Thermostatic-Expansion Valve (Bi-Flow Txv); Coil Removal; Liquid-Line Strainer (Heat Pumps Only); Accumulator - Carrier R-22 Application Manual And Service Manual

Table 21—TXV Superheat Setting at Outlet of Evaporator Coil
INSTALLATION
Field Accessory
Field Accessory
Field Accessory/Factory-Installed
Factory-Installed
Factory-Shipped/Field-Installed
Step 8—Thermostatic-Expansion Valve (Bi-Flow TXV)
The standard TXV is a metering device that is used in condensing
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:
1. Superheat temperature sensed by cap-tube sensing bulb on
suction tube at outlet of evaporator coil. As long as this bulb
and cap tube contains some liquid refrigerant, this temperature
is converted into suction pressure pushing downward on the
diaphragm, which tends to open the valve via the pushrods.
2. The suction pressure at the outlet of the evaporator coil is
transferred via the external equalizer tube to the underside of
the diaphragm.
3. The needle valve on the pin carrier is spring-loaded, which
also exerts pressure on the underside of the diaphragm via the
pushrods, which tends to close the valve. Therefore, bulb
pressure equals evaporator pressure (at outlet of coil) plus
spring pressure. If the load increases, the temperature in-
creases at the bulb, which increases the pressure on the topside
of the diaphragm, which pushes the pin carrier away from the
seat, opening the valve and increasing the flow of refrigerant.
The increased refrigerant flow causes increased leaving
evaporator pressure, which is transferred via the equalizer tube
to the underside of the diaphragm. This tends to cause the
pin-carrier spring pressure to close the valve. The refrigerant
flow is effectively stabilized to the load demand with negli-
gible change in superheat. The bi-flow TXV is used on
split-system heat pumps. In the cooling mode, the TXV
operates the same as the standard TXV previously explained.
(See Fig. 51.)
However, when the system is switched to the heating mode of
operation, the refrigerant flow is reversed. The bi-flow TXV
has an additional internal-check valve and external tubing.
(See Fig. 52.) These additions allow the refrigerant to bypass
the TXV when refrigerant flow is reversed with only a 1- to
2-psig pressure drop through the device. When the heat pump
switches to the defrost mode, the refrigerant flows through a
completely open (unthrottled) TXV, and the bulb senses the
residual heat of the outlet tube of the coil that had been
operating in the heating mode (about 85°F and 155 psig). This
temporary, unthrottled valve decreases the indoor-pressure
drop, which in turn increases the refrigerant-flow rate, de-
creases overall defrost time, and enhances defrost efficiency.
Step 9—Coil Removal
Coils on this family of units are easy to remove, if required for
compressor removal or replacement coil.
Wear safety glasses and gloves when handling refrigerants.
To remove or replace coil:
1. Shut off all power to unit.
TXV TYPE PRODUCT USAGE
RPB/HSO Air Conditioner Indoor Unit
RPB/HSO Heat Pump Indoor Unit
HSO Indoor Fan Coil Unit
HSO 2-Speed Heat Pump Outdoor Unit
HSO 2-Speed Indoor Unit
2. Remove and recover refrigerant from system through service
valves.
3. Remove top cover. (See Remove Top Cover section.)
4. Remove screws in base pan to coil grille.
5. Remove coil grille from unit.
6. Remove screws on corner-post (WeatherMaker™) service-
valve panel (Cube unit) holding coil-tube sheet.
Cut tubes to reduce the possibility of fire and personal injury.
7. Use midget-tubing cutter to cut liquid and vapor lines at both
sides of coil. Cut in convenient location for easy reassembly
with copper-slip couplings.
8. Lift coil vertically from basepan. Place aside carefully.
9. Reverse procedure to reinstall coil.
Step 10—Liquid-Line Strainer (Heat Pumps Only)
The liquid-line strainer is upstream of the heating piston. The
strainer catches debris in the liquid line during heating mode. If it
becomes plugged, system operation and pressure become abnor-
mal, and the compressor may become hot and cycle off on the
overloads or pressure relief.
If the strainer must be replaced, shut off all power to the unit. See
Fig. 47 for strainer location.
Step 11—Accumulator
The accumulator is a device always found in heat pumps and found
in some condensing-unit models. Under some light-load condi-
tions on indoor coils and on outdoor coil with heat pump in heating
mode, some 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 replac-
ing. 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 refriger-
ant. 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. Bleed hole is so
tiny that cleaning efforts are usually not successful. The accumu-
lator has a fusible element located in the bottom-end bell. (See Fig.
53.) This fusible element melts at 430°F and vents 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. Remove and reclaim all refrigerant from system.
43
SUPERHEAT SETTING
10°
6°
6°
4°
4°