Superheat High Limit Detection; Variable Speed Compressor Modulating Current Limit; Compressor Low Suction Saturated Temperature Speed Limit; Coil Frost Protection - Trane IntelliPak RA 20 Installation, Operation And Maintenance Manual

S S e e q q u u e e n n c c e e o o f f O O p p e e r r a a t t i i o o n n

Superheat High Limit Detection

A unit may experience refrigerant leakage at any point
in its service life. Moderate refrigerant loss reduces
capacity and causes low suction saturated temperature
but excessive refrigerant loss may lead to compressor
damage. Excessive refrigerant loss is detected by a
suction superheat condition which exceeds a high limit
threshold.
This function detects excessive refrigerant loss by
comparing suction superheat to a high limit threshold.
The detection method reduces the potential for
nuisance trips by providing a startup delay time that is
a function of the outdoor air temperature.
This approach helps to mitigate false detection trips
because low suction saturated temperature and high
superheat is normal following startup at low outdoor
air temperature conditions.
When this function is active, an indication will be made
to the user interface and the circuit will shutdown.
Variable Speed Compressor Modulating
Current Limit
This Adaptive Control action will first reduce variable
speed compressor RPM which, in turn, reduces
discharge pressure. This will reduce the variable speed
compressor motor current thus preventing an A49
Speed Limit VFD trip. This function works in concert
with Current Limit Staging Command which can
subtract, hold or allow adding compressor capacity.
When this function is active, an indication will be made
to the user interface.
Compressor Low Suction Saturated
Temperature Speed Limit
This is an Adaptive Control feature for variable speed
compressors. Low suction saturated temperature can
occur following startup at low outdoor air temperature
conditions. This circuit-level feature applies to variable
speed compressors and enforces a minimum and
maximum speed limit.

Coil Frost Protection

This is a circuit level protection and is active whenever
one or more compressors on a circuit are running. This
feature is always available with all compressor types.
As frost builds on a coil, thermal resistance increases
and airflow decreases. Frost buildup on the coil can
also cause instabilities in superheat control. If suction
saturated temperature drops below a threshold, Frost
Protection will inhibit capacity in an attempt to melt the
frost and allow the unit to run more efficiently. Once
the frost is removed the unit will be returned to normal
operation. To avoid nuisance trips at startup, a
"ignore" time delay is applied. The delay time is
variable and is based on outdoor air temperature
80
Modulating Coil Frost Limit Control applies to circuits
with a variable speed compressor. This function will
limit variable speed compressor capacity according to
evaporator coil frosting conditions. If the suction
saturated temperature (SST) is sufficiently above the
Coil Frost Threshold, the Symbio 800 reverts to normal
control.
Modulating Coil Frost Limit Control is active following
compressor startup and is not subject to the ignore
time.
There are no diagnostics for this feature. When this
function is active, an indication will be made to the user
interface.

Head Pressure Control

The refrigeration system will utilize condenser fan
control to maintain the discharge pressure of operating
compressors on a given circuit to desirable levels.
Pressure sensors will be utilized to provide the control
signal. As the pressure of the condenser section
increases more stages of fan capacity will be used to
prevent excessive operating conditions that could open
the High Pressure Cutout switch causing nuisance trips
of the refrigeration circuit. Condenser fan capacity will
be reduced to prevent excessive low pressures that
could cause un-stabilized refrigeration conditions. A
unit can have up to two refrigeration systems (circuits)
that are completely isolated from each other and are
controlled independent from each other.
L L o o w w A A m m b b i i e e n n t t O O p p e e r r a a t t i i o o n n
Units configured with Low Ambient Operation will
utilize a VFD for the first condenser fan.

Hot Gas Bypass

Hot gas bypass is a unit configuration option that, at
some predetermined suction pressure, initiates a
modulated flow of hot refrigerant gas from high side to
the low pressure portion of the refrigeration system.
This adds "false load" to avoid low suction pressures
that would otherwise result in coil frosting. This feature
can be disabled, but when enabled is transparent to the
normal staging control provided by the Symbio 800
controller. Regardless, Hot Gas Bypass operation
indirectly places operating constraints on the system
that have to be addressed by the unit controls in the
areas of EXV superheat control and dehumidification/
reheat operation.
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