Vertiv Liebert XD Series, System Design Manual

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6.3 Heat Rejection Piping
6.3.1 Piping for Direct Expansion (DX) Circuit—R-407C Air-Cooled Units
Risk of refrigerant system explosion or rupture from overpressurization. Can cause equipment damage,
injury or death.
WARNING! 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.
6.4 Install Double Discharge Risers
For air-cooled systems, double discharge risers must be installed in the hot gas lines that have vertical
heights of 15 feet (4.6m) or more (see Figure 6.4 on the next page). This will allow proper oil return to the
compressors 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).
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