CA1139346A - Condensation-free freezer door assembly - Google Patents

Abstract

Abstract A freezer door assembly with a viewing area and that requires no electric heating source to main-tain the assembly free from condensation under standard conditions. The assembly includes inner and outer spaced parallel glass panes and a panel spaced between the panes, the panes and panel having confronting sur-faces of which at least one is provided with coatings highly transmissive of visible light and highly reflec-tive of long-wave infra-red radiation. The assembly includes an insulating frame at the edges of the panes, the frame having sufficiently low thermal conductivity as to in turn maintain outer surfaces of the frame and outer pane free from condensation under standard condi-tions.

Description

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CONDENSATION-FREE FPEEZER DOOR ASSEMBLY
-Technical Field The present invention relates to freezer door -semblies of the type employed, for example, in su~er-markets.
Back~round Art Food freezcrs of the type normally employed in supermarkets commonly are provided with doors havin~
vie~ing areas, e.g., ~lindows through which a customer may view frozen food products. Freezers of this type may be upright freezers (in which the door opens on vertically aligned hinges), or horizontal (in which the door opens on horizontally aligned hinges).
Accepted industry design conditions for super-market refrigcration equipment contemplates room telnp-erature air (as in a supermarket) to be controll~d in the summertime at 75 F (about 24 C) and 55~O rel~ti~e humidity, and a freezer compartment to have a tempelc:-ture of -12 ~ (about-24.4 C). Air at a tPmperature of 75 F (24 C) and 55~ relative humidity has a de~
point of approximately 58 F (14.4 C). If the expo.sed surfaces of the fre~zer door, that is, exterior surf~ces of the frame or the window, drop below the de~ point, undesirable condensat-ion may form on these surfac~s.
Condcnsation on the ~indo~ portion is p~rticul~ ly undiesirable in thclt i~ hecomes difficult Ol impo_a.bl~
for a customer to view frozen goods throug}i the ~iinc'o::.
If the tem~eraturc of the e~terior surfaci- of the fr~ e i~.3~

or window drops below the freezing point, then frost is formed.
Only one practical soluiion to the condensa-tion problem appears to have won favor in the industry.
This solution requires electrical heating elements to be placed in the frame so that the exterior surface of the frame remains above the dew point temperature.
Simularly, the window itself may be provided with an electrically conductive film heated b~ electrical resistance heating to raise the outer surface of the glass above the dew point. The heaters often are not carefully controlled, and exterior frame surfaces are often warm to the t~ h.
The amount of energy, overall, required to accomplish heating o a freezer doGr to prevent conden-sation is surprisingly large. One must consider not only the electrical energy required for heating the door, but also the energy required to dissipate such heat. For example, a supermarket may have thirtv or more self-contained freezer units. If one assumes that each freezer includes a freezer door approximately two feet (0.61m.) in ~Jidth and five feet (1.52m.) in heighth, and if one further assumes that the energy requirements for the glass ~indow portion of the free-zer is eight watts per square foot (0.093m ) and thatthe frame of the door requires 10 watts per lineal foot (0.3m.) of perimeter, 220 watts of electrical energy are required to heat the freezer door. One ma~ furt~er assume that the freezer itself absorbs 70~ of the heating energy, or 154 watts. If it is now further assumed that the ratio of the energy required to remove heat from the freezer is a~proximately three times the quantity of heat energy removed, then an additional 462 watts is required to remove the heat transferl-~a to the freezer b~ the heated frame an~ ~;ndow. Thc additional load on the freezer com~ressor (46~ watts) G~

represents heat cner~y that will be dissipated to the interior of the supermarket. Employing the above ratio, 1386 watts are required to remove the additional heat from the sup~rmarket. Since the freezer must be opera-ted continuously throughout the year, the energy re~uire-ments of 220 watts and 462 watts set out above provides a total energy consumption during the year of about 600~ kilowatt hours for one freezer. Assuming that the air conditioning system of the supermarket is used only for about six months of each year, then the additional energy requirement for the air conditioning unit is approximately 6000 kilowatt hours per year. The total energy cons~lption of about 12,000 kilowatt hours per year, at an assumed cost of 5.5 cents per kilot~att hour, leads to a cost pcr door of approxima~ely $660 U.S. per year. If thirty of such freezer doors are conside.ed, the total direct and indirect cost of ~lectrically heating the freezer doors rises to nearly $20,000 U.S.
rhe above exam,~le is not intended to be a factual analysis of any particular case, but is intended simply to illustrate the fact that the electrical resis-tance heating of fr~ezer doo.s to prevent condensation is highly wasteful of energy. A freezer door whi~h does not require additional ener~y input to avoid condensation hence is much to be desired.
Disclosure of Invention The present inventic~n relates to a gla2ed freezer door assembly having a viewing area and that requircs no electrical heatiny source to maintain the assembly free from condensation. The assembly comprises inner and outer spaced, parallel g]ass panes, and a panel substailtially transpaxent to visible lic~llt and positioned betweel7 ~nd parc~llel to bu. spaced frc,m tl-le panes, the panes and panel defining t~;o spaces tnere-betwe2n. The panes and panel have confronti.7y sur~ac~Sof which one or more (preferably t~o) ~;I fa_es, and ~3~1t~

desirably surfaces facing separate spaces, are provided with thin coatings highly transmissive of visible light and highly reflective of long wave infra-red radiation, the panes and panel providing a U value (overall heat-transfer coefficient) of not more than about 0.20 Bta/(hr)(ft2)(F) (about 1.136 Watts/(m2)(K). The assembly includes an insulating frame that extends a~out and seal-ingly supports the panes and panel at their edges. The frame has an inwardly extending flange parallel to and along the outer surface of the outer pane, the flange havin~ an inner edge defining a viewing area through ~h~
panes and panel, such area being at least about 80O of the area of the glass panes. The frame has sufficicntly low thermal conductivity as to coact with the panes and panel assembly to maintain the exterior surface of the frame and the adjacent exterior ~lass pane surface at a temperature of not less than about 58CF(14.4 C) when the exterior surfaces of the frame and outer glass pane are exposed to air at 75 F (23.9 C) and 55% relative hur,~i-dity, and the inner surface of the inner pane is exposed to air at -12 F (-24.4 C).
Brief Descrip~ion of Drawin~s Figure 1 is a perspective view of a freezer equipped with a door of the invention;
25Figure 2 is a perspective view, partially broken-awa~r, of the freezer door of Fi~lre 1;
Figure 3 is a partially broken-away cross-sectional view ta~en along line 3-3 of Figure l; and Fi~ure 4 is a bro~en-aw~iy, cross-sectiolal diagrammatic view of a spacer and associated structure, as shown also in-Figure 3.
B~.st ~lode for Carr~ing Out the Invention l'eferring now to the drawing, a freezer door asscmbly of the invention is designate-' 10, an~ inc~a'es inner and outer, spaced, parallel glass l~anes (12 anc.
12.1) and a panel (14) that is substan~ially transparent to visible light and that is positioned between and parallel to but spaced from the panes (12 and 12.1).
The panes and panel define two spaces (16 and 16.1) therebetween.
As used herein, "inside" or "inner" refers to that portion of the assembly nearer the inside of the freezer with which the door is used, and the terms "outside", "outer" and "exterior" refer to the portion or portions of the assem~ly tha are furthest from or face away from the freezer interior. The glass panes may be of any practical thickness, and thicknes~es of about 1/8 to about 3/8ths of an inch (about 3.175 to a~out 9.525 n~.j are preferred. Pref~rably, the panes (12 and 12.1) are of tempered or he~,t-strengthened glass. The confronting surfaces of the glass panes preferably are spaced on the order of at least a~out 3/4 inches (about l9mm.) and the t.hus-described viewin~
area comprising the glass panes and panel may have an overall thickness of at least about one inch (25.4mm.).
The panel (14) may be of substantially any transparent material such as glass or plastic, but preferably is of polyester film having a thickness of from about .0005 to about 0.006 inches (about 0.013 to about 0.152mm.). Plastic film such as polyester film is preferred for the internal panel (14) because it is of li~ht weight and because of the ease of manufacture of the assem~ly, which will be described more fully bclow.
The pane and parel assembly provid~s two exter:ior surface~ (3?.2 ~-ld 12.3) and four int~rnal surfaces of which two, (14.1 and 14.2) ar~ provided ~y the panel with the remaining two (12.4 and 12.5) plO-vided l~y the panes. ~t least one and pref~rably ~.
least two of the four internal sur~accs are prot,-ided with coatings "C" that are higllly trans.,~:issive of visible light and highly reflective of long wave infra-red radiation. Preferably, at least one coating confronts each of the two spaces (16 and 16.1), and if only one coatins is usec', this coating preferably confronts the air space (16) nearer the interior of the freezer. For ease of manufacture, it is preferred that the coatings be applied to each of the surfaces (14.1 and 14.2) of the panel (14), and the latter preferably is spaced an equal distance from the internal surfaces (12.4 and 12.5) of the glass panes. The panel 14 pre-ferably also is provided with means such as a smallorifice (14.3) permitting air or other gas to move between the two spaces (16 and 16.1).
Coatings of the type descri~ed that are higllly transmissive of visible light and highly reflective of long wave infra-red radiation are known to the art, and have been popularly referred to as "heat mirrors". A
number of such coatings are commercially available, and for the most part consist of two or more layers, the inner layer co~monly being a metallic layer such as a vacuum-depositec gold, and an out~r layer typically being an oxide of til:anium. CoaLings of this type have successfully been used as fog-preventive coatings for aircraft and automobile windshields, for ski goggles, etc. The coatings should be highly transmissive of visible light, that is, light of a wave length in the ranye of from about 0.3 to about 0.75 microns, and the transmissivity of such coatings should be in excess of 80~ over a reasonably broad ranse of wave lengtlls of visible ligll'. On the other hand, the coatings should be highly reflective o, long wave infra-red radiation, that is, racliation having wave lengths in the rang~ of from about five to about twenty microns.
The reflcctivity of such coat;ngs LO a reasona~ly bl-,ad band of wave lengths of long wave infra-red radiation should b^ at le~st about 85~ and prefer~lly greatel-than about 90~. The hemispllerical emissivity of cac' such coating should not exceed about 0.18.
Each pane is spaced at its edges from the panel ~y a spacer (18.6) that includes a generally hollow, elongated, tubular shape (18), typically of aluminum, having generally parallel, flat outer sur-faces (18.4 and 18.5), as shown best in Figure 4.
Thicknesses of the ~ nel and coatings have, for clarity, been exaggerated in Figure 4. The tubular shape is provided with a passageway (18.1) in its interior, and the interior itself is provided with a desiccant such as a molecular SiQVe or silic2 gel (designated lS.2 in the drawing). Strips (18.3) of polyisobutylene are provided between the edges of the tubular shap~s and the adjacent surfaces of the glass panes and panel, as ~5 shown best in Figures 3 and 4. The tubular shapes terminate short of the edges of the panes and panel, providing a recessed trough which in turn is filled with a rubber-like sealant (20) of a type con~onl~7 used _ in the art and w~ich may ~e typified as Silicon2 Insula-.~ 20 tin~ Glass Sealant IGS 3204 (a product of the G~ne~al El~cl:ric Compan,~). About the edges of the thus p.re pared panel-pane ass~mbly is provided a frame, includ-ing a gasket of rubber or of polyvinyl chlori.de or o.t other con~only used substances for this purpose, th~
gasket being sho.~n as 21 in the drawin~. About the frame gasket is assemblcd a rigid outer frame, designa-ted generally as 22 in the drawing, the frame havi.ng a peripheral recess (22.1) shaped to receive the gas~eted edge of the glass-panel assembly. ~ primary purpose of the gasket (21),as will now be evid~nt, is to the edges of the glass pane-panel assembly within tl rigid fra,me.
The outer ~ralne 22 is desirably of mat~ri;~l having a low coefficient of therma3. conductivi~y, i.- , not greater t}~all about 0.7 ~tu~(}lr)(~t )~~
or about 0.1 ~att/(m)(l~). One SUCIl IllatCrial i'` WC~
-~ I r~ '` k i~ 3~ 4~;

Frames of the assemblies of the invention preferably are made of polyurethane structural foam, however, the foam desirably having a closed-cell interior designated generally as "P", and a hard, solid skin designated generally as "S" in Figure 3. Polyurethane foam mater-ials of this ty~e are known to the art, and are typified by "Baydur" brand polyurethane structural foam, a trade-marked commercial product of the Mobay Chemical Corpora-tion. The coefficient of thermal conductivity for such material is 0.55 Btu/(hr)(ft2)(F/in), or about 0.03 Watts/(m)(K). The frame is provided with at least one outer flange (22.2), and desirably with an inner flanye (22.~), the flanges defining generally the walls of the peripheral recess (22.1) of the frame. The frame may be provided with an internal stiffening extrusion (22.4) of e.c3., aluminum, of a type known to the art. The extrusion (22.4) tends to stiffen the frame structure, and may include a generally circular groove (22.5) to accep, a toi;ion bal- which ultimately is fastened to the ~amb of the freezer compartment, the ~orsion bar providing the necessary springiness to the door so that when it is released, it will shut. The extrusion also provides an inner channel (22.6) into which may be inserted a magnetic gasket designated generally as (22.7). The large, hollow interior (22.8) of the aluminum extrusion is employed, as is known in the art, to accept corner keys (not shown). The aluminum ex-tr~l ion (22.4), corner leys, the torsion bar spring system and the magnetic gasket are known to the art, and need not be described in further detail.
A typical freezer door assembly of the inven-tion r~ay be manuEactured by first providing each of t~o eq~lally dimensionecl glass panes with peripheral elon~,ct~d tubular shapes (18), typically o~ al~uminum, attached ~
adherrent polymer (e.g., polyisol)utylene) strips (1~,.3), the tubular shapes b~inc3 attached at corllers by corner c~

ke~s of known design. To the exposed, gener~lly flat surfaces (18.4 and 18.5) of the tubular shapes are adhered adherrent polymer (polyisobutylene) strips (18.3).
A flexible plastic sheet having coatings "C" on both surfaces and defining the panel 14 is liyhtly stretclled over the latter strips, care being takt?n to p~o~7ide a tin~, desirably nearl~ imperceptible hole ~14.3) in the panel near its edge. The panel should be stretched fairly tightly over the strips (18.3). The other pan~
is then mounted in the configuration shown in ~igure 3, the exposed strip (18.3) carried by the tubular ShapC? of the second pane coming into contact ~.ith the panel ahout its periphery and in opposed relationship to the fir ~-mentioned adherrent polymer strip. ~ rapidly curing silicone sealant (20) is then provide~ about the peri-phery of the assembly to occupy the space defined b~
the confronting surfaces of the glass panes and the peripheral urfaces of the tubular shapes (18). When the silicone sealant is fully cured, the thus-described assembly is heated as a substantially sealecl, in-tegxal unit by placing it in an ove~ at a temperaturc oE from about 195 F (about 90.5 C) to abou' 250 F (about 121.1 C). It ~ill be noted that the se~lant is so chosen as to be able to withstand this range of ten~era-tures without si~nificant softening or degradation. Theoven employed desirably is a forced air oven, and the entire assembly is raised to the oven te~peraturc? il~
a period of ~-20 minutes. Tlle polyester film that is cmployed as a substrate for tht? panel (1~) is a heat-shrinkablc film; th~lt is, it is ~ilm so srientc?~l in itsmanufacture that upon further application of hc~, it wi]l tend to shrinl~ in its ~'..ane. . Tne ovcn t~?mpera~lr~
is selectei so as to sihrink the panel ara to CaUsc the samo to be drawn tightl~ betweell 'he supportinc, Sl) lcerS
(1~.6). The ~ssembly is then rcm~ ?d from thc O~ ;n~l ~ 46 allowed to cool slowly, (e.~., over a period of an hour or so), following which the panel (14) is found to be taut and flat and devoid of noticeable waves or wrinkles in its surface, and the coatings "C" are found to be substantially unimpaired from the standpoint of trans-parency to light and reflectivity of lony-wave infra-red radiation.
It should be noted that the interior spaces (16 and 16.1) communicate with one another throu~h the tiny orifice (14.3) formed in the panel so as to avoid pressure differentials across the panel. The spaces (16 and 16.1) may be air spaces or may be filled with a ~as such as nitrogen. The desiccant carried by thc tub~lar shapcs (18) tends to adsorb water from the spaces (16 and 16.1), and hence condensation on the internal surfaces of the assembly is avoided.
The pane and par.el assembly is enclosecl within the gasket (21), as shown in the drawing, and to this assembly in turn is mounted the outer frame (22), the gasketed portion of the pane and panel assembly being received within the peripheral recess (22.1) of the outer frame. Hingcs, handles, the magnetic gasket (22.7), a torsion bar and other desired hardware is then attached to the frame.
The inner surfaces of the freezer door asscmbly of the invention commonly will be exposed to the interior of a freezer maintained at approximately -12 F (-24.4 C).
llhe ou, r surfaces of the assembly, under current desi~n condition, will be exposed to air at approximately 75 F
(23.9 ~) and 55% relative humidity (hercin sometim~s referred to as "standac~i conditions"). To a oid con~en-SatiOIl UpOIl either the e~terior surface (12.23 of ~he ~lass panc (1?.1), or upon the extcrior surfacc of the outer framc (22), such surfaces must be maintained at least sliglltly abov- 5S ~ (about 14.4 C). ~eat loss from the outer surE~ces of the frame (22) and pane (12.1) occurs primarily at the edcjes of the unit, although of course some heat transfer occurs tllroughout the entirc unit. It is believed that the coatings "C" tend to reduce heat loss from the outer surface (12.2) of the 5 pane (12.1), and such radiation as is reflected from the coatings may be paritally absorbed by that pane to warm the same. The outer frame (22) is characterized by very low thermal conduc~ivity. By combining the pane-panel assembly and the frame unit into a single freezer door structure, it has been found that the outer surface of the door (that is, the outer surfaces of th~
frame and pane (12.1)) are maintained above about 58i' (about 14.4 C) under standard conditions, and that as a result the freezer door G. the inventioll requires no 1' auxiliary heating system such as electric power to avoid condensation or frost forma~ion. As poi.nted out above, the energy savings whi.ch can be realized with the freezer door assemb''y of the invention are si~nificant.
Thus, the instant invention provides a freezer door assembly ~Ihich is free of any external heatinc~
source, and which combines a glass pane and interior panel structure with a highly insulating peripheral frame in a manner preventing condensation from air at 75 F (23.9 C) and 55% R.H. on exterior surfaces o the door when the interior surfaces are exposed to a freczer compart~lent maintained at -12 F (-24.4~ C).
While a preferred embodim~nt of the present -invention has beerl described, it should be unders'~ood that various chanc~es, adaptations, and mGdifica~.ions may be made therei.n without departinc3 from the spir.it of the invention and the scope of the appended clain

Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A glazed freezer door assembly having a viewing area and requiring no electric heating source to maintain the assembly free from condensation, comprising a) a viewing assembly including inner and outer spaced, parallel glass panes, and a panel substantially transparent to visible light and positioned between and parallel to but spaced from the panes, the panel having at least one surface bearing a substantially continuous thin coating highly transmissive of visible light and highly reflective of long-wave infra-red radiation, the viewing assembly having a U value not exceeding about 0.20 Btu/(hr) (ft2)(°F); and b) an insulating frame extending about and supporting the panes and panel at their edges and having a coefficient of thermal conductivity not greater than about 0.7 Btu/
(hr)(ft2)(°F/in.), and including a flange extending parallel to and along the outer surface of the outer pane; the viewing assembly and the frame each being suffi-ciently resistant to heat transfer and each coacting with the other so as to maintain outer surfaces of the outer pane and frame at a temperature above about 58°F when said outer surfaces are exposed to air at 75°F
and 55% relative humidity, and the inner sur-face of the inner pane is exposed to air at -12°F.

2. The assembly of Claim 1 in which the panel comprises a sheet of polyester film heat shrunk between said panes to a taut, wrinkle-free condition.

3. The assembly of Claim 1 wherein the frame includes a rigid outer portion of a material having a coefficient of thermal conductivity not greater than about 0.7 Btu/(hr)(ft2) (°F/in.) and wherein said flange defines a viewing area not less than about 80% of the predetermined surface area of the panes.

4. A glazed freezer door assembly having a viewing area and requiring no electric heating source to maintain the assembly free from condensation, comprising a) a viewing assembly including inner and outer spaced, parallel glass panes, and a panel parallel to but spaced from the panes and comprising a sheet of polyester film heat-shrunk between the panes to a taut, wrinkle-free condition, the panel bearing on each of its sides a thin coating highly transmissive of visible light and highly reflective of long-wave infrared radiation, the viewing assembly being characterized by a U value not exceeding about 0.20 Btu/(hr)(ft2)(°F);
b) an insulating frame including a rigid outer frame having a hard, dense outer skin and a cellular insulating interior and having a coefficient of thermal conductivity not exceeding about 0.7 Btu/(hr)(ft2) (°F/in.); the frame extending parallel to and along the outer surface of the outer pane;
the viewing assembly and the frame in which the viewing assembly is supported each being sufficiently resistant to heat transfer and each coating with the other so as to maintain the outer surface of the outer pane and frame at a temperature above about 58°F when said outer surfaces are exposed to air at 75°F and 55%
relative humidity, and the inner surface of the inner pane is exposed to air at -12°F.