Carrier 69NT40-541-300, Operation And Service

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6 − 13 T − 318
6.13 FILTER − DRIER
On units equipped with a water-cooled condenser, if the
sight glass appears to be flashing or bubbles are
constantly moving through the sight glass when the
suction modulation valve is fully open, the unit may have
a low refrigerant charge or the filter-drier could be
partially plugged.
a. To Check Filter-Drier:
1. Test for a restricted or plugged filter-drier by feeling
the liquid line inlet and outlet connections of the drier
cartridge. If the outlet side feels cooler than the inlet
side, then the filter-drier should be changed.
2. Check the moisture-liquid indicator, if the indicator
shows a high level of moisture, the filter-drier should
be replaced.
b. To Replace Filter-Drier:
1. Pump down the unit (refer to paragraph 6.4) and
replace filter-drier.
2. Evacuate the low side in accordance with paragraph
6.6.
3. After unit is in operation, inspect for moisture in sys-
tem and check charge.
6.14 THERMOSTATIC EXPANSION VALVE
The thermostatic expansion valve (TXV) (see
Figure 2-2) is an automatic device that maintains
constant superheat of the refrigerant gas leaving the
evaporator, regardless of suction pressure.
TXV functions are:
1. Automatic control of the refrigerant flow to match the
evaporator load.
2. Prevention of liquid refrigerant entering the com-
pressor.
Unless the valve is defective, it seldom requires
maintenance other than periodic inspection to ensure
that the thermal bulb is tightly secured and properly
insulated. See Figure 6-18.
NOTE
TXV Bulb Clamp is soldered to the suction line.
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2
3
1
4
5
1. Suction Line
2. Thumbscrew
3. TXV Bulb Clamp
4. TXV Bulb
5 Foam Insulation
Figure 6-18 Thermostatic Expansion Valve Bulb
6.14.1 Checking Superheat
NOTE
Proper superheat measurement should be
completed at -18
°
C (0
°
F) container box tem-
perature where possible.
a. Open the upper right (EFM#1) access panel (see
Figure 2-1) to expose the expansion valve.
b. Attach a temperature sensor near the expansion
valve bulb and insulate. Make sure the suction line is
clean and that firm contact is made with the sensor.
c. Connect an accurate gauge to the service port direct-
ly upstream of the suction modulating valve.
d. Set the temperature set point to -18 ° C (0 ° F) and run
unit until conditions stabilize.
e. The readings may cycle from a high to a low reading.
Take readings of temperature and pressure every
three to five minutes for a total of 5 or 6 readings.
f. From the temperature/pressure chart (Table 6-9), de-
termine the saturation temperature corresponding to
the evaporator outlet test pressures at the suction
modulation valve.
g. Subtract the saturation temperatures determined in
step f from the temperatures measured in step e. The
difference is the superheat of the suction gas. Deter-
mine the average superheat. It should be 4.5 to 6.7 ° C
(8 to 12
°
F).
6.14.2 TXV Replacement
NOTES
1. The TXV is a hermetic valve, it does not have
adjustable superheat (See Figure 6-19).
2. All connections on the hermetic TXV are
bi-metallic, copper on the inside and
stainless on the outside.
3. All joints on the hermetic TXV (inlet, outlet
and equalizer lines) are brazed.
4. Bi-metallic connections heat up very quickly.
1
6
5
4
3
2
1. Hermetic Thermostatic Expansion Valve
2. Non-adjustable Superheat Stem
3. Equalizer Connection
4. Inlet Connection
5. Outlet Connection
6. Hermetic Expansion Valve Bulb
Figure 6-19 Thermostatic Expansion Valve