Carrier 38TN Service Manual page 13

The service valve is a combination front seating/back seating
valve, which has a metal-to-metal seat in both the open and closed
positions. When it is fully back seated, the service port is not
pressurized. To pressurize the service port, this valve must be
moved off the back seating position. This valve does not contain a
Schrader fitting. Both types of service valves are designed for
sweat connection to the field tubing.
The service valves in the outdoor unit come from the factory front
seated. This means that the refrigerant charge is isolated from the
line set connection ports. The interconnecting tubing (line set) can
be brazed to the service valves using either silver bearing or
non-silver bearing brazing material. Consult local codes. Before
brazing the line set to the valves, the belled ends of the sweat
connections on the service valves must be cleaned so that no brass
plating remains on either the inside or outside of the bell joint. To
prevent damage to the valve and/or cap "O" ring, use a wet cloth
or other acceptable heat-sinking material on the valve before
brazing. To prevent damage to the unit, use a metal barrier
between brazing area and unit.
After the brazing operation and the refrigerant tubing and evapo-
rator coil have been evacuated, the valve stem can be turned
counterclockwise until it opens or back seats, which releases
refrigerant into tubing and evaporator coil. The system can now be
operated.
Back seating service valves must be back seated (turned counter-
clockwise until seated) before the service port caps can be
removed and hoses of gage manifold connected. In this position,
refrigerant has access from and through outdoor and indoor unit.
The service valve stem cap is tightened to 20 ± ft/lb torque and the
service port caps to 9 ± 2 ft/lb torque. The seating surface of the
valve stem has a knife set edge against which the caps are
tightened to attain a metal-to-metal seal.
The service valve cannot be field repaired, therefore, only a
complete valve or valve stem and service port caps are available
for replacement.
If the service valve is to be replaced, a metal barrier must be
inserted between the valve and the unit to prevent damaging the
unit exterior from the heat of the brazing operations.
Wear safety glasses and gloves when handling refrigerants.
PUMPDOWN PROCEDURE — Service valves provide a conve-
nient shut-off valve useful for certain refrigeration system repairs.
System may be pumped down to make repairs on low side without
losing complete refrigerant charge.
1. Attach pressure gage to suction service valve gage port.
2. Front seat liquid line valve.
3. Start unit in cooling mode. Run until suction pressure reaches
5 psig (35kPa). Do not allow compressor to pump to a
vacuum.
4. Shut off unit. Front seat suction valve.
NOTE: All outdoor unit coils will hold only factory-supplied
amount of refrigerant. Excess refrigerant, such as in long-line
applications, may cause unit to relieve pressure through internal
pressure relief valve (indicated by sudden rise of suction pressure)
before suction pressure reaches 5 psig (35 Kpa). If this occurs, shut
off unit immediately, front seat suction valve, and recover remain-
ing refrigerant.
Step 5—AccuRater (Bypass Type) Components
AccuRater piston has a refrigerant metering hole through it. The
retainer forms a stop for piston in refrigerant bypass mode, and a
sealing surface for liquid line flare connection. To check, clean or
replace piston follow these steps. (See Fig. 18.)
1. Shut off power to unit.
2. Pump unit down using pumpdown procedure described in this
service manual.
3. Loosen nut and remove liquid line flare connection from
AccuRater.
4. Pull retainer out of body, being careful not to scratch flare
sealing surface. If retainer does not pull out easily, carefully
use locking pliers to remove it.
5. Slide piston out by inserting a small soft wire with small kinks
through metering hole. Do not damage metering hole, sealing
surface around piston cones, or fluted portion of piston.
6. Clean piston refrigerant metering hole.
7. Install a new retainer O-ring or retainer assembly before
reassembling bypass-type AccuRater.
PISTON BODY
INTERNAL STRAINER
Fig. 18—AccuRater Components
Step 6—Thermostatic Expansion Valve (TXV)
The thermostatic expansion valve used in the 38TN condensing
unit is specifically designed to operate with refrigerant R134A.
HARD SHUT-OFF (HSO) — Has no bleed port and allows no
bleed through after system shutdown. No pressure equalization
occurs. Because of unequalized system pressures, a start capacitor
and relay must be installed on single-phase reciprocating compres-
sors to start the compressor.
See Table 1 for thermostatic expansion valve superheat settings.
These settings are factory set at 10° and are not field adjustable.
The setting is for Carrier-approved accessories only.
The standard TXV is a metering device that is used in condensing
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: (See Fig. 19.)
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
exerts pressure on the underside of the diaphragm via the
13
PISTON
O-RING
PISTON RETAINER
FLARE NUT
A94012