Abstract:
A refrigeration system trapping device and method traps airborne liquid water and ice particles before they accumulate on evaporator coils, reducing frost deposition on the evaporator. The frost on the trapping device can be actively removed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not applicable.  
       STATEMENT AS TO FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable.  
       FIELD OF THE INVENTION  
       [0003]     This invention relates generally to methods and devices for prevention of frost or ice deposition on cold surfaces such as evaporator and freezer coils in refrigeration devices. More specifically, the invention relates to methods, systems and devices for trapping airborne water and ice crystals prior to their deposition on such surfaces.  
       BACKGROUND  
       [0004]     Refrigerators, freezers and air conditioners rely for their cooling ability on a vapor compression cycle in which a refrigerant liquid such as ammonia, various chlorofluorocarbons, Freon, or a combination of such refrigerants, is caused to evaporate by having the liquid absorb the heat from inside the refrigerator or freezer compartment or from inside the conditioned air space. Typical refrigerants evaporate at extremely low temperatures, and thus can potentially create subfreezing temperatures inside the refrigeration unit. The cooled refrigerant vapor is subsequently circulated through the compressor portion of the unit where both the pressure and temperature are raised, and then through a condenser, thus returning the refrigerant vapor to the liquid state.  
         [0005]     As is well known, evaporator coils within refrigeration and freezer units are subject to the buildup of frost or ice. Frost forms when water vapor in the air condenses and deposits on the cooled surfaces of the evaporator coils due to the existence of a cold surface whose temperature is below both the dew point temperature and the freezing point. Ice forms due to a slow transformation of the frost layer into a more dense layer over time, due to a series of complex processes that may involve melting and refreezing of the frost layer and the subsequent seeping of the melt into the pores of the frost sublayers. Ice may also form if supersaturated air exists in the freezer which may result in deposition of these crystals on the coil surface due to a complex convection-dominated phenomenon. Accumulation of frost and ice decreases the efficiency of the evaporator, necessitating removal of the buildup on a periodic basis.  
         [0006]     In order to maintain efficient operation of refrigeration units, methods and devices have been developed for removing frost from the cooling coils of evaporator units. In a conventional arrangement, a heating device is disposed proximate to the cooling coils and is used to periodically melt the accumulated ice or frost. The heater is generally controlled by a timer to initiate a defrost cycle at given intervals, in some cases determined electromechanically on the basis of accumulated compressor operating time. Other methods are also known for defrosting the evaporator coils. In all cases, overall energy efficiency and cooling capability of the unit are decreased substantially by the need to include a defrost cycle. In larger units such as walk-in coolers and freezers, automatic defrosting cycles can be impractically lengthy, thus requiring the unit to be completely shut down for defrosting.  
         [0007]     For improved energy efficiency in refrigeration units, a need exists to minimize frost buildup on evaporator coils, reducing the necessity for, and duration of, defrost cycles and for shutdowns due to frost and ice buildup.  
       SUMMARY  
       [0008]     The invention provides a trapping device, system and method for trapping, in a refrigeration or freezer unit, airborne water or ice particles that would otherwise accumulate on the evaporator. The trapping device is disposed between the evaporator and the flow of air within the unit directed toward the evaporator.  
         [0009]     Accordingly, the invention includes in one aspect a device for trapping airborne water or ice particles in a refrigeration unit. The device includes an evaporator including a plurality of coils, and a trapping device disposed between the evaporator and airflow directed toward the evaporator. The trapping device intercepts liquid water or ice particles that would otherwise accumulate on the plurality of coils. The device can be used within a refrigeration unit such as a freezer, and particularly a walk-in freezer.  
         [0010]     In some embodiments of the device, the trapping device of the invention can include a filter having at least one metallic element on a surface or within the media of the filter. The metallic element can be a wire. The device can include a motor for translating the filter relative to the airflow.  
         [0011]     The device can further include at least one scraper. The scraper can be spring-loaded.  
         [0012]     The device can further include at least one electrical power source, wherein the metallic element is heated by the power source to melt ice on the filter. The electrical power source can be an electrical contact brush. The electrical contact brush can be spring-loaded.  
         [0013]     The device can further include at least one control unit. The control unit can include a time clock, a differential pressure controller and/or an optical sensor.  
         [0014]     The device can include condensate drainage piping. The condensate drainage piping is preferably heat-traced.  
         [0015]     In another aspect of the invention, a system for trapping airborne water or ice particles in a refrigeration unit is disclosed. The system includes an evaporator including a plurality of coils, and a trapping device disposed between the evaporator and airflow directed toward the evaporator. The trapping device intercepts liquid water or ice particles that would otherwise accumulate on the plurality of coils.  
         [0016]     In yet another aspect of the invention, a method for trapping airborne water or ice particles in a refrigeration unit is disclosed. The method includes the steps of providing a trapping device interposed between an evaporator comprising a plurality of coils and airflow directed toward the evaporator, and directing the airflow toward the trapping device, wherein the trapping device intercepts liquid water or ice particles that would otherwise accumulate on the plurality of coils. The method can further include the step of actively removing ice that accumulates on the trapping device.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     A fuller understanding of the present invention and the features and benefits thereof will be accomplished upon review of the following detailed description together with the accompanying drawings, in which:  
         [0018]      FIG. 1  is a perspective view of a system showing a device for trapping airborne water or ice particles in relation to an evaporator, according to an embodiment of the invention.  
         [0019]      FIG. 2  is a schematic elevation view of a device for trapping airborne water or ice particles including a motor and a control unit, according to an embodiment of the invention.  
         [0020]      FIG. 3  is an elevation view of a trapping device that includes an electrical power source, a scraper, a condensate collection pan and drainage piping, according to an embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     During the operation of refrigeration units, and in particular large units of the walk-in type, especially freezers, chilled air can become supersaturated with an “ice fog” that includes ice particles and water droplets sufficiently small (on the order of microns or less) to remain suspended in the air. Existence of an ice fog is a well recognized phenomenon, although its mechanism of formation remains a subject of research. An ice fog is exacerbated, for example, by opening the door of the refrigeration unit, with resultant influx of warmer, moist air from the exterior. Suspended water and ice particles in the supersaturated environment of refrigeration units can create efficiency problems for the refrigeration unit. By condensing on the coils of evaporators, water droplets from these ice and water particles add to the formation of frost in the system, which must be periodically removed to maintain efficient operation of the system. The faster the rate at which the frost builds up, the more frequently frost removal must be undertaken.  
         [0022]     To mitigate this problem, the invention provides in one embodiment a device for trapping airborne water particles before they accumulate on the evaporator, and a system based on this device. The device can be used in newly manufactured refrigeration systems or retrofitted to existing refrigeration systems.  
         [0023]      FIG. 1  illustrates an embodiment of a refrigeration system  100  showing a trapping device  205  of the invention and an evaporator  110 . The system  100  also includes a condensing unit (not shown) and refrigerant lines (not shown). The trapping device  205  is interposed between the evaporator  110  and the flow of air  120  directed toward the evaporator  110 . Preferably the width and height of the trapping device  205  and the evaporator  110  of the system  100  are similarly proportioned. However, the trapping device  205  may be of any size or shape adequate to screen the evaporator coils  130  from airborne water particles in the incoming airflow  120 . Although a preferred embodiment is described herein in the context of a freezer, the principle of operation of the trapping device  205  of the invention is applicable to any refrigeration unit that includes an evaporator subject to deposition of frost on the coils under conditions of supersaturation of the air with suspended ice crystals or water droplets.  
         [0024]     During normal operation of refrigeration units such as walk-in freezers, a flow of air  120  is directed toward the evaporator  110 , typically by a fan (not shown). The trapping device  205  intercepts suspended water and ice particles contained in the air directed toward the evaporator  110  before these particles have an opportunity to contact and freeze on the coils  130  of the evaporator  110 . The trapping function can be performed by a filter  210  that includes at least one metallic element  220  suitably positioned for trapping the water particles, for example, by condensation and freezing onto the surface of the metallic elements  220 . The metallic elements  220  of the filter  210  are designed to “frost up” during operation of the refrigeration unit. Thus, the metallic elements  220  of the filter  210  present a preemptive, or alternative site, other than the evaporator coils  130 , for preferential deposition of a substantial portion of the frost which would otherwise accumulate on the evaporator coils  130  from water particles suspended in the air stream  120 . The suspended water particles can also accumulate on the filter  210 .  
         [0025]     The trapping device  205  of the invention can be secured in position proximate to the evaporator coils  130  by any suitable attachment mechanism. For example, the trapping device  205  can be positioned within an open framework or other suitable housing (not shown) that can be fastened to the coils  130  or other component of the evaporator  110 , for example using bolts or screws.  
         [0026]     The metallic elements  220  of the filter  210  can be of any material, shape and construction suitable for acting as a site of deposition of airborne water droplets or ice crystals. Preferably, a plurality of metallic elements  220  is included on or within the filter  210 . In the embodiment of the trapping device  205  shown in  FIG. 1 , the metallic elements  220  are fibers, such as nichrome wires, oriented perpendicularly to the long axis of the filter  210 . When constructed as fibers, the metallic elements  220  can be fibers of any suitable length, width, shape, thickness, and metal composition. Alternatively, the metallic elements  220  need not be in the form of wires, and can be constructed in any other shape appropriate for trapping of airborne water particles or ice crystals. Combinations of different types of metallic elements  220 , such as wires and metallic elements of other shapes are also within the invention. In some embodiments, the metallic elements  220 , or a portion thereof, may be embedded within the matrix of the filter  210 .  
         [0027]     The matrix of the filter  210  can be made of any suitable filter material, provided that it allows adequate airflow therethrough to the evaporator  110 . The mesh size of the filter  210  can be any size suitable for the purpose. An exemplary material for the filter  210  is layered fiberglass of the type typically used for commercial air conditioning filters. Where reusability of the filters  210  is desired, the filter matrix can be constructed of a washable mesh material formulated, for example, from plastic.  
         [0028]     The filter  210  can be movable in relation to the air flow  120 . For example, in the embodiment of the trapping device  205  illustrated in  FIG. 1 , interception of airborne water droplets or ice particles can be achieved by revolving the filter  210  using upper and lower rollers  230  and  240 , respectively. By revolving the filter  210 , a new surface is continuously exposed to the air flow  120  and thus is available for more efficient capture of suspended water droplets and ice particles and deposition of frost on the metallic elements  220 . Concurrently, as described below, the rotation of the filter  210  can be advantageously utilized to remove accumulated frost deposited on the surface of the filter  210  during the previous cycle of exposure to the air flow  120 .  
         [0029]      FIG. 1  illustrates one exemplary embodiment of a device  100  of the invention including a revolving filter  210  and an electrical power source in contact with the filter  210 . In this embodiment, the electrical power source is a pair of standard electrical contact brushes  250 . The position of an electrical contact brush  250  relative to the filter  210  can be flexible. For example, an electrical contact brush  250  can be held under tension against the filter  210 . In the device  100  shown, the electrical contact brushes  250  are spring-loaded by attachment to springs  260 .  
         [0030]     During active operation of the trapping device  205 , the filter  210  rotates on the rollers  230  and  240 . Once per cycle of rotation, the metallic elements  220  of the filter  210  come into contact with the electrical contact brushes  250 . In some embodiments of the invention (described below) portions of the metallic elements  220  are embedded within the matrix of the filter  210 , in which case suitably positioned extensions of the metallic elements  220  make contact with the electrical contact brushes  250 .  
         [0031]     Contact of one or more metallic elements  220  with the electrical contact brushes  250  allows a current to flow through the metallic elements  220 , causing them to heat up. Heating of the metallic elements  220  melts any adherent frost, which then drains from the bottom of the filter  210  to a gravity fed drain line (described below). Upon movement of the metallic elements  220  away from the electrical contact brushes  250 , the metallic elements  220  rapidly lose heat, once again chilling to the ambient temperature within the interior of the refrigeration unit.  
         [0032]     From the standpoint of energy conservation, it is an advantageous feature of the design of the trapping device  205  that the metallic elements  220 , despite their continuous use during the operation of the device  100 , are heated only briefly during the cycle of operation. As described above, the metallic elements  220  carry out their primary “cold” function in the chilled state existing in the interior of the refrigeration unit, serving to trap and freeze airborne water and ice particles. In the preferred embodiment illustrated in  FIG. 1 , the metallic elements  220  are subjected to heating only once per rotational cycle, i.e., during their brief period of “hot function,” brought about by contact with the electrical contact brushes  250 . In this way, energy use is minimized. Moreover, the metallic elements  220  have a thermal mass that is a small fraction of that of the coils  130  of the evaporator  110 .  
         [0033]     Whereas only one site of heating is illustrated in the device  100  of  FIG. 1 , it will be appreciated, however, that more than one site of contact of the metallic elements  220  with an electrical source could be provided at a given point in time without departure from the overall concept of minimizing the duration of the period of “hot function” vs.“cold function” of the metallic elements  220 . Moreover, although the above-described embodiment includes a revolving filter  210 , those of ordinary skill in the art will appreciate that many mechanisms can be envisioned for driving movement of the filter  210 .  
         [0034]     In embodiments of the devices of the invention incorporating revolving or otherwise moving filters, movement of the filters (for example on rollers  230  and  240 , as illustrated in  FIG. 1 ) can be driven by a motor.  FIG. 2  shows a device  300  of the invention including a trapping device  305  with a filter  310  and a motor  370  disposed within a housing  380  positioned external to the trapping device  305 . The motor  370  can be operably connected to the movement mechanism in any suitable manner.  
         [0035]     The device  300  of the invention can be further configured to include at least one control unit  390  for control of various operations of the device  300 . Suitable control devices are well known to those having ordinary skill in the refrigeration, defrosting and air conditioning arts, and can include, among others, a time clock, a differential pressure controller, and/or an optical sensor. Such devices can be used, for example, to automatically activate a motor for driving movement of the filter  310 . As is well known, activation can be initiated by these devices on the basis of various parameters, for example, duration of compressor operation, changed pressure conditions within the refrigeration unit, or optical detection of an exceeded level of frost accumulation.  
         [0036]     Some embodiments of the devices of the invention can include one or more structures for removing frost from the filter of the trapping device. Referring to  FIG. 3 , device  400  includes a trapping device  405  with a filter  410  in which metallic elements  420  are embedded in the matrix of the filter  410 . The device  400  includes a structure for removal of frost in the form of a scraper  425  positioned to be in contact with the surface of the filter  410  exposed to the air flow. The device  400  also includes a condensate collection pan  455  for capture and drainage of the removed frost and water.  
         [0037]     In the operation of the device  400 , the scraper  425  functions to mechanically remove the frost  445  that has built up on the filter  410  after it has been exposed to the airflow (not shown). Scraping is accomplished by positioning the scraper  425  so as to be in contact with the outer surface of the moving filter  410 , or proximate thereto. In the embodiment of the device  400  shown, the scraper  425  is spring-loaded by attachment with a spring  435  to a housing, such as a condensate collection pan  455 . Any suitable shape, size and material can be used in the manufacture of a component designed to accomplish the function of the scraper  425  of the device  400 . As an alternative to stationary scrapers, either rigidly affixed or flexibly attached to a housing such as by springs, it will be apparent to those skilled in the art that the design of the devices of the invention could also encompass, for example, a trapping device with a stationary filter and one or more moving scrapers.  
         [0038]     Removal of accumulated frost  445  from the filter  410  can be further facilitated by including a source of electrical power, as described above, to periodically heat the metallic elements  420  of the filter  410 . Frost removal can be effected by heating alone, or preferably in combination with the mechanical action of one or more scrapers  425 . The device  400  shown in  FIG. 3  includes both a scraper  425  and an electrical contact brush  450  for heating the metallic elements  420 . In embodiments such as device  400  in which the metallic elements  420  are included within the matrix of the filter  410 , portions of the metallic elements  420  can extend beyond the edges of the filter  410 , to make contact with the electrical contact brushes  450 .  
         [0039]     For optimal energy conservation in embodiments incorporating rotating filter mechanisms, such as the device  400  shown in  FIG. 3 , the electrical contact brush  450  or other suitable electrical source should generally be positioned close to the site of scraping of the filter  410 , i.e., closely “downstream” of the scraper  425  in the direction of rotation (shown by arrows) of the filter  410 . According to such an arrangement, the majority of the deposited frost  445  is eliminated mechanically by the scraper  425 , thereby minimizing the energy required to melt the remaining adherent frost by heating the metallic elements  420  by contact with the electrical contact brushes  450 .  
         [0040]     The embodiment shown as device  400  further includes a condensate collection pan  455 , which can be of any appropriate size, shape and material. The condensate collection pan  455  can be connected to condensate drainage piping  465  used for directing the flow of frost and/or water condensate removed from the filter  410  by gravity, or during an active cleaning process such as by the above-described scraping of the filter  410  and/or by heating of the metallic elements  420 . The condensate drainage piping  465  can include a heat-trace  475 , preferably on the exterior of the refrigeration unit, to facilitate free flow and removal of collected condensate. As is well known in the art, a condensate trap  485  can be included in the condensate drainage piping  465 , preferably exterior to the housing of the refrigeration unit.  
         [0041]     While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as described in the claims.