24
REFRIGERANT PIPING cont.
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.
See figures RP-5 through RP-10 for hot gas
bypass piping configurations.
Hot Gas Bypass Piping Considerations for
Evaporator Above Condensing Unit:
• Pitch the hot gas bypass 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, 5
feet in length, from the drain leg to the suction
line. Connect the oil return line below the sight
glass, 1 inch above the bottom of the drain leg.
• HGBP valves are adjustable. Factory HGBP
valve settings will be sufficient for most
applications, but may require slight adjustments
for some make up air or other process cooling
applications.
• Insulate the entire length of the HGBP line with
a minimum 1 inch thick Armaflex insulation.
• Refer to figure RP-5 for piping diagram
Hot Gas Bypass Piping Considerations for
Evaporator Below Condensing Unit:
• The line must slope downward from the hot gas
bypass valve toward the evaporator.
• Refer to figure RP-6 for piping diagram
Hot Gas Bypass Line Guidelines:
• Choose a small size line to ensure oil return,
and minimize refrigerant charge.
• Maintain velocities below a maximum of 4000
fpm. A general minimum velocity guideline to
use is approximately 1000 fpm.
Hot Gas Reheat:
• The AAON modulating hot gas reheat system
diverts hot discharge gas from the condenser to
the air handling unit to supply the reheat coil
and/or the hot gas bypass valve. Size this line 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.
• Insulate the entire length of the hot gas line
with a minimum 1 inch thick Armaflex
insulation.
• Refer to figures RP-7 through RP-10 for piping
diagrams.
Hot Gas Reheat Guidelines:
• Maintain velocities below a maximum of 3500
fpm. A general minimum velocity guideline is
2000 fpm.
Predetermined Line Sizes:
• To aid in line sizing and selection, AAON has
predetermined line sizes for comfort cooling
applications.
• In order to generate this information, the
following cycle assumptions are made:
Saturated suction temperature = 50°F, Saturated
condensing temperature = 125°F, Sub-cooling =
10°F, Superheat = 15°F.
• The liquid lines have been chosen to maintain
velocities between 100 and 350 fpm. The
suction line diameters are selected to limit
velocities to a 4000 fpm maximum, while a
minimum velocity restriction is imposed by the
ability to entrain oil up vertical suction risers
