ES2600474T3 - Thermodynamic installation with improved lubrication - Google Patents

Abstract

Thermodynamic installation comprising a first compressor (1, C1), a second compressor (2, C2), a multi-way circuit for the circulation of a refrigerant fluid that unites them, lubricant receptacles (100, 200) designed to receive a lubrication fluid of said compressors, selection means (3; 50, 51, 52) that allow operation of the installation according to at least one of the following three configurations: - a) with the refrigerant fluid passing through the first compressor without pass through the second compressor, - b) with the refrigerant fluid passing through the second compressor without passing through the first compressor, - c) with the two compressors operating in series, the second compressor then being arranged downstream of the first compressor in the direction of circulation of the refrigerant fluid, locating the, or some of said selection means, which are adapted to create a pressure drop, in the circuit of the refrigerant fluid: * upstream of the second compressor, in the configuration in which the refrigerant fluid passes through it without passing through the first compressor, * and between said first compressor and said second compressor, when they work in series, characterized in that it comprises means (40, 55 , 56, 57, 59) for selective circulation of the lubricant comprising a conduit that joins said receptacles and in which a valve means (55) is interposed, managed by a regulator (40), to selectively open and close the circulation of the lubricant between the compressors and to allow a circulation of the lubricant from the receptacle (100) of the first compressor to that (200) of the second in a situation in which, operating the installation according to configuration b), the pressure drop of said means of selection(3; 50, 51, 52) creates a pressure difference between said lubricant receptacles.

Description

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DESCRIPTION

Thermodynamic installation with improved lubrication

The present invention relates to a thermodynamic installation for refrigerant and its operating procedure.

From document FR-A-2889733, an installation of this type is known which presents the characteristics of the preamble of claim 1, comprising a first compressor and a second compressor, a circuit of several ways of circulation of a cooling fluid, lubricant receptacles in order to receive a lubrication fluid from said compressors and selection means that allow an operation of the installation according to some at least of the following three configurations, and preferably these three possible configurations:

- with the cooling fluid that passes through the first compressor without passing through the second compressor;

- with the refrigerant flowing through the second compressor without passing through the first compressor;

- with the two compressors operating in series, the second compressor then being arranged downstream of the first compressor according to the direction of circulation of the cooling fluid.

During the operation of said two-stage thermodynamic system, the refrigerant fluid often carries lubricant (eg lubrication oil) for the compressors.

It is wanted to avoid that this alters the operation of the installation.

An objective of the invention is to avoid this. This objective is achieved with an installation having the characteristics of claim 1 and a method according to claim 8.

Thus, it is proposed that, in an operating situation of the installation such that there is a pressure difference between the lubricant receptacles of said first compressor and said second compressor, lubricant is circulated between the (receptacles of the) compressors , a priori outside said circulation circuit of the refrigerant fluid.

To guarantee this "under pressure" circulation of the lubricant, in a technically efficient and safe way, said selective circulation means comprise:

- a specific conduit that joins together the lubricant receptacles of the first compressor and the second compressor;

- and a valve means interposed at a point of this conduit to selectively open and close the circulation of the lubricant between the compressors.

A consequent problem also arises in terms of how to guarantee the type of valve means that must be used so that lubricant balancing can be performed efficiently and simply at least in the second configuration and in the third configuration mentioned above.

For this, it is proposed to use a medium with adapted shutter:

- in a closed state, to establish a sealing tightness that opposes the circulation of the lubricant between the compressors so much stronger as the pressure of the lubricant of the second compressor is high with respect to that of the first compressor; Y

- to be opened by drive and to remain open regardless of the difference in lubricant pressures established between the compressors.

By means of the solution of the invention, it will be avoided that if during the operation of the installation the cooling fluid carries lubricant, this causes an improper migration of this lubricant from one of the compressors to the other with wear, and even destruction, of Mechanical compression systems.

In addition, with the essential characteristics of the installation, of type Heat Pump (BdC), on the other hand chosen, limited CO2 emissions associated with both individual and collective heating will be guaranteed, within the framework of a technical solution with a low energy consumption and with a very controlled cost.

It should also be noted that the architecture of the system will preferably allow the use of a simple 4-way valve (favorably with a retention clapper) to move from the two-stage operating cycle to the one-stage cycle, depending on the needs of heating of the building in said application.

Other features of the invention will be shown in the description that follows, made in reference to the attached drawings and in which the examples of embodiment do not have a limiting character, but merely illustrative. In these drawings:

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- Figures 1, 2, 3 are diagrams representing an embodiment of the heat pump system according to the present invention in operation according to respectively a two-stage cycle, a one-stage cycle with the BP compressor (low pressure) alone, and a one-stage cycle with the AP compressor (high pressure) alone;

- Figure 4 is a MOLLIER diagram representing the operation of the system according to the two stage cycle;

- Figure 5 shows the regulator of the installation and its environment;

- Figures 6, 7, 8 show the operation of Figures 1 -3 with the four-way valve replaced by several solenoid valves;

- and Figure 9 is a specific balancing joint scheme between the lubricant reservoirs of the BP and AP compressors.

In Figures 1-3 and 6-8 the thick line indicates a circulation of the cooling fluid in the shaded conduit.

Thus, the embodiments of Figures 1, 2 and 3 refer to two-stage and one-stage operating solutions of a heat pump installation 100 as a function of the outside temperature. In this preferred example, it is an installation that can operate according to the three configurations mentioned above.

When the outside temperature (Text) is low, traditionally between -20 ° C and 0 ° C, it is necessary to operate the heat pump in two-stage mode to obtain the necessary heating power in the condenser 10 and a high performance coefficient.

Figure 1 shows said operation in two stages. The low pressure compressor (BP) 1 aspirates the refrigerant fluid that evaporates in the evaporator 9 thereby recovering the heat generally from the outside air.

The cold source may eventually be soil or water. However, it is preferred to use outside air.

Compressor 1 compresses the refrigerant fluid, for example a mixture of HFCs such as R-407C, at half the pressure of the system that sets the high pressure compressor (AP) 2. This intermediate pressure is set by the ratio of the swept volumes of compressors 1 and 2.

The four-way valve 3 therefore relates its inlet 31 to its outlet 34, as in Figure 2. It will be understood that this valve is mounted to make the installation work according to one of said three possible configurations, being arranged between the two compressors 1, 2 when they work in series. The low pressure discharge tubing 5 is thus directly connected with the suction tubing 6 of the compressor 2. The retention flap 4 prevents the high pressure refrigerant flowing out of the compressor 2 through the tubing 21 back into low system pressure through pipes 8 and 7.

In the condenser 10, the water of the heating circuit of the environment to be heated here is reheated by the cooling fluid that gives it heat. The water enters the condenser 10 after cooling in the heating system (radiators, convectors ...) through the pipeline 19 and exits, at a higher temperature traditionally between 4 and 5 ° C, through the pipeline 20. The Orifice 17 in the condenser outlet high pressure pipe 18 allows a certain flow f3 of fluid to be extracted that evaporates in the subcooling exchanger 11, after expansion by means of the expander 12, in order to sub-cool the flow f2. The flow f2 is equal to f1-f3. The flow f1 is therefore the total flow compressed by the compressor 2 and condensed in the condenser 10. The flow f3 is much smaller than f2. The relation of the flows (expressed in m3) is determined by the following relation: f3 (h13-h 17) = f2 (h17-h25) in which h3, h17 and h25 are the respective enthalpies of the cooling fluid at point 13 located in the branch circuit 26, at the outlet of the exchanger 11, at point 17 located in the fourth pipe 18, at the exit point of the branch circuit 26, before entering the exchanger 11, at point 25 located in the fourth pipe 18 at the outlet of the exchanger 11.

Point 25 is the exit point of the flow f2 of the subcooling exchanger 11. The enthalpies at the respective points 13, 17 and 15 are shown in Figure 4 in a diagram called Mollier, pressure-enthalpy, where the cycle is represented Two-stage corresponding to the operation presented in Figure 1.

The hole 16 of the pipe 26 on the pipe 6 allows the high voltage compressor 2 to also aspirate the flow f3 evaporated in the subcooling exchanger 11.

The flow f2 is then conducted through the pipe 14 to the evaporator 9, the cooled fluid expanding in the expander 15.

When the weather conditions are milder, traditionally for an outside temperature above 0 ° C, it is then much more efficient to make the heat pump run in a single stage mode.

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The four-way valve 3, according to its position, therefore allows either the first compressor 1 to work alone, or the second compressor 2 to work alone. The operation of one or the other of the compressors depends on the necessary calorific power, since the first compressor 1 has a sweep volume greater than the AP 2 compressor and, therefore, a higher power, taking into account its sizing for operation in two stages.

Figure 2 shows the operation with the first compressor 1 in operation and the second compressor 2 stopped. In this case, the inlet 31 of the four-way valve 3 is connected to the outlet 32, directly connecting the discharge pipeline 5 of the first compressor 1 with the pipeline 8 which derives from the second compressor 2 and joins the pipeline 21 before from the input to the condenser 10 through the hole 22.

On the other hand, in this case, there is no flow extraction for subcooling since the second compressor 2 is stopped. The total flow f1 passes through the exchanger 11 without any subcooling.

Figure 3 shows the operation of the second compressor 2 alone, the first compressor 1 being stopped. The second compressor 2 aspirates the refrigerant fluid that has evaporated in the evaporator 9 through the pipe 7 and the hole 23 of the evaporator outlet pipe 24 9. The four-way valve 3 then communicates this pipe 7 with the suction pipe 6 of the second compressor 2 through the second inlet 33 and the first outlet 34 of this four-way valve 3. The rest of the circulation of the refrigerant fluid is done identically. Here the subcooling exchanger 11 is not in operation either. Therefore, the total flow f1 crosses it.

The four-way valve 3 is electrically controlled, to displace an inner piston, thus communicating the first inlet 31 either with the first outlet 34, or with the second outlet 32. In the latter case, the valve of four-way 3 communicates the second inlet 33 with the first outlet 34. This unique four-way valve 3 favorably associated with the retention clapper 4 thus allows a simple regulation to be developed depending on the outside ambient temperature.

The regulator 40 of the installation that controls its configuration receives through the input channel 41 an electric serial corresponding to the analog value of the external temperature measured, for example, by a probe, schematized in 46, in Figure 5. When this temperature outside is below a threshold temperature T1, for example, 0 ° C, the four-way valve 3 is in position 1 of the following table A corresponding to the operation presented in figure 1 in which the pipe 5 is in communication with the tubing 6, through the first inlet 31 and the first outlet 34 of the four-way valve. The regulator 40 then sends an electric serial via track 42 to put the four-way valve 3 in position 1. The regulator has also put the compressors 1 and 2 into operation via tracks 44 and 45 of the regulator 40. On the other hand, the heat output of the heat pump is set by two thresholds, lower and upper, of the water inlet temperature to the condenser. This Tagua temperature is measured in the pipeline 19 by means of a probe, schematized at 47, and its value is received in an analogous manner by the regulator 40, via track 43. When the Tagua temperature measured in the pipeline 19 is greater than the threshold higher regulation, which indicates the satisfaction of heating needs, at least one of compressors 1 and 2 stops. When the water temperature, always measured by the probe 47 in the pipeline 19, becomes lower than the trip threshold, then the regulator 40 puts the compressors 1 and 2 into operation together in two-stage operation, or one of the compressors 1,2, depending on the outside temperature, as indicated below in table A.

When the Text outside temperature is higher than the threshold temperature T1, but lower than the threshold temperature T2, for example 5 ° C, then the regulator 40 sends, via track 42, the setpoint position 2 to the valve four lanes 3, as defined in the following table. The regulator 40 controls in parallel the operation of the first compressor 1 alone, as shown in Figure 2. The first input 31 is then communicated with the second output 32.

When the outside temperature Text is higher than the threshold T2, the regulator 40 sends, via track 42, the setpoint of the same position 2 to the four-way valve 3, as defined again in the following table. In parallel, the regulator 40 controls the operation of the second compressor 2 alone, as shown in Figure 3. The second input 33 is then communicated with the first output 34.

In figures 2 and 3, the four-way valve 3 is in position 2 without changes. It is the compressor, 1 or 2, in operation that causes the cooling fluid to circulate as indicated in Figure 2 or the corresponding Figure 3.

Finally, when the Text outside temperature is higher than the non-heating threshold temperature, T3, the system stops.

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Table A

 Text

 Text <T1 Text <Text <T2 T2 <Text T3 <Text

 Compressor 1

 March March Stop Stop

 Compressor 2

 March Stop March Stop

 Four way valve position

 1 2 2 -

With the previous installation, therefore it will be possible to:

- in the configuration in which the output of the first compressor 1 is connected to the input of the second compressor 2, make this second compressor work downstream AP in series with the first compressor, at the same time therefore the latter, of according to a two stage cycle;

- and in one or other of the other configurations, make the first condenser alone or the second compressor work alone (parallel compressors), in a one-stage cycle;

- moving from one configuration to another using the four-way valve 3 and the clapper 4, depending on the heating needs.

The second expander 12 may, in a classical manner, be a capillary tube. In this case, it is necessary to provide in the branch circuit 26 a solenoid valve, schematized in 27, in Figures 1 to 3, to close the circuit 26, in particular in the way of operation schematized in Figure 3.

The capillary tube could be replaced by another type of expander.

In addition, although it allows an economic solution, as well as efficient and simple in terms of regulation, the four-way valve 3 could be replaced by equivalent means that fulfill the same functions (selection means).

Thus, it is possible to use the solution with three (electro) valves 50, 51,52, as illustrated in Figures 6, 7, 8 (installation 101).

The first valve 50 is mounted, upstream of the hole 16, between the pipes 5 and 6 connected together.

The second valve 51 is mounted in the second branch 8 then connected between the orifice 54 located upstream of the first valve 50 and the orifice 22.

The third valve 52 is mounted in the first branch 7 then connected between the orifice 23 located upstream of the inlet of the compressor BP 1 and the orifice 53 connected between the orifice 16 and the inlet of the compressor AP 2.

Figure 6 shows the two-stage configuration, therefore with the two compressors in series. Valves 51, 52 are closed, 50 is open.

Figure 7 shows the configuration of one stage with the AP 2 compressor running, while the other 1 is stopped. Valves 50, 51 are closed, 52 is open.

Figure 8 shows the other configuration of a stage with the compressor BP 1 in operation, the other 2 being stopped. Valves 50, 52 are closed, 51 is open.

Then you will return to an important point of the invention and installation presented here, whatever the version considered (figures 1 -3 or figures 6-8).

In these figures, it is in fact shown in an interchangeable manner that selective circulation means are provided in 55, 57, 59 which allow a circulation of lubricant between the BP / AP compressors 1, 2, in an operating situation of the installation such that there is a pressure difference between the lubricant receptacles 100, 200 (see Figure 9) of this first compressor and this second compressor.

As clearly seen in the aforementioned figures, the selective circulation means comprise a specific duct 56 with two tubing 57, 59 that join together the lubricant receptacles of the compressors, and a valve means 55 interposed at a point of this conduit to selectively open and close the circulation of the lubricant between the compressors. In Figure 9, in each of the two compressors, the tubing in question is coupled in the crankcase at the level of the oil level (used herein as a lubrication fluid), therefore connected with the lubricant receptacles 100, 200.

The valve means 55 that it is recommended to use is of the two-way solenoid valve type which, when directly actuated, operates in such a way that the solenoid's magnetic field forces

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then a movement of the plunger and in this way causes the opening of the shutter means 550 that it contains, without any pressure difference being necessary. In this way, this preferred means with shutter will be adapted to be opened by (direct) drive and to remain open regardless of the pressure difference of the lubricant established between the compressors 1,2.

On the contrary, in a closed state, it will preferably establish a sealing tightness that opposes the circulation of the lubricant between the compressors so much stronger as the pressure (P2) of the lubricant of the second compressor 2 will be high with respect to (P1 ) in the first compressor.

This will impose a sense of assembly of the valve means 55, as shown more clearly in Figure 9 in which the arrow 551, directed from the AP 2 compressor towards the BP 1 compressor, therefore indicates this sealing tightness. of shutter 550 the stronger the more P2 will be higher with respect to P1.

A two-way shutter medium with solenoid from the company “ALCO CONTROLS” type 110RB, without pressure differential, would be suitable.

Traditionally, the operation of the installation together with the pressure balancing sought could be the following, in the operating hypotheses corresponding to figures 1 and 3 (the information will be easily adapted in the cases of corresponding figures 6 and 7) :

Balancing the BP 1 compressor to the AP 2 compressor:

The installation is started here from the compressor 2 (C2) with the four-way valve 3 in one-stage configuration (figure 3). The suction pressure of the C2 (pressure of the C2 crankcase) is equal to the suction pressure of the compressor 1 (C1) minus the pressure drop of the valve 3. With the opening (under direct drive) of the solenoid valve 55, it goes therefore, the passage of C1 to C2 of the eventual excess oil present in the lubricant receptacle 100 of C1 will occur. The management of the opening time of the valve 55 and the frequency of this operation are defined by the regulator 40.

This allows to avoid the accumulation of oil in the C1 compressor and always maintain the optimum oil level for the correct operation of the compressors.

Balancing the AP 2 compressor to the BP 1 compressor:

Just after stopping, after a two-stage cycle operation (figure 1), the pressure accumulated in the compressor housing C2 is higher than in C1. An opening of the solenoid valve 55 at this time will allow the transfer of oil from C2 to C1, if there has been an accumulation in C2.

As before, regulator 40 will intervene so that a correct amount of oil is preserved in each compressor for reliable operation.

It should be understood that the action of the valve 3 could be replaced by that of the valves 50 or 52, for the loss of load, and that of the three valves 50, 51,52 for the circulation of the cooling fluid.

Thus, to at least participate in the existence of the expected pressure difference for the circulation of the lubricant and the equilibrium sought, the, or some of the, selection means mentioned above 3; 50, 51.52:

- will therefore be located in the refrigerant fluid circuit:

* upstream of the second compressor (2, C2), in the configuration in which the cooling fluid passes through it without passing through the first compressor (1, C1);

* and between said first compressor and said second compressor, when these operate in series,

- and will be adapted, in these circumstances, to create a favorable load loss.

Furthermore, by using the selective circulation media mentioned above (55, 56, 57, 59), in this way it will be possible:

- Carry out a balancing circulation of the lubrication fluid between the lubricant receptacles (from the first 100 to the second 200) in the case of a start-up of the installation that is made, therefore, that the cooling fluid passes through the second compressor (C2, 2) without passing it through the first compressor, but this circulation is not carried out in the opposite case;

- and / or, when stopping the installation, as the two compressors (1, C1; C2, 2) have worked together in series, open the valve 55 or equivalent so as to allow said balancing circulation that will then be operated from the receptacle of lubricant 200 to the other 100.

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Balancing Frequency:

This frequency will depend on the double stage operation modes of the machine; several possibilities:

- Stop / Start defined by the PAC regulator 40 (set point reached);

- Stop / start linked to the defrosting cycle (ice is formed on the air exchanger 11 of the PAC when the latter is in evaporator mode, the humidity present in the low temperature air obstructs the passage of air in the exchanger. Periodically, it is it is necessary to remove this ice, with heat input.This mode of operation is called defrosting);

- Forced stop with equilibrium if the operating time in continuous mode of the installation is longer than 1 h (and even 2 h if it is not too requested).

In this way, it will be avoided that, during the operation of the thermodynamic system in two stages (figure 1 or 6), and then later, the lubrication oil of the compressors, dragged by the refrigerant, causes an excessive migration of the lubricant from one of the the compressors towards the other with wear and destruction of the mechanical compression systems.

As a complement to the previous system of active balancing of the lubricant level of each compressor, it is even advisable to use passive lubricant separators 62, 64 in each compressor discharge 5, 21, with reinjection in the aspiration of the compressor concerned. Thus, in all the figures, except 4, 5, it is seen, at the outlet of each of the first compressor 1 and the second compressor 2, a means 62, 64 passive separator between the entrained lubricant and the compressed refrigerant fluid, to recover lubricant and reinject it into the inlet (aspiration) of the compressor concerned, respectively 240, 210, through a reinjection line, respectively 66, 68.

It is understood that the present invention is not limited to the embodiments described and represented, and that modifications can be made without departing from the scope of the invention, as long as it remains within the limit of what is claimed.

The duration of balancing could be a few seconds.

Claims (9)

5 10 fifteen twenty 25 30 35 40 Four. Five fifty 55 60 1. Thermodynamic installation comprising a first compressor (1, C1), a second compressor (2, C2), a circuit of several circulation paths of a cooling fluid that joins them, lubricant receptacles (100, 200) provided for receiving a lubrication fluid from said compressors, selection means (3; 50, 51, 52) that allow an operation of the installation according to some at least of the following three configurations: - a) with the refrigerant flowing through the first compressor without passing through the second compressor, - b) with the refrigerant flowing through the second compressor without passing through the first compressor, - c) with the two compressors running in series, the second compressor then being arranged downstream of the first compressor according to the direction of circulation of the cooling fluid, placing the, or some of said selection means, which are adapted to create a loss of charge, in the refrigerant fluid circuit: * upstream of the second compressor, in the configuration in which the cooling fluid passes through it without passing through the first compressor, * and between said first compressor and said second compressor, when these operate in series, characterized in that it comprises means (40, 55, 56, 57, 59) for selective circulation of the lubricant that comprise a conduit that joins said receptacles and in which a valve means (55) is interposed, managed by a regulator (40 ), to selectively open and close the circulation of the lubricant between the compressors and to allow a circulation of the lubricant from the receptacle (100) of the first compressor to the (200) of the second in a situation in which, operating the installation according to configuration b ), the loss of load of said selection means (3; 50, 51,52) creates a pressure difference between said lubricant receptacles. 2. Installation according to claim 1, characterized in that the valve means comprises a shutter means (550) adapted: - in a closed state, to establish a sealing tightness that opposes the circulation of the lubricant so much stronger as the pressure of the lubricant in the second compressor is high with respect to that of the first compressor, and - to be opened by drive and to remain open regardless of the difference in lubricant pressures established between the compressors. 3. Installation according to one of the preceding claims, characterized in that the valve means (55, 550) is two-way and with solenoid without pressure differential. 4. Installation according to one of the preceding claims, characterized in that the selection means (3; 50, 51,52) comprise a four-way valve (3) mounted to make the installation work according to one of said three configurations possible, being arranged between the two compressors (1, 2) when they work in series. 5. Installation according to one of the preceding claims, characterized in that at the outlet of each of the first compressor (1) and the second compressor (2) this comprises a passive separator (62, 64) between the entrained lubricant and the compressed refrigerant fluid, to recover lubricant and reinject it into the compressor inlet concerned, through a lubricant reinjection line. 6. Installation according to one of the preceding claims, characterized in that: - said circuit of several coolant circulation circuits comprises a first outlet (5) of the first compressor (1), a second inlet (6) of the second compressor (2), a third outlet (21) of the second compressor (2) extending to the inlet of a condenser (10), a fourth tubing (18, 14) that joins the outlet of the condenser (10) at the inlet of an expander (12, 15) arranged upstream of the evaporator (9), a fifth pipe (24) that joins the outlet of an evaporator (9) at the inlet of the first compressor (1), a first branch (7) of the first compressor (1) that extends from the inlet of the first compressor (1) in the direction of the second pipe (6), a second branch (8) of the second compressor (2) extending from the third pipe (21), in the direction of the first pipe (5) and provided of a retention clapper (4), - and the selection means (3; 50, 51,52) are adapted to join: * in the configuration in which the two compressors (1,2) operate in series, the first pipe (5) with the second pipe (6), and the first branch (7) with the retaining clapper (4) of the second derivation (8), and 5 10 fifteen twenty 25 30 35 * in the configuration in which the refrigerant fluid passes through the first compressor (1) without passing through the second compressor (2), the first pipe (5) with the third pipe (21) and the first branch (7) with the second tubing (6). 7. Installation according to claims 4 and 6, characterized in that the four-way valve (3) has a first inlet (31) attached to the first pipe (5), a first outlet (34) attached to the second pipe (6), a second inlet (33) attached to the first branch (7), a second outlet (32) attached to the retaining clapper (4) of the second branch (8), and an adapted mobile means (35) to join, in a first configuration, the first input (31) with the first output (34) and the second input (33) with the second output (32) and, in a second configuration, the first input (31) with the second exit (32) and the second entrance (33) with the first exit (34). 8. Procedure for promoting the circulation of lubricant between a first compressor (C1, 1) and a second compressor (C2, 2) of a thermodynamic installation comprising a circuit of several routes of circulation of a cooling fluid, lubricant receptacles ( 100, 200) to receive a lubrication fluid from said compressors and selection means (3; 50, 51, 52) that allow an operation of the installation according to the following configurations: - a) with the refrigerant flowing through the first compressor without passing through the second compressor, - b) with the refrigerant flowing through the second compressor without passing through the first compressor, - c) with the two compressors running in series, the second compressor then being arranged downstream of the first compressor according to the direction of circulation of the cooling fluid, procedure in which a loss of charge is created in the refrigerant fluid circuit by means of, or some of said selection means, characterized in that said pressure drop is used to create a pressure difference between the lubricant receptacles (100, 200) and thus, out of said refrigerant fluid circuit (specific duct 56), a fluid balancing circulation of lubrication between said receptacles, this only from (100) of the first compressor to (200) of the second in the case of a start-up of the installation in which the cooling fluid passes through the second compressor (C2, 2) without passing by the first compressor (C1, 1). 9. Method according to claim 8, characterized in that said circulation of balancing the lubrication fluid is also carried out when the installation is stopped, although the two compressors (1, C1; C2, 2) have operated together in series, opening said selective circulation means (55, 56, 57, 59) arranged between the receptacles (100, 200) so that this circulation is operated from the lubricant receptacle (200) of the second compressor to that of the first compressor (C1, 1) .