GB2567233A

GB2567233A - Apparatus and method for controlling a near-critical fluid

Info

Publication number: GB2567233A
Application number: GB1716489.8A
Authority: GB
Inventors: Hildenbeutel Jan; Treite Philipp
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2017-10-09
Filing date: 2017-10-09
Publication date: 2019-04-10
Also published as: GB201716489D0; WO2019072739A1

Abstract

The present invention pertains to a process related control of flow characteristics in a convectional flow of a near-critical fluid, in particular a process heating arrangement 1 for controlling a convectional flow of a near-critical fluid in a conduit 30. The process heating arrangement for controlling a convectional flow of a near-critical fluid in a conduit comprises a first heating device 10, which is configured to output heat to a first heated fluid conducting surface of the conduit and a second heating device 12, which is configured to output heat to a second heated fluid conducting surface of the conduit. The process heating arrangement furthermore comprises a control unit 20 configured to control the heat output of said first and second heating devices. Accordingly, the process heating arrangement is configured to control a process flow and in particular to remedy or prevent the occurrence of oscillations preferably by adjusting the heat output or adjusting a heat flux density of said first and second heating devices locally and area-specific. The device may feature pressure valves 34 between the two heating devices or temperature/pressure sensors 32 before each heating device.

Description

Apparatus and method for controlling a near-critical fluid

Technical Field

The invention relates to an apparatus and a method for controlling flow of a near-critical fluid. In particular, the invention relates to a process related control of flow characteristics in a convectional flow of a near-critical fluid, in particular a process heating arrangement for controlling a convectional flow of a near-critical fluid in a conduit.

Technological Background

For simulating heat loads, in particular isothermal heat loads in e.g. a refrigeration system or plant a variety of configurations are used. Such heat load simulations, wherein the evaporation enthalpy is commonly used, may e.g. be performed in cavities, vessels, cryomodules, or other applications.

To control the thermal energy, a heating device is commonly provided for a flow comprising a supercritical, gaseous, two-phased, or liquid fluid. For example, a heating device may be provided along a fluid conduit in a process, wherein the conduit comprises a pressure release valve downstream of the heating device. The fluid may comprise a supercritical fluid, such that the heating device may provide heat to control a thermodynamic and physical state of the super critical fluid, i.e. prevent the fluid from entering a liquid state. The pressure release valve may then reduce the pressure to establish a two-phased fluid, e.g. having both a supercritical phase and a gas phase or a one-phased fluid, e.g. by successively heating the supercritical fluid from a first supercritical process point to a second supercritical process point and then providing an expansion of the fluid such that the fluid is transferred to gas phase.

In alternative configurations, the heating device may be arranged downstream of a pressure release valve at a vessel, wherein the pressure release valve relaxes a supercritical state of the fluid to establish a two-phased fluid and wherein the heating device is disposed at the vessel around a liquid phase of the fluid.

The introduction of heat in a flow of a process, in particular supercritical process flows, however, may be accompanied with large density shifts, which cause large changes in the volume flow of the fluid lengthwise to the heating device. The phase shift and change of flow and fluid characteristics together with the heat transfer from the heating device to the process medium results in boundary interactions, for example local boundary surface or boundary layer disruptions, which are noticeable in the dynamic behavior of the process flow as oscillations.

Such oscillations are undesirable and may damage and impair the operation of downstream devices, for example, rotation devices, such as e.g. expansion turbines, pumps or turbocompressors. Accordingly, such oscillations, in particular oscillations in relation to near-critical forced convection, are to be prevented.

However, since the surface of the heating device is dependent on the provided geometry of the process and conduit and the global or total heat input is provided as a fixed process parameter, modifications of the surface specific heat flux provided by the heating device are difficult to implement.

Accordingly, a need exists to control a convectional flow of near-critical fluids and reduce the occurrence of oscillations caused by heat transfer.

Summary of the invention

It is an object of the present invention to provide a device for an improved controlling of a convectional flow of a near-critical fluid, in particular a cryogenic fluid, in a conduit.

This object is solved by a process heating arrangement for controlling a convectional flow of a nearcritical fluid in a conduit according to the independent claims. Preferred embodiments are given by the dependent claims.

Accordingly, in a first aspect, a process heating arrangement for controlling a convectional flow of a near-critical fluid in a conduit is suggested, wherein the process heating arrangement comprises a first heating device configured to output heat to a first heated fluid conducting surface of the conduit, a second heating device configured to output heat to a second heated fluid conducting surface of the conduit, and a control unit configured to control the heat output of said first and second heating devices.

The process heating arrangement hence comprises at least two heating devices, which are communicatively connected with a control unit. The control unit is preferably configured to adjust flow characteristics of the convectional flow within the conduit and may accordingly adjust the heat output of both the first and second heating devices. This at least has the advantage that the fluid flow is no longer dependent on a single heating device and any undesirable flow characteristics downstream of the first heating device may hence be remedied or corrected for by the second heating device. For example, a near-critical fluid may be present in the conduit in two physical states or phases, e.g. a liquid phase and a gas phase. Should the heat output of the first heating device lead to an amount of the liquid phase to enter the gas phase, the volume of the fluid is spontaneously expanded, which may cause a disruption of a boundary layer. Such disruptions and accompanying oscillations are undesirable since they may impair the functioning of downstream devices such as e.g. compressors, turbo-compressors, pumps, expansion turbines, etc. Accordingly, instead of only adjusting the heat output of the first heating device, the controller may also adjust the heat output of the second heating device to compensate for any undesirable conditions. The heat output of the first heating device may e.g. be reduced while the heat output of the second heating device may be accordingly increased to ensure that the liquid phase and gas phase remain constant and the gas phase does not enter the liquid phase. This mutual adjusting at least has the advantage that the occurrence of oscillations or vortices within the convectional flow may be remedied or at least minimized by the implementation of the second heating device.

The adjusting of the heating devices by the control unit may be based on e.g. previous data available for the process that is compared and/or correlated with current data or settings and processed via control algorithms to define the proper adjustment.

Preferably, the first heating device is arranged to output heat to a circumferential first heated fluid conducting surface of the conduit and/or the second heating device is arranged to output heat to a circumferential second heated fluid conducting surface of the conduit. Such circumferential conducting surface has the advantage that the fluid in the convectional flow may be homogenously heated and hence the heat convection is controlled.

The control unit of the process heating arrangement may furthermore be configured to adjust a heat flux density of said first and second heating devices by adjusting the heat output of said first and second heating devices, wherein the control unit is preferably configured to adjust the heat flux density locally and area-specific. By adjusting the heat flux density, the flow of energy per area is controlled over time. Accordingly, the exact heat transfer may be calculated and controlled for each heated fluid conducting surface, so that by implementation of e.g. Fourier algorithms for tubular systems such as conduits the amount of energy being transferred to the fluid in the system is known. Should e.g. irregular flow characteristics occur, for example, due to expansion behavior of the fluid by entering the gas phase from the liquid phase, the adjusting of the local and area-specific heat flux density may reduce the occurrence of such behavior and re-establishes or maintains a flow without irregular flow characteristics.

To further control the heat output of the first and second heating devices and maintain a controllable fluid state of the near-critical convectional flow, each of the first and second heating devices may be dimensioned to output a predetermined total thermal energy. The total thermal energy output of each heating device may be the same, such that the second heating device forms a redundant heating device with regard to the first heating device. However, the heating devices may also have dimensions, such that e.g. the second heating device is dimensioned larger than the first heating device and vice versa. For example, the first heating device may output a total thermal energy of 500 W, whereas the second heating device may be dimensioned to output a total thermal energy of 1000 W. Accordingly, the second heating device may be configured to compensate for any occurring flow characteristics due to a reduced and/or an insufficient thermal energy output of the first heating device.

To control the convectional flow of a near-critical fluid the first and second heating device may furthermore commonly output a continuous total thermal energy, wherein the controller is configured to distribute the total thermal energy between the first heated fluid conducting surface of the conduit and the second heated fluid conducting surface of the conduit by adjusting the heat output of said first and second heating devices. Hence, the first and second heating devices may together provide a constant heat output to the fluidic system in the conduit while the heat output of each heating device on itself may vary. For example, the process flow may require that a total energy level of 1000 W is continuously transferred into the process flow, however, the first heating device may be dimensioned to output 200 W, while the second heating device may be dimensioned to output 800 W. Alternatively, the total energy level of 1000 W may e.g. be evenly distributed over the first and second heating devices.

Accordingly, the heat output ratio of the first heating device to the second heating device with respect to the total thermal energy is preferably between 1:99 percent and 50:50 percent, more preferably between 10:90 percent and 40:60 percent. For example, the ratio of the first heating device to the second heating device with respect to the total thermal energy may hence also be between 20:80 percent and 30:70 percent. The total energy level may hence be evenly distributed over the first and second heating devices or the first heating device is configured to provide a heat output that is lower than the heat output of the second heating device.

The control unit of the process heating arrangement may furthermore be configured to vary the heat output of the first and second heating devices by varying the output duration and/or the thermal energy level, wherein the output is preferably varied continuously and/or periodically and/or in a pulsed fashion. For example, the heat output may occur continuously over time, wherein the amount of heat to be output is varied for time blocks ranging from several milliseconds to one or more minutes, e.g. the first heating device may output heat continuously while the amount of thermal energy is 200 W for a time period of e.g. five seconds continued by a thermal energy output of 400 Wfor a subsequent time period of e.g. five seconds. By the same token, the second heating device may output heat in e.g. a pulsed fashion, such that heat is only output every 10 seconds for a period of time of two seconds. Accordingly, various predefined schemes may be implemented to control the convectional flow of the near-critical fluid in the conduit.

The arrangement of the first and second heating devices may depend on the process. Accordingly, the first heating device and the second heating device may be serially arranged along the conduit, wherein the first and second heating device are configured to provide a heat flux in a direction of flow of the near-critical fluid. The serial arrangement may facilitate a direct correction of the occurrence of an undesirable flow characteristic in a shared conduit section such as e.g. a vortex caused by an expansion and an accompanying change of flow.

Alternatively, or in addition, the conduit may comprise a pressure release valve, wherein the first heating device is arranged upstream of said valve and the second heating device is arranged downstream of said valve. The valve may e.g. be required to lower a pressure of the fluid, such that the fluid is changed from a liquid phase into a two-phased fluid comprising a fluid and a gas phase. The arrangement of the first and second heating devices may for example facilitate the establishment of a two-phased fluid, such that e.g. the first heating device preheats a liquid fluid while the second heating device maintains the two-phased fluid by outputting heat to prevent the gas phase of the fluid to enter the liquid phase. By the same token, the pressure release valve may cause the fluid in the convectional flow to enter a supercritical phase, wherein the second heating device is configured to maintain such phase.

The process heating arrangement may furthermore provide that the conduit comprises a pressure release valve, wherein the first heating device is arranged upstream of said valve and the second heating device is arranged downstream of said valve, and wherein the conduit comprises a vessel downstream of said valve and wherein the second heating device is arranged at said vessel. For example, the pressure valve may be configured to reduce or release the pressure to establish a two-phased fluid, wherein the first heating device is configured to accordingly preheat the fluid. The second heating device may, however, be adapted to maintain a fluid phase, such that it may be configured to provide a lower heat output. Such an arrangement has at least the advantage, that fluid characteristics may be studied and identified by providing a platform to observe and measure the different flow characteristics when switching and transforming between different physical states.

Alternatively, the process may be implemented in an application of refrigeration systems, wherein cavities are provided by means of a vessel or liquid bath. Accordingly, the evaporation enthalpy of the process medium may be used to compensate for the incident heat in the cavity. To maintain a constant level of the vessel, i.e. to ensure a quasi-static process, there must be a balance between the vaporized mass and the mass flow that is liquefied by the refrigeration system.

Instead of a serial arrangement, the first heating device and the second heating device may also be arranged in parallel along the conduit, wherein the first heating device is configured to provide a heat flux in a direction of flow of the near-critical fluid and the second heating device is configured to provide a heat flux in a direction opposite to the flow of the near-critical fluid.

Furthermore, more than two heating devices may be provided in the process heating arrangement. Accordingly, the process heating arrangement can be implemented by having a series of heating units for each heating device, by forming a heating arrangement block in a system comprising multiple heating arrangement blocks, or by addition of a third or fourth heating device downstream of the first heating device and in communication with the control unit.

The fluid may comprise a variety of physical states. Accordingly, the fluid may be a supercritical fluid, a gas, a liquid, or a two-phase fluid. Preferably, the fluid comprises a cryogenic fluid. For example, the fluid may be liquid helium, wherein the flow of the near-critical fluid may be controlled by controlling the heat output of the first and second heating devices by the control unit. Such control is particularly important when a supercritical fluid is used, wherein a viscosity of the fluid equals zero and wherein sudden thermal inconsistencies may cause a large change in the fluid dynamics do to a sudden shift in physical state and corresponding expansion behavior of the fluid.

Instead of only controlling the flow of the near-critical fluid based on e.g. previous data available for the process that is compared and/or correlated with current settings, the process heating arrangement may also comprise at least one of a flow measurement device, a temperature measurement device, a pressure sensor, and/or a means for determining a heat coefficient in the conduit. Accordingly, the control unit is configured to adjust the heat output of said first and second heating device based on a measured flow characteristic, temperature, pressure, and/or heat coefficient. For example, the control may implement an algorithm, which does not require predefined schemes and is configured to control the convectional flow based on input of e.g. measured flow characteristics by properly adjusting the heat output of the first and second heating devices.

Furthermore, the control unit may be configured to determine the heat convection and/or heat conduction in the fluid in the conduit, wherein the control unit is configured to derive a flow parameter from said heat convection and/or heat conduction and to adjust the heat output of said first and second heating devices based on said parameter to control the flow of the fluid in the conduit.

For example, the control unit may process the measured data to derive information regarding the heat transfer in a boundary surface within the fluid and accordingly define a ratio of convective and conductive heat transfer across the boundary layer. As a result, a dimensionless parameter such as the Nusselt number may be calculated, which is related to the Reynolds and Prandtl number and may hence provide an estimate about the flow state within the conduit. For example, should a Reynolds number provide a value indicating a potential disruption of the boundary surface and the near-presence of irregular flow characteristics within the flow, the control unit be configured to adjust the heat output of the first and second heating devices to accordingly adjust the convectional flow.

According to a further aspect of the invention, a method is provided for controlling a convectional flow of a near-critical fluid in a conduit, wherein the method comprises providing a convectional flow of a near-critical fluid in a conduit, heating a first heated fluid conducting surface of the conduit, heating a second heated fluid conducting surface of the conduit, and controlling the thermal energy output by said first and second heated fluid conducting surfaces. Accordingly, the method provides the adjusting of flow characteristics of the convectional flow within the conduit by accordingly adjusting the heat output of both the first and second heating devices.

In addition, the method may further comprise controlling a heat flux density of the first and second heated fluid conducting surfaces, wherein controlling of the heat flux density preferably occurs locally and area-specific.

Furthermore, the method may comprise controlling the thermal energy output by the first and second heated fluid conducting surfaces to output a constant total thermal energy, wherein the heat output ratio of the first heated fluid conducting surface to the second heated fluid conducting surface with respect to the total thermal energy output is preferably between 1:99 percent and 50:50 percent, preferably between 10:90 percent and 40:60 percent.

The proposed method is furthermore described in more detail with regard to the corresponding features of the process heating arrangement, outlined in the above. Accordingly, the method may comprise further features corresponding to the features of the proposed process heating arrangement.

Brief description of the drawings

The present disclosure will be more readily appreciated by reference to the following detailed description when being considered in connection with the accompanying drawings in which:

Figure 1 is a schematic view of a process heating arrangement with two heating devices that are serially arranged;

Figure 2 is a schematic view of the process heating arrangement shown in Figure 1 with additional measurement devices;

Figure 3 is a schematic view of a process heating arrangement with two heating devices that are arranged in parallel;

Figure 4 is a schematic view of the process heating arrangement shown in Figure 2 with a pressure release valve;

Figure 5 is a schematic view of a process heating arrangement with two heating devices that are serially arranged, wherein the conduit comprises a vessel.

Detailed description of preferred embodiments

In the following, the invention will be explained in more detail with reference to the accompanying figures. In the Figures, like elements are denoted by identical reference numerals and repeated description thereof may be omitted in order to avoid redundancies.

In Figure 1 a process heating arrangement 1 is shown having a first heating device 10 and a second heating device 12, which are arranged at a conduit 30. The first and second heating devices 10, 12 are serially arranged and are each connected with a control unit 20, which is configured to adjust the heat output of each of the first and second heating devices 10, 12. The heating devices 10, 12 are configured to output heat respectively to a first and second heated fluid conducting surface of the conduit 30. In order to provide a more homogeneous heat output, both heating devices 10, 12 are arranged to output heat to a circumferential first heated fluid conducting surface of the conduit 30. In other words, the first and second heating devices 10, 12 are configured as flow-through heating devices.

In order to control the flow, the control unit 20 may hence provide an isothermal load to the fluid in the conduit 30 by means of the first and second heating devices 10, 12. For example, for simulation purposes of supercritical fluids, the specific enthalpy in the fluid before the first heating device 10 must be less than the specific enthalpy at the critical point. Accordingly, the fluid may be in a liquid phase. To establish a supercritical fluid or to establish a two-phased fluid, the first heating device 10 may then output heat to equal a supercritical state of the fluid. In addition, the second heating device 10 may adjust any deficits in the required enthalpy or distribute the total enthalpy over the first and second heating devices 10, 12.

The control unit 20 may comprise a processor and a storage medium in communication with the processor and storing algorithms and/or reference data. Accordingly, the control unit may be part of a computer system or network, which is configured to automatically or manually control the first and second heating devices 10, 12.

For example, should the heat output of the first heating device 10 and the second heating device 12 result in a boundary surface disruption and cause oscillations, the control unit 20 may re-distribute the heat to be output by each heating device 10, 12, while the total amount of heat to be output is maintained constant. Accordingly, the flow dynamics and the thermodynamic state of the fluid are controlled by the control unit 20 and the heating devices 10, 12. Thus, by the provision of a second heating device 12, another degree of freedom is provided to adjust the local and area-specific heat flux density.

The process heating arrangement 1 as depicted in Figure 1 is furthermore described in relation to the implementation of measurement devices, e.g. temperature and/or pressure sensors 32. According to the embodiment, the process heating arrangement 1 comprises three temperature and/or pressure sensors 32. In order to facilitate the calculation of a thermodynamic state of the fluid, preferably both a pressure and temperature of the fluid are known, which are then compared with known data. The sensors 32 are arranged upstream of the first heating device 10 and downstream of the first heating device 10, one sensor 32 being upstream of the second heating device, i.e. arranged between the first and second heating devices 10, 12, and one sensor being arranged downstream of the second heating device 12. Although each sensor 32 may comprise both a temperature sensor and a pressure sensor 32, they may also comprise either a temperature sensor or a pressure sensor 32. Alternatively, or in addition, the sensors 32 may comprise a flow measurement device or flow sensor to indicate a mass flow of the fluid. Accordingly, a variety of sensor arrangements may be provided.

Although the embodiment is shown having three sensors 32, more or less sensors 32 may be implemented, e.g. only one sensor 32 downstream of the first heating device, or a plurality of sensors 32 upstream of the first heating device 10 and/or downstream of the second heating device 12, such that the control unit 20 may monitor a larger section of the conduit 30.

Based on the input of the sensors 32, the control unit 20 may adjust the heat output of the first and second heating devices 10, 12 to control a thermodynamic and fluidic state of the fluid. For example, should the sensor upstream of the first heating device 10 indicate a temperature that exceeds a predefined tolerance range, the heat output of the first heating device 10 may be reduced, whereas, in order to provide a constant global heat output, the heat output of the second heating device 12 may be increased. By the same token, the sensor 32 arranged between the first and second heating devices 10, 12 may provide a control measure to the control unit, to indicate whether the adjustment is effective to reduce the occurrence of oscillations. Should the adjustment of the heat output be insufficient, the heat output of the second heating device 12 may be adjusted and the total heat output may be redistributed between the first and second heating devices 10, 12.

Figure 3 shows a process heating arrangement 1 that is similar to the process heating arrangement shown in Figure 1. However, according to the embodiment of Figure 3, the first and second heating devices 10, 12 are arranged to in parallel. Such arrangement may be implemented when e.g. requiring a return conduit or when implementing a circulation of the fluid through the conduit 30. Accordingly, the first heating device 10 is configured to provide a heat flux in a direction of flow of the near-critical fluid and the second heating device 12 is configured to provide a heat flux in a direction opposite to the flow of the near-critical fluid.

In the embodiment according to Figure 4 the process heating arrangement 1 again comprises two heating devices 10, 12 that are serially arranged at the conduit 30, as described in more detail in Figures 1 and 2. In addition, the conduit 30 comprises a pressure release valve 34 that is in fluid communication with the conduit 30 and which is arranged between the first and second heating devices 10, 12, such that the first heating device 10 is arranged upstream of the valve 34 and the second heating device 12 is arranged downstream of the valve 34. The pressure release valve 34 is controlled by the control unit 20 and is configured as a control valve. For example, the pressure release valve may reduce the pressure of the fluid in the conduit 30 or may provide an outlet to reduce the mass flow in the conduit 30.

Alternatively, the pressure release valve may be configured to, in addition, supply pressure, e.g. in situations, wherein the critical point of a fluid is not reached or maintainable by adjusting the temperature of the fluid by adjusting the heat output of the first and second heating devices 10, 12.

Furthermore, although the embodiment only depicts one valve 34, the process heating arrangement 1 may also comprise a plurality of control valves. Thus, the conduit 30 may, in addition to the pressure release valve 34 arranged between the firstand second heating devices 10,12 comprise one or more valves upstream of the first heating device 10 and/or downstream of the second heating device 12. The one or more valves 34 may be fully adjustable or be set to a fixed position.

The embodiment according to Figure 5 comprises in addition to a pressure release valve 34, described in more detail with reference to Figure 4, a vessel 36. Accordingly, the conduit 30 comprises a vessel 36 that may be implemented at one end of the conduit 30 or as a bypass or outlet of said conduit 30. The pressure release valve 34 is arranged downstream of the first heating device 10 and upstream of the vessel 36. Accordingly, a supercritical fluid may be pre-heated by the first heating device 10 to maintain its supercritical state and subsequently be relaxed by means of the pressure release valve 34. In other words, the pressure release valve 34 may reduce the pressure, such that a two-phased or near-critical fluid is provided in the conduit 30 prior to entering the vessel 36. Within the vessel 36, the fluid hence comprises a liquid or supercritical phase and a gas phase. The second heating device 12 is arranged at the bottom of the vessel 36, such that its heat output directly affects the liquid phase of the fluid in the vessel 36. Accordingly, the thermodynamic behavior and the evaporating enthalpy may be studied by adjusting the heat transferred to the fluid and distributed over the first and second heating devices 10, 12. The provision of the second heating device 12 hence provides a more controllable setting to simulate heat loads, in particular isothermal heat loads in e.g. a refrigeration system or plant. Furthermore, the vessel 36 may be configured as a cooling bath, such that the process heating arrangement 1 may be implemented in a refrigeration system to provide cooling by means of a process fluid. To maintain a constant level of the vessel 36, i.e. to ensure a quasi-static process, there must be a balance between the vaporized mass and the mass flow that is liquefied by the refrigeration system, such balance being provided by the provision and control of the first and second heating devices 10, 12 in the process heating arrangement 1.

It will be obvious for a person skilled in the art that these embodiments and items only depict examples of a plurality of possibilities. Hence, the embodiments shown here should not be understood to form a limitation of these features and configurations. Any possible combination and configuration of the described features can be chosen according to the scope of the invention.

List of reference numerals

Process heating arrangement

First heating device

Second heating device

20 Control unit

Conduit

Temperature or pressure sensor

Pressure release valve

Vessel

Claims

1. Process heating arrangement (1) for controlling a convectional flow of a near-critical fluid in a conduit (30), the process heating arrangement (1) comprising:

a first heating device (10) configured to output heat to a first heated fluid conducting surface of the conduit (30);

characterized by a second heating device (12) configured to output heat to a second heated fluid conducting surface of the conduit (30), and a control unit (20) configured to control the heat output of said first and second heating devices (10, 12).

2. Process heating arrangement (1) according to claim 1, wherein the first heating device (10) is arranged to output heat to a circumferential first heated fluid conducting surface of the conduit (30); and/or the second heating device (12) is arranged to output heat to a circumferential second heated fluid conducting surface of the conduit (30).

3. Process heating arrangement (1) according to claim 1 or 2, wherein the control unit (20) is configured to adjust a heat flux density of said first and second heating devices (10, 12) by adjusting the heat output of said first and second heating devices (10, 12), wherein the control unit (20) is preferably configured to adjust the heat flux density locally and area-specific.

4. Process heating arrangement (1) according to any of the preceding claims, wherein each of the first and second heating devices (10, 12) is dimensioned to output a predetermined total thermal energy.

5. Process heating arrangement (1) according to any of the preceding claims, wherein the first and second heating devices (10,12) commonly output a continuous total thermal energy, and wherein the control unit (20) is configured to distribute the total thermal energy between the first heated fluid conducting surface of the conduit (30) and the second heated fluid conducting surface of the conduit (30) by adjusting the heat output of said first and second heating devices (10,12).

6. Process heating arrangement (1) according to claim 5, wherein the heat output ratio of the first heating device (10) to the second heating device (12) with respect to the total thermal energy is between 1:99and 50:50preferably between 10:90and 40:60.

7. Process heating arrangement (1) according to any of the preceding claims, wherein the control unit (20) is configured to vary the heat output of the first and second heating devices (10, 12) by varying the output duration and/or the thermal energy level, and wherein the heat output is preferably varied continuously and/or periodically and/or in a pulsed fashion.

8. Process heating arrangement (1) according to any of the preceding claims, wherein the first heating device (10) and the second heating device (12) are serially arranged along the conduit (30), wherein the first and second heating devices (10, 12) are configured to provide a heat flux in a direction of flow of the near-critical fluid, and/or the conduit (30) comprises a pressure release valve (34), wherein the first heating device (10) is arranged upstream of said valve (34) and the second heating device (12) is arranged downstream of said valve (34).

9. Process heating arrangement (1) according to claim 8, wherein the conduit (30) comprises a pressure release valve (34) the first heating device (10) is arranged upstream of said valve (34) the second heating device (12) is arranged downstream of said valve (34), the conduit (30) comprises a vessel (36) downstream of said valve (34), and the second heating device (12) is arranged at said vessel (36).

10. Process heating arrangement (1) according to any of the claims 1 to 7, wherein the first heating device (10) and the second heating device (12) are arranged in parallel along the conduit (30), wherein the first heating device (10) is configured to provide a heat flux in a direction of flow of the near-critical fluid and the second heating device (12) is configured to provide a heat flux in a direction opposite to the flow of the near-critical fluid.

11. Process heating arrangement (1) according to any of the preceding claims, wherein the fluid is a supercritical fluid, a gas, a liquid, ora two-phase fluid, and/or wherein the fluid is preferably a cryogenic fluid.

12. Process heating arrangement (1) according to any of the preceding claims, further comprising a flow measurement device, a temperature measurement device (32), a pressure sensor (32), and/or a means for determining a heat coefficient in the conduit, wherein the control unit (20) is configured to adjust the heat output of said first and second heating devices (10, 12) based on at least one of the following: a measured flow characteristic, temperature, pressure, heat coefficient.

13. Process heating arrangement (1) according to claim 11, wherein the control unit (20) is configured to determine the heat convection and/or heat conduction in the fluid in the conduit (30), wherein the control unit (20) is configured to derive a flow parameter from said heat convection and/or heat conduction; and to adjust the heat output of said first and second heating devices (10, 12) based on said parameter to control the flow of the fluid in the conduit (30).

14. Method for controlling a convectional flow of a near-critical fluid in a conduit (30), the method comprising: providing a convectional flow of a near-critical fluid in a conduit (30); heating a first heated fluid conducting surface of the conduit (30); heating a second heated fluid conducting surface of the conduit (30), controlling the thermal energy output by said first and second heated fluid conducting surfaces.

15. Method according to claim 14, wherein the method further comprises controlling a heat flux density of the first and second heated fluid conducting surfaces, wherein controlling of the heat flux density preferably occurs locally and area-specific.

16. Method according to claim 14 or 15, wherein the method further comprises controlling the thermal energy output by said first and second heated fluid conducting surfaces to output a constant total thermal energy, wherein the heat output ratio of the first heated fluid conducting surface to the second heated fluid conducting surface with respect to the total thermal energy output is preferably between 1:99 and 50:50, preferably between 10:90 and 40:60.