Bitzer KT-600-4 Manual Del Usuario página 43

The oil distribution to the compressors (controllers) takes place from an oil tank. It must have a minimum volume to
compensate for variations in the oil balance (design according to manufacturer's specifications). Two versions are
on offer:
Low-pressure reservoir as a separate vessel ( see figure 9, page 42 , top half)
• The oil from the separator is fed into the tank under high pressure. There, a controlled pressure reduction takes
place via a degassing line to the suction header. A differential pressure valve (1.4 bar) in the connecting line en-
sures sufficient excess pressure upstream of the oil level controller for "standard refrigerants".
• In 2-stage compressors, the crankcases are under intermediate pressure. The degassing line must therefore for
routed to a transverse connecting line between the connections at the motor case cover (see dimensional draw-
ings in Operating instructions KB-150 or BITZER Software) – level of the connecting line below the connecting
position.
NOTICE
!
!
Mechanical oil level controllers may only be suitable for relatively small pressure differences!
Select a design for a working pressure of at least 6.5 bar.
• Systems for transcritical operation with R744 require oil separators with coalescence filter cartridges. The oil
separators must not be equipped with a float valve. If necessary, the (electronic / optical) level monitoring in the
oil separator opens a solenoid valve in the line to the oil tank. The volume of the oil tank must be twice as large
as the entire oil volume of the compressors. The pressure in the tank must be higher than the highest suction
pressure in the system so that the oil can flow back into the compressors. Degassing is therefore routed into the
corresponding suction gas line.
In systems with parallel compression, degassing always takes place at intermediate pressure level and not, for
example, at the suction pressure level of compressors for medium temperature application. In the degassing
line, discharge gas valves must preferably be used. The required pressure difference depends on the boundary
conditions, pressure differences between 2.5 and 4.5 bar are normally necessary.
In so-called booster systems, the low pressure compressors are supplied with oil from the common tank. This
can result in the excess pressure upstream of the oil level controller being clearly higher than the values permit-
ted for mechanical controllers. This is why, in general, only electronic oil level controllers equipped with integ-
rated solenoid valves that are suitable for the high differential pressures of R744 systems should be used.
To adjust the oil management, the following points must be observed and harmonized with each other:
– Nozzle cross-sections of the oil level controllers used in the respective compressor stage
– Opening times for oil replenishment of the oil level controllers used
– Provided oil volume in the tank
– Nominal maximum pressure difference in the tank
– Length and cross-section of the oil lines from the tank to the oil level controllers
High-pressure reservoir as additional oil reservoir in the lower part of the oil separator ( see figure 9, page 42 , bot-
tom half)
• The oil separator has been designed here without float valve, the oil line leading directly to the oil level control-
lers.
• Only electronic oil level controllers with integrated solenoid valves are suitable for these systems. They must be
suitable for operation with full pressure difference. The nozzles used for limiting the oil flow are adjusted as a
function of the pressure difference according to manufacturer's specifications. This is extremely important be-
cause a large amount of refrigerant is dissolved in the oil due to the high upstream pressure. If pressure is sub-
sequently reduced to the level of the crankcase, the refrigerant becomes gaseous again. If the continuous feed
rate is too high, considerable foaming will occur, resulting in liquid slugging, increased oil carry over, insufficient
oil level control and lack of oil.
KT-600-4 // PDF Download 43