When multiple compressors are piped together with a common suction and discharge header - such as with the arrangement of a parallel rack configuration - oil return to each compressor is a major concern. Typical oil recirculation rates are dependent on three factors: a compressor’s run time, size of the compressor, and wear on a compressor (a worn compressor will circulate more oil per pound of refrigerant pumped per hour.)
On a parallel rack each of these factors affects how oil is returned to
individual compressors. Not all compressors operate at the same time, so run
times will affect how oil is returned to individual compressors. Ideally the
same amount of oil pumped from a compressor will be returned to that
compressor. This may not occur, as multiple compressors are cycled off and on
at differing rates.
The size of the compressor is also a factor in how much oil recirculates. A
new, properly functioning reciprocating compressor will circulate approximately
1 to 3 percent of oil per pound of refrigerant pumped every hour. A typical
15-hp compressor operating at 110°F condensing temperature with a 15°
evaporator recirculates approximately 1,790 pounds of refrigerant an hour; or
at a 1 percent recirculation rate of 17.9 pounds of oil. A typical 5-hp
compressor operating at the same conditions recirculates approximately 445
pounds of refrigerant per hour; or at a
1 percent oil recirculation rate, 4.5 pounds of oil per hour. Some rack systems
are built with various size compressors, so the overall recirculation rate is
not evenly divided among all the compressors. Some of the compressors may get
more or less oil returned.
All compressors eventually wear. This affects the oil circulation rate of a
compressor. For instance, as our 15-hp compressor wears, its pumping rate can
easily increase to 5 percent, meaning our original recirculation rate of 17.9
pounds per hour will increase to 89 pounds of oil an hour.
If no other components are added to this system, oil will return to each
compressor at different rates. This will cause the oil level on some
compressors to become too low, while other compressors will become overcharged
with oil. Both scenarios can lead to compressor damage, resulting in a failed
To ensure each compressor maintains the correct oil level, an oil separator,
reservoir, and oil level controls are added to the rack.
The oil separator is used to limit the amount of oil in circulation. It is
designed to separate a major portion of the oil from the refrigerant as it is
pumped from the compressor. As the refrigerant-oil mixture is discharged from
the compressors to the discharger header, it travels to the oil separator. The
velocity of this mixture is slowed down by the use of internal baffles and
impingement screens. Slowing down causes a major portion of the oil to drop out
of the mixture and fall to the bottom of the oil separator.
At the bottom of the oil separator is a float assembly connected directly back
to the oil reservoir. As the oil level at the bottom of the separator
increases, the float will cause a valve to open and send oil to the reservoir.
The purpose of the reservoir is to store excessive oil not currently needed by
To limit the pressure of the oil and vapor in the reservoir, a copper line with
a spring-load check valve is attached between the top of the reservoir and the
suction header. This check valve has a 20 psi setting, and will permit the
high-pressure vapor coming from the separator to the reservoir to bleed off to
20 psi above the suction pressure. From the oil reservoir another refrigerant
line is connected to each oil level float assembly directly connected to each
compressor. Each compressor has its own float assembly which feeds oil to the
compressor as needed to maintain a proper oil level. The 20 psig difference in
pressure will cause the oil to flow from the float assembly into the compressor’s
crankcase when the float opens. This control will make up for oil that is not
returning through the suction manifold.
On a properly balanced system, a large percentage of oil returns to the
compressor through the suction line. The oil level control will make up for
moderate differences in compressor oil pumping rates due to size or wear,
different oil return rates due to turbulence in the suction manifold and for
compressors which are cycling off and on at different rates.
Ice Breaker: Oil Management
February 7, 2011