Method for supplying cryogenic fluid to a user station, particularly a machining station - Patent application

Abstract

A method for supplying a cryogenic fluid, such as liquid nitrogen, to a user station, wherein a cryogenic fluid storage tank contains the cryogenic fluid in a liquid phase at the bottom of the tank and a gas phase at the top of the tank under a storage pressure higher than atmospheric pressure, the liquid / gas two-phase content arriving at the user station from the tank being a factor affecting the quality of the operations performed by said station, the tank being suitable for supplying the station with liquid taken from the bottom of the tank and also being supplied with fluid from the outside, i) means for gas / liquid phase separation (6), such as a degasser or phase separator, are provided, which are supplied with the cryogenic fluid from the tank (20) and separate the cryogenic fluid into an elementary liquid phase and an elementary gas phase (40), the elementary liquid phase (30) being directed to the user station. j) variations in the two-phase liquid / gas content arriving at the user station over time, e.g. in the course of each operating stage of the user station, are measured by providing a measurement of the gas flow from the gas outlet of the phase separation means and measuring any deviations in said gas flow over time, or by having information provided by a flow sensor placed at the gas outlet of the phase separation means, capable of detecting the flow leaving the separation means and determining the opening time of the sensor over a given time range, being binary information 0 / 1 (8); k) one or more actions (2, 10, ...) are undertaken to inform the user station and / or modify the operating conditions of said user station depending on the result of the decision made in j).

Description

[Technical Field]

[0001] The present invention relates generally to a method using cryogenic liquids such as liquid nitrogen, where the two-phase content of the liquid / gas arriving at the user station is an important factor, i.e. influencing the quality of the product this station performs, which is the case in particular in the technical field of cryogenic machining of machine parts, and also in mixers equipped with a nozzle for injection through the bottom of a cryogenic mixer for food products, for example. [Background technology]

[0002] The case of machining is discussed below.

[0003] Machining is a method for shaping a workpiece by removing material. The mechanical energy required for machining, and therefore the formation of chips, is converted almost entirely into heat. Despite the good thermal conductivity properties of the partially machined and machining materials, the use of cutting fluids -As well as cooling and lubrication of the cutting zone - Removal of shavings from the work area is still essential to ensure safety.

[0004] These cutting fluids are mostly neat or water-soluble mineral or synthetic base oils. The temperatures encountered in the center of the cutting zone (currently +800°C to +1000°C) lead to both the production of vapors or gases that are harmful to the external environment and to chemical contamination of the chips and machined surfaces that can even impair their properties.

[0005] Oils are a significant expense due to their purchase and regeneration costs as well as management. In this context, lubrication methods called "microlubrication" or "dry lubrication" reduce or even eliminate the consumption of cutting fluid. Machining performance is thereby degraded, and for this reason these methods are only applied in machining scenarios that require only slight cooling of the cutting zone (such as machining of aluminum-based materials, high-speed machining, etc.).

[0006] In other machining scenarios, i.e. scenarios requiring significant cooling of the cutting zone, machining by adding cryogenic fluids, hereafter referred to as "cryogenic machining", is a very attractive solution for cooling and lubricating the cutting zone, combining the advantages of oils (chip removal, heat transfer fluid, etc.) with the advantages of dry machining (environmental friendliness, non-polluting generated surfaces, chip regeneration, increased tool life, etc.).

[0007] The cryogenic fluid may be nitrogen and carbon dioxide.

[0008] There is extensive prior art relating to the provision of such machine tools with the aid of cooling fluids (for cutting tools, cutting zones, etc.), particularly with the aid of liquid cryogenics such as liquid nitrogen.

[0009] Cryogenic fluids are generally understood to be fluids that are liquid at temperatures much below 0°C at atmospheric pressure.

[0010] Such cryogenic liquids (e.g. liquid nitrogen), regardless of their type, are conventionally supplied to consumer equipment from cryogenic fluid tanks connected to the consumer equipment for this fluid, the tanks containing the cryogenic fluid in a liquid phase at the bottom of the tank and a gas phase at the top of the tank under a storage pressure higher than atmospheric pressure, the tanks supplying the consumer equipment with liquid removed from the bottom of the tank and designed to be supplied with fluid from the outside.

[0011] So-called "low pressure storage" tanks, i.e. tanks in which the maximum pressure reached at the top of the tank is generally less than about 4 bar absolute, are most commonly used in the industry, but so-called medium pressure storage tanks reaching up to 15 bar, or even high pressure storage tanks reaching up to 30 bar, depending on the intended use, can also be found.

[0012] Since the storage pressure of the tank is higher than atmospheric pressure, the opening of a valve placed on the pipe connecting the tank to a consumer equipment (e.g., machine tool) causes the liquid to move from its point of withdrawal to its point of use without any forced drive means and despite pressure losses in the line (valves, bends, etc.).

[0013] To ensure that the drive of the cryogenic liquid is always effective regardless of the liquid level in the tank, the gas pressure at the top of the tank is conventionally regulated so that this pressure remains substantially equal to a predetermined fixed value, for example about 2 to 4 bar (more broadly 2 to 15 bar).

[0014] However, the pressure of the liquid at the bottom of the tank varies depending on the liquid level inside the tank, so as the liquid level drops, the pressure of the extracted liquid drops and tends to approach the pressure of the gas at the top. For example, in the case of nitrogen, a liquid height of about 10 meters implies a pressure difference of about 0.7 bar between the gas pressure at the top of the tank and the liquid pressure at the bottom at the point of extraction.

[0015] This change in the pressure of the liquid at the point of withdrawal necessarily causes a change in the flow rate of the liquid withdrawn, disrupting the operation of consumer equipment installed downstream. A symmetric effect occurs during the refilling of the tank with fluid.

[0016] For well-known reasons of better "cryogenic quality" in terms of available cold energy, the cryogenic literature and those industries, particularly the machining industry, have become interested in means to provide these user stations with pure or substantially pure liquid or subcooled liquid, i.e., liquid at reduced pressure and at a lower temperature than it would be at higher pressure.

[0017] In particular, considering the example of machining, the higher the atomization pressure in the machining zone, the better the coefficient of heat exchange. However, when a cryogenic liquid, such as liquid nitrogen, is atomized, gas is generated at the atomization nozzle due to its expansion. The amount of gas generated is directly proportional to the temperature of the liquid nitrogen and its pressure upstream of the nozzle. Therefore, the advantage of striving to have a subcooled liquid can be seen.

[0018] It should be noted that, among the large amount of literature available, certain studies have recommended the use of phase separation (degassing) means on the lines connecting the tank to the civil installations, see for example document EP 2347855.

[0019] Other solutions have proposed combining the two tanks and using them after alternatively filling and depressurizing them. The drawback of this solution is that it results in very heavy handling and the combining of the two tanks is very obvious.

[0020] Another solution is to insert a heat exchanger (e.g., a plate heat exchanger) immediately upstream of the point of use, where cooled liquid nitrogen (specifically originally at 3 bar and a temperature of about -185°C) circulates through one of the exchanger's paths (the main path), while decompressed nitrogen circulates through the other path, typically at a pressure of about 1 bar and a lower temperature of about -196°C. The exchange between these two paths allows the nitrogen in the main path to be subcooled in parallel or countercurrent flow. However, temperature control here is difficult to manage and stabilize, and the exchanger must undergo heating and recooling and other steps, especially when downstream consumer equipment operates discontinuously.

[0021] Reference may also be made to the document WO 2004 / 005791 in the name of the applicant, which recommends varying the pressure of the gas in the upper part of the tank depending on the operating state of this tank (consumption phase of downstream user equipment, standby phase, or phase of supplying the tank with cryogenic liquid) and, according to one of its embodiments, legitimately recommends venting the tank during standby periods. In other words, when the tank is not subjected to a withdrawal operation and a priori for a long period of time, for example several hours (for example overnight), the control unit commands the opening of a valve to vent the upper part of the tank. The pressure of the gas in the upper part of the tank then changes from the storage value to a value substantially equal to atmospheric pressure (a residual pressure of several hundred grams). By lowering the nitrogen storage pressure in this way, the change in enthalpy of the nitrogen storage pressure tends to increase, which results in a fluid with a much lower temperature than when it was under pressure. The fluid stored in this way during these periods when the tank is not in use therefore has a lower temperature than usual, ensuring a better cryogenic quality in relation to the available cold energy. In fact, rapid repressurization allows the use of destabilized (supercooled) liquids, for example using a proprietary atmospheric heater or the like.

[0022] Nevertheless, this solution is not without drawbacks: this ventilation necessarily involves losses, and the procedure also has the paradox of requiring repressurization to make the nitrogen usable, thus heating it up. Experiments with this solution have specifically demonstrated an evaporation of 4-9% of the stored capacity. This evaporation is not utilized, and so costs directly affect the user site.

[0023] In summary, two main drawbacks of this venting solution can be deduced: 1) Using unavailable nitrogen for repressurization, 2) The introduction of hot gas into the storage tank to reduce pressure and the creation of thermal bridges.

[0024] It has also been considered to supply a user station, e.g. a machining station, directly from a cryogenic storage tank at medium or high pressure, but it has then been found that large amounts of gas are generated at the exit of the spray nozzle, which reduces heat exchange.

[0025] Finally, one could consider supplying the machines from a low pressure storage tank through a pump, but the difficulties associated with handling such pumps are known, and in addition, it is not possible to supply multiple machining stations at a single site with different pressures and low flow rates.

[0026] The present invention seeks to propose a technical solution for controlling and maintaining the conditions for operation at a desired level in operations using cryogenic liquids, such as machining operations, these conditions being related to the temperature, pressure and two-phase content characteristics of the utilized cryogenic liquid.

[0027] The case of machining will be discussed very specifically below for ease of reference, but it will be recognized that the ideas discussed above and below have broader application to many other applications using cryogenic liquids.

[0028] In this regard, as will be shown in more detail below, the present invention contemplates that a cryogenic machining (or similar) method does not necessarily require knowledge of the two-phase content within the liquid cryogenic temperature, e.g., liquid nitrogen, but rather requires monitoring it and analyzing any changes therein over time, e.g., during each machining operation. Moreover, it is an advantage and feature of the present invention that it proposes to employ this new approach.

[0029] It is known that many studies have been and still are being attempted to develop a velocimeter capable of measuring the flow rate of a liquid cryogenic liquid and its two-phase content. To date, the systems developed have not been satisfactory or have certain drawbacks (cost, size, accuracy, etc.) that are unacceptable in the field of cryogenic machining. Summary of the Invention [Problem to be solved by the invention]

[0030] The present invention therefore proposes not to measure the gas content in a cryogenic liquid, but rather to measure the fluctuations in the content of the two phases in a simple and inexpensive system. [Means for solving the problem]

[0031] For this, the gas flow rate at the gas outlet of the degasser (or other phase separator) is measured and deviations in this gas flow rate over time are measured, which, as will be appreciated, translate directly into variations in the content of the two phases.

[0032] It is known that degassing cylinders are characterized by a maximum flow rate that depends on the pressure of the extracted gas relative to the liquid, and do not guarantee 100% two-phase at the outlet of the degassing cylinder.

[0033] By way of illustration, in one example implementation of cryogenic machining, the flow rate of liquid nitrogen is conventionally about 2 l / min, and therefore the maximum gas flow rate of the degasser can "sweep" about 250 l / min of nitrogen gas at 10 bar, thus providing sufficient sweeping of the gas section in any configuration.

[0034] The degasser is therefore not always open in injection mode. [Brief explanation of the drawings]

[0035] [Figure 1] 1 illustrates an example of an arrangement for implementing the present invention. [Figure 2] Schematically shows the flow rate subject to tool changes and changes in type corresponding to the number of machines served by the same network. DETAILED DESCRIPTION OF THE INVENTION

[0036] The attached Figure 1 shows an example of an installation for implementing the present invention, allowing a better understanding of the present technical proposal.

[0037] The names of the elements in the figure are as follows: -1: "FCV11" = Valve for regulating flow rate (under pressure) -2: "PV13" = Liquid shut-off valve that allows bypass PV14 to be opened (so degassing takes place here and not in the downstream user process), this is a so-called "normally closed" valve. -3: "TT13" = Temperature Transmitter -4: "PT12" = Pressure Transmitter -5: "PSV12" = Overpressure relief valve -6: "TG12" = Phase separator, e.g. degasser -7: "FE" = flow meter or flow sensor (capable of providing a signal that can be used to control other elements of the facility) -8: "FT12" = A flow meter or flow sensor capable of providing a measurement of gas flow (and therefore two-phase flow) (which can provide a signal that can be used to control other elements of the equipment) -9: "PCV12" = Back pressure regulator to ensure back pressure in the deaerator cylinder and to facilitate more reliable measurement of the gas flow rate, this back pressure regulator represents an advantageous option according to the present invention. -10: "PV14" = shut-off valve on bypass, this valve is a so-called "normally open" valve, this valve is a cooling valve, it is possible (and optional) to use this valve to monitor changes in the content of the two phases. -20: Extremely low temperature -30: Cryogenic liquid is released and directed towards the user station -40: Gas outlet of phase separator.

[0038] Thanks to this equipment, the measurement of the content of the two phases via the gas flow or sensor "FT12" can be carried out, for example, by one or more of the following actions: - notifying a user station of changes in the two-phase content of the fluid arriving at the machining station; - stopping the process when the user decides to do so via an action on the liquid shut-off valve PV13, - adjusting the flow rate via the regulating valve FCV11 to keep the amount of cold energy constant, increasing it if necessary (thus increasing the pressure with this type of valve); - Repeating the degassing step (and thus recooling) to remove the two-phase excess via valve PV14 on the bypass.

[0039] As will be apparent to those skilled in the art, the raw signal from the flow meter or flow sensor 8 cannot be used as the operation of the separator / degasser will cause the flow rate to be inconstant through its flotation and will be characterized by oscillations.

[0040] Post-processing of this signal by the control device is therefore advantageously carried out. · Mass flow rate is not constant during line cooling (hot liquid, unstable gas / liquid percentage, etc.). In production mode at a user station, e.g. a downstream machining station, the flow rate is subject to changes in type (variable flow rate) corresponding to tool changes and the number of machines supplied by the same network, as can be seen (schematically) in the attached Figure 2.

[0041] The post-processing carried out by the control device can thus take into account, for example, one or more of the following criteria: The number of degassing cylinder opening cycles / periods for a single given period over a production cycle, e.g. over a machining cycle between two tool changes (start and end of cycle information collected by the control device). The value of the opening period, the opening period of the deaerator each time it opens, for a constant or varying change in this opening period over time. The integral of the gas flow measured during each machining cycle. The average two-phase content can be calculated throughout the use of the cryogenic fluid.

[0042] While it will be readily understood that the flow rate of the liquid cryogenic temperature, e.g., liquid nitrogen, depends on the program selected for the machining operation (and thus, e.g., on the pressure downstream of the inlet valve entering the machining zone) and on the diameter of the injection orifices corresponding to the various machining stages (roughing, finishing, material, pass depth, etc.), it will be appreciated that the method according to the invention does not perform or provide for the measurement of the flow rate of the liquid nitrogen. Comparison of the above criteria between two production cycles and issuing an alarm if a change is detected (under the condition that the pressure conditions of the liquid nitrogen at the inlet to the machining zone and the injection orifice are identical in these two cycles); The standby mode is decoupled from the calculation by collecting the output machining signals from the machine to the controller, etc.

[0043] The original form of modification of the two-phase content during the machining operation can be considered as follows: -Blocking the floating of the deaerator (this effect is very high), - when several appliances are drawn from the same storage tank, i.e. the shutdown of one or more of these appliances may affect other appliances that remain partially operational; - filling of liquid nitrogen storage tanks while machining operations are taking place; -Leaking liquid nitrogen from components of the fluid network.

[0044] The invention thus relates to a method for supplying a user station with a cryogenically cooled fluid, such as liquid nitrogen, in which a cryogenically cooled fluid storage tank contains the fluid in a liquid phase at the bottom of the tank and a gas phase at the top of the tank under a storage pressure higher than atmospheric pressure, the liquid / gas two-phase content arriving at the user station from the tank being a factor affecting the quality of the operations performed by this station, said tank being designed to supply said station with a liquid taken from the bottom of the tank and to be supplied with fluid from the outside, i) providing a gas / liquid phase separation means, such as a degasser or phase separator, supplied with the cryogenic fluid from the tank and effecting separation of a substantially liquid phase and a substantially gas phase of the cryogenic fluid, the substantially liquid phase being directed towards a user station; j) Variations in the two-phase liquid / gas content arriving at the user station over time, e.g., during each operating stage of the user station, by having a measurement of the flow rate of the gas output from the gas outlet of the phase separation means and measuring any deviations of this flow rate of gas over time, or - having information provided by a flow sensor placed at the gas outlet of the phase separation means, this information being a 0 / 1 binary number that detects the presence of a flow at the outlet of the separation means and makes it possible to determine the opening time of the sensor over a given time interval; k) one or more actions are undertaken to inform the user station and / or modify the operating conditions of this user station depending on the outcome of the decision made in j).

Claims

1. 1. A method for supplying a cryogenic fluid, such as liquid nitrogen, to a user station, a station where machining operations are performed, comprising: the two-phase liquid / gas ratio reaching the user station from the cryogenic fluid storage tank is a factor affecting the quality of the machining operations performed by the user station; the tank contains the cryogenic fluid in a liquid phase at the bottom of the tank and in a vapor phase at an upper portion of the tank under a storage pressure greater than atmospheric pressure; the tank is designed to supply the user station with liquid removed from the bottom of the tank and to be supplied with fluid from outside; i) providing a liquid / gas phase separation means (6) supplied with the cryogenic fluid (20) from said tank and for effecting separation of the cryogenic fluid into a substantially liquid phase and a substantially gaseous phase (40); the substantially liquid phase (30) is directed towards the user station; j) the variation in the two-phase proportion of the liquid / gas reaching the user station during each stage of operation of the user station; - measured over time by measuring the flow rate of the gas output from the gas outlet of said phase separation means with a flow meter and measuring the variation of said gas flow rate over time (8); Or, - measured over time by a flow sensor placed at the gas outlet of the phase separation means, providing information in binary format (0 / 1) (8); said information enabling the presence of a flow rate at the outlet of said separating means to be detected and the activation time of said sensor over a given time interval to be determined; k) depending on the results of the measurements made in j), one or more actions (2, 10, etc.) are undertaken to inform the user station and / or modify the operating conditions of the user station.

2. The signal provided by the flow meter or the flow sensor is post-processed by a control device, the post-processing being based on one or more of the following criteria: The method according to claim 1. the number of opening periods / cycles of the phase separation means in the same given period over the machining cycle between changes of two tools of the user station; the value of the opening period and / or the opening period of the phase separation means each time the phase separation means opens, to take into account the change over time of the opening period, whether constant or varying, - The sum of the gas flow rates measured during each production cycle of the user station.

3. The method of claim 1 or 2, wherein the user station performs machining operations.

4. 3. The method of claim 1 or 2, wherein the user station is a food grade cryogenic mixer equipped with a nozzle for injecting the cryogenic fluid through the bottom of the mixer.

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