Frick QUANTUM - Compressor Sequence Overview

FRICK QUANTUM CONTROL PANEL
OPERATION
S90-010 O
Page 43
COMPRESSOR SEQUENCE OVERVIEW
The above settings are used to sequence the compressors
that have been setup for compressor sequence control mode
2. The panels selected for compressor sequence control
mode 2 must be connected via the proper hardware con-
nection. Only a compressor that is in both remote compres-
sor mode and remote Slide Valve mode (and not in shut-
down), will be started, stopped, loaded or unloaded using
this sequence control.
The refrigeration load refers to the demand for loading of
compressors to reduce the rise in capacity that has resulted
from an increased need for refrigeration in the plant. When
the load rises this means the capacity has risen. When the
load falls this means the capacity has fallen. Compressors
need to load if the Leader sequence controller’s capacity
reading is above it’s “Upper Dead Band” setpoint plus the
“Capacity Control” setpoint and compressors need to un-
load if the Leader sequence controller’s capacity reading is
below it’s “Capacity Control” setpoint minus the “Lower Dead
Band” setpoint. During compressor sequencing and in the
description of it, if the Capacity Up output (request to load
the next compressor) is energized, then the Capacity Down
output (request to unload the next compressor) is not ener-
gized; and likewise, if the Capacity Down output is ener-
gized, then the Capacity Up output is not energized.
If one of these compressors has been told to run, but re-
mains off due to remaining Recycle Delay time or other
conditions preventing the compressor from running, then
no sequencing will happen until the compressor is running
or shutdown.
If one of these compressors is not in remote compressor
mode or is not in remote Slide Valve mode or is in shut-
down, that compressor is not available to be sequenced.
However, it will signal its Capacity Control requirements to
the next compressor.
Because the next compressor is forced to load to the “Mini-
mum Capacity when Running” setpoint, excess capacity
could occur causing the previous compressor to adjust by
unloading a little. If the load continues to rise, then the
previous compressor that unloaded to below 98% will load
again to at or above 98% prior to signaling the next com-
pressor to load.
The following is an example of sequencing compressors:
On System Start-up - The Leader compressor will start
if the capacity is greater than or equal to the “Upper
Dead Band” plus the “Capacity Control” setpoint for
the “Next Compressor Start Delay” time period. If the
Leader is not available to be sequenced, then the
Leader's Start Follower output and Capacity Up output
are energized. If the Follower with the Start Follower
input and the Capacity Up input energized is not
available to be sequenced, then the Follower's Start
Follower output and Capacity Up output are
energized.
On an initial start-up of a compressor, the compressor
started will load to the “Minimum Capacity when Run-
ning” setpoint and remain at or above this setpoint until
stopping. A Follower sequence controller energizes
the Slide Valve > Minimum Capacity output when the
Slide Valve is above the “Minimum Capacity when
Running” setpoint + 1.
Load Rises - If the “Actual” reading continues to be
greater than or equal to the “Upper Dead Band” plus
the “Capacity Control” setpoint at the Leader sequence
controller, then the first available compressor in the
order the panels are wired, starting with the Leader
sequence control panel, is selected to load to 100%,
followed by the next.
If the running compressor loads to at or above 98%
Slide Valve position for the “Next Compressor Start De-
lay” time period, then the Start Follower output and
Ca-pacity Up output are energized.
Load Falls - If the “Actual” reading falls to or below the
“Capacity Control” setpoint minus the “Lower Dead
Band” setpoint at the Leader sequence controller, then
the last compressor that is running will be selected for
unloading. If the Start Follower output is energized
(indi-cating another compressor has been told to
run), the Capacity Down output is energized. If the
Start Follower output is not energized and the
Capacity Down input is energized, then this compressor
is unloaded to it’s “Mini-mum Capacity when Running”
setpoint. If the Start Follower output is energized
and the Slide Valve > Mini-mum Capacity input is
not energized (indicating the next compressor has
been unloaded to at or below the “Minimum Capacity
when Running” setpoint), then this compressor is
unloaded to or below its “Minimum Ca-pacity to Stop
Next Compressor” setpoint. When the “Next
Compressor Stop Delay” is timed out then the Start
Follower and Capacity Down outputs are de-
energied.
A running compressor that is signaled to stop will un-
load to or below it’s “Highest Slide Valve Position to
allow starting the compressor” setpoint and then will
stop. The Leader sequence controller is the last com-
pressor to unload. The Leader will unload the com-
pressor to the “Minimum Capacity when Running” set-
point and if the Leader's stop delay has timed out, the
Leader will unload the compressor to it’s “Highest Slide
Valve Position to allow starting the compressor” set-
point before stopping this compressor.