Trane TR-SCS Series Installation, Operation And Maintenance Manual page 14

Installation
Riser #2 should be sized in such way that the inside
diameter of riser #1 and #2 will together have a combined
area allowing for a flow velocity that's suitable to carry the
refrigerant oil to the condenser during peak load
conditions.
Use a trap between the 2 risers so riser #2 will be sealed off
when the trap fills with oil during partial load operation,
thus diverting the flow of refrigerant to riser #1.
Figure 9. Piping installation
INVERTED TRAP
LIQUID
DISCHARGE
LINE
LINE
TRAP IN BOTTOM OF COLUMN
WITH MINIMUM TRAP DEPTH POSSIBLE
14
PITCH 1/4 INCH FOR EVERY 10 FEET OF RUN
PITCH IN DIRECTION OF REFRIGERANT FLOW
(DISCHARGE & LIQUID LINES)
CONDENSER
TRAP EVERY 20 FEET WITH MINIMUM TRAP
DEPTH POSSIBLE
PITCH IN DIRECTION OF REFRIGERANT FLOW
AIR
CONDITIONER
Figure 10. Dual riser piping
INVERTED TRAP
LIQUID
DISCHARGE
LINE
LINE(S)
RISER #2
RISER #1
TRAP IN BOTTOM OF COLUMN
WITH MINIMUM TRAP DEPTH POSSIBLE
Liquid Line
The velocity of refrigerant in the liquid line is less critical
because liquid refrigerant and oil are mixed thoroughly in
the liquid state. The main concern when sizing the liquid
line is to maintain a solid head of liquid refrigerant
entering the thermostatic expansion valve (TXV). If the
refrigerant pressure falls below its saturation
temperature, a portion of the liquid refrigerant may
change into vapor. Vapor will cause flashing and prevent
the TXV from functioning properly. As flashing begins, the
rate of pressure loss increases.
The liquid refrigerant is sub-cooled slightly below its
saturation temperature. Sub-cooling must be sufficient to
allow the necessary pressure drop without approaching a
saturation condition where gas flashing could occur.
Under normal operation the refrigerant is sufficiently
cooled as it leaves the condenser to allow for normal line
pressure drops.
Liquid line size is to be selected based on a pressure drop
equivalent to 2°F sub-cooling.
Operating liquid line velocities should be less than 
300 fpm to avoid liquid hammering during solenoid
operation. If the condenser is installed below the
evaporator section, the installer must observe the
pressure changes that occur if the elevation change. See
Table 4, p. 15 that follows for the vertical pressure drops
for the two types of refrigerant used.
PITCH 1/4 INCH FOR EVERY 10 FEET OF RUN
PITCH IN DIRECTION OF REFRIGERANT FLOW
(DISCHARGE & LIQUID LINES)
CONDENSER
TRAP EVERY 20 FEET WITH MINIMUM TRAP
DEPTH POSSIBLE
PITCH IN DIRECTION OF REFRIGERANT FLOW
AIR
CONDITIONER
SS-SVX006C-EN