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1.3 Refrigerant Piping
Good system piping designs will minimize the possibility
of lubrication failure, flooded starts, and refrigerant
floodback problems. Refrigerant piping systems must
therefore be designed to protect the compressor by:
1. Preventing excessive lubricating oil from being trapped
in the system. Refrigerant piping must be sized for prop-
er velocity, especially in suction lines, to return oil under
all conditions. If capacity control is utilized, piping must
be sized for full and part load conditions. With the
increased use of mechanical subcooling in refrigeration
conditions, the system designer must also consider the
lower refrigerant mass flow that results in systems that
are designed to operate with large amounts of subcool-
ing (30°F to 70°F, 16°C to 39°C). With the new HFC
refrigerants, this is especially important in low tempera-
ture applications using large amounts of subcooling (for
example liquid subcooled from 110°F to 40°F, (43°C to
4°C). This can result in a 40% reduction in mass flow
over a system without subcooling. The lower mass flow
will result in lower refrigerant velocities and can result
in inadequate oil return if not considered during the sys-
tem piping design.
2. Minimizing the loss of lubricating oil from the compres-
sor at all times.
3. Preventing liquid refrigerant from entering the compres-
sor during operation and shut down.
To properly cover the subject of piping design would be
too lengthy to treat here, especially since many excellent
guides to piping design are presently available. For com-
plete details of good system piping practices, the Carrier
System Design Manual (Part 3 Piping Design) is
recommended.
1.4 Vibration Isolation
On installations where noise and vibration must be kept
to a minimum, it is desirable to use vibration mounts
under the compressor unit, even though the compres-
sors may be spring mounted. Proper precautions must
be taken to prevent the transmission of compressor
vibration through the piping system. It is also recom-
mended to design the suction line with sufficient
“spring” so the suction service valve can be moved aside
for access to the suction strainer. Compressors applied
in spring-mounted systems should also have adequate
flexibility in the suction and discharge piping to avoid
the excessive stresses caused by the start and stop “kick”
of the compressor. These excessive stresses can typically
be avoided by adding bends in the piping in different
directions. Many systems have been designed with com-
pressors mounted to the bases. In these cases, it is
important that the compressors be properly torqued to
the base or the compressor may produce a “rattle” or
transmit excessive vibration to the base.
For a more complete review of the system vibration and
piping recommendations, see Carlyle OEM Bulletin
#118.
1.5 Refrigerant Migration and Floodback
Liquid refrigerant, or even excessive amounts of
entrained liquid particles in the suction gas, must be
kept out of the compressor by proper system design and
compressor control. Under running conditions, the pres-
ence of liquid refrigerant in the compressor tends to
break down the oil film on the cylinder walls, resulting
in increased wear to the cylinder walls and piston rings,
and possible compressor damage. Furthermore, exces-
sive liquid in the cylinders causes hydraulic compres-
sion, which can create cylinder pressures as high as
1500 psi (103 bar). This hydraulic loading can cause
suction and discharge valve and gasket failures to occur
while also subjecting the connecting rod, piston, and
main bearings to excessive loading. Although laboratory
testing of 06D and 06E compressors has shown that they
can withstand substantial flooded starts and floodback,
prolonged excessive flooding will eventually cause any
compressor to fail.
Therefore, special care should be taken to ensure that
liquid refrigerant is kept out of the compressor especial-
ly in systems where large quantities of refrigerants are
often used. During compressor operation, the expansion
valve must be properly adjusted to prevent liquid from
entering the compressor.
During compressor shutdown, gravity, thermal action
and refrigerant absorption will result in a refrigerant and
oil mixture in the compressor crankcase. Gravity flow
can be prevented by the use of recommended loops in
the piping, but thermal action and the absorption of
refrigerant by lubricating oil cannot be prevented by pip-
ing design. To minimize the absorption of refrigerant
into the oil, the use of crankcase heaters is strongly rec-
ommended. Because oil dilution is more critical with
HFC refrigerants and POE lubricants, Carlyle requires the
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