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Liquid refrigerant traveling upwards in a riser
loses head pressure. If the evaporator is
below the condenser, with the liquid line
flowing down, the gravitational force will
increase the pressure of the liquid refrigerant.
This will allow the refrigerant to withstand
greater frictional losses without the
occurrence of flashing prior to the TXV.
A moisture-indicating sight glass may be
field installed in the liquid line to indicate the
occurrence of premature flashing or moisture
in the line. The sight glass must 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 must 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
In cooling only systems, 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.
In heat pump systems, when the liquid line is
routed through regions where temperature
losses are expected (all lines exposed to
outside air conditions), insulate with a
minimum 1 inch thick Armaflex insulation,
as this will prevent capacity loss during
heating mode of operation.
Liquid Line Guidelines
In order to ensure liquid at the TXV, the sum
of frictional losses and pressure loss due to
vertical rise 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 -17.2 to -16.7°C
(1-2°F) change in saturation temperature.
The sum of frictional losses (including valve
losses, filter drier losses, other accessories,
and line losses) and pressure loss due to
vertical rise must not exceed -13.3°C (8°F) if
the available sub-cooling is -12.2°C (10°F).
If the velocity of refrigerant in the liquid line
is too great, it could cause excessive noise or
piping erosion. The maximum velocities for
liquid lines are 100 fpm from the condenser
to a receiver to discourage fluid backup, and
500 fpm from receiver tank to the evaporator
(300 fpm if the line includes an electric valve
to minimize valve induced liquid hammer).
Liquid Line Accessories
Liquid line shut off valves and filter driers are
factory provided. 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.
A liquid line receiver is factory installed on
units with modulating hot gas reheat, units
with low ambient control flooded condenser,
and units with heat pump.
Suction Line
The suction line is more critical than the
liquid line from a design and construction
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