EP0925476A1

EP0925476A1 - Method for controlling the gas extraction rate from a cryogenic apparatus and apparatus therefor

Info

Publication number: EP0925476A1
Application number: EP98929512A
Authority: EP
Inventors: Nicolas Viard
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 1997-07-03
Filing date: 1998-06-08
Publication date: 1999-06-30
Also published as: FR2765674B1; US6094924A; FR2765674A1; WO1999001705A1

Abstract

The invention concerns a method for controlling (26) the gas extraction rate of a gas extractor from a cryogenic apparatus (10) provided with a cryogenic fluid supply (18, 20), said apparatus (10) being in contact with the environment. The method comprises the following steps: measuring the environment gas content (mo2) in the chamber atmosphere; defining a reference input content (uo2) for said measured content; computing the extraction rate (rc) of the extractor (26) on the basis of said measured content (mo2) to maintain said content substantially equal to said reference input content (uo2); and controlling said extractor (26) on the basis of the computed rate (rc). The invention further concerns a cryogenic apparatus for implementing the method and is applicable to quick-freezing tunnels.

Description

METHOD FOR CONTROLLING THE GAS EXTRACTION SYSTEM OF A CRYOGENIC APPARATUS AND APPARATUS FOR IMPLEMENTING IT

The present invention relates to a method for controlling the extraction regime of a gas extractor from an enclosure of a cryogenic device provided with a cryogenic fluid supply, said device being in contact with the ambient medium.

The invention further relates to a cryogenic device implementing such a control method.

The invention relates in particular to freezing tunnels comprising means for supplying liquid nitrogen and means for extracting vaporized nitrogen, formed by a chimney provided with an extraction fan. Such a tunnel has loading and extraction openings 5 for articles to be frozen and frozen. Through these openings, gas exchanges take place between the ambient environment and the atmosphere prevailing in the enclosure of the tunnel.

If the extraction rate is too high, hot and humid air enters the tunnel enclosure. Many disadvantages in 0 arise. In particular, the air being at a temperature higher than the temperature prevailing in the enclosure, this results in excessive consumption of cryogenic fluid in order to keep the temperature in the enclosure constant. In addition, the air being charged with moisture, there is formation of ice and frost when the air cools both in the enclosure of the tunnel 5 and in the extraction chimney.

Conversely, if the extraction rate is too low, the cryogenic fluid is not completely evacuated by the chimney and spreads outside the enclosure of the tunnel, thus modifying the composition of the ambient medium. Thus, the current cryogenic tunnels cannot be used in confined spaces because of the risks of creating a dangerous oxygenated atmosphere dangerous for the personnel.

Currently, the extraction rate is adjusted manually. In practice, the person in charge of this adjustment visually assesses the direction of the gas flow established between the enclosure of the tunnel and the ambient environment. It attempts to cancel this flow by acting on the extraction rate. However, it is found that, in practice, in order not to risk an under-oxygenated atmosphere, the adjustment is made so that ambient air continuously penetrates inside the cryogenic tunnel. The above problems then affect the proper functioning of the device.

The invention aims to provide a simple solution to the problem

10 mentioned above resulting from poor control of the gas flows between the outside and the inside of the enclosure of a cryogenic device.

To this end, the subject of the invention is a method for controlling the extraction regime of a gas extractor from an enclosure of a cryogenic device provided with a supply of cryogenic fluid, said device

• 15 being in contact with the ambient environment, characterized in that it comprises the stages consisting:

- to measure the content of the enclosure atmosphere in a gas from the ambient environment;

- to define a target content for said measured content;

- calculating the extraction speed of the extractor as a function of said measured content to maintain this content substantially equal to said setpoint content; and

- controlling said extractor from said calculated extraction regime. According to particular modes of implementation, the method can

25 include one or more of the following characteristics:

- it also includes the steps of:

. determining a value representative of the quantity of cryogenic fluid introduced into the enclosure; and

. calculating the extraction regime as a function on the one hand of the value representative of the quantity of cryogenic fluid measured and on the other hand of said measured content and of said target content; the step of calculating the extraction regime comprises the successive steps consisting of:

. firstly calculating a theoretical extraction regime of said extractor as a function of said value representative of the quantity of cryogenic fluid introduced into the enclosure;

. then correcting by a control loop said theoretical extraction regime calculated as a function of said measured content and said setpoint content;

- it includes the following stages:. measuring the temperature of the gas in the enclosure upstream of the extractor;

. calculating a value representative of the quantity of gas to be extracted from the enclosure from the measured temperature and from the value representative of the quantity of cryogenic fluid introduced into the enclosure; and

. calculating said theoretical extraction regime as a function of said value representative of the quantity of gas to be extracted;

- The enclosure of the cryogenic device is substantially at the pressure of the ambient medium, and the method comprises the stages consisting:. measuring the pressure of the gas upstream of said cryogenic fluid supply;

. calculating a value representative of the quantity of gas to be extracted from the enclosure from the pressure measured and from the value representative of the quantity of cryogenic fluid introduced into the enclosure; and. calculating said theoretical extraction regime as a function of said value representative of the quantity of gas to be extracted;

- it includes the following stages:

. measuring the temperature of the gas upstream of said supply of cryogenic fluid; . calculating a value representative of the quantity of gas to be extracted from the enclosure on the basis of the measured temperature and the value representative of the quantity of cryogenic fluid introduced into the enclosure; and

. calculating said theoretical extraction regime as a function of said value representative of the quantity of gas to be extracted; - the cryogenic fluid is introduced into the device through at least one valve and in that said value representative of the quantity of cryogenic fluid is the valve opening rate;

the ambient medium is atmospheric air and in that said gas, the content of which is measured, is oxygen; and the extraction regime is calculated by a regulator comprising automatic means for optimizing the regulation parameters and it comprises the stages consisting:

. calculating and storing at different times of a period of time the absolute value of the difference between the measured content and the target content; . calculating over said time period the average of said absolute values of the deviations;

. comparing said average with a predetermined threshold value; and. triggering an automatic optimization of the regulation parameters if said average is greater than said predetermined threshold value.

The invention further relates to a cryogenic device comprising a working enclosure open to the ambient environment, a gas extractor with variable extraction regime and a cryogenic fluid supply to the enclosure, characterized in that it comprises: - means for measuring the content of the atmosphere of the enclosure in a gas from the ambient environment;

- Means for defining a target content for said measured content;

- Means for calculating the extraction regime of the extractor as a function of said measured content to maintain this content substantially equal to said set content; and - Means for controlling said extractor from said calculated extraction regime.

According to particular embodiments, the device can include one or more of the following characteristics: - it also includes:

. means for determining a value representative of the quantity of cryogenic fluid introduced into the enclosure; and

. means for calculating the extraction regime as a function on the one hand of the value representative of the quantity of cryogenic fluid measured and on the other hand of said measured content and of said target content;

- said means for calculating the extraction regime comprise:

. means for first calculating a theoretical extraction regime of said extractor as a function of said value representative of the quantity of cryogenic fluid introduced into the enclosure; . means for then correcting, by a regulation loop, said theoretical extraction regime calculated as a function of said measured content and of said setpoint content;

- it comprises :

. means for measuring the temperature of the gas in the enclosure upstream of the extractor;

. means for calculating a value representative of the quantity of gas to be extracted from the enclosure from the measured temperature and from the value representative of the quantity of cryogenic fluid introduced into the enclosure; and. means for calculating said theoretical extraction regime as a function of said value representative of the quantity of gas to be extracted.

the enclosure of the cryogenic device is substantially at the pressure of the ambient medium, and the device comprises:

. means for measuring the pressure of the gas upstream of said supply of cryogenic fluid;

. means for calculating a value representative of the quantity of gas to be extracted from the enclosure from the measured pressure and the value representative of the quantity of cryogenic fluid introduced into the enclosure; and

. means for calculating said theoretical extraction regime as a function of said value representative of the quantity of gas to be extracted;

- it comprises :

. means for measuring the temperature of the gas upstream of said supply of cryogenic fluid;

. means for calculating a value representative of the quantity of gas to be extracted from the enclosure from the measured temperature and from the value representative of the quantity of cryogenic fluid introduced into the enclosure; and

. means for calculating said theoretical extraction regime as a function of said value representative of the quantity of gas to be extracted; - It includes a valve for introducing cryogenic fluid into the device and in that said value representative of the quantity of cryogenic fluid is the valve opening rate;

the ambient medium is atmospheric air and said gas, the content of which is measured, is oxygen; and - it comprises a regulator for calculating the extraction regime, which regulator comprises automatic means for optimizing the regulation parameters and it comprises:

. means for calculating and storing at different times of a period of time the absolute value of the difference between the measured content and the setpoint content;

. means for calculating over said time period the average of said absolute values of the deviations;

. means for comparing said average with a predetermined threshold value; and. means for triggering an automatic optimization of the regulation parameters if said average is greater than said value of predetermined threshold.

The invention will be better understood on reading the description which follows, given solely by way of example and made with reference to the drawings in which: - Figure 1 is a schematic view of a cryogenic device implementing a control method according to the invention; and

- Figures 2 and 3 are schematic views of alternative embodiments of the apparatus of Figure 1.

In FIG. 1, the cryogenic apparatus is, for example, a cryogenic tunnel 10 suitable for the freezing of food articles. The tunnel is traversed right through by a conveyor 12 intended for the introduction and extraction of the articles.

The cryogenic tunnel 10 comprises means 14 for supplying liquid cryogenic fluid, for example liquid nitrogen. These means comprise several injection nozzles 16, supplied by a pipe 18 for supplying cryogenic fluid. A regulation valve 20 is provided on the pipe 18 and is suitable for adjusting the flow of nitrogen introduced into the enclosure of the tunnel. Liquid nitrogen is stored in a container 21 to which the pipe 18 is connected. The regulating valve 20 is controlled for its opening by a control unit 22 connected to a temperature sensor 23 placed in the enclosure of the tunnel. The control unit 22 is adapted to maintain a substantially constant temperature inside the enclosure of the tunnel and forms a temperature regulation loop acting on the opening α of the valve 20.

A chimney 24 ensures the evacuation of the cryogenic fluid vaporized in the enclosure of the tunnel 10. This chimney is connected inside the enclosure of the tunnel. It comprises a fan 26 intended for the extraction of gases from the enclosure. The fan 26 is at variable speed, so that it is possible to electrically control the extraction rate of the chimney.

For this purpose, it is known the characteristic curve of extraction of the fan, giving as a function of the fan rotation frequency, the extracted gas flow. This curve is generally a parabola.

According to the invention, the tunnel comprises a sensor 28 placed in the enclosure of the tunnel and adapted to measure the content m ₀₂ of the atmosphere in the enclosure in a gas from the ambient medium and in particular in oxygen. The sensor 28 is connected to a control unit 30 forming a regulator and adapted to control the fan 26. In particular, the control unit 30 is adapted to calculate an extraction regime r _c for the fan 26 and to generate a control signal so that the fan operates at a rotation frequency corresponding to the desired extraction speed r _c .

To this end, the control unit 30 comprises means for entering a nominal extraction regime r _n set by the user. From this nominal speed and the characteristic fan extraction curve, the control unit 30 determines the control signal corresponding to the speed r _c to be applied to the fan. The unit 30 also comprises means for inputting an oxygen target content denoted u ₀₂ . This content is, for example, fixed at 0.5% of the atmosphere in the enclosure.

According to the invention, the control unit 30 calculates the extraction speed r _c used for controlling the fan 26 so that the oxygen content m ₀₂ in the enclosure, measured by the sensor 28, remains substantially constant and equal to the target content, noted u ₀₂ .

Advantageously, the control unit 30 comprises a PID regulator (proportional, integral and derivative action regulator) provided with automatic means for calculating and optimizing the parameters of the regulator. This type of regulator being known, it will not be described in more detail.

The regulator 30 is associated with means 31 for determining the instant of triggering of the means for calculating and optimizing the parameters of the regulator.

These means 31 include means for storing at a predetermined frequency both the target content u ₀₂ and the content actually measured m ₀₂ in the enclosure of the tunnel. In addition, the triggering means 31 comprise calculation means suitable for determining the average over time γ of the absolute value of the differences between the setpoint content u ₀₂ and the content actually measured m ₀₂ . This average is expressed in the form:

where: Δt is the measurement interval and i is the number of measurements made during the time interval Δt.

The trigger means 31 are suitable for, when the average of the deviations γ exceeds a predetermined threshold γ ₀ , automatically trigger the optimization means of the regulator. Thus, if the regulator as initially set cannot maintain the oxygen content m ₀₂ close to the set value u ₀₂ , the parameters of the regulator are automatically recalculated, without an operator having to intervene.

The trigger means 31 can be integrated into the control unit 30 or be deported to a remote site connected to the control unit 30 by a telephone link.

It is understood that with such an installation, if ambient air enters excessively inside the enclosure of the tunnel, the oxygen content m ₀₂ of the atmosphere of the enclosure will increase so that l 'control unit 30 will reduce the speed r _c of the extractor so that the ambient air is no longer drawn into the enclosure of the tunnel.

Conversely, if the oxygen content m ₀₂ of the atmosphere in the enclosure of the tunnel is lower than the set content u ₀₂ , then the speed r _c of the fan 26 is increased by the control unit 30 of so that the vaporized cryogenic fluid does not escape outside the tunnel.

Thus, the gas flows between the interior of the tunnel enclosure and the exterior are very low, so that the apparatus does not have the disadvantages mentioned above. In particular, such a device can be used in an enclosed space without risk of creating an under-oxygenated atmosphere harmful to the health of the personnel.

In FIG. 2 is shown a more sophisticated embodiment of the tunnel of FIG. 1. In this figure, the similar elements are designated by the same references as in FIG. 1.

In this variant, the solenoid valve 20 is replaced by a regulating valve 50 associated with a sensor 52 adapted to determine the rate of opening of the valve. The latter is noted α and corresponds to the opening angle of the valve shutter in the case of a butterfly valve.

The sensor 52 is connected to an information processing unit 54 comprising a computer and means for memorizing a characteristic curve of the opening of the valve giving, as a function of its opening rate α, the fluid flow cryogenic introduced into the tunnel enclosure. This characteristic curve is provided by the valve manufacturer and is generally a straight line.

The processing unit 54 includes means for calculating a theoretical extraction regime noted r _t for the fan 26. The theoretical extraction regime r is in particular calculated from a value representative of the quantity of fluid cryogenic introduced into the tunnel enclosure, this representative value being determined from the valve opening ratio α and the stored characteristic curve. The processing unit 54 is connected to the control unit 30 in order to provide the calculated value r _t of the theoretical speed for the control of the fan 26. Thus, it is understood that in the apparatus described in FIG. 2, the extraction regime r _c is determined as a function of the quantity of cryogenic fluid introduced into the enclosure of the tunnel and a cascade regulation is performed as a function of the oxygen content measured in the enclosure of the tunnel.

FIG. 3 also shows an improvement to the tunnel shown in FIG. 2.

In this figure, elements similar to those of Figure 2 are designated by identical references.

In the embodiment of FIG. 3, the tunnel also comprises a temperature sensor 56 arranged in the enclosure of the tunnel and adapted to determine the temperature T of the vaporized gas immediately upstream of the chimney 24. The temperature sensor 56 is connected to the processing unit 54.

In addition, a pressure sensor 58 and a temperature sensor 60 are mounted upstream of the control valve 50 on the pipe 18. These sensors respectively determine the pressure Pe and the temperature Te upstream of the valve. They are both connected to the processing unit 54.

Finally, a differential pressure sensor 62 adapted to determine the pressure drop ΔP due to the control valve 50 is mounted on either side of this valve on the pipe 18. It is connected to the unit 54.

In this alternative embodiment, the processing unit 54 comprises means 54A for calculating a value representative V of the quantity of vaporized gas to be extracted from the enclosure from the temperature T in the enclosure measured by the sensor 56, of the pressure Pe measured by the sensor 58, of the temperature Te measured by the sensor 60, of the opening rate α of the valve 50 and of the pressure drop ΔP linked to the valve 50 and to the pipe 18.

Knowledge of the temperature T in the tunnel enclosure, immediately upstream of the exit, of the pressure Pe and of the temperature

Te of the cryogenic fluid upstream of the regulating valve 50 as well as of the pressure drop ΔP, makes it possible to determine with precision the actual amount of gas to be extracted from the tunnel.

The processing unit 54 also comprises calculation means noted

54B suitable for calculating the theoretical speed r _t addressed to the control unit 30 as a function of the calculated value V representative of the quantity of gas to be extracted. In order to perform the calculation, the calculation means 54B use the characteristic extraction curve of the fan.

According to yet another embodiment, the pressure sensor 58 and / or the temperature sensor 60 can be eliminated and the values of the pressure and the temperature of the cryogenic fluid upstream of the valve 50 are then estimated by predetermined constant values.

Claims

CLAIMS 1.- Method for controlling the extraction regime of a gas extractor (26) from an enclosure of a cryogenic device (10) provided with a cryogenic fluid supply (18, 20), said device ( 10) being in contact with the ambient medium, characterized in that it comprises the stages consisting:

- to measure the content (m ₀₂ ) of the enclosure atmosphere in a gas from the ambient environment;

- defining a target content (u ₀₂ ) for said measured content;

- calculating the extraction regime (r _c ) of the extractor (26) as a function of said measured content (m ₀₂ ) to maintain this content substantially equal to said target content (u ₀₂ ); and

- controlling said extractor (26) from said calculated extraction regime

(r ").

2.- Method according to claim 1, characterized in that it further comprises the steps consisting:

- determining a value (α) representative of the quantity of cryogenic fluid introduced into the enclosure; and

- calculating the extraction regime (r _c ) as a function of the value (α) representative of the quantity of cryogenic fluid measured and on the other hand of said measured content (m ₀₂ ) and of said content of deposit

(u ₀₂ ) •

3.- Method according to claim 2, characterized in that the step of calculating the extraction regime (r _c ) comprises the successive steps consisting in: - first calculating a theoretical extraction regime (r _t ) of said extractor (26) as a function of said value (α) representative of the quantity of cryogenic fluid introduced into the enclosure;

- Then correcting by a regulation loop (30) said theoretical extraction regime (r _t ) calculated as a function of said measured content (m ₀₂ ) and said setpoint content (u ₀₂ ).

4.- Method according to claim 3, characterized in that it comprises the stages consisting:

- to measure the temperature (T) of the gas in the enclosure upstream of the extractor;

- calculating a representative value (V) of the quantity of gas to be extracted from the enclosure from the measured temperature (T) and the value

(α) representative of the quantity of cryogenic fluid introduced into the enclosure; and

- calculating said theoretical extraction regime (r _t ) as a function of said representative value (V) of the quantity of gas to be extracted.

5.- Method according to claim 3 or 4, wherein the enclosure of the cryogenic device (10) is substantially at the pressure of the ambient medium, characterized in that it comprises the steps consisting:

- Measuring the pressure (Pe) of the gas upstream of said cryogenic fluid supply (18, 20); - calculating a representative value (V) of the quantity of gas to be extracted from the enclosure from the measured pressure (Pe) and the value (α) representative of the quantity of cryogenic fluid introduced into the enclosure; and

6.- Method according to any one of claims 3 to 5, characterized in that it comprises the steps consisting:

- measuring the temperature (Te) of the gas upstream of said supply of cryogenic fluid (18, 20); - calculating a representative value (V) of the quantity of gas to be extracted from the enclosure from the measured temperature (Te) and the value (α) representative of the quantity of cryogenic fluid introduced into the enclosure; and

7.- Method according to any one of claims 2 to 6, characterized in that the cryogenic fluid is introduced into the device through at least one valve and in that said value representative of the quantity of cryogenic fluid is the opening rate (α) of the valve.

8.- Method according to any one of the preceding claims, characterized in that the ambient medium is atmospheric air and in that said gas whose content is measured is oxygen.

9.- Method according to any one of the preceding claims, characterized in that the extraction regime (r _c ) is calculated by a regulator comprising automatic means for optimizing the regulation parameters and in that it comprises the consisting of:

- to calculate and memorize at different times of a period of time the absolute value of the difference between the measured content (m ₀₂ ) and the target content (u ₀₂ );

- calculating over said time period the average (γ) of said absolute values of the deviations;

- comparing said average (γ) with a predetermined threshold value

- to trigger an automatic optimization of the regulation parameters if said average (γ) is greater than said predetermined threshold value (γ ₀ ).

10.- Cryogenic device comprising a working enclosure open to the ambient environment, a gas extractor (26) with variable extraction regime and a cryogenic fluid supply (18, 20) of the enclosure, characterized in that it comprises: - means (28) for measuring the content (m ₀₂ ) of the atmosphere of the enclosure with a gas from the ambient medium;

- means for defining a target content (u ₀₂ ) for said measured content;

- Means (30, 54) for calculating the extraction regime (r _c ) of the extractor (26) as a function of said measured content (mo ₂ ) to maintain this content substantially equal to said setpoint content (u _Q2 ); and - Means (30) for controlling said extractor (26) from said calculated extraction regime (r _c ).

11.- cryogenic device according to claim 10, characterized in that it further comprises: - means (52) for determining a value (α) representative of the quantity of cryogenic fluid introduced into the enclosure; and

- Means (30, 54) for calculating the extraction regime (r _c ) as a function on the one hand of the value (α) representative of the quantity of cryogenic fluid measured and on the other hand of said measured content (m ₀₂ ) and said setpoint content (u ₀₂ ).

12. Cryogenic device according to claim 11, characterized in that said means (30, 54) for calculating the extraction regime (r _c ) comprise:

- Means (54) for first calculating a theoretical extraction regime (r _t ) of said extractor (26) as a function of said value (α) representative of the quantity of cryogenic fluid introduced into the enclosure;

- Means (30) for then correcting by a regulation loop (30) said theoretical extraction regime (r _t ) calculated as a function of said measured content (m _σ2 ) and of said setpoint content (u ₀₂ ).

13.- cryogenic device according to claim 12, characterized in that it comprises:

- Means (56) for measuring the temperature (T) of the gas in the enclosure upstream of the extractor;

- Means (54A) for calculating a representative value (V) of the quantity of gas to be extracted from the enclosure from the measured temperature (T) and the value (α) representative of the quantity of cryogenic fluid introduced into the enclosure; and

- Means (54B) for calculating said theoretical extraction regime (r _t ) as a function of said representative value (V) of the quantity of gas to be extracted.

14.- cryogenic device according to claim 12 or 13, wherein the enclosure of the cryogenic device (10) is substantially at the pressure of the ambient environment, characterized in that it comprises:

- Means (58) for measuring the pressure (Pe) of the gas upstream of said cryogenic fluid supply (18, 20);

- Means (54A) for calculating a representative value (V) of the quantity of gas to be extracted from the enclosure from the measured pressure (Pe) and of the value (α) representative of the quantity of cryogenic fluid introduced into the enclosure; and

15.- cryogenic device according to any one of claims 12 to 14, characterized in that it comprises:

- Means (58) for measuring the temperature (Te) of the gas upstream of said cryogenic fluid supply (18, 20); - Means (54A) for calculating a representative value (V) of the quantity of gas to be extracted from the enclosure from the measured temperature (Te) and the value (α) representative of the quantity of cryogenic fluid introduced into the enclosure; and

16.- cryogenic device according to any one of claims 11 to 15, characterized in that it comprises a valve (20; 50) for introducing cryogenic fluid into the device and in that said value representative of the quantity of cryogenic fluid is the valve opening rate (α).

17.- cryogenic device according to any one of claims 10 to 16, characterized in that the ambient medium is atmospheric air and in that said gas whose content is measured is oxygen.

18.- cryogenic device according to any one of claims 10 to 17, characterized in that it comprises a regulator for the calculation of the extraction regime (r _c ), which regulator comprises automatic means (31) optimization of the regulation parameters and in that it comprises:

- Means for calculating and memorizing at different times of a period of time the absolute value of the difference between the measured content (m ₀₂ ) and the target content (u ₀₂ ); - means for calculating over said time period the average (γ) of said absolute values of the deviations;

- means for comparing said average (γ) with a predetermined threshold value (γ ₀ ); and

- Means for triggering an automatic optimization of the regulation parameters if said average (γ) is greater than said predetermined threshold value (γ ₀ ).