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Coil Piping and Connections
Suction Lines
Line sizing
Proper line sizing is required to guarantee 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).
Routing
To prevent residual or condensed refrigerant from "free-
flowing" toward the compressor during the off cycle, install the
suction line so it slopes by ¼ in. to 1 inch per 10 feet of run
toward the evaporator.
When the application includes a suction riser, oil must be
forced to travel the height of the riser. Riser traps are
unnecessary in the suction line. They will add pressure drop.
Double risers must not be used. They not only add pressure
drop, but can hold great amounts of oil - oil better used in the
compressor.
Note: If a suction riser is properly sized, oil will return to the
compressor regardless of whether a trap is present. If
a suction riser is oversized, adding a trap will not
restore proper oil entrainment.
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.
Insulation
Any heat that transfers from the surrounding air to the cooler
suction lines increases the load on the condenser (reducing
the system's air-conditioning capacity) and promotes
condensate formation. After operating the system and testing
all fittings and joints to verify that the system is leak-free,
insulate suction lines to prevent heat gain and unwanted
condensation.
Components
Installing the suction line requires field installation of these
components: a filter, access port, and a Frostat™ control when
the refrigerant coil is used with Trane condensing units.
Position them as close to the compressor as possible.
Note: Placement of the Frostat control is illustrated in
Figure 48 on page
•
Filter: The suction filter prevents contaminants, introduced
during installation, from entering the compressor. For this
reason, the suction filter should be the replaceable-core
type, and a clean core should be installed after the system
is cleaned up.
•
Access port: The access port is used to determine suction
pressure. This port is usually a Schraeder valve with a
core.
44
42.
•
Frostat™ coil frost protection: The Frostat control is the
preferred method for protecting evaporator coils from
freezing when the refrigerant coil is used with Trane
condensing units. It senses the suction-line temperature
and temporarily disables mechanical cooling if it detects
frost conditions. The control is mechanically attached to
the outside of the refrigerant line, near the evaporator, and
wired to the unit control panel.
•
Ball shutoff valve: Adding manual, ball-type shutoff valves
upstream and downstream of the filter simplifies
replacement of the filter core.
Expansion Valves
Expansion valves meter refrigerant into the evaporator under
controlled conditions. If there is too much refrigerant, the
refrigerant will not completely vaporize and the remaining
liquid will slug the compressor. If there is too little refrigerant,
there may not be enough cooling for the compressor.
Expansion valve requirements vary based on condensing unit
design. Consult the product literature for the condensing unit
to be used for proper valve selection.
Hot Gas Bypass
Many years ago, hot gas bypass (HGBP) was successfully
added to HVAC systems to correct a number of operational
problems. Hoping to avoid such problems altogether, it
eventually became common practice for designers to specify
hot gas bypass in new systems. Unfortunately, the practice
often degraded rather than improved reliability.
Hot gas bypass increases the minimum refrigerant charge; it
also inflates the first cost of the system. Besides adding more
paths for potential refrigerant leaks, hot gas bypass increases
the likelihood of refrigerant distribution problems. Finally, hot
gas bypass uses excessive amounts of energy by preventing
the compressors from cycling with fluctuating loads.
Trane now has many years of experience in the successful use
of equipment without hot gas bypass in commercial comfort-
cooling applications. To prevent evaporator freeze-up, Trane
equipment typically includes Frostat™ coil frost protection.
Like hot gas bypass, the Frostat system protects the coil from
freezing, but it does so by turning off compressors when a
sensor detects the formation of frost on the evaporator coil.
The compressor is released to operate when the coil
temperature rises a few degrees above the frost threshold. The
Frostat control strategy reduces the overall energy
consumption of the system while maintaining system control.
Systems should be designed to avoid HGBP whenever
possible. For more information about HGBP, refer to the
Engineers Newsletter, "Hot Gas Bypass – Blessing or a
Curse?" (ADM-APN007*-EN).
CLCH-SVX009L-EN
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