Carrier NDK33BN0KB, Operation & Service Manual

Page: 237 / 270

62-61753-21 8-25
8.6.10 Economizer Thermostatic Expansion Valve
The thermostatic expansion valve (TXV) is an auto-
matic device which maintains constant superheat of
the refrigerant gas leaving the economizer heat ex-
changer regardless of suction pressure. The valve
functions are: (a) automatic control of refrigerant flow
to match the economizer heat exchanger load and (b)
prevention of liquid refrigerant return to the compres-
sor. Unless the valve is defective, it seldom requires
any maintenance.
Replacing Expansion Valve
a. To replace a valve, remove the refrigerant charge.
(Refer to section 8.5.1).
b. Ensure the unit is disconnected from the power
source and that SROS is in OFF position. Remove
insulation from the valve bulb and remove bulb.
c. Remove insulation from the expansion valve power
head.
d. Heat inlet and outlet connections to valve body and
remove valve. Clean all tube stubs so new valve fits
on easily.
e. Install new valve.
f. The thermal bulb is located on the bottom of the
equalizer line. This area must be clean to ensure
positive bulb contact. Firmly tighten the straps
around the thermal bulb and equalizer line.
g. Leak check and evacuate the unit in accordance
with sections 8.5.2 & 8.5.3. Reinstall insulation on
bulb and power head and charge in accordance with
section 8.5.4.
h. Check superheat in accordance with the following
section.
Measuring Superheat
a. Loosen the insulation at one end of the expansion
valve bulb.
b. Loosen one TXV bulb clamp and make sure area
under clamp (above TXV bulb) is clean.
c. Place a thermocouple parallel to the TXV bulb and
then secure loosened clamp making sure both bulbs
are firmly secured to suction line as shown in Figure
8-18. Use insulation to completely cover both.
d. Connect an accurate gauge to the 1/4" port on the
economizer line.
e. Run unit until stabilized. Set controller to a frozen
setpoint. The economizer solenoid valve must be
energized (check FET 22 is illuminated) in order to
measure economizer TXV superheat. Several oper-
ating conditions may prevent energizing of the
economizer solenoid valve. These conditions in-
clude high discharge pressure, low suction pres-
sure, high compression ratio and high current draw.
The most common condition that limits energizing of
the valve is maximum allowable current. If current is
above the limit, allow the unit to pull-down to a lower
temperature to reduce power.
1
5
4
3
2
1. Equalizer Line
2. TXV Bulb Clamp
3. Nut and Bolt (Clamp) 4. Thermocouple
5. TXV Bulb
Figure 8-19. Thermostatic Expansion Valve Bulb
and Thermocouple
f. Take five or six readings of pressure and tempera-
ture 10 seconds apart and average readings.
g. From the temperature/pressure chart, determine the
saturation temperature corresponding to the aver-
age economizer heat exchanger outlet pressure.
h. Subtract the average saturation temperature from
the average bulb temperature. The difference is the
superheat of the economizer outlet gas. Refer to
section 2.10 for superheat setting. The valve is not
adjustable if superheat is not correct, replace valve.
8.6.11 High Pressure Switch
Replacing High Pressure Switch
a. Remove the refrigerant charge. (Refer to section
8.5.1).
b. Ensure the unit is disconnected from the power
source and that SROS is in OFF position.
c. Disconnect wiring from switch, and remove switch.
d. Install switch after verifying switch settings. (Refer
to following "Checking High Pressure Switch"
section)
e. Leak check, evacuate the unit and charge in ac-
cordance with sections 8.5.2, 8.5.3 & 8.5.4.