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Pressure transducers convert pressure into an electronic
signal that the microprocessor displays in pounds per
square inch (psi) or bar.
Temperature sensors transmit temperature data
electronically to the microprocessor for display in either
Fahrenheit (°F) or Celsius (°C).
Expansion Valve
An expansion valve is a metering device controlling the
flow of refrigerant to the evaporator based on the
evaporator superheat.
Superheat is factory-set for around 12°F (6.7°C). Close the
valve to increase superheat. To accurately read superheat,
install a temperature sensor at the evaporator outlet. The
sensor bulb should be located at the 4 o'clock or 8 o'clock
positions on the pipe for the most accurate pressure
measurement.
Confirm that the closed-cell insulation covers the thermal
expansion valve sensing bulb. If insulation is missing, the
bulb will tend to feed more refrigerant to satisfy the
superheat setting.
Solenoid Valve
The liquid-line solenoid valve is used in the refrigeration
cycle to provide a refrigerant pump-down cycle at the end
of a compressor cycle. The liquid refrigerant is pumped out
of the evaporator to eliminate liquid refrigerant from
migrating to the compressor in the off cycle.
Sight Glass
When the sight glass shows a green indicator, no moisture
is present. When the sight glass shows a yellow indicator,
there is moisture in the refrigerant line. Bubbles can be
observed whenever chiller cycling causes the pressure to
change up or down.
ARTC-SVX008A-EN
NOTICE
Equipment Damage!
Failure to remove moisture from system could cause
corrosion within the chiller/heater components, and
degrade performance.
Perform vacuum evacuation of system to remove
moisture.
Low Pressure Bypass
Logic that uses a time delay that temporarily bypasses the
low-pressure switch for compressor start up. Once the
delay opens (times out) the normal controls are put back on
line within the control circuit.
Low Pressure Pump Down (LPPD) Controls
Allows the system to start in low ambient conditions and
prevents flood-back to the compressor by pumping the
liquid refrigerant out of the evaporator. The LPPD control
consists of a pressure switch that is set at "cut in" and "cut
out" pressures that depend on the type of refrigerant in the
system and is based on pressure at which the refrigerant
reaches the freezing point of the evaporator water/glycol
mixture.
The set points are typically just below the freezing point.
When using water, the pressure settings for R–454B
refrigerant are 75 psig for the "cut out" and 90 psig for the
"cut in". For any other water/glycol mixture, contact Trane
technical support for proper set points.
High and Low Ambient Controls
Condenser coils must be properly oversized for high
ambient operation (or high elevation). In order to prevent
refrigerant migration to the condensing coil when operating
in low ambient temperatures, it is essential to have
effective head pressure control. If the condenser head
pressure is not properly regulated, the chiller may not
receive sufficient refrigerant, leading to low suction
pressure, evaporator freezing, and ultimately causing the
chiller to shut down. Flooded condenser head pressure
control, fan variable frequency drive, and fan cycling are all
used to allow varying degrees of low ambient operation.
Thermal Capacity
The thermal capacity of the chiller modules is dependent
on the leaving temperature of the chilled water/glycol
mixture, maintaining a minimum flow of water through the
evaporator and keeping debris out of the air-cooled
condenser. In applications where it is desired to operate
with a lower flow rate or higher temperature change,
consult the technical support for recommendations.
Controls Interface
51
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