The invention is related to the field of using ultra low viscosity volatile fluids (ULWF) as lubricant for rolling element bearings in general, and in particular in a refrigeration compressor. In this connection, for instance a liquefied refrigerant is supplied from the condensor to the bearing space in the bearings. The refrigerant which enters the bearing space is at saturation condition and is sprayed onto the bearing races and the rolling elements. The pressure of the liquefied lubricant drops in this process, as a result of which some of the lubricant may evaporate. The remaining liquid lubricant is maintained at saturation level. Thus, despite the fact that ULVVF' s are fluids with a low boiling point and low viscosity, studies have revealed that they are nevertheless able to produce a lubricant film.

However, it appears that an ULVVF lubricant film is very thin in comparison to an oil based lubricant film due to said low viscosity and also due to low pressure- viscosity coefficients. A further problem in this connection is that, more often than not, within the bearing boiling or flashing occurs of the ULWF's, such as refrigerants, which are used as lubricants. This is caused by a decrease of the pressure or by the addition of energy, such as caused by frictional heat. In those cases, the bearing will run dry in an intermittent fashion, depending on said pressure decrease or added energy. Furthermore, flashing or boiling can also lead to cavitation at the entrance or the exit of the rolling contacts in the bearing. As a result of these phenomena, high contact shear stresses will occur which in the end lead to a reduced bearing life.

The object of the invention is to provide a method for lubricating a bearing by means of an ULWF which does not have these disadvantages. This object is achieved by a method for lubricating a rolling element bearing by means of an ultra low viscosity volatile fluid (ULVVF) comprising the steps of:

- injecting a liquefied fluid flow into the bearing,

- lubricating the rolling contacts between the rings and the rolling elements of the bearing by means of the liquefied fluid,

- providing a flow restriction for the liquefied fluid so as to maintain at least part of the fluid at a pressure which is above the evaporation pressure as is defined by

evaporation of the fluid at the temperature or temperatures which prevail(s) in the bearing (subcooling).

The step of providing a restriction in the flow path of the lubricant leads to a relative pressure increase of said lubricant. Such pressure increase may be just sufficient to prevent evaporation of the liquefied ULVVF which is injected in to the bearing as lubricant. The liquid state of the ULVVF is thus prolonged, with the beneficial effect of improved lubrication and avoidance of dry running phenomena.

According to a preferred way of carrying out the method, it comprises the steps of: - providing a bearing cavity,

- accommodating the bearing in the bearing cavity,

- injecting the liquefied fluid into the bearing cavity,

- making the liquefied fluid flow through the bearing,

- delimiting the bearing cavity by a flow restriction. The bearing cavity is defined as a room or area within which one or more rolling element bearings are accommodated. The bearing cavity should be distinguished from the bearing space, which is the area defined between the rings of a rolling element bearing and in which the rolling elements and possible cage(s) are located.

The flow restriction addressed before can take several forms. According to a first possibility, the rolling element bearing itself can be used as a flow restriction. Alternatively, a shield or seal may be applied at the fluid flow outlet side of the bearing, while using said shield or seal as a flow restriction. The invention furthermore comprises the steps of:

- accommodating a further bearing in the bearing cavity, - injecting the liquefied fluid in the bearing cavity defined between the two bearings,

- making the liquefied fluid flow through the bearings,

- delimiting the bearing cavity by flow restrictions at both bearings.

In particular, the invention can be used for lubricating a rolling element bearing in a refrigerant compressor used in a refrigerating cycle, wherein a refrigerant flow is compressed and liquefied in a condensor while generating a temperature increase and subsequently the main part of said refrigerant flow is evaporated in an evaporator while generating a temperature decrease, comprising the steps of:

-obtaining an auxiliary part of the refrigerant flow from the condensor at condensor outlet pressure and in liquefied form,

-increasing the pressure of said liquefied auxiliary part of the refrigerant flow to an increased pressure which is higher than the condensor outlet pressure, -supplying said increased pressure liquefied auxiliary part of the refrigerant flow to said bearing for lubrication thereof,

- delimiting the bearing space by a flow restriction.

By the step of increasing the pressure of said liquefied auxiliary part of the refrigerant flow and thereby subcooling the liquid refrigerant, several advantages are obtained. First of all, the boiling or flashing of the liquefied lubricant in the bearing is delayed having regard to the fact that a larger temperature difference has to be overcome in comparison to the prior art. Thus, a larger amount of liquefied lubricant can be maintained for longer periods within the bearing. Furthermore, the step of subcooling increases the viscosity of the refrigerant and thereby the thickness of the lubricant film. Moreover, it is easier to pump the subcooled refrigerant since pump cavitation can be avoided.

The supply of pressurized refrigerant and maintaining the pressure for a time span which is sufficient for a proper lubrication effect, can be obtained by delimiting the bearing space of the bearing by a flow restricting member. The invention is furthermore related to a bearing arrangement for a refrigerant compressor, said arrangement comprising a housing, at least one bearing accommodated in said housing for supporting a compressor shaft, said bearing comprising at least two ring elements as well as at least one series of rolling elements which are in rolling contact with said rings, said rings defining a bearing space, as well as liquid refrigerant supply means for supplying a liquid refrigerant flow into the bearing space. According to the invention, at least one flow restriction member is provided which delimits the bearing space.

Said flow restriction member can be carried out in several ways. According to a first possibility, the flow restriction member is a seal connected to one of the rings and extending towards the other ring. Said seal can slidably engage the other ring. Furthermore, the seal may be elastically deformable under the influence of the increased pressure liquefied auxiliary part of the refrigerant flow. By selecting a proper elasticity for the seal, a certain pressure can be maintained in the bearing space.

Alternatively, the flow restriction member can be a shield connected to one of the rings and extending towards the other ring. The gap between the shield and the opposite ring provides a choking effect, which enables a specific pressure build up within the bearing space. According to a further embodiment, at least two bearings can be mounted in a common housing and supporting a common shaft, said bearings, housing and shaft enclosing a cavity which at both ends is delimited by a flow restriction, the liquid refrigerant supply means emanating in the cavity.

The invention will now be described further with reference to the embodiments shown in the drawings.

Figure 1 shows an embodiment of a refrigeration installation according to the invention.

Figure 2 shows an alternative embodiment.

The refrigeration installation as shown in figure 1 comprises a compressor which is identified by reference numeral 1. Said compressor 1 comprises a motor 2 as well as compressor means 3 which have been schematically shown. The motor 2 comprises a stator and rotor (not shown in detail), which rotor is supported by means of bearing arrangements 4, 4'.

Furthermore, the installation comprises a condensor 5, which is supplied with pressurized gaseous refrigerant through line 6. In the condensor 5, the pressurized gaseous refrigerant is liquefied while releasing temperature.

Through line 7, which contains the reducing valves 8, 9 as well as economizer 10, the refrigerant is fed to the evaporator 11. In the evaporator 11 , the liquefied refrigerant is evaporated while extracting heat from the environment. The gaseous refrigerant is then returned to the compressor means 3 through line 12.

In a well known way, the liquid refrigerant pressure is reduced by the first pressure reducing valve 8, and subsequently the liquid refrigerant is fed, via economizer 10, to a second pressure reducing valve 9. Any gaseous refrigerant is fed from the economizer 10 through line 13 to the compressor means 3. According to the invention, an auxiliary flow of liquid refrigerant from the condensor 5 is fed through the auxiliary line 14 to the bearing surfaces in the bearings 4, whereby boiling or flashing is prevented or postponed. Thus, a much better lubrication behavior is obtained, as will be explained with reference to figure 2.

As shown in figure 2, the liquid is injected through a nozzle 14 in the bearing cavity 15 which is present between the two bearings 16, 17 of the bearing arrangement 4'. Said bearings 16, 17 each comprise an outer ring 18, an inner ring 19 defining a bearing space 24 and a series of balls 20 contained in said bearing space 24. The outer rings 18 are accommodated in a motor housing 21, and the inner rings 19 support the motor shaft 22. The outer rings 18 each carry a flexible or rigid seal 23.

The fluid injected in the bearing cavity 15 defined between the bearings 16, 17, the motor housing 21 and the shaft 22 has an increased pressure, whereby evaporation of the fluid is prevented or postponed so as to ensure a proper lubrication of the rolling elements 23 and the rings 18, 19 by the refrigerant liquid.

The seals 23 are selected in such a way that they prevent a rapid decrease of the pressure of the liquid in the bearing cavity 15. To that end, the seal may be rigid, flexible or semi-rigid. Also the series of balls 23 themselves together with a possible cage (not shown) both a flow restriction for maintaining a certain overpressure in the bearing space 15.