AG 31-011 • REFRIGERANT PIPING DESIGN 22 www.DaikinApplied.com
Figure 15: Double Suction Riser Detail
shows a double suction riser arrangement that is
more common in refrigeration applications where suction
pressure drops are more critical. Most modern air conditioning
applications can be met without requiring a double suction
riser. Although the operation and design of a double suction
riser is included in this guide, it is strongly recommended that
systems be designed without a double suction riser, even if the
pressure drop in the suction or discharge line is higher than
desired.
In a double suction riser at full capacity, the refrigerant flow
passes through both risers with enough velocity to move
the oil. At minimum capacity, oil in the riser flows backward
and fills the trap at the bottom. Once the trap is full of oil,
refrigerant flow through the large diameter riser is cut off and
only refrigerant gas flows through the smaller diameter riser.
The sum of the two risers is sized for full capacity. The smaller
diameter riser is sized for minimum capacity.
One of the challenges of double suction risers is that they
hold a significant amount of oil within the trap. Refrigeration
compressors often have larger sumps than commercial
compressors, so the oil lost to the trap is less problematic
for refrigeration than commercial compressors. In addition,
when the capacity increases in a double suction riser, a large
amount of oil is “blown” through the piping system back to the
compressor. Either an oil separator or a suction accumulator
(both common in refrigeration systems) may be required for
a double suction riser to operate properly without causing
damage to the compressor.
Tip:
For most air conditioning applications, a single pipe riser will work. In this case, it may be necessary
to undersize the riser pipe by one pipe size to provide better oil management.
Slope in
direction of
refrigerant flow
Small diameter pipe
inverted trap not
required if pipe is
properly sloped
Minimize
Trap
Volume
Large
Diameter
Riser
Small
Diameter
Riser