24VNA6 / 25VNA4: Service Manual
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
38
TXV Operation
The TXV is a metering device that is used in air conditioning and heat
pump systems to adjust to changing load conditions by maintaining a
preset superheat temperature at the outlet of the evaporator coil. The
volume of refrigerant metered through the valve seat is dependent upon
the following:
1. Superheat temperature is sensed by cap tube sensing bulb on
suction tube at outlet of evaporator coil. This temperature is
converted into pressure by refrigerant in the bulb pushing
downward on the diaphragm which opens the valve via the push
rods.
2. The suction pressure at the outlet of the evaporator coil is
transferred via the external equalizer tube to the underside of the
diaphragm. This is needed to account for the indoor coil pressure
drop. Residential coils typically have a high pressure drop, which
requires this valve feature.
3. The pin is spring loaded, which exerts pressure on the underside of
the diaphragm. Therefore, the bulb pressure works against the
spring pressure and evaporator suction pressure to open the valve.
If the load increases, the temperature increases at the bulb, which
increases the pressure on the top side of the diaphragm. This opens
the valve and increases the flow of refrigerant. The increased
refrigerant flow causes the leaving evaporator temperature to
decrease. This lowers the pressure on the diaphragm and closes the
pin. The refrigerant flow is effectively stabilized to the load demand
with negligible change in superheat.
Accumulator
The accumulator is specifically designed to operate with Puron® or R22
respectfully; use only factory-authorized components. Under some light
load conditions on indoor coils, liquid refrigerant is present in suction
gas returning to compressor. The accumulator stores liquid and allows it
to boil off into a vapor so it can be safely returned to compressor. Since a
compressor is designed to pump refrigerant in its gaseous state,
introduction of liquid into it could cause severe damage or total failure of
compressor.
The accumulator is a passive device which seldom needs replacing.
Occasionally its internal oil return orifice or bleed hole may become
plugged. Some oil is contained in refrigerant returning to compressor. It
cannot boil off in accumulator with liquid refrigerant. The bleed hole
allows a small amount of oil and refrigerant to enter the return line where
velocity of refrigerant returns it to compressor. If bleed hole plugs, oil is
trapped in accumulator, and compressor will eventually fail from lack of
lubrication. If bleed hole is plugged, accumulator must be changed. The
accumulator has a fusible element located in the bottom end bell. (See
.) This fusible element will melt at 430°F//221°C and vent the
refrigerant if this temperature is reached either internal or external to the
system. If fuse melts, the accumulator must be replaced.
To change accumulator:
1. Shut off all power to unit.
2. Recover all refrigerant from system.
3. Break vacuum with dry nitrogen. Do not exceed 5 psig.
NOTE:
Coil may be removed for access to accumulator. Refer to
appropriate sections of Service Manual for instructions.
4. Remove accumulator from system with tubing cutter.
5. Tape ends of open tubing.
6. Scratch matching marks on tubing studs and old accumulator.
Scratch matching marks on new accumulator. Unbraze stubs from
old accumulator and braze into new accumulator.
7. Thoroughly rinse any flux residue from joints and paint with
corrosion-resistant coating such as zinc-rich paint.
8. Install factory authorized accumulator into system with copper slip
couplings.
9. Evacuate and charge system.
Pour and measure oil quantity (if any) from old accumulator. If more
than 20 percent of oil charge is trapped in accumulator, add new PVE oil
to compressor to make up for this loss.
A88410
Fig. 38 – Accumulator
Vapor Injection Operation (5 Ton Models)
Some 25VNA4 and 24VNA6 models contain an additional refrigeration
circuit known as a vapor injection circuit. When active this circuit will
increase the capacity of the outdoor unit with a minimal increase in
power consumption. The result is higher capacity at a higher efficiency.
This circuit is only active at extreme heating and cooling temperatures
and only in efficiency mode.
The vapor injection circuit routes a portion of the subcooled liquid from
the condenser through a vapor injection EXV (EXV-VI) and into a
brazed-plate heat exchanger (BPHX) The resulting superheated vapor
will return to the compressor and be "injected" into the mid port.
Meanwhile subcooled liquid leaving the condenser is routed through the
other portion of the BPHX and is further subcooled before exiting to the
evaporator section of system, thus improving the capacity.
A200183
Fig. 39 – Vapor Injection Operation
CAUTION
!
PERSONAL INJURY HAZARD
Failure to follow this caution may result in personal injury.
Wear safety glasses, protective clothing, and gloves when handling
refrigerant.
WARNING
!
ELECTRICAL SHOCK HAZARD
Failure to follow this warning could result in personal injury or death.
Before installing, modifying, or servicing system, main electrical
disconnect switch must be in the OFF position. There may be more than
1 disconnect switch. Lock out and tag switch with a suitable warning
label.
430
°
FUSE
ELEMENT
A88410
BPHX
EXV-VI
COMPRESSOR
MID-PORT
DISCHARGE PORT
SUCTION PORT
CONDENSER
HX
EVAPORATOR
EX VALVE
