DE102018005503A1 - Device for supercooling liquefied gases - Google Patents

Abstract

State-of-the-art subcoolers comprise a container in which a liquefied gas is stored at a lower pressure and therefore at a lower temperature than the liquefied gas to be supercooled, which is thermally connected to the interior of the container on a heat exchanger surface. In the case of greatly fluctuating cooling requirements, if the heat exchanger surface is dimensioned too small, film boiling can occur on the surface of the heat exchanger surface, which drastically reduces the cooling capacity of the subcooler. In addition, with a high coolant requirement, there is an increased pressure loss and the associated occurrence of a new gas phase. In order to overcome this disadvantage, the invention proposes to provide a plurality of heat exchanger surfaces which are arranged parallel to one another and are switched on or off in accordance with the requested cooling capacity. As a result, the size of the heat exchanger surface in the tank is adapted to the respective cooling requirement.

Description

The invention relates to a device for supercooling liquefied gases, with an insulated container for holding a cooling bath, which is fed from a partial flow of a liquefied gas removed from a storage tank and expanded to a low pressure, with a device for withdrawing a gas phase from the insulated container and with a device arranged within the insulated container and fluidly connected to the storage tank via a feed line for transferring heat from the liquefied gas to be supercooled through the feed line to the cooling bath.

Such devices, commonly referred to as “subcoolers”, are used, for example, to cool surfaces for deburring molded parts made of plastic, rubber, elastomers, etc., such as in US Pat EP 2 143 528 A1 described. In order to deburr, in particular, softer molded parts, the knobs and burrs are embrittled at least superficially by exposure to a refrigerant consisting of a low-boiling liquid gas, such as liquid nitrogen, and then mechanically removed in a treatment chamber. To prevent the inlet temperature of the refrigerant from entering the treatment chamber of the deburring system from being dependent on the pressure and the temperature of the refrigerant tank connected to the deburring system, the subject of EP 2 143 528 A1 A device for subcooling the refrigerant is provided in the refrigerant supply line, by means of which the temperature of the refrigerant is brought to a defined temperature below its boiling temperature before the molded parts are treated. This ensures in particular that the refrigerant still reaches the molded parts to be treated in the liquid state, thereby minimizing the consumption of refrigerant and making the heat transfer from the refrigerant to the molded parts more efficient. Furthermore, the subcooling enables the refrigerant to be introduced into the treatment chamber at a precisely defined temperature; the temperature of the refrigerant entered therefore no longer depends on the pressure in the refrigerant storage tank.

Subcooling of a liquefied gas used as a refrigerant is also used in the technology of light metal extrusion, such as in DE 198 57 790 A1 described. The subject of this document is an extrusion process in which liquid nitrogen is used as a refrigerant to remove heat from the die used for forming the profile strand during the pressing process and thus to remove all or part of the forming heat. This method requires a very precise metering of the refrigerant, but this is problematic in the case of a low-boiling liquid gas such as nitrogen or argon insofar as it evaporates to a part that is difficult to calculate when it is fed to the pressing tool. The liquid gas is therefore subcooled with a subcooler to a temperature of, for example, 10K to 15K below its boiling temperature, so that it is ensured that the nitrogen hits the die completely in the liquid state.

Another application of a supercooled is the technique of cold grinding. So in the EP 2 368 638 A1 describes a method and a device for cold grinding of products, in which a cryogenic refrigerant is brought to a temperature below its boiling temperature, that is to say subcooled, before it is fed to a mill grinding the products. As a result, the refrigerant can be metered much more precisely, which increases the efficiency of the flour process.

A device for subcooling which can be used in the aforementioned applications comprises a container with thermally insulated walls, in the interior of which a cooling coil made of stainless steel or copper is arranged as a heat exchanger. A low-boiling liquid gas, for example liquid nitrogen, is fed through the cooling coil from a storage tank and, after passing through the cooling coil, is fed to the treatment device. A branch line also connected to the storage tank opens into the insulated container at a relief valve. The expansion valve is, for example, a float valve in which a mechanism equipped with a floating body releases an inflow opening for supplying liquid nitrogen from the storage tank into the insulated container when the liquid gas level in the container falls below a predetermined level or closes it when the filling level is exceeded , The gas phase in the container is fluidly connected to an exhaust gas line for discharging vaporized liquefied gas, in which a pressure-maintaining valve is mounted, which keeps the pressure in the exhaust gas line upstream of the pressure-maintaining valve and thus at the same time essentially constant in the container. The pressure in the container is always lower than the pressure in the storage tank or in the cooling coil passed through the container. Accordingly, the temperature of the liquefied gas in the container is lower than the temperature of the liquefied gas passed through the cooling coil. Therefore, the liquefied gas passed through the cooling coil gives off heat to the surrounding liquefied gas in the container and subsequently has a temperature that is significantly lower (usually between 1 K and 20 K depending on the pressure difference) than the boiling point of the liquefied gas at there is pressure in the cooling coil. In addition, gas bubbles present in the liquefied gas are recondensed. Instead of a mechanical float valve, a solenoid valve can also be used, by means of which the replenishment of liquid nitrogen in the container is regulated as a function of the fill level of the refrigerant bath in the container measured by suitable sensors. The subcooling should reduce the temperature of the supercooled liquefied gas to such an extent that even after passing through fittings arranged downstream of the subcooler, which lead to a pressure loss in the liquefied gas and thus to a lowering of the boiling point, the temperature is below the respective boiling point remains.

The further development in the EP 0 524 432 A1 described integration of a gas phase separator into the subcooler, which separates nitrogen, which has passed into its gas phase before the supercooling process, from the liquid phase.

With large throughputs of liquefied gas, subcoolers easily experience pressure fluctuations and thus temperature fluctuations within the container. In order to overcome this disadvantage, the EP 2 679 879 A2 It is proposed that the container be equipped with a plurality of feed lines which are immersed at different depths, each having a valve which opens or closes the respective feed line depending on the level of the liquid in the container. In this way, the volume flow of the liquefied gas fed into the container is regulated as a function of the cooling requirement and the occurrence of pressure fluctuations is reduced.

The dimensioning of the heat exchanger, such as the length, diameter and wall thickness of the cooling coil, is based on the required heat transfer performance. The necessary refrigerant throughput, the proportion of gas phase in the liquefied gas used as the refrigerant, the pressure of the refrigerant and the desired cooling flow temperature of the refrigeration application contribute to this. If refrigeration applications are switched on or off, or if gas phase content or gas pressure in the refrigerant to be supercooled varies due to storage conditions that change during the course of the use of the subcooler, for example as a result of an increase in the temperature and / or a decrease in the fill level of the refrigerant in the tank, the heat transfer capacity also varies.

As a result, the cooling coils are generally designed such that, in particular, operating conditions with high refrigerant throughputs and unfavorable storage conditions can also be covered. They therefore have a - compared to the requirements of an average heat transfer performance - a comparatively large pipe length and / or a large pipe diameter. A very long pipe coil, however, leads to a loss of pressure at high refrigerant throughputs, which in turn leads to an increased proportion of gas phase; film boiling can also occur in the cooling bath on the surface of the heat exchanger, which deteriorates the heat transfer. If, on the other hand, the diameter of the pipeline is very large and the flow rate decreases due to a lower cooling requirement, the heat transfer to the pipe coil is reduced due to an established laminar flow.

The object of the present invention is therefore to provide a device for supercooling cryogenic liquids which overcomes the aforementioned disadvantages.

This object is achieved in a device of the type and purpose mentioned at the outset in that the device for heat transfer is equipped with a plurality of heat exchangers connected in parallel, which are at least partially each equipped with a shut-off fitting for switching the flow through the heat exchanger on and off ,

A “plurality of heat exchangers” is to be understood here to mean a number of at least two, preferably three to ten, heat exchangers, each of which can have the same or different heat transfer capacities. At least some of the heat exchangers are equipped with a shut-off valve, by means of which the flow of refrigerant through this heat exchanger can be enabled or blocked. The shut-off valves can be, for example, manually operated valves, but are preferably controllable valves, for example solenoid valves, which can be operated by means of a control unit depending on the respective requirements, for example depending on a measured controlled variable. The device for heat transfer is preferably designed such that the liquefied gas to be supercooled can be subcooled to a temperature of 1K to 25K, preferably 10K to 20K below its boiling point at the pressure prevailing in the device.

The invention thus allows the number of heat exchangers used for heat transfer in the container to be varied by switching individual heat exchangers on or off, and thus the heat transfer capacity being adapted to the respective requirements. For example, in the case of similar heat exchangers Heat transfer capacity by switching on another heat exchanger is doubled compared to that of a single heat exchanger, tripled when two heat exchangers are switched on, etc. Since the refrigerant to be supercooled flows through all connected heat exchangers at the same time due to the parallel arrangement of the heat exchangers, efficient cooling can also be achieved with a large increase in the Refrigerant flow can be achieved with low pressure loss. A new gas phase formation caused by pressure loss within the heat transfer device can thereby be reliably avoided. Conversely, with a reduced required heat transfer capacity, the number of connected heat exchangers can be reduced and the risk of an insufficient flow rate being countered.

A preferred embodiment of the invention provides that at least some of the heat exchangers are connected to one another in such a way that they can be switched on and off independently of one another. In this way, heat exchangers can be individually combined and switched on. In this case, the shut-off valves are at least partially arranged in the region of branch lines which lead away from a common feed line and each establish the flow connection to a heat exchanger. This configuration is particularly advantageous in the case of heat exchangers of different heat transfer capacities, since in this way a total heat transfer capacity which is adapted to the heat transfer capacity required in each case can be set.

In another advantageous embodiment of the invention, the heat exchangers are connected to one another in such a way that some of the heat exchangers can only be switched on when at least one further heat exchanger has already been switched on. In this way, a group of several heat exchanges can be switched on and off in blocks. For this purpose, at least one shut-off valve is arranged upstream of a plurality of branch lines, which each establish the flow connection to a heat exchanger and can in turn be equipped with a shut-off valve. This configuration is an easy-to-implement solution, which is particularly suitable if all heat exchangers have essentially the same heat transfer capacity.

The shut-off valves are expediently operatively connected to a control, in particular computer-aided control, by means of which the heat exchangers can be switched on and off according to a predetermined program or depending on measured parameters. The control is thus designed such that the number and / or the capacity of the heat exchangers to be connected are determined and the shut-off valves are activated accordingly, depending on the respective requirements, for example in the event of a change in the refrigerant flow detected by means of suitable means.

In a preferred embodiment of the invention, an apparatus for continuously detecting the flow rate of the liquefied gas is provided, for example a Coriolis flow meter. This apparatus is arranged, for example, in a feed line, that is, upstream of the heat exchangers. In order to ensure, in particular, reliable flow measurement for liquefied gases with a high proportion of gas phases, it is advisable to install such an apparatus for recording the flow rate downstream of the heat exchangers, for example in a common discharge line into which the heat exchangers flow. The data of the apparatus are preferably transmitted to the control mentioned before and used by the latter according to a predetermined program to control the locking fittings. A likewise advantageous embodiment provides a device arranged downstream of the heat exchangers for detecting the temperature of the supercooled refrigerant, the measured values of which can be used in the control device to determine the number and / or capacity of the heat exchangers to be connected.

Another advantageous development of the invention provides that a gas phase separator is assigned to the feed line for the liquefied gas to be cooled. The gas phase separator is, for example, an object as it is in the EP 0 524 432 A1 is described. A further container (hereinafter referred to as “separator container”) with thermally highly conductive walls is accommodated in the insulated container in the cooling bath, into which the outlet pipe which is connected to the flow with the storage tank opens. In the upper part of the separator container there is a gas outlet which is connected to the gas phase in the storage tank and through which liquid gas which has already evaporated on the way from the storage tank is drawn off. In order to draw off the liquid phase, the separator container has a connection in its lower part from which the supercooled liquid gas is discharged for further use. In terms of flow, the connection for withdrawing the liquid phase is followed by the device for heat transfer arranged in the cooling bath, in which the liquid phase removed from the gas phase separator is further subcooled by thermal contact with the cooling bath in the insulated container. The gas phase separator ensures that the liquid gas passed through the outlet pipe is at least largely in the liquid state and contains no or only a few gaseous inclusions.

Another advantageous development of the invention provides that means are provided to regulate the inflow of liquefied gas to the cooling bath. This also allows the amount of refrigerant in the cooling bath to be adapted to the respective heat transfer capacity. For example, this can be achieved with a plurality of feeds for the refrigerant which can be connected in parallel and are each opened and closed depending on a fill level in the cooling bath. Such an arrangement is in the EP 2 679 879 A2 , to which express reference is made here.

A preferred use of the device is the provision of a supercooled liquefied gas, in particular liquid nitrogen, as refrigerant for cooling a device for extruding light metals, in particular an aluminum extrusion device. Another preferred use of the device is in a device for cold grinding or in a Arrangement of several devices for cold grinding connected in parallel.

A low-boiling liquefied gas (cryogen), such as, for example, nitrogen, oxygen, LNG or a noble gas such as argon or helium, is preferably used as the liquefied gas to be cooled.

An embodiment of the invention will be explained in more detail with reference to the drawing.

The only figure ( 1 ) shows schematically a device according to the invention.

The device 1 for supercooling includes a device 2 for heat transfer in a container 3 is arranged with thermally insulated walls. The facility 2 for heat transfer comprises a plurality of heat exchangers, in the exemplary embodiment three cooling coils 4 . 5 . 6 , which have a pressure-resistant and heat-insulated supply line 7 with a heat insulated storage tank 8th are connected to the flow. In the storage tank 8th becomes a cryogenic medium, for example nitrogen in the cryogenic liquefied state, up to the level 9 stored. The liquid nitrogen is inside the storage tank 8th at its boiling temperature; in the lower area of the storage tank 8th , in the area of a connecting piece 10 for the nitrogen supply 7 , the boiling temperature is in turn determined by the hydrostatic pressure inside the storage tank 8th up to the level 9 co-determining the standing liquid column. At a pressure of 5 bar, the temperature of the liquid nitrogen at the connection piece is 10 for example about minus 180 ° C, at 6 bar even minus -177 ° C. The cooling coils are on the output side 4 . 5 . 6 with a common, thermally insulated rejection 11 flow-connected, via which the liquid nitrogen is subsequently fed to a further use, for example as a refrigerant to a device not shown here.

Inside the container 3 there is a cooling bath 13 into which the cooling coils 4 . 5 . 6 plunge. The cooling bath 13 consists of the same cryogenic medium as that in the storage tank 8th stocked, in the exemplary embodiment thus from liquid nitrogen. For supplying refrigerant to the cooling bath 13 serves a refrigerant supply 14 by the supply line 7 still outside the container 3 branches. The refrigerant supply 14 is on the output side with a float valve 15 equipped. The float valve 15 works in such a way that when falling below a predetermined fill level 16 of the cooling bath 13 in the container 3 liquid nitrogen in the container 3 flows, whereby this depends on the pressure in the container 3 relaxed. In the container 3 exists between the cooling coils 4 . 5 . 6 on the one hand and the cooling bath 13 on the other hand, only a thermal connection, but no flow connection.

Instead of a float valve 15 can also be provided other devices depending on the level 16 of the cooling bath 13 the supply of liquid nitrogen through the refrigerant supply 14 control, for example solenoid valves, which are operatively connected to suitable sensors for level detection, for example superconducting sensors.

In the roof space of the container 3 is an exhaust pipe 18 provided for the discharge of gaseous nitrogen. In the exhaust pipe 18 is a pressure control valve 19 mounted that the pressure in the exhaust pipe 18 upstream to the pressure control valve 19 and at the same time in the container 3 to a predetermined value of, for example, 1 bar. The pressure in the tank 3 can be chosen freely, but must be lower than the pressure in the storage tank 8th in the area of the connecting piece 10 to ensure that the temperature of the cooling bath 13 is lower than the temperature of the liquid nitrogen in the feed line 7 , For example, given an assumed pressure in the container 3 1 bar (abs.) a temperature of the liquid nitrogen in the container 3 of approx. minus 196 ° C, at a pressure of 0.3 bar even approx. minus 204 ° C, and thus a lower temperature than that of the liquid nitrogen in the storage tank 8th at an assumed pressure of, for example, 5-6 bar. Otherwise, a simplified version can also be used on the pressure control valve 19 be dispensed with, with the result that the refrigerant in the container 3 is always at atmospheric pressure. in this case the temperature of the evaporating refrigerant is inside the container 3 however by that the changes in atmospheric pressure are subject to fluctuations.

In operation of the device 1 is inside the container 3 the refrigerant bath 13 up to a filling level 16 above the float valve 15 in front. The pressure in the tank 3 corresponds to that on the pressure control valve 19 set value and is, for example, 1 bar (abs.). The pressure of the liquid nitrogen in the refrigerant supply line 14 and in the supply line 7 corresponds approximately to that in the storage tank 8th in the area of the connecting piece 10 , By opening the valve 21 becomes supercooled nitrogen through the supply line 7 , the establishment 2 and the diversion 11 a consumer, not shown here, for example an extrusion press or a cold grinding device. Due to the higher pressure in the supply line 7 compared to the pressure in the container 3 indicates this through the supply line 7 led refrigerant to a higher temperature than that of the refrigerant in the cooling bath 13 , At the facility 2 is thus heat from the through the supply line 7 led refrigerant to the cooling bath 13 released and the refrigerant in the supply line 7 is hypothermic.

In order to be able to react quickly to fluctuating cooling requirements, the facility can 2 the number of active cooling coils used for heat transfer 4 . 5 . 6 can be varied. For this are the cooling coils 4 . 5 . 6 connected in parallel with each other, the cooling coil 4 in the exemplary embodiment is always available as a heat exchanger surface while the cooling coils 5 . 6 by means of valves 22 . 23 can be switched on or off. By switching on the cooling coil 5 or 6 the refrigerant flows through the cooling coils in equal parts 4 and 5 (respectively. 4 and 6 ) led, which means a compared to the use of only the heat exchanger 4 double heat exchanger surface is available. When both cooling coils are switched on 5 and 6 accordingly, a triple heat exchanger surface is available. The valves 22 . 23 can by means of an electronic control 24 can be controlled, which makes it possible to switch the cooling coils on and off 5 . 6 according to a predetermined program and / or as a function of measured parameters, for example the temperature or the volume flow of the refrigerant supplied to the consumer.

In this way, the available heat exchanger surface can be quickly adapted to fluctuating cooling requirements. The risk of film boiling of refrigerant in the cooling bath 13 on the outer surface of the cooling coil 4 , which would limit the heat transfer capacity, is significantly reduced. Also by switching on the cooling coils 5 and or 6 efficient subcooling even with high required heat transfer capacities without the need to install a long cooling coil that works with a correspondingly high pressure drop.

For the rest, the invention is not based on the provision of three identical cooling coils 4 . 5 . 6 , as shown in the exemplary embodiment, only two or more than three heat exchanger surfaces can be provided within the scope of the invention, each of which has the same or different heat transfer capacities and can be connected in blocks or independently of one another. In the case of different heat transfer capacities of the cooling coils 4 . 5 . 6 it is also beneficial to use the cooling coil 4 Equip with a shut-off valve to prevent the refrigerant from flowing through the cooling coils 5 and or 6 to enable. In this way, the heat transfer capacity can be adapted even better to the heat transfer capacity required in each case.

Through the entry of heat from the through the supply line 7 led refrigerant evaporates refrigerant from the cooling bath 13 , The inflow of refrigerant into the cooling bath 13 can be regulated (not shown here) via a plurality of supply lines and adapted to the requirements, such as in the EP 2 679 879 A2 described.

That when exchanging heat in the tank 3 emerging gaseous refrigerant is via the exhaust pipe 18 dissipated and possibly used for further use. After passing through the cooling coil / s 4 . 5 . 6 owns that through the diversion 11 transported refrigerant at least approximately the temperature of the cooling bath 13 (for example minus 196 ° C), and thus a temperature that is significantly below the boiling point of nitrogen at that in the feed line 7 prevailing pressure.

Liquid nitrogen is used as the refrigerant in the exemplary embodiment, but other cryogenic refrigerants are also conceivable within the scope of the invention, for example LNG, liquid oxygen, liquid hydrogen or a liquefied noble gas.

LIST OF REFERENCE NUMBERS

1.

contraption

Second

Heat transfer device

Third

container

4th

cooling coil

5th

cooling coil

6th

cooling coil

7th

supply

8th.

tank

9th

level

10th

spigot

11th

Recovery

12th

-

13th

cooling bath

14th

Refrigerant supply line

15th

float valve

16th

filling height

17th

-

18th

exhaust pipe

19th

Pressure holding valve

20th

-

21st

Valve

22nd

Valve

23rd

Valve

24th

control

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• EP 2143528 A1 [0002]
• DE 19857790 A1 [0003]
• EP 2368638 A1 [0004]
• EP 0524432 A1 [0006, 0018]
• EP 2679879 A2 [0007, 0019, 0032]

Claims (9)

Device for subcooling liquefied gases, with an insulated container (3) for receiving a cooling bath (13), which is fed from a partial flow of a liquefied gas removed from a storage tank (8) and reduced to a low pressure, with a device (18, 19 ) for withdrawing a gas phase from the insulated container (3), and with a device (2) arranged within the insulated container (3) and flow-connected to the storage tank (8) via a feed line (7) for heat transfer from the through the feed line ( 7) led liquefied gas to be supercooled to the cooling bath (13), characterized in that the device (2) for heat transfer is equipped with a plurality of heat exchangers (4, 5, 6) connected in parallel with one another, which at least partially with a shut-off device (22, 23) for switching the flow through the respective heat exchanger (4, 5, 6). Device after Claim 1 , characterized in that at least some of the heat exchangers (4, 5, 6) are connected to one another in such a way that they can be switched on and off independently of one another. Device after Claim 1 , characterized in that at least some of the heat exchangers (4, 5, 6) are connected to one another in such a way that some of the heat exchangers (4, 5, 6) can only be switched on when at least further heat exchangers (4, 5, 6) already have is switched on. Device according to one of the preceding claims, characterized in that the locking fittings (22, 23) are operatively connected to a controller (24) by means of which the heat exchangers (4, 5, 6) according to a predetermined program or depending on measured parameters. and can be switched off. Device according to one of the preceding claims, characterized in that an apparatus for detecting the flow rate of the liquefied gas and / or a device for detecting the temperature of the supercooled liquefied gas is assigned upstream and / or downstream of the heat exchangers (4, 5, 6) , Device according to one of the preceding claims, characterized in that a gas phase separator is assigned to the feed line (7) for the liquefied gas to be supercooled. Device according to one of the preceding claims, characterized in that the inflow of liquefied gas to the cooling bath (13) can be regulated. Device according to one of the preceding claims, characterized by the use for cooling in an extruded aluminum extrusion device or in a cold milling device. Device according to one of the preceding claims, characterized in that a low-boiling gas is used as the liquefied gas.

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