Txv (Thermostatic Expansion Valve); Txv Bulb Location And Installation - Carrier Aero 39MN Series Installation, Start-Up And Service Instructions Manual

TXV (Thermostatic Expansion Valve)

TXV LOCATION — Thermostatic expansion valves may be
mounted in any position, but they should be installed as close
to the evaporator as possible. If a refrigerant distributor is used
with the TXV, best performance is obtained if the distributor is
mounted directly to the valve outlet. If the distributor cannot be
mounted directly to the valve outlet, the distance between the
valve outlet and distributor should not exceed 24 in. or refriger-
ant distribution problems may occur. Also, the tube connecting
the valve outlet and distributor can be sized smaller to maintain
refrigerant velocity and better distribution. Elbows located be-
tween the expansion valve and distributor will hinder proper
distribution and are, therefore, not recommended.
IMPORTANT: There are no liquid line penetrations
through the casings from the factory. Best distribution is
usually obtained if the expansion valve feeds vertically
up or down into the distributor. Assemble the refrigerant
piping and specialties inside the cabinet to determine the
hole location prior to using a hole saw to penetrate the
cabinet.
While not always convenient or possible, valve Types BI, F,
FB, and O are easier to service if mounted in a vertical and up-
right position. If mounted in a horizontal position, the internal
parts must be carefully reassembled to prevent damage. Some
consideration should also be taken in mounting larger sized ex-
pansion valves. They must be adequately supported since sys-
tem vibration and the weight of the valve can cause valve con-
nections to fracture.
SOLDER TECHNIQUES — It is not necessary to disassem-
ble solder type valves when soldering to the connecting lines.
Any of the commonly used types of solders, e.g., 95-5, Sil-Fos,
Easy-Flo, Phos-Copper, Stay Brite 8 or equivalents, may be
used for copper-to-copper connections. When soldering a brass
refrigerant distributor to the valve, appropriate solders for these
connections, such as 95-5, Easy-Flo, Stay Brite 8 or equiva-
lents must be used. Regardless of the solder used, it is import-
ant to direct the flame away from the valve body and avoid ex-
cessive heat on the diaphragm. See Fig. 99 for details. As an
extra precaution, a wet cloth should be wrapped around the
body and element during the soldering operation.
NOTE: This precaution will prevent overheating the valve
body which could damage the superheat spring and result in
flood-back problems. In addition, the Type O, EBF/SBF,
and EBS valves contain synthetic parts which can be dam-
aged due to overheating, resulting in poor valve perfor-
mance.
a39-3970
Fig. 99 — Solder Technique
TXV BULB LOCATION AND INSTALLATION — The
location and installation of the bulb is extremely important to
the proper performance of the system. Therefore, care should
be taken with its final location. Laboratory testing has shown
that better sensing can be achieved with the sensing bulb
mounted on a vertical riser. However, with proper piping prac-
tices, horizontal mounting will also provide adequate sensing.
When the horizontal mounting location is used, the suction line
must drop below the coil suction outlet, as shown in Fig. 100.
This line is pitched slightly downward, and when a vertical ris-
er follows, a short trap is placed immediately ahead of the ver-
tical line. See Fig. 100. The trap will collect any liquid refriger-
ant or oil passing through the suction line and prevent it from
influencing the bulb temperature.
Steps for using a vertical riser as shown in Fig. 100 are:
1. A minimum of two 90-degree elbows must be installed
upstream of the expansion bulb location. A distance of
8 pipe diameters between the elbows is desirable.
2. The thermal bulb must be strapped to a vertical riser
following the second elbow of item 1. This riser must
be at least 15 pipe diameters high and the bulb should
be fastened 10 pipe diameters above the preceding el-
bow.
3. Size the suction line from near the evaporator through
the 15 pipe diameter long riser for high velocity. It is
recommended that the suction pipe sizing charts in the
Refrigerant Reference Manual be used, entering at cor-
rected design tons (each evaporator split) and 80 feet
equivalent length (for 2°F loss). The chart point usual-
ly falls between 2 sizes; choose the smaller size pipe.
Since the selection of suction piping recommended for
high velocity is approximately 20 feet equivalent
length, this sizing method will result in about 0.5°F
friction loss for that section. If the commonly selected
2°F loss is allowed for the entire suction line, this
leaves 1.5°F for the balance of the suction line and it
should be sized on that basis. Usually, this high veloci-
ty sizing will be adequate for oil return up the 15 diam-
eter long riser, though it should be checked.
The purpose of these recommendations is to achieve good
mixing of the refrigerant leaving the evaporator suction header
for proper sensing by the TXV bulb. Tests have confirmed that
this, in addition to the vertical riser bulb location, results in sig-
nificantly better superheat control over a broader load range.
Figures provided in the Carrier Refrigerant Reference Man-
ual illustrate these recommendations for various coil splits and
compressor configurations.
NOTE: For satisfactory expansion valve control, good ther-
mal contact between the bulb and suction line is essential.
The bulb should be securely fastened with the two bulb
straps, supplied with each expansion valve, to a clean
straight section of the suction line.
NOTE: On multiple evaporator installations, the piping
should be arranged so that the flow from any valve cannot
affect the bulb of another. Approved piping practices,
including the proper use of traps, ensures individual control
for each valve without the influence of refrigerant and oil
flow from other evaporators.
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