GB2276440A

GB2276440A - Rotary tunnel freezer

Info

Publication number: GB2276440A
Application number: GB9405995A
Authority: GB
Inventors: Peter Robert Wilyman
Original assignee: Air Products and Chemicals Inc
Current assignee: Air Products and Chemicals Inc
Priority date: 1993-03-26
Filing date: 1994-03-25
Publication date: 1994-09-28
Also published as: GB9405995D0; ZA942132B; GB9306285D0; DE4410503A1; CA2119994A1

Description

2276440 ROTARY TUNNEL FREEZER This invention relates to rotary tunnel

freezers.

Typically, rotary tunnel freezers comprise a drum which is inclined at an angle of a few degrees to the horizontal and which, in use, is rotated about its longitudinal axis.' Product to be frozen is introduced into the upper end of the drum and, as it passes through the drum, is cooled by direct heat exchange with cryogenic fluid, for example liquid and/or cold gaseous nitrogen. The flow of cryogenic fluid and product through the drum may be cocurrent or counter-current.

Typically, the liquid nitrogen is introduced via a spray header located at one or other end of the drum as shown in, for Example US-A-1 601 298; US-A-1 492 488 and UK-A-1 474 471.

Although there is excellent heat transfer between the cold nitrogen and the product to be frozen in the area where the liquid nitrogen is introduced into the drum the rate of heat transfer diminishes quite rapidly away from this area.

In UK-A-1 474 471 product is cooled in a rotatable drum in countercurrent flow to a stream of nitrogen which is introduced through a spray headed adjacent the product outlet. Part of the cold nitrogen vapour leaving the drum at the product inlet is compressed and blasted axially into the drum via a nozzle mounted adjacent the product outlet. This arrangement has several disadvantages. In particular, the effect of the blast decreases extremely rapidly as the distance from the nozzle increases. In addition, the recirculation fan absorbs energy and, at the same time, imparts heat energy to the recirculating gas.

It will be appreciated that it is impractical to install fans in the rotatable drum because of the rota- 2 tion of the drum, the risk of damage from and to the product and the low temperatures present.

According to the present invention there is provi ded a rotary tunnel freezer comprising a drum which, in use, is rotatable about its longitudinal axis, and means for introducing a,cryogenic fluid into an end of said drum, characterized in that said drum is provided with a plurality of nozzles arranged at intervals along a major portion of the length (F) of said drum for introducing cryogenic fluid into said drum and stirring the atmo sphere therein.

Advantageously, said means for introducing cryo genic fluid into said end of said drum comprises a spray header and said plurality of nozzles are disposed on an extension thereof.

Advantageously, said nozzles for stirring the atmo sphere are disposed to introduce cryogenic fluid into said drum substantially tangentially thereof and, pre ferably, closely adjacent the radially inner surface of the drum.

Conveniently, the nozzles for stirring the atmo sphere are disposed on generally outwardly extending extensions emanating from a common supply bar.

Typically, said nozzles for stirring the atmosphere are disposed at centres of from 250mm to 750mm, prefer ably from 40Omm to 60Omm, and advantageously at 50Omm centres.

Preferably, said nozzles for stirring the atmo sphere are disposed over at least 70% of the overall length of the tunnel, more preferably over at least 85%, and advantageously over that part of the drum which is not occupied by the spray header.

Typically, the spray header will comprise a plural ity of nozzles separated at 25mm - 50mm centres. The output of the nozzles in the spray header and the noz- 3 zles for stirring the atmosphere may be the same for a given pressure or may differ. Typically, The nozzles in the spray header will have an outlet diameter of from 1.5mm to 3.Omm whilst the nozzles for stirring the atmosphere may have an outlet diameter less than or equal to lmm.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawing which shows a schematic perspective view, with part cut away, of a rotary tunnel freezer in accordance with the invention.

Referring to the drawing, there is shown a rotary tunnel freezer which is generally identified by reference numeral 1.

The rotary tunnel freezer 1 comprises a drum 2 which is mounted for rotation about its longitudinal axis on rollers 3. A belt 4 extends around the drum 2 and around the drive shaft of an electric motor 5.

A spray header 6 connected to a supply of liquid nitrogen (not shown) is mounted in the inlet section 7 of the drum 2 and extends a distance equal to approximately 15 of the overall length ' F' of the drum. The spray header 6 is provided with a plurality of nozzles with outlets 2mm in diameter positioned at 50mm centres. 20 An inlet tray 8 slopes gently downwardly into the inlet section of the drum 2 which is itself inclined downwardly at an angle of about 30 to the horizontal. The inlet section 7 is provided with a helical fin which defines a plurality of weirs. 25 At the commencement of a freezing operation motor 5 is actuated to rotate the drum 2, which typically has an external diameter D of from 1.25m to 1.6m at from 1 to 5 rpm. A valve (not shown) is then opened at allow liquid nitrogen to spray downwardly from the spray header 6 into the inlet section 7. As the drum 2 cools down pools of liquid nitrogen form in the inlet section 7 whilst the gaseous nitrogen rolls down and cools the remainder of the drum 2. 35 Once the temperature at the downstream end 9 of the drum 2 reaches the desired level a control system (not shown) adjusts the supply of liquid nitrogen as required.

Product to be frozenO for example particles of food to be individually quick-frozen is then introduced into the inlet section 7 via inlet tray 8. The product falls into the liquid nitrogen in the weirs and a frozen crust forms within a few seconds. Because of the nature of immersion freezing the individual particles do not tend to stick together.

As the drum 2 rotates the product passes slowly down the drum 2 during which time it freezes throughout.

The throughput of the rotary tunnel freezer is largely determined by the time which the product must stay in the freezer, which is itself determined by the rate of heat transfer from the liquid and gaseous nitrogen to the product. The arrangement thus far described is also described in our European Patent Publication No. 20 0 372 354. In order to increase the rate of heat transfer the drum is provided with stirring means which comprise nozzles 10 mounted on arms 11 which extend outwardly from a tube 12 which communicates with the downstream end 13 of the spray header 6.

In use, in addition to being sprayed downwardly onto the inlet section 7, nitrogen is ejected from the nozzles 10 which have an outlet diameter of 0.75mm. Nitrogen expands by approximately 700 times when chan- ging from liquid to vapour and this expansion vigorously stirs the surrounding atmosphere in the drum thereby greatly enhancing the heat transfer.

The nitrogen may be ejected from the nozzle 10 as liquid for maximum cooling. However, where it is desir- able to avoid over-cooling, a valve (not shown) is pre- 6 ferably disposed between the spray header 6 and the tube 12 to limit the flow of liquid nitrogen and to enable it to vaporise in the tube 12 and leave the nozzles 10 as cold vapour. 5 The nozzles 10 are positioned at 50Omm centres and are positioned to introduce nitrogen substantially tangentially to the drum 2 approximately 10 cms from the inside of the drum 2. Initial trials have been extremely encouraging and we have f ound that at product rates where previously product was not frozen throughout on leaving the drum 2, freezing is now complete.

Various modifications to the embodiment described are envisaged, for example whilst the nozzles 10 are preferably simple spray nozzles they could also comprise air movers. However, this alternative is not recommended because of the additional expense.

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Claims

Claims:

1. A rotary tunnel freezer comprising a drum which, in use, is rotatable about its longitudinal axis, and means for introducing a cryogenic fluid into an end of said drum, characterized in that said drum is provided with a plurality of nozzles arranged at intervals along a major portion of the length of said drum f or introducing cryogenic fluid into said drum and stirring the atmosphere therein.

2. A rotary tunnel f reezer as claimed in Claim 1, characterized in that said means for introducing cryogenic fluid into said end of said drum comprises a spray header, and said plurality of nozzles are disposed on an extension thereof.

is

3. A rotary tunnel f reezer as claimed in Claim 2, characterized in that the diameter of the output of the nozzles for stirring the atmosphere in said drum is less than or equal to lmm.

4. A rotary tunnel freezer as claimed in Claim 1, 2 or 3, characterized in that said nozzles for stirring said atmosphere are disposed to introduce cryogenic f luid into said drum substantially tangentially thereof.

5. A rotary tunnel freezer as claimed in any preceding Claim, characterized in that said nozzles f or stirring said atmosphere are disposed to introduce cryogenic fluid into said drum closely adjacent the inner surface thereof.

6. A rotary tunnel freezer as claimed in any preceding Claim, characterized in that said nozzles for stirring said atmosphere are disposed on generally outwardly extending extensions from a common supply bar.

7. A rotary tunnel freezer as claimed in any preceding Claim, characterized in that said nozzles for stirring said atmosphere are disposed at centres of from 250mm to 75Omm.

8 8. A rotary tunnel freezer as claimed in any preceding Claim, characterized in that said nozzles for stirring said atmosphere are disposed over at least 70 of the overall length of said drum.

9. A rotary tunnel freezer substantially as herein before described with reference to and as shown in the accompanying drawing.

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