EP0730719B1 - Refrigerating exchanger, method for controlling same, and refrigeration facility comprising said exchanger - Google Patents

Abstract

PCT No. PCT/FR94/01401 Sec. 371 Date Jul. 2, 1996 Sec. 102(e) Date Jul. 2, 1996 PCT Filed Nov. 30, 1994 PCT Pub. No. WO95/15467 PCT Pub. Date Jun. 8, 1995A refrigerating exchanger (1) for a refrigeration facility such as a cold storage room or a refrigerated display case, including a refrigerant or heat-removing fluid circulating assembly (5) connected to a heat exchanging assembly (6) with a large surface area. The assemblies (5, 6) form an array of several layers (7) and several rows (8) in the form of a plurality of mutually similar parallel members (9) separated by spacers (10) for providing heat insulation and collecting defrosting runoff water. Each member (9) comprises at least one layer (7) as well as a respective inlet (11) and outlet (12) forming part of the circulating assembly (5). At least three members (9) of the array each have a refrigerating capacity that is a fraction of the overall standard rating of the exchanger (1) so that at any one time during the operation of the exchanger (1), at least one of the members (9) can be in defrosting mode while at least two of the members (9) are in refrigerating mode, the members (9) being in turn in defrosting mode and in refrigerating mode.

Description

The invention relates to a refrigeration installation according to the preamble of claim 1 and a method of controlling such an installation refrigerator.

The refrigeration system can be a cold room, a freezing room or deep freezing, a refrigerated sales unit or a display case refrigerated or similar.

Such an installation comprises a body which delimits an enclosure inside which a given temperature - low - must be maintained. In this Indeed, there are provided air circulation means suitable for circulate, in this enclosure, air and at least a refrigerant exchanger through which a fluid flows refrigerator or refrigerant.

Such an exchanger is usually presented under the form of a battery having several layers and several ranks.

According to document US-A-5 031 413, which discloses the characteristics of the preamble of claim 1, the battery is arranged in two elements similar to each other, mounted in parallel, separated from each other by partitions. Each element has its own air flow and its own fan. This structure allows to maintain a degree satisfactory humidity, the evaporators operating always alternately.

GB-A-2 164 133 also relates to a structure tending to maintain a hygrometric degree satisfactory in a refrigerated display case. The structure includes two evaporators mounted in parallel, one providing a cooling function, while the other provides a humidification function.

According to document US-A-4 373 353 there is described a air conditioning system comprising a plurality evaporators so as to avoid overloading the compressor.

The invention aims to ensure defrosting an exchanger of a refrigeration installation while, simultaneously, now in the enclosure to which is associated this installation a temperature of acceptable refrigeration and meets required requirements and avoiding having a cooling capacity in excess of needs.

Indeed, it should be avoided that the defrosting lead to a rise in temperature which, if it is too large, is a disadvantage especially if the products inside the enclosure are sensitive to thermal point of view.

Furthermore, it is desirable, for reasons that the cooling capacity of the installation does not go too far beyond actual needs.

To this end, the invention proposes, according to a first aspect, a refrigeration installation as it was previously mentioned, containing the characteristics of the characterizing part of the claim 1.

According to another characteristic, the exchanger, which includes heat exchange means, further comprises air circulation means on the exchange means thermal arranged to form a common air flow to the inlet and a common air flow at the outlet of the drums.

According to another characteristic and a variant of realization the exchanger also includes means selective air circulation or non-circulation on the heat exchange means of each element so air circulation is allowed for some elements and prohibited for others.

These selective means are associated with means of ordered. Air circulation is permitted for elements in cold production situation and prohibited for elements in a defrost situation. Means of heating of the heat exchange means for each of the elements are planned so as to be active for a element for which air circulation is prohibited and inactive for an element through which air circulation is allowed.

According to a second aspect, the invention relates to a method of controlling such refrigeration installation according to claims 14 or 15.

The refrigeration system comprises on the one hand a refrigeration exchanger controlled by the process which comes to be mentioned and secondly a circuit for the fluid refrigerant or refrigerant, between the outlets and the battery cell inputs.

Such an installation includes means selective circulation or non-circulation of the fluid refrigerant or refrigerant in each of the elements of the battery so that circulation is ensured for elements in a cold production situation and the circulation is prohibited for the element (s) in defrosting situation.

Such selective means include valves or equivalent and control means of said valves.

The circuit for the refrigerant or at least partly refrigerant branches mounted in parallel, in a number equal to that of battery cells, one branch per cell.

In a first possible embodiment, it is planned in part several branches mounted in parallel and partly a single branch or a bundle unique of several branches. According to another mode of realization, the circuit comprises on its entirety several branches mounted in parallel.

In the case of an installation intended for a fluid refrigerant, a common regulator can be provided many elements. And, in particular, a unique regulator mounted on a single branch of the circuit.

In this case, a refrigerant distributor is interposed between this single branch downstream of the regulator and the plurality of branches upstream of the means selective circulation or non-circulation.

• la figure 1 est une représentation schématique et en perspective d'une partie d'installation de réfrigération selon l'invention.
• La figure 2 est une vue schématique en coupe verticale d'un échangeur pour plafond de chambre froide à température négative conforme à une variante de réalisation de l'invention.
• La figure 2a est une vue similaire à la figure 2, d'un autre mode de réalisation de cette variante.
• La figure 3 est une vue schématique en coupe d'un distributeur à vannes intégrées pour une installation conforme à l'invention.
• La figure 4 est une vue en perspective d'un mode réalisation de distributeur.
• La figure 5 est une vue en coupe axiale selon la ligne V.V de la figure 4.

The characteristics of the invention will result from the description which follows with reference to the accompanying drawings in which:

• Figure 1 is a schematic perspective view of a portion of the refrigeration installation according to the invention.
• Figure 2 is a schematic vertical sectional view of a heat exchanger for cold room ceiling at negative temperature according to an alternative embodiment of the invention.
• Figure 2a is a view similar to Figure 2, of another embodiment of this variant.
• Figure 3 is a schematic sectional view of a distributor with integrated valves for an installation according to the invention.
• Figure 4 is a perspective view of an embodiment of a dispenser.
• FIG. 5 is a view in axial section along the line VV of FIG. 4.

A refrigeration installation conforming to the invention comprises a refrigeration exchanger 1 and a circuit 2 for the refrigerant or refrigerant circulating in the exchanger 1.

Such an installation is intended to be associated to a refrigeration enclosure 3. Means of air circulation 4 are able to put the air in the enclosure 3 in circulation while ensuring its passage over the exchanger 1. These means 4 can include a fan 13.

Naturally, it may be a gas other than the air.

The refrigeration chamber 3 can be a room cold, a freezing or deep-freezing chamber, a refrigerated display cabinet or a refrigerated display case or another similar provision. This enclosure 3 can be brought to a temperature of the order of a few degrees celsius, for example typically around 2 ° C for application to the preservation of food products fresh (Figure 1). In other applications, the temperature in enclosure 3 may be negative (figure 2).

The refrigeration exchanger 1 comprises means 5 for circulation of a refrigerant or refrigerant under tube shape. Means 5 are associated with means 6 surface exchange heat exchangers. These means 6 are for example successive fins, spaced apart from each other, mounted transversely around the tubes means 5.

The means 5 for circulating refrigerant or refrigerant and the heat exchange means 6 are arranged in the form of a battery having several tablecloths 7 and several rows 8.

In the embodiment of FIG. 1, provision is made five layers 7a, 7b, 7c, 7d and 7th and five rows 8a, 8b, 8c, 8d and 8e. In FIG. 1, the layers 7a to 7e are placed substantially horizontally. Rows 8a to 8e are placed substantially vertically.

These plies 7 and rows 8 are arranged in a plurality of elements 9 similar to each other from the point of view structural which together form the battery.

In the embodiment of FIG. 1, provision is made five elements 9a, 9b, 9c, 9d, 9e which each correspond to a sheet 7a, 7b, 7c, 7d and 7e. Each item includes five rows 8a, 8b, 8c, 8d and 8e.

The elements 9 are mounted in parallel in what concerns means 5. They form a compact and are separated from each other by partitions 10. In particular, the heat exchange means 6 are separate each other for the different elements 9 of so as to avoid any thermal bridge between them.

Partitions 10 provide some isolation between the elements 9. They also ensure the collection of defrost water to prevent these only pass into a neighboring element. To this end, the partitions 10 can be in the form of solid plates with good insulation capacity thermal. Preferably, the partitions 10 and the means 6 heat exchangers do not have thermal bridges between them.

The partitions 10 are arranged substantially horizontally or slightly inclined on the horizontal to ensure the flow of water from defrosting.

Although Figure 1 shows the case of an element comprising a single sheet 7, it must be understood that the invention applies to the case where each element 9 comprises several tablecloths 7.

In order to allow the parallel mounting of elements 9, each of them includes an entry 11 and a own exit 12, belonging to means 5 of circulation refrigerant or refrigerant.

The battery 5, 6 which has just been described forms a all unitary, the elements 9 which compose it being placed one next to the other.

Battery 5, 6 includes at least three elements 9.

Each element 9 has a cooling capacity which is only a fraction of the total nominal power of exchanger 1.

By refrigeration capacity of element 9 the cooling capacity that this element is able to provide in a normal operating situation.

The term nominal cooling capacity total of the exchanger 1 the power that the exchanger has in a normal operating situation. This situation is such that the exchanger then makes it possible to reach in enclosure 3 the desired temperature.

So at some point in the operation of the exchanger 1, at least one of the elements 9 can be in defrosting situation and at least two elements 9 in cold production situation. Elements 9 are round alternately in defrost and in de cold production. The refrigeration capacity developed by all of the elements 9 in a situation of production of cold corresponds to the cooling capacity required for that enclosure 3 is at the required temperature.

In the case of a battery 5, 6 comprising three elements 9, each element 9 is arranged to have a power equal to half the total normal power of the exchanger. Indeed, two elements 9 work together.

In the case where the battery 5, 6 comprises four elements 9, these each have a cooling capacity equal to a third of the total normal power of the exchanger.

In the case of Figure 1 where there are five elements 9, four operating for the production of cold and one in a defrosting situation, each element 9 has a cooling power equal to a quarter of the power total normal heat exchanger.

More generally, in the preferential case of a battery 5, 6 having n elements 9 of which n-1 in situation of production of cold and one in defrosting situation, the cooling capacity of each element 9 is equal to P / n-1 where P is the total nominal cooling capacity of the exchanger 1. In this case, each element 9 is in defrosting situation one cycle every n cycles and in cold production situation n-1 cycles every n cycles.

The air circulation means 4 ensure the passage of air over the heat exchange means 6. These means 4 are arranged to define and form an air flow which, both at the input and at the output of the battery 5, 6 is common to the different elements 9.

Consequently, the air flow both at the inlet and at the output is not shared according to elements 9.

One or more fans 13 interposed on the common air flow of the battery 5, 6, to entry and / or exit. When more than one is planned fans these all act on the air flow common.

An exchanger 1 as just described is ordered as follows:

In a given N operating cycle, we make at least two elements 9 for production of cold while placing at least one of the elements 9 in defrosting situation.

When the defrost of the element (s) 9 in defrosting situation is complete or sufficient, we go to another similar cycle N + 1.

In the N + 1 cycle, one or more elements 9 in cold production situation in the N cycle are brought in a defrosting situation. The element (s) 9 in defrost situation in cycle N are brought, conversely, in a cold production situation.

The exchanger control is reiterated according to this process, until each element 9 has been at least once in defrosting situation.

After that, the ordering process is repeated.

We order the passage from a cycle N to the cycle according to N + 1 either by a prior adjustment of the duration of each cycle, either by a command linked to the operation of the exchanger 1 or of the installation which incorporates it and in particular the degree of icing of the exchange media thermal 6. In this second case, the cycle times successive may be different.

The control process which has just been described applies under normal conditions when the installation has been in operation for some time and one or more elements 9 have frost on the exchange means thermal 6 corresponding.

The control process which has just been described applies under normal conditions when the installation has been in operation for some time and one or more elements 9 have frost on the exchange means thermal 6 corresponding.

At the start of exchanger 1, and of the installation which includes it, no element 9 presents frost. By elsewhere, the temperature in enclosure 3 is generally higher than in normal operating conditions and this requires greater cooling capacity or longer refrigeration time.

In addition, it is not excluded to order exchanger 1 so that only part of the number of elements 9 to operate in normal operating mode at some point or time. This is the case if the refrigeration requirement is limited.

According to the invention, in a start-up cycle of exchanger 1 and while no element 9 should be defrosted, all of the elements 9 until a given temperature is obtained satisfactory for air flow at battery outlet 5, 6 and in enclosure 3.

For example, in the case of a battery 5, 6 comprising three elements 9, the cooling capacity temporarily implemented is one and a half times the total nominal power of the exchanger 1.

In the case of the embodiment of FIG. 1, the starting power is 25% higher than total nominal heat exchanger power 1. More selective means 14 for circulation or not circulation of the refrigerant or refrigerant in each of the elements 9 of the battery 5, 6.

These means 14 are such that circulation is insured for elements 9 in production of cold and that traffic is prohibited or not ensured for elements 9 in a defrost situation.

The selective means 14 include valves 15 or equivalent and means 16 for controlling said valves 15.

The valves 15 are associated with the different items 9.

The selective means 14 are located either on the side inputs 11 of the elements 9 of the battery 5, 6 or output side 12.

Circuit 2 has at least several branches 17 mounted in parallel, in a number equal to that of elements 9 of the battery 5, 6. At each element 9 corresponds to a branch 17.

In the embodiment of FIG. 1, the circuit 2 partly comprises several branches 17 mounted in parallel on the side of the inputs 11, and a single branch 18 on the outlet side 12. In another variant not shown, it can be substituted a single bundle of several branches to the single branch 18.

In another variant not shown, the circuit 2 comprises on its entirety several branches such as 17.

We now refer more particularly to the Figure 1 which relates to an installation intended for a fluid refrigerant. The exchanger 2 is then an evaporator since it allows a change of phase of the fluid gas phase refrigerant at inputs 11 and phase liquid at the outlets 12.

In this embodiment, a regulator is provided single 19 mounted on the branch 18. This regulator 19 allows also a phase change. Upstream liquid, the refrigerant goes into the gaseous phase downstream.

Downstream of the regulator 19 and between the single branch 18 and the plurality of parallel branches 17 is interposed a distributor 20 of the refrigerant.

The circuit or exchanger 2 also includes -en the occurrence upstream of the holder 19- means 21 of refrigeration production suitable in particular for vacuuming at low pressure the refrigerant from a manifold 22 connecting the parallel outputs 12.

In the case of an installation intended for a fluid there is no expansion valve such as 19 and the means 21 such as a pump then make it possible to put circulating the fluid in the installation.

The installation may also include means not shown such as probe, clock, calculator having to identify the degree of icing of the elements 9 during cold production as well as the degree of defrosting of the elements 9 in a defrosting situation. These locating means can be coupled to the means selective 14 for circulation or non-circulation of the fluid refrigerant or refrigerant. The degree of icing can be measured, for example, by the pressure drop in the air flow in an element 9 between its input and its output. Indeed, plus the heat exchange means 6 are covered with the more difficult it is to pass air. he means are then provided for measuring this loss of charge.

Likewise, the installation may include control means, not shown, means 4 of air circulation which can also be coupled to the means selective 14 for circulation or non-circulation of the fluid refrigerant or refrigerant.

Finally, the installation may include sensors or temperature and / or flow sensors as well as means such as clock or time delay and, more generally control or regulating or safety useful or necessary for the operation of this type of installation.

The partitions 10 ensure according to one embodiment possible some retention of water from defrost. Although it does not appear in the application considered essential, it then follows that the circulating air can be humidified, in a certain measure, by contact with defrosting water.

In another embodiment, on the contrary, partitions 10 ensure the evacuation of water from defrosting. For this purpose, the partitions 10 are inclined on the horizontal and the defrost water eliminated at low collection point.

There is shown in Figure 1 by the arrow F the direction of flow of the fluid in the exchanger 1 and the circuit 2. The upstream and downstream qualifiers mentioned refer to this sense.

Although the number of elements 9 is not limited, from a theoretical point of view, we understand that for reasons constructive, it is preferable that it remains limited.

In practice, excellent results have been obtained, for refrigerated display cases or rooms cold, with a number of elements 9 equal to three, four or five.

This allows a potential overpower in starting, equal respectively to half, to third, to quarter of the total nominal power of the exchanger.

Naturally, the invention also includes the case where several batteries such as 5, 6 mounted are provided in series or in parallel.

In Figure 2 is shown a part installation comprising the exchanger 1 and the means of air circulation 4, intended to be placed in a part at negative room temperature. For example, this part is the enclosure which is intended to be at temperature negative.

The installation part considered includes a casing 23 fixed to a wall 24 which constitutes a ceiling. The layout of the exchanger and the installation which features, shown in Figure 2, is similar to that of figure 1.

Therefore, only the specifics of the Figure 2 are described here.

In this embodiment, the exchanger further comprises the components already described, means 25 selective circulation or non-circulation of air on the heat exchange means of each element 9.

The means 25 are such as air circulation is authorized for certain elements 9 and prohibited for others.

The selective means 25 comprise flaps mobile 26 or equivalent associated with means of ordered. These control means are in particular the means 16 provided in connection with the selective 14 means of circulation or non-circulation of refrigerant or refrigerant.

The control means are such as the means selective 25 are controlled so that air circulation be authorized for elements 9 in situation of production of cold and prohibited for elements 9 in defrosting situation.

In the embodiment shown in FIG. 2, the system exchanger further comprises means heating 27 of the heat exchange means 6. The heating means 27 are associated with each of the elements 9 of the battery. They are also associated with the means of control of selective means 25. Control of means of heating 27 is such that these heating means 27 are active (i.e. heating) for element 9 for which air circulation is prohibited (element in defrost situation) and are inactive for an element 9 for which air circulation is authorized (element in cold production situation).

Consequently, the heating means 27 are functionally coupled to the selective means of air circulation or non-circulation.

The movable flaps 26 of the selective means 25 can be pivotally mounted between an extreme position opening and an extreme opposite closing position. In FIG. 2, four flaps 26a are in position opening and a flap 26b in the closed position.

Preferably, the flaps 26 are located on the upstream side with respect to the general flow direction of the air represented by arrow D.

Another series of shutters can be arranged in downstream of the elements 9, to completely isolate from the air flow the defrosting element (s).

According to an alternative embodiment, not shown, the selective means 25 are in the form of fans. A fan is provided for each element 9. During operation, the fans ensure the passage of air. When stopped, a fan prevents or in any case hinders the passage of air through the corresponding element.

The heating means 27 can be the subject of different variants. According to an achievement possible, these are heating resistors or air hot blown.

In the embodiment illustrated in FIG. 2, the partitions 10 each have collection means 28 water from defrosting and means of disposal 29 of these waters are also provided.

Figure 2a shows an exchanger such as that shown in Figure 2 with a direction wall 30 vertical rather than horizontal for the wall 24.

In this arrangement, the means 4 of circulation are placed in the upper position while the partitions 10 are placed in the lower position. The partitions 10 are, in this embodiment, directional general vertical.

The defrost water then falls vertically and can be collected by a bin 31 having means evacuation 32.

In Figures 1, 3, 4 and 5 is shown the distributor 20 in an embodiment where it integrates two functions. The first function is to distribute the refrigerant or refrigerant to the branches 17. The second function is to allow cut off the flow of refrigerant or refrigerant on each branch 17 independently of each other.

The distributor 20 includes an inlet 33 for the fluid 1 which communicates with a distribution chamber 34. This distribution room communicates, opposite the entrance 33, with a plurality of outputs 35.

The valves 15 are arranged near the outputs 35.

The outputs 35 are themselves connected to the branches 17 of the installation.

The distributor 20 is equipped with solenoid valves 36 the coils of which are mounted on armature sleeves 37, and a removable plate 38 is integral with said shirts 37. Each shirt 37 is tightly fixed on the plate 38. A movable core or armature 39 depending on the arrow direction N is shown in the lower position on Figure 5 and then closes the way out of the distributor when the coil is energized. The core or armature 39 clears the passage or way out of the fluid when the coil is de-energized.

Pipe from production equipment installation refrigerant and opening into the chamber 34 is preferably made of metal and brazed or welded on the distributor 20.

Claims (15)

1. A refrigerating unit such as a cold storage or a salesroom refrigerator cabinet, of the type comprising, in a circuit (2), means (5) for the circulation of a refrigerant with which are associated surface-extended heat exchanging means (6); these means (5, 6) being disposed in the form of a battery having several layers (7) and several rows (8) arranged as a plurality of elements (9) similar to one another, mounted in parallel, separated from each other by separations (10) ensuring the heat insulation and defrosting water collection functions; each element including at least one layer (7) as well as a specific inlet (11) and outlet (12) belonging to the circulation means (5),
      characterised in that the battery comprises at least three elements (9) as well as a single pressure reducer (19) common to the elements (9), mounted on a single branch (18) of the circuit (2) and located upstream of the inlet (11) of the elements (9), whereas a plurality of branches mounted in parallel is located downstream of said pressure reducer (19); each element (9) mounted on one of the branches (17) mounted in parallel having a refrigerating power which is a fraction of the rated total power of the exchanger (1), so as to make it possible, at a given moment of the operation of the exchanger (1), for at least one of the elements (9) to be in defrosting condition and for at least two of the elements (9) to be in cold production condition, the elements (9) being in turn in defrosting condition and in cold production condition.
2. A unit according to claim 1, characterised in that the exchanger (1) further includes air circulation means (4) on the heat exchanging means (6) arranged to form a common air flow at the inlet and a common air flow (D) at the outlet of the battery.
3. A unit according to claim 2, characterised in that the air circulation means (4) comprise at least one ventilator (13) inserted in an air flow (D) common to the battery.
4. A unit according to claim 1, characterised in that the elements (9) of the exchanger (1) form a compact assembly.
5. A unit according to claim 1, characterised in that the separations (10) also ensure either the defrosting water retention or evacuation function.
6. A unit according to claim 2, characterised in that the exchanger (1) further comprises selective air circulation or non-circulation means (25) on the heat exchanging means (6) of each element (9), so that air circulation is authorised for certain elements and forbidden for others and, for example, mobile flaps (26) or equivalent, associated with control means (16).
7. A unit according to claim 6, of negative temperature type such as a chill room, characterised in that the selective air circulation or non-circulation means (25) are arranged so that the circulation is authorised for the elements (9) in cold production condition and forbidden for the elements (9) in defrosting condition.
8. A unit according to claim 7, characterised in that the exchanger (1) further comprises heating means (27) of the heat exchanging means (6) for each of the elements (9) of the battery associated with control means (16) so as to be active for an element (9) for which air circulation is forbidden and inactive for an element (9) for which air circulation is authorised.
9. A unit according to claim 1, characterised in that it includes, downstream of the pressure reducer (19), selective refrigerant circulation or non-circulation means (14) in each of the elements (9) of the battery so that the circulation is ensured for the elements (9) in cold production condition and the circulation is forbidden for the element(s) (9) in defrosting condition.
10. A unit according to claim 9, characterised in that the selective circulation or non-circulation means (14) comprise valves (15) or equivalent and means (16) for controlling said valves, arranged downstream of the pressure reducer (19).
11. A unit according to claim 1, characterised in that the circuit (2) includes, at least partially, several branches (17) mounted in parallel, in a quantity equal to the number of elements (9) of the battery, one branch (17) per element (9).
12. A unit according to claim 9, characterised in that a refrigerant distributor (20) is inserted between the pressure reducer (19) and the plurality of branches (17), upstream of the selective refrigerant circulation or non-circulation means (14).
13. A unit according to any of claims 1 to 12, characterised in that it is associated with a cold storage cell (3), air circulation means (4) being capable of putting the air of the cell (3) into circulation while ensuring its passage on the heat exchanging means (6).
14. A method for controlling a unit according to any of claims 1 to 13, in which:

    during a given operating cycle N of the exchanger (1), at least two of the elements (9) are made to operate for cold production, whereas at least one of the elements (9) is placed in defrosting condition;

    when the defrosting of the element(s) (9) in defrosting condition is completed or sufficient, another similar cycle N+1 is switched to, in which the element(s) (9) in cold production condition in cycle N are in defrosting condition whereas the element(s) (9) in defrosting condition in cycle N are in cold production condition, whereas during an exchanger (1) start-up cycle, even though there is no element (9) to be defrosted, all of the elements (9) are made to operate temporarily until a given output air flow (D) temperature is obtained, by means of a temporary excess of power.
15. A method for controlling a unit according to any of claims 1 to 13, in which:

    during a given operating cycle N of the exchanger (1), at least two of the elements (9) are made to operate for cold production, whereas at least one of the elements (9) is placed in defrosting condition;

    when the defrosting of the element(s) (9) in defrosting condition is completed or sufficient, another similar cycle N+1 is switched to, in which the element(s) (9) in cold production condition in cycle N are in defrosting condition whereas the element(s) (9) in defrosting condition in cycle N are in cold production condition, and in which the switch from one cycle to the next is controlled by means of a prior adjustment or by means of an operation-related command (16), particularly the degree of defrosting of the heat exchanging means (6), measured in terms of the head loss of the air flow in an element (9) between its inlet and its outlet, for example.

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