Method for supplying a user station, in particular a machining station, with cryogenic fluid

Abstract

A method for supplying cryogenic fluids, such as liquid nitrogen, from a cryogenic fluid storage tank to a user station, wherein the liquid / gas two-phase contents arriving at the user station are a factor affecting the quality of operations performed by the station, the tank containing a liquid phase of the cryogenic fluid at the bottom and a gas phase of the cryogenic fluid at the top of the tank at a storage pressure above atmospheric pressure, the tank being adapted to supply the station with liquid drawn from the bottom of the tank and fluid suitable for external supply, characterized in that: i) a gas / liquid phase separation device (6), such as a degassing vessel or phase separator, is provided, and the gas / liquid phase separation device supplies (20) cryogenic fluid from the tank and separates the substantially liquid phase and substantially gas phase (40) of the cryogenic fluid, the substantially liquid phase (30) being diverted (i) To guide to the user station; j) For example, during each operational phase of the user station, measure (8) the fluctuation of the liquid / gas two-phase contents arriving at the user station over time by: - ​​providing a measurement of the gas flow rate from the gas outlet of the phase separation device and measuring any deviation of the gas flow rate over time; or - acquiring information provided by a flow sensor located at the gas outlet of the phase separation device, i.e., 0 / 1 binary type information, so as to enable the flow rate leaving the separation device to be detected and the opening time of the sensor within a given time range; k) Based on the results of these determinations made in j), take one or more measures (2, 10, ...) to inform the user station and / or modify the operating conditions of the user station.

Description

[0001] This invention generally relates to methods using cryogenic liquids such as liquid nitrogen, in which the liquid / gas two-phase contents arriving at the user station are an important factor (i.e., an important factor affecting the quality of the product or the performance of the station), particularly in the field of cryogenic machining of mechanical parts, and for example, for food-grade cryogenic mixers equipped with nozzles for injection through the bottom of the mixer.

[0002] Machining will be discussed below.

[0003] Machining is a method of shaping a workpiece by removing material. The mechanical energy required for machining, and thus for forming chips, is almost entirely converted into heat. Although some machined materials and machining processes have good thermal conductivity, the use of cutting fluids remains mandatory to ensure:

[0004] - Cooling and lubrication of the cutting zone;

[0005] -And it also removes chips from the work area.

[0006] These cutting fluids are primarily pure or soluble mineral oils or synthetic base oils. The temperatures encountered at the center of the cutting zone (currently +800°C to +1000°C) result in the generation of fumes or gases harmful to the external environment, as well as chemical contamination of the chips and machined surfaces, which may even damage the properties of the chips and machined surfaces.

[0007] Oil is a major expense due to its purchase and recycling costs, as well as its management costs. In this context, lubrication methods known as "micro-lubrication" or "dry lubrication" reduce or even eliminate the consumption of cutting fluid. Consequently, machining performance is reduced, and for this reason, these methods are only applicable to machining situations requiring minimal cooling of the cutting zone (such as machining aluminum-based materials, high-speed machining, etc.).

[0008] In other machining scenarios (i.e., those requiring significant cooling of the cutting zone), machining with the addition of cryogenic fluids (hereinafter referred to as "cryomachining") is a very attractive solution for cooling and lubricating the cutting zone, combining the advantages of oil (chip removal, heat transfer fluid, etc.) with those of dry machining (environmental protection, pollution-free surfaces, chip recovery, extended tool life, etc.).

[0009] This cryogenic fluid can be nitrogen and CO2.

[0010] A large number of existing technologies involve supplying such machine tools with cooling fluids (for cutting tools, cutting zones, etc.), and in particular with liquid coolants such as liquid nitrogen.

[0011] Cryogenic fluids are generally understood to be fluids that are liquid at atmospheric pressure and temperatures well below 0°C.

[0012] Traditionally, this cryogenic liquid (e.g., liquid nitrogen) is supplied to the consumer device from a cryogenic fluid tank connected to the consumer device, regardless of the type of the consumer device. The tank contains a liquid phase of cryogenic fluid at the bottom and a gaseous phase of cryogenic fluid at the top of the tank at a storage pressure above atmospheric pressure. The tank is designed to supply the consumer device with liquid drawn from the bottom of the tank and fluid supplied from the outside.

[0013] The most commonly used in industry are so-called “low-pressure storage” tanks (i.e., tanks where the maximum pressure reached at the top of the tank is usually less than about 4 bar of absolute pressure), but depending on the intended application, so-called medium-pressure storage tanks reaching up to 15 bar, or even high-pressure storage tanks reaching up to 30 bar, have also been found.

[0014] Because the storage pressure of the can is higher than atmospheric pressure, in the absence of a forced actuation device, although there is pressure loss in the pipeline (valve, bend, etc.), opening the valve placed on the pipeline that connects the can to a consumer device item (e.g., a machine tool) causes the liquid to move from its extraction point to its point of use.

[0015] To ensure that the cryogenic liquid drive remains effective regardless of the liquid level in the tank, the gas pressure at the top of the tank is typically adjusted to maintain a predetermined fixed value, such as about 2 to 4 bar (more broadly, 2 to 15 bar).

[0016] However, the liquid pressure at the bottom of the tank varies depending on the liquid height inside the tank, causing the pressure of the extracted liquid to decrease as the liquid level drops and tend to approach the gas pressure at the top. For example, in the case of nitrogen, a liquid height of approximately 10 meters means that the pressure difference between the gas pressure at the top of the tank and the liquid pressure at the bottom of the tank, at the extraction point, is approximately 0.7 bar.

[0017] This pressure change in the liquid at the extraction point inevitably leads to a change in the extracted liquid flow rate, thus interfering with the operation of downstream consumer equipment. A symmetrical effect occurs during the resupply of fluid to the tank.

[0018] For well-known reasons, namely the superior "low-temperature quality" in terms of available cold energy, the literature and industries that utilize refrigerants, particularly the machining industry, have shown interest in devices for supplying pure or substantially pure liquids, or subcooled liquids (i.e., liquids at reduced pressures and at a lower temperature than when they are at higher pressures), to these user stations.

[0019] Specifically, considering the example of machining, the higher the injection pressure in the machining zone, the better the heat exchange coefficient. However, when a refrigerant (e.g., liquid nitrogen) is injected, gas is generated at the injection nozzle due to the expansion of the refrigerant. The amount of gas generated is proportional to the temperature of the liquid nitrogen and the pressure of that liquid nitrogen upstream of the nozzle. Therefore, it will be understood that there are advantages to having a subcooled liquid.

[0020] In the extensive existing literature, it should be noted that some studies have recommended the use of phase separation (degassing) devices on pipelines connecting canisters to consumer device items; see, for example, document EP-2 347855.

[0021] Other solutions have been proposed, which involve connecting the two tanks and using them alternately after filling and depressurization. The drawbacks of this solution are quite obvious: it introduces a very large amount of manipulation and requires the use of two tanks.

[0022] Another solution is to insert a heat exchanger (e.g., a plate heat exchanger) just upstream of the point of use: the liquid nitrogen to be cooled (typically, initially 3 bar and at a temperature of about -185°C) circulates in one path of the exchanger (the main loop), while depressurized nitrogen (typically at a pressure of about 1 bar and a lower temperature of about -196°C) circulates in the other path. It is this co-current or counter-current exchange between these two paths that allows the nitrogen in the main loop to be subcooled. However, temperature control here is difficult to manage and stabilize, especially when downstream consumer equipment operates discontinuously, forcing the exchanger to undergo heating and recooling stages.

[0023] Reference may also be made to the applicant's document WO 2004 / 005791, which suggests varying the gas pressure at the top of the tank depending on its operating state (consumption phase of downstream user facilities, standby phase, or phase of supplying cryogenic liquid to the tank), and that the document reasonably suggests, based on one of its embodiments, that the tank be vented during standby periods. In other words, when the tank is not undergoing venting operations and will not be used for a considerable period of time, such as several hours (e.g., overnight), the control unit commands the valve to open to vent the top of the tank. The gas pressure at the top of the tank then changes from its stored value to a value substantially equal to atmospheric pressure (with a residual pressure of several hundred grams). Therefore, by reducing the nitrogen storage pressure in this way, the enthalpy change of nitrogen tends to increase, which is equivalent to the fluid temperature being much lower than when the fluid is under pressure. Thus, the fluid stored in this way during these periods when the tank is not in use has a lower temperature than usual, thereby ensuring better cryogenic quality in terms of available cold energy. In fact, rapid repressurization (using, for example, its own atmospheric pressure heater) makes it possible to use unstable (supercooled) liquids.

[0024] However, this solution is not without its drawbacks. Such venting inevitably results in losses, and furthermore, this process is paradoxical in that it requires repressurization to utilize nitrogen, thus allowing heat to enter. Experiments with this solution have specifically demonstrated that 4% to 9% of the stored volume vaporizes. Because this vaporization is not utilized, the cost directly impacts the user site.

[0025] In summary, this leads to two main drawbacks of this exhaust solution:

[0026] 1) Use nitrogen that cannot be used for repressurization.

[0027] 2) Allow hot gas to enter the storage tank to reduce pressure and create thermal bridges.

[0028] Supplying the refrigerant directly from the refrigerant storage tank to the user station (e.g., a machining station) at medium or high pressure was also considered, but it was subsequently observed that a large amount of gas was generated at the outlet of the injection nozzle, which reduced heat exchange.

[0029] Finally, supplying the machine from a low-pressure storage tank via a pump could be considered, but the difficulties associated with operating such a pump are known, and in addition to these difficulties, it is not possible to supply several machining stations at a single site at different pressures and low flow rates.

[0030] This invention aims to provide a technical solution for controlling and maintaining operating conditions at desired levels in operations using cryogenic liquids (e.g., machining operations), where these conditions are related to the temperature, pressure, and characteristics of the two-phase contents of the refrigerant used.

[0031] For the sake of simplicity, the following discussion will focus very specifically on the case of machining; however, it should be understood that the considerations discussed above and below are applicable to and more broadly suited to many other applications using cryogenic liquids.

[0032] In this regard, as will be shown in more detail below, the present invention proposes that cryogenic machining (or similar machining) methods do not necessarily require knowledge of the two-phase contents in the liquid refrigerant (e.g., liquid nitrogen), but rather require, for example, monitoring of the two-phase contents during each machining operation and analysis of any deviations in the two-phase contents over time. Moreover, this novel approach is presented as an advantage and feature of the present invention.

[0033] As is well known, many studies have attempted, and continue to attempt, to develop a flow meter that can measure both the flow rate of a liquid refrigerant and its two-phase contents. To date, the systems developed have been unsatisfactory or have certain weaknesses (cost, size, accuracy, etc.) that are prohibitive for the field of cryogenic machining.

[0034] Therefore, this invention proposes to measure the fluctuations of two-phase contents instead of measuring the gaseous contents in cryogenic liquids, using a simple and inexpensive system.

[0035] In this regard, the gas flow rate at the gas outlet of the degassing vessel (or other phase separator) is measured, and the deviation of this gas flow rate over time is measured. As will be understood, these measurements are directly related to the fluctuations in the contents of the two phases.

[0036] As is well known, the maximum flow rate of a degassing vessel depends on the pressure of the extracted gas relative to the liquid, and there is no guarantee that the outlet of the degassing vessel is 100% two-phase.

[0037] For illustration, in one embodiment of cryogenic machining, the flow rate of liquid nitrogen is typically about 2 l / min. Therefore, the maximum gas flow rate of the degassing vessel allows for the "purification" of nitrogen at 10 bar at a rate of about 250 l / min, and is thus sufficient to purify the gas portion in any configuration.

[0038] Therefore, in injection mode, the degassing vessel will not always be open.

[0039] The attached [ Figure 1 Examples of facilities for implementing the present invention are presented to enable a better understanding of the present technical solution.

[0040] The components in the diagram are named as follows:

[0041] -1: "FCV11" = Valve used for regulating flow (under pressure).

[0042] -2: "PV13" = a liquid shut-off valve that allows the bypass PV14 to be opened (therefore, degassing occurs here instead of in the downstream user process), which is a so-called "normally closed" valve.

[0043] -3: "TT13" = Temperature transmitter

[0044] -4: "PT12" = Pressure transmitter

[0045] -5: "PSV12" = Overpressure relief valve

[0046] -6: "TG12" = Phase separator (e.g., degassing vessel)

[0047] -7: "FE" = Flow meter or flow sensor (capable of providing signals that can be used to control other components of a facility).

[0048] -8: "FT12" = Flow meter or flow sensor (capable of providing signals that can be used by other components in the control facility), capable of providing a measurement of gas flow rate (therefore, a measurement of two-phase flow).

[0049] -9: "PCV12" = Back pressure regulator for ensuring back pressure in the degassing vessel and for facilitating more reliable measurement of gas flow rate, this back pressure regulator represents an advantageous option according to the invention.

[0050] -10: "PV14" = Bypass shut-off valve, which is a so-called "normally open" valve; this valve is a cooling valve that can (and optionally) be controlled to observe changes in the contents of the two phases.

[0051] -20: Refrigerant enters

[0052] -30: The cryogenic liquid leaves and is guided toward the user station.

[0053] -40: Gas outlet of the phase separator.

[0054] By using this facility to measure the two-phase contents via gas flow or sensor "FT12", one or more of the following measures can be taken:

[0055] - Inform the user station of the changes in the two-phase contents of the fluid arriving at the machining station;

[0056] -If the user decides to stop, the method is stopped by actuating the liquid shut-off valve PV13;

[0057] - The flow rate is adjusted via the regulating valve FCV11, and if necessary, the flow rate is increased to keep the amount of cooling energy constant (therefore, for this type of valve, the pressure increases).

[0058] - Repeat the degassing phase (and thus recool) to eliminate the two-phase excess via valve PV14 on the bypass.

[0059] As will be apparent to those skilled in the art, the raw signal from the flow meter or flow sensor 8 is unavailable because the operation of the separator / degassing vessel via its float results in an inconsistent flow rate, characterized by oscillations.

[0060] Therefore, advantageously, the controller performs post-processing on this signal:

[0061] ●The mass flow rate is not constant during pipeline cooling (hot liquid, unstable gas / liquid percentage, etc.).

[0062] ● In the production mode of a user station (e.g., a downstream machining station), the flow experiences as described in the attached […]. Figure 2 The types of changes that can be seen (illustratively) correspond to tool changes and the number of machines supplied by the same network (variable traffic).

[0063] Therefore, post-processing performed by the controller can take into account one or more of the following criteria:

[0064] ● The number of times / cycles the degassing vessel is opened within a production cycle (e.g., a machining cycle between two tool changes (information collected by the controller about the start and end of the cycle) within a single given duration).

[0065] ●The duration of the degassing vessel's opening whenever it is opened: the value of the opening duration, how this opening duration changes over time, and whether the opening duration is constant or variable.

[0066] ● The integral of the gas flow rate measured during each machining cycle. The average value of the two-phase contents can be calculated throughout the entire cryogenic fluid usage period.

[0067] As will be readily understood, the flow rate of the liquid refrigerant (e.g., liquid nitrogen) will depend on the program selected for the machining operation (and therefore, for example, on the pressure downstream of the inlet valve in the machining zone) and the diameter of the injection hole corresponding to each machining stage (roughing stage, finishing stage, material stage, depth stage, etc.). Meanwhile, as will be understood, the method according to the invention does not perform the measurement of the liquid nitrogen flow rate and does not supply the measured value of the liquid nitrogen flow rate.

[0068] ● Compare the above criteria between two production cycles, and if a change is observed (under the condition that the liquid nitrogen pressure conditions at the inlet leading to the machining area and the liquid nitrogen pressure conditions at the injection port are the same for both cycles), an alarm is issued.

[0069] ●Standby mode separates calculations from the process by collecting ongoing machining signals from the machine toward the controller, etc.

[0070] The reasons for modifying the contents of the two phases during machining operations can be attributed to the following factors:

[0071] - Blockage of the float in the degassing vessel (this has a very significant impact).

[0072] When several equipment items are drawn from the same storage tank, the cessation of one or more of these equipment items may affect other equipment items that are still in operation.

[0073] - The filling of the liquid nitrogen storage tank occurred while machining operations were in progress.

[0074] A member of the liquid nitrogen fluid network experienced a leak.

[0075] Therefore, the present invention relates to a method for supplying cryogenic fluids, such as liquid nitrogen, from a cryogenic fluid storage tank to a user station, wherein the liquid / gas two-phase contents arriving at the user station are a factor affecting the quality of operations performed by the station, the tank containing the cryogenic fluid in a liquid phase at the bottom and in a gaseous phase at the top of the tank at a storage pressure higher than atmospheric pressure, the tank being designed to supply the station with liquid drawn from the bottom of the tank and fluid supplied from the outside, characterized in that:

[0076] i) A gas / liquid phase separation device, such as a degassing vessel or phase separator, is provided, which supplies the cryogenic fluid from the tank and performs a separation of the cryogenic fluid into a substantially liquid phase and a substantially gas phase, the substantially liquid phase being directed toward the user station;

[0077] j) For example, during each operational phase at the user station, the fluctuation of the liquid / gas two-phase contents arriving at the user station over time is measured by the following:

[0078] - Obtain a measurement of the gas flow rate output from the gas outlet of the phase separation device and measure any deviation of this gas flow rate over time; or

[0079] - Obtain information from a flow sensor located at the gas outlet of the phase separation device. This information is of 0 / 1 binary type, which allows the presence of flow at the outlet of the separation device to be detected and the opening time of the sensor within a given time interval to be determined.

[0080] k) Based on the determined results in j), take one or more measures to inform the user station and / or modify the operating conditions of the user station.

Claims

1. A method for supplying cryogenic fluid from a cryogenic fluid storage tank to a user station, wherein, The liquid / gas two-phase contents arriving at the user station are a factor affecting the quality of operations performed by that station. The tank contains a liquid phase of the cryogenic fluid at the bottom and a gaseous phase at the top, at a storage pressure above atmospheric pressure. The tank is designed to supply the station with liquid drawn from the bottom and fluid supplied from the outside. Its characteristics are: i) A gas / liquid phase separation device (6) is provided, which is supplied (20) with cryogenic fluid from the tank and performs liquid and gas phase (40) separation on the cryogenic fluid, the liquid phase (30) being directed toward the user station; j) The fluctuation of the liquid / gas two-phase contents arriving at the user station over time is measured as follows: (8) - Obtain a measurement of the gas flow rate output from the gas outlet of the phase separation device, provided by the flow meter, and measure any deviation of this gas flow rate over time; or - Obtain information from a flow sensor located at the gas outlet of the phase separation device. This information is of 0 / 1 binary type, which enables the detection of flow at the outlet of the separation device and the determination of the sensor's on time within a given time interval. k) Based on the results determined in j), take one or more measures to inform the user station and / or modify the operating conditions of the user station.

2. The method as described in claim 1, characterized in that, The signal supplied by the flow meter or flow sensor is post-processed by the controller, and this post-processing takes into account one or more of the following criteria: ● The number of times the phase separation device is opened / number of cycles within the same given duration during the production cycle of this user station. ● The duration for which the phase separation device is opened each time it is opened, in order to take into account the value of the opening duration and / or any changes in the opening duration over time, whether the opening duration is constant or variable. ● The integral of the gas flow rate measured during each production cycle at this user station.

3. The method as described in claim 1 or 2, wherein, This user station performs machining operations.

4. The method as described in claim 1 or 2, wherein, The user station is a food-grade cryogenic mixer equipped with nozzles for injecting the cryogenic fluid through the bottom of the mixer.

5. The method as described in claim 1 or 2, wherein, The cryogenic fluid is liquid nitrogen.

6. The method as described in claim 1 or 2, wherein, The gas / liquid phase separation device (6) is a degassing vessel or phase separator.

7. The method as described in claim 1 or 2, wherein, During each operational phase of the user station, the fluctuation of the liquid / gas two-phase contents arriving at the user station is measured (8).

8. The method of claim 2, wherein the production cycle of the user station is a machining cycle between two tool changes.

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