4-18
valve with a wet cloth while brazing. The coil need not be
removed.
4. Install new suction service valve gasket and
install bolts in suction service valve. Torque to a value of
2.77 to 4.15 mkg (20 to 30 ft/lb).
5. Solder all connections and leak check same.
6. Dehydrate and evacuate the unit per section 4.3
and then add refrigerant charge per section 4.4.2.
4.26 THERMOSTATIC EXPANSION VALVE
The thermal expansion valve is an automatic device
which maintains constant superheat of the refrigerant gas
leaving the evaporator regardless of suction pressure.
The valve functions are: (a) automatic response of
refrigerant flow to match the evaporator load and (b)
prevention of liquid refrigerant entering the compressor.
Unless the valve is defective, it seldom requires any
maintenance.
6
2
5
3
7
1
4
1. Power Assembly
2. Body Flange Gaskets
3. Seat Gasket
4.
Bulb
5. Cage Assembly
6. Body Flange
7. Body Flange Screws
Figure 4-24. Thermostatic Expansion Valve
a. Removing Expansion Valve
1. Store the refrigerant in an evacuated container
by attaching a hose to the liquid line valve.
2. Remove insulation (Presstite) from expansion
valve bulb and power assembly and then remove thermal
bulb from the suction line.
3. Loosen flare nut and disconnect equalizing line
from expansion valve.
4. Remove capscrews and lift off power assembly
and remove cage assembly. Check for foreign material in
valve body.
5. The thermal bulb is located below the center of
the suction line (4 o’clock position). This area must be
clean to ensure positive bulb contact. Strap thermal bulb
to suction line and insulate both with “Presstite.”
b. Installing Expansion Valve
1. Replace all gaskets, make sure to lightly coat
with oil. Insert cage and power assembly and bolts.
Tighten bolts equally. Fasten equalizer flare nut to
expansion valve.
2. Leak check the unit per section 4.2. Evacuate
and dehydrate unit per section 4.3 and add refrigerant
charge per section 4.4.2.
3. Clean suction line with sandpaper before
installing bulb to ensure proper heat transfer. Strap
thermal bulb to suction line, making sure bulb is firmly
against suction line. The bulb is located in the 4 o’clock
position on the suction line.
4. Check superheat. (See Table 4-6) Superheat
setting is 4.4 to 5.5
_
C (8 to 10
_
F) at 0
_
C (32
_
F) container
temperature.
c. Checking Superheat
NOTE
It is not recommended adjusting internal
adjustable valves unless absolutely necessary.
If a replacement valve has the wrong superheat
setting, the valve may be adjusted if you do not have
another replacement valve on hand. Due to the time
involved in adjusting the superheat, replace the valve
rather than adjusting it.
To Measure Superheat
:
1. Open access panel to expose the expansion valve
and 1/4” port (see Figure 1-1). The 1/4” port is equipped
with a Schrader fitting.
2. Attach a temperature tester sensor near the
expansion valve bulb and insulate. Make sure the suction
line is clean and firm contact is made with tester.
3. Connect an accurate gauge to the 1/4” port.
4. Run unit until unit has stabilized. Set controller
5.5
_
C (10
_
F) below container temperature.
NOTE
Suction pressure must be 0.5 kg/cm
@
(6 psig) below
valve M.O.P. (maximum operating pressure).
Example: if valve rated at 55 MOP, suction
pressure must be below this MOP. Recommended
pressure is below 3.44 kg/cm
@
(49 psig).
5. From
the
temperature/pressure
chart
(Table 4-6), determine the saturation temperature
corresponding to the evaporator outlet pressure.
6. Note the temperature of the suction gas at the
expansion valve bulb.
7. Subtract the saturation temperature determined
in Step 6 from the average temperature measured in Step
5. The difference is the superheat of the suction gas.
d. Adjusting Superheat
1. Remove the refrigerant and then remove the two
bolts holding the valve body together.
2. Remove the cage assembly and rotate the
adjusting nut to increase or decrease the superheat.
Turning the nut to compress the valve spring will increase
the superheat and decrease refrigerant flow through the
valve. Decompressing the spring will decrease the
superheat and increase the refrigerant flow through the
Downloaded from