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Sequence of Operation
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
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.
Low Ambient Operation
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.
80
Figure 40. Piping diagram with hot gas bypass
Hot Gas Bypass Control
Activation
If a unit is configured with hot gas bypass, the hot gas
bypass system is functional whenever circuit 1is operating.
The hot gas bypass valve is closed during normal
operation. The valve will open, passing the amount of hot
gas required to produce a suction pressure that is high
enough to prevent coil frosting, when necessary. If Hot Gas
Bypass Enable is Disable, the Hot Gas Bypass valve is
always closed.
Interaction
Hot Gas Reheat and Hot Gas Bypass options are installed
on circuit 1, however, hot gas bypass is not allowed to
operate if the "Hot Gas Reheat Valve Reheat" is open
(>0%). The hot gas bypass valve will open when suction
pressure is lower than the calculated target. In addition, the
valve control has interactions with coil defrost protection,
EXV control and condenser fan control.
HGBP Valve Calibration
HGBP valve calibration (also known as overdrive closed)
procedure is initiated in the following circumstances:
•
Whenever the unit controller is power cycled. This is
under the control of the unit controller software, as part
of the software startup procedure. Note: under the most
common scenario when both the HGBP valve
electronic controller and unit controller are power
cycled, HGBP valve recalibration may occur twice.
•
When the circuit cycles off and the circuit has run for at
least EXV Recalibration Time since the previous
calibration.
Note: EXV calibration time is used for both HGBP and
EXV valve calibration.
•
Whenever the HGBP valve electronic controller is
power cycled. The HGBP valve electronic controller
does this automatically.
RT-SVX063G-EN
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