standpoint. More care must be taken to
ensure that adequate velocity is achieved to
return oil to the compressor at minimum
loading conditions. However, reducing the
piping diameter to increase the velocity at
minimal load can result in excessive pressure
losses, capacity reduction, and noise at full
load.
Suction Line Routing
For cooling only systems, pitch the suction
line in the direction of flow (about 1 inch per
20 feet of length) to maintain oil flow
towards the compressor, and keep it from
flooding back into the evaporator.
For heat pump systems, do not pitch lines
since they will be flowing in one direction in
cooling mode and the opposite direction in
heating mode.
Crankcase heaters are provided to keep any
condensed refrigerant that collects in the
compressor from causing damage or wear.
Make sure to provide support to maintain
suction line positioning, and insulate
completely between the evaporator and
condensing unit.
It is important to consider part load operation
when sizing suction lines. At minimum
capacity, refrigerant velocity may not be
adequate to return oil up the vertical riser.
Decreasing the diameter of the vertical riser
will increase the velocity, but also the
frictional loss.
For difficult line routing applications, a
double suction riser can be applied to the
situation of part load operation with a suction
riser. A double suction riser is designed to
return oil at minimum load while not
incurring excessive frictional losses at full
load. A double suction riser consists of a
small diameter riser in parallel with a larger
diameter riser, and a trap at the base of the
large riser. At minimum capacity, refrigerant
velocity is not sufficient to carry oil up both
risers, and it collects in the trap, effectively
closing off the larger diameter riser, and
diverting refrigerant up the small riser where
velocity of the refrigerant is sufficient to
maintain oil flow. At full load, the mass flow
clears the trap of oil, and refrigerant is carried
through both risers. The smaller diameter
pipe must be sized to return oil at minimum
load, while the larger diameter pipe must be
sized so that flow through both pipes
provides acceptable pressure drop at full
load.
Figure 9 - Double Suction Riser
Construction
A double riser can also be used for heat pump
operation. The specific volume (ft
3
/lb) of
refrigerant at the discharge temperature and
pressure (heating mode line conditions) is
significantly lower than the specific volume
at the suction temperature and pressure
(cooling mode line conditions). To
compound the issue, the capacity in heating
mode is lower than the capacity in cooling
mode. The discharge velocity in the riser
during heating mode is much lower than the
suction velocity during cooling mode. Often,
a double riser is necessary to get acceptable
velocities for the discharge mode and
acceptable velocities for the suction mode. In
the example diagrams (See Figure 10 &
Figure 11), the cooling mode will use both
lines, and the heating mode will use only one.
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