Standard Screw Compressor - RWB Series
3-8
M&M
R
EFRIGERATION
I
NC
.
Control Groups
Four groups of control setpoints can be defined for use with automatic capacity con-
trol. Each setpoint group consist of a cut-in, cut-out, control, and low alarm and fail-
ure setpoints. The control associated with these pressures is as follows;
Cut-Out Setpoint - The CCS will stop the compressor when the suction pressure or
process temperature goes below the cut-out setpoint.
Cut-In Setpoint - The CCS will start the compressor when the suction pressure or
process temperature goes above the cut-in setpoint.
Control Setpoint - The CCS will adjust the slide valve position in an attempt to main-
tain the suction pressure or process temperature at the control setpoint.
Low Alarm/Failures Setpoints - These are the low suction pressure or process tem-
perature alarm and failure values associated with the control setpoint. In Sequenced
mode, the local setpoints are replaced by the Sequencer low alarm and failure set-
points transmitted by the master sequencer controller.
Time Proportional Control Strategy
The length of time the slide valve is moved during automatic capacity control is deter-
mined by a time proportional control strategy. The slide valve move time is propor-
tional to the difference between the control parameter and the control setpoint. The
parameters that define the move time are bandwidth, deadband, short pulse, long
pulse, and slide valve update time. These parameters are defined for both load and
unload conditions by the operator. The definition and description of these parameters
is as follows:
Deadband - When the difference between the control setpoint and the control param-
eter is less than the deadband value no slide valve movement will take place. If the
deadband values are too small, the slide valve may continuously hunt for the proper
setting very close to the control setpoint resulting in excessive slide valve wear. If the
deadband values are too large, the compressor may not hold the control parameter
close enough to the control setpoint resulting in inefficient capacity control.
Short Pulse - This is the shortest amount of time the slide valve is moved to load or
unload the system. The short pulse time is the fine adjustment for control parameter
differentials slightly above or below the deadband values. If the shortest pulse is too
long the slide valve may continuously overshoot the optimal position resulting in
excessive slide valve wear.
Bandwidth - This parameter adjusts the overall sensitivity of the control strategy. The
slide valve move time is computed to be between the shortest and longest pulse while
the control parameter is outside the deadband but inside the bandwidth. The move
time approaches the longest pulse time as the difference between the control param-
eter and the control setpoint approaches the bandwidth value. If the bandwidth value
is too small, the compressor will respond quickly and may constantly overshoot the
optimal position. If the bandwidth values are too large the compressor will react
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