1
0.5% slope,
13 ft/s or
more
max. 13ft
max. 13ft
26 to 39 ft/s
0.5% slope,
13 ft/s or
more
SYSTEM DESIGN RECOMMENDATIONS
Piping design
Suction lines
Oil in a refrigeration circuit is required to
lubricate moving parts in the compres-
sor. During normal system operation
small quantities of oil will continually
leave the compressor, with the dischar-
ge gas. With good system piping design
this oil will return to the compressor.
As long as the amount of oil circula-
ting through the system is small it will
contribute to good system operation
and improved heat transfer efficiency.
Excess oil in the system, however, will
have a negative effect on condenser
and evaporator efficiency. If, in a poorly
designed system, the amount of oil re-
turning to the compressor is lower than
the amount of oil leaving the compres-
sor, the compressor will become star-
ved of oil and the condenser, evapora-
tor and/or refrigerant lines will become
filled with oil. In such situations, addi-
tional oil charge will only correct the
compressor oil level for a limited period
of time and increase the amount of sur-
plus oil in the rest of the system.
Only correct piping design can ensure a
good oil balance in the system.
Horizontal suction line sections shall
have a slope of 0.5% in the direction
of refrigerant flow (5/8" for every 10' of
pipe). The cross-section of horizontal
suction lines shall be such that the re-
sulting gas velocity is at least 13 fps. In
vertical risers, a gas velocity of 26 to 40
fps is required to ensure proper oil re-
turn. A U-trap is required at the foot of
each vertical riser. If the riser is higher
than 13 ft, additional U-traps are requi-
red for each additional 13 ft. The length
of each U-trap must be as short as possi-
ble to avoid the accumulation of exces-
sive quantities of oil (see figure below).
For compressors mounted in parallel,
the common suction riser should be de-
signed as a double riser. Also refer to the
News bulletin "Mounting instructions
for installation of Maneurop® compres-
sors in parallel" and "Parallel application
guidelines".
Gas velocities higher than 40 fps will
not contribute significantly to better oil
return. They will, however, cause higher
noise levels and result in higher suction
line pressure drops which will have a
negative effect on system capacity.