Trane TTA240F Series, Application Manual, Page: 13 / 28

SS-APG008-EN 7
Line Sizing, Routing, and Component Selection
Gas Line
Line Sizing
Proper line sizing is required to guarantee that the oil returns to the compressor throughout the
system’s operating envelope. At the same time, the line must be sized so that the pressure drop
does not excessively affect capacity or efficiency. To accomplish both objectives, it may be
necessary to use two different line diameters: one for the horizontal run and for the vertical drops,
and another for the vertical lifts (risers).
Note: Preselected suction-line diameters shown in Table 2 and Table 3 are independent of total
line length for properly charged 6- through 20-ton TTA/TWA in normal air-conditioning
applications.
Routing
Route the line as straight (horizontally and vertically) as possible. Avoid unnecessary changes of
direction. To prevent residual or condensed refrigerant from “free-flowing” toward the
compressor, install the gas line so that it slopes by ¼ to 1 inch per 10 feet of run (1 cm per 3 m)
toward the indoor coil.
Do not install riser traps. With field-supplied air-handler coils, what appears to be a riser trap is
located at the coil outlet; see
Figure 5 for an example. This piping arrangement, which isn’t a riser
trap, is the result of two requirements:
• Drain the coil to the common low point.
• Rise at least 1 ft (30 cm) from the common low point to prevent any off-cycle condensed
refrigerant in the coil from attempting to flow to the compressor.
Double risers must not be installed. All 6- through 20-ton TTA and TWA units unload such that a
single gas line size, preselected in
Table 2, p. 20 , or Table 3, p. 21 , provide sufficient velocity to push
entrained oil up the permissible riser height.
Note: If a gas riser is properly sized, oil will return to the compressor regardless of whether a trap
is present. If a gas riser is oversized, adding a trap will not restore proper oil entrainment.
Figure 5. Gas-line arrangement at the outlet of a field-supplied indoor coil
Avoid Underground Refrigerant Lines
Refrigerant condensation during the off cycle, installation debris inside the line (including
condensed ambient moisture), service access, and abrasion/corrosion can quickly impair
reliability.