EP2221559B1 - Thermodynamic installation with improved lubrication. - Google Patents

Description

The present invention relates to a thermodynamic refrigerant plant and its method of operation.

• avec le fluide frigorigène passant par le premier compresseur sans passer par le deuxième compresseur,
• avec le fluide frigorigène passant par le deuxième compresseur sans passer par le premier compresseur,
• avec les deux compresseurs fonctionnant en série, le deuxième compresseur étant alors disposé en aval du premier compresseur suivant le sens de circulation du fluide frigorigène.

Of FR-A-2889733 such an installation is known having the features of the preamble of claim 1, comprising first and second compressors, a multi-lane refrigerant circulation circuit, lubricant receptacles for receiving a lubricating fluid from said compressors, and selection means allowing operation of the installation according to at least three of the following configurations, and preferably these three possible configurations:

• with the refrigerant passing through the first compressor without going through the second compressor,
• with the refrigerant passing through the second compressor without going through the first compressor,
• with the two compressors operating in series, the second compressor then being disposed downstream of the first compressor in the direction of circulation of the refrigerant.

During the operation of such a two-stage thermodynamic system, lubricant (such as lubricating oil) for the compressors is often driven by the refrigerant.

We want to prevent this disrupting the operation of the installation.

An object of the invention is to avoid this. This object is obtained with an installation having the features of claim 1 and a method according to claim 8.

It is thus proposed that, in an operating situation of the installation such that there exists a pressure difference between the lubricant containers of said first and second compressors, lubricant is circulated between the (receptacles of) compressors, a priori out of said circuit of circulation of the refrigerant.

• un conduit dédié reliant entre eux les réceptacles à lubrifiant des premier et deuxième compresseurs,
• et un moyen vanne interposé en un endroit de ce conduit pour sélectivement ouvrir et fermer la circulation du lubrifiant entre les compresseurs.

To ensure this "pressurized" circulation of the lubricant, in a technically efficient and reliable manner, said selective circulation means comprise:

• a dedicated duct interconnecting the lubricant containers of the first and second compressors,
• and a valve means interposed at a location of said conduit for selectively opening and closing the flow of lubricant between the compressors.

A corollary problem also arises as to how to ensure the type of valve means to be used so that the lubricant balancing can be achieved effectively and simply at least in the second and third aforementioned configurations.

• dans un état fermé, à établir une étanchéité de fermeture qui s'oppose à la circulation du lubrifiant entre les compresseurs d'autant plus fortement que la pression du lubrifiant du deuxième compresseur est élevée par rapport à celle dans le premier compresseur, et,
• à s'ouvrir sur commande et à demeurer ouvert indépendamment de la différence de pressions du lubrifiant établie entre les compresseurs.

For this, it is proposed to use a suitable valve means:

• in a closed state, to establish a closing seal which opposes the circulation of the lubricant between the compressors all the more strongly that the pressure of the lubricant of the second compressor is high compared to that in the first compressor, and,
• to open on command and to remain open regardless of the difference in lubricant pressures established between the compressors.

Thanks to the solution of the invention, it will be avoided that, during the operation of the installation of the lubricant is driven by the refrigerant, this causes inappropriate migration of this lubricant from one of the compressors to the other with wear, even destruction, mechanical compression systems.

In addition, with the essential characteristics of the installation, of type Heat Pump (PAC), by Elsewhere, we will ensure limited emissions of CO ₂ associated with both individual and collective heating, as part of a technical solution that is energy efficient and well-controlled.

It should also be noted that the architecture of the system will preferably make it possible to use a simple 4-way valve (favorably with a non-return valve) to go from the two-stage operation cycle to the single-stage cycle, depending on the needs of the system. building heating in such an application.

• les figures 1, 2, 3 sont des schémas représentant un mode de réalisation du système de pompe à chaleur selon la présente invention en fonctionnement selon respectivement un cycle bi-étagé, un cycle mono-étagé avec le compresseur BP (basse pression) seul, et un cycle mono-étagé avec le compresseur HP (haute pression) seul ;
• la figure 4 est un diagramme de MOLLIER représentant le fonctionnement du système selon le cycle bi-étagé,
• la figure 5 montre le régulateur de l'installation et son environnement ;
• les figures 6, 7, 8 montrent les fonctionnements des figures 1-3 avec la vanne quatre voies remplacée par plusieurs électrovannes,
• et la figure 9 est un schéma de liaison d'équilibrage dédiée entre les réservoirs à lubrifiant des compresseurs BP et HP.

Other features of the invention will become apparent from the description which follows, made with reference to the accompanying drawings and wherein the embodiments are not limiting, but only illustrative. On these drawings:

• the Figures 1, 2 , 3 are diagrams showing an embodiment of the heat pump system according to the present invention in operation respectively according to a two-stage cycle, a single-stage cycle with the LP (low pressure) compressor alone, and a single stage cycle with HP compressor (high pressure) alone;
• the figure 4 is a diagram of MOLLIER representing the operation of the system according to the two-stage cycle,
• the figure 5 shows the regulator of the installation and its environment;
• the Figures 6, 7 , 8 show the functions of figures 1-3 with the four-way valve replaced by several solenoid valves,
• and the figure 9 is a dedicated balancing link diagram between the lubricant tanks of the BP and HP compressors.

On the figures 1-3 and 6-8 the fat indicates a circulation of the refrigerant in the blackened pipe.

The achievements of Figures 1, 2 and 3 thus refer to two-stage and single-stage operation 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 aforementioned configurations.

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

The figure 1 shows such a two-stage operation. The low pressure (LP) compressor 1 draws in the refrigerant which has evaporated in the evaporator 9 thus recovering the heat generally from the outside air.

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

The compressor 1 compresses the refrigerant, for example a mixture of HFCs such as R-407C, at the medium pressure of the system which is fixed by the high pressure compressor (HP) 2. This intermediate pressure is set by the ratio of the volumes swept compressors 1 and 2.

The four-way valve 3 then relates its input 31 to its output 34, as figure 2 . It will be understood that this valve is mounted to operate the installation according to one of said three possible configurations, being arranged between the two compressors 1, 2 when they operate in series. In this way, the low-pressure delivery pipe 5 is in direct connection with the suction pipe 6 of the compressor 2. The non-return valve 4 prevents the high-pressure refrigerant leaving the compressor 2 by the pipe 21 to iron at low system pressure by the pipes 8 and 7.

In the condenser 10, the water of the heating circuit of the habitat here to be heated is heated by the refrigerant which gives in heat there. The water enters the condenser 10 after cooling in the heating system (radiators, convectors ...) by the pipe 19 and out, at a temperature typically higher than 4 to 5 ° C, through the pipe 20. The tapping 17 on the high-pressure pipe 18 of the condenser outlet makes it possible to take a certain flow rate d3 of fluid which is evaporated in the subcooling exchanger 11, after expansion by the expander 12, in order to sub-cool the flow rate d2. The flow d2 is equal to d1 - d3. The flow d1 is then the total flow compressed by the compressor 2 and condensed in the condenser 10. The flow d3 is much lower than d2. The ratio of flow rates (expressed in m3) is fixed by the following relation: d3 (h13 - h17) = d2 (h17 - h25) in which h13, h17 and h25 are the respective enthalpies of the refrigerant at point 13 located on the circuit branch 26, at the outlet of the heat exchanger 11, at the point 17 located on the fourth pipe 18, at the departure point of the bypass circuit 26, before entering the heat exchanger 11, at the point 25 located on the fourth tubing 18 at the outlet of the exchanger 11.

The point 25 is the outlet point of the flow d2 of the subcooling exchanger 11. The enthalpies at the respective points 13, 17 and 25 are shown in FIG. figure 4 on a so-called Mollier diagram, pressure - enthalpy, where is represented the two-stage cycle corresponding to the functioning presented figure 1 .

The quilting 16 of the pipework 26 on the pipe 6 allows the high-pressure compressor 2 to suck also the flow rate d3 evaporated in the subcooling heat exchanger 11.

The flow d2 is then brought by the pipe 14 to the evaporator 9, the refrigerant being expanded in the expander 15.

When the weather conditions are milder, typically for an outdoor temperature above 0 ° C, it is much more efficient to operate the heat pump in single-stage mode.

The four-way valve 3, according to its position, then allows either the first compressor 1 to operate alone, or the second compressor 2 to operate alone. The operation of one or the other of the compressors depends on the heat output required, since the first compressor 1 has a swept volume greater than the HP compressor 2 and therefore a higher power, given their size for the two-stage operation.

The figure 2 presents 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 in connection with the outlet 32, thereby directly connecting the discharge pipe 5 of the first compressor 1 with the pipe 8 which bypasses the second compressor 2 and joins the ducting 21 before entering the condenser 10 via the quilting 22.

On the other hand, in this case, there is no flow sampling for the subcooling since the second compressor 2 is stopped. The total flow d1 passes through the exchanger 11 without subcooling.

The figure 3 shows the operation of the second compressor 2 alone, the first compressor 1 being stopped. The second compressor 2 sucks up the refrigerant which has evaporated in the evaporator 9 via the piping 7 and the tap 23 of the outlet pipe 24 of the evaporator 9. The four-way valve 3 then communicates this piping. 7 with the suction pipe 6 of the second compressor 2 via the second inlet 33 and the first outlet 34 of the four-way valve 3. The rest of the circulation of the refrigerant is identical. Again the sub-cooling exchanger 11 is not in operation. It is therefore crossed by the total flow d1.

The four-way valve 3 is electrically controlled to move an inner piston, thereby communicating the first input 31 with either the first output 34 or the second output 32. In the latter case, the four-way valve 3 in communication the second input 33 with the first output 34. This single four-way valve 3 associated favorably with the non-return valve 4 then allows to develop a simple regulation according to the external temperature of the environment.

The regulator 40 of the installation which controls its configuration receives, via the input channel 41, an electrical signal corresponding to the analog value of the outside temperature, measured for example by a probe, shown schematically in FIG. figure 5 . When this outside temperature is below a threshold temperature T1, for example 0 ° C., the four-way valve 3 is in position 1 of Table A below corresponding to the operation presented. figure 1 where the tubing 5 is in communication with the tubing 6, via the first inlet 31 and the first output 34 of the four-way valve. The regulator 40 then sends an electrical signal via the channel 42 to put the four-way valve 3 in position 1. The controller has also operated the compressors 1 and 2 via the channels 44 and 45 of the regulator 40. On the other hand , the heating capacity of the heat pump is set by two thresholds, low and high, from the water inlet temperature to the condenser. This water temperature is measured on the pipe 19 by a probe, schematized at 47, and its value is received analogically by the regulator 40, via the channel 43. When the water temperature measured on the pipe 19 is greater than the high threshold of regulation, which indicates the satisfaction of the heating needs, at least one of the compressors 1 and 2 stopped. When the temperature of the water, still measured by the probe 47 on the pipe 19, becomes lower than the threshold of engagement, then the regulator 40 puts into operation either the compressors 1 and 2 together in two-stage operation, or the one of the compressors 1, 2, this as a function of the outside temperature, as indicated below in Table A.

When the outside temperature Text is greater than the threshold temperature T1 but lower than the threshold temperature T2, equal for example to 5 ° C., then the regulator 40 sends, via the channel 42, the setpoint position 2 to the four-way valve 3 as defined in the table below. The regulator 40 controls in parallel the operation of the first compressor 1 alone, as shown figure 2 . The first input 31 is then put into communication with the second output 32.

When the external temperature Text is greater than the threshold T2, the regulator 40 sends, via the channel 42, the instruction of the same position 2 to the four-way valve 3, as again defined in the table below. In parallel, the regulator 40 controls the operation of the second compressor 2 alone, as shown figure 3 . The second input 33 is thus placed in communication with the first output 34.

Figures 2 and 3 , the four-way valve 3 is in its position 2 unchanged. It is the compressor, 1 or 2, in operation that circulates the refrigerant as indicated in FIG. figure 2 or 3 corresponding.

Finally, when the outside temperature Text is higher than the threshold temperature of no heating, T3, the system is stopped. Table A text Text <T1 T1 <Text <T2 T2 <Text T3 <Text Compressor 1 Walk Walk Stop Stop Compressor 2 Walk Stop Walk Stop Four way valve position 1 2 2 -

• dans la configuration où la sortie du premier compresseur 1 est reliée à l'entrée du second compresseur 2, faire fonctionner ce deuxième compresseur aval HP en série avec le premier compresseur, en même temps donc que ce dernier, selon un cycle bi-étagé,
• et dans l'une ou l'autre des autres configurations, faire fonctionner le premier compresseur seul ou le second compresseur seul (compresseurs en parallèle), en cycle mono-étagé,
• ceci en passant d'une configuration à l'autre en utilisant la vanne à quatre voies 3 et le clapet anti-retour 4, en fonction des besoins de chauffage.

With the installation above, we will be able to:

• in the configuration where the output of the first compressor 1 is connected to the input of the second compressor 2, operate this second HP downstream compressor in series with the first compressor, at the same time so that the latter, in a two-stage cycle,
• and in either of the other configurations, operating the first compressor alone or the second compressor alone (compressors in parallel), in a single-stage cycle,
• this passing from one configuration to another using the four-way valve 3 and the check valve 4, depending on the heating requirements.

The second regulator 12 may conventionally be a capillary tube. In this case, it is necessary to provide on the bypass circuit 26 a solenoid valve, shown diagrammatically in Figures 1 to 3 , to close the circuit 26, in particular in the operating mode shown schematically in FIG. figure 3 .

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

In addition, although allowing a cost-effective solution, as well as efficient and simple in terms of regulation, the four-way valve 3 could be replaced by equivalent means fulfilling the same functions (selection means).

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

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

The second valve 51 is mounted on the second bypass 8 then connected between the tapping 54 situated upstream of the first valve inlet 50 and the tapping 22.

The third valve 52 is mounted on first bypass 7 then connected between the tap 23 located upstream of the inlet of the compressor BP 1 and the tap 53 connected between the tap 16 and the inlet of the compressor HP 2.

The figure 6 shows the two-stage configuration, with the two compressors in series. The valves 51, 52 are closed, the one 50 is open.

The figure 7 shows the single-stage configuration with the HP 2 compressor running, while the other 1 is stopped. The valves 50, 51 are closed, that 52 is open.

The figure 8 shows the other single-stage configuration with the BP 1 compressor in operation, the other 2 being stopped. The valves 50, 52 are closed, the 51 is open.

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

In these figures, it can indeed be seen that are provided at 55, 57, 59 selective circulation means for a circulation of lubricant between the compressors BP / HP 1, 2, in an operating situation of the installation such that there is a pressure difference between the receptacles 100, 200 to lubricant (see figure 9 ) of these first and second compressors.

As can be seen clearly in the figures mentioned above, the selective circulation means comprise a dedicated conduit 56 with two pipes 57, 59 interconnecting the lubricant receptacles of the compressors, and a valve means 55 interposed at a location of this selectively to open and close the circulation of the lubricant between the compressors. Figure 9 , on each of the two compressors, the tubing concerned is connected to the casing at the level of the oil level (used here as a lubricating fluid), thus in connection with the lubricant receptacles 100, 200.

The valve means 55 that is recommended to use is of the two-way solenoid valve type which, when under direct control ("directly actuated"), operates in such a way that the magnetic field of the solenoid then forces a movement of the plunger and thus causes the opening of the valve means 550 that it contains, this without requiring any pressure difference. Thus, this preferred valve means will be adapted to open on command (direct) and remain open regardless of the difference in lubricant pressures established between the compressors 1, 2.

On the other hand, in a closed state, it will preferably establish a closing seal opposing the circulation of the lubricant between the compressors all the more strongly that the pressure (P2) of the lubricant of the second compressor 2 will be high compared to that (P1) in the first compressor.

This will impose a mounting direction of the valve means 55, as shown more clearly figure 9 where the arrow 551, directed from the compressor HP 2 to the compressor BP 1, therefore indicates this closing tightness all the stronger the valve 550 that P2 will be higher compared to P1.

A two-way valve solenoid valve from "ALCO CONTROLS" type 110RB, without pressure differential, will be suitable.

Typically, the operation of the installation in connection with the pressure equalization sought may be the following, in the operating hypotheses corresponding to the figures 1 and 3 (information will easily be adapted to cases of Figures 6, 7 corresponding):

Balancing the BP 1 compressor to the HP 2 compressor:

The start of the installation is carried out here from the compressor 2 (C2) with the four-way valve 3 in single-stage configuration ( figure 3 ). The suction pressure of the C2 (C2 housing pressure) is equal to the suction pressure of the compressor 1 (C1) minus the pressure drop of the valve 3. On opening (under direct control) of the solenoid valve 55, so it goes have the passage from C1 to C2 of the possible excess oil present in the lubricant receptacle 100 C1. The management of the opening time of the valve 55 and the frequency of this operation are defined by the regulator 40.

This avoids the accumulation of oil in the C1 compressor and always maintain an optimum oil level for the proper operation of the compressors.

Balancing the HP 2 compressor to the BP 1 compressor:

Just stopped, after a two-stage cycle operation ( figure 1 ), the pressure accumulated in the crankcase of the compressor C2 is higher than in the 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 accumulation in C2.

As above, the regulator 40 will intervene so that a correct amount of oil is stored 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 pressure drop, and that of the three valves 50, 51, 52 for the circulation of the refrigerant.

• seront donc situés, sur le circuit du fluide frigorigène:
  • * en amont du deuxième compresseur (2, C2), dans la configuration où le fluide frigorigène passe par lui sans passer par le premier compresseur (1, C1),
  • * et entre lesdits premier et deuxième compresseurs, lorsqu'ils fonctionnent en série,
• et seront adaptés, dans ces circonstances, à créer une perte de charge favorable.

Thus, to at least participate in the existence of the pressure difference provided for the circulation of the lubricant and the balancing sought, the, or some of the aforementioned selection means 3; 50, 51, 52:

• will therefore be located on the refrigerant circuit:
  • * upstream of the second compressor (2, C2), in the configuration where the refrigerant passes through it without passing through the first compressor (1, C1),
  • * and between said first and second compressors, when operating in series,
• and will be adapted, in these circumstances, to create a favorable pressure drop.

• d'effectuer une circulation d'équilibrage du fluide de lubrification entre les réceptacles à lubrifiant (du premier 100 vers le second 200) dans le cas d'un démarrage de l'installation s'effectuant en faisant donc passer le fluide frigorigène par le deuxième compresseur (C2, 2) sans le faire passer par le premier compresseur, cette circulation n'étant par contre pas réalisée pas dans le cas inverse,
• et/ou, à l'arrêt de l'installation, alors que les deux compresseurs (1, C1;C2, 2) ont fonctionné ensemble en série, d'ouvrir la vanne 55 ou équivalent de sorte à permettre ladite circulation d'équilibrage qui va alors pouvoir s'opérer du réceptacle à lubrifiant 200 vers l'autre, 100.

In addition, by using the aforementioned selective circulation means (55, 56, 57, 59), it will thus be possible:

• to carry out a balancing circulation of the lubricating fluid between the lubricant receptacles (from the first 100 to the second 200) in the case of a start of the installation being carried out by thus passing the refrigerant through the second compressor (C2, 2) without passing through the first compressor, this circulation is not against cons not in the opposite case,
• and / or, at the end of the installation, while the two compressors (1, C1; C2, 2) have operated together in series, to open the valve 55 or equivalent so as to allow said balancing circulation which will then be able to operate from the lubricant receptacle 200 to the other, 100.

Frequency of the balancing:

• Arrêt/Démarrage définis par le régulateur 40 de la PAC (point de consigne atteint),
• Arrêt/Démarrage liés au cycle de dégivrage (du givre se forme sur l'échangeur à air 11 de la PAC lorsque ce dernier est en mode évaporateur, l'humidité présente dans l'air à basse température vient obstruer le passage d'air dans l'échangeur. Périodiquement, il faut éliminer ce givre, avec apport de chaleur. Ce mode de fonctionnement est appelé le dégivrage) ;
• Arrêt forcé avec équilibrage si le temps de fonctionnement en continu de l'installation est supérieur à 1h (voire 2h si elle n'est pas trop sollicitée).

This frequency will be a function of the two-stage operating modes of the machine; several possibilities :

• Stop / Start defined by the PAC controller 40 (set point reached),
• Stop / Start related to the defrosting cycle (frost is formed on the heat exchanger 11 of the heat pump when the latter is in evaporator mode, the humidity present in the the air at low temperature obstructs the passage of air in the exchanger. Periodically, it is necessary to eliminate this frost, with heat input. This mode of operation is called defrosting);
• Forced shutdown with balancing if the continuous running time of the installation is greater than 1h (or 2h if it is not too much stressed).

Thus, it will be avoided that, during the operation of the two-stage thermodynamic system ( figure 1 or 6 ), and subsequently, the lubricating oil of the compressors, driven by the refrigerant, causes excessive migration of the lubricant from one compressor to the other with wear and destruction of the mechanical compression systems.

In addition to the above system of active balancing of the lubricant level of each compressor, it is even advisable to use passive lubricant separators 62, 64 on each compressor discharge, 5, 21, with reinjection at the suction of the compressor concerned. Thus, in all the figures except 4, 5, it is seen, at the outlet of each of the first and second compressors 1, 2, a means 62, 64, a passive separator between the driven lubricant and the compressed refrigerant, to recover lubricant. and reinjecting it into input (suction) of the compressor concerned, respectively 240, 210, through a reinjection duct, respectively 66, 68.

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

The duration of the balancing may be a few seconds.

Claims (9)

1. Thermodynamic installation comprising a first compressor (1, C1), a second compressor (2, C2), a circuit with several circulation channels for a refrigerant fluid that connects them, lubricant containers (100, 200) provided to receive a lubricant from said compressors, means for selecting (3; 50, 51, 52) that allow for an operation of the installation according to some at least of the three configurations hereinafter:

   - a) with the refrigerant fluid passing through the first compressor without passing 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 according to the direction of circulation of the refrigerant fluid,
   the, or some of said means for selecting, which are able to create a loss of load, being located on the circuit of the refrigerant fluid:

   - upstream of the second compressor, in the configuration wherein the refrigerant fluid passes through it without passing through the first compressor,

   - and between said first and second compressors, when they are operating in series,

   characterised in that it comprises means (40, 55, 56, 57, 59) for the selective circulation of the lubricant comprising a duct that connects said containers and wherein is interposed a valve means (55), managed by a regulator (40), in order to selectively open and close the circulation of the lubricant between the compressors and in order to allow for a circulation of the lubricant from the container (100) of the first compressor to that (200) of the second in a situation wherein, with the installation operating according to the configuration b), the loss of load of said means for selecting (3; 50, 51, 52) creates a pressure difference between said lubricant containers.
2. Installation according to claim 1, characterised in that the valve means comprises a pressure relief valve (550) adapted to:

   - in a closed state, establish a closing seal which opposes the circulation of the lubricant all the more so that the pressure of the lubricant in the second compressor is high with respect to that in the first compressor, and,

   - open as controlled and remain open independently of the difference in pressure of the lubricant established between the compressors.
3. Installation according to one of the preceding claims, characterised in that the valve means (55, 550) has two channels and a solenoid without pressure differential.
4. Installation according to one of the preceding claims, characterised in that the means for selecting (3; 50, 51, 52) comprise a four-way valve (3) mounted in order to operate the installation according to one of said three possible configurations, by being arranged between the two compressors (1, 2) when they are operating in series.
5. Installation according to one of the preceding claims, characterised in that at the outlet of each one of the first and second compressors (1, 2) it comprises a passive separator (62, 64) between the driven lubricant and the compressed refrigerant fluid, in order to recover the lubricant and reinject it into the inlet of the compressor concerned, through a duct for reinjecting lubricant.
6. Installation according to one of the preceding claims, characterised in that:

   - said circuit with several circulation channels of the refrigerant fluid comprises a first outlet hose (5) of the first compressor (1), a second inlet hose (6) of the second compressor (2), a third outlet hose (21) of the second compressor (2) extending to the inlet of a condenser (10), a fourth hose (18, 14) connecting the outlet of the condenser (10) to the inlet of a pressure reducer (12, 15) arranged upstream of the evaporator (9), a fifth hose 24) connecting the outlet of an evaporator (9) to the inlet of the first compressor (1), a first by-pass (7) of the first compressor (1) extending from the inlet of the first compressor (1) in the direction of the second hose (6), a second by-pass (8) of the second compressor (2) extending from the third hose (21), in the direction of the first hose (5) and provided with a non-return valve (4),

   - and the means for selecting (3; 50, 51, 52) are adapted to connect:

   - in the configuration wherein the two compressors (1, 2) operate in series, the first hose (5) to the second hose (6), and the first by-pass (7) to the non-return valve (4) of the second by-pass, (8), and,

   - in the configuration wherein the refrigerant fluid passes through the first compressor (1) without passing through the second compressor (2), with the first hose (5) to the third hose (21) and the first by-pass (7) to the second hose (6).
7. Installation according to claims 4 and 6, characterised in that the four-way valve (3) has a first inlet (31) connected to the first hose (5), a first outlet (34) connected to the second hose (6), a second inlet (33) connected to the first by-pass (7), a second outlet (32) connected to the non-return valve (4) of the second by-pass (8), and a mobile means (35) adapted to connect, in a first configuration, the first inlet (31) to the first outlet (34) and the second inlet (33) to the second outlet (32), and, in a second configuration, the first inlet (31) to the second outlet (32) and the second inlet (33) to the first outlet (34).
8. Method for favouring the circulation of lubricant between first and second compressors (C1, 1, C2, 2) of a thermodynamic installation comprising a circuit with several circulation channels for a refrigerant fluid, with lubricant containers (100, 200) for receiving a lubricant from said compressors and means for selecting (3; 50, 51, 52) allowing for an operation of the installation according to the configurations hereinafter:

   - a) with the refrigerant fluid passing through the first compressor without passing 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 being then arranged downstream of the first compressor according to the direction of circulation of the refrigerant fluid,

   in which method, a loss of load is created on the circuit of the refrigerant fluid by the intermediary of the, or of certain of said means for selecting,
   characterised in that said loss of load is used to create a difference in pressure between the lubricant containers (100, 200) and as such carry out, outside of said circuit of the refrigerant fluid (dedicated duct 56), a balancing circuit of the lubricant between said container, and this solely from the one (100) of the first compressor to the one (200) of the second in the case of a start-up of the installation wherein the refrigerant fluid passes through the second compressor (C2, 2) without passing through the first compressor (C1, 1).
9. Method according to claim 8, characterised in that said circulation for balancing the lubricant is also carried out when the installation is stopped, while the two compressors (1, C1; C2, 2) operated together in series, by opening said means for selective circulation (55, 56, 57, 59) arranged between the containers (100, 200) in such a way that this circulation takes place from the lubricant container (200) of the second compressor to that of the first compressor (C1, 1).

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