Title of Invention

A VACUUM INSULATED REFRIGERATOR CABINET AND METHOD FOR ASSESSING THERMAL CONDUCTIVITY THEREOF

Abstract A vacuum insulated refrigerator cabinet comprises an evacuation system for evacuating an insulation space (10) of the cabinet when pressure inside such space is higher than a predetermined value. It comprises a sensor device having an insulation reference element (14) located on one side of said insulation space (10) and temperature sensors (A, B, C) for assessing the differences of temperature (ΔT1, ΔT2) across the insulation space (10) and across the insulation reference element (14), such sensor device being suitable for providing the evacuation system with a signal related to the ratio of the above differences of temperature.
Full Text

A VACUUM INSULATED REFRIGERATOR CABINET AND METHOD FOR ASSESSING THERMAL CONDUCTIVITY THEREOF
The present invention relates to a vacuum insulated refrigerator cabinet comprising an evacuation system for evacuating an insulation space of the cabinet when pressure inside such space is higher than a predetermined value. With the term "refrigerator", we mean every kind of domestic appliance in which the inside temperature is lower than room temperature, i.e. domestic refrigerators, vertical freezers, chest freezer or the like. A vacuum insulated cabinet (VIC) for refrigeration can be made by building a refrigeration cabinet that has a hermetically sealed insulation space and filling that space with a porous material in order to support the walls against atmospheric pressure upon evacuation of the insulation space. A pump system may be needed to intermittently re-evacuate this insulation space due to the intrusion of air and water vapour by permeation. A solution of providing a refrigerator with a vacuum pump running almost continuously is shown in EP-A-587546, and it does increase too much the overall energy consumption of the refrigerator. It is advantageous for energy consumption to re-evacuate only when actually needed. Therefore there is in the art the need of a simple and inexpensive insulation measurement system that would be applicable to operate a refrigerator cabinet vacuum pump or similar evacuation system only when actually needed.
The present invention provides a vacuum insulated refrigerator cabinet having such insulation measurement system, according to the appended claims. According to the invention the sensor system is a system that compares the insulating value of the vacuum insulated cabinet to a standard insulation. Temperature measurements are made all at the same point on the cabinet. A pad of a material with known properties, preferably a standard non-ageing insulation, covers this point. The insulation performances of such standard insulation do not preferably change with time. Non-ageing insulators would be for instance rigid, open celled PU and rigid glass fibre insulation. Closed cell insulation such as PS or PU is less preferred since their insulation performances may change with age due to change in cell gas composition. The

temperature measurements are preferably made at a point on or near the outer surface of the insulation pad, at the interface of the pad and the cabinet liner (or alternatively to the wrapper, i.e. the outside surface of the cabinet) and at a point the opposite side from the pad. The temperature difference across the pad is compared to the temperature difference across the vacuum insulation. When the ratio of the temperature differences changes, it will indicate that the vacuum insulation is deteriorating. A criterion for vacuum pump operation based on this temperature ratio will assure that the insulation is always operating in an efficient manner. The function of the sensor system according to the invention is not affected by changing ambient conditions, as it would be affected a sensor system based on temperature values. Anyway, due to such changing ambient conditions, averages may have to be taken. Any of various temperature measuring devices may be used, some of which can measure the differences directly. Thermocouples and resistance thermometers are useful examples of such devices.
The invention will now be explained in greater detail with reference to drawings, which show:
Figure 1 is a schematic cross-view of a vacuum insulated cabinet according to the invention;
Figure 2 is an enlarged view of a detail of figure 1; and
Figure 3 is a schematic diagram showing the relationship between the
ratio of temperature differences across the cabinet and across the
insulation pad and the insulation performances.
With reference to figures 1 and 2, a refrigerator cabinet comprises a insulated
double wall 10 comprising two relatively gas impervious walls 10a (liner) and
10b (wrapper) filled with an insulation material 12 that can be evacuated. Both
liner 10a and wrapper 10b may be of polymeric material. The insulation material
12 can be an inorganic powder such as silica and alumina, inorganic and
organic fibres, an injection foamed object of open-cell or semi-open-cell
structure such as polyurethane foam, or a open celled polystyrene foam that is
extruded as a board and assembled into the cabinet. The insulation material 12
is connected to a known evacuation system (not shown) that can be a physical

adsorption stage (or more stages in series) or a mechanical vacuum pump or a
combination thereof.
According to the invention, on the wrapper 10b of the double wail 10 it is glued
or soldered an insulation pad 14 of a standard, non-ageing insulation, for
instance a rigid glass fibre pad. Temperature sensors, such as thermocouples,
are placed at points Af B and C of figure 2 and they are connected to a central
process unit of the appliance (not shown) in order to provide it with a ratio
AT1/AT2 between temperature difference across points A, B and B, C
respectively.
In the central process unit of the appliance every ratio AT1/AT2 is compared to a
minimum threshold value indicative of an increased pressure inside the cabinet
double wall 10. In figure 3 there is an indication of how the heat transmission
coefficient A changes with time, showing an increase of pressure inside the
double wall. In figure 3 the threshold value of A7YAT2 is indicated with reference
K.
A technical explanation behind the above behaviour may be found in the
Fourier's law for heat diffusion q=kxA*ar/ai (for steady-state heat diffusion
across the refrigerator walls), solved for one-dimensional conditions as is typically the case in domestic refrigerators where one of the dimensions (thickness) is usually much smaller then the other two (height and width). Fourier's law reveals that the temperature ratio of the differential temperatures across the vacuum wall and across a pad of standard insulation - AT1/AT2- can be ultimately expressed as ((*2xfl)/(ifclx/2)), where "/c" stands for the thermal
conductivity, and"I" stands for thickness.
From that, it is immediately evident that by keeping all the terms constant but k\%
the parameter described in the present invention to measure the insulation characteristics - again, A"TVAT2 - will increase as k \ decreases, and will decrease as k\ increases, as shown in fig. 3.
Some other observations may be made regarding the measurement system according to the present invention. Under steady state conditions, the equation AT1/AT2 is independent on temperatures inside the refrigerator and that of the ambient, so appropriately reflecting the variation of the uk factor" (thermal conductivity) of the vacuum insulation.

By increasing the thickness of the pad 14, or decreasing its thermal conductivity, the accuracy of value calculated by equation AT^ATawill improve. Secondly, although the proposed scheme does not depend upon the temperature history of the measured sites, it may be sensitive tc transient. In order to eliminate or reduce the above side effects, it is preferred to define a trigger value for vacuum pump switching-on based on a 10 % increase in k value.
This may be suitable from insulation maintenance standpoint, and could be implemented with reasonable accuracy.
Moreover it is preferred to use a "standard insulation pad" as thick as possible and with the lowest possible thermal conductivity (/c) for the sake of temperature measurement accuracy. Thermistors for temperature measurement should be preferably chosen with accuracy better than 0.2 °C, and door opening effect should be preferably eliminated through door sensors for awareness of "door status". As an alternative, it is possible to use the strategy of several consecutive measurements for confirming the degradation of the thermal insulation (vacuum degradation) and avoid the peaks in value since the door opening effect tend to be concentrated in a short period of time and vanishes quickly. If ambient temperature variation can be an issue (as for example in locations close to air conditioning/heating outlets), an external temperature sensor can help to purge those variations off the ATi/AT2 calculation.


A VACUUM INSULATED REFRIGERATOR CABINET AND METHOD FOR ASSESSING THERMAL CONDUCTIVITY THEREOF
The present invention relates to a vacuum insulated refrigerator cabinet comprising an evacuation system for evacuating an insulation space of the cabinet when pressure inside such space is higher than a predetermined value. With the term "refrigerator", we mean every kind of domestic appliance in which the inside temperature is lower than room temperature, i.e. domestic refrigerators, vertical freezers, chest freezer or the like. A vacuum insulated cabinet (VIC) for refrigeration can be made by building a refrigeration cabinet that has a hermetically sealed insulation space and filling that space with a porous material in order to support the walls against atmospheric pressure upon evacuation of the insulation space. A pump system may be needed to intermittently re-evacuate this insulation space due to the intrusion of air and water vapour by permeation. A solution of providing a refrigerator with a vacuum pump running almost continuously is shown in EP-A-587546, and it does increase too much the overall energy consumption of the refrigerator. It is advantageous for energy consumption to re-evacuate only when actually needed. Therefore there is in the art the need of a simple and inexpensive insulation measurement system that would be applicable to operate a refrigerator cabinet vacuum pump or similar evacuation system only when actually needed.
The present invention provides a vacuum insulated refrigerator cabinet having such insulation measurement system, according to the appended claims. According to the invention the sensor system is a system that compares the insulating value of the vacuum insulated cabinet to a standard insulation. Temperature measurements are made all at the same point on the cabinet. A pad of a material with known properties, preferably a standard non-ageing insulation, covers this point. The insulation performances of such standard insulation do not preferably change with time. Non-ageing insulators would be for instance rigid, open celled PU and rigid glass fibre insulation. Closed cell insulation such as PS or PU is less preferred since their insulation performances may change with age due to change in cell gas composition. The

temperature measurements are preferably made at a point on or near the outer surface of the insulation pad, at the interface of the pad and the cabinet liner (or alternatively to the wrapper, i.e. the outside surface of the cabinet) and at a point the opposite side from the pad. The temperature difference across the pad is compared to the temperature difference across the vacuum insulation. When the ratio of the temperature differences changes, it will indicate that the vacuum insulation is deteriorating. A criterion for vacuum pump operation based on this temperature ratio will assure that the insulation is always operating in an efficient manner. The function of the sensor system according to the invention is not affected by changing ambient conditions, as it would be affected a sensor system based on temperature values. Anyway, due to such changing ambient conditions, averages may have to be taken. Any of various temperature measuring devices may be used, some of which can measure the differences directly. Thermocouples and resistance thermometers are useful examples of such devices.
The invention will now be explained in greater detail with reference to drawings, which show:
Figure 1 is a schematic cross-view of a vacuum insulated cabinet according to the invention;
Figure 2 is an enlarged view of a detail of figure 1; and
Figure 3 is a schematic diagram showing the relationship between the
ratio of temperature differences across the cabinet and across the
insulation pad and the insulation performances.
With reference to figures 1 and 2, a refrigerator cabinet comprises a insulated
double wall 10 comprising two relatively gas impervious walls 10a (liner) and
10b (wrapper) filled with an insulation material 12 that can be evacuated. Both
liner 10a and wrapper 10b may be of polymeric material. The insulation material
12 can be an inorganic powder such as silica and alumina, inorganic and
organic fibres, an injection foamed object of open-cell or semi-open-cell
structure such as polyurethane foam, or a open celled polystyrene foam that is
extruded as a board and assembled into the cabinet. The insulation material 12
is connected to a known evacuation system (not shown) that can be a physical

adsorption stage (or more stages in series) or a mechanical vacuum pump or a
combination thereof.
According to the invention, on the wrapper 10b of the double wail 10 it is glued
or soldered an insulation pad 14 of a standard, non-ageing insulation, for
instance a rigid glass fibre pad. Temperature sensors, such as thermocouples,
are placed at points Af B and C of figure 2 and they are connected to a central
process unit of the appliance (not shown) in order to provide it with a ratio
AT1/AT2 between temperature difference across points A, B and B, C
respectively.
In the central process unit of the appliance every ratio AT1/AT2 is compared to a
minimum threshold value indicative of an increased pressure inside the cabinet
double wall 10. In figure 3 there is an indication of how the heat transmission
coefficient A changes with time, showing an increase of pressure inside the
double wall. In figure 3 the threshold value of A7YAT2 is indicated with reference
K.
A technical explanation behind the above behaviour may be found in the
Fourier's law for heat diffusion q=kxA*ar/ai (for steady-state heat diffusion
across the refrigerator walls), solved for one-dimensional conditions as is typically the case in domestic refrigerators where one of the dimensions (thickness) is usually much smaller then the other two (height and width). Fourier's law reveals that the temperature ratio of the differential temperatures across the vacuum wall and across a pad of standard insulation - AT1/AT2- can be ultimately expressed as ((*2xfl)/(ifclx/2)), where "/c" stands for the thermal
conductivity, and"I" stands for thickness.
From that, it is immediately evident that by keeping all the terms constant but k\%
the parameter described in the present invention to measure the insulation characteristics - again, A"TVAT2 - will increase as k \ decreases, and will decrease as k\ increases, as shown in fig. 3.
Some other observations may be made regarding the measurement system according to the present invention. Under steady state conditions, the equation AT1/AT2 is independent on temperatures inside the refrigerator and that of the ambient, so appropriately reflecting the variation of the uk factor" (thermal conductivity) of the vacuum insulation.

By increasing the thickness of the pad 14, or decreasing its thermal conductivity, the accuracy of value calculated by equation AT^ATawill improve. Secondly, although the proposed scheme does not depend upon the temperature history of the measured sites, it may be sensitive tc transient. In order to eliminate or reduce the above side effects, it is preferred to define a trigger value for vacuum pump switching-on based on a 10 % increase in k value.
This may be suitable from insulation maintenance standpoint, and could be implemented with reasonable accuracy.
Moreover it is preferred to use a "standard insulation pad" as thick as possible and with the lowest possible thermal conductivity (/c) for the sake of temperature measurement accuracy. Thermistors for temperature measurement should be preferably chosen with accuracy better than 0.2 °C, and door opening effect should be preferably eliminated through door sensors for awareness of "door status". As an alternative, it is possible to use the strategy of several consecutive measurements for confirming the degradation of the thermal insulation (vacuum degradation) and avoid the peaks in value since the door opening effect tend to be concentrated in a short period of time and vanishes quickly. If ambient temperature variation can be an issue (as for example in locations close to air conditioning/heating outlets), an external temperature sensor can help to purge those variations off the ATi/AT2 calculation.



CLAIMS
1. A vacuum insulated refrigerator cabinet comprising an evacuation system for evacuating an insulation space (10) of the cabinet when pressure inside such space is higher than a predetermined value, characterised in that it comprises a sensor device having an insulation reference element (14) located on one side of said insulation space (10) and temperature sensors (A, B, C) for assessing the differences of temperature (ATi, AT2) across the insulation space (10) and across the insulation reference element (14), such sensor device being suitable for providing the evacuation system with a signal related to the ratio of the above differences of temperature.
2. A vacuum insulated refrigerator cabinet according to claim 1, characterised in that the insulation reference element (14) is located on the external side of the cabinet.
3. A vacuum insulated refrigerator cabinet according to claim 1 or 2, characterised in that temperature sensors are three thermocouples (A, B, C) located on a surface of the insulation space (10) opposite the insulation reference element (14)t between the insulation space and the insulation reference element and on a surface of the insulation reference element opposite the insulation space.
4. A vacuum insulated refrigerator cabinet according to claim 1 or 2, characterised In that temperature sensors (A, B, C) are resistance thermometers.
5. A vacuum insulated refrigerator cabinet according to claim 4, characterised in that temperature sensors (A, B, C) have an accuracy at least of 0,2°C.
6. A vacuum insulated refrigerator cabinet according to claim 1, characterised in that the evacuation system is adapted to be triggered when the ratio of the above difference of temperature corresponds to a change in heat transfer coefficient higher than 10%.
7. Method for assessing the pressure inside an insulation space (10) of a vacuum insulated cabinet of a refrigerator, characterised in that it comprises the steps of evaluating the differences of temperature across

the insulation space (10) and across an insulation reference element (14) placed on a side of such insulation space, such evaluation being carried out on the same zone of the vacuum insulated cabinet where the insulation reference element is also placed, and providing a control system of the refrigerator with a signal related to the ratio (ATVAT2) of the above differences of temperature, such ratio being indicative of pressure value inside the insulation space.

8. A vacuum insulated refrigerator cabinet substantially as herein described with reference to the accompanying drawing.


Documents:

2964-CHENP-2004 AMENDED PAGES OF SPECIFICATION 21-06-2011.pdf

2964-CHENP-2004 AMENDED CLAIMS 21-06-2011.pdf

2964-chenp-2004 form-1 21-06-2011.pdf

2964-chenp-2004 form-3 21-06-2011.pdf

2964-CHENP-2004 OTHER PATENT DOCUMENT 21-06-2011.pdf

2964-CHENP-2004 POWER OF ATTORNEY 21-06-2011.pdf

2964-CHENP-2004 CORRESPONDENCE OTHERS 10-12-2010.pdf

2964-CHENP-2004 EXAMINATION REPORT REPLY RECEIVED 21-06-2011.pdf

2964-chenp-2004-claims.pdf

2964-chenp-2004-correspondnece-others.pdf

2964-chenp-2004-correspondnece-po.pdf

2964-chenp-2004-description(complete).pdf

2964-chenp-2004-drawings.pdf

2964-chenp-2004-form 1.pdf

2964-chenp-2004-form 3.pdf

2964-chenp-2004-form 5.pdf

2964-chenp-2004-pct.pdf


Patent Number 248234
Indian Patent Application Number 2964/CHENP/2004
PG Journal Number 26/2011
Publication Date 01-Jul-2011
Grant Date 29-Jun-2011
Date of Filing 31-Dec-2004
Name of Patentee WHIRLPOOL CORPORATION
Applicant Address 2000 M 63 BENTON HARBOR MI 49022
Inventors:
# Inventor's Name Inventor's Address
1 KIRBY, DAVID 1775 MEADOW GROVE ST JOSEPH MI 49085
2 MARTINELLA, LUIGI VIA RISORGIMENTO 127 I-28823 GHIFFA ITALY
3 GIUDICI, GIORGIO VIA FIUME 6 21015 LONATE POZZOLO ITALY
PCT International Classification Number F25D 23/06
PCT International Application Number PCT/EP03/06865
PCT International Filing date 2003-06-27
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 02014061.2 2002-07-01 EUROPEAN UNION