GB2023789A - Method and apparatus for cooling or freezing - Google Patents

Abstract

A method of cooling or freezing articles, particularly foodstuff items of a type which if refrigerated in mass tend to stick together undesirably. Such as extruded scampi 7, are deposited individually onto the surface of a rotating thermally-conductive drum 2, the interior of which is exposed to a cryogenic medium such as a pool of liquid nitrogen 3. The scampi receive a crust freeze whereby they adhere to the drum as it rotates, until they are brought towards the lowest point of its orbital path. Here they encounter a higher rate of heat loss due to the proximity of the cryogen, and the consequent thermal contraction breaks the bond with the drum, allowing the scampi to drop off whence they are collected by a conveyor 8. Cold vapour derived from the cryogen and which is permitted to escape from the drum is directed over the scampi by fans 10 and 13. <IMAGE>

Description

SPECIFICATION Method and apparatus for cooling or freezing The present invention relates to a method and apparatus for cooling or freezing articles, using as the source of refrigeration a cryogenic medium.

Particularly, though not exclusively, the invention is concerned with the freezing of foodstuffs. Hitherto, the most commonly used commercial method of freezing foodstuffs has been by conveying the items to be frozen through a tunnel wherein they are exposed to streams of cold air refrigerated by a conventional vapour compression system. Such a method is, however, generally unsuitable for the rapid freezing necessary to maintain fresh food products in peak condition, typical freezing times of commercially available tunnels of this type being in the order of an hour.

Furthermore, tunnel freezers which make use of conventional vapour compression refrigeration need to be physcially long to provide a sufficient residence time for the items to become frozen, which means that a freezer of this type represents a large capital outlay.

In order to achieve more rapid freezing it is known to use tunnels generally similar to those described above but in which, in place of a vapour compression system, refrigeration is achieved by spraying the items to be frozen directly with a cryogenic liquid, e.g. with liquid nitrogen. Tunnel freezers using cryogenic liquid injection are generally less bulky than their more conventional counterparts and represent less in terms of capital cost. Because they are continually expending refrigerant, however, they are considerably more costly to operate.

One particular problem with which the invention is concerned arises inter alia in the food processing industry when it is required to freeze large numbers of items which, if refrigerated in mass, stick together undesirably.

As examples can be quoted soft fruit, hamburgers and the like meat preparations, comestibles coated in (uncooked) batter, discrete doses of egg preparations and the like viscous fluids, and extruded scampi. Hitherto, the usual procedure when freezing such items has been to convey them in discrete form through a tunnel freezer as described above, frequently with additional sheets of nylon or stainless steel interposed between them and the surface of the conveyor in order to prevent them adhering to the conveyor when frozen.

In practice, however, when handling such goods it is difficult to fully utilise the available surface area of the slatted conveyors usually incorporated in freezing tunnels and this, coupled with the additional thermal mass present in the tunnel when additional substrates are used, means that a considerable proportion of the available refrigeration power is wasted. As will be appreciated, this uneconomic type of operation is particularly undesirable in the case of a tunnel freezer refrigerated by cryogenic liquid injection.

The present invention accordingly seeks to provide a method and apparatus for rapidly and economically cooling or freezing articles and which will in particular be found useful in the freezing of foodstuffs such as those listed above.

In a first aspect the invention resides in a method of cooling or freezing articles which comprises depositing the articles on the outer surface of a rotatable drum the inner surface of which is exposed to a cryogenic medium, the drum providing for heat transfer between said inner and outer surfaces, and rotating the drum so that each article is carried through at least a portion of the orbital path of the drum in contact with its outer surface.

In a second aspect the invention resides in apparatus for cooling or freezing articles comprising a drum providing for heat transfer between its inner and outer surfaces, means for rotating the drum, and means for exposing the inner surface of the drum to a cryogenic medium.

The cryogenic medium may be, for example, a liquefied gas such as liquid nitrogen which is sprayed onto the inner surface of the drum or a pool of which is maintained within the drum, or a solidified gas such as solid carbon dioxide suitably supplied to the interior of the drum.

In operation, the drum will usually be rotated about a fixed horizontal axis, each article to be treated being deposited on the drum near the highest point of its orbital path and subsequently being removed from the drum near its lowest point. The tendency of a freezing article to adhere to the surface of the drum is desirable in this context as clearly there is a need for the articles to be positively located on the drum during treatment. If the apparatus is to be used with articles which do not have this tendency, suitable means for impaling or otherwise locating the articles on the drum may be provided.

The invention will now be more particularly described, by way of example, with reference to the accompanying drawing which is a schematic illustration of an apparatus according to the invention for use in freezing extruded scampi.

Referring to the drawing, the apparatus comprises a thermally insulated cabinet 1 housing a thermally conductive drum 2 of e.g.

stainless steel arranged for rotation in the anticlockwise sense (as viewed) by means not shown. The drum has a flange 2A at each end by which to retain a pool of liquid nitrogen 3.

The liquid nitrogen is supplied to the interior of the drum through a valved hose 4 controlled in response to a sensor 5 to keep the liquid within the drum at a substantially constant level. In an alternative embodiment, solid carbon dioxide may be supplied to the interior of the drum e.g. by means of an auger.

A feed device 6 is provided which in the illustrated embodiment extrudes onto the radially outer surface of the rotating drum, close to its 'top dead centre' position, discrete portions 7 of a paste made from small pieces of meat collected during the processing of whole scampi. Extruding the scampi portions directly on to the moving drum is of advantage as it can be arranged that the leading end of each portion is drawn out from the rest of the portion by the rotating drum to give the articles a wedge profile resembling that of whole shelled scampi. The feed device may have a plurality of parallel extrusion heads by which parallel rows of extruded scampi portions are deposited onto the drum. It will be appreciated, however, that any form of feed device may be provided as appropriate to the particular type of goods to be frozen, of which extruded scampi are but one example.Furthermore, although the drum is shown as having a cylindrical outer surface it may instead be of polygonal form so that each article to be frozen can be deposited onto a flat, rather than an arcuate, surface.

As the drum rotates, each part of its circumferential wall 2B in turn encounters the pool of liquid nitrogen 3. The surface of the drum onto which the extruded scampi portions (hereinafter referred to simply as 'scampi') are deposited is accordingly very cold and on contact with the drum the underside of each scampi is given an immediate crust freeze by which the scampi adheres to the drum. As the scampi is carried round by the drum towards its 'bottom dead centre' position the underside crust freeze is reinforced by heat exchange through the wall 2B with the cold nitrogen vapour which fills the interior of the drum, and subsequently by heat exchange through the wall 2B with the liquid nitrogen itself.The rate of heat loss from the scampi as it nears 'bottom dead centre' and encounters the pool of liquid nitrogen increases rapidly and the consequent thermal contraction is usually sufficient to crack the bond between the scampi and the drum, thereby allowing the scampi to drop onto a conveyor 8. Any scampi which do not drop off the drum in this way are removed mechanically when they subsequently encounter a scraper 9.

In use of the apparatus cold nitrogen vapour is continually boiling off from the pool 3 and escaping into the cabinet 1 through the ends of the drum 2. While each scampi is being carried round by the drum and losing heat through its underside (i.e. side in contact with the drum) it is also being cooled on its opposite side by streams of the cold nitrogen vapour which are directed onto the scampi by fans 1 0. If required, liquid nitrogen may additionally be injected into these streams through spray headers 11. Consequently, by the time the scampi reach the conveyor 8 they have been given a crust freeze on both sides and, when stabilised, this results in the scampi becoming frozen through.

The conveyor 8 leads the frozen scampi out to a collection point through a short tunnel 1 2 equipped with fans 13, wherein the remaining available 'cold' is extracted from the nitrogen vapour before it is exhausted to atmosphere through an extraction duct 14.

Advantages which an apparatus of the type described above has over known tunnel freezers are that it is more compact and likely to require less capital cost; it utilises well the available refrigeration power of the cryogenic medium; it turns to good effect the tendency of a freezing article to adhere to a substrate; problems of utilising the available surface area of a drum when treating discrete articles are less than those which occur with a slatted conveyor and longitudinally moving conveyors in general; and it is accordingly able to achieve rapid freezing of discrete articles while consuming substantially less refrigerant than a comparable tunnel freezer working with cryogenic liquid injection.

Claims (14)

1. A method of cooling or freezing articles which comprises depositing the articles on the outer surface of a rotatable drum the inner surface of which is exposed to a cryogenic medium, the drum providing for heat transfer between said inner and outer surfaces, and rotating the drum so that each article is car ried through at least a portion of the orbital path of the drum in contact with it outer surface.

2. A method according to claim 1 wherein the cryogenic medium is a liquefied gas which is sprayed on to the inner surface of the drum.

3. A method according to Claim 1 wherein the cryogenic medium is a liquefied gas a pool of which is maintained within the drum.

4. A method according to claim 1 wherein the cryognic medium is solidified gas a quantity of which is maintained within the drum.

5. A method according to any preceding claim wherein the drum is rotated about a generally horizontal axis, each article to be treated being deposited on the drum near the highest point of its orbital path and subse quently being removed from the drum near the lowest point of its orbital path.

6. A method according to any preceding claim wherein articles to be treated adhere to the surface of the drum during their passage through at least a portion of the orbital path of the drum by virtue of a crust freeze im parted to the articles by the drum.

7. A method according to claims 5 and 6 wherein a quantity of cryogenic medium is maintained with the drum in the region of the lowest point of its orbital path, and wherein the rate of loss of heat from such articles as they encounter such region is such that thermal contraction causes the bond between the articles and drum to be broken, thereby permitting the articles to drop from the drum.

8. A method according to any preceding claim wherein the outer sides of the articles carried by the drum are exposed to streams of cold vapour derived from said cryogenic medium and which is permitted to escape from the interior of the drum.

9. A method according to claim 8 wherein said streams are created by impeller means located without the drum.

1 0. A method according to claim 9 wherein further quantities of cryogenic medium are injected into the streams created by said impeller means.

11. A method according to any preceding claims wherein treated articles removed from said drum are subsequently fed through a tunnel within which they are exposed to cold vapour derived from said cryogenic medium and which is permitted to escape from the interior of the drum.

1 2. A method according to any preceding claim wherein the articles to be treated are extruded scampi or the like.

1 3. Apparatus for cooling or freezing articles comprising a drum providing for heat transfer between its inner and outer surfaces, means for rotating the drum, and means for exposing the inner surface of the drum to a cryogenic medium.

14. Apparatus according to claim 1 3 and adapted to perform the method according to any one of claims 2 to 12.

1 5. A method of cooling or freezing articles substantially as hereinbefore described with reference to the accompanying drawing.

1 6. Apparatus for cooling or freezing articles substantially as hereinbefore described with reference to the accompanying drawing.