Dry oil pan type reciprocating piston compressor with lubricant circuit

Abstract

An extremely compact dry sump type reciprocating compressor includes a lower external reservoir in which lubricant is collected by gravity, and a positive displacement pump which draws lubricant from the reservoir and supplies the lubricant to a distribution circuit.

Description

Technical Field

[0001] The present invention belongs to the technical field of reciprocating piston compressors, which are used in the refrigeration industry and heat pumps and more generally in the fields of commerce, industry, transportation, process cooling, etc.

[0002] More specifically, the present invention relates to a reciprocating compressor of the type preferably used in combination with a closed refrigeration circuit, which is of the type that sucks in a gaseous refrigerant fluid and compresses the refrigerant fluid into the refrigerant circuit, where the gas expands and then re-enters the compressor at a lower pressure.

[0003] The present invention is specifically applied to the field of two transcritical carbon dioxide compressors. Background Art

[0004] Generally, inside a reciprocating compressor, a low-pressure part and a high-pressure part are identified. The low-pressure part is the suction pressure of the gas inside the machine, and the high-pressure part is the discharge pressure of the refrigerant fluid compressed by the piston in the cylinder.

[0005] The piston is connected to a connecting rod, which is in turn driven by a crankshaft; these crank mechanisms require lubrication, which is provided by supplying lubricating oil to a specific circuit that delivers the lubricating oil to various positions inside the compressor.

[0006] Different from other types of compressors, such as screw compressors, in a reciprocating compressor, the lubricating oil does not participate in the sealing between the high-pressure part and the low-pressure part, and this sealing is provided by a sealing ring between the piston and the cylinder.

[0007] It should be noted that in this patent text, the expressions "refrigerant circuit" or "refrigeration circuit" commonly refer to a set of pipes and equipment in which the refrigerant fluid discharged from the compressor circulates. Hereafter, this term refers to both the refrigeration circuit itself and the heat pump circuit.

[0008] In a so-called closed refrigerant circuit system, the process fluid never comes into contact with the outside world: the refrigerant fluid enters the compressor and undergoes an increase in pressure and temperature, and then is discharged from the compressor through a delivery pipe and sent to a condenser, where the refrigerant fluid releases heat to the external environment; the fluid leaving the condenser first passes through a lamination unit and then is transported to an evaporator, where the refrigerant fluid absorbs heat from the environment to be cooled, and finally the fluid returns to the compressor through a suction pipe.

[0009] Reciprocating compressors, which are commonly used in systems with a closed refrigerant circuit, such as those used in refrigeration systems or heat pump systems, include a crankcase that is adapted to present an internal collection area in the low-pressure section where lubricating oil required for lubricating the crank mechanism and various components within the compressor itself accumulates. The lubricant is withdrawn from the collection area by a pump and supplied into a lubrication circuit within the crankcase.

[0010] Unlike in an open-circuit system, in a system with a closed refrigerant circuit, the lubricant is not considered a contaminant, so there is no need to maintain a neat physical separation between the refrigerant fluid and the lubricant. As has been anticipated, the lubricant is actually directly stored in a specifically defined collection volume in the lower part of the crankcase within the compressor.

[0011] In an open compressor driven by an external motor, the lubricant is stored in the lower part of the crankcase that houses the crank mechanism, while in a hermetic or semi-hermetic compressor, the electric motor is also housed within the crankcase, and the lubricant is collected below the crank mechanism and the motor rotor. The pump ensures the circulation of the lubricant from the collection area to the bushings and, if necessary, to other mechanical components of the compressor. Gradually, the lubricant applied to the various mechanical components returns to the crankcase by the effect of gravity or due to the relative movement of the components themselves.

[0012] However, the proximity between the refrigerant fluid and the lubricant results in an unwanted drag effect on the lubricant when the fluid flows from the low-pressure section to the high-pressure section within the crankcase: A portion of the lubricant that is mixed with the refrigerant fluid is then gradually carried into the refrigeration circuit, where the lubricant deposits on the walls of the heat exchanger and / or in the pipes, leading to a decline in system performance and a gradual reduction in the lubricant stored within the crankcase.

[0013] The mixing between the lubricant and the refrigerant fluid is also facilitated by the fact that the lubricant used in the compressor must be at least partially miscible with the refrigerant fluid, because otherwise, there would be a risk of freezing when the lubricant passes through the sections of the circuit at lower temperatures.

[0014] Compressors commonly used in refrigeration or heat pump systems are very compact machines. In fact, in these fields, there is a great need to keep the size and weight of the crankcase as small as possible, also to reduce production costs. However, as the size of the crankcase is minimized, the internal volume allocated for lubricant collection also decreases, so that the free surface of the lubricant is located just below the crank mechanism and even submerges the lower part of the motor rotor, especially in semi-hermetic compressors.

[0015] The close proximity between the free surface of the lubricant and the moving mechanical parts increases the undesirable drag effect of the refrigerant fluid on the lubricant. This drag increases as the displacement and speed of the compressor increase.

[0016] The lubricant dragged by the refrigerant fluid partially re-enters the compressor after passing through the refrigerant circuit, however, a certain amount of lubricant is dispersed in the circuit outside the compressor itself, and the more extensive and complex the refrigerant circuit is, the greater the amount of lubricant dispersed. An excessive reduction in the lubricant content in the crankcase can cause a sudden stall of the compressor and / or cause excessive wear of the mechanical parts, especially when the remaining lubricant is insufficient to ensure continuous lubrication and / or a sufficiently thick lubricating film.

[0017] Another disadvantage of compressors of the type described here relates to excessive heating of the lubricant collected in the crankcase due to heat transfer from the compressor delivery duct. As the temperature increases, the viscosity of the lubricant decreases significantly and the lubricating properties decrease accordingly.

[0018] These problems are particularly evident in semi-hermetic compressors with integrated motors using CO2 as the process fluid, where the motor actually reaches very high temperatures during the compression phase, some of which is transferred to the compressor crankcase and then to the lubricant contained in the compressor crankcase.

[0019] To at least partially remedy these problems, oil separator devices have been developed for recovering a portion of the lubricant entrained from the compressed fluid. Such devices are typically placed just at the inlet of the refrigerant circuit, downstream of the compressor, and are capable of separating a portion of the lubricant mixed with the fluid, for example by returning the lubricant to the compressor crankcase through one or more filters.

[0020] However, not all systems include an oil separator; furthermore, no current oil separator is able to fully recover the lubricant discharged from the compressor and, therefore, cannot prevent the lubricant from being gradually dispersed into the refrigerant circuit.

[0021] Another unsolved problem in systems of the type described above is that such oil separators must be installed on the hottest part of the refrigerant circuit, i.e. at the inlet of the refrigerant circuit, immediately downstream of the cock separating the compressor from the circuit, primarily to intercept the lubricant mixed into the fluid before it enters the refrigerant circuit. However, the compressed fluid leaving the compressor is very hot, so the lubricant recovered from the separator and returned to the crankcase is also at a very high temperature. Due to the low viscosity of the hot lubricant, this has a very negative impact on the functioning of the compressor and the life of the components.

[0022] U.S. Patent US5277564 describes a hermetic screw compressor coupled to an external oil separator, which is adapted to be a tank equipped with a filter for intercepting lubricant carried by the fluid flowing out of the compressor. The compressed and very hot fluid leaving the compressor is conveyed to the separator, where the oil mixed with the fluid is intercepted by the filter and returned to the compressor crankcase for lubrication. The oil recovered from the separator is at a high temperature, thus having poor lubricating properties and poor adhesion to the components to which it is applied.

[0023] In known types of systems where the oil separator is located on the delivery manifold of the compressor, one or more additional devices, such as valves or other pressure control devices, are also required to allow the oil collected in the separator to return to the compressor crankcase while maintaining the pressure difference existing between the separator itself at delivery pressure and the circuit returning the oil to the crankcase at suction pressure. Therefore, this solution results in an increase in the number of required components and the complexity of the plant, leading to higher implementation and maintenance costs, as well as a reduction in the reliability of the entire system. Summary of the Invention

[0024] Therefore, the problem is felt to be the use of a dry-sump reciprocating compressor in a closed refrigerant circuit, in which the influence of the lubricant dragged by the refrigerant is minimal or more preferably zero.

[0025] Another object of the present invention is to provide an extremely compact reciprocating piston compressor in which the lubricant collection volume is located far enough from the parts of the compressor at higher temperature and pressure to avoid undesired heating of the lubricant without the need to install an auxiliary lubricant cooling system.

[0026] Another object of the present invention is to provide a reciprocating compressor in which the free surface of the lubricant is located far enough from the moving parts of the compressor to prevent a lubricant drag effect due to contact and / or proximity with the moving mechanical components and the crank mechanism.

[0027] In known solutions, the lubricant collection area is located in the lower part of the crankcase, as shown, for example, in EP4187090 and US5591011. However, in this type of configuration, the free oil surface is still in contact with the gas in the compressor crankcase, and the risk of the free oil surface being dragged by the lubricant is high.

[0028] A less important object of the present invention is to provide a compressor of the above type in which the low-pressure part of the crankcase, in which the connecting rod and the crankshaft are housed, can be inspected without having to shut down the compressor and stop the system.

[0029] Finally, the object of the present invention is to provide a reciprocating piston compressor for a refrigerant circuit and / or a heat pump, the reciprocating piston compressor having a crankcase, the weight and dimensions of which are very small compared to a conventional type compressor and which does not require additional internal volume for storing lubricant.

[0030] By reading this text, those skilled in the art will clearly understand these and other objects, which are achieved by a dry sump type reciprocating compressor including a lubricant distribution circuit including an external lubricant collection reservoir located below the crankcase of the compressor and communicating with the crankcase through a discharge hole defined in the outer wall of the crankcase.

[0031] In this patent text, the term "dry sump" refers to a crankcase that may or may not have an internal lubricant collection area in its lower part. However, the internal lubricant collection area is not used for storing lubricant because the lubricant is collected in a separate reservoir and supplied from the reservoir to the interior of the compressor by a pump to lubricate various mechanical parts and components, such as main bushings and connecting rods.

[0032] The same inventive concept applies both to reciprocating compressors with a motor mounted outside the crankcase, also known as open compressors, and to reciprocating compressors with a motor integrated inside the crankcase, known as semi - hermetic or hermetic compressors.

[0033] Advantageously, the compressor object of the present invention includes a crankcase in which one or more discharge holes are defined in the lower part thereof at low pressure, the discharge holes communicating with a reservoir below, the reservoir being separated from the compressor. The lubricant applied to the mechanical components inside the crankcase falls by gravity to the bottom of the crankcase and then is discharged through one or more of the said discharge holes into the reservoir below. Any lubricant mixed with the refrigerant fluid, once it re - enters the low - pressure part of the crankcase, also separates from the fluid by gravity and is discharged through the discharge holes into the reservoir below.

[0034] Very advantageously, the solution which is the subject of this patent application enables the dimensions and weight of the compressor crank to be minimized by storing the lubricant in a reservoir located below the compressor crankcase and at low pressure and low temperature, the low - pressure and low - temperature state of the reservoir substantially corresponding to the state of the low - pressure part of the crankcase.

[0035] Preferably, but not exclusively, the lubrication circuit pump is a self-priming mechanical gear pump and is integrally connected to the crankshaft of the compressor. The pump draws the lubricant stored in the reservoir below the compressor and delivers it to a plurality of lubricant distribution ducts preferably formed at least in part inside the crankshaft, thus ensuring a continuous flow of lubricant to the bushings and other compressor components requiring lubrication.

[0036] The inlet of the oil suction pipe is located in the lower part of the reservoir to avoid also sucking in the vapor phase.

[0037] During the operation of the compressor, the oil level can vary significantly. Therefore, in order to avoid the possibility of sucking in steam even when the oil level in the reservoir is particularly low, an object of the present invention includes a subdivision inside the reservoir, which is configured to create a suction chamber in the part of the reservoir surrounding the inlet of the suction pipe. The suction chamber is arranged to communicate with the rest of the reservoir only through a small channel, and the small channel is positioned close to or adjacent to the bottom wall of the reservoir. Due to this configuration, only the oil in the liquid phase can enter the suction chamber.

[0038] According to a particularly complete embodiment, the compressor crankcase includes one or more openings defined in the outer wall of the low-pressure part, which are normally closed by corresponding doors. The doors are preferably connected to the crankcase in an airtight manner by screws or bolts. After the compressor stops operating, the doors can be removed to inspect the interior of the crankcase without first pumping out the lubricant stored in the reservoir.

[0039] In some embodiments, the lubricant distribution circuit advantageously includes an overpressure valve, which opens when the lubricant pressure value exceeds a predetermined value, diverting the fluid from the lubrication circuit and directly into the crankcase so that the lubricant can return to the reservoir below through the discharge holes. Description of the Drawings

[0040] Figure 1 A side view showing an embodiment of a dry sump type compressor with an external oil reservoir is shown.

[0041] Figure 2 A front view of the compressor is shown.

[0042] Figure 3 A longitudinal section A-A of the compressor is shown, in which the baffle (71), the suction chamber (73) and the channel (72) defined in the lower part of the baffle (71) can be seen; the flow of the lubricant is indicated by arrows.

[0043] Figure 4 A B-B section of the compressor is shown.

[0044] Figure 5 A C-C section of the compressor is shown. Detailed implementation mode

[0045] The embodiment of the open dry sump reciprocating piston compressor which is the subject of this patent application is as follows Figure 1 The shown embodiment includes a crankcase (1), in which a low-pressure part (4) and a high-pressure part (2) are defined. A suction pipe (5) is connected to the low-pressure part (4), and the high-pressure part (2) is connected to a delivery pipe (3). The refrigerant fluid compressed by the piston is sent to the refrigerant circuit through this delivery pipe (3). The low-pressure part (4) of the crankcase (1) houses the motor and the crank mechanism, and particularly houses the crankshaft (6) attached to the piston connecting rod.

[0046] Two discharge holes (11) are defined in the lower wall of the crankcase (1). The discharge holes (11) connect the low-pressure part (4) of the crankcase (1) to the lower reservoir (7) for collecting lubricant. The reservoir (7) is separated from the crankcase (1) of the compressor and is located outside the crankcase (1). Lubricant required for lubricating the various mechanical components of the compressor is stored in the said reservoir (7), and the mechanical components are, for example, connecting rod bushings and crankshaft bushings (6). The lubricant is taken out from the reservoir (7) by a positive displacement pump (9), and the positive displacement pump (9) supplies the lubricant into the conduit (8) of the lubricant distribution system inside the crankcase (1).

[0047] The lubricant - dripping from the lubricated components and / or separating from the refrigerant fluid when re-entering the suction chamber, falling on the inner wall of the low-pressure part (4) of the crankcase (1) and passing through the said discharge holes (11) - is transferred to the lower reservoir (7) by gravity. The inner wall of the crankcase (1) is appropriately shaped to facilitate the transfer of the lubricant to the lower reservoir (7). The crankcase (1) is connected to the lower reservoir (7) by a flanged connector.

[0048] In the preferred embodiment described herein and shown in the drawings, the positive displacement pump (9) is a gear-type pump and is directly driven by the said crankshaft (6), and the positive displacement pump (9) is coaxial with the crankshaft (6). The pump (9) sucks the lubricant from the reservoir (7) through a suction manifold (91), and supplies the lubricant into the conduit (8) formed in the hub of the pump (9). The conduit (8) is integrally connected to the wall of the crankcase (1) and is connected to the inside of the crankshaft (6).

[0049] In the attached drawings, an embodiment of the compressor is shown, which includes an elongated reservoir (7) arranged with its longitudinal axis parallel to the longitudinal axis of the compressor; of course, other embodiments with reservoirs of different configurations are also possible.

[0050] In a reservoir (7) for collecting lubricant, a compartment is provided which delimits a suction chamber (73). The suction chamber (73) is separated from the rest of the reservoir (7) by a partition (71). In the partition (71), a small passage (72) is defined which is positioned near or adjacent to the bottom wall of the reservoir. Through this small passage (72), the lubricant travels into the suction chamber (73). In this way, the compressor can always suck the oil in the liquid phase without sucking gas.

[0051] In a particularly complete embodiment, the reservoir (7) includes an outer surface that is at least partially finned or a different cooling system. This solution allows the temperature of the lubricant stored in the reservoir (7) to be very advantageously reduced without changing the temperature of the refrigerant fluid flowing in the crankcase (1).

[0052] The lubricant distribution circuit further includes a safety bypass valve (10). The function of the safety bypass valve (10) is to keep the pressure in the circuit below a preset maximum threshold. In the embodiment shown in the drawings, the valve (10) is positioned in a conduit (8) upstream of a section formed in the crankshaft (6). When the lubricant pressure exceeds the preset maximum threshold, the valve (10) intervenes by opening a discharge pipe which releases the lubricant outside the lubrication circuit and directly into the crankcase (1). Once the lubricant pressure returns to the optimal range, the valve (10) automatically closes again. A possible embodiment of the bypass valve (10) includes a shutter which is kept closed by a spring or other similar elastic counteracting means and which opens when the thrust exerted by the lubricant on the shutter exceeds the counteraction of the spring, allowing the lubricant to flow through the discharge passage. Depending on the possible embodiments of the compressor, such a valve (10) can be positioned at other locations in the lubrication circuit.

[0053] Four openings are defined in the low-pressure part (4) of the crankcase (1), two openings being defined on each side of the crankcase (1), for inspecting and maintaining the components integrated in the compressor. Usually, these openings are closed by corresponding doors (12) which are attached to the crankcase (1) in a sealed manner by screws or bolts. In the technical solution which is the subject of the present patent application, there is no accumulation of lubricant in the low-pressure part (4) of the crankcase (1), which makes it possible to open the above-mentioned doors (12) without first removing the lubricant from the compressor.

Claims

1. A dry sump type reciprocating piston compressor for a closed refrigerant circuit, the dry sump type reciprocating piston compressor comprising: A crankcase (1), wherein a high-pressure portion (2) communicating with a delivery pipe (3) and a low-pressure portion (4) communicating with a suction pipe (5) are defined in the crankcase (1); a crankshaft (6), wherein the crankshaft (6) drives one or more pistons to slide in corresponding cylinders via corresponding connecting rods; and a lubricant distribution circuit located in the crankcase (1), characterized in that the lubricant distribution circuit comprises: - a lubricant collecting reservoir (7) located below the crankcase (1) and communicating with the crankcase (1) via one or more drain holes (11) defined in the outer wall of the lower part of the crankcase (1); - one or more ducts (8) leading to the interior of the crankcase (1), into which lubricant flows in order to be delivered to one or more points inside the crankcase (1); a positive displacement pump (9) which draws the lubricant from the reservoir (7) through a suction line (91) and supplies the lubricant to one or more of the ducts (8); And, a compartment is provided in the lubricant collecting reservoir (7), the compartment defining a suction chamber (73), the suction chamber (73) being separated from the rest of the reservoir (7) by a partition (71), a small channel (72) being defined in the partition (71) for transferring the lubricant into the suction chamber (73).

2. Reciprocating compressor according to the preceding claim, characterized in that The channel (72) is defined in the partition (71) near or adjacent to the bottom wall of the reservoir, via which the lubricant passes into the suction chamber (73).

3. Reciprocating compressor according to the preceding claim, characterized in that The lubricant distribution circuit comprises an overpressure bypass valve (10) adapted to discharge the lubricant out of the circuit and directly into the crankcase (1) when the lubricant pressure reaches a certain predetermined threshold.

4. Reciprocating compressor according to one of the preceding claims, characterized in that The low pressure portion (4) of the crankcase (1) is internally adapted to allow the lubricant to be gravity transferred from the crankcase (1) to the reservoir (7) below through one or more drain holes (11).

5. Reciprocating compressor according to one of the preceding claims, characterized in that The positive displacement pump (9) is coaxial with the crankshaft (6) of the compressor.

6. Reciprocating compressor according to one of the preceding claims, characterized in that One or more of the conduits (8) are at least partially defined within the crankshaft (6) of the compressor.

7. Reciprocating compressor according to one of the preceding claims, characterized in that One or more of the conduits (8) are at least partially defined within a hub of the positive displacement pump (9).

8. Reciprocating compressor according to one of the preceding claims, characterized in that One or more of the discharge holes (11) are bounded by respective flange portions for coupling with corresponding flange portions protruding upward from the tank (7) by means of screws or bolts.

9. Reciprocating compressor according to one of the preceding claims, characterized in that The reservoir (7) comprises cooling means for reducing the temperature of the lubricant.

10. Reciprocating compressor according to the preceding claim, characterized in that The cooling device comprises an at least partially finned outer surface of the reservoir (7).

11. Reciprocating compressor according to one of the preceding claims, characterized in that The crankcase (1) comprises one or more inspection openings defined in an outer wall of the low pressure portion (4).

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