Replacing Expansion Valve; Table 5-3 Optimum Operating Superheat Temperature Refrigerant Pressure And Type - Carrier Supra 950MT Operation & Service Manual

Table 5–3 Optimum Operating Superheat Temperature Refrigerant Pressure and Type
Operating Superheat (°F)
34°
35°
36°
37°
38°
39°
40°
41°
42°
43°
44°
45°
46°
47°
8. If adjustment is required, there is an external adjustment screw on the side of the TXV.
9. One complete turn of 360 degrees will equate to approximately 7°F of superheat. To decrease superheat, turn
the screw counter-clockwise; to increase superheat, turn it clockwise. After adjustments are made, the unit
should be allowed to operate for approximately 10 minutes for system stabilization before rechecking superheat.
The optimum superheat setting is 10 to 34°F (-12.2 to 1.1°C). This is not always possible depending
on the application and desired temperatures in each compartment. Since the type of evaporator, com-
partment size and location varies greatly with each application, it is recommended that the superheat
be checked and adjusted, if necessary, at final unit pre-delivery inspection.
Superheat may be set higher in some conditions if the unit is unable to maintain the required temperature. Increas-
ing superheat in the remote evaporator reduces the amount of refrigerant flow through the TXV, which conse-
quently reduces the capacity of that evaporator.
In some applications, the superheat may be set slightly lower; this will increase capacity of the evaporator by allow-
ing more refrigerant to flow through the TXV. This is needed in some cases where higher suction line pressure is
required from the evaporator. The superheat should never be set low enough to allow frost on the compressor. This
indicates liquid refrigerant returning to the compressor.
5.16.2

Replacing Expansion Valve

a. Check superheat and adjust valve (if adjustable) in accordance with the preceding steps. If valve requires
replacement, pump down the unit. (Refer to
b. Slowly loosen the nut at the base of the valve to relieve any remaining pressure. Pull the line away from the
valve sufficient to remove the orifice and strainer. Check condition of orifice and strainer, clean as neces-
sary. If no foreign material is found, then proceed with replacing the valve. Retain orifice for reassembly.
c. Remove insulation from expansion valve bulb and then remove bulb from suction line.
d. Using inert gas brazing procedures (refer to Technical Procedure 98-50553-00), unbraze the equalizer line,
and if required, distributor.
e. Remove the strainer from the replacement valve and wrap in damp rags to prepare for brazing. Braze
replacement valve in place. Install nut(s), orifice and strainer.
f. Strap thermal bulb to suction line and insulate both. It is recommended that the thermocouple required to
check superheat be reinstalled at this time.
g. Leak check and evacuate low side by connecting at the suction and discharge service valve. Refer to
5.4.1 and Section 5.5
h. Re-check superheat.
62-11863
R-22 (Psig)
NOTE
Section
for general procedure.
60.2
61.5
62.9
64.3
65.7
67.1
68.6
70.0
71.5
73.0
74.5
76.1
77.6
79.2
5.4, a)
5-20
R404A (Psig)
76.5
78.1
79.7
81.3
83.0
84.7
86.4
88.1
89.9
91.6
93.4
95.3
97.1
99.0
Section