Oil Management; Oil And Gas Equalisation Between Crankcases; Parallel Compounding With Optimized Suction Header - Bitzer KT-600-4 Manuel D'installation

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Oil management

4.1 Oil and gas equalisation between crankcases

For compressors with small to medium capacity, previously oil and gas equalisation between crankcases was fre-
quently selected. This is a low-cost option, but has decisive drawbacks: Due to the pipe joint between the crank-
cases, suction gas (in the bypass) will keep flowing also through shut-off compressors. Under unfavourable condi-
tions, this may lead to the migration of oil into compressors that are in operation. This can result in faults to the oil
balance and to damage to the compressor ‒ a pressure differential of as little as 0.01 bar corresponds to an oil
level difference of approximately 11 cm. This is why in particular the methods described below are recommended
( see chapter Parallel compounding with optimized suction header, page 35 , see chapter Parallel compounding
with oil level controllers, page 41 ).
BITZER compressors of series BE5 and BE6 (4JE-13 .. 6FE-50) can be designed with separate, generously sized
oil and gas equalising lines between the crankcases. Suitable connections are available as standard (see operating
instructions KB-104 ) and suitable adaptors are available as accessories. The connecting lines should be kept as
short as possible. Deviations from the exactly straight horizontal connection by slight bends are allowed in the gas
equalising line only upwards and in the oil equalising line only downwards. However, in view of the remarks made
above, this version, too, is rather suitable for systems which (on the basis of practical experience and tests) are
built either as standard or at least repeatedly.

4.2 Parallel compounding with optimized suction header

This version was previously used in particular in factory-made liquid chillers or specially tested systems. Although
this prevents a direct oil migration between compressors, uniform oil distribution requires, however, special meas-
ures and requirements.
The internal oil recirculation system of the compressors has been designed such that the oil carry over changes as
a function of the oil level in the crankcase. In connection with an optimized suction header and an intelligent com-
pressor rack controller, this leads to a self-regulation effect.
General requirements and recommendations
• The concept is applicable to systems with
– ≤ 4 compressors of equal capacity and without capacity control
– identical lubrication system for the compressors and monitoring of the oil supply by an oil level switch (centri-
fugal lubrication) or oil pressure limiter (pump lubrication). Compressors with centrifugal and pump lubrication
may only be combined when designed with oil level controllers ( see chapter Parallel compounding with oil
level controllers, page 41 ).
• Load sequence switching at a rate of max. 1 h is required to ensure that the temperature of shut-off compressors
does not drop too much. Ideally, the cylinder heads should not be cooled below the condensing temperature, in
order to avoid back condensation. A higher temperature also reduces the refrigerant concentration in oil, thus re-
ducing oil carry over during the compressor start.
• System design / Pipe works: The prerequisite for safe operation is a uniform oil transport in the system during
full load and part load. To ensure this, the pipe works must be carefully designed with respect to the minimum
flow velocities.
• System control: An intelligent control logic should be used which guarantees an automatic sequence change and
uniformly long compressor running times at minimum switching frequency.
In case of deviating specification or for widely branched pipe works with critical oil balance, systems with oil level
controllers are recommended ( see chapter Parallel compounding with oil level controllers, page 41 ). Exceptions
are possible in case of tested concepts for the application in question.
KT-600-4 // PDF Download 35