Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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2
This invention relates to a method and means for providing
in-transit refrigeration for goods.
The term "refrigeration°' as used herein should be
interpreted as lowering of the temperature of goods and/or
maintaining goods at a relatively low temperature and/or reducing
the rate of increase in the temperature of goods.
Carbon dioxide "snow" as used herein is the term commonly
used for carbon dioxide in a powder form, and which is
conventionally made by means of an appropriate device, typically
a lance provided with a nozzle, through which liquid carbon
dioxide at an appropriate temperature and pressure is forced.
This process is commonly known as "snow shooting°' which is a
trade mark of The BOC Group,
1
3
The use of carbon dioxide snow for in-transit refrigeration
is known. Conventionally, carbon dioxide snow is introduced
directly into a transport container through a port in a wall or
door of the container. The carbon dioxide snow settles onto the
goods being transported and onto the container walls, and can
thereby cause severe thermal shock to the goods and to the walls
of the container. Furthermore, a significant proportion of the
carbon dioxide snow particles sublimes when coming into contact
with relatively warm container walls and goods, and most of the
cold gaseous carbon dioxide formed by sublimation is immediately
swept out of the container with the main snow shooting gas
stream. The cooling potential of this gas is thereby lost.
The applicant is also aware of containers having permanent
solid wall bunkers or "plenums" as they are called. Carbon
dioxide snow is injected into the permanent bunker which is
usually located in the upper region of the container near the
ceiling. The thermal shock problems of the abovementioned
conventional method as far as the goods are concei~-ied, are
reduced thereby and a longer lasting cooling effect results.
However, disadvantages of this method include thermal shock to
the bunker (ie plenum) walls and further include relatively high
installation costs and a reduced payload in the container to
compensate for the extra weight of the permanent bunker wall and
resulting reduced loading space. Furthermore, the plenum has a
relatively high thermal absorption capacity, similar to that of
the container walls, and also insulates the carbon dioxide snow
4
it contains from heat which leaks into the container (hereinafter
referred to as "heat in=leak"). Thus there is a significant
delay before the heat in-leak is compensated for by sublimation
of some of the carbon dioxide snow in the plenum, thereby
reducing the efficiency of this system for maintaining g~ods at a
low and constant temperature.
According to the invention there is provided a method of
providing a container for goods with a refrigeration facility,
which method includes
placing in gaseous communication with the interior of the
container, a holder for holding carbon dioxide snow, which holder
or a continuous wall region thereof constituting at least about
15% of the total wall area of the holder, is of a flexible
material that is at least substantially impermeable to the carbon
dioxide snow and permeable to gaseous carbon dioxide.
Preferably the continuous wall region which is of a flexible
material constitutes at least 25% of the total wall area of the
holder, more preferably 60 to 100%, and ideally 1000 thereof.
By "total wall area'° is meant the total area of the outside
surface of the holder wall(s).
The method may include locating the holder in the container
so that it is readily removable from the container, and may
further include providing a support structure for the holder
a i
~i~l~.l~~
within the container and locating the holder in the container by
attaching it to the support structure. Where the container has a
ceiling, the method may include locating the holder at or
0
adjacent the ceiling of the container so that it is situated
above the goods in use.
Where the holder is readily locatable in a container and,
further, readily removable from the container at will, then the
method of the invention may include introducing the carbon
dioxide snow into the holder before it is located in the
container.
Instead, the holder can be charged with carbon dioxide snow
after it has been appropriately located in the container.
According to a further aspect of the invention, there is
provided a holder for holding carbon dioxide snow in gaseous
_ communication with the interior of the container for goods, which
holder or a continuous wall region thereof constituting at least
about 15% o_f the total wall area of the holder, is of a flexible
material that is at least substantially impermeable to the carbon
dioxide snow and permeable to gaseous carbon dioxide.
The holder preferably has an insignificant thermal
absorption capacity. By this is meant that the thermal
absorption capacity of the holder is low relative to that of the
container walls and related equipment and relative to the goods
6
intended to be refrigerated in the container, so that any heat
in-leak is experienced by the carbon dioxide snow with minimal
delay to cause substantially immediate sublimation of carbon
' dioxide snow to compensate for the heat in-leak.
The holder may be of a fabric of cotton, wool or plastics
material, or may be of wire mesh. For example, the fabric may be
a woven polypropylene, polyethylene or nylon cloth.
A preferred holder according to the invention comprises a
bag of a gas permeable material which acts as a phase separator
by allowing the gaseous carbon dioxide to pass through it while
retaining the carbon dioxide snow which, while the holder is
being charged with carbon dioxide snow, eg by snow shooting, is
forced outwardly towards the walls) of. the bag by the escaping
gaseous carbon dioxide. Semi-compaction of the solid carbon
dioxide snow phase results.
The holder may have attaching means whereby it can be
attached to the support structure within the container. The
attaching means may be removably attachable to the support
structure to enable the holder readily to be removed from. the
container.
The applicant envisages that the method and holder of the
invention will be particularly useful for in-transit
refrigeration. Such applications will involve, inter alia,
maintenance of the relatively low temperatures of a variety of
frozen or chilled goods, particularly beverages and foodstuffs
such as meat, seafood, confectionery, poultry, vegetable, fruit,
yeast, and dairy products while they are in bar/drinks trolleys,
hand held containers, etc. or while they are being transported in
road vehicles, railcars, shipping or aircraft containers or the
like.
The amount of carbon dioxide snow and hence the size of the
holder required for a particular application will be influenced
by a variety of parameters which include the size of the
container, its specific heat and insulative properties (ie the
material of which the container is made and the nature and
thickness of any additional insulation), the nature and mass of
any goods being transported in the container, ambient
temperature, the respective initial and desired temperatures
within the container, the respective initial and desired
temperatures of the goods, the expected duration of
transportation to destination, the expected number of times the
container is expected to be opened before it reaches its
destination, etc.
The invention extends to a container for goods which
comprises a holder according to the invention or which has been
provided with a refrigeration facility according to the method of
the invention.
2~~~~~~
8
The invention extends further to a support structure/holder
combination for use in a method of providing a container for
goods with a refrigeration facility, which combination c~mprises
a holder for holding carbon dioxide snow, which holder or a
continuous wall region thereof constituting at least about 150 of
the total wall area of the holder, is of a flexible material that
is at least substantially impermeable to the carbon dioxide snow
and permeable to gaseous carbon dioxide; and
a support structure for supporting the holder and which is
attachable to the container in a position in which, in use, the
carbon dioxide snow in the holder is in gaseous communication
with the interior of the container.
The invention is now described by way of the following non-
limiting examples and with reference to the accompanying drawings
in which
Figure 1 is a schematic diagram of a first embodiment of a holder
according to the invention in the form of a bag, which is being
charged with carbon dioxide snow;
Figure 2 is a schematic section through a container which has
been provided with a refrigeration facility.according to the
method of the invention;
Figure 3 is a schematic section through a container which is
being provided with a refrigeration facility according to the
method of the invention;
Figure 4 is a schematic diagram of a typical carbon dioxide snow
making device;
9
Figure 5 is a schematic section through a container which has
been provided, in an alternat~.ve manner with a refrigeration
facility in the form of a second embodiment of a holder according
' to the invention;
Figure 6 is a three-dimensional view of a support
structure/holder combination according to the invention, with a
container in which it is mounted being shown in broken lines;
Figure 7 is a partly sectioned side view of a container in which
is mounted the combination of Figure 6;
Figure 8 is a section through 8 - 8 on Figure 7;
Figure 9 is a three-dimensional view of a third embodiment of a
holder according to the invention;
Figure 10 is a graphical illustration of respective temperature
profiles of the atmosphere within a container with a
refrigeration facility according to the invention in use with
carbon dioxide snow, and chilled goods contained therein;
Figure 11 is a graphical illustration for comparison purposes, of
_ respective temperature profiles of a conventional container and
chilled goods contained therein, and into which container carbon
dioxide snow has been injected directly;
Figure 12 is a graphical illustration of respective temperature
profiles of the atmosphere within a container with a
refrigeration facility according to the invention in use with
carbon dioxide snow, and frozen goods contained therein;
Figure 13 is a graphical comparison between a temperature
profile of the atmosphere within a container with a refrigeration
facility according to the invention in use with carbon dioxide
~~7~"~~~
snow, and a temperature profile of the atmosphere within a
conventional container into which a similar mass of carbon
dioxide snow has beer injected directly;
Figure 14 is a graphical comparison of respective oxygen
profiles of the atmospheres within three different containers
each having a refrigeration facility according to the invention,
in use with carbon dioxide snow; and
Figure 15 is an elarged cross-sectional view through a side wall
of the holder of Figure 9. '
EXAMPLE 1:
In Figures 1 and 2, reference numeral 10 generally
indicates a holder according to the invention in the form of a
bag for carbon dioxide snow.
The bag 10 is of a nylon, polyethylene or polypropylene
material 12 which is tightly woven so that the apertures are in
the order of 0,5 mm in size. This material 12 is substantially
impermeable to carbon dioxide snow but is permeable to gaseous
carbon dioxide.
When expanded to its greatest extent, the size of the.bag 10
is approximately 2,0 m x 2,0 m x 0,25 m and holds 400 kg of
carbon dioxide snow.
11
In order to charge the bag 10 with carbon dioxide snow 14
according to the method of the invention, liquid carbon dioxide
(indicated by arrow 16) at approximately -20°C and 20 bar pressure
' is forced, via a conventional snow shooting lance 18, through an
inlet port 20 into the bag 10. Carbon dioxide snow 14 is driven
,against the inside wall 22 of the bag 10 by gaseous carbon dioxide
(indicated by arrows 24) escaping through the permeable wall 22.
As the snow shooting process continues, carbon dioxide snow 14 is
semi-compacted along the inside wall of the bag 10.
By weighing the bag 10 after it had been charged with carbon
dioxide snow 14 and measuring the mass of liquid carbon dioxide 16
used to charge the bag 10, a conversion rate of liquid carbon dioxide
to carbon dioxide snow can be determined. By using the method of
this example, a conversion rate approximating to 2 : 1 is obtainable.
Tt will be appreciated that this is significantly better than
conventional conversion rates of approximately 2.5 to 3 . 1.
Tndeed, according to the method of the invention, a conversion rate
approximating to the theoretical value is obtainable.
A conventional snow making device can be used to charge the bag
with carbon dioxide snow 14. Such a device is shown
schematically in Figure 4 in which reference numeral 30 indicates
an insulated vessel containing liquid carbon dioxide at typically
minus 20°C and 20 bar pressure, reference numeral 32
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indicates a liquid carbon dioxide supply line comprising a 25 mm
diameter copper tube or flexible stainless steel hose, numeral 34
indicates an isolation valve (typically a 25 mm diameter ball valve) ,
' and reference numerals 18 and 36 indicate a snow shooting lance and
a 6,35 mm diameter Venturi nozzle which is capable of delivering 20
kg carbon dioxide snow per minute.
In use, the bag 10, charged with carbon dioxide snow 14, is
suspended from the walls or ceiling of a container 40 and adjacent
the ceiling thereof, in the manner shown in Figure 2. The container
40 is a standard international container having dimensions 2.4 m x
2.4 x 6.0 m and having 75 mm polyurethane insulated walls. The
container 40 has a 100 mm diameter inlet port.(not shown in Figure
2) for carbon dioxide snow and a gas vent (not shown in Figure 2)
which is 150 mm square. The container 40 is loaded, on average,
with 10 to 18 ton weight of frozen foodstuff 41 at -18°C.
For such a container, on average 6,0 kg snow is consumed per
hour to compensate for heat inleak through the insulated walls.
Thus, far a 24 hour journey, 144 kg of carbon dioxide snow is
required to deliver frozen foodstuff at approximately the same
temperature at which it starts the journey (ie at approximately -
18°C).
13
Accordingly the bag 10 is suitable, for example, for use in
a standard international container such as the container 40, for
the type of application described above.
luring the journey, the carbon dioxide snow 14 sublimates to
form gaseous carbon dioxide which escapes through the wall 22 of
the bag 10.
EXAMPLE 2:
Figure 3 shows a container 40 being provided with a
refrigeration facility according to the method of the invention
and in a similar manner to what is described in Example.l above.
The only difference is that in the present example, the bag 10
is installed in the container 40, adjacent the ceiling, before it
is charged with carbon dioxide snow 14. After installation of
the bag 10, carbon dioxide snow 14 is introduced into the bag 14
via the lance 18 which is inserted into the bag 10 via the inlet
port 42 in the container walle The gaseous carbon dioxide being
formed during this snow shooting process, escapes through the
wall 22 of the bag 10 and passes out of the container 40 via the
gas vent 44 in the container wall. When the bag 10 has. been
charged with the desired mass (in this case 144 kg) of carbon
dioxide snow 14, the lance 18 is removed and the inlet port 42
and gas vent 44 are closed.
~~"~~."~~~
14
EXAMPLE 3:
Figure 5 shows an alternative holder 50 according to the
invention in the form of a sheet of metal mesh, or in the form of
a sheet of polyethylene or polypropylene or nylon material
similar to the material of which the bag 10 is made. The sheet
50 is suspended from the ceiling or upper walls of the container
40 to define a space immediately below the ceiling of the
container 40, for accommodating carbon dioxide snow 14.
144 kg of carbon dioxide snow 14 is produced by the snow
making device 30 and charged directly into the space between the
carbon dioxide-permeable sheet 50 and the ceiling of the
container 40 via the inlet port 42 (not shown in Figure 5) in the
container wall, and using the lance 18 (not shown in Fj~gure 5) .
After completion of snow shooting, the lance 18 is withdrawn and
the inlet port 42 and gas vent 44 (not shown in Figure 5) are
closed.
Upon arrival at the final destination, the doors of the
container 40 which have been refrigerated according to the method
of the invention described in any one of examples 1 to 3 above,
should be left open for a few minutes to allow any remaining
carbon dioxide gas to escape so that safe offloading can be
effected.
CA 02071792 1999-12-30
15
EXAMPLE 4:
In Figure 6, reference numeral 60 generally
indicates a combination support structure/holder
according to the invention. The combination 60
comprises a bag 62 similar to the bag 10 and a
framework 64 for supporting the bag 62. The framework
64 is mountable within a container for goods 66 by
means of brackets 68 provided for that purpose on
opposed walls 70 of the container 64 adjacent the
container ceiling 72.
The size of the bag 62 is 2 m x 1,75 m x 0,3 m. It
has attachment means in the form of VELCRO ~J strips 74
whereby it is attached to the framework 64. In use, the
bag 62 is charged with carbon dioxide snow via an inlet
tube 76.
"VELCRO" is a trade mark of Velcro Industries B.V.
As mentioned previously, sufficient snow is
provided in the container to maintain the temperature
of the load during the entire journey. This snow
therefore compensates for heat ingress from the
relatively warm environment (at ambient atmospheric
temperature) into the cooled interior of the insulated
container. Heat in-flow occurs as a result of (1) heat
in-flow through the container walls and (2) heat in-
flow and carbon dioxide gas loss through openings in
the container (eg open doors and faulty seals).
a . ~\.
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1. Heat in-flow through insulated container walls:
The following formula is useful in estimating this heat flow.
Heat in-flow ~~ = 3600 AU T (kJ/h.)
d
Where A = External container surface area (mz)
U = Coefficient of conductivity of insulating
material (ku.m)
(T
T =Temperature difference between external ambient
atmospheric temperature and load temperature (°C)
d = Thickness of insulating material in
container walls.
2. Heat in-flow and loss of cold COZ gas from container throw
2.1 Opening of doors (eg for deliveries)
Estimation formula:
Net heat inflow QZ = d9V~CP T n (kJ/h. )
Where 89 = Density of COZ gas (kg/ 3)
m
V~ = Container internal free volume (m3)
CP = Specific heat of COZ gas (k~ )
kf°C
a
17
T - Temperature difference (as above)
(°C)
n - Frequency of door openings
(number/h.)
2.2 Leaks:
Estimation formula:
Net heat in-flow Q3 = MCP T (kJ/h. )
Where M - COZ gas leakage rate (kg/h.)
CP = Specific heat (as above) (k,
(kg°C)
T = Temperature difference (as above) (°C)
Total heat in-flow into container Q = Q~ + QZ + Q3 (kJ/h.)
Snow required S = Q~ + QZ + Q~ (kg/hr)
R
Where R = COZ snow refrigeration capacity (kJ/kg)
Liquid COZ required L = S x f (kg/h.)
Where S = snow required (as above) (kg/h.)
f = liquid to snow conversion factor
and is 2 for permeable.snow holder
and is 2.7 for conventional snow injection
The preferred holders 10, 50 and 62 are designed to hold
sufficient snow in such a way that it will absorb heat (by way of
sublimation) at a rate similar to the total net rate of heat in-flow.
into the container, thereby maintaining load temperature at any desired
value in a typical range of -20°C (for frozen goods) to y5°C
(for
chilled goods).
CA 02071792 1999-12-30
18
The following three snow holder parameters are
relevant:
Volume:
Estimation formula for desired volume (VH):
Holder volume UH = S x t (m3)
~s
Where S - Snow required (as above) (kg/h.)
t - Desired maximum journey duration (h.)
Density of semi-compacted snow in
holder ( kg/m3 )
Dimensions (or surface to volume ratio):
Dimensions of snow holder should be compatible with
load and method of .loadinq (refer examples).
Permeable surface to volume ratio of the snow holder is
a factor that determines the rate of heat absorption of
the snow holder when it is in use with carbon dioxide
snow and which should substantially match the rate of
heat in-flow into the container in order to maintain
load temperature.
High heat in-flow rates will require relatively high
permeable surface to volume ratio snow holders and vice
versa (refer examples).
Material:
The following requirements should be met:
1. An ability to withstand low temperature of
snow(-76°C).
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19
2. An appropriate heat buffer (if required) bet~eeen sno;~:
on the one hand and the container atmosphere and goods
on the other hands. The nature and thickness of the
' material is another factor that determines the rate of.
heat absorption of the snow holder.
3. Sufficient permeability such that:
Carbon dioxide snow is contained within the holder
and carbon dioxide gas passes through the holder during
injection of the snow into the holder, and
COz gas can escape from the holder at an appropriate
rate to help maintain load temperature in use.
Examples A and B below, are examples of how the approximate amount of
carbon dioxide snow required for use in a particular application of the
method of the invention, can be calculated.
This enables the desired dimensions of a holder according to the
invention for the particular application to be determined.
EXAMPLE A
RAILWAY MINI CONTAINER
Surface area (excluding floor) A~ = 23.7 m2
U - 2.8 x 10 5 kWm/m2°C (polyurethane foam)
~~'~~."~~
d - 0,085 m
T = ambient minus frozen load temperatures
- 25 -(-18)°C
- 43°C
Heat in-flow through walls and roof
Q1A = 3600 x 23.7 x 2.8 x 10 5 x 43
0.085
- 1209 kJ/h.
Surface area of floor Az = 3.6 m2
U - 2.8 x 105 kWm/m2°C
d - 0.01
T = 43°C
Heat in-flow through floor
Q1e = 3600 x 3.6 x 2.8 x 10 5 x 43
0.01
-- 1560 kJ/h.
Qz = 0 because the container doors remained closed during the trip.
However tests revealed a relatively high value for COZ gas loss because
of inadequate door seals, of 11 kg/h. ( = M )
CP = 0.63 kJ/kg°C -
T = 43°C
Heat in-flow Q3 = 11 x 0.63 x 43
- 298 kJ/h.
Total net heat in-flow
Q Q1A + Q1g + Q2 + Q3
CA 02071792 1999-12-30
21
- 1209 + 1560 + 0 + 298
- 3067 kJ/h.
Snow refrigeration capacity R = 640 kJ/h.
Therefore: Snow required S - 3067
640
- 4.8 kg/h.
Liquid COz required L = 4.8 x 2
- 9.6 kg/h.
Required trip duration t - 30 hours
~s - 400 kg/m3
Snow bag volume V = 4.8 x 30
400
- 0.36 m3
Desired bag dimensions were determined as follows:
- The bag should occupy minimum cargo space.
- It should have a high permeable surface to
volume ratio of 16 to 4 in order to
compensate for high rates of heat in-flow
through the floor and because of leaking
doors.
A permeable bag similar to the bag 60 but having
dimensions 2 m x 1.2 m x 0.15 m covered most of
the container roof area and was found to satisfy
the above requirements.
Back material:
Tests showed that woven polypropylene material, single
end warp, twill weave, 30 x 16 constitution, meet the
requirements of
- ability to withstand low temperatures
2~~~
z2
- carbon dioxide gas permeability
° low heat absorption capacity
EXAMPLE E
DOOR-TO-DOOR DELIVERY TRUCK
Truck container surface area - 23.5 m2
U - 2.8 x 10 5 km/mZ°C (polyurethane)
d - 0.05 m
T = ambient - chilled load temperature
25 - 5
- 20°C
Heat in-flow through walls
Q~ = 3600 x 23.5 x 2.8 x 105 x 20
0. 05
- 948 kJ/h.
An average of four deliveries per hour (ie n = 4) were to be made to
deliver chilled goods.
Container free volume U~ = 7. 5 m3 total - 0.4 m3 load = 7 . i m3
d9 = 1.84 kg/m~
CP = 0.63 kJ/kg°C
T = 20°C
Heat in-flow through door openings
QZ = 1.84 x 7.1 x 0.63 x 20 x 4
- 658 kJ/h.
Q3 = 0 because doors seal well
Total heat in-flow = Q~ + QZ + Q3
- 948 + 658 + 0
23
- 1606 kJ/h.
Snow refrigeration capacity R = 640 kJ/kg
Therefore:
- Snow required S = 1606
640
- 2.5 kg/h.
Liquid COz required L = 2.5 x 2
- 5 ka /h _
Max trip duration = 6 hours
ds = 400 kg/m3
Snow holder volume VH = 2.5 x 6
400
- 0.0375 m3
EXAMPLE 5
xn Figure 9, reference numeral 80 generally indicates a third
. embodiment of a holder according to the invention. The holder 80
comprises a standard plastics crate 81 with wall apertures 81.1; 0.04
m3 in volume, which is lined with 30 x 16 weave polypropylene material
82. A sheet of the polypropylene material 82 covers the top opening
of the crate and provides a flexible wall 80.1 of the holder 80, the
wall 80.1 being permeable to carbon dioxide gas and substantially
impermeable to carbon dioxide snow.
Tnstead of the polypropylene material, open (14~') steel mesh can be
used to provide the lining and the flexible wall 80.1.
'~4
One of the side walls 80.2 of the holder 80. is shown in cross
section in Figure 15. Fdeference numerals 81.2 and 82.1 indicate
the wall of the plastics crate 81 and a regien of the sheet 82,
respectively.
In use the holder 80 is charged through an opening 84 in the
polypropylene sheet 82, with carbon dioxide snow injected therein
via a conventional snow shooting lance 86.
A particular advantage of the holder 80 is the relative ease with
which it can be manually loaded into and removed from a
container.
An appropriate number of holders 80 were charged with carbon
dioxide snow and loaded into a container as described in Example
E above, with 0.04 m3 plastics crates containing chilled goods.
The number of holders 80 containing carbon dioxide snow was
determined according to the expected duration of the journey and
the expected number of deliveries (ie door openings) to be
undertaken. (In this case an average of four deliveries per hour
were to be made.) It was expected that the relatively low
surface area to volume ratio of each of the holders 80 and the
relatively short periods of time between consecutive door
openings, would necessitate using a fan within the container to
speed up the carbon dioxide sublimation reaction to heat in-leak.
~~~~ ~~'~e~
It was found that the resulting refrigerated container was
effective throughout the journey in maintaining the crates of
goods at a suitably low chilled temperature while avoiding
' undesired freezing of the goods.
The efficacy of a refrigerated container comprising a holder
according to the invention which has been charged with an
appropriate mass of carbon dioxide snow, is illustrated
graphically in Figures l0 and 12.
Furthermore, a comparison of Figures 10 and 12 (the
invention) with Figure 11 (prior art), and consideration of
Figure 13 which illustrates refrigeration conditions of both the
invention and the prior art, show that a container refrigerated
according to the method of the invention does not undergo the
thermal shock to which a container refrigerated by direct snow
injection (prior art) is subjected. Furthermore, chilled
products in the refrigerated container of the invention (Figure
10) is not subjected to freezing as are the chilled products in
the container which is refrigerated according to prior art
techniques (Figure 11). It will be appreciated that products
comprising fresh fruit, vegetables, dairy products, etc. are
damaged, or at least the quality thereof is adversely affected,
by freezing.
Figure 10 graphically illustrates the efficacy of the above
exemplified means and method of the invention in maintaining
~
\
r 8
26
chilled goods at suitable low temperatures above freezing point, fo
extended periods of time. As can be seen from Figure 11, conventiona
means and methods of charging carbon dioxide snow directly int
conventional containers loaded with chilled (ie nonfrozen) goods a
temperatures in the range up to about 7°C cause freezing of the goods
This adversely affects the quality of goods such as fresh fruit
vegetables and dairy products.
Figure 12 graphically illustrates the efficacy of the abov
exemplified means and method of the invention in maintaining froze
goods at temperatures below 0°C. The apparently anomolous decrease i
the temperature of the frozen goods as the temperature cf the containe
atmosphere increases can be explained by the experimental conditions
Tn fact the respective temperatures of the goods and containe
atmosphere respectively were taken at re:rote locations from, each o;.ha
within the container and, expectedly, the temperature of the goods
not uniform throughout the container; nor was the tempErature of th
container atmosphere. The respective temperatures of gceds an
atmosphere positioned adjacent each other doubtless would have bea
substantially the same.
Figure 13 graphically compares the respective temperature profile.
of atmosphere within a container refrigerated according to th~
invention, and a conventional container which is
~'~~S~a~~
27
refrigerated by direct injection of carbon dioxide snow. The sudden
reduction in temperature of the container atmosphere from about o°C to
-80°C in the conventional case causes thermal shock to the container
walls. In contrast, no sucn tnermal snocx is experiencea by container
walls when the means and method of the present invention are used.
Graphs 88, 89 and 90 in figure 14 indicate the respective oxygen
profiles of the atmospheres within the following containers which have
been provided with refrigeration facilities according tc the invention:
Reference numeral 88 relates to a 6 m container with new door sews.
Reference numeral 89 relates to a 6 m container with suspect door
seals.
Reference numeral 90 relates to a 1,5 m container with inadequate door
seals.
As can readily be seen, the proportion of oxygen in the container
atmosphere relatively quickly increases to about 4~ and is r,.aintainsd
in the range 5 to 15% for an extended period of time, ie in excess of
r
40 hours.
The advantages of the invention, at least as exemplified, include
the greater efficiency in the conversion of liquid carbon dioxide to
carbon dioxide snow obtained according to the method of the invention
and the resulting saving in costs. This is
2~
believed to be the result of a reduction in sublimation losses
of snow particles during the actual snow shooting operation.
Because of the abovementioned semi-compacted state of the snow
particles on the inner side of the gas permeable wall of the
holder according to the invention, the rate of snow sublimation
is reduced. Tt will be appreciated that the usefulness of the
semi-compacted snow as a coolant, particularly as a means for
maintaining and not necessarily reducing the temperature of
goods, is thereby enhanced.
Furthermore, snow shooting of carbon dioxide into a holder
of the type envisaged herein for use in the method of the
invention, enables the snow shooting lance to be inserted into
the holder through an inlet port approximating to that of the
cross-sectional dimensions of the lance. Thus loss of carbon
dioxide which occurs during conventional direct snow shooting
into containers for goods (eg because of leaking door seals,
etc.) is obviated or, at least, minimised.
An advantage of the carbon dioxide atmosphere which results
from using carbon dioxide snow as a coolant in the method of the
invention, is its biocidal or at least its bacteriostatic
properties for applications in which products such as foodstuffs
are being refrigerated. Furthermore it will be appreciated that
the holder according to the invention serves to control the rate
of sublimation of the carbon diaxide snow which allows the
proportion of oxygen in the atmosphere within the container to be
~~'~.~~z~
29
maintained between approximately 5% and 15%. This proportion of
oxygen inhibits the deterioration of respiring fresh produce
which would otherwise occur in a more concentrated carbon dioxide
atmosphere.
Further advantages include a reduction in the thermal shock
on transport container walls and goods because of containment of
snow within the holder according to the invention. Furthermore,
a more uniform time vs temperature profile within the transport
container and load is obtained because of the relatively low rate
of snow sublimation and separation of the snow from the goods.
Furthermore, the means for containing the snow and keeping it
separate from the goods, ie the holder according 'to the
invention, is light weight, space efficient, low in cost, and
readily locatable in and removable from conventional containers.
Thus the method and means according to the invention are
versatile and adaptable to suit any of a large number of possible
t applications.
