RT-SVX063E-EN
81
Figure 40.
Piping diagram with hot gas bypass
Hot Gas Bypass Control
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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.
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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.
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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.
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e:: EXV calibration time is used for both HGBP
and EXV valve calibration.
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Whenever the HGBP valve electronic controller is
power cycled. The HGBP valve electronic controller
does this automatically.
EXV Control (Electronic
Expansion Valve)
Electronic expansion valves (EXVs) control suction
superheat to ensure the complete and efficient
evaporation of refrigerant in the evaporator. Excess
liquid refrigerant in the compressor suction (low
suction superheat) can be harmful to the compressor,
and a deficit of liquid in the evaporator (high suction
superheat) is inefficient. The benefit of EXV control,
through controlling suction superheat, suction
pressure, suction saturated temperature, or pressure
ratio, promotes energy-efficient operation and long-
term reliability of the compressor(s).
Suction superheat is the difference between suction
temperature and saturated suction temperature. A
temperature sensor measures suction temperature.
The saturated suction temperature is determined from
the measured suction pressure by a refrigerant
property calculation. Typically, the EXV control
maintains a small, desired amount of suction superheat
by opening or closing the EXV(s), which adjusts the
refrigerant flow rate into the evaporator.
Figure 41.
Component location — Circuit 1 of
staggered coil unit
In certain situations such as cold ambient starts, warm
supply air starts, or DOAS applications, the EXV control
can automatically move into a suction pressure control
mode, allowing the suction superheat to deviate from
the superheat setpoint. The goal of the suction
pressure control mode is to maintain suction pressure
or differential pressure within the compressor
operating map.
In situations where the superheat rapidly falls to very
low values, special control modes are necessary to dry
out excess refrigerant liquid in the evaporator, suction
line, and compressor sumps, and transition back to
normal control.
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