EP3230663A1

EP3230663A1 - Method for managing the supply of cryogenic liquid to a truck for transporting heat-sensitive products

Info

Publication number: EP3230663A1
Application number: EP15808749.4A
Authority: EP
Inventors: Cécile CLEMENT; Patricia Privat; Maxime Lambert; Jonathan Macron; Thierry Duboudin
Original assignee: 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: 2014-12-10
Filing date: 2015-11-26
Publication date: 2017-10-18
Also published as: FR3030025B1; FR3030025A1; WO2016092177A1

Abstract

The invention relates to a method for managing the supply of cryogenic liquid to a truck (20) for transporting heat-sensitive products, and to a truck implementing a so-called indirect injection method wherein the liquid is sent into a heat-exchanger system (3) located inside the truck, characterised in that when the refrigerating system of the truck is started up, for example at the start of a round or after a prolonged halt of the refrigerating system for any given reason, a mode of rapidly lowering the temperature of at least one of said chambers as follows is adopted: determining the magnitude AT = T_int - T_cons(T_int: the internal temperature of the chamber, and T_cons: a setpoint value for said internal temperature) and, as soon as AT is lower than a high setpoint value AT_cons-H, ordering the closing or the reduction of the opening of a proportional valve (10) located downstream from the exchanger system of the chamber under consideration, the high setpoint value AT_cons-H being no higher than 20 °C, and preferably no higher than 15 °C.

Description

Method for managing the supply of cryogenic liquid of a truck for transporting heat-sensitive products

The present invention relates to the field of transport and distribution of thermosensitive products, such as pharmaceuticals and foodstuffs, in refrigerated trucks.

It is particularly interested in one of the techniques used in this type of truck, called "indirect injection" (technology called "CTI"), which implements one or more heat exchanger (s) in the inner chamber where the products are transported (also known as "chamber", "box", "box" isothermal ...), exchanger in which circulates a cryogenic fluid such as liquid nitrogen or liquid CO ₂ . This cryogenic fluid is conveyed from an onboard cryogenic tank, usually under the refrigerated truck, to one or more heat exchangers located inside the cold rooms of the truck, exchangers equipped with air circulation means. These exchangers allow the cooling of the internal air of the chamber storing the products, air surrounding the exchangers, to the desired temperature.

The heat extracted from the air allows, first of all, a complete evaporation of the cryogenic fluid flowing in the exchanger, then an increase in its temperature to a temperature close to that of the chamber. The cryogenic fluid exiting the exchanger is then rejected outside after giving up a maximum of cooling energy.

In a manner well known to those skilled in the art, solutions exist today to control the temperature of the air internal to the crate storing the products transported,

It may be recalled that the process control most often implemented for such trucks operating by indirect injection is as follows:

- When starting the refrigeration system of the truck (for example at the start of a tour or after a prolonged shutdown of the refrigeration system for any reason) or after an opening of door, we adopt a mode of "rapid descent" in temperature (this industry names this phase "pull-down");

- Once a set temperature has been reached in the product storage chamber, a control / regulation mode is adopted which makes it possible to maintain the temperature of the product storage chamber at the value of the setpoint ("hold" phase) .

The existing control modes are mainly based on control algorithms for the opening / closing of the cryogenic fluid supply valves of the exchanger or heat exchangers, or on taking into account the temperature differences between the inlet and outlet temperature. output of the fluid in and the exchanger.

However, one of the constant concerns of the person skilled in this field is to reduce the consumption of cryogen, for example of liquid nitrogen, of such refrigerated trucks cryogenically.

One of the objectives of the present invention is then to propose a new management of the cryogen supply of such an indirect injection process, in particular making it possible to optimize the quantity of cryogen (for example of liquid nitrogen) required for lowering the internal air temperature to the chambers at or below a required setpoint, and maintaining these conditions during the various required transport phases.

And as will be seen in more detail in the following, the present invention proposes the implementation, at the output of the circuit (downstream of the exchanger or exchangers) of one or proportional valves, and the piloting of this or some of these proportional valves in an optimized way, control which, one conceives it, has a direct impact on the consumption of cryogen and on the profile of temperature obtained.

These valves can be controlled using their opening rate, opening rate which can vary in a range from 0 to 100% (100% corresponding to the maximum rate of opening of the valve). And remarkably, the steering recommended here according to the invention proposes, to some moments and under certain conditions combined, to deliberately degrade the temperature conditions in the storage space and thus voluntarily agree to move away from the target temperature initially targeted. It can then be considered that with respect to the conditions of regulation traditionally practiced, as mentioned above, the invention provides the user / operator of the truck a mode of regulation that can be described as "ECO mode , Additional mode, and that the operator or the system can implement instead of the usual mode (in accordance with the prior art) when he deems it adequate or acceptable.

The invention thus relates to a method for managing the cryogenic liquid supply of a truck for transporting thermosensitive products, of the type in which the truck is equipped with:

at least one product storage chamber,

a reserve of a cryogenic fluid such as liquid nitrogen,

a system of heat exchanger (s) internal to said at least one chamber, in which circulates the cryogenic fluid,

an air circulation system, for example of the fan type, capable of bringing the air inside the chamber into contact with the cold walls of the heat exchanger system,

temperature sensors capable of determining the temperature of the atmosphere internal to said at least one chamber (Tj _n t);

- as well as a management and control unit, able to regulate the internal temperature Tj _nt to a set value T _con s by ordering a closing or opening, or the degree of such opening / closing, a proportional valve located downstream of the exchanger system of a chamber considered;

characterized in that whatever the value of the magnitudeAT = Tint - T _CO ns. said management and control unit orders the maintenance of the opening of said valve located downstream of the exchanger system of the chamber in question at a rate of less than 80%. As will be clear to those skilled in the art, to the extent that the power of the exchangers developed, and therefore the consumption of cryogen, is generally much greater than the need of the application when they are installed on a truck of "average" size , the invention introduces a voluntary limitation of the opening of the proportional valve in "standard mode", regardless of the deviation from the setpoint. This operating mode is therefore similar to a deliberately degraded mode of operation of the exchanger with the particular aim of minimizing the consumption of cryogen, while maintaining cold performance that can be described as sufficient.

In accordance with the invention, said unit is of course able to overcome this voluntary limitation / degradation and thus to restore the full power of the exchanger with an opening rate greater than 80% and in particular being in the vicinity of 100% as it is traditionally practiced, when the operator deems it necessary for various reasons including the environment of the truck.

According to one of the preferred embodiments of the invention, during the startup of the truck refrigeration system, for example at the start of a tour or after a prolonged shutdown of the refrigeration system for any reason, or after a door opening, a mode of rapid descent in temperature of at least one of said chambers is adopted as follows:

the magnitude ΔΤ = Tj _n t - T _CO ns is determined;

and in that as soon as ΔΤ is lower than a high setpoint value ATcons-H, it is ordered to close or reduce the opening of said valve located downstream of the exchanger system of the chamber in question, the setpoint value high AT _CO ns-H being less than or equal to 20 ° C, and preferably less than or equal to 15 ° C.

According to one of the embodiments of the invention, in the case of frozen products, as soon as ΔΤ is below a high set point freezing AT _CO ns-H-con _g one orders the closure or the decrease of the opening of said valve located downstream of the exchanger system of the chamber in question, AT _CO ns-H-con _g being greater than or equal to 10 ° C, preferably greater than or equal to 15 ° C, for example equal to 15 ° C ± 5 ° C.

According to another of the embodiments of the invention, in the case of fresh products, as soon as ΔΤ is lower than a high fresh charge reference value ATcons-H-charges, it is ordered to close or reduce the opening of said valve located downstream of the exchanger system of the chamber in question, ATcons-H-charge being greater than or equal to 2 ° C, preferably greater than or equal to 5 ° C, for example equal to 5 ° C ± 3 ° C.

By way of illustration, to better visualize what the invention proposes, consider the case of frozen products, for which the required set point in the chamber is generally close to -20 ° C., a high freezing set point AT _CO ns-H-con is adopted. _g of 15 ° C ± 5 ° C, and therefore typically from an internal temperature of -5 ° C (or even from an internal temperature of the room close to 0 ° C) it is agreed to reduce the flow supplying the exchanger, without wait to get closer to -20 ° C (as does the prior art) and thus accepting the induced temperature degradation and in particular the degradation of the kinetics of descent in temperature.

Still for illustrative purposes to better visualize what the invention proposes, consider the case of fresh products, for which the deposit required in the room is generally close to 4 ° C, we adopt a high charge AT _CO ns-H-charge from 5 ° C ± 3 ° C, and therefore typically from 9 ° C (or even from an internal temperature to the room close to 12 ° C) it is agreed to reduce the flow supplying the exchanger, without waiting to get closer to 4 ° C (as does the prior art) and thus by accepting the induced temperature degradation and in particular the degradation of the kinetics of descent in temperature. In practice, temperature drop-off kinetics are observed in this field during the "pull down" phase of the order of 0.6 ° C. to 1.2 ° C./min. According to the present invention, this kinetics can be degraded in favor of consumption.

As will be understood from the foregoing, the system thus applies, below the high setpoint AT _CO ns-H (which is understood to be high compared to the limits causing a reaction according to the previously used modes of regulation) a reduced opening rate, compared to what it was before (Tauxo, in general the opening rate was after such an event according to the prior art to its maximum or close to its maximum , for example in the range of 80-100%) and thus the new (reduced) rate can be expressed as follows:

Or

- Rate 1 = a Rateo

To illustrate the foregoing, Table 1 below gives an example of implementation of the invention by providing the new rate of opening Rate 1 of the valve as a function of ΔΤ with a coefficient a1 which is a fixed coefficient = 100/15:

AT m Rate 1

15,100

14.5 96.7

14,933

13 S6.7

12 N / A

11 73.3

10 66 _f 7

S 60.0

S 533

? 46.7

6 40.0

S 33.3

4 26 _f 7

3 20.0

2,133

1 6.7

0 0.0

Table 1

According to one of the embodiments of the invention, the unit takes into account besides the magnitude ΔΤ = Tj _n t -T _CO ns, a notion of operating time of the cold supply system between the last opening of the truck doors and a new opening of these doors. For this, the unit evaluates for example the time elapsed in the closed position, that is to say the time elapsed between the last opening of the doors of the truck and a new opening of these doors, for example by the use of a timer (stopwatch, timer). The longer this time is, the more the opening rate of the valve will be reduced after restarting the system.

According to this mode of implementation, the system will therefore apply, below the high setpoint AT _CO ns-H a reduced opening rate (Rate 1), compared to what it was previously (Tauxo) s 'expressing as follows:

- Rate 1 = βι α _χ ΔΤ

Or

- Rate 1 = β a Rateo the factor β1 (or β) depending on the elapsed time as mentioned above.

To illustrate the above, Table 2 below gives an example of implementation of the invention by providing the values of the coefficient β1 adopted as a function of the elapsed time ("timer" = TE) with:

β1 = (1- (TE * 1/3) / 100)

^" Wf (l E} β1

5 1

10 0.97

15.95

20 0.93

25 0.92

30 0.90

35 0.88

40 0.87

45 0.85

50 0.83

55 0.82

60 0.80

65 0.80

70 0.80 Table 2

Here we have adopted conditions where we decide to restrict the "timer" (TE) over a range of 10 to 60 minutes: in other words if TE> 60 minutes β1 remains unchanged and equal to what it was at 60 minutes.

The question of "resetting" the "timer", which is otherwise familiar to those skilled in the field of process controls (for example, automatically resetting during the extended stopping phase of the truck), will not be developed at length. .

According to another of the embodiments of the invention, the unit takes into account besides the magnitude ΔΤ = Tint-T _CO ns, a notion of number opening of doors during a given period (for example since the end of the previous period of "pull down"). According to this mode of implementation, the system will therefore apply, below the high setpoint AT _CO ns-H a reduced opening rate (Rate 1), compared to what it was previously (Tauxo) s 'expressing as follows:

- Rate 1 = γ1 ¾ ΔΤ

Or

- Rate 1 = there is Rateo

Or, if it takes into account the two aspects of time elapsed between two openings of doors and numbers of openings occurring over a given period:

or

- Rate 1 = γ β a Rateo

The fact that the person skilled in the art is otherwise familiar with the systems that enable him to detect an opening of doors on such trucks will not be developed here at length:

for example by a door contactor (which makes it possible to emit a signal and thus to increment an aperture number counter);

- The system can also count the number of door openings by measuring the rise in indoor temperature in the box and the lowering temperature of the latter when it closes. And the identification of an extended stop can be effected by the difference in temperature between the inside temperature of the body and the outside temperature; if this difference is close to 5 ° C for example, we can consider that we are in the case of a prolonged stoppage and from this observation order the resetting of the counter. To illustrate this embodiment, the following is an example of implementation carried out under the following conditions: - We take into account a door opening counter that can vary from 1 to 12 (beyond 12 door openings the variable will remain locked on 12).

the closure of the valve is limited by 2% per door opening, which is illustrated in Table 3 below which gives the corresponding values of the coefficient γ1 (γ1 = 1 - (counter ^* 0.02), if Counter> 12 then γ1 remains at the value adopted for the counter equal to 12).

Table 3

Other features and advantages of the present invention will appear more clearly in the following description, given by way of illustration but in no way limiting, in connection with the appended drawings for which:

- Figure 1 is a partial schematic representation of a CTI installation according to current practice (prior art).

Figure 2 is a schematic representation of the inner body to a transport truck according to the prior art, here comprising two product storage chambers, and in particular to better visualize the operation of the exchangers and the position of temperature sensors T1.

Figure 3 is a partial schematic representation of a CTI installation according to the present invention.

FIG. 4 shows the temperature curves (temperature difference ΔΤ = T i _n t - T _con s) and of consumption over time for two different control modes, based on two maximum opening rates of the proportional valve (Tmaxl <Tmax2).

Figure 5 shows the opening rate of the valve - in% of the maximum opening rate - as a function of the ΔΤ (temperature difference between the indoor temperature and the set temperature), for two different values of the maximum rate.

Figure 1 hereinafter appended, is a partial schematic representation of an indirect injection installation according to current practice (prior art).

The regulation of the quantity of cryogen, for example of liquid nitrogen, supplying such a CTI process (chamber internal to the truck, equipped with exchangers 3) is today made using at least two valves all or nothing (TOR) 1 and 6, an input and an output, the method then comprises at least the following elements, seen in the following order: a liquid nitrogen tank (not shown in Figure 1), a valve TOR 1 input, normally closed, which allows the supply of cryogen, for example nitrogen, the circuit;

means for distributing the liquid nitrogen (for example of the clarinet type, "2" in the figure),

heat exchangers 3 internal to the truck,

a clarinet 4 for collecting the nitrogen gas leaving the exchangers,

a pressure sensor 5,

a digital valve 6 output, normally open,

a pipe of given diameter which connects these elements.

In room 20 we find moreover:

ventilation systems (not shown in the figure for the sake of clarity but better visualized in the context of Figure 2 attached) positioned at the exchangers whose flow rates are regulated, to intensify the heat exchange between l ambient air of the chamber and exchangers (by sucking the air through the exchangers and forcing it to be in contact with the exchangers) and to homogenize the temperature of the air internal to the chamber.

A temperature sensor (T1) controls the opening and closing of the digital input valve 1; it is located for example at the entrance of the air path in the exchangers and measures the air temperature of the chamber before cooling in the exchangers.

For each additional chamber, a new supply circuit comprising, for example, a normally closed input digital valve, heat exchangers, a normally open digital output valve, etc. (an example of a two-chamber and position of the temperature probes is shown in Figure 2 attached).

Refrigeration in the previous TOR mode typically takes place in two phases:

1 - At start-up or after a door opening, we adopt a rapid temperature descent mode.

2- Once the set temperature has been reached (probe T1 in the chamber), a control / regulation mode is adopted which makes it possible to maintain the temperature of the chamber at the value of the setpoint.

The operation of the CTI process in this discrete mode is typically the following: when the measured temperature T1 is greater than the set temperature, the inlet valve 1 opens (the outlet valve 6 is already open by default) thus allowing the supply of exchangers in cryogen. The liquid nitrogen transforming into gas releases frigories which are absorbed by the air in contact with these exchangers. The fans recover this cooled air to circulate it in the room. Nitrogen gas is then released outside the chamber into the surrounding atmosphere. When the measured temperature T1 reaches the set temperature, the inlet valve 1 closes, thus stopping the supply of the exchangers in cryogen and thus the cooling of the air internal to the chamber. The reduction of the temperature of the chamber and its maintenance are obtained by opening and closing cycles of the valve 1. The frequency and duration of opening of the valve 1 will be higher during the fast descent phase than during the control / regulation phase. When the valve 1 opens, whatever the phase considered, the rate of cryogen introduced into the heat exchangers will depend solely on the nitrogen pressure of the tank and the pressure drops of the various components of the installation. Consequently, this flow rate of cryogen is related to the design of the system and is, for a given installation, identical to each valve opening and this whatever the phase of the process.

In other words, the flow rate of nitrogen not being adjustable, the amount of nitrogen is not optimized; which leads to overconsumption of nitrogen.

This discontinuous flow of nitrogen and the opening and closing reaction time of the valve also lead to a high amplitude of the air temperature of the chamber; which is not satisfactory.

In addition, when the inlet valve 1 is closed, the nitrogen which is upstream of this valve, heats up and leads to an increase in the pressure of the tank. When the inlet valve opens again, some of the nitrogen will be used to cool the nitrogen supply line; which reduces the thermal efficiency of the evaporators. FIG. 2 makes it possible to better visualize the detail of a present example of an internal box to a transport truck (in view of side), here comprising two product storage chambers (for example a room for fresh products and another chamber for frozen products), and in particular to better visualize the operation of the heat exchangers and the position of temperature sensors T1 for the mode exemplified here.

For each chamber, upstream of a normally closed ("NC") inlet digital valve, each chamber is equipped with heat exchangers (vertical for chamber 1, horizontal at the top of chamber for chamber 2), where the cryogen circulates from the tank located under the truck, the gas streams obtained at the outlet of each chamber are sent to a collection pipe, here provided with a single normally open ("NO") exit valve. And here is clearly shown an embodiment where in each chamber there is a temperature sensor (T1) which manages the opening and closing of each digital input valve; she is placed :

for the chamber 1 at the inlet of the air path in the exchangers (the fans 21 being situated on the other side of the exchangers and drawing towards them the air through the exchangers), the probe thus measuring the temperature of the air in the chamber before cooling in the exchangers;

- For the chamber 2 here again at the entrance of the air path in the exchangers considered i.e. substantially at the level of the fans 21 which here push the air inside the exchangers.

FIG. 3 illustrates, in partial view, an embodiment according to the invention, with the following elements:

a liquid nitrogen tank (not shown in FIG. 3), - a normally closed digital inlet valve 1 which allows the supply of cryogen, for example nitrogen, of the exchanger system 3 (constituted for this mode implementation of several vertical exchangers in parallel, but this is only one of the many configurations of exchangers commonly used in this industry);

a means for distributing the liquid nitrogen (for example of the clarinet type, "2" in the figure),

the evaporators 3 (or heat exchangers) internal to the truck,

a clarinet 4 for collecting the nitrogen gas leaving the exchangers,

a pressure sensor 5,

a proportional analog valve 10, normally open, which allows the opening, closing and regulation of the feed of the exchangers 3;

- A digital valve 1 1 output (downstream of the proportional valve), normally open, output digital valve that is only optional according to the invention.

a pipe of given diameter which connects these elements. We will not rethink here the ventilation systems positioned at the exchangers and the presence of the temperature sensor (T1) able to measure the temperature of the air internal to the product storage chamber.

According to the invention, the power management is based on the percentage of opening of the proportional valve 10, as a function of the air temperature of the chamber (Tj _n t) and the desired target temperature. (Tset) -

During a phase of rapid rise in temperature (due to a door opening for example), the measured temperature (Tint) is much higher than the setpoint (T _CO ns), the proportional valve 10 is then commanded to to open (aperture percentage which is then generally close to 100%), the exchangers 3 are then supplied with nitrogen with a maximum flow rate and releases frigories which are absorbed by the air of the chamber (phase of

"Pull down" or fast descent). Then, as Tj _n t approaches T _CO ns _> the system examines the positioning of the magnitude ΔΤ = Tj _n t - Tcons for, as soon as ΔΤ is lower than a high setpoint AT _CO ns -H order closing or decreasing the opening of the valve 10 located downstream of the exchanger system of the chamber in question.

Without it being necessary to detail further, data acquisition and processing means (for example an automaton) are used here, to acquire all the necessary data (and in particular the pressure, temperature, internal to the chamber etc ..) and to feedback by giving orders to the system, in particular to close such or such valve, or to vary the rate of opening of the valve 10. The attached figure 4 shows the temperature curves ( temperature difference ΔΤ = Tj _n t - T _con s) and consumption over time (phase lowering of temperature, then stabilization and opening of doors) for two different modes of regulation, based on two maximum rates of opening of the proportional valve (Tmax1 and Tmax2, Tmax1 <Tmax2): the curves in thick line show the observation ΔΤ for these two valve opening rates while the thin lines show the associated cryogen consumptions.

A difference in temperature profile and a difference in consumption (a reduction of consumption in this case) are then clearly observed when the regulation adopts a reduced rate of opening (Tmax1 reduced compared to Tmax2).

FIG. 5 shows the opening rate of the valve - in% of the maximum opening rate - as a function of the ΔΤ (difference in temperature between the inside temperature and the set temperature), for two different values of the maximum rate, Max rate 1 and Max rate 2, in the case of regulating a fresh product storage chamber.

It should be noted that on this graph, the abscissa represents the difference in temperature between the internal temperature of the chamber and the set temperature, a difference which is reduced to% knowing that 100% corresponds to a temperature delta of 15%. ° C.

We then note in this figure the following points:

in the case of the maximum rate 1, Tmax1, the regulation is not sufficiently adjusted to allow the flow rate to be reduced (the aperture rate applied is always at the maximum rate),

- while with the maximum rate 2, Tmax2, the regulation has been modified to allow a variation in the opening rate of the valve and therefore a reduction in consumption.

Claims

claims

1. Method of managing the cryogenic liquid supply of a truck for transporting thermosensitive products, of the type where the truck is equipped with:

at least one chamber (20) for storing the products,

a reserve of a cryogenic fluid such as liquid nitrogen,

a heat exchanger system (3) internal to said at least one chamber, in which the cryogenic fluid circulates,

an air circulation system, for example of the type of fans (21), able to put the air inside the chamber into contact with the cold walls of the heat exchanger system,

temperature sensors (T1) able to determine the temperature of the atmosphere inside said at least one chamber (Tj _n t);

- as well as a management and control unit, able to regulate the internal temperature Tj _nt to a set value T _con s by ordering a closing or opening, or the degree of such opening / closing, a proportional valve (10) downstream of the exchanger system of a chamber considered;

characterized in that regardless of the magnitude value ΔΤ =

Tint - T _CO ns. said management and control unit orders the maintenance of the opening of said valve located downstream of the exchanger system of the chamber in question at a rate of less than 80%.

2. Management method according to claim 1, characterized in that said unit is able to overcome this voluntary limitation and thus to restore the full power of the exchanger with an opening rate greater than 80%, and especially in the neighborhood of 100%.

3. Management method according to claim 2, characterized in that at the start of the refrigeration system of the truck, for example at the start of a tour or after a prolonged shutdown of the refrigeration system. for any reason, or after a door opening, a mode of rapid descent in temperature of at least one of said chambers is adopted as follows:

the magnitude ΔΤ = Tj _n t - T _con s is determined;

- and as soon as ΔΤ is lower than a high setpoint value AT _CO ns-H it is ordered to close or reduce the opening of said valve located downstream of the exchanger system of the chamber in question, the high setpoint ATcons-H being less than or equal to 20 ° C, and preferably less than or equal to 15 ° C.

4. The management method as claimed in claim 3, characterized in that the chamber in question carries frozen products and in that as soon as ΔΤ is lower than a high freezing setpoint value AT _CO ns-H-cong the closure or reducing the opening of said valve located downstream of the exchanger system of the chamber in question, AT _CO ns-H-con _g being greater than or equal to 10 ° C, preferably greater than or equal to 15 ° C, for example equal to 15 ° C +/- 5 ° C.

5. The management method as claimed in claim 3, characterized in that the chamber in question carries fresh products and in that as soon as ΔΤ is lower than a high fresh charge reference value AT _CO ns-H-charges the closure or decreasing the opening of said valve located downstream of the exchanger system of the chamber in question, AT _CO ns-H-cool being greater than or equal to 2 ° C, preferably greater than or equal to 5 ° C, for example equal to at 5 ° C +/- 3 ° C.

6. Management method according to one of claims 2 to 5, characterized in that the unit takes into account, besides the magnitude ΔΤ = Tj _n t -

Tcons, a parameter of operating time of the cryogenic fluid supply between the last opening of the doors of the truck and a new opening of these doors, by the evaluation of the time elapsed in the closed position, that is to say the time elapsed between the last opening of the doors of the truck and a new opening of these doors.

7. Management method according to one of claims 2 to 6, characterized in that the unit also takes into account a parameter of the number of door openings made during a given period of the tour of the truck, for example since the end of the fast descent period that occurred earlier.