AAON M2-036 - Manual (Page 43)

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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
15 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
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 for
Evaporator above Condensing Unit
Pitch the hot gas bypass (HGB) line
downward in the direction of refrigerant
flow, toward the evaporator.
When installing hot gas bypass risers, a
drain leg must be provided at the lowest
point in the system. The drain leg must be
vertical, its diameter should be the same as
the diameter of the riser, and it should be 1
foot long. Install a sight glass in the drain
leg for observation. Run an oil return line,
using 1/8 inch capillary tube, 10 feet in
length, from the drain leg to the suction line.
Connect the oil return line below the sight
glass and 1 inch above the bottom of the
drain leg.
HGB valves are adjustable. Factory HGB
valve settings will be sufficient for most
applications, but may require slight
adjustments for some applications, including
some make up air applications.