AAON M1-011 - Manual (Page 22)

22
1. Avoid trapping oil at part-load operation,
and
2. prevent condensed refrigerant and oil in
the line from draining back to the head of
the compressor, and
3. avoid developing excessive noise from
either hot-gas pulsations, compressor
vibrations, or both.
Pressure loss in hot-gas lines increases the required
compressor power per unit of refrigeration, and
decreases the compressor capacity. Pressure drop is
minimized by over sizing the lines for low friction
losses while maintaining refrigerant line velocities to
entrain and carry oil at all loading conditions. Normally,
pressure drop is designed not to exceed the equivalent
of a 2°F change in saturation temperature, while
recommended sizing is based on a 1°F change in
saturation temperature.
Other Piping
Hot Gas Bypass Lines (Optional)
The hot gas bypass option is a system that maintains
evaporator pressure at or above a minimum value in
order to prevent the coil from freezing, and to keep
refrigerant velocity high enough for proper oil return
when operating at a light load.
Pressure drop in the hot gas line is normally designed
not to exceed the equivalent of a 2
°F change in
saturation temperature with R-22. See Table 5.4
below that is based on a 1
°F change in saturation
temperature.
Hot gas bypass lines must be insulated to minimize
heat loss and condensation of gas inside the piping
and to prevent injury from high temperature surfaces.
See Section 8 of this manual for more information
about hot gas bypass.
Table 5.4, R-22 Hot Gas Bypass Line Capacity w/ 3 PSI
(1
°F) Pressure Loss per 100 Feet at 40°F Saturated
Suction
Line Size (In.) Tons
1/2 .9
5/8 1.6
7/8 4.1
1 1/8 8.4
1 3/8 14.2
Minimum Gas Velocities for
Oil Transport in Risers
On multiple compressor installations, the lowest
possible system loading should be calculated with a
riser size selected to give at least the minimum
capacity for successful oil transport. Some
installations will have excessive pressure drop at
maximum load when multiple compressors exist with
capacity control, a vertical hot-gas line, and that are
sized to transport oil at minimum load. A double riser,
or a single riser with an oil separator can be used to
correct this problem.
Double Hot-Gas Risers
A double hot-gas riser can be used the same way it is
used in a suction line. Figure 5.1 shows the double
riser principle.
Single Riser and Oil Separator
Alternatively, an oil separator located in the discharge
line, just before the riser, permits sizing the riser for a
low-pressure drop. Any oil draining back down the
riser accumulates in the oil separator. With large
multiple compressors, the capacity of the separator
may dictate the use of individual units for each
compressor located between the discharge line, and
the main discharge header. Horizontal lines should be
level, or pitched downward in the direction of gas flow
in order to facilitate travel of oil through the system,
and back to the compressor.
Piping to Prevent Liquid and Oil from Draining to
Compressor Head
Whenever the condenser is located above the
compressor, the hot-gas line should be trapped near
the compressor before rising to the condenser,
especially if the hot-gas riser is long. This minimizes
the possibility that refrigerant, condensed in the line
during off cycles, will drain back to the head of the
compressor. Also, any oil traveling up the pipe wall
will not drain back to the compressor head.
The loop in the hot-gas line serves as a reservoir and
traps liquid resulting from condensation in the line
during shutdown, thus preventing gravity drainage of
liquid, and oil back to the compressor head. A small
high-pressure float drainer should be installed at the
bottom of the trap to drain significant amounts of
refrigerant condensate to a low side component such
as a suction accumulator, or low-pressure receiver.