Discharge Line Traps
Include traps every 3.7 meters (12 feet) in
vertical discharge riser sections. Include a
trap at the bottom of the vertical run.
Discharge Line Insulation
Although a typical discharge line does not
need to be insulated, the suction line does.
Since the same line is used for both, the line
must be insulated as described in the Suction
Line Insulation section.
Discharge Line Guidelines
For proper performance, keep discharge line
velocities less than a 17.8 m/s (3,500 fpm).
The minimum velocity required to return oil
is dependent on the pipe diameter, however,
a general guideline of 4.6 m/s (900 fpm)
minimum may be applied.
When discharge flow is up, variable capacity
compressors require a minimum velocity of
4.6 m/s (900 fpm) at full load.
Tandem compressors must be considered for
full load operation (both compressors
operating) and at partial load (only one
compressor operating). When discharge flow
is up, and the tandem has a variable capacity
compressor, the velocity for only one
compressor in operation must be greater than
4.6 m/s (900 fpm). For on/off compressors,
the velocity must be greater than the
minimum velocity required to return oil.
Heat pump vapor lines must be checked for
suction flow (cooling mode operation) and
discharge flow (heating mode operation).
The same line must be used for both modes
of operation.
In a fashion similar to the suction line, a
common 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.
For split system piping with long horizontal
runs and short vertical risers, a smaller pipe
size can be used to provide sufficient velocity
to return oil in vertical risers at part loads, and
a larger size pipe can be used on the
horizontal runs and vertical drop sections.
This helps with oil return, yet keeps the
pressure drop to a minimum.
Hot Gas Bypass Line
Hot Gas Bypass is available for use with DX
systems that may experience low suction
pressure during the operating cycle. This may
be due to varying load conditions associated
with VAV applications or units supplying a
large percentage of outside air. The system is
designed to divert refrigerant from the
compressor discharge to the low pressure
side of the system in order to keep the
evaporator from freezing and to maintain
adequate refrigerant velocity for oil return at
minimum load.
Hot discharge gas is redirected to the
evaporator inlet via an auxiliary side
connector (ASC) to false load the evaporator
when reduced suction pressure is sensed.
Field piping between the condensing unit
and the evaporator is required.
Hot Gas Bypass Piping Considerations
Pitch the hot gas bypass (HGB) line
downward in the direction of refrigerant
flow, toward the evaporator.
DISCHARGE RISER TRAPS
Circuits require discharge riser traps
every 12 feet.
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