THREE-PIPE THERMAL NETWORK

Abstract

Thermal network comprising at least one plant, at least one end-user location (14,15), a pipe system (11-13) and a medium contained within said pipe system (11-13) said plant and said end-user location; said end-user location(s) (14,15) being connected to the plant through the pipe system (11-13). The thermal network according to the invention is characterized in that it comprises three main pipes (11-13) that are each connected to said plant(s) and wherein the medium is in a liquid state in the first and the third main pipe (11,13), and in a gaseous state in the second main pipe (12).

Claims

1. Thermal network comprising at least one plant, at least one end-user location, a pipe system and a medium contained within said pipe system said plant and said end-user location; said end-user location(s) being connected to the plant through the pipe system wherein it comprises three main pipes that are each connected to said plant(s) and wherein the medium, when it is operated, is in a liquid state in the first and the third main pipe, and in a gaseous state in the second main pipe.

2. Thermal network according to claim 1 configured in a way that the first main pipe is a supply pipe that has a monodirectional flow from the plant towards the end-user locations, and that the third main pipe is a return pipe that has a monodirectional flow from the end-user locations towards the plant.

3. Thermal network according to claim 1, wherein the pressure in the first main pipe is higher than the pressure in the second main pipe, and wherein the pressure in the second main pipe is higher than the pressure in the third main pipe.

4. Thermal network according to claim 1 comprising a liquid pipe bypass connecting the first main pipe to the third main pipe, that guarantees a minimum flow of the medium in said pipes.

5. Thermal network according to claim 1 comprising a medium receiver with a part, such as a lower part, connected to the first main pipe and another part, such as an upper part, connected to the second main pipe.

6. Thermal network according to claim 1 comprising a condenser configured to generate said liquid state.

7. Thermal network according to claim 1 comprising an evaporator configured to generate said gaseous state.

8. Thermal network according to claim 1 comprising one end-user location connected via an inlet and an outlet to at least two of the said main pipes and configured in a way that the thermodynamic state of medium at the outlet corresponds to the one desired in the main pipe it is connected to.

9. Thermal network according to claim 1, wherein the medium is CO.sub.2 and is used as an energy transfer medium and wherein at least one end-user location comprises one or several outlets which are adapted for releasing the said CO.sub.2 to connected devices in a continuous or intermittent processes, such as fire extinguishing, Carbon capture and sequestration (CCS) applications, dry ice production, chemical processes, food and beverage processes.

10. Thermal network according to claim 1, wherein the medium is CO.sub.2 and is used as an energy transfer medium and wherein at least one end user location comprises one or several inlets which are adapted for injecting into the network CO.sub.2 from connected devices in a continuous or intermittent processes, such as fire extinguishing, Carbon capture and sequestration (CCS) applications, dry ice production, chemical processes, food and beverage processes.

11. Thermal network according to claim 1 comprising a liquid trap on the second main pipe configured for extracting any medium in liquid phase from said second main pipe.

12. Thermal network according to claim 1 comprising a gas trap on the third main pipe configured for extracting any medium in gaseous phase from said third main pipe.

13. Thermal network according to claim 1 comprising a gas trap on the first main pipe configured for extracting any medium in gaseous phase from said first main pipe.

14. Thermal network according to claim 1 comprising a cooling system configured for extracting heat from the flow entering the plant via the third main pipe.

15. Thermal network according to claim 1 comprising a subcooling apparatus configured for extracting heat from a liquid entering the plant via the third main pipe and/or leaving a receiver.

16. Thermal network according to claim 14, wherein the cooling system comprises a heat pump used, if present, with the said subcooling apparatus.

17. Thermal network according to claim 1 comprising a compressor capable of extracting gas build-up before a condensate extraction pump and compress it at the corresponding pressure within the second main pipe at the plant side.

18. Thermal network according to claim 1 comprising a set of receiver isolation valves and a set of receiver flash purge valves operated in such a way as to extract gas build-up before a condensate extraction pump.

19. Use of a thermal network as defined in claim 1, wherein when the heating requirements are higher than the cooling requirements (e.g. in winter), the gas flows from the receiver to the second main pipe at the plant.

20. Use of a thermal network as defined in claim 1, wherein when the cooling requirements are higher than the heating requirements (e.g. in summer), the gas flows from the second main pipe into the receiver at the plant.

21. Use of a thermal network as defined in claim 1, wherein when the cooling requirements are equal to the heating requirements (e.g. in summer), there is no flow in the second main pipe that goes into or comes from the receiver.

22. Use of a thermal network as defined in claim 1 comprising the generation of a liquid sate medium and gaseous state medium, wherein the liquid state medium is transferred to the first and third main pipes and wherein the gaseous state medium is transferred to the second main pipe.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0052] The invention will be better understood in this chapter, with the support of non-limiting examples illustrated by the following figures:

[0053] FIG. 1: 1.sup.st configuration of a thermal network according to the invention

[0054] FIG. 2: 2.sup.nd configuration of a thermal network according to the invention

[0055] FIG. 3: 3.sup.nd configuration of a thermal network according to the invention

[0056] FIG. 4: 4.sup.th configuration of a thermal network according to the invention

NUMERICAL REFERENCES USED IN THE FIGURES

[0057] 1. Intermediate pressure receiver [0058] 2. Medium pressure liquid pump [0059] 3. Low pressure receiver [0060] 4. Low pressure compressor [0061] 5. Low pressure liquid pump [0062] 6. Sub-cooling apparatus [0063] 7. Anti-flash condenser [0064] 8. Heat pump apparatus [0065] 9a. Intermediate pressure evaporator [0066] 9b Source preheater [0067] 10. Intermediate pressure condenser [0068] 11. Liquid supply pipe [0069] 12. Gas pipe [0070] 13. Liquid return pipe [0071] 14. Cooling user [0072] 15. Heating user [0073] 16. Intermediate pressure compressor [0074] 17. Refrigeration apparatus [0075] 18. Liquid pipe bypass [0076] 19. Gas pipe trap [0077] 20. Liquid return pipe gas trap [0078] 21. Liquid supply pipe gas trap [0079] 22. Receiver isolation valve [0080] 23. Receiver flash purge valve

[0081] In all examples, the thermal network comprises a plant, end-user locations 14,15 three main pipes, two of which, namely the first 11 and the third 13, respectively form a liquid supply pipe and a liquid return pipe. The medium is preferably CO.sub.2: The second main pipe 12 contains the same medium but in a gaseous state. The energy transfer is realized by evaporation/condensation of the said medium.

[0082] Both first and third main pipes 11, 13 are arranged and connected in a fashion such that the medium flows in a single direction between the end user locations 14, 15 and between the plant and the end user locations 14,15.

[0083] The second main pipe 12 is arranged and connected in a fashion such that the medium can flow indifferently in both directions in every segment connecting the end-user locations 14,15 between them, as well as those connecting the plant and end-user locations 14,15.

[0084] Preferably, the end-user locations 14,15 are provided with any suitable technology that can: [0085] extract the network medium from one or several lines and/or [0086] inject the network medium into one or several lines in a thermodynamic state close to the one desired in said lines.

[0087] For instance, when considering the examples illustrated in the following figures, at end-user locations 14 the medium is extracted from the first main pipe 11 in a thermodynamic state corresponding to that prevailing in said pipe at this location and time, ideally saturated or slightly subcooled liquid but in some cases a liquid/gas mixture at the saturation with a gas content as low as possible. Within the equipment at the end-user location 14, the medium can undergo all sorts of thermodynamic processes, the simplest of which being an evaporation in a heat exchanger device in which the flow of the medium is regulated, preferably using a valve upstream of the heat exchanger inlet, so as to guarantee that the thermodynamic state of the medium, at the outlet of the end user location corresponds to that desired in the gas pipe 12 to which said outlet is connected. In that case the desired state is saturated gas or slightly superheated gas with no liquid content.

[0088] In the other end-user location 15, the medium is extracted from the second main pipe 12 in a thermodynamic state corresponding to that prevailing in said pipe at this location and time, ideally saturated or slightly superheated gas but in some cases a liquid/gas mixture at the saturation with a gas content as high as possible. Within the equipment at the end-user location 15, the medium can undergo all sorts of thermodynamic processes, the simplest of which being the condensation in a heat exchanger device in which the flow of the medium is regulated, preferably using a valve downstream of the heat exchanger device's outlet, so as to guarantee that the thermodynamic state of the medium, at the outlet of the end user location 15, corresponds to that desired in the third main pipe 13, to which said outlet is connected. In that case the desired state is saturated or slightly subcooled liquid with no gas content.

[0089] Advantageously, the plant includes any suitable element that allows to: [0090] maintain pressure at suitable set points by exchanging energy with an energy source/sink, denominated The Source. [0091] ensure the circulation of the medium in the different pipes 11-13, in accordance to the flow requirements generated at end-user locations 14,15. [0092] ensure that the medium supplied by the plant to the first (supply) main pipe 11 is in liquid state and, when applicable, the medium supplied to the second main pipe 12 is in gaseous state. [0093] Ensure that the plant can continue its operation even in the event that the medium collected from the third (return) main pipe 13 departs from the desired liquid state (within reasonable bounds). [0094] Ensure that the plant can continue its operation even in the event that the medium collected from the second main pipe 12 departs from the desired gas state (within reasonable bounds).

[0095] In those examples, the network is arranged to ensure that the medium in the first main pipe 11 is always and everywhere at a higher pressure than that in the second main pipe 12 and the pressure in this later one always and everywhere higher than the pressure in the third pipe 13.

[0096] At end user locations 14,15, the above-mentioned pipe arrangement and their respective pressure allows for the medium to flow either from the first main pipe 11 to the second main pipe 12 or from the second main pipe 12 to the third main pipe 13, without the need of any active machine such as a pump or a compressor. This provides an important benefit for the end-user locations dedicated to cooling 14, heating 15 or both. Among other advantages, it reduces the footprint of the equipment installed at said locations 14,15 as well as the operational risk link to active machines that tend to be more sensitive than passive equipment (p.ex. valve or heat exchangers). Also, active machines generally require more maintenance than passive equipment and, if located within the premisses of end-users, such machines will likely be more difficult to service than if located in the plant, for basic access right reasons.

[0097] For a system where the end-user locations 14,15 exploit the network only for cooling purposes 14 or heating purposes 15 or both, there is no medium that leaves or enters the system (the system is closed, it does not exchange mass with its environment, only energy).

[0098] During steady state operation the mass flow of the gas leaving, respectively entering the plant, exactly compensates the difference between the mass flow of liquid entering the plant from the third main pipe 13 and the mass flow of liquid leaving the plant through the first main pipe 11. During transient, the mass flows will not compensate each other until a new steady state is reached. As an example if the end-user locations cumulated demand for cooling generates a mass flow in the liquid supply line of X kg/s and the cumulated demand for heating generates a mass flow in the liquid return line of Y kg/s, then a mass flow of Z=XY kg/s entering the plant from the gas line will be observed (provided that the system is in steady state). In reverse, if the mass flow from the liquid return line is Y kg/s and that to the liquid supply line is X kg/s, a mass flow of Z=YX kg/s of gas leaving the plant will be observed (provided that the system is in steady state.) The flow of liquid entering the plant from the third main pipe 13 is first directed into a receiver 3, the function of which is to separate the liquid from the gas. Indeed, because of the pressure drops along the third main pipe 13 but also through some of the valves at the end-user locations 14,15 and also, in some conditions, because of heat input coming from the environment through the pipe wall, a certain amount of flash gas will enter the plant. Therefore, a separation receiver must be used to allow the pump 5 being fully fed with liquid. However, over time, gas will build up in the receiver and a suitable mean must be used to either send it back to the gas line, for instance by condensing it, with an anti-flash condenser 7 (see FIG. 1) or using a dedicated compressor 4 (see FIG. 2).

[0099] The anti-flash condenser 7 can be cooled by any suitable mean. For instance, if a source cold enough is available, it can be used directly to cool down said condenser. In the absence of source at a suitable temperature for direct cooling, a heat pump apparatus 8 can be used for providing the necessary cooling to the anti-flash condenser via its cold source. A combination of both direct cooling and cooling via the heat pump apparatus can be used, which would be particularly suited for cases where the temperature of the source available varies significantly over the time.

[0100] To improve the operational reliability and energy efficiency of the system several devices can be incorporated in the system.

[0101] One or several liquid pipe bypass 18 can be installed, preferably with at least one bypass at the furthest point of the pipe system from the plant. Said bypass consist in a valve, the opening of which can be fixed, manually set or automatically actuated, that connects the first main pipe 11 to the third main pipe 13. It ensures that even in absence of medium being extracted from, respectively injected into said pipes at the end-user locations 14, 15 a minimum flow of the medium is guaranteed within the network. The purpose of having a minimum flow of medium guaranteed always and everywhere in the pipe system is two-fold: [0102] to ensure that the medium supplied by the first main pipe 11 at the inlet of the end-user locations 14 is always in an acceptable thermodynamic state, ideally saturated or slightly subcooled liquid, so as to allow for a start-up with no or minimal time delay of the device at the end-user location. Without this minimal flow, thermal input from the environment could evaporate the liquid in the pipe when the medium is at rest and the start-up of the equipment at end-user locations 14 would be delayed of the time needed to bring back liquid to said end-user location from the plant, via the first main pipe 11. [0103] to ensure that the medium coming back to the plant via the third main pipe 13 is always in the adequate thermodynamic state, ideally saturated or slightly subcooled liquid but it is also conceivable to have a liquid/gas mixture at saturation with a gas content as low as possible. This is particularly of importance when there is no or very little medium injected in the third main pipe 13 by the device at the end-user location 15 since in such cases thermal energy from the environment (e.g heating of liquid pipe or equipment) could evaporate a too large fraction of liquid in the pipe for the low pressure liquid pump 5 to be able to operate, even with the help of the other devices that are incorporated in the system to mitigate that problem, namely the low pressure receiver 3, the anti-flash condenser 7 or the low pressure compressor 4 or the low pressure receiver isolation and flash purge valves 22, 23.

[0104] One or several gas traps 19 can be installed to collect and drain the gas bubbles that may be present in the medium flowing in the first main pipe 11. These traps could for instance be advantageously placed at locations where there is a local maximum in elevation in the network. Such a trap comprises a receiver connected via a pipe to the Liquid Supply Pipe 11, and connected via an automatic valve to the gas pipe 12 (second main pipe). The arrangement is made in such a way as to ensure that the automatic valve will drain gas from the receiver and not liquid. During normal operation the valve is closed and gradually the receiver of the trap will fill up with the gas collected. Once the liquid level in the receiver reaches a value low enough, the automatic valve opens and drains the gas towards the gas pipe 12. When the liquid level in the receiver of the trap reaches a value high enough the valve closes back. It is also possible to use a modulating valve commanded via a suitable control loop, to stabilize the level of liquid in the receiver at a desired value. With the latter option the drainage of the gas would be a continuous process instead of a batch one.

[0105] One or several gas traps 20 can be installed to collect and drain the gas bubbles that may be present in the medium flowing in the third main pipe 13. These traps could for instance be advantageously placed at locations where there is a local maximum in elevation in the network. Such a trap comprises a receiver connected via automatic valves to all three main pipes of the pipe system 11, 12 and 13. The arrangement is made in such a way that the valve that connects the receiver to the second main pipe 12 will drain gas and not liquid. During normal operation the valve connecting the receiver of the trap to the pipes 11 and 12 are closed and the one connecting said receiver to the third main pipe 13 is open. In this way the receiver will gradually fill-up with gas collected from the third main pipe 13. Once the liquid level in the receiver of the trap reaches a value low enough, the valve that connects the receiver to the third main pipe 13 is closed, the valves that connects the receiver to the main pipes 12 and 11 are opened. Because of the higher pressure in the pipe 11 than in the pipe 12, some liquid mixture is admitted from the pipe 11 into the receiver, acting as a liquid piston that pushes out the gas from the receiver through the valve and into the second main pipe 12. Once the level of liquid in the receiver reaches a value high enough, the valves that connect it to the main pipe 11 and 12 closes back, the one that connects it to pipe 13 opens again and normal operation resumes.

[0106] One or several liquid traps 21 can be installed to collect and drain liquid that may be present in the medium flowing in the second main pipe 12. These traps could for instance be advantageously placed at locations where there is a local minimum in elevation in the network. Such a trap comprises a receiver connected via a pipe to the second main pipe 12 and connected via an automatic valve to the third main pipe 13. The arrangement is made in such a way as to ensure that the automatic valve will drain liquid from the receiver and not gas. During normal operation the valve is closed and gradually the receiver of the trap will fill up with the collected liquid. Once the liquid level in the receiver reaches a value high enough, the automatic valve opens and drains the liquid towards the liquid return pipe 13. When the liquid level in the receiver of the trap reaches a value low enough the valve closes back. It is also possible to use a modulating valve commanded via a suitable control loop, to stabilize the level of liquid in the receiver at a desired value. With the latter option the drainage of the liquid would be a continuous process instead of a batch one.

[0107] In order to further increase the reliability of the system one can help the pump 5 be fed fully with liquid at its inlet (which equivalent to say that the net positive suction head available must be higher than the net positive suction head required by the pump) using a subcooling apparatus 6 located either between the low-pressure receiver 3 and the pump 5 or alternatively directly in the receiver. Alternatively it is also possible to impose some subcooling at the anti-flash condenser outlet 7 either by the mean of a controlled valve that would be operated in such a way as to impose actively said subcooling, or by the mean of a syphon at the anti-flash condenser outlet that would guarantee that at any time the bottom of the anti-flash condenser is filled with an appropriate amount of liquid that will be subcooled. Another advantage of providing subcooled liquid at the pump inlet is to reduce the necessary height difference between the bottom of the receiver and the inlet of the pump.

[0108] Similarly to the anti-flash condenser 7, the cooling circuit of the subcooling apparatus 6 can be fed directly if a source cold enough is available. Alternatively, it can also be fed by the cold source of heat pump apparatus 8, It may be advantageous in term of space to use the same heat pump apparatus for both the subcooling apparatus and the anti-flash condenser. The heat exchangers can be connected in series or in parallel to the heat pump apparatus and/or the cooling source.

[0109] In the case where a heat pump apparatus 8 is used, during its operation, the waste heat discharged at its hot sink can advantageously be used to preheat the source before it enters the intermediate pressure evaporator 9a. Alternatively it is also possible to install a dedicated flooded evaporator on the intermediate pressure receiver 1. It is particularly well suited since the maximum load on the anti-flash condenser 7 and subcooling apparatus 6 will occur simultaneously with the maximum demand for gas to be provided by the plant, through the second main pipe 12, to the concerned end-user locations, 15. Meaning that said discharged heat can always be fully valorised within the system.

[0110] The liquid from the low-pressure receiver 3 is pumped back into the intermediate pressure receiver 1 using the low-pressure liquid pump 5.

[0111] As an alternative to the anti-flash condenser 7 it is possible to use a compressor 4 that extracts the flash gas from the low-pressure receiver compress it and send it into the intermediate pressure receiver 1 or directly into the second pipe 12. In any case the liquid pump 5 is still required and the beneficial effects of having a subcooling apparatus 6 remains even in the absence of the anti-flash condenser 7.

[0112] At the intermediate pressure, the gaseous phase from the gas line and/or in the intermediate pressure receiver can be condensed 10 (the case when gas comes back to the plant from the pipe 12) or the liquid be evaporated 9a in the intermediate pressure receiver (the case when gas is sent from the plant into the pipe 12). Depending on the temperature of the source available for the evaporator's heating circuit 9a, respectively the condenser's cooling circuit 10, said source can either feed directly those heat exchangers or a heat pump apparatus, respectively a refrigeration apparatus 17 (see FIG. 3), can be used to supply said heat exchangers, thus decoupling the saturation temperature of the intermediate pressure gas in the network to that of the source. The source feeding the evaporator's heating circuit 9a can also be pre-heated in heat exchanger 9b using the heat available from the heat pump apparatus 8. In cases where the temperature of the source varies significantly over time, a combination of direct heat exchange and use of a heat pumping apparatus or a refrigeration apparatus can be realised advantageously.

[0113] As an alternative to a separate refrigeration apparatus 17 that would interface the intermediate pressure receiver to a source with a temperature too hot for direct condensation, it is possible to use a compressor 16 that extract gas from the intermediate pressure receiver, compress it and sends it to the condenser 10 where it will condense and be sent back through an expansion valve to the intermediate pressure receiver 1. In term of energy efficiency, this solution may be especially of interest when the temperature difference required between the source available and the desired saturation temperature in the intermediate pressure receiver is relatively small.

[0114] In a rather similar way, it is possible to decouple the temperature in the intermediate pressure receiver 1 from that in the evaporator 9 by extracting liquid from said receiver, expand it in a valve, evaporate the liquid in said evaporator and recompress it and send it back to the intermediate receiver. Here also, this solution might be of a particular interest if the temperature difference required is relatively small.

[0115] A pump 2 is also used to extract from the intermediate pressure receiver 1, pressurise and send the liquid demanded by the end-user locations via the first main pipe 11. A subcooling apparatus may also be installed for improving the performance and reliability of said pump as well as reducing the static head required. The subcooling can be imposed and the cooling be provided by means analogous to those described for the subcooling apparatus at the low pressure 6.

[0116] As an alternative to both methods of elimination of the flash gas in the low-pressure receiver 3, namely via the use of the anti-flash condenser 7 or the low-pressure compressor 4, it is possible to provide the same functionality using a suitable arrangement of the receiver 1 and 3 namely: [0117] By installing the intermediate pressure receiver 1 at a slightly higher elevation than that of the low-pressure receiver 3, in order to allow for a gravity driven gas purge of the latter [0118] By installing two automated Receiver Isolation Valves of the low-pressure receiver 22 one located on the third main pipe 13 and one on the pipe upstream of the low-pressure pump 5 [0119] By installing two automated receiver flash gas purge valves 23, one located on a pipe that connects a liquid filled part of the intermediate pressure receiver 1 to the low-pressure receiver 3 and the other located on a pipe that connects a gas filled part of the intermediate pressure receiver 1 to a part of the low-pressure receiver 3 that also contains gas.

[0120] During normal operation, the receiver isolation valves 22 are open and the receiver flash gas purge valves 23 are closed. Ideally the third main pipe 13 returns saturated liquid or even slightly subcooled, however because of pressure drops, thermal energy input from the environment and/or possible injection of medium in an inadequate thermodynamic state from end-user locations 15, it can be expected that some gas is also returned to the receiver 3. Gradually the gas will build up in said receiver until the liquid level reaches a value low enough to trigger a purge cycle by closing the isolation valves 22 and opening the purge valves 23. As a result, the pressure of the low-pressure receiver 3 will rise up to that of the intermediate pressure receiver 1 and thanks to the lower density of the gas with respect to that of the liquid phase, the volume of gas in the receiver 3 will migrate through the purge valve 23 into receiver 1 and be replaced by liquid flowing down from the intermediate pressure receiver 1 into the low-pressure receiver 3. Once the liquid level in the low-pressure receiver 3 reaches a value high enough the purge valves 23 close back and the isolation valve 22 reopen and normal operation can resume.

[0121] During the time gas is being purged from the low-pressure receiver 3, the flow from the third main pipe 13 and the flow through the low-pressure pump 5 are both interrupted. This may be detrimental to the stability of operation of the whole system but can be overcome by installing in parallel two or more aggregates that comprise the receiver low-pressure 3, the isolation valves 22 and the purge valves 23 (and possibly the subcooling apparatus 6). That way it is possible to continue operating the system while one of the aggregates proceeds to a purge cycle. Alternatively (see FIG. 4), it is also possible to change the geometry of the low-pressure receiver 3 by having one part of the volume of said receiver, preferably smaller, installed above the main part of the low-pressure receiver 3, both parts being linked by a pipe on which is installed the receiver isolation valve 22. When the said valve is open both volumes constitute the low-pressure receiver 3 and during normal operation the gas coming from the third main pipe 13 will accumulate within the top part of said receiver by buoyancy. Once the liquid level in the top part of low-pressure receiver 3 reaches a value low enough because of gas build-up in said part, a purge cycle analogue to the one described previously is carried out by closing the receiver isolation valve 22 and opening the purge valves 23. Once the liquid level in the top part of the low-pressure receiver 3 reaches a value high enough, valves 23 close back and the valve 22 opens up again to resume normal operation. The advantage of this version of the receiver gas purge is to avoid the interruption of the flow coming from the third main pipe 13 and going to the pump 5 while also avoiding the necessity of putting aggregates in parallel as described earlier. Moreover, the number of isolation valves 22 is reduced to only one valve. Note that the purge valves 23 can also be installed on pipes that link the low-pressure receiver 3 to respectively the first main pipe 11 and second main pipe 12 instead of the intermediate pressure receiver 1. This latter solution could be advantageous if it is not possible to install the intermediate pressure receiver 1 slightly above low-pressure receiver 3.

[0122] The invention is of course not limited to those four illustrated examples but to any alternative covered by the claims.