EP0826937B1

EP0826937B1 - Cooling unit

Info

Publication number: EP0826937B1
Application number: EP97114434A
Authority: EP
Inventors: Franz Dr Lürken
Original assignee: Messer Griesheim GmbH
Current assignee: Messer Griesheim GmbH
Priority date: 1996-08-29
Filing date: 1997-08-21
Publication date: 2003-07-16
Anticipated expiration: 2017-08-21
Also published as: DE69723515D1; ATE245270T1; EP0826937A1; ZA977372B; NL1003915C2

Abstract

Autonomously operating cooling unit for cooling a storage space (9) during a certain time. The cooling unit (1) contains a container (2) for a coolant and a number of pipes (14) and is provided with means (13) for making the air in the storage space circulate by using the increase in pressure caused by the evaporation of the coolant; the storage space (9) is equipped for the transport of foods in a frozen or cooled condition. The evaporation of a coolant causes: the cooling of the storage space (9); the circulation of the ambient air in the storage space (9); the drying of the ambient air; and thermally controlled cooling. <IMAGE>

Description

The invention relates to a cooling unit for cooling a storage space.
Such cooling units are familiar and are used in particular for cooling frozen foods during transport and distribution. The familiar cooling units contain a container for a coolant, e.g. liquid nitrogen or carbon dioxide in the solid or fluid state. Heat is exchanged because of the evaporation of the coolant and the storage space is cooled.
For example, in the US 4 576 010 a cryogenic control system for cooling a storage space within a road truck is described, where liquid nitrogen, stored in a tank, is evaporated by heat exchange with the air inside the storage space. The evaporated gas is conducted to a venturi where, due to the increase of pressure during evaporation, ambient air is sucked in, thus creating an air circulation in the storage space. In this way, the storage space will be cooled with high efficiency. However, due to the the direct contact with cold evaporated coolant, moisture contained in the ambient air sucked into the venturi will freeze out and finally block the venturi.
In the US 3 447 336 a cooling unit is described where a cryogenic coolant - like liquid nitrogen - is fed to a venturi in its liquid state. To avoid the problem of freezing out of moisture inside the venturi, the ambient air sucked in is cooled down inside a moisture trap before it is fed to the venturi, leading to the effect that a mayor part of the moisture freezes out before it reaches the venturi. However, a minor part of the moisture still reaches the venturi and freezes out there. A cooling unit according to the preamble of claim 1 is disclosed in document US 3 447 336.
New EU-directives on the storage and transport of frozen foods prescribe a temperature of -18°C during the entire process of storage and transport. The familiar cooling units are unable to ensure such a stable cooling temperature.
The invention envisages overcoming these objections and to this, provides a cooling unit for cooling a storage space comprising a container for a cryogenic coolant, a first heat exchanger fluidly connected with said container for cooling said storage space by evaporating said cryogenic coolant, a moisture trap having an inlet and an outlet for cryogenic coolant and an inlet and an outlet for ambient air, for freezing moisture out of an airflow of air of said storage space by the means of thermal contact with said cryogenic fluidic, a venturi connected to said outlet for cryogenic coolant and to said outlet for ambient air of said moisture trap, for creating an air circulation in said storage space using the increase in pressure caused by said evaporation of the cryogenic coolant fluidly, by that a second heat exchanger for heating cryogenic coolant is located between the freezing element and the venturi.
Hence, the cooling fulfils three functions:

the cooling of the storage space through heat exchange with the evaporated or liquid coolant,
freezing the moisture from the air that is drawn in by an element with a matching volume, termed the moisture trap, and
circulating the ambient air by the suction caused by the acceleration of the evaporated coolant by the way of the venturi.

Furthermore, temperature measurements make it possible to cause an additional direct influx of evaporated coolant by way of pressure-regulating elements and pipes. This provides thermally controlled cooling. In this way the cooling unit provides thermally controlled cooling and the temperature in the storage space will remain constant independent of the external ambient temperature. Moreover, the influx of this additional flow of evaporated coolant will cause a better mix with the cooled ambient air which is drawn in and returns to the storage space.
The invention also relates to a storage space fitted with a mobile or stationary cooling unit according to the invention. Such a storage unit is suitable for the transport of frozen food.
Finally, the invention also relates to a process for cooling a storage space according to claim 7.
An embodiment of the invention will be described in greater detail on the basis of the following descriptions of the figures illustrating a number of preferred specimen designs of the cooling unit according to the invention.
Figure 1 shows a front view in cross-section of a preferred design of a cooling unit in accordance with the present invention;
Figure 2 shows the same preferred design as in Figure 1 in a side view;
Figure 3 contains a flow diagram of the ambient air and the evaporated coolant in a cooling unit according to the invention;
Figure 4 contains a flow diagram similar to that of Figure 3, in which an additional flow of evaporated coolant causes thermostatic operation.
Figure 5 contains a diagrammatic cross-section, in which the operation of the moisture trap and the venturi is shown in a T-piece connector.
Figure 1 shows the cooling unit 1 in which a container 2 for a coolant 3 is connected to a number of pipes. The container 2 is filled with a coolant by way of a filling tube 4. Since the wall 5 of the container 2 is largely insulated by a vacuum sheath 6, the base 7 of the container 2 functions as a heat exchanger. This base 7 of the container 2 is not insulated, so that evaporation of the coolant is caused by way of the difference in temperature, so that pressure is built up. This overpressure is caused by the evaporation of the liquid nitrogen 8. Since the container 2 has not been closed off to the liquid nitrogen 8, nitrogen in the gaseous phase flows from the container 2 because of the overpressure into an element with a matching volume 10, hereinafter to be termed a moisture trap, after which this gas flows through a heat exchanger and a venturi connection. This results in acceleration of the flow, so that underpressure is caused in the T-piece connection, which causes the ambient air to be drawn in through the pipe 26.
The venturi 13 is protected against freezing by the moisture trap 10. The ambient air which is drawn in is collected in the moisture trap and the moisture in the ambient air is frozen solid by the evaporated coolant. The moisture trap may be regarded as a heat exchanger, by means of which moisture in the ambient air is temporarily frozen solid so that the operation of the venturi is not impaired.
The most important function of the evaporated coolant 3 (here nitrogen gas 8) is to cool the storage space 9 by ensuring the air flow which is drawn in, together with the nitrogen gas 8 which is accelerated by the venturi 13, is forced into the storage space.
After the heat exchange in the moisture trap 10, the nitrogen gas 8 is compelled to exchange heat by way of a heat exchanger 14: cooling the storage space on the one hand and heating the nitrogen gas on the other hand.
Another heat exchanger 15 is connected to the container at the base and provides additional cooling, e.g., if this is necessary in warmer periods.
The design according to the embodiment of the invention has the same cooling power because of an adaptation of the pressure-regulating elements, irrespective of the level of the coolant in the container 2.
Because the venturi 13 causes the ambient air to be drawn in, circulation is caused as long as evaporation takes place, so that the temperature in the storage space is homogeneous.
Figure 2 shows a lateral cross-section view of the preferred design of the cooling unit shown in Figure 1. The uppermost 14 and the lowest 15 heat exchangers can be clearly seen; the latter is an additional heat exchanger. The uppermost one reaches to the ceiling of the storage space 9. Cooling is obtained in the heat exchangers by the transfer of heat by way of the evaporated nitrogen. The level in the container 2 can be determined by a probe.
Figure 3 contains a flow diagram of the preferred design of Figures 1 and 2. There are two types of flows: that of the evaporated coolant 16 (completely black triangle) and that of the ambient air 17 (triangle with O). The liquid nitrogen 3 in the container 2 evaporates and provides cooling of the storage space by way of the first heat exchanger 7. The ambient air is drawn in through a venturi 13, so that circulation is created in the storage space, with the ambient air mixing with the evaporated coolant which has passed through the moisture trap. This mixture is returned to the environment. A third heat exchanger 20 is fitted to the base of the container 2.
Figure 4 contains a flow diagram comparable to that of Figure 3. A different pressure-regulating system is used in this design, in which pressure-regulating elements 21 and a cut-off valve 22 are switched in series or in parallel. A further flow of liquid nitrogen is introduced into the storage space by other pressure-regulating elements 24 and temperature measurements 23 by way of pipe 25. Thermally controlled cooling of the storage space is caused in this way. The additional flow may cause additional mixing with the output flow - ambient air - of the evaporated nitrogen.
Figure 5 contains a diagram of the moisture trap and the venturi according to the invention. The moisture trap 27 consists of a pipe 30 for the ambient air; the low temperature T_V dominates the moisture trap 27 is caused by the presence in direct or counter-flow of evaporated coolant, through which the moisture in the ambient air is frozen solid to the wall of the pipe 30. In this way ice 29 is formed on this wall 30. The flow of evaporated coolant is then led to the input of the venturi 28 by way of heat exchangers. The temperature T_W is the temperature of the coolant after exchanging heat with the storage space and for preference is higher than the temperature T_L of the ambient air from the moisture trap in order to exclude all phenomena of freezing from the venturi. Ambient air is drawn in through the constriction and mixing with the evaporated coolant and the ambient air which has been drawn in takes place; this is released into the storage space at point 31.
A cooling unit according to the invention is used for preference in a storage space, e.g., a container or the body of a lorry used for the transport and distribution of foods. A calculation using the logistical data, such as transport time and ambient temperature and the volume of the storage space will enable the coolant level in the container to be adjusted. This level can be read off by means of a probe determining the level, for example. In this way regular cooling over time and in the storage space which satisfies all European standards for the transport of cooled and frozen foods.

Claims

Cooling unit for cooling a storage space (9) filled with ambient air comprising:

a container (2)for a cryogenic coolant,

a first heat exchanger (7) for cooling said storage space (9) by evaporating said cryogenic coolant,

a moisture trap (10,18,27) for freezing moisture of the ambient air

the moisture trap having

a first inlet and a first outlet for cryogenic coolant, the first inlet being connected to the container (2) and

a second inlet and a second outlet for the ambient air,

a venturi (13,28) for creating an air circulation in said storage space (9) using the increase in pressure caused by said evaporation of the cryogenic coolant

the venturi (13,28) having

a third inlet for the ambient air connected to the second outlet of the moisture trap (10,18,27),

fourth inlet for the evaporated cryogenic coolant in communication with the first outlet of the moisture trap and

a third outlet for the mixture of the ambient air and the evaporated cryogenic fluid,

characterised in that the first heat exchanger is formed by the base of the container and in that a second heat exchanger (14,19) for exchanging heat between the evaporated cryogenic coolant and the storage space (9) is located between the moisture trap (10,18,27) and the venturi (13,28).
Cooling unit according to claim 1, characterised in that said inlet for cryogenic coolant of said moisture trap is fluidly connected with the first heat exchanger so that evaporated coolant is conducted through the moisture trap.
Cooling unit according to claim 1 or 2, characterised in that pressure regulation means (21) are provided for controlling the evaporation of cryogenic coolant.
Cooling unit according to one of the previous claims, characterised in that temperature sensors are provided.
Cooling unit according to one of the previous claims, characterised in that the cooling unit is provided with means for moving the cooling unit from one storage space to another.
Storage space fitted with a cooling unit according to one of the previous claims.
Process for cooling a storage space (9) comprising the steps:

- a cryogenic coolant stored in a container (2) is evaporated in a first heat exchanger (7) formed by the base of the container so that the storage space (9) is cooled,

- the evaporated cryogenic coolant flows from the container (2) because of the overpressure into

a moisture trap (10,18,27), after which the evaporated cryogenic coolant flows through

a second heat exchanger (14,19) for exchanging heat between the evaporated cryogenic coolant and the storage space (9) and

a venturi (13,28),

- the evaporated cryogenic coolant is accelerated in the venturi (13,28), so that an underpressure is caused in the venturi (28,13), which causes the ambient air of the storage space (9) to be drawn in,

- the ambient air which is drawn into the venturi (28,13) together with the evaporated cryogenic coolant which is accelerated by the venturi (28,13) is forced into the storage space (9), so that the ambient air in the storage space (9) is caused to circulate,

- the ambient air before being drawn into the venturi (13) passes the moisture trap (10,18,27), so that the moisture in the ambient air is frozen solid by the evaporated cryogenic coolant.
Process according to claim 7, characterised in that liquid nitrogen and/or solid or liquid carbon dioxide is used as cryogenic coolant.
Process according to one of the claims 7 or 8, characterised in that thermally controlled cooling is created by regulation of pressure and/or regulation of temperature.