6-9
T -309
2. C heck the moisture-liquid indicator if the indicator
showsahighlevelofmoisture,thefilter-driershould
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. Afterunitisinoperation,inspectformoistureinsys-
tem and check charge.
6.14 EXPANSION VALVES
Two expansion valves are used, the evaporator
expansion valve (item 9, Figure 2-2), and the
economizer expansion valve (item 26, Figure 2-4) .
The
expansionvalvesareautomaticdeviceswhichmaintain
constant superheat of the refrigerant gas leaving at the
pointofbulbattachmentregardlessof suctionpressure.
The valve functions are:
1. Automatic control of the refrigerant flow to match
the load.
2. Prevention of liquid refrigerant entering the com-
pressor.
Unless the valve is defective, it seldom requires
maintenance other than periodic i nspection to ensure
thatthethermalbulbistightlysecuredtothesuctionline
and wrapped with insulating compound. (See
Figure 6-10.)
1
2
3
4
1. Suction Line
2. TXV Bulb Clamp
3. Nut and Bolt
4. TXV Bulb
Figure 6-10 Thermostatic Expansion V alve Bulb
6.14.1 Checking Superheat.
NOTE
Propersuperheatmeasurementfortheevapora-
tor expansion valve should be completed at
-- 18 _C(0_F) container box temperature where
possible. If the economizer valve is suspect, it
should be replaced.
a. Open the heater access panel (see Figure 2-1) to ex-
pose t he evaporator expansion valve.
b. Attachatemperaturesensorneartheexpansionvalve
bulb and insulate. Makesurethe suctionline isclean
and that firm contact is made with the sensor.
c. Connectanaccurategaugetotheserviceportdirectly
upstream of the suction modulating valve.
d. Sett hetemperaturesetpointto--18_C(0_F),andrun
unit until conditions stabilize.
e. Thereadingsmaycyclefromahigh toalowreading.
Take readings of temperature and pressure every
three to five minutes for a total of 5or 6 readings
f. From the temperature/pressure chart (Table 6-7), de-
termine the saturation t emperature corresponding to
the evaporator outlet test pressures at the suction
modulation valve.
g. Subtract the saturation temperatures determined in
stepf.fromthetemperaturesmeasuredinstep e..The
difference is the superheat of the suction gas. Deter-
minethe averagesuperheatIt should be 4.5 to 6.7°C
(8 to 12 °F)
6.14.2 V alve Replacement
a. Removing the Expansion Valve
NOTES
1. TheTXV’sarehermeticvalvesanddonot
have adjustable superheat.
2. Allconnections on the evaporator TXVare
bi---metallic, copper on the inside and
stainless on the outside. When brazing,
bi---metallic connections heat up very
quickly.
1
6
5
4
3
2
1. Evaporator Expansion Valve
2. Non-adjustable Superheat Stem
3. Equalizer Connection
4. Inlet Connection
5. Outlet Connection
6. Expansion Valve Bulb
Figure 6-11 Evaporator Expansion Valve
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