The present invention relates to a separator for separating liquid from a gas. The invention in particular relates to conduits where a gas is compressed in a compressor and where the gas discharged from the compressor carries fine droplets of liquid which need to be removed from the gas. The in¬ vention has more particular relevance to compressor driven refrigeration systems wherein oil needs to be removed from a refrigerant.

In the operation of a compressor, e.g. a screw compressor or a scroll compressor, with the purpose of compressing a working fluid, e.g. gaseous ammonia, to be circulated in a closed refrigerant loop of a refrigeration system, it may be necessary to inject oil into the working space inside the compressor where the working fluid is compressed. The oil serves the purposes of lubricating the compressor bearings, sealing the clearances between rotors and housing and cooling the compressor. This has the consequence that the compressed fluid discharged by the compressor comprises gaseous refri¬ gerant mixed with droplets of oil. Since circulation of oil in the remaining portion of the refrigerant loop is detrimen¬ tal to system performance, it is a general practice to convey the fluid discharged by the compressor through an oil separator wherein the oil is separated from the working fluid and to recycle the oil to oil inlets at the compressor adjacent the suction side by means of a shorter oil return line.

The fluid leaving the compressor is generally gaseous refrigerant, such as ammonia with fine droplets of oil. Separation of oil from the compressed ammonia is achieved by a process wherein the compressed ammonia is taken to a vessel with an enlarged cross section in order to reduce the velocity of the flow and conveyed through a filling of wire meshing. By passing through the filling of wire meshing, the

ammonia gas stream loaded with oil droplets undergoes many changes of directions whereby the oil droplets collide with the wires of the meshing to create a film of oil which may flow off downwards or which may form larger droplets which in turn may be carried away with the gas stream. The larger droplets are more likely to drop out of the gas stream due to the force of gravity, provided flow conditions allow it.

It is a general practice to arrange at least two stages of oil separators based on mutually similar principles, the first stage, referred to as the agglomerator stage, compri¬ sing fillings of relatively coarse wire meshings, the second stage, referred to as the coalescer stage, comprising a filling of very fine wire meshing. Generally, the greater part of the oil content is removed in the first stage, whereas the second stage is necessary to obtain a very low relative oil content in the ammonia leaving the separator. Oil returned from the two separator stages has to be conveyed in separate conduits up to the compressor as there is a pressure differential between the two stages which makes it necessary to avoid shunting the oil discharge connections.

Since the pressure of the working fluid, at least at the compressor discharge side, may reach a substantial level, e.g. 20 Bars, it is often preferred to fit the two stages of the oil separator inside a common pressure vessel, e.g. an elongate cylindrical vessel, wherein the two stages are separated by a partition wall extending perpendicularly to the axis of the pressure vessel. The partition wall should be capable of withstanding the pressure differential between these two stages, e.g. about 0.2 Bars, whereas the pressure vessel as a whole should be capable of safely withstanding 20 Bars internal pressure.

Manufacture of an oil separator of this kind is relatively complicated since the fitting and securing of the partition wall and the fitting of the wire mesh fillings has to be

performed manually inside the pressure vessel. Furthermore, the restricted access makes inspection, testing, and service of these components difficult. Certain repair procedures, e.g. welding to mend oil leakages between the stages, may be harmful to the integrity of the pressure vessel, therefore requiring renewed certification before the vessel can be put into service again.

US patent 5,214,937 discloses an oil separator for use in a screw compressor driven refrigeration system which oil separator comprises an essentially cylindrically shaped casing provided with internal divider plates or partitions and external connections so as to define a flow path taking the entering gas through muffler chambers, demister pads from where removed oil is collected in a main oil sump and through a coalescer from where removed oil may be collected in a secondary oil sump while the cleaned gas leaves the casing through a gas outlet. A drain from the main oil sump is arranged in the casing wall near one end of the cylindrical vessel while a drain for the secondary oil sump is similarly arranged in the casing near the opposite end of the vessel. The publication does not clarify how this structure could be manufactured or serviced.

EP-A-0 583 770 discloses a two-stage oil separator wherein the gas is caused to flow in a helical flow path sweeping a steel mesh screen and then through an aperture in a dividing wall internally of the oil separator and through a coalescing filter in a second stage of the oil separator. This structure is enclosed into a cylindrical pressure vessel which is provided with a removable header plate at one end. Removal of the header plate permits access to and removal of the coalescing filter. Since the coalescing filter is a part of the membrane separating the two stages, and since a pressure drop across the coalescing filter needs to be present for the intended functioning, it is essential that a sealing engage¬ ment between the coalescing filter and the internal dividing

wall can be obtained once the coalescing filter has been inserted. However, access to the sealing surfaces for inspection and possibly for repair is narrowly restricted.

US patent 4,662,190 discloses an assembly where a screw compressor is partially integrated into the housing of an oil separator. However, the oil separator features only one stage.

It is the object of the invention to provide a liquid separator which is simpler in manufacture and maintenance. It is a further object of the invention to provide a liquid separator which is easily modified to satisfy changing demands. It is a still further object of the invention to provide a liquid separator which exhibits longterm stability and superior performance. These objects are achieved by the apparatus as recited in claim 1.

In the apparatus according to the invention the core struc- ture forms no part of the pressure bearing structure consti¬ tuted essentially of the casing parts. Standards and codes concerning the safety of pressure vessels therefore apply solely to the casing parts. Maintenance repair, or modifica¬ tion of the core structure will therefore not necessitate new certification of the oil separator.

On the other hand, modification of the casing parts, necessi¬ tated e.g. due to diferring national safety codes, does not require redesign of the core structure. The invention also makes it commercially viable to manufacture more complicated versions of the core structure, e.g. to satisfy more sophi¬ sticated performance requirements.

According to a preferred embodiment, a two-stage liquid separator is constructed with a substantially closed chamber forming essentially the second stage arranged as a removable core structure fitted inside a pressure vessel, the first

stage of the liquid separator occupying generally the annular space externally of the removable unit but internally of the pressure vessel jacket. The removable core structure is preferably adapted for sealing engagement with the removable inspection cover of the pressure vessel when the cover is installed.

Access to the second stage for external connections, such as gas outlet, liquid outlet, level gauges, sight glasses, meters etc. is conveniently gained at the end cover adjacent the removable cover. Access to the first stage is generally possible everywhere else. Partition walls and holders for wire mesh fillings for the first as well as for the second stage are easily fitted as part of the removable core structure away from the pressure vessel to form a unit or subassembly which may subsequently be introduced into the pressure vessel from one end and sealed by the securing of the end cover. All membranes essential to the separation between the stages may be tested and repaired separate from the pressure vessel.

The flow pattern is preferably arranged with the working fluid being introduced through a peripheral opening in the pressure vessel jacket close to one end into the annular chamber externally of the inner vessel. From here the gas flow passes a first wire mesh filling to enter into an intermediate section with larger cross section, to pass a second wire mesh filling, to pass a volume wherein larger droplets are collected in a sump. At a point close to the opposite end of the pressure vessel the gas enters a ge¬ nerally centrally arranged second stage entry pipe from where the gas flows axially in the opposite direction to enter the second stage of the liquid separator, passing here a fine cylindrical wire mesh in the direction from the center axis and radially outwards into the annular space externally of the fine cylindrical wire mesh wherein it is conveyed on

axially to leave the inner vessel and the pressure vessel through an axial outlet in the removable end cover.

According to preferred embodiments, access for the outlet for gas is gained in the cover and the oil outlet for one of the separator stages is arranged in the cover and the other one in the end cap. This simplifies the manufacture as it is generally simpler to make the connections in the end cover or in the end cap than it is to make similar installations in the cylindrical part of the vessel.

According to a preferred embodiment, the core structure supports baffle plates adapted for defining a plurality of chambers with different acoustical properties. This creates a muffler inside the liquid separator which will serve to reduce pulsations in the refrigerant flow which may be generated by the compressor, thereby reducing the noise radiated from the oil separator and from other parts of the refrigeration system.

According to preferred embodiments, the cover and the end cap comprise respective substantially planar plates. This is an advantage in manufacturing as it permits the use of automatic computer controlled tools and welding fixtures for the fitting of the connections etc.

Further advantages and benefits of the inventions will appear from the appended detailed description of preferred em¬ bodiments which is given with reference to the drawings, wherein

Fig. 1 shows a longitudinal view, partially in section through the pressure vessel and the end cover in empty but assembled state,

Fig. 2 shows a longitudinal section through the core structure, and

Fig. 3 shows a longitudinal section through the assem¬ bled oil separator as installed in the refrigera¬ tion system.

All drawings are schematic and not necessarily to scale and illustrate only features essential to enable those skilled in the art to understand and practice the invention, whereas such features which may readily be suggested or implemented by those skilled in the art have been omitted from the drawings for the sake of clarity. Throughout the drawings similar references are used for identical or similar fea¬ tures.

Reference is first made to fig. 1 showing a pressure vessel with the end cover fitted in a longitudinal and partially sectional view. The vessel 1 features an essentially cylin¬ drical jacket 3 which is circular and generally symmetrical about axis 2. The jacket is closed to the right hand side in fig. 1 by attached end cap 4 comprising a substantially circular planar plate. The end cap 4 may be joined to the circular edge of the jacket 3 by welding. The end cap 4 also features an oil outlet 9 in the form of a pipe introduced actually through the end cap and angled downwards and with an aperture placed close to the inside wall 5 of the jacket. During normal service the vessel will be oriented so that the pipe aperture will be situated towards the vessel lower portion in order that any oil collected may be drained through the pipe. The end cap 4 may be provided with other equipment such as level gauges, sight glasses, instrumenta- tion, etc. Such auxiliary equipment is considered known in the art and is therefore not illustrated in the figures.

The vessel 1 is further provided with a pipe fitting joined to the jacket 3 at a position somewhat to the left of the center of the jacket and upwards in the service position which is illustrated in fig. 1. This is the fitting for the inlet 8 for gas mixed with oil.

The left hand circular edge of the jacket 3 is fitted with a peripheral flange 6 which is in a preferred embodiment welded to the edge of the jacket. This flange 6 provides a sub¬ stantially planar sealing surface 7 towards the left in fig. 1. The flange is also provided with walls permitting inser¬ tion of bolts.

According to a preferred embodiment, the flange is arranged so as to substantially leave the aperture defined by the jacket inside 5 unrestricted in order to facilitate insertion of various equipment to be explained later on into the interior of the vessel.

To the left hand side in fig. 1 is further illustrated removable cover 10 featuring a substantially planar and circular plate or disc with a sealing surface 11 and with bores for insertion of bolts adapted for mating engagement with the flange 6 as illustrated in fig. 1.

Flange 6 is provided with an axial pipe fitting used for gas outlet 12. Flange 6 is also provided with a throughport situated as illustrated in fig. 1 below the gas outlet 12 but spaced somewhat above the lowermost part of the jacket 3. This port serves as the oil outlet 13 for the second stage to be explained later.

Reference is now made to fig. 2 for a brief explanation of the core structure which is a part of the oil separator. The core structure 14 comprises cylindrical chamber jacket 15 fitted in the left hand end in fig. 2 with peripheral chamber flange 16, externally of the chamber jacket and connected at the opposite end to chamber end wall 17 which is in the form of a disc like plate with a central aperture. A brace 31 welded to the chamber end wall 17 bridges the central aperture and supports axial stay bolt 23 extending from the chamber end wall and leftwards as illustrated in fig. 2. The stay bolt is provided with threads for engaging a nut which

holds coalescing filter end plate 25 which compresses the cylindric coalescing filter 24 against the chamber end wall 17.

The chamber jacket 15 outside supports peripheral coarse wire mesh filling 18 held between the marginal portion of the chamber end wall and wire mesh supporting rim 19 welded peripherally to the outside of the chamber jacket 15. The marginal portions of the chamber end wall and the rim are provided with perforations (not shown) in order to allow throughflow of gas.

The chamber jacket 15 outside further supports baffle plates 32 in the form of radial rim sections with through openings arranged in a staggered pattern.

Chamber end wall 17 further supports second stage entry pipe 20 which registers with the aperture in the end wall and which extends towards the right in fig. 2 to end in a bended portion, the opening of the pipe turned upwards.

The entry pipe 20 structurally supports fine wire mesh filling 21 arranged peripherally to the entry pipe and sandwiched between two wire mesh supporting plates 22. The wire mesh supporting plates are provided with perforations (not shown) in order to allow throughflow of gas.

Reference is now made to fig. 3 for an illustration of a section through the assembled oil separator.

In order to assemble the parts illustrated in figures 1 and 2 to form the oil separator 30 as illustrated in fig. 3, core structure 14 is introduced into the vessel 1 through the opening which emerges while the end cover is removed. As may be understood referring to fig. 3, the wire mesh fillings fit snugly against the jacket inside 5 and the edges of the wire mesh supporting plates 22, the edge of the chamber end wall

17, the edge of the wire mesh supporting rim 19 and the edges of the baffle plates 32 fit inside the jacket in a manner where they are slideable but divide the volume enclosed inside the vessel into separate compartments.

The cover 10 is registered over the flange 6 and the bolts are inserted and tightened. Chamber flange 16 sealingly engages flange sealing surface 7 and sealingly engages also the cover sealing surface 11 in order that the assembled structure will be capable of withstanding substantial internal pressures, e.g. in the order of 20 Bars.

The pressure differentials supported by the partitions which are part of the core structure are much smaller, e.g. below 0.2 Bars.

The flow path defined inside the oil separator is essentially as follows: Gas mixed with droplets of oil enters gas inlet 8 leading to the annular chamber exteriourly of the chamber jacket 15 and interiourly of the vessel jacket 3. In the preferred embodiment, this space is provided with baffle plates 32 in order to provide subchambers with different acoustic properties. From this chamber the gas passes the perforations in wire mesh supporting rim 19, the coarse wire mesh filling 18, the perforations in the peripheral portion of chamber end wall 17 to enter the volume just to the right of chamber end wall 17.

The coarse wire mesh filling serves as an agglomerator essentially agglomerating fine droplets of oil into larger droplets of oil most of which are carried away from the wire mesh filling in the gas stream. Subsequently the gas enters the fine wire mesh filling 21 by way of the perforations in the wire mesh supporting plates 22 to pass on to the space to the right of the fine wire mesh filling 21. The portions of the flow path mentioned so far are generally referred to as the first stage 26.

The cross sectional area of the fine wire mesh filling is larger than the cross sectional area of the coarse wire mesh filling, e.g. by a factor of five, therefore the gas flow velocity is correspondingly lower. This combined with the finer meshing serves to make the greater proportion of the entrained oil droplets collect on the wire meshing and flow downwards to collect in the oil sump 27 of the first stage.

The gas moves on to enter the end opening of second stage entry pipe 20 from where it moves leftwards as illustrated in fig. 3 to enter the space axially of coalescing filter 24. Coalescing filter 24 generally comprises a very fine wire meshing or a glass fiber meshing which serves to remove the greater proportion of any remaining oil content from the gas. The gas moves radially outwards through the coalescing filter into the chamber peripherally of the coalescing filter but internally of the chamber jacket 15 in which chamber the gas moves leftwards, cornering coalescing filter end plate 25 to leave the oil separator by the actual gas outlet 12. These portions of the flow path are generally referred to as the second stage 28.

Oil left in the coalescing filter 24 flows downwards to be collected in the oil sump 29 of the second stage from which oil may be removed through the oil outlet 13, which is located at a position just above the lowermost portion of the chamber jacket.

An oil separator manufactured according to the invention has proven capable of treating ammonia mixed with 1% of oil entering at a pressure of 14 Bars, ammonia being discharged after a pressure drop of 0.18 Bars in the oil separator and with a remaining content of oil at just 10 parts per million.

Although in the preferred embodiment the volume externally of the removable chamber defines the first stage while the volume internally defines the second stage, it will be

obvious to those skilled in the art to suggest an embodiment based on similar principles, but where the functions of these volumes were interchanged. Such modifications are considered to lie within the scope of the invention.

Although in the functional description of the preferred embodiment, particular reference has been made to removal of oil, this is not intended to exclude that the invention could equally well be applied to the removal of any other liquid or mixture of liquids from a gas. Although particular reference has been made to gaseous ammonia as the gas carrying droplets of liquid, this is not intended to exclude that the invention could equally well be applied to the cleaning of other refrigerants, e.g. R22, R134a, R404A, HFC, hydrocarbon refrigerants or any other gas.

Although various components and systems have been explained in particular set-ups above, this is not to exclude that such components or systems might be applied in other set-ups, might be configured differently or might be separately patentable. Although particular examples have been mentioned, the detailed explanation has the sole purpose of facilitating understanding of the invention and is not intended to limit the scope thereof which is defined exclusively by the appended patent claims.