Emerson Vilter VilTech, Руководство по эксплуатации и обслуживанию

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Sequencer Control
The control function is responsible for the management of a group of compressors to control a refrigeration load.
The following sections will present the user with both a technical overview and the operating procedures for
sequencer control.
Sequencer Technical Section
In a typical refrigeration system several compressors may be used to provide the required tonnage to control the
refrigeration load. If the load is high all compressors may be needed. If the load is low only one compressor may be
needed. The sequencer automatically manages the starting and stopping of the compressors to maintain the system
control parameter at the desired setpoint. This process of starting and stopping compressors is referred to as staging
up or down compressors.
The order that the compressors are staged is defined by the operator in a lead list. The sequencer makes decisions
based on operator defined parameters and delay times which can be tailored to match the needs of the refrigeration
system. The sequencer continuously transmits the setpoint and system control parameter values to each compressor.
When a compressor is started by the sequencer it attempts to maintain the system control parameter at the defined
setpoint using its own automatic capacity control. The compressor that is making capacity adjustments in response
to load changes is referred to as the “Trim Machine.” Once the trim machine is fully loaded it informs the sequencer
that it can no longer increase capacity. If the load is still not under control the sequencer will inform the trim machine
to lock at full capacity and will start another compressor, if one is available. The newly started compressor is now the
trim machine. This process will continue until the load is under control or all compressors have been utilized. If the
load decreases to the point where the trim machine is no longer needed it will be stopped and control is passed back
to the previous compressor.
The staging up and down of compressors will be performed at different rates based on the difference between the
system control parameter and the setpoint. If the difference is large, fast staging is performed. If the difference is
small, normal staging is performed. This staging technique ensures that compressors are started when needed and at
the same time minimizes any unnecessary starting or stopping. Independent sets of operator defined parameters are
provided for both the loading and unloading conditions.
The refrigeration control may contain multiple sequencers depending on the configuration of the refrigeration
system. For example, if booster compressors are used to maintain one suction pressure level and high stage compressors
maintain a second pressure level the system would use two different sequencers. In many refrigeration systems a
compressor may be piped such that it can be used on more than one refrigeration load, this is referred to as a swing
machine. For example, using hand valves the operator can configure a compressor as a booster or a high stage
machine. This swing machine would be available to both the high stage and booster sequencers.
The sequencer allows a group of compressors to be used in an efficient manner without operator intervention. The
automatic starting and stopping of compressors ensures that power requirements and operational costs are kept to a
minimum.