Carrier AQUAZONE 50PSH, Installation, Start-Up And Service Instructions Manual

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Thermostatic Expansion Valves
Thermostatic expansion valves (TXV) are used as a means of me -
tering the refrigerant through the evaporator to achieve a preset su -
perheat at the TXV sensing bulb. Correct superheat of the refriger -
ant is important for the most efficient operation of the unit and for
the life of the compressor.
Packaged heat pumps typically use one bi-flow TXV to meter re -
frigerant in both modes of operation. When diagnosing possible
TXV problems it may be helpful to reverse the refrigerant flow to
assist with the diagnosis.
Geothermal and water source heat pumps are designed to operate
through a wide range of entering-water temperatures that will
have a direct effect on the unit refrigerant operating pressures.
Therefore, diagnosing TXV problems can be difficult.
TXV FAILURE
The most common failure mode of a TXV is when the valve fails
while closed. Typically, a TXV uses spring pressure to close the
valve and an opposing pressure, usually from a diaphragm, to
open the valve. The amount of pressure exerted by the diaphragm
will vary, depending on pressure inside of the sensing bulb. As the
temperature of and pressure within the bulb decreases, the valve
will modulate closed and restrict the refrigerant flow through the
valve. The result is less refrigerant in the evaporator and an in -
crease in the superheat. As the temperature at the bulb increases
the diaphragm pressure will increase, which opens the valve and
allows more refrigerant flow and a reduction in the superheat.
If the sensing bulb, connecting capillary, or diaphragm assembly
are damaged, pressure is lost and the spring will force the valve to
a closed position. Often, the TXV will not close completely so
some refrigerant flow will remain, even if there is inadequate flow
for the heat pump to operate.
The TXV sensing bulb must be properly located, secured, and in -
sulated as it will attempt to control the temperature of the line to
which it is connected. The sensing bulb must be located on a dedi -
cated suction line close to the compressor. On a packaged heat
pump, the bulb may be located almost any place on the tube run -
ning from the compressor suction inlet to the reversing valve. If
the bulb is located on a horizontal section, it should be placed in
the 10:00 or 2:00 position for optimal performance.
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 (polyolester
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.
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.
To determine that a TXV has failed, verify the following:
• The suction pressure is low and the valve is non-responsive.
• The TXV sensing bulb can be removed from the suction
line and warmed by holding the bulb in your hand. This ac -
tion should result in an increase in the suction pressure
while the compressor is operating. The sensing bulb can
also be chilled by immersion in ice water, which should re -
sult in a decrease in the suction pressure while the com -
pressor is operating. No change in the suction pressure
would indicate a nonresponsive valve.
• Simultaneous LOW suction pressure, HIGH refrigerant
subcooling and HIGH superheat.
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.