Abstract:
A refrigerated container having an air compressor for maintaining a controlled atmosphere includes provisions for cooling the ambient air being drawn in by the air compressor to thereby prevent corrosion in the air compressor. A condensing line heat exchanger is connected between an inlet port and the compressor and is so disposed that the evaporator fan passes air thereover to provide cooling to the ambient air passing therethrough such that condensation forms within the tube. A water separator is included in the system to remove condensate and allow the drier air to pass to the compressor. The water separator is so constructed as to allow for the free flow of air therethrough with very little restriction. A drain fitting in the water separator is so constructed as to prevent spattering that might otherwise occur.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to refrigerated containers with controlled atmosphere apparatus and, more particularly, to a method and apparatus for preventing the corrosion of an air compressor therein.  
         [0002]     In cargo containers used for transporting food and other perishables it is necessary to maintain an oxygen poor atmosphere in order to prevent food spoilage, and to prevent damage from insects and similar vermin. Limiting the amount of oxygen in the container, while maintaining the internal temperature within a desired cool range inhibits metabolic processes within the food, and can eventually kill organisms that would attack valuable food cargo. In such containers the need to maximize payload volume limits the space that is available for refrigeration and environmental control equipment, including the system for maintaining the controlled internal atmosphere. This system typically includes an air compressor, heater, and filter that feeds a gas separator that discharges oxygen poor effluent into the chamber, and returns the remainder into the ambient. The gas separator is typically a semipermeable membrane. In order to conserve space, and to protect the system from a harsh marine environment it is standard practice to dispose its air compressor inside the container, within the controlled atmosphere. The protection from salt water and particulates afforded by internal placement of the compressor, and the cool internal temperatures of the operating environment extend the useful life of the air compressor and other components of the system.  
         [0003]     Because the air compressor is located within the refrigerated space and the suction port is located outside the container, the ingested air is at a different temperature and humidity ratio then the conditioned space in which the compressor resides. This condition causes condensation to form in the suction line tubing, and the condensation is then drawn into the compressor to cause moisture related problems that reduce bearing and compressor ring life.  
         [0004]     This problem is exacerbated by other design considerations. In order to prevent ring blow-by gases from altering container conditioned atmospheres, the applicants have adopted the practice of back venting the air compressor crank case gases into the suction ports. With reciprocating positive displacement compressors, this practice causes pressure pulsations inside the crank case, and these pressure fluctuations create a continuous flow of warm, and sometimes moist, air into the crankcase by way of the breather tube. A crank case condensation problem is encountered when the compressor is running at low discharge pressures, with a perishable set point, in moderately moist environments. Briefly stated, the heat generated by the fundamental compression and motor work is not sufficient to raise the aluminum crank case above the compressor suction dew points. Continuous running at these conditions causes moisture to collect and corrode on internal components. When the equipment is stored during the down season, this water causes corrosive damage to the compressor.  
       SUMMARY OF THE INVENTION  
       [0005]     In accordance with one aspect of the invention, the compressor suction air temperature and dew point are reduced by employing a tube heat exchanger between the lower temperature container air and the compressor suction air inside the tube. The resulting reduction of suction air temperature tends to eliminate the problems encountered when compressor suction dew points exceed the crank case wall temperature.  
         [0006]     In accordance with another aspect of the invention, provision is made to separate condensed liquids from compressor suction gas without adversely affecting compressor or system performance. This is accomplished by the use of a low restriction water separator system on the vacuum or suction side of the compressor system.  
         [0007]     By yet another aspect of the invention, provision is made for draining the water separator sump by using a radial flowing bulkhead fitting. A phenomenon of vertical back spatter, is eliminated by the use of a plurality of radially extending draining ports and the impaction of liquid condensate with the accumulator wall.  
         [0008]     In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a schematic illustration of the present invention as incorporated into a refrigerated container system with a controlled atmosphere provisions.  
         [0010]      FIGS. 2A and 2B  are top and side views, respectively, of portions of such systems in accordance with the prior art.  
         [0011]      FIGS. 3A and 3B  are top and side views, respectively, of portions thereof in accordance with a preferred embodiment of the invention.  
         [0012]      FIG. 4  is a sectional view of a water separator and drain fitting portion thereof in accordance with a preferred embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     Referring now to  FIG. 1 , there is shown a simplified schematic illustration of a refrigerated container system with the present invention incorporated therein. The container is outlined in dashed lines and is indicated generally at  11 . A refrigerant compression circuit is shown at  12  and comprises a compressor  13 , a condenser  14 , an expansion device  16  and an evaporator  17 , all interconnected in serial flow relationship, with the evaporator  17  being located within the container  11  for purposes of cooling the air therein. In this regard, it should be recognized that the refrigerant compression circuit  12  is different from an air conditioner which simply cools the air sufficiently to provide comfort to persons within a space but is rather designed as a system to refrigerate the container  11  to the extent necessary to preserve food such as in a refrigerator.  
         [0014]     Shown at  18  is a controlled atmosphere system which operates to inject nitrogen rich gas into the container  11  for purposes of maintaining the cargo in a non-ripened condition. In operation, ambient air is brought into the inlet port  24  and is passed through the condensing line/heat exchanger  19  in a manner to be described hereinafter. As the air is cooled in the heat exchanger  19 , the resulting condensate then passes to a water separator/accumulator  21  for removal of the condensate to ambient through the line  26 . The drier air is then passed to the compressor  22  where it is compressed and passed to the gas separator  23  for separation of the oxygen from the nitrogen. The nitrogen rich gas passes into the container  11  by a line  27 , and the oxygen rich gas is passed to ambient by way of a line  28 . This portion of the system will be described more fully hereinafter.  
         [0015]     Referring now to  FIGS. 2A and 2B , there is shown portions of a refrigerated container system with controlled atmosphere in accordance with the prior art. As part of the refrigerant compression circuit  12 , the evaporator fan  29  and its drive motor  31  are shown as installed in such a position to circulate cooling air through the surrounding area  32 . Its function is to circulate air to be cooled over the evaporator  17  (not shown in this view), and has little or no effect on the operation of the controlled atmosphere system  18 .  
         [0016]     As part of the controlled atmosphere system  18  an inlet panel/filter  33  is provided to filter the ambient air being drawn into the suction port  34 , then into the suction line  36  to the air compressor  22 . The ambient air is then compressed and passes along the discharge line  37  to the gas separator (not shown) as described hereinabove. A bypass tube connects the interior of the crank case with the inlet of the cylinders so that blow-by is returned from the crank case to the inlet of the cylinder for recirculation. This prevents ring blow-by gases from altering container conditioned atmospheres.  
         [0017]     It has been found that with the above arrangement, condensation tends to form in the suction line  36  because of the differences in temperature and humidity ratio at ambient and in the conditioned space. The condensation is then drawn into the compressor and causes moisture problems as described hereinabove.  
         [0018]     The above problem is acerbated by the fact that reciprocating positive displacement compressors cause pressure pulsations inside the crank case, and these pressure fluctuations create a continuous flow of warm and some times moist air into the crank case via the breather tube. A crank case condensation problem is encountered when the compressor is running at low discharge pressures, with a perishable set point, and moderately moist environments. Under these conditions, the heat generated by fundamental compression and motor work is not sufficient to raise the aluminum crank case above compressor suction dew points. Current controlled atmosphere compressor suction dew points are in excess of 70-75 F, depending on location in the world, while crank case wall temperatures average around 62-65° F. continuous running at these conditions cause moisture to collect and corrode on internal components. When equipment is stored during the down season, this water causes corrosive damage to the compressor.  
         [0019]     The above problems are overcome by the present invention as shown in  FIGS. 3A and 3B . Rather than the suction line  36  passing directly to the compressor as in the prior art, a condensate line heat exchanger  19  is attached to the suction line  36  as shown and is routed so as to be substantially within the area  32  such that the air passing through the area  32  because of operation of the evaporator fan  29  passes over the condensing line heat exchanger  19  so as to cool the ambient air therein and cause some of it to condense.  
         [0020]     At the downstream end of the heat exchanger  19 , the water separator/accumulator  21  receives the air/condensate mixture and acts to separate the two, with the drier air then passing along an insulated tube  39  to the compressor  22 , and the condensate passing along the line  41  to ambient. Such an arrangement not only removes moisture from the ambient air but also reduces the suction air temperature and thus eliminates the problem encountered when compressor suction dew points exceed the crank case wall temperature.  
         [0021]     It should be recognized that the ambient air being drawn in will vary substantially with the environment. Thus, the present system must be designed to withstand such adverse conditions as is present in a marine salt air environment. Accordingly, the heat exchanger  19  is preferably treated internally such as with a conversion coating or a hard coat anodized process to render it substantially corrosion resistant. Further, such a coating also promotes a drop-wise condensation rather than a film condensation, thereby promoting more effective heat transfer. The length and shape of the heat exchanger  19  is designed to allow for easy assembly into the system.  
         [0022]     Referring now to  FIG. 4 , the water separator/accumulator  21  is shown to include an elongate cylindrical housing  42  disposed with its axis in a vertical orientation. A top member  43  closes the cylindrical housing  42  at its top and includes an inlet opening  44  for the admission of the air and water mixture, and a discharge opening  46  for discharging the relatively dry air to the compressor suction port. A centrally disposed drop tube  47  is connected to the top member  43  and provides fluid communication from the inlet opening  44  to the inner portion of the cylindrical housing  42 . The length of the drop tube may vary, but it has been found that it should preferably reach about half way down the length of the housing  42 . A drain fitting  48  is then provided to drain the condensate from the system.  
         [0023]     In operation, the air and water mixture enters the inlet opening  44  and passes down through the relatively long drop tube  47  to enter the interior of the cylindrical housing  42  at its about midpoint. As the air/water enters the larger diameter housing  42 , the flow area expands, and the flow velocity is reduced. This drop in velocity slows the momentum of the water droplets allowing them to fall from the tip of the suspended drop tube  47 . Additionally, in order for the flow of air to rise up on the outer sides of the drop tube  47  as shown by the arrows and enter the discharge opening  46 , the flow must change directions by 180 degrees. This forced flow direction change causes the momentum of the smaller entrain droplets to push out of the flow stream during its change in directional path. The inside diameter of the cylindrical housing  42  is such that it prevents droplets from being pulled up the wall by velocity pressure. In this way, a combination of volume expansion and the 180 degree change of direction of the gas flow accomplishes substantially 100% effective removal of the moisture from the ambient air.  
         [0024]     It should be recognized that the above described system is a free flowing design with very little restriction in the flow of the air/water mixture on the vacuum or suction side of the compressor. This is to be contrasted with the existing pneumatic systems wherein water separators are used on the high pressure side of the system and therefore have an associated relatively high pressure drop which can impact compressor performance. The present system does not negatively impact compressor performance and doesn&#39;t require a media filter as a liquid impingement method, which would require repeated service for cleaning.  
         [0025]     Another possible problem addressed by the present invention is that of back spattering, which can adversely affect performance. That is, since that water separator sump is in vacuum, a condition exists when the vacuum in the sump cause water from the drain to be pulled back into the separator sump. This cause back spattering which results in lower collection efficiencies. At times the back spattering can reach the top of the accumulator sump and carry over to the outlet port. This problem is addressed by the specific design of the drain fitting  48 .  
         [0026]     The drain fitting  48  comprises a cylindrical body  49  with a central opening  51 . The cylindrical body  49  is secured in the bottom of the cylindrical housing  42  by brazing or the like and is relatively large in its outer diameter as compared with the inner diameter of the cylindrical housing  42  (i.e. about ½). At the top of the cylindrical body  49  is a flat top element  52  with a plurality of radially extending ports  53  as shown. It has been found that six radial ports  58  are satisfactory for this design. A trap  54  is disposed within the central opening  51 .  
         [0027]     In operation, the condensate that falls to the bottom of the cylindrical housing  42  flows radially inwardly through the ports  53  and flows downwardly through the central opening  51 , from where it then passes to ambient as described hereinabove. The spattering effect is therefore eliminated because any condensate that tends to be drawn back into the housing  42  must first flow through one of the radial ports  53  and then is impacted against the inner diameter of the accumulator wall surrounding the ports  53 .  
         [0028]     The drain fitting  48  also includes a jam nut  56  which is used as the primary attachment of the accumulator to the fan deck of the system.  
         [0029]     While the present invention has been particularly shown and described with reference to a preferred embodiment as illustrated in the drawings, it will be understood by those skilled in the art that various changes in detail may be affected therein without departing from the spirit and scope of the invention as defined by the claims.