Patent Publication Number: US-2005138959-A1

Title: Evaporator for a refrigeration device

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This is a continuing application, under 35 U.S.C. § 120, of copending international application No. PCT/EP03/05864, filed Jun. 4, 2003, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German patent application No. 102 27 135.6, filed Jun. 18, 2002; the prior applications are herewith incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an evaporator for a refrigeration device. The surface of an evaporator of this type reaches temperatures of below 0° C. when the evaporator is supplied with refrigerant. Moisture in the air from the interior of the refrigeration device may, therefore, condense and freeze on the surface of the evaporator. This initially results on the surface of the evaporator in the production of a thin, uniformly distributed layer of frost that, provided that it is not defrosted in the meantime, becomes ever thicker over the course of operation of the refrigeration device and, finally, coalesces to form a solid, hard layer of ice. Because the temperature of the evaporator varies over the course of time, depending on whether it is supplied with refrigerant or not, this layer of ice periodically expands over the course of time and contracts again. In the process, stresses build up between the layer of ice and the evaporator situated therebeneath and, finally, cause the layer of ice to slip backward. The cracking noises produced as a result are found to be disturbing.  
     SUMMARY OF THE INVENTION  
      It is accordingly an object of the invention to provide an evaporator for a refrigeration device that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and in which these cracking noises are reduced.  
      With the foregoing and other objects in view, there is provided, in accordance with the invention, an evaporator for a refrigeration device, including a pipeline through which a refrigerant flows and a surface thermally conductively connected to the pipeline, the surface being a heat-exchange surface and having thereon a hydrophobic layer of oil.  
      According to the invention, the surface of the evaporator, on which surface ice could form, is provided with a hydrophobic coating. Such a coating acts to delay the formation of first ice crystals on an ice-free evaporator and, thus, to extend the time required for an amount of ice to accumulate on the evaporator following a defrosting operation, which is thick enough to cause cracking noises. A further effect of the hydrophobic layer is that it prevents the formation of a homogeneous layer of frost on the surface of the evaporator. Instead, icing up begins on a small number of unavoidable nuclei on the surface of the evaporator. The beads of ice that form there grow more or less uniformly over the entire surface and, thus, reach a substantial thickness before they begin to coalesce. That is to say, in comparison to a non-hydrophobic evaporator, a large amount of ice can collect on the evaporator according to the invention before a coherent layer is produced, which layer is capable of producing cracking noises.  
      With the objects of the invention in view, there is also provided a lamellar evaporator for a refrigeration device, including a hollow pipe for conveying a refrigerant therethrough and a lamellar heat-exchange surface thermally conductively connected to the pipe surface, the heat-exchange surface being at least partially coated with a hydrophobic layer of oil.  
      In accordance with another feature of the invention, the pipeline has a pipeline surface having thereon the hydrophobic layer of oil and the heat-exchange surface has thereon the hydrophobic layer of oil.  
      In accordance with a further feature of the invention, the pipeline has an outer surface having thereon the hydrophobic layer of oil.  
      In accordance with an added feature of the invention, the pipeline has an outer surface at least partially coated with the hydrophobic layer of oil.  
      In accordance with an additional feature of the invention, the pipeline and the surface form a lamellar evaporator.  
      A further advantage of the growth of islands of the layer of ice is that surface regions of the evaporator remain free of ice for a relatively long time and, thus, make effective heat exchange possible between the refrigerant and the area surrounding the evaporator even when a substantial amount of ice has collected on other regions of the evaporator surface. The time interval between two defrosting phases can, therefore, be chosen to be longer than in the case of a non-hydrophobic evaporator. This produces an increased convenience for the user of a refrigeration device equipped with an evaporator according to the invention, and a reduction in the power consumption of the refrigeration device and, thus, a reduction in its operating costs.  
      The hydrophobic coating may contain, preferably, a silicone as the hydrophobicizing constituent.  
      The coating may be applied as a strengthening layer of lacquer or as a layer of oil. In such a case, the layer of oil has the particular advantage that, because it is not solid, it promotes the displaceability of the beads of ice that are forming on the surface of the evaporator such that, even in the case of complete icing up of the evaporator with a layer thickness sufficient to produce cracking noises in the case of a conventional evaporator, the cracking noises are reduced. This occurs in the case of the evaporator according to the invention because high thermal stresses that are discharged as loud cracking noises do not need to be built up to displace the crust of ice with respect to the evaporator. Rather, a significantly lower stress is sufficient to allow the layer of ice to slide relative to the evaporator or possibly even bring about a continuous sliding movement of the layer of ice on the evaporator.  
      The evaporator according to the invention is configured, preferably, as a lamellar evaporator suitable for use in a recirculating air refrigerator or freezer.  
      Other features that are considered as characteristic for the invention are set forth in the appended claims.  
      Although the invention is illustrated and described herein as embodied in an evaporator for a refrigeration device, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
      The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
      The FIGURE is a perspective view of the evaporator according to the invention. 
    
    
     DESCRIPTION OF THE PREFFERED EMBODIMENTS  
      Referring now to the single FIGURE of the drawing, it is seen that a partially iced-up section of a lamellar evaporator  4  according to the invention. The lamellar evaporator  4  includes a pipe  1  through which refrigerant flows during operation. A lamella  2  is helically wound around the outer surface of the pipe  1 . The lamella  2  and the outer surface of the pipe  1 , on which surface the lamella  2  is mounted, form the heat-exchange surface of the lamellar evaporator  4  and are provided with a hydrophobic coating of silicone oil.  
      On account of the hydrophobicity of the coating, a large amount of energy is required to wet the surface of the lamellar evaporator  4  and this energy must be released when moisture freezes on the surface. Such a property constitutes a considerable obstacle for the new formation of ice crystals on the surface. Therefore, a considerable proportion of the moisture of a flow of air that flows past the lamellar evaporator does not even condense on the lamellar evaporator  4  itself when the flow of air is supersaturated with moisture, and the reason for this is simply that the moisture does not strike a surface on which freezing would be energetically optimum. The lamellar evaporator  4 , therefore, only slightly dries the air in the interior of the refrigeration device when it is used in a recirculating air or no-frost refrigeration device, this being extremely vulnerable for the storage of vulnerable food products such as, for example, fresh vegetables.  
      For moisture in the air to condense on the lamellar evaporator  4 , the lamellar evaporator  4  must have surface regions on which the condensation is energetically optimum. Such locations are usually condensation nuclei in the form of dust particles, surface discontinuities, or the like that are unavoidably present on the surface of the lamellar evaporator  4  but only make up a very small part of it. During operation of the lamellar evaporator  4 , a large number of compact beads of ice  3  form—as shown in the figure—on these condensation nuclei. Further moisture from the air surrounding the lamellar evaporator  4  preferably condenses on the surface of these beads of ice  3  so that the beads  3  increase in thickness and diameter over the course of time, while regions of the surface of the lamellar evaporator  4  that are situated between the beads remain free of ice for a long time and make efficient heat exchange possible between refrigerant circulating in the lamellar evaporator  4  and the surrounding air.