Heat exchanger and method for operating a heat exchanger

The present invention relates to a heat exchanger and a method for operating a heat exchanger.

Background of the invention

Heat exchangers are used for numerous different applications, for example solar thermal and gas processing applications.

Solar thermal power stations convert solar energy into electrical energy using a thermodynamic cycle process. Herein, a circulating working medium such as water is vapourised, and the vapour generated is used to drive a turbine. While the working medium can be directly heated by means of solar irradiation, it is more common to effect this heating indirectly by heating a heat transfer medium such as a salt melt. Herein, the salt melt, as a first heat transfer medium, is introduced into a heat exchanger, in which it transfers heat to a second heat transfer medium, such as water.

It has proved particularly advantageous to utilise coil wound heat exchangers (CWHE) in order to transfer heat from a salt melt to water or vapour because of faster warm up rates and mechanical robustness. CWHEs are typically provided with a coiled tubing arranged within a shell space, wherein a shell member defines the outer delimitation of the shell space. A salt melt (as first heat transfer medium) is conveyed through the shell space, and water (as second heat transfer medium) through the coiled tubing. Typically, the coiled tubing is surrounded by a shroud, partitioning the shell space into an inner region containing the coiled tubing and an outer region between the shroud and the shell member. This shroud prevents a bypass of the fluid beneath the tube bundle what would lead to a malperformance of the CWHE. During a normal mode of operation of such a CWHE (referred to as first mode of operation in the following), the water conveyed through the coiled tubing is vapourised or steam is superheated by heat transfer from the molten salt conveyed through the shell space. Molten salt will be referred to as salt melt, and molten salt having a temperature sufficient for effecting vapourisation will be referred to as hot salt melt in the following.

The salt melt receives its heat energy from solar irradiation. During the night or periods of bad weather, the situation can occur that there is no sufficient hot salt melt supply available. Under these circumstances, it is necessary to switch the CWHE to a mode of operation referred to as the so called„freeze protection mode". Herein, the temperature within the CWHE is gradually reduced by feeding colder salt melt, simply referred to as cold salt melt in the following, into the coiled tubing, until finally only cold salt melt circulates through the CWHE. Be it noted that the final temperature defined by the cold salt melt is still higher than the freezing point of the salt melt.

Conversely, in case there is sufficient hot salt melt available again, for example when solar irradiation is again available, the process is reversed by gradually feeding hot salt melt into the coiled tubing of the CWHE, until the salt melt again reaches the temperature sufficient for vapourisation, i.e. an operation temperature.

In the following, the phase, in which the temperature is reduced or increased, is referred to as a transitional or second mode of operation, and the freeze protection mode as a standby or third mode of operation.

This switching between modes of operation can typically be performed two times in 24 hours, said two times being determined by sunrise and sunset. It is desirable to effect the switching as quickly as possible in order to maximise use of available solar irradiation.

CWHEs are also used in gas processing applications such as rectisol units, ethylene or LNG (liquid natural gas) plants. For example, the operation temperature of a cryogenic natural gas liquefaction unit comprising one or more CWHEs typically goes down to around - 165° C. When starting up such a plant, the cryogenic equipment has to be cooled down to a certain temperature to avoid mechanical failure due to thermal stress when initiating the cryogenic process flows through the CWHEs.

Depending on the process such cooling is typically executed by externally cooled natural gas, which is routed through the cryogenic CWHEs. Downstream of the CWHEs, this natural gas stream can be routed to a flare. Typically, cooling down of liquid natural gas CWHEs from a warm ambient temperature to an operation temperature requires around five to fifteen hours or more with a typical flare rate in the order of magnitude of 250 t/h. This leads to a large impact on the environment, as flared gas can not be further used. It also constitutes a significant financial loss for an operator.

In the following, the normal mode of operation of a cryogenic natural gas liquefaction unit, in which cryogenic temperatures are realized, is referred to as a first mode of operation, transitional phases, i.e. a cooling down phase, during which a cooling from a warm ambient to cryogenic temperatures is realized, or a warming up, during which a warmup from cryogenic to ambient temperatures is effected, are referred to as second mode(s) of operation, and a standby phase, during which a main CWHE of an LNG unit essentially has ambient temperature, is referred to a third mode of operation.

In other LNG processes the CWHE is cooled down by mixed refrigerant. The flow of the refrigerants coming from the mixed refrigerant cycle is routed via expansion valve on its normal way to the shell side. Due to the expansion colder temperatures occur. Only small flows are allowed during cooling down to prevent a too fast cooling down as this can lead to exceedance of allowable temperature differences and thermal stress. Cooling down can typically take from 12 to 24 hours.

The main reason for such long cooling down (and also heating up) times are that the shell side medium only flows with in the bundle (blocked by the shroud to prevent bypass) and not between shroud and shell wall. As a result the shell wall material with its large wall thickness is not cooled down / heated up directly by the flowing medium and takes much more time to reach the same temperature as the bundle material. This leads to significant temperature differences between shell material and bundle material which leads to thermal stress. In order to limit such thermal stress during cooling down/heating up periods, it must be ensured that specific temperature difference limits are not exceeded. This in effect means that cooling down and/or heating up has to be slow enough for the shell material temperature to be able to follow the bundle temperature. The object of the invention is to provide a heat exchanger and a method for its operation with which switching times (transitional phases) between modes of operation as discussed can be optimized or accelerated. Disclosure of the invention

This object is achieved by providing a heat exchanger and a method for its operation comprising the features of the respective independent claims. According to the invention, there is provided a heat exchanger comprising a shell member defining a shell space and a coiled tubing arranged within the shell space adapted to convey a first heat transfer medium between a first inlet port and a first outlet port, the shell space being adapted to convey a second heat transfer medium between a second inlet port and a second outlet port. The shell space comprises an inner bundle region adapted to provide free flow of the heat transfer medium during a first and a second mode of operation, and an outer region adapted to prevent free flow of the second heat transfer medium during the first mode of operation and to provide free flow of the second heat transfer medium during the second mode of operation. By adapting the outer shroud region of the shell space to prevent free flow of the second heat transfer medium during the first mode of operation and allowing free flow of the second heat transfer medium during the second mode of operation the heat exchanger can be switched between different operating modes in an optimized manner. The inner region surrounds the tube bundle, and can be referred to as bundle region. The outer region can be referred to as shroud region, as its inner boundary is essentially defined by the shroud.

The invention offers significant benefits:

Due to simultaneous flow within the inner and outer regions the tube bundle as well as the shell material are cooled down / warmed up in a similar manner, i. e. essentially simultaneously or synchronously, which leads to a reduction of temperature differences between shell and bundle. As a result cooling down/heating up of the CWHE can be effected in a faster way which leads e. g. to a reduction of shut down times of a plant. For example, within the context of liquefaction of natural gas, flaring times can be significantly reduced by implementation of the invention. Especially, thermal stress occurring in heat exchangers can be minimized, because the shell member can cool down or warm up at a rate essentially similar to that of the coiled tube bundle. This is especially advantageous in the context of heat exchangers used in solar thermal power stations.

Advantageously, the first mode of operation is a normal heat exchanging mode of operation, in which the second heat transfer medium is at its normal operating temperature. Here, by preventing flow of the second heat transfer medium in an outer region, it can be ensured that second heat transfer medium comes into optimized contact with the coiled tubing, which is located within the inner region, in which the second heat transfer medium can freely flow. In the second mode of operation, i.e. a switching or transitional mode between the normal mode of operation and a standby mode, during which second heat transfer medium at a lower or higher temperature than the temperature required in the normal mode of operation is conveyed through the shell space, providing free flow of second heat transfer medium in the outer region of the shell space renders possible a uniform change of temperature of the heat exchanger as a whole and therefore reduces material stress during the transitional phases, especially in the shell member of the heat exchanger, compared to prior art solutions what allow a faster transition time.

According to a preferred embodiment, the inner region and the outer region of the shell space are separated by a shroud member. Providing such a shroud member effectively ensures that the second heat transfer medium flows through the inner region of the shell space in optimal thermal contact with the coiled tubing.

Expediently, there is provided a blocking member, for example an annular blocking member, arranged between the shroud member and the shell member. The blocking member may be adapted to prevent a flow of second heat transfer medium in the outer region of the shell space in the first mode of operation.

Advantageously, the heat exchanger comprises a third inlet port adapted to convey second heat transfer medium into the outer region during the second mode of operation enabling a free flow of second heat transfer medium during the second mode of operation. Essentially, such a third inlet port is provided to bypass a blocking member arranged between the shroud member and the shell member. Providing such a third inlet port in connection for example with a controllable valve provides a simple and robust means for preventing or providing free flow of second heat transfer medium in the outer region of the shell space in direct contact with the shell wall material.

According to a further preferred embodiment, the blocking member is provided with at least one vent member. Typically, a number of vent members are provided in the blocking member. Advantageously, these vent members are provided as manually or automatically operable vent members. Such vent members are typically used for CWHEs in gas processing applications, and can be connected to a flare.

The approach according to the invention is to open these vent members during the second mode of operation, i.e. the transitional mode between a normal mode of operation and a standby mode. This establishes a flow of second heat transfer medium during the second mode, to provide a gas flow between the shroud member and the shell member. This additional flow can increase the cooling down rate of the shell member significantly, for example by a factor of at least two to three. Thus, transitional modes of operation, which, as outlined above, could take between five to fifteen hours in prior art solutions, can be reduced significantly. At the same time, thermally induced stress due to temperature differences within the CWHE can be minimized.

It is noted in this connection that by increasing the flow rate by means of making the outer region of the shell space available for flow, heat transfer to the shell wall is increased as especially two basic heat transfer parameters are positively influenced:

For example, the additional flow increases the driving temperature difference ΔΤ, for example between the relatively warm shell member and the cold gas environment, since new cold gas can continuously be transported through the outer region of the shell space. Furthermore, this flow increases the heat transfer coefficient, since it generates forced convection at the inner wall of the shell member.

Obviously, the same process of opening the vent members in order to provide flow of second heat transfer medium in the outer region of the shell space can be used during warmup modes of operation, for example in connection with a deriming process or before a shut-down. According to a further preferred embodiment, the coiled tubing is adapted to surround a central tube which, during the first mode of operation, is adapted to prevent a free flow of second heat transfer medium therethrough, and, during the second mode of operation, to provide free flow of second heat transfer medium therethrough. This central tube is also referred to as a "mandrel" in the art. Providing a central tube, around which the coiled tubing is wound, provides a heat exchanger with mechanical stability. During the normal mode of operation, there is no flow of either first or second heat transfer medium within this central tube. Providing such a free flow of second heat transfer medium through the central tube during a transitional mode also reduces transition times and material stress to the central tube as the temperature of the heat exchanger as a whole increases or decreases. The central tube or mandrel is preferably provided with inlet and/or outlet nozzles, which can be controlled to allow or prevent said flow of second heat transfer medium through the central tube.

In the molten salt process according to a preferred embodiment, the first heat transfer medium is water or water vapour or a thermal oil, and the second heat transfer medium is a molten salt or salt melt. These heat transfer media are especially suitable in the context of solar thermal power stations.

According to a further preferred embodiment, the first heat transfer medium and the second heat transfer medium are gaseous or liquefied gases or liquids. All heat transfer media can especially be the same gases provided at different temperatures. Providing gases, especially the same gas, as both first and second heat transfer medium is especially useful in connection with cryogenic applications such as liquefaction of natural gas. Be it noted that the terms "first heat transfer medium" and "second heat transfer medium" are meant to respectively comprise a plurality of heat transfer media, in addition alternatively to just one single first or second heat transfer medium.

According to a preferred embodiment, the heat exchanger is provided as a coil wound heat exchanger (CWHE).

Further advantages and embodiments of the invention will become apparent from the description and the appended figures. It should be noted that the previously mentioned features and the features to be further described in the following are usable not only in the respectively indicated combination, but also in further combinations or taken alone, without departing from the scope of the present invention.

In the drawings:

Figure 1 is a schematically simplified side view of a preferred embodiment of a heat exchanger according to the invention.

Figure 1 shows a schematically simplified side view of a coil wound heat exchanger (CWHE) generally designated 100 as a preferred embodiment of the heat exchanger according to the invention. As an example, be it assumed that CWHE 100 is adapted for use in a solar thermal power station.

CWHE 100 is provided with a central tube 102, around which is wound a coiled tubing 104. Coiled tubing 104 is provided with a first inlet port 132 and a first outlet port 134. Coiled tubing 104 is adapted to convey a first heat transfer medium such as water or water vapour from the first inlet port 132 to the first outlet port 134, and from thereon to further downstream applications (not shown).

Central tube 102 and coiled tubing 104 are surrounded by a shell member 1 10. Shell member 1 10 defines a shell space 1 12 and is provided with a second inlet port 1 14 and a second outlet port 1 16. The shell space 1 12 is adapted to convey a second heat transfer medium, such as salt melt, between the second input port 1 14 and the second output port 1 16.

The coiled tubing 104 is surrounded by a shroud member 108. The shroud member partitions the interior of shell 1 10, i.e. shell space 1 12, into an inner region 1 12a and an outer region 1 12b.

Furthermore, there is provided a blocking member 109 between shroud member 108 and shell member 1 10. As shell member 1 10 and shroud member 108 are usually arranged coaxially and are both of a cylindrical shape, blocking element has an annular shape extending between said two cylindrical shapes.

During a normal mode of operation, the CWHE is adapted to provide an indirect heat exchange between hot salt melt, which typically has temperatures around 300-500°C and water. Typically, the heat of the salt melt is used to vapourize the water or superheated steam, so that the medium exiting first outlet 134 is water vapour, which can, for example, drive a gas turbine (not shown). During this normal mode of operation, annular blocking member 109 blocks any flow of salt melt through the outer region 1 12b of the shell space 1 12. The flow of salt melt is effectively guided through the inner region 1 12a, thereby ensuring optimal thermal contact with coiled tubing 104.

If the temperature of the salt melt can not be maintained at a sufficiently high temperature, it is necessary to switch the CWHE into a standby mode of operation, the so-called freeze protection mode. Herein, cold salt melt, which has temperatures of around 300°C is conveyed through shell space 1 12. CWHE 100 is adapted to optimize the transitional mode, during which cold salt melt is added to the hot salt melt within the shell space, between the normal mode of operation and the freeze protection mode. As is shown in Figure 1 , CWHE is provided with a third inlet port 1 18, which can be controlled, i.e. opened or closed, by a valve 120. During the normal mode of operation, as described above, valve 120 is closed, in order to prevent any flow of salt melt through the outer region 1 12b of shell space 1 12. However, in order to effect a rapid and uniform cooling down/warming up of CWHE 100 as a whole during transition between said modes of operation, thereby also minimizing material stress within CWHE 100, valve 120 is advantageously opened in case cold salt melt shall be conveyed through the shell space 1 12 between the second inlet port and the second outlet port. Thus, cold salt melt is simultaneously conveyed through the inner and outer regions of shell space 1 12. Hereby, a rapid cooling down of CWHE 100 can be achieved. Also, thermal stress can be minimized, as the shell member 1 10 is effectively cooled down at the same rate as other components of CWHE.

When the second, lower operating temperature has been reached, and all components of CWHE essentially have the same (lower) temperature, valve 120 can be closed again. The process as described above can be reversed in case hot salt melt again becomes available. As soon as hot salt melt enters first inlet port 1 14, advantageously valve 120 is again opened so that, in an initial phase, salt melt can flow through the outer region 1 12b of shell space 1 12, until the operating temperature has again been reached.

According to a further advantageous embodiment, it is also possible, to provide a flow of salt melt through the inner tube 102 during said transition modes as described. In order to provide such a flow through inner tube 102, a further inlet port 122, which is also controlled by valve 120, can be provided. Obviously, it would also be possible to control inlet ports 1 18, 120 by means of different valves.

Be it noted that the invention as being described especially in connection with CWHE's adapted for use in solar thermal power stations, i.e. using a salt melt as one of the heat transfer mediums. The invention is also applicable to other kinds of CWHE

applications, which especially require a cyclical change of modes of operation characterized by different operating temperatures or which have to be cooled down or heated up fast. A similar CWHE 100 can be used for gas processing applications, such as a liquefaction of for example LNG. Herein, first inlet port 1 14 and first outlet port 1 16 serve to convey LNG, during a normal mode of operation, at a lower temperature through shell space 1 12. At the same time, second inlet port 132 and second outlet port 134 serve to convey NG and mixed refrigerants at a higher temperature through coiled tubing 104, in order to cool down NG and mixed refrigerants flowing through coiled tubing 104. The blocking member 109 prevents the medium flowing between shroud and shell (1 12b). In this application a third inlet port 1 18 allows LNG to bypass the blocking member 109. The valve member 120 can be kept closed during the normal mode of operation as described above, and opened during transitional modes of operation, in which the temperature within CWHE is changed, as described above. In such a transitional mode of operation, during which a temperature of CWHE 100 changes, opening of valve member 120 provides flow of heat transfer medium through the outer regions 1 12b of shell space 1 12. In this context, it is advantageous to also provide a shroud vent valve 127, which is normally closed, but can be opened to effect a venting of the outer region 1 12b, if necessary or expedient.