15
SYSTEM DESIGN RECOMMENDATIONS
Piping design
Suction lines
Oil in a refrigeration circuit is required
to lubricate moving parts in the
compressor. During normal system
operation small quantities of oil will
continuously leave the compressor,
with the discharge gas. With good
system piping design this oil will
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 effi
ciency. However, too
large amounts of oil in the system will
have a negative eff ect on condenser
and evaporator effi
ciency. If, in a
poorly designed system, the amount
of oil returning to the compressor is
lower than the amount of oil leaving
the compressor, the compressor
will become starved of oil and
the condenser, evaporator and/or
refrigerant lines will become fi lled with
oil. In such situations, additional oil
charge will only correct the compressor
oil level for a limited period of time
and increase the amount of surplus 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 fl ow (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 fi gure
below).
Gas velocities higher than 12 m/s will
not contribute to signifi cantly better
oil return. However they will cause
higher noise levels and result in higher
suction line pressure drops which will
have a negative eff ect on the system
capacity.