WO2008116723A1

WO2008116723A1 - Method and device for refrigerating a cold store and also refrigerating vehicle

Info

Publication number: WO2008116723A1
Application number: PCT/EP2008/052480
Authority: WO
Inventors: Franz Lürken; Reinhard Kost; Helmut Henrich
Original assignee: Air Liquide Deutschland GmbH; Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: Air Liquide Deutschland GmbH; Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2007-03-28
Filing date: 2008-02-29
Publication date: 2008-10-02
Anticipated expiration: 2009-09-28
Also published as: DE102007015390A1

Abstract

A device and a method for refrigerating at least one cold store (1), using liquefied cold gas, in particular liquid nitrogen (LIN), with the use of a heat exchanger (2) with a heat accumulator which can be cooled by the liquefied gas (LIN) and via which outside air (AIR) can be supplied to the cold store (1), at least part of the cooling being effected, in a first time segment, by the supply of liquefied gas (LIN) via the heat exchanger (2) to a first outlet, in particular a spraying means (10), in the cold store (1), and, for ventilation, the supply of liquefied gas (LIN) being interrupted and, in a second time segment/ the supply of outside air (AIR) via the heat exchanger (2) taking place. Preferably, in a third time segment, liquefied gas (LIN) is supplied to the cold store (1), at the same time bypassing the heat exchanger (2), and the selected third time segment is as long as is required for defrosting the heat exchanger (2).

Description

METHOD AND DEVICE FOR REFRIGERATING A COLD STORE AND ALSO REFRIGERATING VEHICLE

The invention relates to a method and a device for refrigerating a cold store and also to a refrigerating vehicle. In particular, the invention relates to a method and a device for refrigerating a mobile cold store, such as, for example, a container, and also to a refrigerating vehicle having such a cold store for goods transport, in which refrigeration takes place by the direct spraying of a liquid low-temperature gas in the cold store, and frequent opening and entry are required for loading and unloading.

Nitrogen has been used for a long time for vehicle refrigeration, particularly in vehicles having multi- chamber systems. A method of this type is known by the name of CryogenTrans (CT) . In the CT method, nitrogen is carried on or in the vehicle in liquid low- temperature form in a vacuum-insulated reservoir. The nitrogen is extracted, when cold is required, via a pipeline and is sprayed directly into a cold store by means of the inherent pressure of the medium. The method is particularly simple and unsusceptible to faults. Furthermore, the refrigerating power is always equally high independently of the ambient temperature. It is limited, in principle, only by the transmitting capacity of the spray nozzles. Consequently, CT- refrigerated HGVs, which are used for the distribution of foodstuffs and naturally are multiple openings of the door during refrigeration, exhibit considerable advantages in terms of refrigeration quality. Particularly in high summer, when mechanical refrigeration systems have to deal with power drops on the condensers and with icing-up on the evaporators, the CT method displays its advantages in terms of efficiency, reliability and performance. After the opening of a door, the desired temperature is reached again within seconds. The CT method has the disadvantage that, before the cold store is entered, it has to be ventilated completely for safety reasons. In this case, an unnecessarily large amount of warm air passes into the cold store. Although renewed refrigeration takes place very quickly, it nevertheless consumes more energy and consequently entails more costs than necessary. The otherwise customary installation of cold retention systems such as, for example, a curtain, are ruled out in CT-refrigerated vehicles, since they would dangerously impede ventilation.

A method for keeping a freezer store of a food transport vehicle cold and a corresponding vehicle are known from EP 1 252 471 Bl. In this case, the freezer store is refrigerated by liquid nitrogen being sprayed.

When the freezer store is intentionally to be opened and entered, the supply of nitrogen is interrupted and liquid nitrogen is conducted through a heat exchanger.

A fan presses outside air into the freezer store via the heat exchanger, with the result that the air is cooled down and the freezer store is ventilated. In this case, the nitrogen conducted through the heat exchanger is conducted outwards.

In another embodiment, it is known, further, during loading and unloading, to conduct nitrogen through a pipe coil arranged as an evaporator on the ceiling of the cold store and then to discharge it into the surroundings in order to keep the cold store at a low temperature .

The disadvantage of this prior art is that the full theoretical cooling power of the nitrogen cannot be utilized. A further disadvantage is that the cold gaseous nitrogen discharged into the surroundings may lead to problems, particularly when the vehicle is standing in enclosed unloading stations, such as warehouses or garages, which have a relatively small volume .

Known, furthermore, is indirect refrigeration, in which heat exchangers are used, which are refrigerated, for example, by means of liquid nitrogen and refrigerate the air inside a cold store. Ventilation before entry for safety reasons is then not required.

The disadvantage of this prior art, however, is that the abovementioned advantages of refrigeration with liquid nitrogen are lost. In particular, in summer, an icing-up of the heat exchanger may occur due to the supply of warm air with a high moisture content.

The object of the present invention, therefore, is to make available a method and a device for refrigerating at least one cold store and also a corresponding refrigerating vehicle, in which refrigeration takes place by the introduction of liquid low-temperature gas and, for entry during the loading and unloading of the cold store, ventilation with ambient air takes place efficiently and cost-effectively.

This object is achieved by means of a method according to Claim 1, a device according to Claim 6 and a refrigerating vehicle according to Claim 15. Advantageous refinements and developments which may in each case be combined individually or suitably with one another, as desired, are specified in the subclaims.

In the method according to the invention, in a first time segment, a cold store is refrigerated by the supply of liquefied gas, in particular liquid nitrogen, the liquefied gas simply being sprayed into the cold store. However, the liquefied gas is first routed through a heat exchanger which in this case serves as a refrigeration accumulator, with the result that this heat exchanger cools down to about the temperature of the liquefied gas. When the cold store is to be entered by a person, first the supply of liquefied gas is interrupted and outside air is supplied to the cold store for ventilation. Depending on the outside temperature, in the case of a direct supply, this outside air would warm up the cold store relatively quickly, which, according to the invention, is prevented in that the outside air is then supplied in a second time segment by the heat exchanger. In this case, this heat exchanger acting as a refrigeration accumulator discharges cold accumulated in it to the supply of outside air relatively quickly, so that the latter passes into the cold store only after being cooled down sharply. The cold previously accumulated in the heat exchanger can therefore be utilized almost completely for the further refrigeration of the cold store when the latter is ventilated in order to prepare for entry by a person. This gives rise in the cold store to an easily breathable atmosphere which, however, is considerably colder than in the case of direct ventilation.

The outside air supplied by the heat exchanger typically contains a certain fraction of moisture which leads to the formation of ice in the heat exchanger. For this reason, after the end of entry to the cold store, in a third time segment liquefied gas is supplied to the cold store, at the same time bypassing the heat exchanger. As a result, cold losses caused by entry can be compensated very quickly, but at the same time the heat exchanger can be defrosted. Correspondingly, the selected third time segment is at least as long as is required for defrosting the heat exchanger .

So that the method according to the invention can be employed particularly advantageously, the heat exchanger is equipped as a heat accumulator with a suitable heat capacity and surface. It is particularly suitable to use at least one pipe coil, preferably consisting of a copper alloy, through which the liquefied gas flows in the first time segment and around which the outside air flows in the second time segment, in succession.

The method according to the invention is particularly suitable in the case of the use of liquid nitrogen as the refrigerant of a cold store of a refrigerating vehicle having one or more refrigerating chambers.

A device according to the invention for refrigerating at least one cold store by means of a liquefied gas has a heat-accumulating heat exchanger which comprises at least one primary inlet and at least one primary outlet for liquefied cold gas and at least one secondary inlet and one secondary outlet for ambient air. In this case, the primary inlet is connectable to a reservoir for liquefied gas. The secondary inlet is connectable to the surroundings. The primary outlet and the secondary outlet are connected to the cold store. Moreover, means for shutting off the primary inlet are present. This equipment makes it possible first to supply liquefied cold gas from a reservoir through the heat exchanger to the cold store and spray it into the latter. While the liquefied gas is flowing from the primary inlet through the heat exchanger to the primary outlet, it cools down the entire mass of the heat exchanger approximately to the temperature of the liquefied gas. When liquid nitrogen is used this means a cooling down to approximately minus 196°C. After the means for shutting off the primary inlet have been actuated, liquefied cold gas no longer flows into the heat exchanger. Outside air can then be conducted through the heat exchanger to the cold store, the outside air cooling down very sharply in the heat exchanger. Residues of liquefied cold gas which are possibly present in the heat exchanger evaporate and simultaneously escape into the cold store. The cold store can thus be flooded with ambient air in a relatively short time, for example in less than 90 seconds, and in this case nevertheless remains relatively cold.

The heat exchanger preferably has a heat capacity which makes it possible to cool down an ambient-air volume corresponding approximately to the volume of the cold store by at least 5 Kelvin, preferably by more than 10 Kelvin. Depending on the temperature requirements in the cold store and on refrigeration continuity, substantially sharper cooling may also be achieved by means of a correspondingly higher heat capacity of the heat exchanger.

So that the possibility of entering the cold store can be established as quickly as possible, it is advantageous if the heat exchanger has between the secondary inlet and secondary outlet a heat exchanger surface around which ambient air flows, and a design, which, when ambient air flows through, makes it possible to discharge of 50% of its accumulated cold in less than 120 seconds, preferably in less than 60 seconds, particularly preferably less than 30 seconds.

It has proven to be beneficial if the heat exchanger contains at least one pipe coil connecting the primary inlet to the primary outlet and preferably consisting of a copper alloy. Copper has a suitable heat capacity and heat conductivity and is particularly suitable as a material for heat exchangers in refrigerating assemblies .

If appropriate, the heat capacity, that is to say the refrigeration accumulator capacity, in the heat exchanger can be increased by means of an additional latent heat accumulator which is preferably integrated into the heat exchanger. Latent heat accumulators contain a material which has a phase transition in the desired temperature range and in this range can absorb and discharge a particularly large amount of heat or cold. In the present instance, a latent heat accumulator with a phase transition in the vicinity of the desire temperature of the cold store can be used.

During the ventilation of the cold store by the supply of ambient air through the heat exchanger, the formation of ice may occur in the heat exchanger, depending on the duration of ventilation, since the moisture contained in the ambient air for the most part freezes out there. Although this has the advantage that the moisture does not pass into the cold store and form ice there, it would nevertheless eventually lead to complete icing-up in the case of frequent brief ventilations and subsequent immediate cooling of the heat exchanger. For this reason, the device according to the invention has, in a particular refinement, bypass means for supplying liquid cold gas into the cold store, at the same time bypassing the heat exchanger. Thus, after the end of entry, liquefied gas can be sprayed into the cold store directly again, without cooling the heat exchanger down directly again. The latter then has sufficient time for defrosting before the method described commences from the outset by the liquefied gas being supplied again through the heat exchanger to the cold store. The bypass means may consist of a simple bypass line between the primary inlet and primary outlet of the heat exchanger. Of course, a completely separate line from the reservoir of liquefied gas to the cold store may also be used. Depending on the design, in this case, the same inlet orifices as or inlet orifices other than those used during operation in the first time interval may be used.

While the primary system of the heat exchanger is designed for the throughflow of liquefied gas, that is to say may have a relatively small line cross section, the secondary system must be designed for the passage of relatively large quantities of ambient air, so that it can serve as an air cooler. The secondary inlet of the heat exchanger is preferably connected to the surroundings via a connecting line having a blower. To prepare for entry to the cold store, the blower conveys a large volume of ambient air into the cold store in a short time.

In an alternative embodiment of the invention, the secondary inlet does not suck in an ambient air, but, instead, sucks in air from inside the cargo hold. In this case, air circulation serves for assisting convection which also, of course, ventilates the vehicle body, in particular when the door is open. With this type of ventilation, heat likewise penetrates into the cargo hold and is cooled down again by the accumulated cold of the heat exchanger.

To control or regulate the sequences described, the device according to the invention preferably has a central control which is connected via signal lines to valves and/or to the blower and/or to sensors in the cold store or heat exchanger. The central control typically receives information on the temperature and/or on the oxygen content in the cold store or on the temperature in the heat exchanger. It then controls both normal operation, which takes place during the first time segment, and, on demand, ventilation in the second time interval, and also cooling down again, initially bypassing the heat exchanger, in the third time interval.

The invention also relates, in particular, to a refrigerating vehicle with a cold store for goods transport and with a device, such as was described above .

The invention is suitable for the ventilation of all types of cold stores with direct refrigeration by low- temperature liquid gas. In a vehicle, however, particular advantages arise when a very large number of cold store entry cycles are required here, for example during deliveries. In this case, the cold store may be installed permanently in the vehicle or, for example, be transported as a refrigerating container.

An exemplary embodiment of the invention, to which, however, this is not restricted, and the features and advantages of the method according to the invention are explained by way of example with reference to the following drawing.

Fig. 1 shows, as a diagram, the set-up of a refrigerating device according to the invention .

Fig. 2 shows a corresponding refrigerating vehicle.

Fig. 1 shows a device according to the invention in a diagrammatic illustration. A cold store 1 has a spraying means 10 by which a liquefied gas LIN, preferably liquid nitrogen, can be sprayed for refrigerating the cold store 1. Moreover, the cold store 1 has an air inlet 11 through which ambient air AIR can be supplied for ventilation before entry by a person. During normal operation, liquefied gas LIN is supplied to the cold store 1 from a reservoir 17. By means of a cold-store sensor 19, the temperature is measured and is supplied to a central control 20 via a signal line 25. The liquefied gas LIN passes via a supply line 12 and an inlet valve 7 into the primary inlet 3 of the heat exchanger 2 and in the latter flows through a pipe coil 15 as far as a primary outlet 4. The liquefied gas passes from there to the spraying means 10. A bypass line 13 with a bypass valve 8 serves as a bypass of the heat exchanger 2 and allows the direct supply of liquefied gas to the cold store 1, at the same time bypassing the heat exchanger 2. In a first time segment which corresponds to normal operation, the bypass valve 8 is kept constantly closed by the central control 20, while the inlet valve 7 is opened by the central control 20 in each case when the temperature in the cold store 1 overshoots a predetermined desired value. During normal operation, therefore, liquefied gas repeatedly flows through the pipe coil 15 and consequently keeps the heat exchanger 2 at a very low temperature, at a temperature of about -196°C when liquid nitrogen is used.

When entry to the cold store 1 by a person is imminent, a corresponding demand signal must be sent to the central control 20. This then continues to keep the bypass valve 8 closed and additionally closes the inlet valve 7, so that further liquefied gas is no longer supplied to the heat exchanger 2. A blower 9 is then switched on via a signal line 23 and conducts ambient air AIR through a supply duct 14 to the secondary inlet 5 of the heat exchanger 2. The ambient air AIR flows through the heat exchanger 2 and at the same time cools down sharply until it arrives at the secondary outlet 6. It is transferred from there to the air inlet 11 into the cold store 1. This situation is maintained by the central control 20 during a second time segment which is sufficient for a reliable ventilation of the cold store 1. The cold store 1 can thereafter be entered freely through a door 26, but is at a markedly lower temperature than the outside temperature.

After the end of entry to the cold store 1, which, despite the measures described, has led to a temperature rise in the cold store 1, the central control 20, in so far as this has not yet happened, switches off the blower 9 and opens the bypass valve 8. Liquefied gas thereby flows from the reservoir 17 through the supply line 12 and the bypass line 13 again to the spraying means 10 and cools the cold store 1 down quickly to the desired temperature. This takes place during a third time segment which must be sufficiently long to allow a defrosting of the heat exchanger 2. During the third time segment, the central control 20 regulates the temperature in the cold store

1 by the corresponding opening and closing of the bypass valve 8, with the inlet valve 7 closed. After the end of the third time segment, the central control 20 switches back to normal operation, as was described for the first time segment. Of course, the cold store 1 must have at least one exhaust-gas outlet 27 which opens in the event of a slight excess pressure in the cold store 1, in order to discharge outwards the re-evaporated sprayed-in liquefied gas LIN or excess blown-in ambient air AIR.

In order to reinforce the action of the heat exchanger

2 as a heat accumulator, a latent heat accumulator 16 may be integrated into the latter. This should have its phase transition in the region of the desired temperature of the cold store 1, so that, during the ventilation of the cold store 1, the said latent heat accumulator can cool down the supplied ambient air AIR to the desired temperature of the cold store 1 for at least some time.

A corresponding refrigerating vehicle 28 with a cold store 1 and with a heat exchanger 2 is illustrated diagrammatically in Fig. 2. Typically, a reservoir 17 for liquid nitrogen is located in the lower region of the refrigerating vehicle 28, and the latter has a door 26 in the rear region.

The use of the system according to the invention, particularly including the heat exchanger serving as a cold accumulator, makes it possible to have a particularly economical type of operation of cold stores which have to be entered relatively frequently for loading or unloading. This occurs particularly in refrigerating vehicles making deliveries, where high savings of liquefied gas, mostly liquid nitrogen, can be achieved by virtue of the invention.

LIST OF REFERENCE SYMBOLS

Cold store Heat exchanger Primary inlet Primary outlet Secondary inlet Secondary outlet Inlet valve Bypass valve Blower Spraying means Air inlet Supply line Bypass line Supply duct Pipe coil Latent heat accumulator Reservoir for liquefied gas Temperature sensor Cold-store sensor Central control Signal lines Cold-store door Exhaust-gas outlet Refrigerating vehicle

Liquefied gas, in particular liquid nitrogen Ambient air

Claims

PATENT CLAIMS

1. Method for refrigerating at least one cold store (1), using liquefied cold gas (LIN), with the use of a heat exchanger (2) which can be cooled by the liquefied gas (LIN) and via which outside air (AIR) can be supplied to the cold store (1), characterized in that at least part of the cooling is effected, in a first time segment, by the supply of liquefied gas (LIN) via the heat exchanger (2) to the cold store (1), and in that, for ventilation, the supply of liquefied gas (LIN) is interrupted and, in a second time segment, the supply of outside air (AIR) via the heat exchanger (2) takes place.

2. Method according to Claim 1, characterized in that, in a third time segment, liquefied gas (LIN) is supplied to the cold store (1), at the same time bypassing the heat exchanger (2) .

3. Method according to Claim 2, characterized in that the selected third time segment is as long as is required for defrosting the heat exchanger (2).

4. Method according to Claim 1 or 2, characterized in that the heat exchanger (2) used is a heat accumulator with a suitable heat capacity and surface, in particular with a pipe coil (15), preferably consisting of copper, through which heat exchanger the liquefied gas (LIN) flows in the first time segment and around which heat exchanger the outside air (AIR) flows in the second time segment, in succession.

5. Method according to one of the preceding claims, characterized in that nitrogen is used as liquid cold gas .

6. Device for refrigerating at least one cold store (1) by means of a liquefied gas (LIN), with a heat- accumulating heat exchanger (2) which has at least one primary inlet (3) and at least one primary outlet (4) for liquefied cold gas (LIN) and at least one secondary inlet (5) and one secondary outlet (6) for ambient air (AIR) , the primary inlet being connectable to a reservoir (17) for liquefied gas (LIN), the secondary inlet (5) being connectable directly or indirectly to the surroundings, the primary outlet (4) and the secondary outlet (6) being connected to the cold store (1), and means (7) for shutting off the primary inlet

(3) being present.

7. Device according to Claim 6, characterized in that the heat exchanger (2) has a heat capacity which makes it possible to cool down an ambient-air volume, corresponding approximately to the volume of the cold store (1), by at least 5 Kelvin, preferably by more than 10 Kelvin.

8. Device according to Claim 6 or 7, characterized in that the heat exchanger (2) has between the secondary inlet (5) and secondary outlet (6) a heat exchanger surface around which ambient air (AIR) can flow, and a design, which, when ambient air (AIR) flows through, makes it possible to discharge 50% of its accumulated cold in less than 120 s, preferably in less than 60 s, particularly preferably in less than 30 s.

9. Device according to one of Claims 6 to 8, characterized in that the heat exchanger contains at least one pipe coil (15) connecting the primary inlet (1) to the primary outlet (6) and preferably consisting of a copper alloy.

10. Device according to one of Claims 6 to 10, characterized in that the heat exchanger (2) contains a latent heat accumulator (16).

11. Device according to one of Claims 6 to 10, characterized in that bypass means (8, 13) for supplying liquid cold gas (LIN) into the cold store (1), at the same time bypassing the heat exchanger (2), are present.

12. Device according to one of Claims 6 to 11, characterized in that a connecting line (14) having a blower (9) is present as a connection between the secondary inlet (5) of the heat exchanger (2) and the surroundings .

13. Device according to one of Claims 6 to 11, characterized in that the secondary inlet (5) of the heat exchanger (2) is connected to the cold store (1), so that, in particular with the door (26) open, ambient air (AIR) penetrating into the cold store (1) can be circulated and cooled by the heat exchanger (2) .

14. Device according to one of Claims 6 to 13, characterized in that a central control (20) is present, which is connected to valves (7, 8) and/or to a blower (9) and/or to sensors (18, 19) in the cold store (1) and/or heat exchanger (2) via signal lines (21, 22, 23, 24, 25) and which controls or regulates these elements (7, 8, 9) by a method according to one of Claims 1 to 4.

15. Refrigerating vehicle with a cold store (1) for goods transport and with a device according to one of Claims 6 to 14.