EP1446619A1

EP1446619A1 - A method and an apparatus for refrigerating and/or freezing products

Info

Publication number: EP1446619A1
Application number: EP02760896A
Authority: EP
Inventors: Stig Are Karlsen
Original assignee: Norsk Hydro ASA
Current assignee: Yara International ASA
Priority date: 2001-10-29
Filing date: 2002-09-09
Publication date: 2004-08-18
Also published as: NO20015299D0; MY136060A; NO316630B1; NO20015299L; WO2003038354A1

Abstract

The present invention concerns a method and an apparatus for refrigerating and/or freezing products using a cryogenic agent. The cryogenic agent is mixed into a gas flow to form a cryogenic gas which is used to treat the products. At least one flow generator such as a fan (14) is used to generate the gas flow. During the treatment, the cryogenic gas flow is conducted past the products and on towards the intake side of the flow generator. The present invention contributes to homogeneous refrigeration/freezing of theproducts treated and has good capacity and low consumption of cryogenic agent.

Description

A Method and an Apparatus for Refrigerating and/or Freezing Products

The present invention concerns a method and an apparatus for refrigerating and/or freezing products. The present invention relates in particular to the industrial treatment of foods using a cryogenic agent which is brought into direct contact with them. The apparatus may consist of a long chamber or tunnel containing at least one belt conveyor. Alternatively, the apparatus may comprise a chamber with stationary support surfaces for the products. The latter are then expediently mounted on a board or similar. The present invention allows products to be refrigerated or frozen in an efficient and homogeneous manner with minimum consumption of cryogenic agent.

EP 0 667 502 A 1 concerns a tunnel freezer in which one or more turbulence-initiating fans are arranged under a longitudinal belt conveyor which conveys products through the tunnel from its inlet opening to its outlet opening. The fans are equipped with screens which cause gas to be sucked into the fan from the space below the belt conveyor. The gas is conveyed away from the fan to the upper side of the belt via two ducts located on either long side of the belt. The gas flow is stated as meeting in a central area located above the belt, where eddies are formed with mutually opposing directions of rotation. The roof of the tunnel in this area may be designed in such a way that the stated rotation is stimulated by means of specially designed guide surfaces. Liquid nitrogen may be injected in the area above the belt conveyor. One problem with the stated flow pattern may be that, in the area between the eddies, on a level with the belt, an area may arise with low gas speed. This may result in the transfer of cold to the products being uneven in relation to the transverse direction of the belt. Moreover, if so many products are placed on the belt that they block the gas flow, areas may arise with relatively stationary gas and subsequently low emission of thermal energy from the products to the refrigerating medium.

With the present invention, the above problems may be reduced or avoided. With the present invention, heat may be removed more rapidly from the products, which will produce an increase in capacity, and the specific consumption of cryogenic agent will be reduced. In an embodiment of the present invention, it will also be possible to achieve a further capacity increase by inserting two or more support surfaces in a stationary chamber or two or more belts if a tunnel is used. A basic condition of the present invention is that the gas circulates in the transverse direction of the belt(s). One advantage of this is that it is possible to install several belts above one another while maintaining good gas circulation at all belt levels. Good gas circulation means rapid refrigeration/freezing of the products to be treated. The above means that the available area of the entire belt may be covered with products as the gas does not need to pass through the belt itself. It is also possible to use belts which are non-permeable to gas, for example plate link belts.

The above and further advantages may be achieved with the present invention as it is defined in the attached claims.

The present invention will be described in further detail in the following with examples and figures, where:

Fig. 1 shows a schematic view of an apparatus in accordance with the present invention, seen in perspective,

Fig. 2 shows a longitudinal section through an apparatus in accordance with the present invention, comprising a belt conveyor,

Fig. 3 shows a transverse section through an apparatus in accordance with the present invention, comprising a belt conveyor,

Fig. 4 shows a flow diagram for an apparatus in accordance with the present invention, comprising a belt,

Fig. 5 shows a longitudinal section through an apparatus in accordance with the present invention, comprising three belt conveyors, Fig. 6 shows a transverse section through an apparatus in accordance with the present invention, comprising three belt conveyors,

Fig. 7 shows a transverse section through an apparatus as shown in Figure 6 which has been modified somewhat,

Fig. 8 shows a flow diagram for an apparatus in accordance with the present invention, comprising three belts.

Figure 1 shows a schematic view of an apparatus in accordance with the present invention, seen in perspective. The apparatus comprises a long chamber or tunnel 1 with an inlet end 2, an outlet end 3 and a belt conveyor 4. At the inlet end 2, an extraction screen 5 may be installed, which is connected to an extraction pipe 10 to extract any gas which emerges from the opening. A similar arrangement may be installed at the outlet end of the tunnel but is not shown in this figure. Alternatively, the extraction system may be installed so that it is located above the inlet and outlet ends of the tunnel (not shown). The extraction system will then be turned so that the opening faces downwards. The tunnel may be supported by legs 6, 7, 8 (only 3 are shown in the figure). In one side wall of the tunnel 1a, motor-driven fans may be installed. Their respective motor housings 9, 9', 9", 9'", 9"" are shown on the outside of the wall.

Figure 2 shows a longitudinal section through an apparatus in accordance with the present invention, comprising a belt conveyor 4 to convey products 12, 13 through the tunnel 1. The products are placed on the belt 4 at the inlet end 2 and are subsequently removed at the outlet end 3 in a manner not specified. Moreover, an extraction screen 5 is shown at the inlet end 2 and a similar extraction screen 5' at the outlet end 3. The tunnel may be equipped with thermal insulation to reduce cold loss to the surroundings.

Figure 3 shows a transverse section through an apparatus in accordance with the present invention, comprising a belt conveyor 4 installed in a tunnel 1. The belt shown is endless with an upper part 4a and a lower part 4b. A fan 14, preferably a radial fan, is installed in the tunnel's side wall 1a. The fan is driven by a motor 9 via a shaft 15. Nozzles 16, 17 for the introduction of cryogenic agent may be installed above and below the fan. Carbon dioxide or nitrogen in liquid or gaseous form, for example, may be used as the cryogenic agent. The use of other cryogenic agents in either liquid or gaseous form is also possible. If cryogenic CO₂ in liquid form is used, this will be converted into gaseous form containing small solid particles (CO₂ snow). If liquid nitrogen (LIN) is used, this will be converted into gaseous form when it is expanded at the nozzle outlet.

As shown in the figure, a nozzle 16 may be connected to an adjustable valve such as a solenoid valve 19 via a pipe 18 for the desired injection of cryogenic agent in the proximity of the fan's outflow side. The solenoid valve is connected to a supply of cryogenic agent (not shown). The fan 14 will set up a flow of gas, which is conducted towards the upper and lower parts of the tunnel 1b and 1c respectively. The flow is conducted in the transverse direction of the tunnel by means of screens 21 and 22. The screens are designed so that they are terminated towards a certain radial level of the fan 14 and extend in the transverse direction of the tunnel towards the opposite side wall 1 b, thereby creating flow ducts for the gas in the downstream direction of the fan. The screens are terminated at wall 1 b in such a way that an opening is formed between the ends of the screens and wall 1 b. This allows the flow to turn so that it is returned towards the intake side of the fan 14. When it returns, the gas is conducted along the inside of the screens (the side facing the belt conveyor), ensuring that a constant flow of gas passes the products on the belt conveyor. The screens may extend along the full length of the tunnel and be equipped with openings towards the fan(s) 14. The return flow will generally pass over the top of the belt but may also be permitted to flow between its top and bottom 4a and 4b as well as below the belt.

As shown in the figure, a flow director 22, in the form of an outward-facing, convergent part, is installed by the wall 1 b. The other parts of the tunnel's internal walls may also be designed to produce optimal flow conditions which cause an upper and a lower sub-flow to be set up in the downstream direction of the fan which meet at the wall 1b to return towards the fan's intake side. The flow is expediently at least partially laminar in the areas where the products are located during the treatment. However, the presence of the products on the belt will contribute to creating a certain turbulent flow in their immediate proximity. This contributes to more rapid and more even removal of thermal energy from the products during the treatment. One way to ensure good flow control is by the gas which passes the products during the treatment being sucked in towards the fan's relative low pressure area and not its over pressure area. Moreover, the divergent part of the flow director 22 may be designed in such a way that it points towards the belt conveyor's top 4a.

It has been shown that effective refrigeration of the belt before the products are placed on it will contribute to the products not adhering to the belt and allow them to be refrigerated/frozen more rapidly. This may be ensured by means of an efficient return gas flow between the top and bottom of the belt and possibly below the belt.

Figure 4 shows a flow diagram for an apparatus in accordance with the present invention, comprising a belt with a top and a bottom, 4a and 4b. No products are placed on the belt. The flow diagram shows that the gas flows downstream from the fan 14 in ducts 23, 24 and returns in ducts 25, 26, 27. The flow diagram shows a schematic view of flow vectors which indicate flow speeds and directions. The scale of the speed vectors is such that 5 mm is equivalent to 1 m/s gas speed.

Figure 5 shows a longitudinal section through an apparatus in accordance with the present invention, which consists of a tunnel 101 comprising three belt conveyors 104, 104', 104", which convey products 112, 113, 112', 113', 112", 113". Like the previous embodiment, this one is equipped with extraction screens 105, 105' at its inlet and outlet ends 102, 103. The conveying direction of the upper and lower belt conveyors 104, 104" may be the same but opposite to that of the central belt 104'. A belt located above another belt is preferably terminated in such a way that the products will fall down onto the lower belt when they reach the end of the belt. Further equipment may be installed at the inlet and outlet ends of the tunnel for automatic input and output of products.

Figure 6 shows a transverse section through an apparatus in accordance with the present invention, comprising a tunnel with three belt conveyors 104, 104', 104". Like the previous examples, this embodiment comprises nozzles 116, 117 for injection of cryogenic agent, a fan 114 and screens 120, 121. The cryogenic agent may be injected on the basis of a temperature which, for example, is measured in the area by the flow director and which is compared with a setpoint value. The cryogenic agent may be added via nozzles using a regulator circuit (not shown), which opens/closes solenoid valves 119, 119'. As the figure shows, the screens extend towards the fan's intake side, which is fixed to the wall 101a, and radially within its periphery. Moreover, at their other end, towards the wall 101b, the screens are terminated short of the wall so that a flow passage is formed between the wall and the screens. The wall 101b is designed with flow directors 122', 122, 122", which are designed to contribute to conducting the return flow back to the fan's intake side in an optimal manner in relation to the belt conveyors 104, 104', 104".

The flow director 122' is designed to contribute to an optimal part of the return flow being conducted into the passage formed between the inside of the screen 121 and the top 104a of the belt 104. Moreover, at least one other sub-flow will be conducted along the wall 101 b down to the flow director 122 so that gas is conducted into the passage between the bottom 104b of the belt 104 and the top 104'a of the belt 104'. The flow director 122" has a similar task and is designed to contribute to an optimal part of the return flow being conducted into the passage formed between the bottom 104'b of the belt 104' and the top 104"a of the belt 104". Moreover, at least a sub-flow of the return gas will be conducted via the flow director 122 into the passage formed between the bottom 104b of the belt 104 and the top 104'a of the belt 104'. In addition to this, some return gas may flow between the inside of the screen 120 and the bottom 104"b of the belt 104" and in any passages formed between the tops and bottoms of the belts.

Figure 7 shows a transverse section through an apparatus similar to that shown in Figure 6 which has been modified somewhat. As in the previous figure, a tunnel is shown with three belt conveyors 104, 104', 104". Like the previous example, this embodiment comprises nozzles 116, 117 for injection of cryogenic agent, a fan 114 and a screen 121. In this embodiment, the second screen 120' is designed so that it does not extend over to the opposite side of the tunnel but is terminated towards the bottom 104"b of the lower belt 104". This solution may be relevant if many belts are to be used in the tunnel and there are restrictions with regard to the available construction height.

Figure 8 shows a flow diagram for an apparatus in accordance with the present invention, comprising three belts (without products). The flow diagram shows that the gas flows downstream from the fan 114 in ducts 123, 124 and returns in ducts 125, 126, 127. In addition, smaller sub-flows may return via duct 128 and in any spaces between the tops and bottoms of the belts 129, 130, 131. The flow diagram shows a schematic view of flow vectors which indicate flow speeds and directions. The scale of the speed vectors is such that 5 mm is equivalent to 1 m/s gas speed.

Another advantage of circulating the gas across the belt is that the product temperature is more constant regardless of where the product is located on the belt. A typical gas speed may be 4-5 m/s while the typical width of the belt may be 1 m. It follows from this that the gas will circulate fast enough to contribute to evening out any temperature gradients over the belt.

When long tunnels are designed, they may have different temperature zones, for example one for each 4 m. Several fans will then be used and their speed may be controlled individually in each zone.

Tunnels which are designed with more than one belt may have the belts arranged so that the products fall from one belt down onto one below or the belts may be separate/individual, i.e. the products pass right through the tunnel, in and out on the same level. In the latter case, different products may be treated on separate belts in the same tunnel, thus creating a flexible solution. This principle may also be utilised to achieve an increase in capacity if just one type of product is to be treated.

A tunnel with three separate belts makes it possible to run three different products through the tunnel at the same time. For example, the first belt may contain chicken legs which are frozen from +76°C to -18°C during treatment in the tunnel. The next belt may contain chicken legs which are refrigerated from +76°C to +2°C. The third belt may be used for chicken nuggets which are frozen from +76°C to -18°C. The products are exposed to the same temperature through the tunnel but the speed of the belts may be adjusted individually so that different quantities of thermal energy are removed from the products.

In tunnels which have more than one belt, it is also possible to control the gas so that some belts receive more gas circulation than others. This may be controlled with the design of the flow director located on the opposite side of the circulation fans. A possible case might be a tunnel with two separate belts. The first belt might contain buns which require refrigeration from +90°C to +20°C. The buns are then removed and receive a coating of icing sugar with sugar sprinkled on top. The buns then go onto the other belt for further refrigeration to 0°C. On the second belt, it will be important for the gas circulation not to be so strong that the sugar sprinkled on top is blown off. The return gas there must be conducted in such a way, using the flow director, that the speed is kept below a maximum level. A similar situation applies when one belt contains small, light products and another belt contains large, heavy products.

In the examples, the apparatuses are described with one and three belts respectively. However, the present invention is not restricted to a certain number of belts. Apparatuses with 2, 4, 5 or more belts will also fall within the scope of the attached claims. The present invention is not restricted to an apparatus based on a tunnel containing belts either. For example, the principle for refrigerating and/or freezing in accordance with the present invention may be used in a chamber with stationary support surfaces to support products.

Alternatively, the flow generator may consist of a supply duct for pressurised gas and an extraction duct designed to establish the flow conditions shown in the examples.

It should be understood that apparatus of a certain vertical extension may have plural flow generators arranged at different vertical levels.

Claims

A method for refrigerating and/or freezing products using a cryogenic agent which is mixed into a gas flow to form a cryogenic gas which is used to treat the products, in which at least one flow generator such as a fan (14) is used to generate the gas flow, characterised in that the cryogenic gas flow is conducted past the products and on towards the intake side of the flow generator.

2. A method in accordance with claim 1 in which the treatment of the products is carried out in a tunnel with at least one belt conveyor, characterised in that the flow generator(s) is(are) installed in a side wall of the tunnel in which a gas flow is created above and/or below the belt conveyor(s) and returns from the opposite side wall, where at least one return gas flow is designed to pass the top of the belt(s) back towards the intake side of the flow generator.

3. A method in accordance with claim 2, characterised in that another return gas flow is designed to pass between the top and bottom of the belt(s) back towards the intake side of the flow generator.

4. A method in accordance with claim 2, characterised in that another return gas flow is designed to pass the bottom of the belt(s) back towards the intake side of the flow generator.

5. A method in accordance with claim 1 , characterised in that the cryogenic agent is mixed into the gas flow directly after the flow generator.

6. An apparatus for refrigerating and/or freezing products using a cryogenic agent which is mixed into a gas flow to form a cryogenic gas which is used to treat the products, in which at least one flow generator such as a fan (14) is used to generate the gas flow, characterised in that the cryogenic gas flow is conducted past the products and on towards the intake side of the flow generator.

7. An apparatus in accordance with claim 6, comprising a tunnel with at least one belt conveyor (4), characterised in that the flow generator(s) is(are) installed at one side wall (1a) of the tunnel, where screens (20, 21) conduct the gas over to the opposite side wall (1b) above and/or below the belt conveyor(s), after which the gas returns to the flow generator via one or more passages (25, 26, 27) in which the products are located.

8. An apparatus in accordance with claim 7, characterised in that one passage (27) is located between an upper screen (21 ) and the top of the belt(s) (4a).

9. An apparatus in accordance with claim 7, characterised in that one passage (26) is located between the top of the belt(s) (4a) and the bottom of the belt(s) (4b).

10. An apparatus in accordance with claim 7, characterised in that one passage is located between a lower screen (20) and the bottom of the belt(s) (4b).

1. An apparatus in accordance with claim 7, characterised in that at least one flow director (22) is installed at the opposite side wall to guide a defined quantity of the gas flow into the passage(s).