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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 in this
situation. The double suction riser is designed to
return oil at minimum load while not incurring
excessive frictional losses at full load. The 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. Size the
smaller diameter pipe to return oil at minimum
load, and size the larger diameter pipe for
acceptable pressure drop at full load.
Figure 8 - Double Suction Riser
Suction Line Traps:
Include a trap immediately after the evaporator
coil outlet to protect the TXV bulb from liquid
refrigerant.
Include traps every 20 feet in vertical riser
sections.
Suction Line Insulation:
Insulate the entire suction line with a minimum 1
inch thick Armaflex insulation. This prevents
condensation from forming on the line, and
reduces any potential loss in capacity associated
with heat gain placing additional load on the
system.
Suction Line Guidelines:
For proper performance, keep suction line
velocities less than a 4000 fpm. The minimum
velocity required to return oil is dependent on the
pipe diameter, however a general guideline of
1000 fpm minimum may be applied.
Tandem compressors must be considered for full
load operation (both compressors operating) and
at partial load (only one compressor operating).
When suction flow is up, the velocity at partial
load must be greater than the minimum velocity
required to return oil.
In a fashion similar to the liquid line, a common
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.
At points where small pipe size can be used to
provide sufficient velocity to return oil in vertical
risers at part loads, greater pressure losses are
incurred at full loads. This can be compensated
for by over sizing the horizontal and vertical drop
SUCTION RISER TRAPS
Circuits require suction riser traps every
20 feet.
CAUTION
