Abstract:
An ultra low temperature freezer evaporator (ULT) apparatus that has a triple feed capillary tube system. The ULT apparatus prevents the low stage compressor from running in a vacuum. The invention significantly reduces the “pull down” time. Further, the ULT freezer evaporator apparatus significantly increases the transfer area between the evaporator tubing and the contact surface on the unit to assist the transfer of heat from the refrigerated unit through the condenser to the surrounding room.

Description:
[0001]    This application claims benefit of U.S. Provisional Application Ser. No. 61/581,234 filed Dec. 29, 2011, pursuant to 35 USC §119(e). 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to ultra low temperature freezer units, in particular, ultra low temperature (ULT) freezers that have an improved evaporator with an improved capillary feed system on the low stage compressor assembly. 
       BACKGROUND OF THE INVENTION 
       [0003]    Ultra low temperature (ULT) freezers are typically designed to store and protect critical biological materials. Minus 86° C. freezers are a common product produced by several manufacturers. This type of freezer as well as other ULT freezers operating at even colder temperatures is used for the storage of blood component additives, bone marrow, insect cell culture, mammalian cell culture, nucleic acids (DNA/RNA), sperm, fertilized ova, tissues and viruses. 
         [0004]    Referring to  FIGS. 1 and 2 , a typical prior art ULT freezer  80  has one long continuous run of evaporator tubing  20  fed by one extremely long piece (preferably more than 20 feet in length) of capillary tube  22 . Such a tube  22  is typically 0.036 inches in diameter. Evaporator tube  20  is typically a ⅜ inch copper tube. 
         [0005]    This single length of capillary tube  22  causes the suction pressure of the low stage compressor to run in a substantial vacuum. This vacuum can cause accelerated wear and tear on the crankshaft and connecting rod. In turn, this will drive up the compression ratio of the compressor. Thus, a higher compression ratio can result in the compressor to be running at a pressure exceeding the compressor manufacturer&#39;s recommended operating envelope. Of course, operating in such a manner is likely to adversely affect the reliability of the compressor. Further, the flow rate of the refrigerant is reduced in a negative manner. Consequently, longer “pull down” times (the time required for the unit to reach the desired temperature) are experienced. This requires the compressor to run longer as well thus reducing the lifespan of the unit and also increasing operating costs. 
         [0006]    As shown in  FIG. 3 , a lot of transfer area  28 , that is, between the contact surface  24  and tube  20 , is reduced when evaporator tube  20  is routed around corners  26 . Therefore, this design requires a greater amount of time to absorb the heat from inside the unit and transfer this heat from the condenser to the surrounding room. This will also increase the time the compressor must run and increases operating cost. 
         [0007]    There is no ULT freezer unit presently available that solves the problems noted above. 
       SUMMARY OF THE INVENTION 
       [0008]    It is an aspect of the invention to provide an ULT freezer evaporator apparatus that has a triple feed capillary tube system. 
         [0009]    It is another aspect of the invention to provide an ULT freezer evaporator apparatus that prevents a low stage compressor from running in a vacuum. 
         [0010]    Another aspect of the invention is to provide an ULT freezer evaporator apparatus that significantly reduces the “pull down” time. 
         [0011]    Finally, it is still another aspect of the invention is to provide an ULT freezer evaporator apparatus that significantly increases the transfer area between the evaporator tubing and the contact surface on the unit to assist the transfer of heat from the refrigerated unit through the condenser to the surrounding room. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is an isometric rear view of a prior art freezer illustrating the evaporator tubing configuration in place on the freezer unit. 
           [0013]      FIG. 2  is a detailed isometric view of the area of evaporator tubing identified in  FIG. 1  as area “B”. 
           [0014]      FIG. 3  is a detailed side view of evaporator tubing identified in  FIG. 1  as area “C”. 
           [0015]      FIG. 4  is a left top isometric view illustrating the evaporator tubing configuration in place on the freezer unit in accordance with the invention. 
           [0016]      FIG. 5  is a right bottom isometric view illustrating the evaporator tubing configuration in place on the freezer unit in accordance with the invention. 
           [0017]      FIG. 6  is a detailed isometric view of capillary tubes  22 A,  22 B, and  22 C brazed into their respective evaporator tubes  20 A,  20 B, and  20 C indentified in  FIG. 4  as area “D”. 
           [0018]      FIG. 7  is a detailed isometric view of distributor  32  with capillary tubes  22 A,  22 B, and  22 C leading into raceway  30  identified in  FIG. 4  as area “A”. 
           [0019]      FIG. 8  is a schematic of the invention used with a preferred ULT freezer evaporator apparatus. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    Referring now to  FIG. 8  where invention  10  is shown in combination with a typical ULT freezer such as manufactured by Nor-Lake, a Wisconsin Company. This freezer is a two-stage compressor system as shown. This unit is powered by a low noise, high performance cascade refrigeration system using two 1 Horsepower hermetically sealed compressors  40 ,  50 . The high stage compressor  40  is an Emerson Model No. RFT42CIE-PFV for 230-volt units or the RFT42CIE-PFA for the 115-volt unit. The low stage compressor  50  is also made by Emerson using the same model as above. 
         [0021]    When the freezer sensor units call for cooling, high stage compressor  40  runs by itself until heat exchanger  42  reaches a temperature of −34 degrees Centigrade. At that time, the controller will start low stage compressor  50  to run with the high stage. The low stage refrigerant will begin to circulate through oil separator  56 , downstream to heat exchanger  42  and through filter dryer  60  then to distributor  32  where the refrigerant will be dispersed evenly into three equal length sections  13 ,  14  and  15  of evaporator invention  10  with each section having capillary tubes  22 A,  22 B, and  22 C inside copper tubing  20 A,  20 B, and  20 C, respectively. 
         [0022]    The capillary tubes  22 A,  22 B, and  22 C are of a predetermined diameter and length to cause a predetermined temperature/pressure drop of the refrigerant as it reaches the ⅜ copper tubing  20 A,  20 B, and  20 C that is attached to freezer liner  12 . In the example shown in  FIG. 8 , once the refrigerant is at the evaporator invention  10 , the refrigerant will start to absorb heat from the interior of the invention  10  through the freezer liner walls  12 . 
         [0023]    The three-piece evaporator invention  10  is attached to freezer liner walls  12  with aluminum tape (not shown) to provide better heat transfer. Care must be taken to attach evaporator  10  either level or slightly sloping downhill to aid in refrigerant/oil to return to low stage compressor  50 . Refrigerant is fed at the top of the freezer cabinet providing a down feed design, thus letting gravity assist the refrigerant/oil back to compressor  50 . 
         [0024]    Two sections of evaporator invention  10  are mirror images of each other.  FIG. 4  shows evaporator sections  14  and  15 . The third section ( 13 ) of evaporator invention  10  is shown in  FIG. 5  as well as section  14  again. Thus,  FIG. 4  shows the back ( 14 ), top and left side ( 15 ) of the freezer box which corresponds to sections  14  and  15 .  FIG. 5  shows the bottom, right ( 13 ) and again the back ( 14 ) of the freezer box which corresponds to sections  13  and  14 . Note that the door of the freezer box  80  (in direction  35 ) is not shown. 
         [0025]    The back section  14  is adjusted by reducing the radius of the turns to achieve the same length as the other two sections  13  and  15 . The three-evaporator sections tees into a manifold  34 , then back to the compressor  50  as shown  FIG. 8 . 
         [0026]    As shown in  FIG. 6 , capillary tube  22 A is brazed into evaporator tube  20 A; capillary tube  22 B is brazed into evaporator tube  20 B; and finally capillary tube  22 C is brazed into evaporator tube  20 C to form evaporator sections,  13 ,  14 , and  15  respectively. 
         [0027]    As shown in  FIG. 7 , low stage distributor  32  provides refrigerant in direction  36  up raceway  30  where it is split into the three-evaporator sections  13 ,  14 , and  15  as shown in  FIGS. 4 and 5 . 
         [0028]    The use of evaporator invention  10  provides an accelerated “pull down” by providing increased contact area. In fact, when the inventor tested a similar freezer model without evaporator invention  10 , it was found that runtime was approximately 40% less to go from ambient temperature to −80 degrees Centigrade. 
         [0029]    The high capacity air-cooled condenser  49  features rifled tubing. Having rifled tubing will spin the refrigerant to keep more liquid against the tubing walls for improved heat rejection to the surrounding environment. 
         [0030]    Again, referencing  FIG. 8 , the high stage compressor starts and refrigerant exits compressor  40  through the discharge line to the heat exchanger suction accumulator  44 . Part  44  has both low and high temperature refrigerant entering in the dome of the canister. The hot gas makes a couple of passes in ⅜″ tubing inside the canister to boil off any liquid that might be present from the return gas. This heat exchange is to prevent any liquid from entering compressor  40  and causing damage to the bearing surfaces. The refrigerant exits part  44  and travels to condenser  49  where cooler air is drawn across it to lower the temperature of the refrigerant and condense it. Now the liquid refrigerant exits condenser  49  and enters filter drier  48  where particles and moisture are filtered from the refrigerant. The refrigerant enters capillary tube  47  and achieves the right temperature/pressure drop then onward to heat exchanger  42  to absorb heat from the low stage circuit. The refrigerant exits and makes a pass through heat exchanger suction accumulator  44  to boil off any liquid before entering compressor  40  where the refrigerant is drawn into the combustion chamber. Heat of compression will add heat and raise the pressure of the refrigerant where it exits through the discharge line and the cycle will start again. 
         [0031]    The low stage compressor  50  will start once heat exchanger temperature reaches −34° c. The refrigerant passes through oil separator  56  where the oil is retained through a coalescing filter and falls to the bottom of oil separator  56 . The filtered refrigerant exits and enters heat exchanger  42  where heat is rejected to the high stage circuit. The refrigerant exits and enters filter drier  60  where particles and moisture are filtered from the refrigerant. The refrigerant now enters distributor  32  where the pressure evenly disperses the refrigerant into capillary tubes  22 A,  22 B and  22 C to achieve the right temperature/pressure drop and then onward to evaporator sections  13 ,  14  and  15 . Here the refrigerant will absorb heat from conditioned area  12 . The refrigerant enters manifold  34  and returns to low stage compressor  50  where the refrigerant is drawn into the combustion chamber. Heat of compression will add heat and raise the pressure of the refrigerant where it exits through the discharge line and the cycle will start again. Both compressors will run until the cabinet sensor is satisfied. 
         [0032]    Although the present invention has been described with reference to certain preferred embodiments thereof, other versions are readily apparent to those of ordinary skill in the preferred embodiments contained herein.