FR3046669A1 - OPTIMIZATION OF THE DEFROSTING OF A HEAT EXCHANGER OF REFRIGERATING TRUCKS - Google Patents

Abstract

A method for managing the operation of a refrigerated transport truck for thermosensitive products, of the indirect injection type, in which the truck is provided with: at least one product storage chamber, a fluid reservoir cryogenic liquid nitrogen, - a heat exchanger system (1) in which circulates the cryogenic fluid, exchanger system comprising at least one internal heat exchanger to said at least one chamber, - and a air circulation system (2), for example of the type fans, able to put the internal air in contact with the chamber with the cold walls of the exchanger (s) internal (s) at said at least one chamber; characterized in that a defrosting operation of said exchanger is triggered when two temperature differences characterizing the exchanger (its nip and a monitoring indicator of the temperature of the air being blown) are greater than a given instruction during a time cumulated t1.

Description

The present invention relates to the field of processes for the transport and distribution of heat-sensitive products, such as pharmaceutical products and food products, in refrigerated trucks, and it is particularly interested in one of the techniques used in this type of truck, so-called "indirect injection" which implements one (or more) heat exchanger (s) (for example coils or finned coils), in which circulates a cryogenic fluid such as liquid nitrogen or liquid CO 2, the enclosure internal (cold room) to the truck is also provided with an air circulation system (fans) putting this air in contact with the cold walls of the exchanger, thereby cooling the air inside the chamber cold truck, the cryogenic fluid that feeds the exchanger (s) from a liquid cryogen tank traditionally located under the truck.

The present invention relates to a method for managing the operation of such a refrigerated transport truck of thermosensitive products.

The atmospheres maintained inside the cold room can be provided for both fresh products (typically a temperature of 4 ° C) that for frozen products (typically a temperature of -20 ° C). As an illustration, these exchangers or refrigerating units may consist of coil (s) exchanger (s) with surface extension type tubes / fins (continuous or independent fins), copper and / or aluminum, the battery being supplied with cryogen liquid, for example liquid nitrogen. This battery is for example placed in a box that guides the flow of air sucked by the fan mentioned above. The sucked air is thus cooled during its passage through the battery in contact with the cold fins and tubes fed with liquid nitrogen.

We then observe the following phenomenon: the humidity contained in the atmosphere inside the chamber, causes, in operation, the formation of a freezing fog and the formation of frost that accumulates on the tubes and fins of the battery . This frost layer which is deposited on the constituent elements of the battery forms a resistance (insulating layer) to the heat transfer between the hot air to be cooled and the cryogenic fluid, with a consequent decrease in the efficiency of these exchangers.

The consequence of this phenomenon is that in a conventional cycle of operation of these exchangers, for example more than 20 hours, several defrosting during this cycle will be necessary.

The present invention then relates more particularly to the defrosting sequence required by such exchangers, which integrates, on the one hand, its triggering with the associated parameters, and on the other hand the actual deicing phase which consists in removing or melting the frost accumulated on the exchanger.

One of the de-icing solutions listed in the literature is de-icing by means of electrical resistances placed near the heat exchanger or, more generally, by using heat generation, for example sending "hot" air towards the heat exchanger. exchanger.

Another solution described in the literature for de-icing the exchanger consists in applying a mechanical action on the exchanger, such as for example vibrations, so as to detach and drop the frost formed on the surface of the exchanger.

The present invention relates more particularly to the first mode of defrosting (heat input). The duration of a defrost cycle is an important parameter vis-à-vis the overall objective of maintaining the temperature of the body to the setpoint, typically close to 4 ° C or close to -20 ° C depending on the products considered .

Indeed, during the defrosting period and therefore the power of the electrical resistors, the injection of cryogen and therefore the exchanger must be stopped. As a result, the temperature of the body will increase during the defrosting time. It is therefore important to minimize the duration of defrosting as much as possible. Another interest in minimizing this phase is related to the electrical consumption of such deicing because the operation and control of the cryogenic exchanger or exchangers is provided by electric batteries or accumulators.

Moreover, in order to guarantee a good efficiency of the defrosting and to allow the heat exchanger to regain its initial efficiency, it is necessary to stop the power supply of the electric resistances at the right moment, ie neither too early because then the defrosting would not be enough effective, nor too late for the reasons mentioned in the previous paragraph.

One of the objectives of the present invention is therefore to propose a strategy for optimizing the "defrosting sequence", that is to say triggering it, and preferably also stopping it at the appropriate moment.

As will be seen in more detail in the following, according to the invention, the triggering criteria are based on taking into account the following temperatures: - Tint: the air temperature at the inlet of the exchanger (which returns substantially to the internal temperature of the chamber); Fluid outlet: the temperature of the refrigerant at the outlet of the exchanger (i.e the temperature of the cold vapors exiting the exchanger); and - blown air: the temperature of the air "blown" i.e the temperature of the air, become cold, after crossing the exchanger. and in particular the taking into account of two quantities linking some of these three temperatures.

It will be recalled here that, as is well known to those skilled in the art, two of the temperatures mentioned above characterize the "pinching of the heat exchanger": the "nip" of the heat exchanger represents the difference between the temperature of the heat exchanger. the air at the inlet of the exchanger and the temperature of the refrigerant at the outlet of the exchanger (ie the temperature of the cold vapors leaving the exchanger), ie the difference Tjnt "TSorte fluid

In general, depending on the conventional operating mode imposed on the exchanger using the known process control algorithms, the pinch is limited to a value close to 15K, mainly in order to maintain a level of high energy efficiency. In practice, when this pinch value approaches 15K, the system will act on the cryogenic flow control valve reaching the exchanger to limit it so as to maintain a nip less than or equal to 15K (control mode pinch).

And the system is generally sized so that the exchanger has a pinch called "natural", that is to say with the cryogen flow control valve open at 100%, for example between 5 and 10K.

At the beginning of the operation of the exchanger, that is to say when it is not fouled by frost, it can deliver its maximum power with a pinch Tjnt "TSorte fluid measured between 5 and 10K according to the dimensioning.

But in operation, opening after opening the truck doors, frost will accumulate on the heat exchanger with the consequence of creating a thermal resistance that will reduce its effectiveness. Therefore, the value of the nip will gradually increase to approach 15K, resulting in a control action of the exchanger in "pinch regulation" (limiting the cryogen flow to have a nip less than or equal to 15K).

The cumulative time during which the exchanger is controlled in "pinch regulation" mode is therefore an indicator of the fouling condition of the exchanger by frost. In practice, by cumulating the time when the pinch

Tint - Fluid outlet is greater than for example 13 ° C (value of the maximum pinch of which one subtracts a margin) one can know the state of fouling of the exchanger and trigger its defrosting after a period t, t being for example determined by a series of tests.

In addition, the frost setting of the exchanger creates a restriction of the free passage for air circulation through the battery, with additional losses that will lead to a decrease in air flow. The air blown out of the exchanger, will therefore become increasingly cold as the decrease in air flow, so the frost setting of the exchanger. The follow-up in time of the blowing temperature (blown Tajr) is therefore a second indicator of the state of fouling of the exchanger by the frost. In practice, it is therefore conceivable to follow the temperature difference Tjnt - Tajr soufflé

The temperature measurement probe Tajr blown advantageously positioned in the path where the air undergoes the greatest pressure losses, in other words on the trajectory where the majority of ice formation is formed so as to have increased sensitivity.

A second interesting indicator for triggering the defrost is therefore the monitoring of the temperature difference Tjnt - Tajr souffé - Defrosting will then be advantageously triggered when this temperature difference will be greater than a given setpoint, ConSTint - blown air. for a given time t, t and ConSimt - blown air being parameters determined for example by means of tests.

In summary, the present invention recommends, and it is quite new compared to deicing procedures listed in the literature, to take into account two indicators for triggering the defrost: - the tracking of the pinch: Tjnt "TSortie fluid, and more preferably cumulative time where the nip is greater than a set point

ConSpincement, for example at 13 ° C, which orders the initiation of the defrost when the nip is greater than this setpoint for a given time t, t being for example determined by a series of tests. and monitoring the temperature of the blown air, and more preferably the temperature difference Tjnt-Tajr blown, which orders the initiation of the defrosting when this difference is greater than a given setpoint, ConSiint - blown air, for the duration given t.

One can think that the interest of following these two differences, in combination, resides in particular in the following aspects: - the first difference (pinch) informs on the efficiency of the exchanger and in particular informs us on the moment where the exchanger is charged with frost over a large part of its exchange surface, which impacts both the aeraulic exchanger and the transfer of the cold to the air flowing through the exchanger, where a power reduced refrigerant (TSortized fluid rises). the second difference (blown air) informs, for its part, on a characteristic phenomenon of cryogenic cryogenic / humid air exchangers, that of the production of a freezing fog which, once produced, blocks the air passages and alters cancels the cooling capacity. - The monitoring of both factors is particularly advantageous to detect the right moment to trigger a defrost.

To better understand the invention, let us refer to FIG. 1, which illustrates an example of an exchanger configuration, with the location of the temperatures taken into account: reference 1 denotes a battery-type exchanger tube and fins. - The reference 2 designates a fan forcing the air (the air to be cooled, i.e of the refrigerating chamber of the truck) to pass through the exchanger. - Reference 3 designates a possible position for the temperature measuring probe Tjnt (air inlet of the exchanger). the reference 4 designates the fins. reference 5 denotes the arrival tube of the cryogen. - Reference 6 designates a possible position for the probe for measuring the temperature Tajr blown (air blown out of the exchanger). - The reference 7 designates the return tube of the refrigerant. the reference 8 designates a possible position for the probe for measuring the fluid temperature.

An example of an algorithm chosen according to the invention is described below to trigger the defrosting of the exchanger: ~> (Tint - Fluid outlet> ConSpincement) and (T jnt - Blown air> Consent - Blown air) for a time t1

With below examples of parameters determined by test campaigns:

- ConSpjncement = 13 ° C - Consent - Shortness of breath - 40 ° C -11 = 20 mn

The appended FIG. 2 illustrates an example of application of this algorithm on a test with 3 door openings of the truck.

As can be seen in FIG. 2, this test comprises a first cooling phase of the refrigerated box from ambient temperature to about -20 ° C. (set temperature of the frozen products), which takes about 1 hour, then stabilization phase at -20 ° C for about 1h30.

A first door opening (OP1) is performed at t = 2h30, then a second (OP2) at t = 3h06, then a third (OP3) at t = 3h46.

It may be noted that during the cooling phase occurring after each door opening, the temperature difference Tjnt - Tajr blown increases with the number of door openings: -Zone A: between 20 ° C and 30 ° C after OP1-zone B: between 40 ° C and 50 ° C after OP2 - zone C: greater than 50 ° C after OP3.

This can be explained by a decrease in the blowing temperature opening after door opening related to the frost setting of the exchanger (decrease in air flow).

Moreover, the zones D (after OP2) and E (after OP3) represent two periods during which the exchanger is controlled in "pinch regulation" mode, which means a loss of efficiency of the exchanger linked to the training. of frost.

With the algorithm defined above, the defrost is triggered at t = 4:18 (F mark). This triggering takes place at a very relevant moment insofar as one observes after the third door opening a significant decrease in the rate of descent in temperature, and difficulties to reach the set temperature (-20 ° C).

Another aspect of the invention relates to the actual deicing phase, that is to say the phase during which we will seek to remove the frost accumulated on the battery of the exchanger in order to allow it to find, as soon as possible, an efficiency close to its initial effectiveness.

As indicated above, while the heat exchanger is in defrost mode, it is stopped and can no longer produce cold. The duration of the defrosting phase should therefore be as short as possible so that the exchanger can operate again.

A compromise must therefore be found between the duration of deicing and its effectiveness.

The defrosting strategy to optimize its duration and its effectiveness, is based firstly on a positioning of the resistors adapted to the distribution of frost on the battery and secondly, on a temperature measurement with a probe placed in the battery in a place where frost will accumulate mainly. Generally, it is observed that the accumulation of frost on the battery is not homogeneous, it tends to deposit preferentially on the coldest parts of the battery, that is to say in this case, in the zone of arrival of the liquid nitrogen, on the first tubes / fins where the cryogen, for example liquid nitrogen, will vaporize.

Also, when we are in conditions where there is formation of a freezing fog in the cold box, micro-droplets of liquid water in the state of supercooling will solidify when they will hit an obstacle such as the edge of attack the vanes of the battery. The entrance of the battery will thus gradually clog up to form a wall of frost.

Frost fog is considered to be an atmosphere of between 0 ° and -40 ° C, saturated or not saturated with moisture and loaded with microdroplets of liquid water in a state of supercooling. This state allows the droplets to freeze instantly when they hit an obstacle, allowing frost formation on the leading edges of objects subject to airflow (for example: suction grilles, fan blades, fins edges of the exchangers). This phenomenon can foul and "choke" forced convection heat exchangers in minutes, making them inefficient.

Consider the example of the exchanger shown in Figure 3 attached, with an arrival (9) of the refrigerant on the upper tubes of the battery (the fluid outlet, gaseous, intervenes with reference 10), the frost will preferentially be deposited on the upper part of the battery and also, in the case where we have the formation of a freezing fog, on the leading edge of the fins. Frost will therefore be deposited preferentially according to the zone denoted 11 in the figure.

According to the invention, a temperature sensor that can indicate the defrost stop (with the aid of the resistors 13) can be positioned with reference 12, ie close to the arrival of the refrigerant, here for example between the tubes of liquid nitrogen, and 5 to 20 cm inside the battery. This temperature measurement will make it possible to detect the end of the defrosting phase at the appropriate time.

As the heat exchanger operates, the battery will progressively charge in frost until it completely covers the probe 12. At the moment when the defrosting will be triggered, according to the criteria defined above, the temperature indicated by the probe will be very low, of the order of -100 ° C in the example considered, since it is taken in the frost mass, the frost mass being in contact with the tubes in which circulates liquid nitrogen to a temperature between -180 and -196 ° C.

Once the defrost is triggered, the resistors will gradually increase in temperature and provide the necessary energy to first heat the frost then melt. The evolution of temperature "12" during defrosting can be broken down into 3 phases: -. The first phase (A) where the temperature will gradually increase corresponds to the heating of the frost. - The second phase (B), during which the temperature reaches a level close to 0 ° C, is related to the liquefaction of the frost. - Finally, the beginning of the third phase (C), where the temperature will increase again, coincides with the end of the frost melting and thus the end of the defrosting stage.

Also, according to the invention, a defrost with an optimized time will be automatically stopped when the probe temperature 12 becomes positive.

It is therefore noted that according to advantageous embodiments of the invention: the instruction CONSpmcement is between 5 and 40K, and more preferably between 10 and 15K. - The ConSTint instruction - Blown air is between 10 and 40K, and more preferably between 10 and 20K. the cumulative time t1 is between 10 and 50 minutes, and more preferably between 10 and 20 minutes.

Claims (5)

claims 1. A method for managing the operation of a refrigerated transport truck for thermosensitive products, of the indirect injection type, in which the truck is provided with: - at least one product storage chamber, - a supply reservoir of one cryogenic fluid such as liquid nitrogen or liquid CO2, - a heat exchanger system (1) in which the cryogenic fluid circulates, the exchanger system comprising at least one heat exchanger internal to the at least one chamber, as well as an air circulation system (2), for example of the fan type, capable of bringing the internal air of the chamber into contact with the cold walls of the exchanger (s) internal to said at least one bedroom ; characterized by the implementation of the following measures: a) temperature probes (3, 6, 8) are available capable of measuring the following temperatures: - Tint: the temperature of the inlet air of the exchanger; - Fluid exhaust: the temperature of cold vapors leaving the exchanger; and - blown air: the temperature of the air "blown" i.e the temperature of the air, become cold, after crossing the exchanger. b) and a defrosting operation of said exchanger is ordered when the following conditions are reached: - the nip of the exchanger: Tjnt - the fluid outlet, is greater than a given setpoint CONSpmcement, for a cumulated time t1, and - l Temperature difference Tjnt - Tajr blown, is greater than a given set point ConSimt-Tair blown, during the cumulative time t1. 2. Method according to claim 1, characterized in that the instruction CONSpmcement is between 5 and 40K, and more preferably between 10 and 15K. 3. Method according to claim 1 or 2, characterized in that the setpoint ConSimt - blown air is between 10 and 40K, and more preferably between 10 and 20K. 4. Method according to one of the preceding claims, characterized in that said cumulative time t1 is between 10 and 50 minutes, and more preferably between 10 and 20 minutes. 5. Method according to one of the preceding claims, characterized in that one puts an end to the deicing operation thus triggered, as follows: - it has a temperature sensor (12) adapted to measuring the temperature of the air in the zone of the exchanger or between the cryogenic fluid; the temperature supplied by this temperature probe is monitored during the deicing operation; the defrosting operation is stopped when this temperature becomes positive again after having passed through the following phases: a first phase (A) occurring after the start of the deicing operation, phase where the temperature increases and which corresponds to the heating of the previously formed frost; a second phase (B), during which the temperature reaches a plateau close to 0 ° C., which phase is related to the liquefaction of the frost; - a third phase (C), where the temperature starts to increase, phase which is linked to the end of the frost melting.