EP2182305A1

EP2182305A1 - Method for controlling the distribution of lubricating oil in a compressor unit and compressor unit

Info

Publication number: EP2182305A1
Application number: EP08425703A
Authority: EP
Inventors: Andrea Verondini
Original assignee: Zanotti SpA
Current assignee: Zanotti SpA
Priority date: 2008-10-31
Filing date: 2008-10-31
Publication date: 2010-05-05

Abstract

A method for controlling the distribution of lubricating oil in a compression unit (1), which comprises at least one first and a second compression stage (10, 20), wherein the first and second compression stage (10, 20) respectively comprise at least one compressor (11, 21) provided with an oil sump (110, 210) for the lubricating oil and in which the second compression stage (20) is connected in series to the first compression stage (21), comprises the steps of:
- picking up lubricating oil from the oil sump (210) of the at least one compressor (21) of the second compression stage (20);
- feeding the picked up lubricating oil to the oil sump (110) of the at least one compressor (11) of the first compression stage (10).

A compression unit (1) for carrying out such a method is also described.

Description

The present invention refers to a method for controlling the distribution of lubricating oil in a compression unit, said unit having a preferred, although not exclusive, use in refrigerating units.
The invention also refers to a compression unit for carrying out such a method.
It is known that inside a compressor there has to be a certain amount of lubricating oil, typically received in a suitable oil sump, to ensure the necessary lubrication of the moving parts. For the compressor to operate correctly, the level of the lubricating oil in the oil sump must remain within predetermined values, dependent upon the type geometry of the compressor and usually specified by the manufacturer. If the level falls below a predetermined minimum value seizure may happen. If, on the other hand, the level exceeds the predetermined maximum value, there can be so-called flooding of the oil sump, in which lubricating oil enters into the cylinders in an amount above that foreseen by the design conditions and it can therefore, due to its low compressibility, cause damage like breaking of the valves and/or of the connecting rods.
Changes of the level of the lubricating oil in the oil sump of a compressor can of course occur during its operation and therefore it is necessary to ensure that such changes are in any case contained within the limits indicated by the manufacturer.
In particular, in the case of compressors used in refrigerating units the lubricating oil tends to mix with the coolant fluid subjected to compression. The transportation of lubricating oil by the coolant fluid, essentially proportional to the mass flow rate of the coolant fluid, on the one hand causes the level of the lubricating oil in the oil sump of the compressor to decrease, and on the other hand it causes the volumetric efficiency of a predetermined mass of coolant fluid to decrease, since the lubricating oil mixed in it does not produce a refrigerating effect.
In order to limit the migration of the lubricating oil from the compressor to the entire refrigerating circuit, it is known to arrange a separation device of the lubricating oil downstream of the compressor suitable for separating lubricating oil from the coolant fluid and for sending the newly separated lubricating oil to the compressor.
The compression units used in refrigerating units, above all those of medium-large size, often comprise many compression stages connected in series, each of which comprises one or more compressors. This makes it possible to respond with greater flexibility to variations in the refrigerating load and thus in the flow rate of coolant fluid that must be treated by the compression unit. Typically, in multi-stage compression units an intermediate cooling of the coolant fluid is arranged between the compression stages, through which, for the same compression ratio, it is possible to lower the temperature of the coolant fluid at the discharge of the compression unit. This makes it possible, on the one hand, to increase the coefficient of performance (COP) of the refrigerating cycle, and on the other hand to keep the temperatures at the end of compression within values prescribed by the producers of lubricating oils even in the case of high compression ratios. Compression units with two or more compression stages in series and intermediate cooling are also known by the term: compound compression unit.
In compression units with many compression stages the problem of controlling the distribution of the lubricating oil between the compressors is even greater. Indeed, due to the connection in series of the compressors and the transportation of the lubricating oil by the coolant fluid, there is typically a decrease in lubricating oil in the oil sumps of the compressors of the initial compression stages and an increase in the lubricating oil in the oil sumps of the compressors of the final compression stages. Added to this, there may be further unbalances in the distribution of the lubricating oil due to the fact that the various compression stages must sometimes operate with different flow rates of coolant fluid to satisfy loads with different cooling requirements from one to the next (for example due to different operating temperatures, different volumes to be cooled or, furthermore, different external environmental conditions).
The known methods for controlling the distribution of the lubricating oil in compression units with many compression stages substantially follow what is done in the case in units with a single compressor. In particular, according to a known method a separation device of the lubricating oil is arranged downstream of each compression stage and the lubricating oil separated through each separation device is sent to the respective compression stage. According to an alternative known method, on the other hand, a single separation device of the lubricating oil is used downstream of the compression unit and the lubricating oil separated through this separation device is sent in parallel to all of the compression stages of the compression unit.
The aforementioned methods do not ensure effective control of the distribution of the lubricating oil in the compression unit and in particular they do not provide a sufficient safeguard against possible flooding of the oil sumps of the compressors of the high-pressure stages. Such flooding is in practice avoided through periodic inspection of the compressors and possible manual bleeding of the excess lubricating oil from the oil sumps.
The technical problem forming the basis of the present invention is to provide a method for controlling the distribution of lubricating oil in a compression unit and a compression unit for carrying out such a method, through which the level of lubricating oil in the oil sumps of the compressors can be kept within an admissible range for the correct operation of the compressors themselves - avoiding, in particular, flooding of the oil sumps -, irrespective of the operating conditions of the compression unit and without the need for manual interventions from the outside.
The present invention therefore concerns a method for controlling the distribution of the lubricating oil in a compression unit according to claim 1 and a compression unit suitable for carrying out such a method according to claim 7. Preferred characteristics of the method and of the compression unit are indicated in the respective dependent claims.
In particular, in a first aspect thereof, the invention concerns a method for controlling the distribution of lubricating oil in a compression unit comprising at least one first and a second compression stage, wherein said first and second compression stage respectively comprise at least one compressor provided with an oil sump for the lubricating oil and wherein the second compression stage is connected in series to the first compression stage, said method comprising the steps of:

picking up lubricating oil from the oil sump of the at least one compressor of the second compression stage;
feeding the picked up lubricating oil to the oil sump of the at least one compressor of the first compression stage.

The method of the invention advantageously makes it possible to effectively control the distribution of the lubricating oil in the compression unit in any operating condition thanks to the fact that lubricating oil is picked up from one compression stage and fed to the immediately preceding stage, i.e. the very compression stage from which the compression stage in which the pick-up is carried out receives lubricating oil transported together with the operative fluid, in particular a coolant fluid. This makes it possible to simultaneously avoid both the occurrence of flooding of the oil sumps in the final (high-pressure) compression stages, and the occurrence of conditions of absence or low level of the lubricating oil in the initial (low-pressure) compression stages. In this way, therefore, the unbalances in distribution of lubricating oil between the various compression stages of the compression unit, which, as explained above, inevitably occur in many operating conditions of the compression unit, are counteracted in a simple way and without the need for interventions from the outside.
Preferably, the second compression stage comprises a plurality of compressors connected together in parallel and the method of the invention comprises:

placing the oil sumps of the compressors of said plurality of compressors in fluid communication through a common manifold, and
in said pick-up step, picking up the lubricating oil from said common manifold.

Preferably, the step of placing the oil sumps in fluid communication comprises equalizing the pressures present in them.
This advantageously allows the levels of the lubricating oil in the oil sumps of the compressors of the second compression stage to be equalised and thus ensures that the pick-up of lubricating oil in a single point at the common manifold has the same effect in all of the oil sumps.
Preferably, the method of the invention also comprises the step of:

detecting the level of the lubricating oil in the oil sump of the at least one compressor of the first compression stage,
and the aforementioned feeding step is carried out when the detected level is below a predetermined level.

The pick-up of the lubricating oil from the second compression stage and the feeding to the first compression stage can in this way be advantageously carried out automatically, based upon an easily detectable magnitude that can be used to control the opening/closing of induction valves of the lubricating oil in the oil sump of the at least one compressor of the first compression stage.
In a preferred embodiment thereof, the method of the invention also comprises the steps of:

at least partially separating the lubricating oil from an operative fluid compressed through said compression unit downstream of said second compression stage, and
feeding the separated lubricating oil to the oil sump of the at least one compressor of said second compression stage.

Preferably, in such a preferred embodiment, the method of the invention also comprises the step of:

detecting the level of the lubricating oil in the oil sump of the at least one compressor of the second compression stage,
and the step of feeding the separated lubricating oil is carried out if the detected level is below a predetermined level.

Even in this case the feeding of the separated lubricating oil can be carried out automatically, based upon an easily detectable magnitude that can be used to control the opening/closing of induction valves of the lubricating oil of said oil sump.
In a second aspect thereof, the invention concerns a compression unit comprising at least one first and a second compression stage, wherein said first and second compression stage respectively comprise at least one compressor having an oil sump for lubricating oil and said second compression stage is connected in series to the first compression stage, characterised in that it comprises at least one duct extending between an outlet opening in the oil sump of the at least one compressor of said second compression stage and an inlet opening in the oil sump of the at least one compressor of said first compression stage, said at least one duct being suitable for placing said oil sumps in direct fluid communication.
Such a compression unit allows the method of the invention described above to be carried out and therefore allows all of the advantages in terms of the possibility of controlling the distribution of the lubricating oil in the compression unit to be obtained. In particular, thanks to the aforementioned characteristics it is possible to keep a correct level of lubricating oil in the oil sumps of the compressor in any operating condition of the compression unit in a simple way and without the need for interventions from the outside.
Preferably, second compression stage comprises a plurality of compressors connected together in parallel and having respective oil sumps for the lubricating oil in fluid communication with each other through a common manifold, and said at least one duct extends between an outlet opening in the common manifold and an inlet opening in the oil sump of the at least one compressor of said first compression stage.
The common manifold is preferably associated with the oil sumps in a position such as to allow the pressures present in them to be equalised.
Advantageously, it is thus possible through a single component both to place the oil sumps in fluid communication, and to ensure that, by the communicating vessels principle, in the common manifold there is a single level of the lubricating oil.
However, this does not rule out design solutions in which the equalisation of the pressures is obtained through a connection between the oil sumps that is distinct and in addition to the aforementioned common manifold.
In a preferred embodiment, the compression unit of the invention comprises heat exchange means arranged between the first and the second compression stage to cool an operative fluid compressed through the compression unit during the compression.
Such heat exchange means advantageously allow a compression with intermediate cooling to be carried out.
Preferably, the compression unit comprises a separation device suitable for at least partially separating the lubricating oil from an operative fluid compressed through the compression unit, said separation device comprising an oil separator arranged downstream of the second compression stage.
Preferably, the separation device is in fluid communication with the oil sump of the at least one compressor of the second compression unit.
In a third aspect thereof, the invention concerns a refrigeration unit comprising a compression unit with the characteristics described above.
Further characteristics and advantages of the present invention shall become clearer from the following detailed description of preferred embodiments thereof, given hereafter with reference to the attached drawings. In such drawings:

figure 1 is a block diagram of a preferred embodiment of the compression unit of the invention;
figure 2 is a block diagram of an alternative preferred embodiment of the compression unit of figure 1;
figure 3 is a schematic side view partially in section of a compressor of the second compression stage of the compression unit of figure 1, and
figure 4 is a block diagram of a refrigerating unit comprising the compression unit of figure 1.
In such figures a compression unit according to the invention is wholly indicated with reference numeral 1. Moreover, the continuous connection lines indicate the path of the lubricating oil, whereas the broken connection lines indicate the path of an operative fluid compressed by the compression unit 1, for example a coolant fluid.
In the embodiment shown here the compression unit 1 comprises a first compression stage 10 (low-pressure stage) and a second compression stage 20 (high-pressure stage), connected in series to the first compression stage 10. Each compression stage 10, 20 comprises at least one respective compressor. In the example shown here, the first compression stage 10 comprises two compressors 11 connected together in parallel, whereas the second compression stage 20 comprises three compressors 21 connected together in parallel.
The compressors 11 and 21 comprise respective oil sumps 110, 210 for the lubricating oil. The specific type and geometry of the compressors 11 and 21 is not relevant for the purposes of the invention. They can, for example, be of the hermetic, semi-hermetic or open type, and, in relation to the way in which the compression is carried out, they can be of the piston, screw or scroll type, and they are in any case know to the man skilled in the art.
Each oil sump 110 or 210 is provided with an inlet opening with an induction valve 111, 211 to enter lubricating oil in it from the outside. The induction valves 111, 211 can be actuated mechanically, electromechanically or electronically and are preferably slaved to level sensors (not shown) suitable for detecting the level of lubricating oil present in the respective oil sump 110 or 210.
The oil sumps 210 of the compressors 21 of the second compression stage 20 are preferably placed in fluid communication with each other through a common manifold 22. The position, and in particular the height, of the common manifold 22 with respect to the compressors 21 is selected so that, as well as the fluid communication, there is also equalisation of the pressures present in the oil sumps 210, and therefore, by the communicating vessels principle, equalisation of the levels of lubricating oil. As can be seen in particular in figure 3, in a preferred embodiment the common manifold 22 is positioned so that a middle line of its cross section is at the same height as a "correct level" reference 212 of the lubricating oil in the sumps 210, a reference that is normally indicated by the manufacturer on the oil sump of a compressor.
In an alternative embodiment of the compression unit 1, not shown, the oil sumps 110 of the compressors 11 of the first compression stage 10 can also be placed in fluid communication with each other through a common manifold, as described above regarding compressors 21 of the second compression stage 20.
In accordance with the invention, the compression unit 1 comprises a duct 2 extending between an outlet opening in the common manifold 21 of the second compression stage 20 and the induction valves 111 at the inlet openings in the oil sump 110 of the compressors 11 of the first compression stage 10. The duct 2 places the common manifold 22, and therefore the oil sumps 210 of the compressors 21, in direct fluid communication - and, in particular, without passing through the compressors 11 and 21
with the oil sumps 110 of the compressors 11. The passage of lubricating oil from the oil sumps 210 to the oil sumps 110 depends upon the state of the induction valves 111, and therefore upon the level of oil detected in the oil sumps 110. When the induction valves 111 are open, the flow of lubricating oil occurs by pressure difference between the common manifold 22 and the oil sumps 110.

In the preferred embodiments illustrated here the compression unit 1 also comprises a separation device 3 suitable for separating the lubricating oil from a compressed fluid with which it has mixed during the compression and is therefore transported away from the compressors 11, 21. The separation device 3 preferably comprises at least one oil separator 30 arranged downstream of the second compression stage 20. The separation device 3 is in fluid communication with the oil sumps 210 of the compressors 21 to feed the separated lubricating oil to them. The flow of lubricating oil from the separation device 3 to the oil sumps 210 is controlled by the induction valves 211, and therefore by the level of lubricating oil in the oil sumps 211 and in the common manifold 22. When the induction valves 211 are open, the flow of lubricating oil occurs by pressure difference.
In a preferred embodiment of the compression unit 1, shown in figure 1, the separation device 3 comprises a collection tank 31 for the separated lubricating oil integrated in the oil separator 30. In this case the pressure difference that makes the separated lubricating oil flow out from the collection tank 31 to the oil sumps 210 is substantially equal to the increase in pressure determined in the fluid compressed through the second compression stage 20.
In an alternative preferred embodiment, shown in figure 2, the separation device 3 comprises a collection tank 31 for the separated lubricating oil outside of the oil separator 30. In this case a differential pressure valve 32 is preferably arranged to keep the collection tank 31 at a pressure slightly above that in the oil sumps 210 and in the common manifold 22. The dotted and dashed lines in figure 2 indicate the connections of the differential pressure valve 32 for detecting the pressures.
In the preferred embodiments of the compression unit 1 shown here, each oil sump 210 is provided with an inlet opening with respective induction valve 211 and the separation device 3 is in fluid communication with each oil sump 210. This constructive solution has a substantial reliability with respect to malfunctions of the induction valves 211 or of the associated actuators and level sensors, since it ensures that in the case of malfunction of one of such induction valves 211 the separated lubricating oil can in any case be fed to all of the oil sumps 210, possibly subject to passage through the common manifold 22. However, should a simpler and more cost-effective constructive solution be desired, in part at the expensive of reliability, it is possible to foresee alternative embodiments in which the oil sumps 210 of the compressors 21 have no inlet openings and relative induction valves to enter lubricating oil from the outside and a single inlet opening is foreseen with relative induction valve for the lubricating oil at the common manifold 22.
The compression unit 1 is preferably a so-called compound compression unit, i.e. it also comprises heat exchange means 4 arranged between the first compression stage 10 and the second compression stage 20, to make compression with intermediate cooling possible. The heat exchange means 4 are of the conventional type and their characteristics and performances can be easily selected by a man skilled in the art based upon the specific application requirements of the compression unit 1. A preferred embodiment of the heat exchange means 4 shall be described hereafter with reference to the use of the compression unit 1 in a refrigeration unit (figure 4).
Figure 4 illustrates a preferred, although not exclusive, use of the compression unit 1 in a refrigeration unit 100. The block diagram shown in such a figure refers, as an example, to a typical refrigerating unit for the conservation of fresh and frozen foods in commercial activities.
As well as the compression unit 1 described above, the refrigeration unit 100 essentially comprises a condensation unit 5 to condense the compressed coolant fluid, a collection tank 6 for the condensed coolant fluid, a plurality of units 7 for the conservation of fresh foods connected together in parallel and a plurality of units 8 for the conservation of frozen foods. The units 7 and 8 can for example be refrigerated cases for conserving and displaying the aforementioned foods. Each unit 7 and 8 comprises a respective expansion element (not illustrated explicitly and in any case of the conventional type), suitable for laminating a certain mass flow rate of coolant fluid taking it to the desired pressure for evaporation in the respective unit 7 or 8.
The coolant fluid evaporated in the units 7 for the conservation of fresh products goes back in vapour state to the compression unit 1 upstream of the second compression stage 20 and the heat exchange means 4, whereas the coolant fluid evaporated in the units 8 for the conservation of frozen foods goes back in vapour state to the compression unit at the intake of the first compression stage 10.
Part of the condensed coolant fluid is also sent to the heat exchange means 4 of the compression unit 1 to carry out the intermediate cooling between the first and the second compression stage 10, 20.
In the preferred embodiment shown here, the heat exchange means 4 are suitable for carrying out both the intermediate cooling of the coolant fluid in the compression step, and a sub-cooling of the condensed coolant fluid intended to feed the units 8 for the conservation of frozen foods. They comprise a tank 43, preferably consisting of a so-called anti-liquid bottle, provided on the inside with one or more ducts 44 for the passage of the condensed coolant fluid.
The intermediate cooling is carried out through controlled mixing in the tank 43 between coolant fluid in the compression step and another coolant fluid in vapour state, obtained through lamination of a flow rate of the condensed coolant fluid. The pick-up of the condensed refrigerated fluid from the collection tank 6 is controlled through an on-off valve 41, for example a solenoid valve. The dosaging and the lamination to take the condensed coolant fluid to the desired pressure and temperature conditions for the intermediate cooling are preferably carried out through a thermal expansion valve 42 with fixed point temperature adjustment, which detects the temperature of the coolant fluid at the intake of the second compression stage 20.
The sub-cooling is carried out by making condensed coolant fluid pass into the duct(s) 44, and then carrying out a heat exchange with the coolant fluid in vapour state that is located in the anti-liquid bottle 43, before sending it to the units 8 for the conservation of frozen foods.
It is clear that in the refrigeration unit 100 the compression stages 10 and 20 of the compression unit 1 generally operate with different flow rates of coolant fluid, due to the different cooling requirements of the units 7 and 8. Of course, deriving from this there is an unbalance in the distribution of the lubricating oil between the oil sumps 110 and 210, with potential negative consequences on the operation of the compressors 11 and 21, as explained above. Typically, there is a decrease in the lubricating oil in the oil sumps 110 of the compressors 11 of the first compression stage 10 and an increase in the lubricating oil in the oil sumps 210 of the compressors 21 of the second compression stage 20.
In the compression unit 1 the aforementioned unbalance of distribution of the lubricating oil can be controlled in a simple manner, irrespective of the operating conditions of the compression unit 1 and without the need for interventions from the outside through the method of the invention for controlling the distribution of lubricating oil in compression units. A preferred embodiment of such a method shall now be described with particular reference to the embodiment of the compression unit 1 ' shown in figure 1.
The method essentially foresees the steps of picking up lubricating oil from the oil sumps 210 of the compressors 21 of the second compression stage 20, and of feeding the picked up lubricating oil to the oil sumps 110 of the compressors 11 of the first compression stage 10. This is obtained by means of the duct 2 described above.
If, like in the embodiment shown here, the second compression stage 20 comprises more than one compressor 21, preferably the method also comprises the step of placing the oil sumps 211 of the compressors 21 in fluid communication, preferably so as to obtain an equalisation of the pressures in the oil sumps. This is obtained through the common manifold 22 described above. In this case, the pick-up of the lubricating oil preferably takes place at the common manifold 22.
Preferably, the method also comprises the step of detecting the level of lubricating oil in the oil sumps 110 of the compressors 21 of the first compression stage 10, and foresees carrying out the aforementioned step of feeding the lubricating oil picked up from the second compression stage 20 when the level detected in at least one of the oil sumps 110 is below a predetermined level. This is carried out automatically through the induction valves 111 and the associated level sensors described above.
Preferably, the method also comprises the steps of at least partially separating the lubricating oil from the compressed fluid downstream of the second compression stage 20, and of feeding the separated lubricating oil to the oil sumps 210 of the compressors 21 of the second stage 20. Also in this case, there is preferably a step of detecting the level of the lubricating oil in the oil sumps 210, i.e. in the common manifold 22 when provided, and the step of feeding the separated lubricating oil is carried out when the detected level is below a predetermined level. This is carried out automatically through the induction valves 211 and the associated level sensors described above.
In an alternative embodiment of the method, the feeding of the separated lubricating oil in the second compression stage can take place at the common manifold 22 instead of at the oil sumps 210.
If necessary, it is also possible to foresee a cooling step of the separated lubricating oil before it is fed into the oil sumps 210 or into the common manifold 22.
In the detailed description above, for the sake of simplicity we have referred to a compression unit 1 comprising two compression stages. A man skilled in the art will recognise that what has been described and illustrated with reference to this embodiment can easily be transferred to cases in which the compression unit 1 comprises more than two compression stages. In these cases, in accordance with the basic principles of the invention, it shall be ensured that the oil sumps of the compressors of a generic intermediate compression stage receive lubricating oil from the oil sumps (or, preferably, from a common manifold associated with them) of the compressors of the compression stage immediately downstream and, in turn, they feed lubricating oil (preferably from a respective common manifold) to the oil sumps of the compressors of the compression stage immediately upstream.

Claims

Method for controlling the distribution of lubricating oil in a compression unit (1) comprising at least one first and a second compression stage (10, 20), wherein said first and second compression stage (10, 20) respectively comprise at least one compressor (11, 21) provided with an oil sump (110, 210) for the lubricating oil and wherein said second compression stage (20) is connected in series to the first compression stage (21), said method comprising the steps of:
- picking up lubricating oil from the oil sump (210) of the at least one compressor (21) of said second compression stage (20);

- feeding the lubricating oil that was picked up to the oil sump (110) of the at least one compressor (11) of said first compression stage (10).
Method according to claim 1, wherein said second compression stage (20) comprises a plurality of compressors (21) connected together in parallel, comprising:
- placing the oil sumps (210) of the compressors (21) of said plurality of compressors in fluid communication through a common manifold (22), and

- in said pick-up step, performing the picking up of the lubricating oil from said common manifold (22).
Method according to claim 2, wherein said step of placing the oil sumps (210) in fluid communication comprises equalising the pressures present in said oil sump (210).
Method according to any one of the previous claims also comprising the step of:
- detecting the level of the lubricating oil in the oil sump (110) of the at least one compressor (11) of said first compression stage (10),
wherein said feeding step is carried out when the detected level is below a predetermined level.
Method according to any one of the previous claims, also comprising the steps of:
- at least partially separating the lubricating oil from a operative fluid compressed through said compression unit (1) downstream of said second compression stage (20), and

- feeding the separated lubricating oil to the oil sump (210) of the at least one compressor (21) of said second compression stage (20).
Method according to claim 5, also comprising the step of:
- detecting the level of the lubricating oil in the oil sump (210) of the at least one compressor (21) of said second compression stage (20),
wherein said step of feeding the separated lubricating oil is carried out if the detected level is below a predetermined level.
Compression unit (1) comprising at least one first and a second compression stage (10, 20), wherein said first and second compression stage (10, 20) respectively comprise at least one compressor (11, 21) having an oil sump (110, 210) for lubricating oil and
wherein said second compression stage (20) is connected in series to the first compression stage (10), characterised in that it comprises at least one duct (2) extending between an outlet opening in the oil sump (210) of the at least one compressor (21) of said second compression stage (20) and an inlet opening in the oil sump (110) of the at least one compressor (11) of said first compression stage (10), said at least one duct (2) being suitable for placing said oil sumps (210, 110) in direct fluid communication.
Compression unit (1) according to claim 7, wherein said second compression stage (20) comprises a plurality of compressors (21) connected together in parallel and having respective oil sumps (210) for the lubricating oil in fluid communication with each other through a common manifold (22), and wherein said at least one duct (2) extends between an outlet opening in said common manifold (22) and an inlet opening in the oil sump (110) of the at least one compressor (11) of said first compression stage (10).
Compression unit (1) according to claim 8, wherein said common manifold (22) is associated with said oil sump (210) in a position such as to allow the pressures present in said oil sump (210) to be equalised.
Compression unit (1) according to any one of claims 7 to 9, comprising heat exchange means (4) arranged between said first and said second compression stage (10, 20) to cool an operative fluid compressed through the compression unit (1) during the compression.
Compression unit (1) according to any one of claims 7 to 10, comprising a separation device (3) suitable for at least partially separating the lubricating oil from an operative fluid compressed through said compression unit (1), said separation device (3) comprising an oil separator (30) arranged downstream of said second compression stage (20).
Compression unit (1) according to claim 11, wherein said separation device (3) is in fluid communication with the oil sump (210) of the at least one compressor (21) of the second compression unit (20).
Refrigerating unit (100) comprising a compression unit (1) according to any one of claims 7 to 12.