GB2263760A - Refrigeration using solid carbon dioxide - Google Patents

Abstract

Apparatus for refrigeration of perishable or heat-sensitive goods such as on a milk-float, comprises a container (2) within a store of the goods, the container (2) having a chamber (4) for the storage of solid carbon dioxide. The chamber (4) has a thermally uninsulated wall (14), thermally insulated walls (8 and 10), and a thermally insulated lid (16). A fan (24) is associated with the chamber (4) and is operable to pass ambient air from the store across the thermally uninsulated wall (14) and return it to the store. An electric motor (26) drives the fan. Cold air is thereby provided to the store. Alternatively, an uninsulated duct may extend through the chamber containing solid carbon dioxide. Cold air blown through the duct may be used for cooling pre-packed meals on a trolley. <IMAGE>

Description

REFRIGERATION METHOD AND APPARATUS This invention relates to a refrigeration method and apparatus. In particular, it relates to a method and apparatus able to be used to refrigerate perishable goods in transit.

It is well known that a large quantity of milk is delivered directly to the individual consumer in the United Kingdom. Large dairies typically operate fleets of delivery vehicles, generally battery driven, known as "milk floats". The direct delivery of milk to individual consumers is a capital intensive operation. Dairies have therefore sought ways of improving the returns they obtain from the capital invested. Accordingly, while the daily milk round has been a traditional feature of British life for several decades, more recently dairies have been using milk floats to deliver a range of perishable goods, particularly foodstuffs, directly to the consumer. To this end, a large number of milk floats have been fitted with thermally insulated chests or cabinets for the storage of a range of products typically including yoghurts, pies, cheese and sausages.Such items are therefore loaded into the chests or cabinets at the start of a milk round from a central refrigerated store. Accordingly, the temperature of such perishable goods gradually rises throughout the duration of the milk round, and on hot days, in particular, the temperature within the insulated chest or cabinet can become unacceptably high. There is therefore a need to provide such chests with a source of refrigeration.

An analogous problem arises when handing pre-packed meals before they are loaded onto an aircraft. Such pre-packed meals are stacked in a cart on ground before a flight. The cart is then loaded onto the aircraft and the meals served from the cart during the flight. On occasions, particularly if there are delays to the aircraft taking off, this procedure may result in the meals being held outside a refrigerated environment for too long a period of time to be acceptable.

Various methods of in-transit refrigeration are known which are suitable for use in the storage spaces of large lorries. Such methods include fitting the storage compartment with large mechanical refrigerators or spraying in a controlled manner into the storage space a cryogenic liquid which evaporates therein to create a storage atmosphere at the required temperature. Such methods are however unsuitable for use in association with thermally-insulated chests or cabinets of a size suitable for their deployment on milk floats.

More simple refrigeration methods, for example the simple packing of perishable foodstuffs in ice or solid carbon dioxide suffer from the disadvantage that there is no control over the temperatures produced. Ice will also tend to damage the foodstuffs or their packaging as it melts, while solid carbon dioxide cannot be used in direct contact with products, such as those based on milk, that are damaged by being frozen.

According to the present invention there is provided apparatus for the refrigeration of a store of perishable or heat-sensitive goods, comprising a container able to be located in flow communication with the store having a chamber for the storage of solid carbon dioxide, the chamber having a portion of its walls thermally insulated and another portion of its walls relatively uninsulated thermally; a fan associated with the chamber operable to pass air from the store across a thermally uninsulated portion of its walls and return it to the store; and means for driving the fan.

The fan is preferably operatively associated with a temperature sensor typically positioned so as to sense the temperature of the air upstream of its passage across said thermally uninsulated portion of the walls of the chamber. The fan is preferably operable at two speeds, its operative association with the temperature sensor being such that the fan is operated at the lower speed when the sensed temperature is at or below a chosen ceiling, say 50C, and at the higher speed when the sensed temperature is above the ceiling.

Preferably, in a first embodiment of the apparatus according to the invention, the container has a first chamber for the storage of solid carbon dioxide, and a second chamber for accommodating an assembly comprising the fan and associated means for driving the fan. Such means typically include an electric motor for driving the fan; a battery, preferably rechargeable, for providing electrical power to the motor; the temperature sensor; and suitable electrical or electronic control means for controiling the speed of the fan in response to the temperature sensor andi for effecting the energisation and de-energisation of the motor. Such components of the assembly are preferably all provided in a housing which is able readily to be inserted in and removed from the second chamber.

In the said first embodiment of the apparatus according to the invention, the second chamber preferably communicates with or contains an unobstructed air way bounded at least in part by a thermally-uninsulated wall of the first chamber, the fan being positioned relative to the airway such that in operation air is caused to flow over said thermally uninsulated wall. Preferably, such wall and the air way extend generally vertically. In this embodiment of the apparatus according to the invention the air way is preferably closed at its top but is open at its bottom being associated with a guide plate beneath and spaced from the floor of the first chamber and arranged to guide air passing out of the air way over the undersurface of said floor. Desirably, in such an embodiment the said floor is thermally uninsulated.

Preferably, said generally vertically extending thermally uninsulated wall has apertures therein affording communication between the air way and the first chamber.

Preferably, the floor of the first chamber has apertures therein, whereby, in use, to permit gas to pass out of the chamber.

The first chamber may receive loose pellets or slabs of solid carbon dioxide. Solid carbon dioxide (also known as dry ice) is typically sold in cartons of different sizes. Preferably, the first chamber simply receives a carton of dry ice, for example, one containing 10 kg of dry ice.

Thus, when the apparatus according to the invention is to be used upon a milk float or other vehicle a carton of dry ice may simply be loaded into the first chamber in its carton, thereby obviating the need to empty the solid carbon dioxide from the carton. In order to facilitate insertion of the solid carbon dioxide, whether packaged or unpackaged, into the first chamber, the first compartment is preferably fitted with a simple lid which is preferably thermally insulated.

In a second embodiment of the apparatus according to the invention, the container is adapted to be located outside the store and has a duct which has an inlet communicating with a first port in the store and an outlet communicating with a second port in the store, at least part of the duct being thermally uninsulated and forming a wall of the chamber for the storage of solid carbon dioxide.

The apparatus according to the invention is able to be operated so as to maintain the temperature within the store within a desired range for an extended period of time sufficiently long to enable a daily delivery round to be completed before the solid carbon dioxide has all been sublimed.

The invention also provides a method of refrigerating a store holding perishable or heat-sensitive goods by operation of the aforementioned apparatus.

The term 'store' as used herein means any closed container vessel or housing defining a space for the storage of foodstuffs or other perishable or heat sensitive items.

The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic perspective view of a container in accordance with the invention; and Figure 2 is a schematic side elevation, partly in section of the container shown in Figure 1.

Figure 3 is a schematic perspective view, partly cut away, and partly exploded, of a second apparatus according to the invention.

The drawings are not to scale.

Referring to Figures 1 and 2 of the drawings, a container 2 is of generally cuboidal shape. The container 2 has a larger first chamber 4 and a smaller second chamber 6. The container 2 also has thermally-insulated pairs 8 and 10 of side walls. Typically (but not shown) each of the pairs of walls 8 and 10 has an inner skin, an outer skin and thermal insulation, such as a foamed plastics material, for example, of foamed polyurethane, located between the skins. The container 2 has a slightly raised, thermally uninsulated floor 12. The floor 12 is typically formed of aluminium or stainless steel or other thermally-conducting metal. The first chamber 4 is separated from the second compartment 6 by a vertical common wall 14.

The wall 14 typically comprises a relatively thin sheet of heat conductive metal (e.g. aluminium) and is not thermally insulated. The first chamber 4 is provided with a lid 16 which is thermally-insulated and is, for example, of the same double-skinned structure as the pairs 8 and 10 of walls. The lid 16 is typically fitted with a handle (not shown) (and/or is hinged along one of its edges to a vertical wall of the container 2) so as to enable it to be lifted to permit access to be gained to the interior of the first chamber 4. The second chamber 6 accommodates a separate and readily removable assembly 7 comprising a generally cuboidal housing 9 containing a rotary fan 24, a motor 26 for driving the fan 24, a battery 28 for providing electrical power to the motor 26, and a control box 30.The housing 9 has a lid 18 which is of the same double-skinned construction as the walls 8 and 10 of the container 2. The other walls of the housing 9 may for example be made of high impact strength polystyrene. The lid 18 is provided with a grid 20 to enable the interior of the housing 9 to communicate with the atmosphere in the store (not shown).

The fan 24 is positioned so that air is able to be drawn in through the grid 20 in the lid 18 and pass into a vertically-extending air way 13 defined between the common wall 14 and a wall 15 of the housing 9 facing it. The wall 15 has a slot 17 formed in it to enable air to pass from the interior of the housing 9 into the air way 13. The air way 13 is closed at its top by a thermally-insulated lid 19 but is open at its bottom. There is thus, in operation, a generally downward flow of air through the air way 13. At the bottom of the air way 13 is an elongate slot 34 in the floor 12 of the container 2. Air, in operation, passes through the 34 and impinges upon a guide plate or baffle 36 which is beneath and spaced from the floor 12 such that air is guided over the undersurface of the floor 12.

The common wall 14 is provided near its top with a row of apertures 38.

This permits a portion, typically relatively small, of the air drawn into the air way 13 to flow into the first chamber 4. A region of the floor 12 bounding the first chamber 4 has a row of apertures therein to permit the flow of gas out of the first chamber 4 into the air flowing beneath the floor 12.

The control box 30 is able to receive signals from a temperature sensor 32 located such that air flowing from the fan impinges upon it, typically before entering the air way 13. The sensor 32 may take the form of a thermocouple or thermostat. The sensor 32, control box 30, battery 28 and electric motor 26 are associated with one another such that rotation of the fan 24 is at a higher speed'if the sensed temperature exceeds a chosen limit and at a lower speed if the sensed temperature is at or falls below the limit. The lower speed is chosen not to be so great as to create to a temperature in the store, e.g. OOC or below, which would have a deleterious effect on the goods being stored. Such arrangements are conventional and need no further description here.

In operation of the apparatus shown in the drawing, the fan 24 is operated to induce a flow of air from within a store (not shown) in which the container 2 is located through the housing 9 into the air way 13. The greater part of the induced flow of air then passes in a downwards direction along the wall 14 and gives up heat to solid carbon dioxide in the first chamber 4. The air is thus cooled. The cooled air then leaves the air way 13 through the elongate slot 34 in the floor 12 of the container 2. It is then guided over the undersurface of the floor 12 beneath the chamber 4 and thus gives up further heat to the solid carbon dioxide in the first chamber 4. The air is thus returned to the interior of the store (not shown) at a lower temperature that at which it enters the second chamber 6.

Some of the air caused to flow into the air way 13 by operation of the fan 24 passes upwardly therethrough and into the first chamber 4 through the row of apertures 38. This air thus passes or sweeps over the top of a batch of solid carbon dioxide (not shown) stored within the first chamber 4, and thus gives up heat to the carbon dioxide to cause it to sublime.

Thus, the apparatus according to the invention enables a batch of solid carbon dioxide to be heated in a controllable manner from below, from above, and along one side. Such an arrangement facilitates the refrigeration of perishable items such as foodstuffs that are held in the store.

The container 2 is of a size and shape such that it can be readily loaded into and out of a store (not shown) for perishable foodstuffs. Typically, the store is itself thermally insulated. In operation, the temperature within the store can readily be maintained within chosen temperature limits over a chosen period of time by charging the first chamber 4 with a suitable quantity of solid carbon dioxide. The solid carbon dioxide may be in pellet or slab form and may be loaded into the first chamber 4 in a package or loose. During operation, the solid carbon dioxide absorbs heat from the ambient environment and sublimes at a rate proportional to the rate of heat absorption. Sublimed C02 is permitted to pass out of the first chamber 4 through the row of apertures 40.The rate of absorption of heat by the solid carbon dioxide depends in part on the air temperature within the store in which the container 2 is located, and in part on the amount of thermal insulation provided for the compartment 4. It is desirable that the thermal insulation so provided is such that the rate of absorption of heat is markedly greater in periods when the fan 24 is operating at its higher speed than in periods when the fan 24 is operating at its lower speed.By appropriately selecting the capacity of the first chamber 4, the thermal conductivity of the insulation employed in the pairs 8 and 10 of walls of the container 2 and in the lids 16, 18 and 19, the relative size of the thermally uninsulated surface area, and the speeds at which the fan 24 rotates, it is a simple matter to ensure that during normal weather conditions, the apparatus shown in the drawing is able to keep a thermally insulated store on a milk float in a desired temperature range for substantially the entire duration of a delivery round.

The battery 28 is preferably of a kind which can be recharged from a mains electricity supply each night. Typically, to facilitate recharging of the battery, the assembly 7 is first withdrawn from the second chamber 6.

After the battery 28 has been recharged, and the assembly 7 inserted again into the second chamber 6, the container is ready for use the next day.

Immediately before use, a suitable carton of dry ice (solid carbon dioxide) may be placed in the first chamber 4. We have found that if the first chamber 4 has a capacity sufficient to enable it just to take a lOkg carton of dry ice, there is sufficient refrigeration latent in the solid carbon dioxide for the container 2 to be used to maintain the temperature of a thermally insulated store for the storage of foodstuffs at a temperature within the range of 2 to 5 C throughout the normal duration of a milk delivery round in normal weather conditions. Once the carton of dry ice has been inserted in the first chamber 4, the container 2 may be located within the store (typically a cabinet) together with foodstuffs which it is desired to refrigerate.These foodstuffs are typically taken from a cold store, so there is no need to provide refrigeration for the purpose of lowering the temperature of the foodstuffs themselves. The cabinet itself typically forms part of a milk float. At the end of a delivery round, the carton of solid carbon dioxide, now largely empty through sublimation of most of the solid carbon dioxide may be thrown away. The assembly 7 may then be taken out and the battery recharged to enable the container 2 to be used the next day.

In order to facilitate its handling, the apparatus shown in the drawing may be of a shape and size such that it fits into a conventional milk crate.

Such an arrangement is advantageous since thermally insulated chests or cabinets used on milk floats are typically sized so as to take a given number of milk crates.

Various changes and additions may be made to the apparatus shown in Figures 1 and 2 of the drawing. For example, the container 2 may be constructed so that it is located outside a store for the storage of perishable goods. In such examples of the invention, the container 2 has an appropriate inlet and outlet to enable air to be withdrawn from and returned to the store.

In another embodiment of the apparatus according to the invention, the air way 13 is defined not between the wall 14 and a complementary wall 15 of the housing 9, but instead between the wall 14 and a complementary additional wall (not shown) extending parallel thereto and having a slot formed therein in a posit-ion enabling it to co-operate with the slot 17 in the wall 15 of the housing 9 (when the assembly 7 is in position in the second chamber 6). It will be appreciated that in this embodiment the first and second chambers 4 and 6 are spaced from one another and do not share a common wall with one another.

Referring now to Figure 3 of the drawings, there is shown a trolley or cart 50 of a well known kind that is used to carry pre-packed meals on board an aircraft. The cart 50 has (typically in its rear wall 52) air vents in the form of a lower port 54 of rectangular shape and an upper port 56, also of rectangular shape. A container 58 is adapted to be mounted on the cart 50 such that a wall 60 of the container 58 engages and is held flush with the rear wall 52 of the cart 50. The container 58 is generally cuboidal in shape and has dimensions such that when mounted on the cart 50 it does not impede or hamper the ability of the cart 50 to be wheeled from one location to another. As shown in Figure 3, the container is relatively tall and thin.

The container 58 has an aluminium duct 62 that extends through its interior from an inlet 64 in the wall 60 to an outlet 66 also in the wall 60. The inlet 64 and outlet 66 are complementary to the ports 54 and 56 respectively and are positioned such that when the container 58 is engaged with the cart 50 the inlet 64 coincides with the port 54 and the outlet 66 with the port 56. A chamber 68 is defined about the duct 62 within the container 58 and the duct 62 therefore provides an inner wall of the chamber 68. The chamber 68, as shown in Figure 3, extends from the bottom to substantially the top of the duct 62. If desired, however, the chamber 68 may surround essentially only a part of the duct 62. The chamber 68 communicates at its top, as shown in Figure 3, with a hopper 70. Access to the hopper 70 can be obtained by removing an insulated lid 72 from the top of the container 58.The lid 72 may be formed with apertures (not shown) therethrough for the escape of gas from the chamber 68. In operation, pellets of solid carbon dioxide may be fed to the hopper 70 and hence to the chamber 68 so as to charge, and preferably fill, the chamber 68 with such pellets. Once so charged, the lid 72 can be fitted again in the top of the container 58.

A fan 76 is located in the duct 62 in the vicinity of the outlet 66 thereof. Operation of the fan causes air to be withdrawn from the cart 50 through its lower port 54 and into the inlet 64 of the duct 62. The air then flows along the duct 62 and return to the interior of the cart 50 via the outlet 66 and the port 56. As the air flows along the duct 62 so it gives up heat to pellets of solid carbon dioxide packed around the exterior of the duct 62. The circulation of the air through the duct 62 is thus able to chill pre-packed meals in the cart 50. As a result of the heat absorbed by these pellets, some solid carbon dioxide sublimes and the resulting gas is free to escape from the chamber 68 through the apertures in the lid 72.

The fan 76 is operatively associated with a thermostat 78 typically located at the inlet 64 to the duct 62. The thermostat 78 is set so as to maintain the temperature it senses within chosen limits, say between +2 and +4 or +60C. The arrangement is such that if the sensed temperature reaches the maximum limit, the speed of rotation of the fan is increased to a maximum and if the temperature falls again to the minimum the speed of rotation of the fan is decreased to a minimum (or alternatively the rotation of the fan can then be stopped altogether). The fan 76 is supplied with electric power from a battery 80 located in a compartment 82 at the bottom of the container 58.

In operation of the apparatus shown in Figure 3, there may be a tendency for relatively warm air to leak into the container 58 through gaps between firstly the lower port 54 and the inlet 64 and secondly between the upper port 56 and inlet 66. Accordingly, when the container 58 is in position mounted on the cart 50 a first soft rubber sealing member 84 is held under compression between the lower port 54 and the inlet 64 and a similar soft rubber sealing member 86 is held under compression between the upper port 56 and the outlet 66 so as to prevent such ingress of ambient air into the container 58.

In operation, the capacity of the chamber 68 may be such as to hold sufficient solid carbon dioxide for the apparatus to be able to maintain the interior of the cart 50 (and hence the pre-packed meals therein) within a chosen temperature range for a period of up to 5 hours. It is therefore possible to maintain the meals in refrigerated state even if loading of the cart onto an aircraft is delayed for several hours.

The apparatus according to the invention is particularly suitable for the refrigeration of relatively small quantities of perishable foodstuffs or other items, such as pharmaceuticals, that tend to be adversely affected if stored at ambient temperatures greater than, say, 100C.

Claims (19)

1. Apparatus for the refrigeration of a store of perishable or heat-sensitive goods, comprising a container, able to be located in flow communication with the store, having a chamber for the storage of solid carbon dioxide, the chamber having a portion of its wall thermally insulated and another portion of its walls relatively uninsulated thermally; a fan associated with the chamber operable to pass air from the store across the thermally uninsulated portion of its walls and return it to the store; and means for driving the fan.

2. Apparatus as claimed in claim 1, in which the fan is operatively associated with a temperature sensor.

3. Apparatus as claimed in claim 2, in which the temperature sensor is typically positioned so as in use to sense the temperature of the air upstream of its passage across said thermally uninsulated portion of the walls of the chamber.

4. Apparatus as claimed in claim 2 or claim 3, in which the fan is operable at two speeds, its operative association with the temperature sensor being such that the fan is operable at the higher speed when the sensed temperature is at or above a chosen maximum and at the lower speed when the sensed temperature is below such maximum.

5. Apparatus as claimed in any one of the preceding claims, in which the container has a first chamber for the said storage of solid carbon dioxide and a second chamber for accommodating an assembly comprising the fan and the drive means.

6. Apparatus as claimed in claim 5, in which the components of the assembly are provided in a housing able readily to be inserted in and removed from the second chamber.

7. Apparatus as claimed in claim 5 or claim 6, in which the second chamber communicates with or contains an air way bounded at last in part by a thermally-insulated wall of the first chamber, the fan being positioned relative to the air way such that in operation air is caused to flow over said thermally uninsulated wall.

8. Apparatus as claimed in claim 7, in which said thermally uninsulated wall and said air way extend generally vertically.

9. Apparatus as claimed in claim 8, in which the air way is closed at its top but is open at its bottom being associated with a guide plate beneath and spaced from the floor of the first chamber, which guide plate is arrange to guide, in operation, air passing out of the air way over the undersurface of said floor.

10. Apparatus as claimed in claim 9, in which said floor is thermally uninsulated.

11. Apparatus as claimed in any one of claims 8 to 10, in which said vertically extending wall has apertures therein affording communication between the air way and the first chamber.

12. Apparatus as claimed in any one of claims 5 to 11, in which the floor of the first chamber has apertures therein, whereby, in use, to permit gas to pass out of the chamber.

13. Apparatus as claimed in any one of the preceding claims, in which the container is able to be located within the store.

14. Apparatus as claimed in any one of claims 1 to 4, in which the container is adapted to be located outside the store.

15. Apparatus as claimed in claim 14, in which the container has a duct which has an inlet communicating with a first port in the store and an outlet communicating with a second part in the store, at least part of the duct being thermally uninsulated and forming a wall of the chamber for the storage of the said carbon dioxide.

16. Apparatus for the refrigeration of a container of perishable goods, substantially as herein described with reference to Figures 1 and 2 or Figure 3 of the accompanying drawings.

17. A method of refrigerating a container of perishable or heat-sensitive goods by operating an apparatus as claimed in any one of the preceding claiins.

18. A method as claimed in claim 17, wherein the said apparatus is operated with the chamber or first chamber holding a carton of dry ice.

19. A method of refrigerating a container of perishable or heat-sensitive goods, substantially as herein described with reference to the accompanying drawings.