INSTALLATION AND SERVICE
Oil charge and oil level
Suction gas superheat
The oil charge must be checked before
commissioning (1/4 to 3/4 of the oil si-
ght glass). Check the oil level again after
a minimum of 2 hours operation at no-
minal conditions. In most installations
the initial compressor oil charge will
be sufficient. In installations with line
runs exceeding 20 m or with many oil
traps or an oil separator, additional oil
may be required. Normally the quantity
of oil added should be no more than
2% of the total refrigerant charge (this
percentage does not take into account
oil contained in accessories such as oil
separators or oil traps). If oil has alrea-
dy been added, and the oil level in the
compressor keeps decreasing, the oil
return in the installation is insufficient.
Refer also to section "Piping design".
In installations where slow oil return is
likely such as in multiple evaporator or
multiple condenser installations, an oil
separator is recommended. Refer to the
table on page 17 to select the correct
oil.
Optimum suction gas superheat is
15
o
F. A lower superheat will contribute
to better system performance (higher
mass flow and more efficient use of eva-
porator surface). Low superheat values
however increase the risk of unwanted
liquid floodback to the compressor.
For very low superheat values an elec-
tronically controlled expansion valve is
recommended.
Maximum allowable superheat is about
54
o
F. Higher values can be accepted but
in these cases, tests have to be perfor-
med to check that the maximum dis-
charge temperature of 265
o
F will not
be exceeded. Note that high superheat
values decrease the compressor ap-
plication envelope and system perfor-
mance.
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