37
Suction Line Traps
Include traps every 6.1 meters (20 feet) in
vertical suction riser sections for cooling only
systems and every 3.7 meters (12 feet) for
heat pump systems. Include a trap at the
bottom of the vertical run.
Suction Line Insulation
The entire suction line must be insulated with
a minimum 1 inch thick Armaflex insulation.
This prevents condensation from forming on
the line, and reduces any potential loss in
capacity associated with heat gain placing
additional load on the system. This line must
still be insulated in heat pump systems even
though it acts as both a discharge and suction
line.
Suction Line Guidelines
For proper performance, keep suction line
velocities less than a 4,000 fpm. The
minimum velocity required to return oil is
dependent on the pipe diameter, however, a
general guideline of 1,000 fpm minimum
may be applied.
When suction flow is up, variable capacity
compressors require a minimum velocity of
1,500 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 suction flow is
up, and the tandem has a variable capacity
compressor, the velocity for only one
compressor in operation must be greater than
1,500 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 liquid 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.
Suction Line Accessories
If the job requirements specify suction
accumulators, they must be separately
purchased and field installed. Heat pump
units will include a factory installed suction
accumulator.
Discharge Line
The discharge line is similar to the suction
line from a design and construction
standpoint. 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.
Pressure loss in the discharge line has less of
an impact on capacity than pressure loss in
SUCTION RISER TRAPS
Circuits require suction riser traps
every 20 feet. (every 12 feet for heat
pumps)
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