Refrigeration – Display type – With air controlling or directing means
Reexamination Certificate
2001-09-10
2002-12-10
Tanner, Harry B. (Department: 3744)
Refrigeration
Display type
With air controlling or directing means
C062S256000
Reexamination Certificate
active
06490878
ABSTRACT:
The present invention relates to a device for simultaneously improving the performance of cabinets, windows, counters, chests, stalls, or units for displaying and selling foodstuffs that are conserved at a temperature lower than ambient temperature. Refrigerated or deep-freeze cabinets are in very widespread use in stores for dispensing foodstuffs, in pharmaceutical outlets, or more generally whenever it is necessary to conserve goods at a temperature lower than ambient. In the text below, refrigerated or deep-freeze cabinets, windows, counters, chests, stalls, or units are referred to generically as “cold sales cabinets”.
In the present state of the art, refrigerated cabinets, windows, or counters, and more particularly so-called “open” cabinets, all present the feature of operating by virtue of curtains of refrigerated air subjected to forced circulation along the open face. Said open face can be horizontal or vertical. Whatever the precautions taken to guide such curtains of air, it is found that some mixing takes place between the refrigerated air and the air of the premises in which said cabinets are placed. A consequence of this partial mixing is that a significant quantity of water vapor is contained in the flow of air driven by the fans and put into thermal contact with the evaporator of a cold sales cabinet. This water vapor is inevitably trapped at said evaporator where it becomes deposited as a progressive buildup of ice. This buildup of ice has the effect of progressively constituting an insulating sheath around the evaporator and its fins, thereby reducing the heat exchange coefficient between said element and the circulating air, and also increasing head losses, thereby significantly reducing the flow rate of refrigerated air.
After cold sales cabinets have been in operation for a few tens of minutes (which period is a function of the humidity of the ambient air, the dew point temperature, and the evaporation temperature of the refrigerating fluid), a significant drop is observed in the efficiency of the curtain of air, and consequently a rise is observed in the temperature of the inside of the cabinet and of the foodstuffs it contains.
To combat that phenomenon of frost appearing and the associated losses of performance, various solutions have been proposed which have turned out to be relatively unsatisfactory.
Firstly, it has been proposed to warm up the evaporator periodically, either by stopping the circulation of refrigerating fluid within said component, or by providing it with heater cords or electric resistances, or indeed by heating the air (WO 98/06987 to Hislop, FR-2 610 708 to Bosch Siemens, EP-0 768 053 to Sanyo, EP-0 403 459 to Electrolux).
Proposals have also been made to duplicate the evaporator, with one of the elements being defrosted while the other one is in operation (WO 94/26154 to Hussmann).
Proposals have also been made to dehumidify the air prior to passing over it over the evaporator (application No. 91/09664 - 2 679 988 to A.R.M.I.N.E.S.).
Proposals have also been made to build up a store of low temperature, using a large mass of ice or a eutectic for the purpose of taking over from the evaporator for the length of time it requires to be defrosted (application No. 88/02314 - 2 611 383 to Toshiba).
Finally, numerous dispositions and combinations of single, double, or triple air curtains have been proposed in which said air curtains are either heated or cooled for the dual purposes of limiting uptake of water vapor in the flow of air and limiting condensation on the evaporator (WO 97/17003 to Bonnet Neve, EP-0 709 046 to Sanyo, FR-2 615 082 to Bonnet Refrigeration, WO 94/23620 to Northampton Refrigeration).
Proposals have also been made to defrost the evaporator by using jets of superheated gas or steam.
None of those solutions turns out to be genuinely satisfactory, whether from the point of view of the energy consumption of cold sales cabinets or the genuine performance of such apparatuses and their ability to keep foodstuffs under temperature conditions that comply fully with the legislation in force.
In terms of cost, it is found that the need to combat frosting phenomena on evaporators leads to evaporators, fans, and air circuits being over-dimensioned. The length and the frequency of the essential defrosting cycles in presently-available apparatuses are such that these pieces of equipment are used for their refrigeration function during less than 80% of their total operating time. The remainder of the time they are used for a function that is the exact opposite from that for which they are designed, i.e. to be subjected to or to cause heating which temporarily and partially cancels their effects. Experience shows that presently-manufactured and sold cold sales cabinets are subjected to large temperature variations over time, which makes it very difficult to comply with the standards in force; these variations are particularly large between the beginning and the end of the defrosting period, which period inevitably gives rise, in the present state of the art, to significant heating of the air curtain and thus of the foodstuffs. Experience also shows that in the present state of the art and regardless of the particular solution implemented to combat evaporator frosting phenomena, presently-manufactured and sold cold sales cabinets cannot provide a temperature that is uniform throughout their inside volume: the phenomenon of evaporator frosting does not take place identically on all parts of said element, so the resulting head losses are necessarily non-uniform, and as a result the temperature and the flow rate of the air curtain(s) are themselves non-uniform. This state of affairs leads to certain inside portions of the cabinet being at a temperature that is far from its nominal temperature, and that is prejudicial to good conservation of foodstuffs.
It is also observed in the present state of the art that evaporators and elements situated close to evaporators in cold sales cabinets can be the subject of microbial or bacterial proliferation phenomena, with pockets of air or collections of water warmed by the action of the defrosting means presently in use providing media that encourage the development of germs, bacteria, or microbes. Since, by construction, these evaporators and their immediate vicinity are difficult to access with any kind of cleaning or decontamination means, it is unfortunately observed that these organisms can become transferred to the foodstuffs or goods conserved inside cold sales cabinets, with the vector being air which is circulated by the fans.
That said, non-published experiments performed by the authors of the present application concerning cold sales cabinets fitted with defrosting devices included in the prior art have shown that the formation of frost on the outside surfaces of evaporators does not have a totally negative effect on the performance of such cabinets: it is found that initially the appearance of a thin film of frost on the outside surface of an evaporator has the effect of increasing its real heat exchange area and thus of increasing heat exchange efficiency to a significant extent; this applies in particular when the layer of frost covering the outside surfaces of the evaporator is of a thickness that does not exceed 1 millimeter (mm). After a few minutes of operation, this positive effect is overtaken by the negative effect of the insulation caused by depositing a layer of frost and/or ice having a thickness of several millimeters; thereafter, this layer of frost and/or ice creates an obstruction, and then a complete blockage between the fins of the evaporator, which ends up by preventing all air circulation and thus preventing any heat exchange.
The invention proposes means for taking advantage of the positive effects of the appearance of frost on the surface of the evaporator and for eliminating the negative effects of said phenomenon.
More generally, the invention proposes making cold sales cabinets that mitigate all of the drawbacks described above.
An object of the invention is to
Consigny Denis
Corfec Michel
Luminet Philippe
Teissedre Bernard
Ali Mohammad M.
Tanner Harry B.
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