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Capacity Control
CAPACITY CONTROL on a RECIP compressor is performed by adding or removing stages. Loading the
compressor adds stages which increases the compressor capacity and Unloading the compressor
removes stages which decreases the compressor capacity. A stage can consist of one or more Loaders
which energize to add pistons, or Unloaders which de-energize to remove pistons. Capacity Control can
be performed manually or automatically depending on the mode of operation.
CAPACITY SETPOINTS there is a Max Time at 0% capacity which is a timer that is intended to NOT
allow the compressor to run too long at 0% capacity. The timer is activated in automatic and remote
modes. If the compressor runs at 0% capacity longer than the allowed time then the compressor will
perform a “Normal” stop and a log entry will be documented. When Auto Start/Stop is enabled then the
compressor will restart when the suction pressure rises above the cut-in setpoint. The compressor may
restart immediately, when two cases occur. Case #1- When the capacity ramp start delay is programmed
lengthy and a low max time is at 0% capacity, it is possible that the Max 0% capacity timer will expire
before the capacity ramp allows the machine to load. If, the suction pressure is above the cut-in setpoint
and auto start/stop is enabled, compressor will restart immediately. Case #2- When the suction pressure
is above cut-in and discharge pressure is above the limit/unload may be holding the compressor at 0%
capacity so the 0% capacity timer will expire and the compressor will restart immediately.
Manual Capacity Control
In MANUAL mode, the capacity is adjusted using the keypad on the Viltech. The up-arrow is used
to add stages and the down-arrow is used to remove stages. Each time an arrow key is hit, one
stage is added or removed until the compressor is fully loaded or unloaded. Manual capacity
control can only be performed when viewing the Main Status screen.
Automatic Capacity Control
AUTOMATIC CAPACITY CONTROL is accomplished by adding or removing stages to load and
unload the compressor in response to changes in the control parameter (suction pressure or process
temperature). Capacity changes are achieved using a time proportional control strategy with
setpoints for Control Deadband, Normal Bandwidth, Normal Stage Delay, and Fast Stage Delay.
Separate setpoints are provided for loading and unloading the compressor so that the operator may
tune the system differently for increasing or decreasing load conditions.
The operator can also define up to four Control Groups which consist of Cut-In, Cut-Out, Control,
Low Alarm, and Low Failure setpoints. 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 capacity value which is used to limit capacity changes at the lower limit.
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