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AIRFLOW
THERMISTOR
LEGEND
COAX
— Coaxial Heat Exchanger
Airflow
Refrigerant Liquid Line Flow
CAUTION
Use caution when tightening the strap. The strap must be tight
enough to hold the bulb securely but caution must be taken not
to over-tighten the strap, which could dent, bend, collapse or
otherwise damage the bulb.
The bulb must be secured to the pipe using a copper strap. The use
of heat transfer paste between the bulb and the pipe will also help
ensure optimum performance.
The bulb must also be properly insulated to eliminate any influ-
ence on valve operation by the surrounding conditions. Cork tape
is the recommended insulation as it can be molded tight to the
bulb to prevent air infiltration.
Causes of TXV Failure
The most common causes of TXV failure are:
1.
A cracked, broken, or damaged sensing bulb or capillary can
be caused by excessive vibration of the capillary during ship-
ping or unit operation.
If the sensing bulb is damaged or if the capillary is cracked or
broken, the valve will be considered failed and must be
replaced. Replacement of the TXV "power head" or sensing
bulb, capillary, diaphragm assembly is possible on some
TXVs. The power head assembly screws onto most valves,
but not all are intended to be replaceable. If the assembly is
not replaceable, replace the entire valve.
2.
Particulate debris within the system can be caused by several
sources including contaminated components, tubing, and ser-
vice tools, or improper techniques used during brazing opera-
tions and component replacement.
Problems associated with particulate debris can be com-
pounded by refrigerant systems that use POE (polyol ester
oil). POE oil has solvent-like properties that will clean the
interior surfaces of tubing and components. Particulates can
be released from interior surfaces and may migrate to the
TXV strainer, which can lead to plugging of the strainer.
3.
Corrosive debris within the system may happen after a fail-
ure, such as a compressor burn out, if system was not prop-
erly cleaned.
AIR
COIL
AIRFLOW
(°F)
(°F)
EXPANSION
VALVE
FP2
CONDENSATE
OVERFLOW
LIQUID
(CO)
LINE
AIR COIL
FREEZE
PROTECTION
Fig. 31 — FP1 and FP2 Thermistor Location
COAX
FP1
WATER IN
WATER OUT
WATER
COIL
PROTECTION
4.
Noncondensables may be present in the system. Non-con-
densables includes any substance other than the refrigerant or
oil such as air, nitrogen, or water. Contamination can be the
result of improper service techniques, use of contaminated
components, and/or improper evacuation of the system.
Symptoms
The symptoms of a failed TXV can be varied and will include one
or more of the following:
•
Low refrigerant suction pressure
•
High refrigerant superheat
•
High refrigerant subcooling
•
TXV and/or low pressure tubing frosting
•
Equalizer line condensing and at a lower temperature than
the suction line or the equalizer line frosting
•
FP1 faults in the heating mode in combination with any of
the symptoms listed above
•
FP2 faults in the cooling mode in combination with any of
the symptoms listed above. Some symptoms can mimic a
failed TXV but may actually be caused be another problem.
Before conducting an analysis for a failed TXV the following
must be verified:
•
Confirm that there is proper water flow and water tempera-
ture in the heating mode.
•
Confirm that there is proper airflow and temperature in the
cooling mode.
•
Ensure coaxial water coil is clean on the inside; this ap-
plies to the heating mode and may require a scale check.
•
Refrigerant may be undercharged. To verify, subcooling
and superheat calculations may be required.
Diagnostics
Several tests may be required to determine if a TXV has failed.
The following tools may be required for testing:
1.
Refrigerant gage manifold compatible with the refrigerant in
the system.
2.
Digital thermometer, preferably insulated, with wire leads
that can be connected directly to the tubing.
3.
Refrigerant pressure-temperature chart for the refrigerant
used.
48
SUCTION
COMPRESSOR
DISCHARGE
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