AAON M2-036 - Manual (Page 42)

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may provide additional sub-cooling to the
refrigerant. When routing the liquid line
through high temperature areas, insulation of
the line is appropriate to avoid loss of sub-
cooling through heat gain.
Liquid Line Guidelines
In order to ensure liquid at the TXV,
frictional losses must not exceed available
sub-cooling. A commonly used guideline to
consider is a system design with pressure
losses due to friction through the line not to
exceed a corresponding 1-2°F change in
saturation temperature.
If the velocity of refrigerant in the liquid line
is too great, it could cause excessive noise or
piping erosion. The recommended
maximum velocities for liquid lines are 100
fpm from the condenser to a receiver tank to
discourage fluid backup, and 300 fpm from
receiver tank to the evaporator to minimize
valve induced liquid hammer.
Liquid Line Accessories
Liquid line shut off valves and filter driers
are factory provided. Filter driers must be
field installed on 2-6 ton units. The total
length equivalent of pressure losses through
valves, elbows and fittings must be
considered when adding additional
components in the field. It is a good practice
to utilize the fewest elbows that will allow
the mating units to be successfully joined.
Suction Line Sizing
The suction line is more critical than the
liquid line from a design and construction
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
Pitch the suction line in the direction of flow
(about 1 foot per 100 feet of length) to
maintain oil flow towards the compressor,
and keep it from flooding back into the
evaporator. 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.
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
Circuits with variable capacity scroll
compressors require suction riser
traps every 10 feet.
CAUTION