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Refrigerant Oil
In the DX refrigeration systems covered by this guide, some
amount of compressor lubricating oil travels with the refrigerant
throughout the piping system. The system design must
promote oil return or the compressor sump will run dry and
damage the compressor.
Recall, refrigerant piping should be pitched to promote
adequate oil return. Fittings and piping layout that traps and
retains oil must be avoided. Compressor capacity reduction
contributes to the challenge of designing the system.
For example, a screw compressor may reduce refrigerant
flow (unload) down to 25%. At this reduced refrigerant flow
rate, the refrigerant velocity is reduced to the point that the oil
may not be pushed through the piping system and back to the
compressor.
Examples of compressors that unload include:
• Scroll compressors often have multiple compressors on a
common refrigeration circuit. The circuit can unload to the
smallest compressor size. For example, 4 equally sized
compressors can unload down to 25%.
• Individual reciprocating compressors unload down to as
low as 33%. There can be multiple compressors on a
common circuit allowing even more unloading.
• Screw compressors may unload down to 25%.
Always check the manufacturer's information to determine
circuit unloading.
More piping typically requires more oil. This is particularly
true for long liquid lines. Residential split systems are often
pre-charged at the factory with enough oil and refrigerant for
a specified line distance. When that distance is exceeded,
additional refrigerant and oil will be required. For commercial
split systems, the equipment may come pre-charged or it may
be provided with either nitrogen or a small holding charge. The
refrigerant and oil charge is then provided in the field.
To confirm if more oil is required, the system refrigerant charge
must be calculated.
Table 19 on page 50
on page 50
provide the charge per 100 feet(30.5 m) length
for various refrigerants. Generally, the oil charge should be 2
to 3% of the liquid line charge. Consult the manufacturer for
the correct volume of oil in the system and the amount of oil
shipped in the compressor sump. The required oil that needs
to be added is the calculated total oil requirement less the oil
shipped in the equipment.
Required oil = Total oil required – oil shipped in equipment
HFC refrigerants use synthetic POE oils. These oils cannot
be mixed with mineral oils. Refer to the manufacturer's
instructions for the correct type of oil to use.
AG 31-011 • REFRIGERANT PIPING DESIGN
Suction Line Sizing
Suction lines contain gaseous refrigerant that moves oil along
the piping and back tothe compressor. Over-sizing suction
pipes increases the initial costs and may reduce the refrigerant
gas velocity to the point where oil is not returned to the
compressor. Recall, under-sizing suction pipes reduces system
capacity. Oil movement is also impacted negatively by risers,
because gravity prevents oil from returning to the compressor.
through
Table 22
20
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