Patent Application
20180128522
The present invention relates to cooling systems, and more particularly to bearings used with cooling systems.
Cooling systems are already known, for example from EP 0 664 424 A2, in which a method for lubricating bearings in refrigerant compressors is to be improved. To this end, a small amount of a refrigerant/oil mixture is introduced into the vicinity of a bearing, wherein the refrigerant is vaporized due to the bearing temperature and a lubricant including at least 75 volume percent of oil is deposited onto the bearing. Thereby, the flow path of the refrigerant and the configuration of the bearing need to be designed such that a sufficient volume of refrigerant with at least 75% oil is deposited in all circumstances. In particular, if the refrigerant compressor has not yet reached its operating temperature, the bearing environment needs to be able to vaporize a sufficient amount of refrigerant from the refrigerant oil mixture.
Further, from EP 1 729 055 B1, lubrication systems for rolling elements in refrigerant compressors are known, in which the lubrication medium consists of an ultra-low viscous volatile fluid (ULVVF). For lubricating and guaranteeing a sufficiently thick liquid lubrication film, it is proposed to inject the liquefied fluid and to keep the fluid at least partly above the evaporating pressure by using a flow restriction. The disadvantage in this case is that, even if it has been ensured that the fluid for lubricating the rolling elements forms a liquid lubrication film and the bearing does not run dry, the bearing has to withstand extreme demands due to the poor lubrication characteristic of the fluid, which does not include lubrication oil, and therefore only high resistant and thus expensive bearings may be used in this application area.
Further, rolling element bearings are already known from EP 0 711 929 B1, in which at least one rolling element consists of a material which is harder or more rigid than the steel material of the other rolling elements, which results in a greater hardness of the at least one rolling element with respect to the other rolling elements.
It is an object of the invention to provide a cooling system which is operated by means of a mixture of refrigerant and lubrication oil, and which is cost-effectively presentable and operates reliably in all operating states.
This object is solved by a cooling system according to claim 1.
The viscosity ratio κ at operating temperature serves as measure for the effectiveness of the lubrication. It indicates the ratio of the actual kinematic viscosity ν to the kinematic viscosity ν1, which is required for a sufficient lubrication.
Until now the general teaching has assumed so far that, at viscosity ratios of κ<1, a sufficiently sustainable hydrodynamic or elasto-hydrodynamic lubrication film cannot be established and, therefore, a boundary layer lubrication with direct touching of the bearing components in the rolling contact occurs. In contrast to that, the invention provides an oil separator which reduces the percentage of the refrigerant in the operating medium to a value between 15% by weight and 60% by weight, and provides this operating medium for lubrication such that, in a first operating state, a viscosity ratio of κ<1 is present.
Here, the viscosity ratio is defined as κ=ν/ν1, wherein ν1 is the nominal viscosity which indicates the required kinematic viscosity of the lubricant at operating temperatures in dependence on the average bearing diameter and the circumferential speed. It turned out that, in good approximation, the nominal viscosity for different speed ranges may be given by two equations. For rotational speeds of the bearing n<1000 r/min, the nominal viscosity is given as follows:
ν1=45000n−0,83Dpw±05
For rotational speeds n≥1000 r/min, the nominal viscosity is given as follows:
ν1=45000n−0,5Dpw−0,5
wherein Dpw is the pitch diameter of the roller bearing.
Further, ν is the actual kinematic viscosity of the lubricant at operating temperature. For values with κ<1, the actual kinematic viscosity is therefore below the nominal viscosity. For values with κ>1, the actual kinematic viscosity is therefore above the nominal viscosity. It may then be assumed that a sufficiently formed sustainable hydrodynamic lubrication film is provided.
Thus, the viscosity ratio κ is an indirect measure of the film thickness of the lubrication film in the rolling contact between the rolling elements and the raceways of the bearing rings. The film thickness of the lubrication film is directly depending on the actual kinematic viscosity of the lubricant, which is the operating medium for lubrication. The actual kinematic viscosity is determined at atmospheric pressure. However, the viscosity of the lubricant is dependent on the pressure acting on the lubricant, wherein the viscosity increases with increasing pressure. The viscosity of the lubricant in the lubrication film in the rolling contact is therefore higher than the viscosity of the lubricant at ambient pressure. A measure for the pressure dependency of the lubricant is the pressure coefficient, which is considerably higher for a lubrication oil than for a refrigerant, approximately twice as high. Thus, the viscosity of the lubricant, which is a mixture of refrigerant and lubrication oil, in the rolling contact decreases with an increasing percentage of refrigerant in the mixture, not only due to the lower actual kinematic viscosity of the refrigerant but also due to the lower pressure coefficient of the refrigerant. The calculated viscosity ratio κ of the refrigerant oil mixture is thus not a direct proportional measure of the film thickness in the rolling contact. For the described reasons, the actual film thickness is lower than it could be assumed based on the value for κ. In other words, the film thickness of a pure lubrication oil having the same viscosity ratio as a considered refrigerant lubrication oil mixture is greater than that of the mixture. Further, the viscosity ratio, or the film thickness, respectively, depends on the rotational speed of the bearing. The lower the speed, the lower is the viscosity ratio and thus the film thickness. This is due to the fact that the nominal viscosity decreases with increasing rotational speed, as stated above.
According to an advantageous embodiment, the viscosity ratio is κ>1 in a second operating state. First operating states may be for example operating states in which a low rotational speed is present, whereas second operating states may be indicated for example by a higher rotational speed compared to the first operating states. For example, the rotational speed parameter for the angular ball bearing may be below 300.000 mm/min in a first operating state, and above 1.000.000 mm/min in a second operating state.
Alternatively or additionally, the first operating states may also be indicated by higher temperatures compared with second operating states. For increasing temperatures, the viscosity of an operating medium having a constant percentage of refrigerant decreases, so that the viscosity ratio decreases. On the other hand, for decreasing temperatures, the percentage of refrigerant in the operating medium may increase so that in general the viscosity decreases due to this effect. It may also be possible that for increasing temperatures the viscosity and hence the viscosity ratio increases due to the decreasing percentage of refrigerant in the operating medium at first, but decreases for further increasing temperatures due to the temperature dependency of the viscosity. Thus, the first operating states may be present in a first temperature range and the second operating states may be present in a second temperature range which differs from the first temperature range.
Preferably, a lubrication oil enriched second operating medium may not only be used for lubricating the angular ball bearing but also for separately lubricating further components of the refrigerant compressor such as the rotating screw conveyors in case of a screw compressor. Thereby, the lubrication oil enriched second operating medium preferably serves for cooling and for sealing of gap tolerances of further components. An advantage of the invention is that, in contrast to conventional solutions, the demands on the oil separator for providing the second operating medium are reduced since the percentage of refrigerant in the lubricant may be substantially higher than 20% by weight. Preferably, the angular ball bearing is configured as single-row angular ball bearing which may support axial forces in one direction. Alternatively, the angular ball bearing may be configured as 4 point bearing, which may support axial forces in both directions. By use of at least a first ball which consists at least partially of a ceramic, the bearing becomes more resistant against deficiencies of the lubrication film for viscosity ratios κ<1, which would otherwise result in damages on the raceway. According to the invention, at least the surface of the first ball consists of a ceramic. Preferably, silicon nitride Si3N4 is used. The inventive first ball hereby provides a surface which is harder than the raceways of the inner ring and of the outer ring. According to the invention, thereby, micro damages, such as micro pitting, which occur due to insufficient lubrication conditions, are partly removed by rolling the harder surface of the first ball on the raceway and the raceway is smoothed, thereby prolonging the lifespan of the bearing also during poor lubrication conditions. Preferably, the inner and the outer ring and their respective raceways are made from conventional ball bearing steel wherein it is not necessary that the steel meets any special requirements due to the poor lubrication conditions.
In a preferred cooling system, the refrigerant includes derivatives of alkenes. As particularly preferred derivatives, derivatives of Hydrofluoroolefins, also referred to as HFOs, or Hydrochlorofluoroolefins, also referred to as HCFOs, are provided. Also a refrigerant, which includes inter alia HFOs and HCFOs, may be employed as refrigerant according to a preferred embodiment. Hereby, it is advantageous that the preferred derivatives of alkenes are particularly eco-friendly as their GWP value is lower than that of conventional refrigerants. The Global Warming Potential is referred to as GWP value, which indicates the direct contribution of the refrigerant to the greenhouse effect. However, the preferred refrigerants have a lower lubrication potential than conventional refrigerants. In addition, they are more volatile.
According to an advantageous embodiment of the cooling system, the oil separator reduces the percentage of the refrigerant in the operating medium to a value between 20% by weight and 40% by weight so that the lubrication oil enriched second operating medium provides a percentage of refrigerant between 20% by weight and 40% by weight. Particularly, when using HFOs and/or HCFOs as refrigerant, it is advantageous that the oil separator does not have to be laboriously configured to reduce the percentage of the refrigerant to <20% by weight in the segregated operating medium, in all circumstances. Thus, according to the invention, oil separators of a conventional configuration may be used.
According to a further advantageous embodiment of the cooling system, a joint operating medium circuit of the first and second operating medium is provided, wherein the bearing site is sealed against the first operating medium. Theoretically, it would be desirable to lubricate the bearing site of the rotor by means of a separate lubricant circuit, which is completely separated from the refrigerant circuit. However, for this purpose, complex sealing systems would be needed to permanently ensure a reliable separation of refrigerant and lubricant. As a reliable separation would be extremely complex and expensive, a joint operating medium circuit of the first and second operating medium is provided at least in a subarea, in which a mixing of the two operating mediums takes place. The oil separator according to the invention serves for separating the joint operating medium circuit into two circuits each of which having one of the two operating mediums. For reliably preventing the second operating medium to unintentionally mix with the first operating medium in the region of the bearing site, a sealing arrangement is provided. This sealing arrangement is preferably arranged between a rotor shaft of the rotor and a housing and seals the bearing site against a high pressure side of the compressor.
According to a further embodiment, the angular ball bearing provides, besides the first ball being made from ceramic or having a ceramic surface, a plurality of second balls having surfaces which are softer than the surface of the at least one first ball. Advantageously, thereby the number of ceramic balls, which are particularly costly to manufacture and thus are more expensive than balls made from ball bearing steel, may be reduced. Balls made from conventional ball bearing steel have a softer surface than ceramic balls. It has been found that only some balls or even only a single ball made from ceramic is sufficient for realizing the preferred effects on the surface quality. All in all, thereby, the angular ball bearing may be manufactured cost-efficiently, having a sufficiently long life span with a percentage of refrigerant in the operating medium between 15 and 60% by weight. According to the invention, with the preferred range of refrigerant in the operating medium being between 20 and 40% by weight, even a single ceramic ball would be sufficient for achieving the required life span.
According to a further preferred embodiment of the angular ball bearing, the angular ball bearing provides an inner ring, an outer ring and balls rolling therebetween, wherein the inner ring and/or the outer ring provide a nitrided or carbonitrided raceway. The advantage is that the raceway has an improved surface resistance during insufficient lubrication conditions due to the nitrided or carbonitrided raceway. The lifespan of the angular ball bearing may hereby be further increased. Alternatively, the inner ring and/or the outer ring may be case-hardened or may have a case-hardened raceway. This may also increase the surface resistance during insufficient lubrication conditions, thereby prolonging the lifespan. A combination of at least one ceramic ball and of a nitride or carbonitrided raceway is particularly preferred. Preferably a carbonitrided raceway is provided. According to a further preferred embodiment of the raceways, the raceways are burnished. Due to the burnished layer on the raceway, the raceway provides a coating, which positively influences the running-in behavior of the bearing. Hereby, it is accepted that the burnished layer is not fatigue endurable during operation of the refrigerant compressor and is gradually consumed, however, this is of subordinate importance with respect to the positive characteristics of the running-in behavior of the bearing. In summary, with the present lubrication conditions, the lifespan of the bearing may be positively influenced, namely prolonged.
According to a preferred embodiment of the raceway, the raceway is carbonitrided, wherein preferably the surface may additionally be burnished after having been carbonitrided. Hereby, it is advantageous that the positive effects of both methods may be combined on the surfaces and hence the lifespan further increases.
According to an embodiment of the invention, the bearing site provides at least a second bearing, wherein the second bearing is a cylindrical roller bearing. Due to the configuration of the second bearing as cylindrical roller bearing, radial forces acting on the bearing site are supported by the cylindrical roller bearing, whereby the angular ball bearing primarily has to support axial forces. Alternatively or additionally, a further angular ball bearing or a radial ball bearing may be used. Particularly, the use of a third bearing, configured as single-row angular ball bearing, is advantageous as thereby, axial forces in both directions may be supported by the two angular ball bearings used. Alternatively or additionally, a needle bearing may be used instead of a cylindrical roller bearing for supporting radial forces.
According to a preferred embodiment of the second bearing, the second bearing provides an inner ring, an outer ring and rolling elements rolling therebetween, wherein at least one rolling element is made from ceramic. Preferably, the ceramic used is Si3N4. According to a particularly preferred embodiment, not every rolling element of the first and of the second bearing consists of ceramic; such an embodiment is referred to as semi-hybrid bearing. Designing both the first and the second bearing as semi-hybrid bearing is particularly cost-effective with the present lubrication conditions.
According to a preferred embodiment of the refrigerant compressor, the refrigerant compressor is operated with varying rotational speeds during operation of the cooling system. Hereby, it is advantageous that the cooling system may be operated according to request so that the rotational speed of the compressor and thus the performance may be reduced during a lower performance request, leading to an energy-optimized use. As the dominant viscosity ratio of the operating medium directly depends on the rotational speed of the bearing, the viscosity ratio decreases for a decreasing rotational speed. Hence, the lubrication conditions deteriorate accordingly. Due to the configuration of the at least first angular ball bearing as semi-hybrid bearing, the compressor may be used with variable rotational speed without sustainably damaging the bearing due to the resulting lubrication conditions which change in accordance with to the rotational speed.
The invention further refers to a method for operating a cooling system, wherein the rotor of the refrigerant compressor is operated with varying rotational speeds.
In the following, the invention is further illustrated based on the
Here,
In
The now gaseous first operating medium is then supplied from the vaporizer to a suction side of the refrigerant compressor 2, which re-compresses the cold gaseous first operating medium and re-supplies it to the circuit. The second operating medium, which is separated by the oil separator, is guided to the compressor, and is from there guided to bearing sites via injection pipes so that the second operating medium forms a lubrication film between rolling elements and raceways of the bearings and thus lubricates the bearings. After passing through the bearings, the second operating medium being supplied to the bearings is returned to the suction side of the compressor via an outlet pipe. Alternatively, it may also be provided that at least a part of the second operating medium is directly returned to an input side of the oil separator via an outlet pipe. Further, a part of the second operating medium provided by the oil separator is directly supplied to the screws of the screw compressor via injection pipes for lubricating the engaging screw windings or additionally cool and seal against each other. From there, the second operating medium immediately mixes with the compressed first operating medium.
In
In
The oil separator being functionally integrated into the vaporizer reduces the amount of refrigerant in the first operating medium to a value between 15% by weight and 60% by weight, and provides the oil enriched second operating medium to the outlet of the vaporizer from where it is supplied to the oil reservoir and from there to the refrigerant compressor by means of the oil pump 38. The remaining refrigerant enriched other part of the first operating medium which is separated by the vaporizer 36 and the oil separator 4, respectively, is supplied to the suction side of the centrifugal compressor via a pipe portion.
In
