AAON V3-E - Manual (Page 30)

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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.
A double suction riser can be applied to the
situation of part load operation with a
suction riser. A double suction riser is
designed to return oil at minimum load
while not incurring excessive frictional
losses at full load. A double suction riser
consists of a small diameter riser in parallel
with a larger diameter riser, and a trap at the
base of the large riser. At minimum
capacity, refrigerant velocity is not sufficient
to carry oil up both risers, and it collects in
the trap, effectively closing off the larger
diameter riser, and diverting refrigerant up
the small riser where velocity of the
refrigerant is sufficient to maintain oil flow.
At full load, the mass flow clears the trap of
oil, and refrigerant is carried through both
risers. The smaller diameter pipe should be
sized to return oil at minimum load, while
the larger diameter pipe should be sized so
that flow through both pipes provides
acceptable pressure drop at full load.
Suction Line Insulation The entire suction line should be insulated.
This prevents condensation from forming on
the line, and reduces any potential loss in
capacity associated with heat gain.
Suction Line Guidelines
For proper performance, suction line
velocities less than a 4,000 fpm maximum
are recommended. 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.
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 1-2°F change in saturation
temperature.
At points where small pipe size can be used
to provide sufficient velocity to return oil in
vertical risers at part loads, greater pressure
losses are incurred at full loads. This can be
compensated for by over sizing the
horizontal runs and vertical drop sections.
This will however require additional
refrigerant charge.
Circuits with variable capacity scroll
compressors require suction riser traps every
10 feet.
Suction Line Accessories
If the job requirements specify suction
accumulators, they must be separately
purchased and field installed.
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
Circuits with variable capacity scroll
compressors require suction riser
traps every 10 feet.
CAUTION