16
Suction Riser —
A suction riser condition exists when the outdoor unit is
located above the indoor (evaporator) unit and suction
vapor must flow vertically up to return to the compressor.
Oil return is a conce rn when the suction tube siz e is too
large to produc e the minimum refrigerant velocity to
ensure oil return at minimum load conditions.
38AUZ (All) and 38AUD (A,B) WITHOUT HGBP
Check Table 8 for maximum suction tube size for 38AU
units at minimum load conditions without factory--installed
hot gas bypass. Consider suction speed riser (reduced tube
size for vertical segment only) or double suction riser
arrangement if the planned suction tube size does not
provide necessary minimum flowrates for this riser .
Table 8 –
38AU Maximum Suction Tube Size
Model: Sizes Maximum Suction Line
38AUZ 16---25 1
5
/
8
38AUDA,B 16---25 1
5
/
8
38AUDC 16---25 See Table 7
38AUCD WITH OPTIONAL HOT GAS BYPASS
Special conside ration for suction r iser requirements must
be considered when a pplying this factory--install ed hot gas
bypass. It is extremely importa nt to consider the lowest
possible refrigerant tons of refrigerant flow when hot gas
bypass is functioni ng to assure proper oil return up suction
risers.
Table 7 provides recommended tube sizes for model
38AUDC units which include the factory--installed hot gas
bypass system on Circuit 1. The select ions in this ta ble are
based on evaluations of system minimum load operating
conditions for comfort cooling applications, wit h
compressor saturated suction pressure to 28_F (--2.2_C)
minimum.
Table 7 includes liquid line siz e selections based on use of
RTPF coil option only.
Table 7 includes three different suction line size selec tions
for Circuit 1 based on the elevation difference conditions
betwee n the 38AUD unit and the evaporator coil location.
Outdoor unit ABOVE evaporator coil: This installation
will have at least one suction riser segment. If the vertical
elevation difference is less than 20 --ft (6.1 m) linear feet
and requires only one or two segments, consider a speed
riser selection for Circuit 1; use the line marked VAPOR
RISER REQUIRED; NOT GREATER THAN 20 FT
(6.1 M). See Fig. 6. Tube S is the horizontal line size;
tube A is the reduced diameter riser size.
CONDENSING
UNIT
20-FT (6.1 M) MAX
A
A - Suction Riser Without Trap
LEGEND
S - Suction Line to Condensing Unit
S
C12404
Fig. 6 -- Suction Line Piping -- Speed Riser
If the vertical elevation difference is greater than 20--ft
(6.1--m) linear feet or re quires more than two short lift
segments, select the Circuit 1 suction line size from
Double Suction Riser lines data under VAPOR RISER
GREATE R THAN 20 FT (6.1 M). See Fig. 7. Tube S is
the horizontal line size. Tube A is the reduced diameter
riser size without bottom t rap; Tube B is the parallel riser
size with bottom oil trap.
FROM
EVAPORATOR
A
A - Suction Riser Without Trap
B - Suction Riser With Trap
S - Suction Line to Condensing Unit
LEGEND
B
S
S
C12405
Fig. 7 -- Suction Line Piping -- Double Riser
Outdoor unit BELOW evaporator coil and no vertical
riser segments: Select Circuit 1 suction line size from
NO VAPOR RISER line.
Circuit 2 suction line: Because there is no hot gas bypass
system in Circuit 2, no special riser selections are
necessary. Select Circuit 2 suction line from bottom line
for all 38AUDC installations.
Vertical Separation (outdoor unit above indoor unit) –
Vertical elevation difference of 200 ft (60 m) is permitted
when the outdoor unit (38AUZ or 38AUD) is located
above the indoor unit.
38AU
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