi slide stop that dynamically changes the dis-
tance of travel of the capacity slide valve. The slide valve position presented to the oper-
ator on the main status screen and other screens for reference is automatically adjusted
based on the Vi Slide stop position. Thus the slide valve position shown is always ranged
from 0-100% corresponding to minimal load and full load regardless of Vi position. The
actual sensor readings can be viewed and on the View analog screen.
NOTE: Calibration of the slide valve position should only be done when the Vi slide stop is in the low
Vi position.
Manual Capacity Control
In Manual mode or Local mode the slide valve is adjusted using the keypad on the CCS.
The up-arrow is used to load the slide valve and the down arrow to unload the slide valve.
The CLEAR button is used to stop loading or unloading. Manual capacity control can only
be performed when viewing the main Status screen or the Graphics Status screen.
Automatic Capacity Control
Automatic Capacity Control is accomplished by moving the slide valve to load and unload
the compressor in response to changes in the control parameter (suction pressure or pro-
cess temperature). The slide valve movement is achieved using a time proportional con-
trol strategy. The operator can define Cut-In, Cut-Out, and Control setpoints. These
setpoints are referred to as a control group. The active control group can either be
selected manually by using the active group screen or automatically through setpoint
scheduling. The operator can also define a minimum and maximum slide valve position
which is used as lower and upper limits for the slide valve during automatic capacity con-
trol operations.
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