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Sequence of Operation
compressor running per circuit the compressor with a
higher wear calculation will be chosen to stop.
Ties
If multiple compressors with equal wear are available for
the next stage, the smallest compressor will start. The
largest compressor will stop.
If Balanced Starts is Disabled from the Front Panel,
compressor staging follows the factory staging sequence.
The nominal unit size configuration setting maps to the
factory staging sequence selection.
Compressor Protections
There are a number of functions which provide protection
of the refrigerant systems during compressor operation.
These protections typically involve reducing, if not
completely removing, one or more compressors from
operation on a given refrigeration circuit. There are
basically two types. The first are Lockouts which are those
associated with diagnostics and are persisted until human
intervention or significant unit mode change occurs, The
second are Inhibits which are typically temporary, self-
correcting, and may not involve a diagnostic.
Working in concert with compressor protection are the
general algorithms of compressor staging (timing) as well
as refrigeration and electrical capacity control.
Low Ambient Lockout
Low Ambient Lockout prevents compressor operation
when Outdoor Air Temperature Active falls below an
adjustable setpoint. The low ambient lockout setpoint
range and default value is dependent on unit configuration.
When the outdoor air temperature rises above the Low
Ambient Lockout Setpoint + 5˚F, inhibit will be removed and
the compressors will be available.
Low Compressor Suction Pressure
Protection
The Low Compressor Suction Pressure diagnostic, Low
Compressor Suction Pressure Start Inhibit, and Low
Compressor Suction Pressure Limit work together to
prevent damage to compressors and suction cooled
compressor motors operating with little or no refrigerant
charge.
The Low Compressor Suction Pressure diagnostic protects
against an inadvertent compressor start when the system
is in a vacuum (such as can be produced during service
procedures). It also avoids compressor failures and
extremely inefficient operation due to total charge loss. The
Low Compressor Suction Pressure diagnostic will shut
down the affected circuit, if already running, when the
circuit's Compressor Suction Refrigerant Pressure
measurement drops below a factory determined pressure
threshold value for normal operation (normal threshold
value). When the circuit is not running, and for a period of
time (in seconds) immediately after the circuit starts
running, the effective start-up pressure threshold is lower
78
than the normal threshold value. These values are
dependent on unit model.
The Low Compressor Suction Pressure Start Inhibit and
Low Compressor Suction Pressure Limit work together to
reduce the likelihood of Low Compressor Suction Pressure
diagnostic, which will lock out the circuit and force a
manual diagnostic reset. When either functions are active,
an indication will be made to the user interface.
The Low Compressor Suction Pressure Start Inhibit is
designed to prevent a circuit from starting when low
compressor suction pressure exists, but will not shut down
a circuit that is already running. When the function is in the
Active state, the circuit will be prevented from operating
(locked out). The threshold value for this function is 1.2
times the normal threshold value.
Low Compressor Suction Pressure Limit only applies to
circuits which have more than 1 compressor installed. This
function prevents the addition of circuit capacity any time
the circuit is running and Compressor Suction Refrigerant
Pressure is less than 1.4 times the normal threshold value.
Temperature and Pressure Sensor
Failures
Each refrigeration circuit has a number of sensors that are
required for proper control of compressors and associated
head pressure control components. Should these sensors
fail, or exceed operational limits, compressors on the given
circuit will be inhibited until the sensor values return to
normal operational ranges. An indication will be made to
the user interface.
Oil Management Protection
Trio Manifolded compressors on 25 and 30 ton standard
efficiency units require an additional level of protection for
the small compressor installed in the "A" position. When the
"A" compressor has been energized and an additional
compressor has been commanded on, a thirty minute timer
is started. When the thirty minute timer has expired, the "A"
compressor is de-energized for thirty seconds. At the end
of the thirty seconds the compressor is released to normal
control. The thirty minute timer is reset at the end of oil
management cycle.
Compressor Discharge Pressure Limit
Compressor Discharge Pressure Limit is a control feature
of Adaptive Control. This feature prevents a circuit
shutdown when the discharge pressure approaches the
high pressure cutout switch setting by decreasing
compressor capacity. This is done by staging off fixed
speed compressors on circuits with high discharge
pressure. This feature will provide maximum capacity while
preventing a high pressure cutout.
The discharge pressure limit is based on the discharge
pressure percentage relative to the high-pressure cutout
setting. This limit is active when the circuit is running.
RT-SVX063G-EN
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