Liebert Extreme Density Chiller XDC - Manual (Page 28)

Piping and Filling with Refrigerant: R-134a and R-407c Circuits
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2.5.2 Piping for Direct Expansion (DX) Circuit—R-407c Air Cooled Units
2.6 Install Double Discharge Risers
For air-cooled systems, double discharge risers must be installed in the hot gas lines that have verti-
cal heights of 15 feet (4.6m) or more (see Figure 23). This will allow proper oil return to the compres-
sors when the system is running at low loads. A double riser system is constructed of a large diameter
riser with a trap at the base and a riser with a smaller diameter in parallel.
At full-load operation, the refrigerant vapor flows up both risers at velocities that are adequate to
carry the oil. At low loads, refrigerant vapor velocities are lower and the trap at the bottom of the
riser becomes filled with oil. When this happens, refrigerant flows up only the smaller riser.
The trap at the top of the riser must be tied into the top of the line to prevent oil from filling the larger
riser. For each double riser used, the maximum height of the riser must not exceed 15 feet (4.6m).
Multiple risers must be installed in series as the height of the hot gas line increases. Total maximum
height of the hot gas line must not exceed 60 feet (18.3m).
Horizontal discharge lines should be pitched downward in the direction of flow to aid in oil drainage
with downward pitch of at least 1/2 inch in 10 feet (13mm in 3m).
Figure 23 Double discharge riser layout
Contact your local Liebert representative for factory approval whenever a refrigerant piping run
exceeds 200 feet (60m) equivalent length or when condensers must be installed below the level of the
cooling coil.
!
WARNING
Risk of refrigerant system explosion or rupture from overpressurization. Can cause death,
injury and equipment damage.
Installer must install a 400 psig pressure relief valve in each of the two R-407c refrigerant
circuits of the Liebert XDC system. Do not install shutoff valves between the compressors and
the pressure relief valves.
For systems requiring EU CE compliance, the pressure relief valves must be CE-certified by a
notified body to the EU Pressure Equipment Directive.
Smaller riser is
5/8" O.D. copper
Trap size is 1-1/8" large riser size with minimum internal
Connection point of upper trap
is at the top of the line
Larger riser 1-1/8"
O.D. copper