Safety Device For Multi-pane Glass Refrigerator Doors

Abstract

A safety device for a multi-pane electrically heated glass refrigerator door. The glass unit forms a pressurized chamber. A pressure sensor and associated circuit breaking means removes electrical power from the door when damage to the unit causes the pressure within the chamber to vary.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention.

This invention relates to safety devices and more particularly to power cut-out safety devices for use in commercial refrigerated display cabinets having multi-pane glass doors wherein electric power is applied to a conductive coating positioned on at least one pane to enable heating of the glass.

2. Description of the Prior Art.

Refrigerated display cabinets are used in food stores to store and display food products which would spoil at ordinary room temperatures. To enhance the display characteristics of these cabinets, the doors are typically made of glass. To maintain the cold temperature within the cabinet, the glass doors are fabricated from sealed multi-pane glass units with space being provided between each pair of glass panes. The space defines a sealed thermal insulation chamber.

For extreme low temperature operation the surface of one of the planes within the sealed chamber typically has a thin film of optically transparent conductive coating material deposited thereon. The coating is connected to electrical power lines via metallic conductive strip leads positioned on the glass. The resultant heating of the glass by the electric current passing through the conductive coating effectively prevents frosting of the glass surfaces, thus permitting the displayed foods to be seen clearly at all times.

Since one or more of the glass panels has electric power applied to conductive metallic strips contacting the conductive coating disposed on the inner surface of the glass, should a break occur in any of the glass panels there is danger of shock or more severe consequences to anyone coming in contact with the unit. To obviate this hazard, a means must be provided to remove electric power should any damage occur affecting the integrity of the glass unit.

Known safety devices require that the glass panel fractures sufficiently to cause a break in one of the metallic conductive strip leads before such devices are activated. These types of safety devices are generally inadequate to detect potentially dangerous conditions as where a hairline fracture of one of the glass panels fails to break a metallic lead.

One of the prior art safety devices utilizes the breaking of the conductive strip lead on the glass to trip an in-line connected relay system, thereby opening the relay contacts connected in the power line. Such a system will fail, as noted above, if a hairline or other crack in the glass fails to break the conductor strip which is usually adhesively attached to the glass. Another prior art safety measure utilizes a bimetallic thermal thermal switch element with normally open contacts in the power line. The bimetallic element is heated by current passing through the conductive glass and then a resistor. When the points are closed a spring links into the system holding the points closed. The entire pane of glass must be broken to open the switch. In addition to the above noted defect of such a system, this approach suffers from the additional disadvantage that there is an inordinately long time delay before the bimetallic element is cooled sufficiently to open its power line contacts. The inherent danger to persons coming in contact with the conductive glass or the conductors thereon immediately after the glass breaks is a serious problem with bimetallic type safety units.

SUMMARY OF THE INVENTION

In accordance with the present invention, the sealed thermal insulation chamber located between the glass panels is pressurized, or, in the alternative, evacuated. A pressure sensitive switching element is positioned within the chamber. The switch is responsive to a change in the pressure condition of the chamber resulting from damage to the sealed unit to provide an output signal. This signal operates a circuit breaker system in line connected with the source of electrical energy to instantaneously remove electrical power upon the sensed change in the pressure condition of the chamber.

If the chamber is initially evacuated, the pressure sensor produces an output signal when an increase in pressure occurs. If the chamber is initially pressurized, the pressure sensor produces an output signal when a decrease in pressure occurs. In either embodiment, the safety system instantaneously disconnects the power applied to the door upon the occurrence of a break in any of the glass panels.

The operation of the present invention will become apparent from the drawings and specifications which follow in which a preferred embodiment is shown and described. It should be noted that the embodiment shown is merely illustrative of a form of the invention and that the invention is not limited thereto. Those skilled in the arts appertaining to the invention in the light of teachings herein may devise other embodiments within the scope of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway perspective view of a multi-panel glass refrigerator door incorporating a pressure sensitive safety switch system in accordance with the principles of the present invention.

FIG. 2 is a partial cross-sectional view of a suitable exemplary pressure switch in a first operative position.

FIG. 3 is a partial cross-sectional view of the pressure switch of FIG. 2 in a second operative position.

FIG. 4 is a cross-sectional view as seen through section 4--4 of FIG. 1.

FIG. 5 is a block diagram and circuit schematic of one embodiment of a safety system constructed in accordance with the principles of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1 of the drawings, there is shown generally at 11 a refrigerator or freezer door constructed in accordance with the principles of the present invention. The door comprises a frame 18 in which is mounted a multiple pane glass unit 14. As best seen in FIGS. 1 and 4, the glass unit includes individual glass panes 10 and 12 secured together in spaced relation by metallic separator strips 16, extending around the peripheral edges of the unit to form a thermal insulation chamber 16 therebetween. A suitable adhesive sealant 60, such as polysulfide or other well-known sealant, is applied to the peripheral portion of the unit to form a hermetic seal. A rubber mounting strip 38 is preferably positioned between glass unit 14 and frame 18, as shown.

While the embodiment shown in the drawings is directed to the so-called two-pane glass-metal type unit, wherein two glass sheets are secured in space relation by metallic separator strips, the invention, with minor modifications, could be applied to other types of well-known two or more pane glass units. For example, in the mastic type unit, the glass panes are maintained in space relationship by a separator strip of plastic or similar material cemented thereto by an adhesive material, whereas, in the all glass multi-pane units the marginal edge portions of the glass sheets are heated sufficiently to become pliable and are thereafter urged into contact with one another to form a sealed edge wall extending around the unit.

Irrespective of the kind of multi-pane glass unit utilized, in accordance with the present invention, before being sealed the thermal insulation chamber 17 is preferably either partially evacuated to produce an internal pressure lower than atmospheric, or, alternatively, pressurized to produce an internal pressure greater than atmospheric.

A transparent electrically-conductive film or coating 15 is adhered to the inner surface of the pane 10 within the chamber 17. In order to supply electrical power to the film 15, metallic strip electrodes 22 and 24 are affixed to pane 10 in contact with the film. The strip electrodes are preferably positioned adjacent opposed sides of the film. Electrodes 22 and 24, when connected to a source of electrical energy, enable current flow through the film thus heating the film, the glass panes, and the atmosphere within thermal insulation chamber 14.

As explained above, should one of the glass panes be broken or in the event of some other damage affecting the integrity of the unit, there is danger of electrical shock to one coming in contact with the damaged unit. Known safety devices attempt to obviate this problem by utilizing the conductive strip electrodes as an integral portion of the power supply circuit and causing the power supply to be disconnected in the event of a break in one of the strips. While such an approach does provide effective protection for many failure modes experienced by glass units of the type described, there are other failure modes where the metallic contact strips are not open circuited and for such failures there would be no disruption in the applied power.

In accordance with the present invention, a safety mechanism operational independently of the portion of the circuit by which power is supplied to the film 15, but, nonetheless, effective to disconnect the power supply from the unit upon the occurrence of a defect in the integrity of the unit, is provided.

For this purpose, a pressure sensor 33 is positioned, as shown in FIG. 1, to operatively sense the pressure condition within chamber 17. Pressure sensor 33 is adapted, as will hereinafter be described, to provide a suitable output signal to a circuit breaker 30 upon the occurrence of a change in the pressure condition within chamber 17. Circuit breaker 30 thereafter disconnects the power source from strip electrodes 22 and 24. Power is thus removed from the conductive film system, obviating the possibility of shock to a person coming in contact with the defective glass unit.

Referring now to FIG. 2, detail of a suitable exemplary pressure sensitive device 33 is illustrated. As shown, chamber 37 of pressure sensor 33 protrudes through one wall of the metallic separator strip 16 to communicate with the interior of the thermal insulation chamber 17 via apertures 34 and 35. A sealant 37 maintains the hermetic integrity of the unit. Within pressure sensor 33, a weighted electrical contact 40 is positioned in the central portion of a flexible diaphragm 36. A second electrical contact 44 is positioned on a non-conductive mounting post 47. Both contacts have suitable electrical leads 41, 46 connected thereto.

Assuming that thermal insulation chamber 17 is normally in an evacuated condition, that is, has a pressure lower than atmospheric, contact 40 will normally be held spaced from and out of electrical communication with contact 44 by virtue of the pressure differential acting upon diaphragm 46. Should the multi-pane unit 11 become defective, thus having its hermetic integrity destroyed, the pressure within upper chamber 37 of the pressure sensor will increase causing contact 40 to come into electrical contact with lower contact 44.

Referring now to FIG. 5, one suitable system for connecting the conductively coated glass unit to a supply of electric power in accordance with the invention is illustrated. As shown, a line from one side of conventional power source 84 is connected to metallic strip lead 22 via on-off switch 80 and contact 81A of double pole relay 81. The other side of power source 84 is connected to contact strip lead 24 via contact 81B of relay 81. Also connected across the power source are the two contacts of the pressure sensor 33 in series with a switch activator 87.

Switch activator 87 may, in the present embodiment, comprise a solenoid drive for the double pole relay 81. Normally, pressure sensor 33 acts as an open circuit preventing current from initiating switch activator 87. However, upon an increase in pressure in chamber 14, the pressure sensor electrically becomes a short circuit completing the current path through switch activator 87, thus opening normally closed relay 81. As the contacts 81A and 81B open, the glass unit is effectively disconnected from the power source.

While the above discussion has been with reference to a thermal isolation chamber normally below atmospheric pressure, the invention disclosed herein is obviously capable of operation where chamber 14 is pressurized above atmospheric pressure with suitable changes in the structure of pressure detector 33. Further, the present invention is not limited to the use of any particular kind of pressure transducer and is obviously capable of being used in connection with other known pressure transducers. For example, pressure transducers of the capacitive, resistive, inductive, semiconductor, electronic, piezoelectric and photoelectric variety may be utilized with suitable modification of the circuitry utilized.

Claims

I claim:

1. In a refrigerated cabinet door adapted to be heated by a source of electrical energy, the combination comprising:

at least two glass panels;

means for sealing said panels in mutually spaced relationship to define at least one sealed chamber therebetween, said chamber being normally maintained at a predetermined pressure different from atmospheric;

an electrically conductive coating adhered to the interior surface of one of said panels within said chambers;

means for electrically connecting spaced portions of said coating to opposite sides of the source of electrical energy;

means responsive to the pressure within said chamber for providing an output signal when said pressure varies from said predetermined pressure;

means responsive to said output signal to disconnect said conductive coating from the source of electrical energy.

2. The combination of claim 1 wherein said means for electrically connecting said coating to the source of energy comprises:

first and second electrodes adhered to the surface of said conductively coated panel in spaced relationship.

3. The combination of claim 2 wherein said means for electrically connecting said coating to the source of electrical energy further comprises:

a first normally closed relay contact connected between said first electrode and one side of the source of electrical energy.

4. The combination of claim 3 wherein said means for electrically connecting said coating to the source of electrical energy further comprises:

a second normally closed relay contact connected between said second electrode and the other side of the source of electrical energy.

5. The combination of claim 3 wherein said means responsive to said output signal comprises:

a relay contact activator responsive to the receipt of said output signal to open said relay contact and disconnect said first electrode from said source of electrical energy.

6. The combination of claim 4 wherein said means responsive to said output signal comprises:

a relay contact activator responsive to the receipt of said output signal to open said relay contacts and disconnects said first and second electrodes from said source of electrical energy.

7. In a multi-pane glass refrigerated cabinet door wherein the glass panes are joined around their peripheral edges in spaced relation to form a chamber isolated from the external atmosphere, and wherein an electrically conductive coating adheres to the interior surface of one of the glass panes within the isolated chamber, and wherein at least two electrode means are adapted to connect spaced portions of said coating to opposite terminals of a source of electrical energy, a safety device comprising:

means for normally maintaining said chamber at a predetermined pressure different than atmospheric pressure;

a pressure sensor operatively positioned within said chamber to sense the pressure therein, said sensor providing an output signal when the sensed pressure varies from the predetermined pressure condition; and

means responsive to the receipt of said output signal to disconnect the electrode means from said source of electrical energy.

8. The glass refrigerated cabinet door defined in claim 7 wherein the chamber is normally at an elevated pressure with respect to the external atmosphere, said pressure sensor being responsive to a reduction in sensed pressure, to provide said output signal.

9. The glass refrigerated cabinet door defined in claim 7 wherein said chamber is normally at a pressure below the external atmospheric pressure, said pressure sensor being responsive to an increase in sensed pressure to provide said output signal.