Danfoss MLM116 Application Manuallines page 43

Application Guidelines Oil management concept
This is one of the simplest and cheapest ways of
manifolding compressors. It is very popular in air
conditioning applications, but in refrigeration this
kind of system needs to be paid special attention
due to severe operating conditions.
Danfoss has qualified only the even tandem
(two same compressors) for passive solutions.
Compressor sumps and low-pressure shells are
interconnected. An interconnecting pipe on
the lower part of the compressor (installed on
the oil equalization port) ensures oil balancing.
The suction header design is critical, as it
ensures a pressure drop balancing and an equal
distribution of oil returning from the system
when both compressors are running.
The success of such a system relies very much
on the sizing and design of the pipe work, as
small differences in sump pressure can result in
significant oil level variations. This system needs
perfect suction tube balancing.
For an MLM/MLZ116 even tandem, oil return
can be secured by an oil separator with float
ball valve, which will return the oil to the main
suction line. The oil equalization line goes
through the oil equalization port with an adaptor
on the compressor. To avoid refrigerant back flow
from high pressure, it is always recommended to
have a non-return valve on the discharge line of
each compressor, as well as one non-return valve
on the outlet of the oil separator in the system to
prevent refrigerant migration.
The following are recommendations from
Danfoss application engineering using an oil
equalization line system without active control:
Danfoss could provide piping drawings for even
tandem passive system, please contact Danfoss
for more information.
Manifold compressors
• An adequately (generously) sized suction
header is needed to provide for equal
distribution of returning refrigerant gas and oil
to each individual compressor; also the suction
header should be installed horizontally.
• To secure sufficient oil return to the compressor,
Danfoss suggests below dimension requirement
of the oil injection position: H>5D or H>111 mm.
Main suction line
D
Oil injection port
H
• The oil equalization tube is recommended to
have an outer diameter of 1"3/8.
• If the unit runs in a very cold situation, both
compressors need to be switched on after
a period of single running for better oil
circulation, especially in low load conditions.
• Care must be taken to mount all the
compressors on the same horizontal level and
also to provide adequate liquid flood back
protection when using this method.
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FRCC.PC.035.A4.02