Application guidelines
17
FRCC.PC.002.A5.22
System design recommendations
Suction line
Piping design
Horizontal suction line sections shall have a slope
of 0.5% in the direction of refrigerant flow (5/8"
per 10 ft of pipe).The cross-section of horizontal
suction lines shall be such that the resulting gas
velocity is at least 13 ft/s. In vertical risers, a gas
velocity of 26 to 40 ft/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 13 ft, additional
U-traps are required for each additional 13ft.
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 40 ft/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.
0.5 % slope,
13 ft/s or more
0.5 % slope,
13 ft/s or more
To condenser
max. 13 ft
max. 13 ft
26 to 40 ft/s
U-trap, as short as possible
U-trap, as short as possible
U-trap
Evaporator