the suction line. Pressure loss in the
discharge line causes the compressors to
work harder and thus use more power.
Discharge Line Routing
For cooling only remote condenser systems,
pitch the discharge line in the direction of
flow (about 1 inch per 20 feet of length) to
maintain oil flow towards the compressor.
In a heat pump system, the field installed
suction line is also used as a discharge line in
the heating mode of operation so the line
must be sized to meet both the suction line
conditions in cooling mode and the discharge
line conditions in heating mode.
Because it is used in both directions for a heat
pump unit, the line must be installed level,
not pitched, to facilitate oil return in both
modes of operation.
It is important to consider part load operation
when sizing discharge 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 discharge riser can be applied to the
situation of part load operation with a
discharge riser. A double discharge riser is
designed to return oil at minimum load while
not incurring excessive frictional losses at
full load. A double discharge 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. (See the Double Suction Riser
Construction Figure 9)
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, the cooling mode will
use both lines, and the heating mode will use
only one. See the following schematics that
illustrate how the double discharge riser can
work for heat pump applications.
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