Oil Separation in a Cooling Circuit

Abstract

The invention concerns a method and a cooling system for oil separation in a cooling circuit, including compression means (4) that supply coolant under a first high pressure through means for oil separation (10) and further on through condensing means (12), from where coolant is applied restriction means (16), from where coolant is conducted to flooded evaporation means (18), from where the compression means (4) suck gaseous coolant back, where remaining oil in the flooded evaporation means is returned to the cooling circuit through an oil removal circuit (26). It is the purpose of the invention to achieve an efficient oil separation from a flooded evaporator (18). This may be attained by the cooling system according to the invention, in that the oil removal circuit (26) includes a pump (24), the suction side of which being connected to the flooded evaporator (18), where the pressure outlet of the pump is connected to a pressure connection (9) between the pressure outlet of the compressor (4) and the oil separator (10).

Description

The invention concerns a method for oil separation in a cooling circuit, including compression means that supply coolant under a first high pressure through means for oil separation and further on through condensing means, from where coolant, mainly in liquid state, is applied restriction means, from where coolant is conducted to flooded evaporation means under a second lower pressure, from where the compression means suck gaseous coolant back, where remaining oil in the flooded evaporation means is returned to the cooling circuit through an oil removal circuit.

Moreover, the invention concerns a cooling system containing at least one compressor having a pressure outlet, which is connected to an oil separator, where the oil separator is further connected to at least one condenser, from where the coolant is conducted through a coolant line to at least one restriction element communicating with at least one flooded evaporator, from where primarily gaseous coolant flows to the suction side of the compressor, where an oil removal circuit from the coolant circuit is provided in connection with the flooded evaporator.

In cooling systems, coolant and oil used for lubricating the compressor come into contact with each other. Thus a certain amount of oil is conducted on to the cooling system. In well-known systems with so-called flooded evaporators, oil will be concentrated and accumulated in the evaporator. Coolant vapours sucked from the flooded evaporator to the compressor contain practically no oil. Therefore, oil from the flooded evaporator is to be returned to the compressor.

Depending on properties and quantitative ratio between oil and coolant, the oil may remain in a separate state or be mixed partly or wholly with the coolant. In the first case, the oil is immiscible; in the second case, the oil is wholly or partly miscible. Irrespectively of the miscibility, oil is normally unwanted in the evaporator as the oil reduces the heat transmission coefficient and thus the efficiency of the evaporator, and the oil disappearing from the compressor is to be substituted by refilling. The oil is therefore to be removed from flooded evaporators. If the oil is not miscible, and if the oil is heavier than the coolant, the oil is collected on the bottom of the evaporator due to the force of gravity. From here, the oil is removed manually by draining off. Due to the manual operation, this method is rarely used today. Alternatively, the draining can be automatised in that the oil by means of the force of gravity is collected in a small container connected in proximity of the evaporator, from where it is pressed to the suction side or the crank housing of the compressor, preferably by means of hot gas. The system can be activated when the collected amount of oil exceeds a determined value.

The disadvantage of the last-mentioned solution is that compressors, and particularly piston compressors, cannot stand up to liquid coolant or oil at the suction side of the compressor due to the risk of hammering. Therefore, strict requirements are made for indicating when only oil is present in the collecting container. It is also a problem that the only force driving the oil to the container is the force of gravity, which means that the driving force may be insufficient at low temperatures when the oil viscosity is high.

For systems with coolant miscible with oil, there is described a solution in “ASHRAE HANDBOOK System Practices for Halocarbon Refrigerants”, 2.29 (FIG. 1.), published 2002. By this solution, the suction line of the compressor is to be passed under the liquid level of the evaporator in order to ensure sufficient driving height for liquid transport, implying a difficult disposition of the suction line.

Another solution for removing miscible oil is found in “Pohlmann Taschenbuch der Kältetechnik”, 16. Auflage punkt 7.9.4.4. Bild 7-138, published 1978. In this case, a small dry evaporator is used, also called an oil heater for coolant evaporation, which is connected to the coolant circulation pump and in parallel with the flooded evaporator. A dry evaporator is capable of transporting the oil due to a high coolant gas speed from the dry evaporator/oil heater. The capacity of the small dry evaporator/oil heater is selected so that the maximum oil content in the flooded evaporator is not exceeded. The oil heater is mounted below liquid level in the flooded evaporator. If the oil heater is filled up with clean oil in case by a interruption of operation, the system cannot start up again by itself as the clean oil does not evaporate and therefore has to be removed manually from the oil heater. Furthermore, it is rare that flooded evaporators have a coolant recirculation pump.

In connection with miscible oils and with regard to both solutions there is, however, still the drawback that hammering may occur in the compressor.

A common problem to all solutions for both miscible and immiscible oils is that oil is transported to the suction side of the compressor. Since flooded evaporators form saturated vapours practically without superheating, there is only very little opportunity of adding extra liquid coolant for oil removed from the evaporator. As the added liquid cannot evaporate in saturated vapours, it remains in liquid state and may cause hammering which may destroy the compressor.

It is the purpose of the invention to achieve an efficient oil separation and returning of oil from a flooded evaporator. A further purpose may be to control the oil level in a compressor.

This may be attained by the cooling system according to the invention, in that the oil removal circuit includes at least one pump, the suction side of which being connected to the flooded evaporator, where the pressure outlet of the pump is connected to a pressure connection between the pressure outlet of the compressor and the oil separator.

By the invention, the oil accumulated in the evaporator is not supplied to the suction side of the compressor with the resulting risk of hammering. The pump delivers a mixture of cold liquid coolant and oil to the pressure side of the compressor in the circuit, where the coolant is present in superheated gas state, and therefore the coolant in gaseous state may absorb and evaporate the liquid coolant while the oil remains in liquid state. When the mixture of superheated coolant and oil subsequently passes through the oil separator, the larger part of the oil is removed. At the same time it happens that the efficiency of the oil separator rises, as the coolant gas mixture has lower temperature due to the heat energy used for evaporating the liquid supplied to the compressed hot coolant gas.

The oil removal circuit may advantageously be designed so that the pressure side of the pump communicates with pressure control valves from where excess coolant is conducted to the flooded evaporator. Hereby may be achieved that the oil removal circuit can remove a substantial amount of liquid coolant from the bottom of the evaporator, without substantially reducing the efficiency of the cooling system.

The cooling system may contain a number of compressors having coolant communication through oil separators and further on through at least one condenser to a number of expansion elements, from where coolant may be conducted to a number of independent evaporators where flooded evaporators advantageously may be connected to the oil removal circuit, where an outlet connections from the oil removal circuit may form a connection to a central oil distribution circuit, from where oil is conducted to the active compressors, depending on the actual oil level in each individual compressor. Hereby may be achieved an efficient automatic regulation of the oil supply to a number of compressors. The need for checking and refilling oil on individual compressors is thus reduced.

The mixture of oil and liquid and/or gaseous coolant may advantageously flow through a preheater. Before the mixing, the pressure connection between the pressure outlet of the compressor and the oil separator is provided. Hereby may be achieved that the mixture of oil, liquid and gaseous coolant returning from oil removal circuit is preheated, whereby a part of the fluid coolant is provided in gaseous state.

Advantageously, the preheater may be contained in an oil separator. Hereby may be achieved a cooling of the oil collected in the oil separator.

The method and circuit according to the invention may be used for both miscible and immiscible oil, and the pumps used in the oil removal circuit may transport both of these oils. Furthermore, it is possible to suck an oil/coolant mixture from the evaporator, where the amount of coolant is substantially greater than the amount of oil, and thus a coolant circulation arises in the evaporator itself as replacement to the sucked off coolant. This extra internal coolant flow also entrains oil located far from the suction branch. In that way, the oil removal action is increased in case of immiscible oil, compared with other technical solutions where the oil flow toward the oil removal circuit is only based on the force of gravity. This has particularly significance with low temperatures where the viscosity of the oil can be high and the oil flow thus be impeded.

By using screw compressors in the cooling circuit, a considerably amount of oil may also be circulated for compressor lubrication and for cooling pressurised gases. The oil itself may therefore also be cooled. In many cases, this is effected by injection of coolant. If the amount of the pumped coolant is selected so great that, besides removing the oil, the need for cooling pressurised gases is also covered, the extra equipment for cooling pressurised gases may be done without. In that way, a combined oil removal/oil cooling system may be provided.

As pump in the oil removal circuit, one may use the common mechanical pumps, e.g. gear wheel pumps, but other alternatives are possible, e.g. ejector pumps, MHD-pumps, gas-driven pumps etc.

In the following, the invention is explained from

FIG. 1 showing a possible embodiment of the invention, and from

FIG. 2 showing an alternative embodiment of the invention.

On FIG. 1 is shown a cooling system 2 containing at least one compressor 4 having a coolant inlet 6 and a coolant outlet 8. The coolant outlet of the compressor 4 communicates with an oil separator 10, where the oil separator 10 conducts oil back to the compressor 4 over a connection 20. From the oil separator 10 the coolant is conducted through a condenser unit 12, where the coolant changes its state from mainly gaseous to mainly liquid state. With a coolant connection 14, coolant is conducted to a restriction element 16, whereafter coolant is conducted to a flooded evaporator 18 where it evaporates. From the evaporator 18, the coolant is conducted to the compressor 4 through coolant inlet 6. At the bottom of the flooded evaporator 18 there is provided an oil drain in the shape of a connection 22 communicating with a pump 24. A connection 26 is shown from the pump 24, connected to a restriction element 28 from where a connection 30 forms connection further on to a point 9 in the connection between the pressure outlet 8 of the compressor 4 and the oil separator 10. Possibly redundant coolant is conducted from the connection 26 back to evaporator 18 through restriction element 32 and connection 34.

The exemplified cooling system 2 can be provided with a miscible or an immiscible oil.

An oil removal circuit 20, 22, 26, 30 is arranged for transporting oil mixed with coolant away from the flooded evaporator 18. The connecting line 22 may advantageously be situated below the liquid level in the evaporator 18 and form a connection to a point 9 in the line or the flowpath between the pressure outlet 8 of the compressor and the inlet of the oil separator 10. The liquid transport in the oil removal circuit is effected by means of a gear wheel pump 24 that imparts a suitably high pressure to the oil/coolant mixture, the pressure being further regulated by a orifice plate/control valve 28 for controlling the circulated amount of liquid before discharge at the point 9.

In order to separate the possible excess coolant, this is returned to the evaporator through a bypass line 34, and through a control valve 32.

By the method according to the invention, the pump 24 sucks a limited amount of coolant/oil mixture from the flooded evaporator 18 and pumps it into the pressure line of the compressor at the point 9, whereby hammering in the compressor 4 is avoided, as the superheated gas in the pressure line can absorb liquid which is evaporated due to the high temperature. At the same time, the superheating of the coolant is reduced, enhancing the efficiency of the oil separator. In the existing oil separator 10, the oil is separated and led to the compressor 4.

The system may be automatically restarted, as the pump may start with clean oil.

FIG. 2 shows an alternative embodiment of the invention, where the compressor 4, as shown on FIG. 1, has a pressure outlet 8 connected to an oil separator 110. From the oil separator 110, coolant is conducted on towards a not shown condenser via a connection 14. A pump 24 is shown with a connection to a not shown flooded evaporator. From the pump 24, there is a connection to restriction elements 28 and 32. From the restriction element 28 there is shown a connection 130 containing a heat exchanger 42 disposed inside the oil separator 110. From the heat exchanger, the connection 130 continues to a connecting point 9 in the pressurised gas connection 8. The oil separator 110 contains an oil separator element 40 disposed uppermost in the oil separator, and below there is shown an oil level 44 where oil is conducted to the compressor 4 via the connection 20.

By the embodiment shown on FIG. 2 is achieved that the mixture of oil, liquid and gaseous coolant contained in the connection 130 is preheated, whereby a part of the liquid coolant is provided in gaseous state. At the same time, cooling of the oil collected in the oil separator may be achieved.

Claims

1. A method for oil separation in a cooling circuit (2), including compression means (4) that deliver coolant under a first high pressure through means for oil separation (10) and further on through condensing means (12), from where coolant in liquid state is applied restriction means (16), from where coolant under a second lower pressure is conducted to at least one flooded evaporation means (18), from where coolant, which is mainly in gaseous state, is conducted to the suction side of the compression means (4), where remaining oil in the flooded evaporation means (18) is returned to the cooling circuit (2) through an oil removal circuit (22, 24, 26, 28, 30), characterised in that the oil removal circuit (22, 24, 26, 28, 30) includes means (24) for pumping oil from the flooded evaporation means (18) and to a point (9) in a pressure connection forming a coolant connection between the pressure outlet of the compression means (4) and at least one oil separation means (10).

2. A cooling system (2) containing at least one compressor (4) having a pressure outlet (6), which is connected to an oil separator (10), where the oil separator (10) is further connected to at least one condenser (12), from where the coolant is conducted through a coolant line (14) to at least one restriction element (16) communicating with at least one flooded evaporator (18), from where primarily gaseous coolant flows to the suction side (6) of the compressor, where an oil removal circuit (22, 24, 26, 28, 30) connected to the coolant circuit (2) is provided in connection with the flooded evaporator (18), characterised in that the oil removal circuit (22, 24, 26, 28, 30) includes a pump (24), the suction side (22) of which being connected to the flooded evaporator (18), where the pressure outlet of the pump is connected to a point (9) in a pressure connection between the pressure outlet of the compressor (8) and the oil separator (10).

3. Cooling system according to claim 1, characterised in that the outlet (8) of the pump (24) communicates with pressure control valves (28-32), from where excess coolant is conducted to the flooded evaporator (18) via pressure control valves (32).

4. Coolant (2) according to claim 1, characterised in that the cooling system (2) contains a number of compressors (4) having coolant communication through oil separators (10) and further on through at least one condenser (12) to a number of expansion elements (16), from where coolant is conducted to a number of independent evaporators (18) where flooded evaporators (18) are connected to the oil removal circuit (22, 24, 26), where an outlet connection (26) from the oil removal circuit forms a connection to a central oil distribution circuit, where oil is conducted to the active compressors depending on the actual oil level in each individual compressor.

5. Cooling system according to claim 1, characterised in that the mixture of oil and fluid and/or gaseous coolant flows through a preheater (42) before the mixture is supplied to the pressure connection (8) between the pressure outlet of the compressor and the oil separator (110).

6. Cooling system according to claim 5, characterised in that the preheater (42) is contained in the oil separator (110).