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Liquid line sizing:
When sizing the liquid line, it is important to
minimize the refrigerant charge to reduce
installation costs and improve system reliability.
This can be achieved by minimizing the liquid line
diameter. However, reducing the pipe diameter
will increase the velocity of the liquid refrigerant
which increases the frictional pressure drop in the
liquid line, and causes other undesirable effects
such as noise.
Maintaining the pressure in the liquid line is
critical
to
ensuring
sufficient
saturation
temperature, avoiding flashing upstream of the
TXV, and maintaining system efficiency.
Pressure losses through the liquid line due to
frictional contact, installed accessories, and
vertical risers are inevitable. Maintaining
adequate sub-cooling at the condenser to
overcome these losses is the only method to
ensure that liquid refrigerant reaches the TXV.
Liquid risers decrease head pressure. If the
evaporator section is below the condenser, and the
liquid line does not include risers, 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 factory
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:
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.
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 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 accessories including sight glasses and
filter driers are available and factory 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:
For cooling only systems, pitch the suction line in
the direction of flow (about 1 inch per 20 feet of
