Oil management system

Field of the Invention

The present invention relates to a method and a system for oil management primary related to a refrigeration system, where compression means has an discharge for refrigerant connected to means for oil separation, from which oil separation means, oil is led back towards the compression means.

Background of the Invention

US 5347821 describes a method and apparatus for monitoring oil charge loss for use with a refrigeration system. The refrigeration system has a compressor for compressing a refrigerant gas, an oil/gas separator for separating compressed refrigerant from lubricating oil, a condenser for condensing the compressed refrigerant gas, an oil cooler for cooling oil separated from the refrigerant, the refrigerant and oil both having known and differing coefficients of heat transfer, and an injection system for injecting the cooled oil into the compressor.

WO 03/027586 Al concerns a liquid purger for purging liquid from a gas where the liquid purger has a constantly open passage for flow of gas and purged liquid, the passage being connected to a return duct where the flow is controllable, and where the return duct can lead gas and the purged liquid back to e.g. a cooling compressor or to a reservoir in the closed circuit. The invention is based on the view that by establishing a weak fluid flow from the bottom of a pressurised vessel in which purged liquid may occur, it is possible to lead this liquid flow through a narrow passage in a pressure loaded valve piston in such a way that this through-flow may occur without any visible influence on the position of the valve piston as long as there is only gas, whereas an increased pressure is built up against the valve piston upon the appearance of liquid in the flow due to the higher density and viscosity of the liquid, whereby the piston is pushed back against the pressure load and thereby opens a wide return duct for the supplied fluid so that it is returned to its area of application.

EP1434022A2 describes a heat exchanger comprising an elongated housing, a group of parallel pipes placed in the housing for passing a cooling medium there through, on which pipes a series of cooling plates have been arranged for cooling a medium to be cooled in the housing, wherein the housing has a round cross-section, wherein the cooling plates in their plane have a geometry composed of several plate portions, particularly plate portions having a separately distinguishable geometry. Preferably the cooling plates have a geometry composed of several rectangular plate portions originating from the same web of material and placed adjacently to each other.

Ob j ect of the Invention

The object of the invention is to provide method and a system for oil management where a common pressure shield contains all oil management functions for treatment of the mixture of oil and refrigerant that leaves the compressor, and returning the oil to the compressor. A further object of the invention is to perform efficient liquid separation and return the oil in the optimum cleaning and temperature condition.

Description of the Invention

This can be achieved in a system where the compressor discharge can operate in conjunction with at least one pressure shield, which pressure shield may comprise at least the following components related to oil management: an oil separator from which oil is flowing to an oil sump; an oil cooler connected to the oil sump, a mixing valve in which the oil from the oil cooler is mixed with oil from the oil sump for achieving an optimised oil temperature, an oil filter for filtering the mixed oil, the oil being returned from the oil filter to the compressor, and where at least the mentioned components can operate at a pressure level substantially equivalent to the discharge pressure at the compressor.

It is hereby achieved that all the oil components can operate in an environment having a high pressure level so that the shielding of the components only needs very limited material strength because there is no or only a small pressure difference to overcome. Also from an energy point of view, it is very efficient to keep the oil at more or less the same pressure level so that the oil that is returned to the compressor has a sufficiently high pressure level related to the oil Met of the compressor. The

combination of the oil components inside the pressure shield reduces the number of pipes that otherwise would be necessary for oil management around a refrigeration system. The reduction of high pressure pipes also reduces the risk of leaks in any of the tubing where a leak could lead to a loss of oil but also to a loss of refrigerant.

The compressor discharge is connected directly to the inside of the pressure shield the bottom of which forms the oil sump. The oil is pressed through oil separators before the refrigerant leaves the pressure shield through a piping. The oil that has been separated from the refrigerant is flowing downwards to the oil sump.

The oil cooler is preferably formed as a longitudinal heat exchanger that can be placed angled at a first angle related to the longitudinal axis of the pressure shield. It can hereby be achieved that the length of the heat exchanger can increase to a length higher than the length of the shielding. It is also possible that one end of the heat exchanger has its inlet submerged in the oil sump, where the upper part of the heat exchanger is placed in the upper part of the pressure shield.

The first angle of the heat exchanger related to the longitudinal axis of the pressure shield might be at least 8 degrees. It is hereby achieved that the heat exchanger can form a thermo siphon.

It is preferred that the heat exchanger comprises a number of longitudinal pipes surrounded by fins, where a refrigerant can flow inside the pipes, where oil can flow outside the pipes, and where heat can be transmitted from the oil towards the refrigerant. It can hereby be achieved that an extremely big surface can be used for the transportation of heat from the oil towards the refrigerant.

The heat exchanger can be formed inside a heat exchanger shield, which shield can have inlet openings in the lower part for forming an access to an oil sump formed in the lower part of the pressure shield. In this way, it is achieved that the oil has access to the oil sump and will fill up at least the lower part of the heat exchanger. Because. of the relatively high pressure inside the pressure shield, this pressure will press the oil

upwards through the heat exchanger, if from the top of the heat exchanger, there is one or another connection towards a lower pressure level.

Longitudinal channels can be formed inside the heat exchanger shield between the shield and the fins of the heat exchanger. The channels inside the heat exchanger shield are necessary for having a flow in an upwards direction inside the shield simply because the fins of the heat exchanger otherwise would block for the flow in the upwards direction. By generating these channels, there can be a flow upwards and depending on how the inlet and the discharge are placed in the heat exchanger, it is possible that all the oil has to pass through the heat exchanger at least once in a direction along the fins and perpendicular to the heat exchanger itself.

Channels between the heat exchanger and the related shield can be partly blocked for forming at least two sections, where oil is forced to flow between the fins for flowing from a first section to a second section. By blocking one of the channels so that only one channel is open, the oil can be forced to pass along the fins perpendicular to the direction of the heat exchanger several times depending on the number of blocking elements that are placed along the heat exchanger. A change in direction two to four times along the heat exchanger is probably preferred.

The heat exchanger can be formed with a separated section, where the heat exchanger shield has an inlet and an outlet for a cooling medium, which can be used for cooling electronic switches, which control a motor connected to the compressor. This can lead to a reduction of the physical size of the electronic equipment that might be used for driving an electric motor, which drives a compressor. By using liquid cooling of the power electronic components, for example IGBTs or other semiconductor switches, an effective cooling can take place which has a relatively small volume compared to what should have been the case if air cooling was used. The amount of heat that has to be removed from the electronic circuit could be as high as a few percent of the total electric power that is used for driving the compressor. Therefore, the cooling demand for the electronic circuit could be several kilowatts.

The invention can also be expressed as a method for oil management primarily related to a refrigeration system, where compression means has a discharge for refrigerant connected to means for oil separation, from which oil separation means, oil is lead back towards the compression means where the compression means is operating in conjunction with at least one pressure shield, which pressure shield comprises at least the following operations related to oil management, separating oil from refrigerant, cooling the oil, filtering the oil, and returning oil to the compression means, which operations are performed at a pressure level substantially equivalent to the discharge pressure at the compression means.

By this method, oil can be managed at the pressure, which it has when it leaves for example a compressor. In this way, all the different components for oil management are placed inside the same shield which means that the casing of the different components do not need to overcome the high pressure. The level of the pressure leaving the compressor depends on the refrigerant, which is used.

The oil can be cooled in a longitudinal heat exchanger placed angled at a first angle related to the longitudinal axis of the pressure shield. This is only one possible way of forming the heat exchanger. Other heat exchangers having a very effective heat transmission between the oil and a cooling medium could be used. The longitudinal heat exchanger has the advantage that at least from one end, it could be possible to get access for refrigerant to the heat exchanger, and probably from the same end both inlet and a discharge can be placed side by side, hi this way, refrigerant from the suction side of a refrigeration system could be used in the oil cooler. This refrigerant could come directly from an evaporator where an increasing temperature of this refrigerant only limits the risk of sending liquid refrigerant towards the compressor inlet. Especially if the compressor is a piston compressor, liquid refrigerant in the suction side of the compressor would destroy the compressor because liquid refrigerant cannot be compressed.

Preferably, the heat exchanger may comprise a number of longitudinal pipes surrounded by fins, where a refrigerant can flow inside the pipes, where oil can flow outside the pipes, where heat can be transmitted from the oil towards the refrigerant.

By letting the refrigerant flow inside the pipes, an effective cooling occurs by the fins, which have a very big surface that is in contact with the oil. The oil flow between the fins could be very turbulent which also will increase the heat transmission from the oil towards the refrigerant.

The oil might flow inside a heat exchanger shield, which shield can be formed with inlet openings in the lower part for forming an access to an oil sump formed in the lower part of the pressure shield. Forming a shield around the heat exchanger can lead to only a certain amount of oil being cooled down by the refrigerant so that only a small amount of oil can continuously be delivered back to the compressor at a low temperature. The great amount of oil that fills up the oil sump can have an increasing temperature without damaging the surroundings, and only the small amount that is used for the compressor is treated by the oil management system as such.

The oil can flow in longitudinal channels inside the heat exchanger shield between the shield and the fins of the heat exchanger. The longitudinal channels simply increase the oil flow inside the shield because the fins of the heat exchanger are partly blocking the flow in the longitudinal direction of the heat exchanger.

The oil can flow in the channels, which between heat exchanger and the related shield are partly blocked for forming at least two sections, where oil can be forced to flow between the fins to flow from a first section to a second section. By separating the heat exchanger into sections, the oil is forced to circulate along the fins perpendicular to the direction of the heat exchanger. This will increase the heat transmission between the oil and the refrigerant.

The heat exchanger can be formed with a separated section for cooling a medium used for cooling electronic switches, which controls a motor connected to the compressor. It can hereby be achieved that the same refrigerant continues in elongated channels into the extra section of the heat exchanger, but where this heat exchanger is cooling a medium, which is used for cooling the electronic circuit. This will lead to a very effective electronic cooling, and big air blowing equipment is not necessary for cooling the power electronic components.

Detailed Description of the Invention

Fig. 1 shows a refrigeration system according to the invention,

Fig. 2 shows a sectional view of a pressure shield comprising an oil cooler formed as a longitudinal heat exchanger,

Fig. 3 shows a sectional view according to the line A-A in Fig. 2,

Figs. 4 and 5 show a sectional view according to the lines D-D,

Fig. 6 shows a sectional view according to the lines C-C, and

Fig. 7 shows a refrigeration system using refrigerant for oil-cooling.

Fig. 8 shows the same refrigeration system as Fig. 1 but modified for use of refrigerant for oil cooling.

Fig. 1 shows a refrigeration system 2 which system comprises an oil management system 4. The system 2 also comprises a motor 5 that drives a compressor 6 which compressor 6 is connected to a suction line 8 which line 8 is connected to valve means 7. The compressor 6 has a discharge 10, which is led into a pressure shield 14. Inside the pressure shield 14, oil-separating means 12 is placed which means 12 comprises a first demister 26 which is built up of a number of travel plates. The refrigerant hereafter passes through an angled demister 28 for further oil separation. A line 30 leads to a fine separator 32 that is able to separate even small particles of oil. Refrigerant leaves the fine separator 32 through a line 34 which is connected to an outlet valve 35. The fine separator 32 has an oil outlet 36 which is connected over a line 37 to an oil return valve 46. From this oil return valve 46, a line 48 leads the oil back to the compressor 6. The pressure shield 14 comprises an oil sump 42. Submerged in this oil sump, oil cooler 22 is placed. This oil cooler has an oil suction line 50 in the oil sump 42 from where oil 16 is sucked through a first raw filter 52

before the oil reaches the oil cooler 22 which might be a heat exchanger where the warm oil is heat exchanged by a medium that is led into the oil cooler 22 from an inlet 38, and where this medium leaves the oil cooler through an outlet 40. The cooled oil leaves the oil cooler 22 through a line 54 which leads to a mixing valve 56. This mixing valve 56 has an oil inlet 58 also connected to the oil sump 42. An outlet 60 from the valve 56 contains oil at the optimised temperature, and this oil is led through the line 60 to a service valve 62. From there, the oil is led to the oil filter 24. The oil filter 24 is connected to a service valve 24 for evacuating the oil filter 24. The oil filter 24 has an outlet 63 which is led to a further service valve 66. From this service valve 66, a line 68 leads to the compressor 6.

During operation, warm refrigerant mixed with oil 16 is led into the pressure shield 14. The refrigerant is pressed out through oil separators 12 which contain demister arrangements 26, 28 for catching as much as possible of the oil 16, which is contained in the refrigerant 8. This oil 16 drips downwards forming an oil sump 42. This oil 16 has nearly the same pressure as the discharge pressure of the compressor 6. From the angled demister 28, the refrigerant is sucked out through a line 30 where the refrigerant is further separated from its content of oil in the next fine demister 32. The oil that is caught in the fine demister 32 is delivered through a line 36 directly to the compressor 6 over an oil return valve 46. hi the oil cooler 22, oil from the oil sump 42 is cooled down and led back over a line 45 to the mixing valve 56 where this relatively cool oil in the line 54 is mixed with warm oil from the oil sump 42. Depending on the temperature in the oil sump, the mixing valve 56 opens or closes the inlet to the line 58 for achieving the optimal temperature in the line 60. In this way, the oil temperature that is led back into the compressor can be kept at a temperature level, which is optimal for the compressor. This optimal temperature can be achieved in nearly all operational situations independently of the load on the compressor.

hi Fig. 2, the oil cooler 122 is formed as a longitudinal heat exchanger 130. The heat exchanger is placed angled with the first angle 132 in relation to the longitudinal axis of the pressure shield 114. The heat exchanger 130 comprises longitudinal pipes 134 surrounded by a perpendicular fins. The heat exchanger is formed inside a shield 138 which shield 138 has openings 140 for inlet of oil. Inside the shield 138, channels 144

for oil circulation are formed. In the lower part of the pressure shield 114, an oil sump 142 is present. The heat exchanger 130 has an elongation 150 in which the pipes 134 continue, but where another cooling medium is surrounding the pipes in the section 150. TMs cooling medium has an inlet 154 and an outlet 156. hi the lower end of the heat exchanger 130, the channels 134 are ending in a chamber 158. Into this chamber liquid refrigerant from the condenser or receiver at a higher geodaetic level is lead and at the other end of the heat exchanger 130, the outlet 161 is connected to the condenser or reciever. The flange 163 is in this configuration blinded. Due to the partly evaporation that occurs at the heat exchange a self-circulating system is established; known as a thermosiphon. Another option is to cool the oil by water or brine pumped by force into 163 and returned at 161 with the cavity 158 acting as a returning chamber. Further, an oil valve 164 that is connected to an oil return line 168 is shown.

During operation of the invention as shown in Fig. 2 the oil flows through openings 140 into channels 144, 146, 148 inside the heat exchanger 130 where pipes 134 are cooled by circulating a cooling media. This cooling media can be water or brine or a refrigerant, hi the inlet, a liquid cooling media might flow, which media is partly or fully evaporated when leaving the heat exchanger. Between the pipes 134, fins are placed so that the oil has to circulate between the fins in order to come from one of the channels 144 to the other channel 146, 148. These channels 144, 146, 148 can be placed so that the heat exchanger 130 is separated into sections 146,148 by shields 145, 147. The oil 116 is circulating upwards along the pipes 134 and changing from the channels 144,146,148 maybe up to 10 times before the oil reaches an oil outlet, which is connected to an oil filter 124. From this oil filter 124, oil is sent towards a valve 16 for mixing the cooled oil with warm oil for achieving a preferred temperature for the compressor. When this valve 164 opens, there is an oil connection through an oil line 168 into the compressor at a pressure level, which is sufficiently lower than the discharge pressure, which will suction the oil into the compressor. Further, an inlet 160 for refrigerant 108 and an outlet 162 for the refrigerant 108 are shown on the figure.

Fig. 3 shows a sectional view of Fig. 2 according to a line A-A. Fig. 3 shows an angled demister 228, which internally has a piping 230 in which refrigerant flows out -from the demister and towards a fine demister, indicated as 32 in Fig. 1. Fig. 3 further shows an inlet 261 and an outlet 263 for a cooling medium that flows in the pipes 134 shown in Fig. 2, which are indicated below the inlet 261 and outlet 263. A valve 262 is shown which valve is a manual closing valve that can isolate the oil filter 224 from all the other parts of the system, see Fig. 1 valve 62. A further valve 266 is placed for closing the outlet of the oil filter, see Fig. 1 valve 66. A valve 264 is also indicated which valve 264 is an oil-mixing valve where cooled oil is mixed with warm oil for achieving the optimized oil temperature for the compressor.

Fig. 4 shows a sectional view of Fig. 3 at the line B-B. Fig. 4 shows the cut-off valve 262, the cut-off valve 266 and the mixing valve 264. Leading away from the shut-off valve 266, the start of piping 268 is shown which piping is the oil pipe that leads purified and temperature-regulated oil back to the compressor.

Fig. 5 shows a sectional view of Fig. 2 at the line D-D. Fig. 5 shows the pipe 238, which comprises pipes 234 for a cooling media. A shield 245 is furthermore shown that shields most of the oil flow longitudinally through the heat exchanger. Only at channel 244 there is an opening where oil can pass from one section of the heat exchanger to the next.

Furthermore, Fig. 6 shows the pipe 238 and the inner pipes 234 where a channel 246 is open for oil circulation.

Fig. 7 shows the same refrigeration system as Fig. 1 but modified for use of refrigerant for oil cooling. Only the differences to Fig. 1 are described in the following:

Liquid refrigerant at a high-pressure level is delivered in pipe towards an expansion valve 70. Expanded refrigerant flows in a pipe 72 to the inlet of the heat exchanger 22. In the heat exchanger 22 the refrigerant evaporates by cooling the oil. Fully or partly evaporated refrigerant leaves the heat exchanger through a pipe 76 which pipe is connected to the compressor, preferably to a port corresponding to a pressure close to

discharge pressure. The opening degree of the expansion valve is controlled by temperature measuring means 74 connected to the pipe 76 containing refrigerant leaving the heat exchanger.

Fig. 8 shows the same refrigeration system as Fig. 1 but modified for use of refrigerant for oil cooling. Only the differences to Fig. 1 are described in the following:

Liquid refrigerant at a high-pressure level is delivered in pipe towards an expansion valve 70. Expanded refrigerant flows in a pipe 72 to the inlet of the heat exchanger 22.

In the heat exchanger 22 the refrigerant evaporates by cooling the oil. Fully or partly evaporated refrigerant leaves the heat exchanger through a pipe 76 which pipe is connected to the compressor 6, preferably to a port corresponding to pressure close to discharge. The opening degree of the expansion valve 70 is controlled by temperature measuring means 75 connected to the oil pipe 54 leaving the heat exchanger 22. This set-up eliminates the need for a mixing valve 56(fϊg. 1 and 7).