AAON F-060 - Manual (Page 31)

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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 in this
situation. The double suction riser is
designed to return oil at minimum load
while not incurring excessive frictional
losses at full load. The 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 4000 fpm maximum
are recommended. The minimum velocity
required to return oil is dependent on the
pipe diameter; however, a general guideline
of 1000 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.
If the job requirements specify suction
accumulators, they must be separately
purchased and installed.
Hot Gas Reheat
The AAON modulating hot gas reheat
system diverts hot discharge gas from the
condenser to the air handling unit through
the hot gas line. Field piping between the
condensing unit and the air handling unit
is required.
The line delivers the hot discharge gas to the
reheat coil and/or the hot gas bypass valve,
so it is sized as a discharge line.
Discharge lines should be sized to ensure
adequate velocity of refrigerant to ensure oil
return, avoid excessive noise associated with
velocities that are too high, and to minimize
efficiency losses associated with friction.
Pitch the hot gas line in the direction of flow
for oil return.
When installing hot gas reheat risers, a drip
leg must be provided at the lowest point in
the system. The drip leg must be vertical, its
diameter should be the same as the diameter
of the riser, and it should be 1 foot long. Run
a drip line, using 1/8 inch capillary tube, 10
feet in length, from the drip leg to the