The Cycle Time setpoint determines the amount of time
the output is on and off, when in the proportional band. At
the completion of the cycle time the actual reading and
necessary response is re-evaluated. If a 4 second period
is selected, then the following will result:
Proportional Distance
Actual Reading is
From Setpoint
Output
Pulsed On
(Seconds)
Output Off
(seconds)
0 0 4
1/4 1 3
1/2 2 2
3/4 3 1
1 4 0
Upper Cycle Time - This setpoint determines the
amount of time in seconds that the load output is on
and off, when in the upper proportional band. Refer to
the above description about cycle time.
Lower Cycle Time - This setpoint determines the
amount of time in seconds that the unload output is
on and off, when in the lower proportional band.
Refer to the above description about cycle time.
Start - A compressor is started at this setpoint. This is
the Start Autocycle setpoint of the active Capacity
Control. A compressor is started at this setpoint
during Compressor Sequencing.
Stop - A compressor is stopped at this setpoint. This
is the Stop Autocycle setpoint of the active Capacity
Control. A compressor is stopped at this setpoint
during Compressor Sequencing.
The load limiting setpoints are used if no compressors are
running and the equalized system is to be slowly pulled
down to normal. These setpoints limit the compressor
loading when starting the first compressor:
Slide Valve Position - the Slide Valve position set
point that the control system will hold the Slide Valve
until the duration timer has timed out.
Duration Time - the amount of time in minutes to
hold the compressor at the Load Limiting Slide Valve
Position setpoint. If load limiting is not desired, set this
time to zero (0).
When the first compressor is started, the
sequence will allow that compressor to load up to
the Slide Valve Position setpoint and hold the
compressor at this setpoint until the Duration
Time has timed out or the current reading of the
process variable falls below the Start setpoint.
The following set points are required for each compressor
on the suction level:
(A - D) Compressor ID - A through D represent the
sequence position. Any machine can be set for any
sequence position. The letter determines the starting
order of the compressors, position A is the first
position. If the Start setpoint is maintained, the lead or
first machine started, will always load up first and then
the next sequence position on up as they are needed.
The stopping of compressors is done in the reverse
order of the starting. Each compressor must have a
unique ID# entered at their control panel. A zero (0)
disables or removes a compressor ID from being
sequenced.
Unload Minimum Slide Valve Position - This is the
Slide Valve position a compressor will be allowed to
unload to. The compressors will unload to the
minimum Slide Valve position for that machine, when
the reading is below the Capacity Control lower
deadband. This set point is only used if more than
one compressor is running. When only one
compressor is running it is allowed to fully unload.
This set point can prevent short cycling of
compressors in the unload sequence. Short cycling
occurs when a compressor is stopped and the current
reading quickly rises and an already running
compressor is unable to compensate because it is
running at full load, so a compressor is cycled back
on very shortly after a compressor was stopped. A
screw compressor has some minimum built-in
capacity that cannot be shed, generally in the range
of 10-15% of full load capacity, when the compressor
is stopped the control system needs to compensate
for this loss. If a compressor that is running at full load
is allowed to unload to a minimum Slide Valve
position, this compressor will then be able to
compensate for the extra load that occurs when
another compressor is stopped.
The computer will not unload a compressor below this
set point during normal system operation. For
example, if compressor A is running at 100 %
capacity and compressor B is running and has been
unloaded to the minimum Slide Valve position, B is
still contributing somewhat to the capacity. To take B
off line would be to lose approximately 10% of the
total capacity. If A was already loaded to 100%
capacity, it would not be able to load to compensate
for losing the 10% B dropped off. The current reading
would rise and B would be cycled on again. If the
current reading remains low (below the lower
deadband) for the predetermined time (stop time
delay), B will then stop. A will have sufficient reserve
capacity to maintain the loss of B. The control of this
set point is in percentage of Slide Valve, not
compressor capacity. With some experimentation, it is
possible to find the right value for the minimum Slide
Valve position set points that neither under-
compensate for the built-in capacity, resulting in short
cycling of the compressors, nor over-compensates for
the built-in capacity, resulting in inefficient operation
(where the sum of the capacity of the compressors
that are running is significantly less than the full load
capacity of the next compressor in the unload
sequence).
Start Time Delay - The minimum time in minutes that
the current reading must be at or above the Start
setpoint before a compressor will start. This timer
helps prevent cycling compressors on and off due to
short sudden changes in load within the system. If the
current reading falls below the Start setpoint for any
reason at any time, the start time delay will be reset to
start counting again if needed. When a compressor is
starting, the start time delays will be reset to start
counting again if needed for a next compressor.
FRICK
QUANTUM™ COMPRESSOR CONTROL PANEL S90-010 O
OPERATION Page 51
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