AAON CB-060 - Manual (Page 16)

16
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,
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 to
discourage fluid backup, and 300 fpm from
receiver tank to the evaporator to minimize
valve induced liquid hammer.
Liquid Line Accessories
Liquid line shut off valves and filter driers
are factory provided. Filter driers must be
field installed. 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 foot per 120 feet 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.
It is important to consider part load
operation when sizing suction lines. At
minimum capacity, refrigerant velocity may
not be adequate to return oil up the vertical
riser. Decreasing the diameter of the vertical
riser will increase the velocity, but also the
frictional loss.
Circuits with variable capacity scroll
compressors require suction riser
traps every 10 feet.
CAUTION