The present invention relates to an interface unit for use with any particular vapour- compression refrigeration system as part of a thermal network. The invention further relates to a refrigeration system for providing cooled media to a location, to a method of providing a multi-mode heat rejection from refrigeration system, and to a method of improving the efficiency of cooling.

Refrigeration systems typically utilise vapour-compression cycles in order to produce the necessary enthalpic changes within a refrigerant so as to produce a cooling effect which can be harnessed for refrigeration purposes. Figure 1 shows a generalised representation of a vapour-compression refrigeration system, indicated globally at 10. The system 10 comprises a compressor 12, a condenser 14 indicated as a heat exchange element, an expansion valve 16, and an evaporator 18, which is also indicated as a heat exchange element.

A refrigerant in the system 10 is pressurised by the compressor 12, and is then passed through a piping circuit 20 of the system 10 to the condenser 14. The refrigerant then condenses into a liquid at the condenser 12, resulting in a decrease in the enthalpy of the refrigerant at constant pressure. The liquid refrigerant is then directed through the expansion valve 16, thereby decreasing the pressure of the refrigerant on the other side of the expansion valve 16, accompanied by a change in enthalpy. The reduced pressure refrigerant equalises with the pressure of refrigerant entering the evaporator 18, and there is a phase transition occurring as the refrigerant intensively absorbs heat from outside, as determined by the evaporation effect, with an associated increase in enthalpy of the refrigerant. The evaporator 18 is then coupled to a thermal energy transfer means, such as a fan for air circulation, or a solid medium for conductive heat transfer. The phase changes associated with a refrigerant are illustrated globally in Figure 2 at 22, indicating the thermodynamic changes occurring in the refrigerant as the cycle progresses, the cycle being defined by the specific pressure envelope. Any change in refrigerant state during compression, condensation, expansion and evaporation can be illustrated by the curved dome line in the phase diagram 22. Refrigerant is present in its vapour form on the right side of the dome and in its liquid form on the left side of the dome. Within the dome, refrigerant is at a mid-point, that is, co-existing liquid and vapour phases.

The top part of the line S-S specifically indicates where refrigerant enters a liquid phase, such that cooling beyond the point of condensation without any associated change in pressure, as defined by the top horizontal line in Figure 2, can be defined as sub-cooling. On a small scale, this effect may occur naturally during a vapour-compression cycle, but a greater magnitude of sub-cooling can be highly beneficial since it will provide excess cooling capability defined by the crossing of line S-S with the lower horizontal line, that is, the evaporation part of the envelope. As such, an extension of the magnitude of the envelope into the left side of line S-S could provide additional potential for cooling.

The engineering challenge in producing an efficient system is to provide a high-quality heatsink so as to be able to achieve the lowest possible discharge pressure regime for a given cooling temperature. A poor quality heatsink requires more work from the compressor in the refrigeration system, by reducing the cooling capacity of a refrigeration system. This in turn creates additional stresses on the system, which can result in failure and/or damage to the components.

Generally, the ability to cool is limited by the phase relationships of the refrigerant; a typical pressure-enthalpy phase diagram is illustrated in Figure 2 for standard refrigerant. The bounded dome represents a liquid-vapour phase, with pure liquid to the left of the dome, and pure vapour to the right.

The compressor does the work in the refrigeration system, increasing the pressure of the refrigerant. At the condenser, the refrigerant is able to condense down to the point of condensation into the liquid phase, that is, the enthalpy can be reduced at constant pressure up to the liquid- vapour to liquid boundary of the dome.

A process known as natural sub-cooling may be possible, in which the enthalpy of the liquid is reduced beyond the point of condensation, and this is a known phenomenon for all refrigerants. Carbon dioxide, when utilised as a refrigerant, may demonstrate larger natural sub-cooling effects due to its relatively high pressure of condensation, as determined by its physical properties. Such sub-cooling permits a more efficient vapour- compression cycle.

It is therefore an object of the invention to provide a refrigeration system in which the effect of sub-cooling can be extended beyond natural limits for all refrigerants, so as to provide a more efficient vapour-compression cycle.

According to a first aspect of the invention, there is provided an interface unit for a thermal network, the interface unit comprising: an interface unit heat exchanger; a plurality of refrigerant pipes; a plurality of intercept valves engagable with a piping circuit of a thermal network; and a controller associated with the plurality of intercept valves; the plurality of refrigerant pipes defining at least two different refrigerant flow paths across the interface unit heat exchanger, the at least two different refrigerant flow paths being selectively activatable by the controller controlling a status of the plurality of intercept valves.

Preferably, one said intercept valve may be provided as a pre-condenser intercept valve, and at least one said intercept valve may be provided as a post-condenser intercept valve, the pre-condenser intercept valve being positionable prior to a condenser in a thermal network, and the at least one post-condenser intercept being positionable following a condenser in a thermal network.

Preferably, a first said refrigerant pipe may be provided as a sub-cooling refrigerant inlet pipe, a second said refrigerant pipe may be provided as a multi-stage refrigerant inlet pipe, a third said refrigerant pipe may be provided as a heat-recovery refrigerant inlet pipe, and a fourth said refrigerant pipe may be provided as a refrigerant outlet pipe, the said first, second and third refrigerant pipes respectively defining a sub-cooling refrigerant flow path, a multi-stage refrigerant flow path, and a heat-recovery flow path through the interface unit heat exchanger and out of the said fourth refrigerant pipe.

By providing an interface unit which can be coupled to an existing refrigeration system, it becomes possible to extent the modes of operation of the existing system, in particular to allow for a dedicated sub-cooling heat exchanger to be provided. This advantageously and significantly improves the efficiency of the refrigeration system.

According to a second aspect of the invention, there is provided a s refrigeration system comprising: at least one compressor; at least one condenser; at least one expansion valve; at least one evaporator; and a piping circuit defining a default refrigerant flow path through the compressor, condenser, expansion valve and evaporator; and an interface unit, preferably in accordance with the first aspect of the invention, wherein one said intercept valve is connected to the piping circuit before the or each condenser as a pre- condenser intercept valve, and at least one said intercept valve is connected to the piping circuit following the or each condenser as a post-condenser intercept valve, the controller permitting selective control of a refrigerant flow through the piping circuit and interface unit, wherein at least one of the refrigerant flow paths through the interface unit is a sub- cooling refrigerant flow path.

The term 'sub-cooling' as used herein and throughout is intended to mean or is defined as a condition where liquid refrigerant is colder than a minimum or saturation temperature required to keep the liquid refrigerant from boiling and thus changing from the liquid to a gas phase.

By providing a dedicated sub-cooling heat exchanger provided as part of an attachable interface unit, it becomes possible for a refrigeration system to achieve sub-cooling with a wide variety of refrigerants which would otherwise only be possible for refrigerants having very specific phase change properties. This allows for refrigeration systems to be improved for efficiency, which can advantageously result in significantly reduced power consumption of refrigeration systems, providing a substantially more cost-effective refrigeration system as a whole. Preferably, the refrigeration system may further comprise a receiver vessel in communication with the piping circuit, the receiver vessel having a volume of the liquid refrigerant therein. In such a scenario, the receiver vessel may be positioned on the piping circuit, one said post-condenser intercept valve being positioned before the receiver on the piping circuit, and one further said post-condenser intercept valve being positioned following the receiver on the piping circuit. The said at least one refrigerant flow path may be defined by the piping circuit and plurality of refrigerant pipes of the interface unit in an order of: compressor; condenser; receiver; interface unit heat exchanger; expansion valve; evaporator; and compressor.

The presence of a receiver vessel beneficially moderates and regulates perturbations on the refrigerant system, making the sub-cooling apparatus more resilient to, for example, extreme changes in the external temperature. Furthermore, the receiver vessel also advantageously encourages separation of the liquid and liquid-vapour phases of the refrigerant, allowing the interface unit heat exchanger to act more effectively to sub-cool liquid refrigerant.

The piping circuit may additionally define a second said refrigerant flow path having a multi-stage refrigerant flow path which extends between the condenser and sub-cooling heat exchanger in which the condenser and sub-cooling heat exchanger are connected in series, the sub-cooling heat exchanger acting as a second stage condenser. The said second refrigerant flow path may be defined by the piping circuit and plurality of refrigerant pipes of the interface unit in an order of: compressor; condenser; sub-cooling heat exchanger; receiver; expansion valve; evaporator; and compressor. An extent of the piping circuit which is between the condenser and interface unit heat exchanger may be greater along the said sub-cooling refrigerant flow path than for the said multi-stage cooling refrigerant flow path.

The piping circuit may additionally define a third refrigerant flow path having a heat recovery refrigerant flow path in which the condenser is bypassed. Said third refrigerant flow path may be defined by the piping circuit and plurality of refrigerant pipes of the interface unit in an order of: compressor; interface unit heat exchanger; expansion valve; evaporator; and compressor.

Optionally, the piping circuit may define a fourth refrigerant flow path having a sub- cooling bypass refrigerant flow path in which the interface unit is bypassed, in which case the said fourth refrigerant flow path be defined by the piping circuit and plurality of refrigerant pipes of the interface unit in an order of: compressor; condenser; expansion valve; evaporator; and compressor.

Providing a plurality of different refrigerant flow paths through the apparatus advantageously allows the refrigeration system to be selectively operated in a plurality of different modes other than the standard sub-cooling mode, thereby significantly increasing the utility of the system for a user.

The present apparatus allows for the provision of a sub-cooling regime for many different types of refrigerant, whereas previous systems may have relied on the natural sub-cooling properties of the refrigerant, such as for carbon dioxide.

In one preferred embodiment, the piping circuit and plurality of refrigerant pipes of the interface unit may be mono-directional to prevent back-flow of refrigerant therethrough.

By preventing back-flow of the refrigerant through the system, it becomes possible to ensure that correct sub-cooling of the refrigerant is achieved, and that liquid refrigerant is directed towards the sub-cooling heat exchanger, rather than liquid-vapour refrigerant, which would otherwise result in multi-stage cooling of the refrigerant.

According to a third aspect of the invention, there is provided a method of providing a multi-mode refrigeration system, the method comprising the steps of: a] providing a refrigeration system, preferably in accordance with the second aspect of the invention, in which the condenser and interface unit heat exchanger are selectably connected both in series and parallel with one another; and b] selecting a refrigerant flow path through the interface unit in accordance with a pre-determined desired system function, wherein one said refrigerant flow path across the interface unit heat exchanger is a sub-cooling refrigerant flow path.

The particular arrangement of condenser and sub-cooling heat exchanger in the present invention advantageously allows for the switching arrangement which allows the user to readily change between the multi-stage cooling or sub-cooling regimes, in addition to the other modes of operation. According to a fourth aspect of the invention, there is provided a method of improving the efficiency of cooling of a refrigerant in a refrigeration system, the method comprising the steps of: a] connecting a dedicated interface unit, preferably in accordance with the first aspect of the invention, to the refrigeration system; b] directing a refrigerant through the at least one condenser to reduce the enthalpy of the refrigerant to the point of condensation; and c] directing the refrigerant to the dedicated interface unit heat exchanger to reduce the enthalpy and therefore sub-cool the condensed liquid refrigerant.

By spacing the condenser and interface unit heat exchanger apart from one another, instead of providing them directly in series with one another, it becomes possible to more effectively separate out the pressurised liquid refrigerant at the sub-cooling heat exchanger in order to more effectively perform sub-cooling.

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a diagrammatic representation of a generic refrigeration cycle;

Figure 2 shows a phase diagram for a refrigerant such as carbon dioxide, indicating the pressure and enthalpy changes during the refrigeration cycle;

Figure 3 shows a diagrammatic representation of one embodiment of an interface unit in accordance with the first aspect of the invention;

Figure 4 shows a diagrammatic representation of one embodiment of a refrigeration system in accordance with the second aspect of the invention, indicating a default or bypass refrigerant flow mode of the system through an interface unit;

Figure 5 shows a diagrammatic representation of the refrigeration system of Figure 4, indicating a heat recovery refrigerant flow mode of the system through an interface unit;

Figure 6 shows a diagrammatic representation of the refrigeration system of Figure 4, indicating a two stage cooling refrigerant flow mode of the system through an interface unit; and

Figure 7 shows a diagrammatic representation of the refrigeration system of Figure 4, indicating a sub-cooling refrigerant flow mode of the system through an interface unit.

Referring firstly to Figure 3, there is shown an interface unit capable of integrating and improving an existing refrigeration system, the interface unit being indicated globally at 50.

The interface unit 50 comprises a housing 52 which encloses an interface unit heat exchanger 54, a plurality of refrigerant pipes, a plurality of intercept or similar flow control valves, and a controller 56.

In the depicted embodiment, there are three different possible refrigerant flow paths which are directed through or across the interface unit heat exchanger 54: a first, sub- cooling refrigerant inlet pipe 58a can direct refrigerant toward the interface unit heat exchanger 54, and out of an outlet refrigerant pipe 60; a second, multi-stage refrigerant inlet pipe 58b directs a second flow path through or across the interface unit heat exchanger 54 and out of the outlet refrigerant pipe 60; and a third heat-recovery refrigerant inlet pipe 58c directs a third flow path through or across the interface unit heat exchanger 54.

Each of the refrigerant pipes 58a, 58b, 58c, 60 is then associated with a respective intercept valve: the sub-cooling refrigerant inlet pipe 58a with a sub-cooling intercept valve 62a; the multi-stage refrigerant inlet pipe 58b with a multi-stage intercept valve 62b; the heat-recovery refrigerant inlet pipe 58c with a heat-recovery intercept valve 62c; and the outlet refrigerant pipe 60 with an outlet intercept valve 62d.

The controller 56 is in communication with the intercept valves 62a, 62b, 62c, 62d, being able to selectively activate or switch the valves 62a, 62b, 62c, 62d, which are here formed as bi-directional valves to allow two-way control of the refrigerant flow therethrough. As such, when the interface unit 50 is coupled to a refrigerant system, a plurality of different refrigerant flow paths across the interface unit heat exchanger 54 can be achieved.

Referring to Figures 4 to 7, one embodiment of a vapour-compression refrigeration system is indicated generally at 110. Only a portion of the whole refrigeration system 110 is illustrated, from the compressor 112 to the portion of the piping circuit 120 leading up to the expansion valve, which is not shown in Figures 4 to 7, but is indicated at 16 in the generalised flow diagram of Figure 1, described above. However, an interface unit 50 has been inserted into the refrigeration system, as indicated. In the depicted embodiment, there are the interface unit 50 has been positioned on the piping circuit 120 near to the condenser 114 following the compressor 112 and before the expansion valve: the condenser 114. A receiver vessel 126 is also provided following the condenser 114. The dedicated interface unit 50 is installed such that the heat recovery intercept valve 62c is installed so as to intercept the piping circuit 120 prior to the condenser 114, the multi-stage intercept valve 62b between the condenser 114 and the receiver vessel 126, with the sub-cooling intercept valve 62a following the receiver vessel 126.

The receiver vessel 126 is installed so as to provide a reservoir of liquid refrigerant therein which is contained at a pressure equal to that of the discharge pressure of the compressor 112. This acts to provide a good separation of the liquid and liquid-vapour phases in the piping circuit 120, and can also compensate for rapid changes in the load within the refrigeration system 110 due to the increased refrigerant volume within the piping circuit 120. It will, however, be appreciated that the receiver vessel 126 is not strictly necessary. In the depicted embodiment, the interface unit heat exchanger 54 may be supplied as a plate heat exchanger, which has been demonstrated to be an efficient heat exchanger for the purposes of sub-cooling; however, other forms of heat exchanger could be provided, such as a vaned or ribbed heat exchanger.

As illustrated, the condenser 114, interface unit heat exchanger 124 and receiver vessel 126 are connected by the piping circuit 120 and the plurality of refrigerant pipes of the interface unit 50 both in parallel and series, with the plurality of intercept valves 62a, 62b, 62c, 62d being provided to change the refrigerant flow path through the various components so as to alter the functionality of the refrigeration system 110.

The first, heat recovery intercept valve 62c may be positioned after the compressor 112 so as to provide selective control of the refrigerant flow path between a direction towards the condenser 114 or towards the interface unit heat exchanger 54.

The second, multi-stage intercept valve 62b may be positioned following the condenser 114 so as to provide selective control of the refrigerant flow path between a direction towards the interface unit heat exchanger 54 or towards the receiver vessel 126, if present. The third, sub-cooling intercept valve 62a may be positioned following the receiver vessel 126 so as to provide selective control of the refrigerant flow path between a direction towards the interface unit heat exchanger 54 or towards the piping circuit 120 leading to the expansion valve. The outlet intercept valve 62d may be positioned following the interface unit heat exchanger 54 so as to provide selective control of the refrigerant flow path between a direction towards the receiver vessel 126, if present, or towards the piping circuit 120 leading to the expansion valve.

Where four such intercept valves 62a, 62b, 62c, 62d are provided, four different refrigerant flow paths can be created, each of which serve a different purpose within the refrigeration system 110. Any or all of the flow control valves 62a, 62b, 62c, 62d may be controlled by the controller 56, either manually or automatically.

Figure 4 illustrates one refrigerant flow path of the refrigeration system 110. Pressurised refrigerant is discharged from the compressor 112 in the vapour phase, and is directed towards the condenser 114 via the first intercept valve 62c. The refrigerant vapour condenses into liquid refrigerant at the condenser 114, resulting in a decrease in the enthalpy of the refrigerant. The refrigerant is then directed to the expansion valve where it will be depressurised, on a path through the piping circuit 120 via the second intercept valve 62b, the receiver vessel 126, and the third intercept valve 62a. The refrigerant is then directed towards the expansion valve, evaporator and back to the compressor 112 to complete the vapour-compression cycle. This is the ordinary vapour-compression cooling route. Some sub-cooling may naturally occur, as a result of the characteristics of the refrigerant itself, but no dedicated sub-cooling can occur. There is therefore defined a default bypass refrigerant flow path as part of the said refrigerant flow path in which the interface unit heat exchanger 54 is bypassed.

Figure 5 illustrates a nominal heat recovery mode of the refrigeration system 110, in which the condenser 114 is isolated from the piping circuit 120 by switching of the first intercept valve 62c so as to direct refrigerant towards the interface unit heat exchanger 54. This can then result in a heat recovery refrigerant flow path in which the condenser 114 is bypassed, the refrigerant flow path flowing from the compressor 112 through the first intercept valve 62c, the interface unit heat exchanger 54, the outlet intercept valve 62d, the receiver vessel 126, and the third intercept valve 62c. The refrigerant is then directed towards the expansion valve, evaporator and back to the compressor 112 to complete the vapour-compression cycle. In this arrangement, the interface unit heat exchanger 54could be configured so as to act as a secondary condenser, if desired, although the intended usage in the depicted embodiment is as a means of heat recovery for the refrigeration system 110. A recovery manifold may therefore be provided in thermal communication with the interface unit heat exchanger 54so as to retrieve and utilise excess thermal energy generated during the vapour-compression cycle, to thereby be put to use.

Figure 6 illustrates a further refrigerant flow path through the piping circuit 120 and refrigerant pipes. The refrigerant is discharged from the compressor 112, and directed through the first intercept valve 62c to the condenser 114. The refrigerant exits the condenser 114 and is directed through the second intercept valve 62b directly towards the interface unit heat exchanger 54 which thereby acts as a secondary condenser in series with the primary condenser 114. Exiting the interface unit heat exchanger 54, the refrigerant is directed towards the receiver vessel 126 via the outlet intercept valve 62d. Following discharge from the receiver vessel 126, the refrigerant is then directed through the third intercept valve 62a towards the expansion valve, evaporator and back to the compressor 112 to complete the vapour-compression cycle. The refrigerant flow path extending between the condenser 114 and the interface unit heat exchanger 54can therefore be considered to be a multi-stage, here two-stage, cooling refrigerant flow path.

This arrangement may act to improve the efficiency of the standard vapour-compression cycle, preventing the primary compressor 114 from overheating, but does not improve the prospects of sub-cooling of the refrigerant; with reference to Figure 2, both stages of the condensation occur whilst the refrigerant exists in the liquid-vapour phase, with each of the condenser 114 and interface unit heat exchanger 54assisting with a percentage of the load of the condensation process.

Figure 7 shows another refrigerant flow path through the piping circuit 120 and refrigerant pipes in which the interface unit heat exchanger 54is able to act to sub-cool the refrigerant flowing therethrough. In the depicted flow path, the refrigerant vapour exits the compressor 112 before being directed via the first intercept valve 62c towards the condenser 114, and via the second intercept valve 62b towards the receiver vessel 126. The separated liquid refrigerant is then diverted via the third intercept valve 62a into the interface unit heat exchanger 54. There can therefore be considered to be a sub-cooling refrigerant flow path extending from the condenser 114 to the interface unit heat exchanger 54, here via the receiver vessel 126. Sub-cooling of liquid refrigerant, as opposed to condensation of vapour or liquid-vapour refrigerant, can then occur at the interface unit heat exchanger 54; this is distinguished from natural sub-cooling, as would be experienced by, for instance, carbon dioxide, since this sub-cooling is applicable to a much wider variety of refrigerants, and/or can improve the efficiency of sub-cooling of refrigerants for which natural sub-cooling is possible. The emergent liquid refrigerant from the interface unit heat exchanger 54 can then be directed via the outlet intercept valve 62d towards the expansion valve, evaporator, and eventually cycled back to the compressor 112.

Notably, the extent of the piping circuit 120 which extends between the condenser 114 and interface unit heat exchanger 54 is significantly increased for the sub-cooling refrigerant flow path when compared with the multi-stage cooling refrigerant flow path, which inhibits the potential of the interface unit heat exchanger 54 to act as a secondary condenser. This means that the full condensation load is provided for by the primary condenser 114, meaning that the interface unit heat exchanger 54 is able to act exclusively as a sub-cooling device, acting solely to cool liquid refrigerant.

The introduction of a dedicated interface unit heat exchanger 54 means that the refrigeration capacity of the refrigeration system 110 can be increased and/or optimised. In particular, the sub-cooling allows for some compensation of the loss of cooling capacity during elevated ambient conditions in which the discharge pressure from the compressor 112 would be increased to a maximum threshold. Additionally, the provision of a dedicated interface unit heat exchanger 54 allows for improved efficiency of the refrigeration system during periods of colder external temperatures; the minimum discharge pressure from the compressor 112 provides a limit to the efficiency of the refrigeration system 110, and therefore sub-cooling extends this efficiency without changing the minimum discharge pressure requirements, by permitting the liquid refrigerant to be cooled, rather than just vapour or liquid-vapour refrigerant at the condenser 114.

The presence of the receiver vessel 126 is beneficial in improving the ability of the system to achieve this goal of dedicated sub-cooling by improving the separation of the liquid refrigerant and liquid-vapour refrigerant such that the interface unit heat exchanger 54is able to solely act to reduce the enthalpy of the liquid refrigerant.

The particular arrangement of the piping circuit 120 and refrigerant pipes beneficially allows a user to operate the refrigeration system 110 in multiple different modes by attachment of an interface unit 50, including at least a sub-cooling refrigerant flow mode, and potentially also including heat recovery refrigerant flow mode, sub-cooling bypass mode, and/or multi-stage cooling mode. This is achieved by providing the condenser 114 and interface unit heat exchanger 54in selectively switchable series and parallel configurations, so as to allow for reconfiguration of the refrigerant flow path therethrough. This advantageously allows for the refrigeration system 110 to operate in different modes so as to suit the needs of the user, but also to permit a more efficient method of operating the refrigeration system 110.

Whilst the refrigerant utilised in the system is likely to be or include carbon dioxide, it will be appreciated that a much wider range of refrigerants can now be sub-cooled as part of a refrigeration system in accordance with the present invention.

It will be appreciated that the directionality of the refrigerant flow through the system is important for the provision of the dedicated sub-cooling as hereto described. It may therefore be advantageous to provide a piping circuit which is mono-direction, so as to prevent or limit back-flow or reflux of refrigerant through the system. This contrasts with prior refrigeration systems, in which the direction of refrigerant through the piping circuit could be reversed.

It is therefore possible to provide a sub-cooling apparatus for a vapour-compression refrigeration system which allows for dedicated sub-cooling of the refrigerant. The apparatus includes a dedicated interface unit heat exchanger for this purpose. This arrangement allows for sub-cooling to be applied to a much wider scope of refrigerants, thereby improving the efficiency of the refrigeration process in both ambient and extreme external conditions.

The words 'comprises/comprising' and the words 'having/including' when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

The embodiments described above are provided by way of examples only, and various other modifications will be apparent to persons skilled in the field without departing from the scope of the invention herein described and defined.