System design recommendations
Suction line
Piping design
Horizontal suction line sections shall have a
slope of 0.5% in the direction of refrigerant flow
(5 mm per meter).The cross-section of horizontal
suction lines shall be such that the resulting
gas velocity is at least 4 m/s. In vertical risers, a
gas velocity of 8 to 12 m/s is required to ensure
proper oil return.
A U-trap is required at the foot of each vertical
riser. If the riser is higher than 4 m, additional
U-traps are required for each additional 4 meters.
The length of each U-trap must be as short as
possible to avoid the accumulation of excessive
quantities of oil (see figure below).
Gas velocities higher than 12 m/s will not
contribute to significantly better oil return.
However they will cause higher noise levels
and result in higher suction line pressure
drops which will have a negative effect on the
system capacity. Note that the suction rotolock
valves, which can be ordered from Danfoss
as accessories, are designed for average pipe
sizes, selected for systems running at nominal
conditions. The pipe sizes selected for specific
systems may differ from these average sizes.
The suction line must always be insulated to limit
suction gas superheat.
Oil in a refrigeration circuit is required to
lubricate moving parts in the compressor. During
normal system operation small oil quantities
will continuously leave the compressor, with
the discharge gas. Therefore, the system piping
shall be designed in a way which allows a good
oil circulation, avoiding oil being trapped in
the system and ensuring a constant oil return
to the compressor. As long as the amount of
oil circulating through the system is small it
will contribute to good system operation and
improved heat transfer efficiency.
Lubricant getting trapped in the evaporator or
suction lines will affect system performance and
will ultimately lead to compressor lubrication
failures. Where a poor oil return situation exists,
adding lubricant will not help safeguard the
compressor. Only a correct piping design can
ensure adequate oil circulation maintaining safe
oil level in the compressor.
Maneurop® NTZ compressors have been
designed and qualified for stationary equipment
using standard alternating power supply.
Danfoss does not warrant the compressors
for use on mobile applications such as trucks,
railways, subways, ships etc.
Discharge line
When the condenser is mounted above the
compressor, a loop above the condenser and a
U-trap close to the compressor are required to
prevent liquid draining from the condenser into
the discharge line during standstill.
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AB245486497018en-000601
Application Guidelines
