Background to the Invention

The present invention relates to an oil separator, a method of making the oil separator and an arrangement of more than one oil separator. It finds particular application in cooling systems such as air conditioning or refrigeration systems, where oil exiting with the discharge gas from a compressor is required to be separated from the gas stream, for efficient heat exchanger operation.

Figure 1 shows an overview of known air conditioning equipment comprising a compressor 14 and an external oil separator 12, these having a connection 17 to heat exchangers (not shown). During a compression process, gas enters the compressor 14 under suction 4. Oil is injected into the compressor 14 to improve efficiency and to provide cooling of the compressor 14. A gas and oil mixture is created in the compressor 14 which is delivered via a first inlet pathway 13 to the oil separator 12. Once separated, the gas is delivered via the connection 17 to the heat exchangers and the oil is delivered via a third pathway 16 back to the compressor 14. The quantity of oil allowed to enter a cooling system such as an air conditioning system must be kept to a minimum if heat exchanger efficiency is to be maintained. For example, oil carry over limits have been approximately 2% for direct expansion system types.

Oil separation is a requirement for efficient operation of any cooling system such as a refrigeration or air conditioning system operating with a compressor 14 which requires oil for lubrication and/ or sealing and/ or cooling, unless the oil is completely isolated from the compression process. Such oil separators 12 tend to adopt one or more of the standard oil separation processes:

1. Impingement

2. Velocity drop out

3. Centrifugal effect

4. Coalescence

Figure 2 shows the oil/ gas separator described in our international application WO 2015/082867. This comprises a duct 20 with one or more bends in it, the duct discharging to a centrifugal cylinder 22. Gas mixed with oil particles is received into an inlet 13 of the duct 20 from the discharge port of a compressor (not shown). In the received mixture 19, particles of oil entrained in the gas stream are both randomly distributed and multi directional. The first separation phase is created using a straight length 31 of the duct 20 combined with a bend section 32 coupling the straight length 31 into the cylinder 22.

The partly separated gas and oil pass into a second separation phase which is an impingement stage, constructed to prevent re-entrainment. This phase incorporates the centrifuging cylinder 22 designed to further separate the oil 23 from the gas stream 26 thereby preventing re-entrainment. Entry into this second stage 22 can be tangentially on one or other side of the cylindrical vessel 22. Separated oil 23 falls to the bottom of the cylinder 22 where it can form an oil reservoir 24 suitable for supply as required. Gas 17 with the oil removed exits through a port 30 at the top of the cylinder 22.

Summary of the Invention

With the aim of improving this known separator, a first aspect of the invention provides an oil/gas separator comprising a duct having an inlet for receiving an oil/ gas mixture, the duct having an inner surface extending at least partially in a horizontal direction and providing a bend or series of bends, the duct providing at least partial separation of the oil/ gas mixture, in use, by both gravitational and centrifugal oil collection onto the inner surface, and the duct being arranged to deliver gas and collected oil into a centrifuging cylinder providing an internal impingement surface, an upper gas outlet, and a lower oil outlet, the cylinder having an internal diameter of less than 180 mm. This internal diameter can be less than 160 mm, less than 140 mm or less than 120 mm. In embodiments of the invention, the minimum value of the diameter is 100mm.

The design of the separator is such as to ensure that the maximum system velocity is less than that required to carry oil particles upwards within the centrifuging cylinder. The centrifuging cylinder may be of copper, aluminium or an alloy of copper or aluminium(in this specification, "an alloy of copper of aluminium'' means an alloy containing at least 50% copper or at least 50% aluminium by weight). Alternatively, the centrifuging cylinder may be of steel, e.g. stainless steel.

The centrifuging cylinder of WO 2015 / 082867 has a significantly larger diameter. This cylinder must be cast from iron or similar metallic material. It is therefore heavy and inconvenient, particularly for rooftop installations. A tapering section may connect the centrifuging cylinder to the upper gas outlet, where this is of smaller diameter than the cylinder.

The duct may be configured to deliver oil and gas tangentially into the centrifuging cylinder at a height sufficient to prevent re-entrainment of oil. The upper gas outlet may comprise a top vent, and the lower oil outlet may be a reservoir outlet or a tangential delivery outlet.

According to a second aspect of the invention, a method of making the oil separator defined above comprises assembling the duct and at least one pipe of copper, aluminium, an alloy of copper or aluminium or stainless steel that defines the centrifuging cylinder. Such a method can avoid casting or the use of a cast part.

Where the upper gas outlet is of a different diameter to the centrifuging cylinder, it can be formed from another pipe of corresponding diameter, and joined to the cylinder, for example by brazing. The tapering section, if present, can be brazed to the cylinder and to the upper gas outlet. Alternatively, a single pipe can be formed into the cylinder, tapering section and upper gas outlet.

A pipe can also be used to form the oil reservoir, and can be attached, for example by brazing, to the lower oil outlet. Any or all of the pipes may be of copper, aluminium, an alloy of copper or aluminium or stainless steel. A third aspect of the invention provides an arrangement of a plurality of the oil separators, a gas/ oil manifold, wherein the gas/oil manifold communicates with the duct of each separator. The arrangement may include an oil outlet reservoir, the lower oil outlet of each separator is arranged to feed oil to the oil outlet reservoir. In one embodiment, the oil outlet reservoir is in the form of a substantially horizontal pipe, the centrifuging cylinders being spaced along the pipe. Alternatively, an oil reservoir may be a component of the heat exchanger of a refrigeration or air conditioning system.

In this way, a suitable number of separators can be ganged to allow operation of a single larger compressor. The cost of the plurality of separators of the invention is lower than the cost of a single separator of the prior art discussed above.

Unlike conventional separators, a separator according to an embodiment of the present invention is not primarily dependant on gas velocity. There are however potential constraints on gas velocity in the duct. The maximum gas velocity is limited by the maximum allowable pressure drop during travel in the duct. In a cooling system, a large pressure drop may mean that the cooling system efficiency falls to an unacceptable level. The pressure drop in a cooling system might lie in the range 200 to 400mbar for example. The minimum gas velocity in the duct is limited by the maximum acceptable separator size since a slower-moving mixture will generally require a larger diameter duct and vessel sizes, which may not suit a final installation. In this invention the pressure loss is > 200mbar.

Embodiments of the invention have been found to provide oil carry over levels as low as 1.5% . They can provide significant improvements in efficiency per unit size oil/ gas separator, together with manufacturing simplicity and can operate across a wide range of operating conditions.

Brief Description of the Drawings Embodiments of the respective aspects of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a block diagram of known air conditioning equipment;

Figure 2 shows the oil/ gas separator described in our international application WO 2015/082867;

Figure 3 shows an oil/ gas separator according to the invention;

Figure 4 is a cut-away view of the separator of Figure 3;

Figure 5 shows an air conditioning system including the separator of Figure 3; and Figure 6 shows a larger air condition system including three oil separators, each according to Figure 3.

Detailed Description of Particular Embodiments

Figures 3 and 4 show an oil/ gas separator having a duct 20 with a bend 32, discharging into a centrifuging cylinder 22. The principle of operation is similar to that of WO 2015/082867, i.e. an inlet 13 of the duct 20 received an oil/ gas mixture from the discharge port of a compressor (not shown). Particles of oil entrained in the gas stream undergo a first separation phase in the bend section 32 of the duct. The outside diameter of the centrifuging cylinder is at most 180 mm, and in certain embodiments is 100 to 150 mm. The cylinder can conveniently be formed from piping sections, such as copper piping sections. In one example, it is formed from a commonly available copper tube of outside diameter 104.8 mm. In another example, it is formed from an aluminium casting and it may alternatively be formed from stainless steel.

The internal diameter of the cylinder 22 might be two or three times the internal diameter of the duct 20. For example, for cylinder of 100 to 150 mm, the duct can have a diameter of 50 mm. The height of the cylinder 22 might be greater than five times the internal diameter of the duct 20.

A tapering part 35 connects the top of the centrifuging cylinder to an upper gas outlet 30. The duct 20 and the upper gas outlet 30 can also be formed from pipe-shaped parts. They and the tapering part 35 can for example be of commonly-available parts. These can be joined by brazing or welding. Alternatively, the cylinder 22, tapering part 35 and upper gas outlet 30 can be formed from a single pipe, formed into the required shape by known machinery.

The bend section 32 could show a single bend or a series of bends providing a total change in direction with respect to the axial direction of the inlet in the range from 100 degrees to 270 degrees. The bend section 32 extends at least partially in a horizontal direction but could be tilted in one or more other directions. For example, the series of bends need not be in the same plane.

In the first separation phase, oil particles are coalesced by a combination of factors: surface tension both between the particles and with the inner surface 21 of the duct 20; the centrifugal forces of the bend 32; and the effect of gravity. This combination ensures a two part oil and gas separation first phase before entering into the centrifugal second separation phase 22.

Entering the second separation phase 22, partly separated gas and oil pass into an impingement separation stage constructed to prevent re-entrainment. This phase incorporates the centrifuging cylinder 22 designed to further separate the oil 23 from the gas stream 26 thereby preventing re-entrainment. Entry into this second stage 22 can be tangentially on one or other side of the cylindrical vessel 22. Separated oil 23 falls to the bottom of the cylinder 22 which communicates with an oil reservoir 24, formed here as a horizontal conduit, suitable for resupply as required. Gas 17 with the oil removed exits through the outlet 30 at the top of the cylinder 22.

As with WO 2015 / 082867, the separator of the invention does not have a vortex finder. Oil for return to the compressor 14 requires velocity. This can be generated by a head of oil collected in the reservoir 24 or by imparting velocity otherwise to the oil. For example, if the cylinder 22 provides sufficient head, the oil can be discharged through a port 28 to the reservoir 24 for delivery back to the compressor 14. Alternatively, the oil 23 can be delivered to a downstream oil reservoir elsewhere. In a further embodiment, the rotation of the oil within the second stage separation cylinder 22 may be used together with a tangential position of the oil outlet 28 to provide the required outlet velocity and thereby reduce the required oil head and thus system oil fill. The relationship between the cross sectional diameter of the inner surface of the duct 20 and the diameter of the cylinder 22 is important for optimum performance. If the duct diameter is too small, pressure drops in the duct 20 are too high which reduces the cooling system efficiency as mentioned above. A duct diameter which is of the order of one third to one half of the diameter of the second stage cylinder 22 has been found to be about the right relationship. A working example is a duct 20 having a nominal bore size of 50 mm with a bend section 32 having duct radius 75 mm to the outside of the pipe 20. The second phase cylinder 22 would then have an internal diameter of 104mm The bend section 32 of the duct 20 provides a change in direction from the straight section 31 to the inlet to the second stage cylinder 22 of approximately 180°. Alternative arrangements are possible, as illustrated in WO 2015 / 082867, where the change in direction can be less than 180° (e.g. as little as 100°), or up to 270° or more. Figure 5 shows an air conditioning system comprising a compressor/heat exchanger unit 50. The inlet 13 of the separator receives the oil/ gas mixture from the compressor. The refrigerant gas is fed from the upper gas outlet 30 to the heat exchanger, and the separated oil passes from the reservoir 24 back to the compressor. A known arrangement for compressors is modular, additional modules being added for larger systems. Figure 6 shows a system comprising three such modules, and a separator of the invention connected to each one. The oil reservoir 24 is a common line accepting oil from all three separators and returning it to the compressor. The oil separator of this invention is in effect formed by the pipework that is required to recirculate refrigerant. A part of that pipework is modified in a simple manner, providing a lightweight and inexpensive separation mechanism instead of the known separator that requires specialised manufacture.