Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
ROTARY TUNNEL FREEZER
FIELD OF THE INVENTION
This invention relates to rotary tunnel freezers.
BACKGROUND OF THE INVENTION
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 co-
' current 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-l 601 298; US-A-l 492 488 and
~` UK-A-l 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 counter-current flcw to a stream of nitrogen
which is introduced through a spray headed ad;acent 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 disadvant-
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ages. In particular, the effect of the blast decreasesextremely rapidly as the distance from the nozzle in-
creases. In addition, the recirculation fan absorbs
energy and, at the same time, imparts heat energy to the
recirculating gas~
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It will be appreciated that it is impractical to
install fans in the rotatable drum because of the rota-
tion of the drum, the risk of damage from and to the
product and the low temperatures present.
SUMMARY OF THE INVENTION
~" 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 cryogen into an end of said drum,
wherein said drum is provided with a plurality of noz-
zles arranged at intervals along a major portion of the
length (F) of said drum for introducing cryogenic fluid
into said drum and stirring the atmosphere therein.
Advantagèously, said means for introducing cryogen
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 cryogen into said drum
substantially tangentially thereof and, preferably,
closely adjacent the radially inner surface of the drum.
Conveniently, the nozzles for stirring the atmo-
sphere are disposed on generally outwardly extending
~i, 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 400mm to 600mm, and advantageously at 500mm
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
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output of the nozzles in the spray header and the noz-
- 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.
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For a better understanding of the present inven-
tion, 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.
~`. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
~,; Referring to the drawing, there is shown a rotary
tunnel freezer which is generally identified by refer-
ence 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 approxima-
tely 15% of the overall length 'F' of the drum. The
spray header 6 is provided with a plurality of nozzles
~`~ 20 with outlets 2mm in diameter positioned at 50mm centres.
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 3 to the horizontal.
~ The inlet section 7 is provided with a helical fin which
,- 25 defines a plurality of weirs.
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.
~, 30 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.
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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 re-
'~ quired.
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Product to be frozen, 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 ni~rogen 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 nitro-
gen to the product.
The arrangement thus far described is also de-
scribed in our European Patent Publication No.
0 372 354.
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In order to increase the rate of heat transfer the
drum is provided with stlrring 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.
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~-` 30 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
~35 liquid for maximum cooling. However, where it is desir-
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able to avoid over-cooling, a valve (not shown) is pre-
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.
The nozzles 10 are positioned at 500mm centres and
are positioned to introduce nitrogen substantially
tangentially to the drum 2 approximately 10 cms from the
inside of the drum 2.
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Initial trials have been extremely encouraging and
~- we have found 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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