AAON F-060 - Manual (Page 30)

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glass should not be used to determine if the
system is properly charged. Use
temperature and pressure measurements
to determine liquid sub-cooling, not the
sight glass.
Liquid Line Routing
Care should be taken with vertical risers.
When the system is shut down, gravity will
pull liquid down the vertical column, and
back to the condenser when it is below the
evaporator. This could potentially result in
compressor flooding. A check valve can be
installed in the liquid line where the liquid
column rises above the condenser to prevent
this. The liquid line is typically pitched
along with the suction line, or hot gas line,
in the direction of the compressor to
minimize the complexity of the
configuration.
Liquid Line Insulation
When the liquid line is routed through
regions where temperature losses are
expected, no insulation is required, as this
may provide additional sub-cooling to the
refrigerant. When routing the liquid line
through high temperature areas, insulation of
the line is appropriate to avoid loss of sub-
cooling through heat gain.
Liquid Line Guidelines
In order to ensure liquid at the TXV,
frictional losses must not exceed available
sub-cooling. A commonly used guideline to
consider is a system design with pressure
losses due to friction through the line not to
exceed a corresponding 1-2°F change in
saturation temperature.
If the velocity of refrigerant in the liquid line
is too great, it could cause excessive noise or
piping erosion. The recommended
maximum velocities for liquid lines are 100
fpm from the condenser to a receiver tank
when used, to discourage fluid backup, and 300 fpm from receiver tank to the
evaporator to minimize valve induced liquid
hammer.
Liquid Line Accessories
The total length equivalent of pressure
losses through valves, elbows and fittings
must be considered when adding additional
components in the field. It is a good practice
to utilize the fewest elbows that will allow
the mating units to be successfully joined.
Suction Line Sizing
The suction line is more critical than the
liquid line from a design and construction
standpoint. More care must be taken to
ensure that adequate velocity is achieved to
return oil to the compressor at minimum
loading conditions. However, reducing the
piping diameter to increase the velocity at
minimal load can result in excessive
pressure losses, capacity reduction, and
noise at full load.
Suction Line Routing
Pitch the suction line in the direction of flow
(about 1 ft. per 100 ft of length) to maintain
oil flow towards the compressor, and keep it
from flooding back into the evaporator.
Crankcase heaters are provided to keep any
condensed refrigerant that collects in the
compressor from causing damage or wear.
Make sure to provide support to maintain
suction line positioning, and insulate
completely between the evaporator and
condensing unit.
Circuits with variable capacity scroll
compressors require suction riser
traps every 10 feet.
CAUTION