System and process for vaporizing a liquefied gas

Abstract

The present invention relates to a process for vaporizing a liquefied gas comprising the following successive steps: - Providing a liquefied gas into a vaporization tank (1), - Transforming within said tank (1) the liquefied gas into a saturated gas, - Exchanging the heat of said saturated gas with a heat transfer fluid and obtaining thereby a heated gas, wherein the freezing point of said heat transfer fluid is above the saturation temperature of said liquefied gas. The invention also concerns a System for vaporizing a liquefied gas comprising: - a vaporization tank (1) with a liquefied gas inlet (A), a heated gas inlet and a saturated gas outlet, - a heat exchanger (2) with an inlet and an outlet, - a first fluid line defined between said saturated gas outlet and the inlet of the heat exchanger (2), - a second fluid line (mainstream) defined between the outlet of the heat exchanger (2) and said heated gas inlet.

Description

[0001] P3846PC00 / 1132-005 dpt

[0002] System and process for vaporizing a liquefied gas

[0003] Field of invention

[0004] The present invention generally relates to system dedicated to the vaporization of a liquefied gas at low temperatures. Applications are typically found in the field of energy distribution, for instance with liquefied natural gas as a fluid being vaporized or for technical gas when stored in a saturated liquid state.

[0005] State of the art

[0006] Natural gas as well as a variety of technical gases are often stored in a saturated liquid state for the obvious reason of having a more compact and lightweight but also safer storage as compared to a storage in a gaseous state at high pressure. It is common for said liquefied gases to be stored at or close to the atmospheric pressure and by doing so, in many cases at a very low saturation temperature.

[0007] For the vaporisation process to occur a heat exchange with a suitable heat transfer fluid is required. Large scale liquefied natural gas (LNG) facilities commonly use an exchange with seawater as these facilities are generally needed at or near the harbours where LNG tankers arrive to be unloaded and the LNG needs to be vaporized prior to being injected into the high pressure natural gas transportation network. Such large-scale vaporization devices process a vast amount of fluid continuously throughout the year except during the necessary maintenance shutdowns. For the LNG vaporization using seawater as a heat transfer fluid, one finds either:

[0008] • Open rack vaporizers, where the LNG flows upwards inside a tube and a film of seawater flows downwards outside of tubes. Significant seawater ice build-up can form on the bottom part of the tubes causing a loss in heat transfer and as result additional surface is required.

[0009] • Intermediate fluid vaporizers, that are fitted with an intermediate fluid operating at a pressure where it will evaporate in a heat exchanger with seawater without causing said seawater to freeze and condense in a heat exchanger with the boiling LNG. A second exchanger with seawater on one side and natural gas vapour on the other side is then used in series to superheat said vapour from its saturation temperature up to the required temperature prior to injection in the treatment and injection facility. The fluid used as the intermediate heat transfer fluid needs to be in a pressure range where it is under the saturation curve while still being above its freezing point when in contact with the coldest surface in the heat exchanger P3846PC00 / 1132-005 dpt where the LNG is being vaporized. Note that sometimes intermediate fluid vaporizers are also made to use ambient air as a heat source instead of seawater in a topologically similar fashion.

[0010] Smaller vaporization facilities, for LNG but also other technical gases such as Nitrogen are using heat exchange in natural convection with ambient air, where the LNG flows in a tube with fins on the outside in contact with the ambient air. These systems are prone to ice build-ups and if operated more than a few hours a day need to have several of them installed in parallel such as to allow for some of them being operated while the others are stopped for de-icing.

[0011] To exchange heat with an evaporating fluid at very low temperature on one side and a fluid at a much higher temperature and that will freeze at a temperature above that of the evaporating fluid is a challenging task because of the risk posed by the temperature cycles that such unit can be submitted to and because of the potential plugging of the heat exchanger with solidified heat transfer fluid. Both issues can lead to a leak of vaporized fluid or even a catastrophic failure of the heat exchanger. The plugging of the heat exchanger also poses the problem of operational availability of the system that can be severely hindered even in the absence of leak.

[0012] Large scale vaporizers can be made to operate continuously, outside of the necessary maintenance shutdowns, such that the number of thermal cycles between the vaporization temperature and high temperatures (generally the ambient temperature) are minimized to a few tens of cycles over the lifetime of a unit. Moreover, large scale continuously operating units can afford to have long and careful preconditioning before being restarted.

[0013] If a larger number of cycles is expected, such as in small ambient air vaporizers or in open rack vaporizers, constructional measures can be adopted to avoid potential problems. Typically, in such units, freezing of the heat transfer fluid can happen without causing detrimental mechanical loading on the device by having enough space between the tubes. Such constructional measures (having more heat exchanger surface than required in a larger volume) and operational measures (having several units in parallel to compensate for the de-icing) are expensive in term of footprint of the unit.

[0014] None of the aforementioned systems has taken into account the fact that the low temperature evaporating fluid represents a potential source of cooling that can be valorised advantageously as a byproduct by providing cooling services to other processes, units or buildings. P3846PC00 / 1132-005 dpt

[0015] General description of the invention

[0016] The present invention addresses several challenges linked to the vaporization of a liquefied gas, when said liquefied gas is stored at a temperature below the freezing point of the heat transfer fluid used to provide the necessary heat for the vaporization.

[0017] The invention more precisely concerns a system and a process as defined in the claims.

[0018] Detailed description of the invention

[0019] The invention will be better understood in the present chapter with a non-exhaustive list of examples.

[0020] List and short description of the figures:

[0021] Figure 1- Example 1 of a system according to the invention.

[0022] Figure 2 - Example 2. The mixing valve for temperature control 4 installed on the main high temperature gas stream.

[0023] Figure 3 - Example 3. The mixing valve for temperature control 4 (Three-way valve) installed at the separation point of the main and secondary high temperature gas stream.

[0024] Figure 4 - Example 4. The gas blower 3 installed between the mixing point and the inlet of the heat exchanger 2 to eliminate the need of a flow mixer.

[0025] Figure 5 - Example 5. Control valve as liquefied gas inflow control device 6.

[0026] Figure 6 - Example 6. A control valve is used as the liquefied gas inflow control device 6 on a gravity liquid feed line from a lliquefied gas intermediate storage tank 9. A pressure equalisation line 10 allows for a smooth transfer of the liquid from said intermediate storage tank 9 to the vaporization tank 1.

[0027] Figure 7 - Example 7. Dual method for the detection of freezing conditions of the heat exchanger 2. Case where the heat transfer fluid is a liquid. Slow degradation of heat transfer indicating a potential partial freeze up of the heat transfer fluid is detected by monitoring the heat exchanger using the inlet and outlet temperatures and a proxy of the gas flowrate using the frequency of the blower 3. Fast occurring freezing condition is detected by a flow switch low (FSL) that shuts down the blower 3 in case the flowrate in the heat transfer fluid is insufficient to avoid it freezing up. P3846PC00 / 1132-005 dpt

[0028] Figure 8 - Example 8. Dual method for the detection of freezing conditions of the heat exchanger 2. Case where the heat transfer fluid is a gas at inlet and liquid at outlet (the heat exchanger 2 is a gas cooled condenser). Slow degradation of heat transfer indicating a potential partial freeze up of the heat transfer fluid is detected by monitoring the heat exchanger using the gas inlet and outlet temperatures, the heat transfer fluid gas inlet temperature and pressure as well as the liquid outlet temperature. A proxy of the gas flowrate using the frequency of the blower 3 is also used. Fast occurring freezing condition is detected by computing the subcooling of the heat transfer fluid liquid at outlet using the pressure (hence the saturation temperature) and the temperature of the liquid at outlet. The value of subcooling is compared to a maximum tolerable value in a Temperature Switch High (TSH) that shuts down the blower 3 in case the subcooling is too high. A high subcooling indicates an insufficient flowrate of the heat transfer fluid at the outlet of the heat exchanger 2.

[0029] Alphanumerical references shown in the figures:

[0030] Components

[0031] 1: Vaporization tank

[0032] 2: Heat exchanger

[0033] 3: Gas blower

[0034] 4: Mixing valve for temperature control

[0035] 5: Gas outflow control device

[0036] 6: Liquefied gas inflow control device

[0037] 7: Gas bubble diffuser

[0038] 8: Flow mixer

[0039] 9: Liquefied gas intermediate storage tank

[0040] 10: Pressure equalisation line P3846PC00 / 1132-005 dpt

[0041] Fluid interfaces with other systems

[0042] A: Liquefied gas inlet port

[0043] B: Gas outlet port

[0044] C: Heat transfer fluid inlet port

[0045] D: Heat transfer fluid outlet port

[0046] The system according to the invention comprises a vaporization tank 1 where the process fluid, a liquefied gas, is admitted through port A. The flow of the liquefied gas into said tank is controlled by a liquefied gas inflow control device 6. That can for instance be a pump (Fig 1-4) or a valve (Fig 5-6). The flow is controlled in such a way as to ensure an appropriate filling level in the vaporization tank 1, for instance using a level transmitter. A gas blower 3 is used to circulate the gas into a heat exchanger with a co-current flow arrangement 2. The "high temperature" gas flow after having been heated by the heat exchangers 2 and some of the inefficiencies of the blower 3 is separated in two streams.

[0047] • The mainstream goes to the gas bubble diffuser 7 at the bottom of the vaporization tank 1 where the bubbles of expelled superheated gas will cool down to the saturation temperature while transferring heat to the surrounding saturated liquid that consequently will evaporate. The flowrate of gas leaving the device via the gas outlet port B is controlled by the gas outflow control device 5 (for instance a valve or a compressor) using the signal from an outlet pressure transmitter or alternatively an outlet flowmeter. To control the amount of liquefied gas being evaporated the rotational frequency of the blower 3 is varied using the signal from a pressure transmitter. In the case of an insufficient amount of liquefied gas being evaporated the pressure in the tank will decrease. To compensate the amount of injected gas via the gas bubble diffuser 7 need to be increased and so does the blower 3 rotational frequency. In the case of an excessive amount of liquefied gas being evaporated the principle is the opposite and said rotational frequency will be decreased.

[0048] • The secondary stream is recirculated from the separation point to a mixing point with the gas at saturation temperature coming from the vaporization tank 1. The amount of recirculated high temperature gas being mixed with the gas at saturation temperature is controlled via the degree of opening of the mixing valve for temperature control 4 in a way as to maintain a suitable intermediate temperature level at the gas inlet port of the heat exchanger 2. For promoting an homogeneous temperature of the flow entering said heat exchanger a flow P3846PC00 / 1132-005 dpt mixer 8 can be fitted additionally. A suitable intermediate temperature level is a temperature that guarantees that the wall temperature in the heat exchanger 2, on the side of the heat transfer fluid, is above that of the freezing point of said heat transfer fluid at its current inlet pressure (at port A).

[0049] The gas bubble diffuser 7 could comprise one or several perforated plates, tubes or profiles of suitable shape. The cross-section and aspect ratio of the perforation should be such that the bubbles generated have a surface to volume ratio high enough in order for the superheated gas within the bubble to cool down (almost) to the saturated state before reaching the free-surface of the liquefied gas within the tank. The number and spatial distribution of the perforations should be such that the pressure drop across the diffuser is relatively small but high enough to reach a relatively homogeneous gas injection over the entire surface of said gas bubble diffuser. Note that it is desirable to have as small a pressure drop as possible to reduce power consumption of the blower 3 but if said low pressure drop comes at the expense of a maldistribution of gas within the gas bubbles diffuser, causing in turn the appearance of a preferential path of large coalesced bubbles rising without exchanging enough heat with the surrounding liquefied gas, then the flowrate of gas would have to be increased to reach the desired evaporation rate which in the end will cause an increased power consumption of the blower 3. Instead of perforated plates, tubes or profiles the diffuser could also be made out of a foam, preferably out of a suitable metal to resist the thermal shock, the gas being injected in the middle of it would flow through said foam towards the edge that are in contact with the liquefied gas were the bubbles would be generated.

[0050] The heat exchanger 2 is arranged in a co-current flow such that gas at the lowest temperature exchanges heat with the heat transfer fluid with the highest enthalpy. This arrangement participates in ensuring that the wall temperature on the side of the heat transfer fluid is always and everywhere in said heat exchanger at a temperature above the freezing point of the heat transfer fluid. Another element in preventing the wall temperature of being too low is that the heat is being transferred to a fully gas stream. In general, the heat transfer fluid could be a liquid, for instance water or a brine. In those cases, the heat exchanger 2 would be a single phase liquid / gas heat exchange. It could also be a refrigerant that will be condensed by said heat exchanger, in that case it would be a gas cooled condenser. The forced convection heat transfer coefficient in a liquid or the heat transfer coefficient in condensation on the refrigerant side will be in general several times higher than the forced convection heat transfer coefficient on the side of the gas. As a result of this imbalance, the wall temperature at a point in the heat exchanger 2 will be closer to that of the heat transfer fluid than that of the gas at said point. This feature further helps protect the heat exchanger against the freezing of P3846PC00 / 1132-005 dpt the heat transferfluid and allowsforthe temperature of the intermediate temperature gas to be lower that it would for instance in gas-gas heat exchanger.

[0051] The combination of a heat exchange with only the gas phase of the process fluid, the co-current arrangement and the active control of the inlet temperature of the fluid provides an effective protection of the heat exchanger 2 against failure caused by excessive thermal cycling and plugging on the heat transfer fluid side caused by said fluid freezing up.

[0052] Furthermore, at the startup of the unit, it would be wise to have 100% recirculation (No flowrate in the main stream) when the blower starts up at minimum rotation frequency, then let the control system modify the opening of the mixing valve 4 gradually enough to have a cooling of the heat exchanger 2 at slow enough rate to avoid any detrimental thermal stress building up. Only when the desired temperature regime is reached will the blower rotation frequency accelerate to provide the desired flowrate of vaporized fluid. Similarly, it would also be wise prior to a shutdown to reduce gradually the rotational frequency to its minimum while letting the control system modify the opening of the mixing valve for temperature control 4 and then change said opening to reach 100% recirculation, all this also gradually enough to avoid any detrimental thermal stress on the heat exchanger 2.

[0053] In the proposed invention, all moving parts except the liquefied gas inflow control device 6 are located where the gas is either at high temperature or at the intermediate temperature level.

[0054] • The blower 3 is preferably installed after the heat exchanger 2 thus it is always submitted to a temperature close to that of the heat transfer fluid leaving said heat exchanger (port D). It means that it is not submitted to significant temperature cycles. In Figure 4, however, the blower 3 is installed before the gas inlet port of the heat exchanger 2. This arrangement submits the blower to potentially large temperature cycles but allows for using the blower simultaneously as a flowrate generator and a flow mixer, therefore removing the need for a dedicated flow mixer 8. However, the potentially incompletely mixed flow arriving at the inlet port of the blower could pose a problem of inhomogeneity in the temperature at the blower inlet, which can be a source of problem (thermal stress).

[0055] • The mixing valve for temperature control 4 is either a two-way valve (Figure 1,2, 4, 5 and 6) or a three-way valve (Figure 3) always located on a stream at high temperature, either the mainstream (Figure 2), or the secondary stream (1, 4, 5 and 6) or at the separation point (Figure 3). These locations all allow for said mixing valve to be protected against any significant temperature cycle. P3846PC00 / 1132-005 dpt

[0056] • The gas outflow control device 5 can be a valve (as depicted in all Figures) but could also be a compressor. It is located on the mainstream at high temperature therefore it is not submitted to any significant thermal cycle.

[0057] All the temperature sensitive components, namely the heat exchanger 2, the blower 3 the valves 4 and 5 and to some extend the flow mixer 8 are installed high enough above the maximum liquefied gas level in the vaporization tank 1, thus creating a system with two zones separated in height, one at the top containing the intermediate and high temperature gas and one at the bottom containing the liquefied gas and the gas both at saturation temperature. The top and bottom zones and their separation level are shown in each figure. When the system is stopped (no forced circulation generated by the blower 3) and since both phases contained in the zone at the bottom have a higher density than the intermediate and high temperature gas, gravity driven stratification will cause said phases to remain within the zone at the bottom, thus preventing low temperature fluid entering in contact with the temperature sensitive components. Moreover, the proposed vertical arrangement in two zones allows for different thicknesses of thermal insulation to be used, namely a thicker insulation in the bottom zone than in the top zone, which further promotes a proper stratification when the system is stopped (For the sake of readability, the thermal insulation is not shown on the Figures).

[0058] In addition to the aforementioned features, to further protect the heat exchanger 2 it is desirable to detect early enough whether said heat exchanger is operating in freezing conditions. A Dual method for the detection of freezing conditions of the heat exchanger 2 can be implemented. In the case were the heat transfer fluid is a liquid the slow degradation of heat transfer can be an indication of partial freeze up of the heat transfer fluid. It can be detected by monitoring the heat exchanger using the inlet and outlet temperatures and a proxy of the gas flowrate using the frequency of the blower 3. Fast occurring freezing conditions are primarily caused by an insufficient flowrate of the liquid heat transfer fluid within the heat exchanger 2 and can be overcome using a Flow Switch Low (FSL) that shuts down directly the blower 3. This dual method can reliably detect slow and rapid excursion of the heat exchanger 2 in condition where the heat transfer fluid can freeze up. An embodiment of the proposed method for the case of the heat transfer fluid being a liquid is provided at Figure 7.

[0059] The heat exchanger monitoring block comprises a computer or a programmable logic control with its accessories (such as data acquisition cards) that runs a computer program based on at least the following principles:

[0060] 1. By knowing the heat transfer area of the heat exchanger (a constant value) and considering the fact that the gas side heat transfer coefficient is about one order of magnitude lower than P3846PC00 / 1132-005 dpt that on the heat transfer fluid side (which is a liquid) one can consider that the overall heat transfer coefficient of the heat exchanger is mostly influenced by the gas side.

[0061] 2. The convection heat transfer coefficient on the gas side varies in first approximation with the 0.8thpower of the flowrate.

[0062] 3. Recalling the fans affinity law that states that flowrate is proportional to the rotational frequency one can build the relationship: Overall heat transfer coefficient of the heat exchanger 2 equals Overall Heat Transfer Coefficient of the heat exchanger 2 at sizing flowrate multiplied by the ratio of actual rotational frequency of the blower 3 to the frequency necessary to reach the sizing gas flowrate of said heat exchanger 2.

[0063] By using those principles it is thus possible to avoid the use of flowmeters to monitor the operation of the heat exchanger. In the case a more accurate characterization of the heat exchanger is required additional measurements can be used to better predict the convective heat transfer coefficient. For instance, in addition to the aforementioned ratio of rotational frequency, the degree of opening of the valves 4 and 5 combined with the liquid level and pressure in tank 1 can be used to obtain a better estimate of the gas flowrate within the heat exchanger 2. For such an improved evaluation to be as precise as possible it might be necessary to provide a model of the pressure drops for the whole gas path, alternatively by defining a multivariate operating map using data from experimental tests or even potentially based on online identification methods. Moreover, using the pressure from the tank 1 and the various temperatures available, an even more accurate evaluation of the heat transfer coefficient could be obtained by integrating the computation of appropriate thermodynamic and transports properties to be used in a suitable heat transfer and pressure drop correlation. In the case of the heat transfer fluid being a gas at the inlet of the heat exchanger 2 and a liquid at its outlet (gas cooled condenser), the use of a flow switch low for detecting fast occurring freezing conditions will likely not yield good results. For such cases the use of a pressure and temperature measurement on the gas inlet and a temperature measurement on the liquid outlet can be used to compute a value of the subcooling of the fluid. That value can then be used in a Temperature Switch High (TSH) and if the value of said subcooling exceeds a preset value the temperature Switch High (TSH) will shut down the blower 3. A high subcooling of the liquid outlet of the heat exchanger 2 on the heat transfer fluid side is symptomatic of an insufficient drainage of said liquid out of said heat exchanger that will within a potentially relatively short amount of time lead to the heat transfer fluid freezing up. An embodiment of the proposed dual method for the case of the heat transfer fluid being a gas at inlet that condenses into a liquid within the heat exchanger 2 is provided at Figure 8. Note that the pressure transmitter could be installed alternatively at the liquid outlet of the heat exchanger 2 at P3846PC00 / 1132-005 dpt the expense of a less accurate evaluation of the saturation temperature caused by the fluctuation in liquid level in said heat exchanger influencing the pressure reading.

[0064] By continuously monitoring the heat exchanger inlet and outlet temperatures and taking care of correcting the overall heat transfer coefficient as described previously one can compare the actual measured log mean temperature difference to the one predicted for a clean and unfrozen heat exchanger. If the actual value is significantly higher than the predicted value and if that trend is increasing it means that heat exchanger 2 is gradually freezing up on the heat transfer fluid side. In that case, a countermeasure could be to throttle up the blower to maximum frequency while simultaneously changing the opening of valve 4 to have 100% recirculation until the log mean temperature reaches back or at least becomes close enough to the predicted log mean temperature difference. Note that other types of fouling of the heat exchanger 2 on the heat transfer side could be detected using the same method. In the case of a known risk of fouling on the heat transfer fluid side additional care should be taken to distinguish between fouling and freezing. The former generally occurring order of magnitudes slower than the latter, the use of appropriates time filtering techniques such as Kalmann filters could be implemented to be able to make this distinction.

[0065] The proposed invention provides a mean to vaporize a liquefied gas (the process fluid) at very low temperature by exchanging heat with a heat transfer fluid the freezing point of which is above the saturation temperature of said liquefied gas. The proposed invention has several key advantages:

[0066] • It allows for realizing relatively compact units over a large range of capacity (from a few kilowatts to several megawatts of vaporization capacity).

[0067] • It allows for changing the vaporization flowrate of the process fluid very dynamically without detrimental effect on the reliability of the system by having a good temperature management of the components sensitive to thermal stress. Said temperature management is ensured by a combination of constructional measures such as a heat exchange with the gas phase of the process fluid, the use of a co-current heat exchanger flow arrangement, the fitting of a flow mixer to homogenize the temperature of the process fluid entering said heat exchanger and finally the separation of the system in two vertically separated zones that promote gravity driven stratification when the system is stopped. The temperature management is further enhanced by the use of an appropriate active control of the frequency of the blower and the mixing valve for temperature control allowing for gradual start-ups and shutdowns as well as effective load-following capabilities.

[0068] • It allows for exchanging heat with a large number of heat transfer fluids. Said heat transfer fluids can be used in such a way as to profit from the cooling effect of the vaporization process P3846PC00 / 1132-005 dpt to transfer and provide cooling services to users in the vicinity that require it (buildings, industrial processes, food storage facilities...)

[0069] • A Dual method for detecting freezing conditions within the heat exchanger can be used to further protect said heat exchanger by providing a reliable and early enough detection of freezing conditions in order to have time to take appropriate countermeasures before any damage consecutive to the heat transfer fluid freezing-up can occur.

Claims

P3846PC00 / 1132-005 dptClaims1. Process for vaporizing a liquefied gas comprising the following successive steps:- Providing a liquefied gas into a vaporization tank (1),- Transforming within said tank (1) the liquefied gas into a saturated gas,- Exchanging the heat of said saturated gas with a heat transfer fluid and obtaining thereby a heated gas, wherein the freezing point of said heat transfer fluid is above the saturation temperature of said liquefied gas.

2. Process according to claim 1 comprising the preheating of said saturated gas before its heat exchange with the heat transfer fluid.

3. Process according to claim 2 wherein the said heated gas is used for the said preheating.

4. Process according to one of the previous claims wherein at least one part of said heated gas is injected into said saturated gas.

5. Process according to claim 4 wherein the injection of the at least one part of said heated gas into the saturated gas is regulated through a mixing valve (4) for temperature control of the resulting mix of said saturated gas and said part of heated gas before the heat exchange between the said resulting mix and the heat transfer fluid.

6. System for vaporizing a liquefied gas comprising:- a vaporization tank (1) with a liquefied gas inlet (A), a heated gas inlet and a saturated gas outlet,- a heat exchanger (2) with an inlet and an outlet,- a first fluid line defined between said saturated gas outlet and the inlet of the heat exchanger (2),- a second fluid line (mainstream) defined between the outlet of the heat exchanger (2) and said heated gas inlet.

7. System according to claim 6 furthermore comprising a third fluid line (secondary stream) defined between the first fluid line and the second fluid line.P3846PC00 / 1132-005 dpt8. System according to claim 7 furthermore comprising a mixing valve (4) located along the third fluid line and a temperature sensor (T) located along the first fluid line, close to the inlet of the heat exchanger (2), said mixing valve (4) being regulated according to the temperature monitored by said temperature sensor (T).

9. System according to one of claims 6 to 8 comprising a top zone and a bottom zone, with a separation level located above the maximal level of liquefied gas within the tank (1), wherein the bottom zone comprises the tank (1) and wherein the top zone comprises the heat exchanger and any optional items such as a blower (3), a flow mixer (8), a mixing valve for temperature control (4) or a gas outflow control device (5).

10. System according to one of claims 6 to 9 comprising one or a plurality of sensors configured to detect freezing conditions within the heat exchanger (2).

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