Abstract:
A flooded evaporator has a plurality of tubes extending through a shell. Process fluid flows through the tubes and refrigerant flows inside of the shell through gaps between the tubes. Filler beads are located in the gaps between the tubes, thus displacing some of the refrigerant and requiring a lower refrigerant charge. The refrigerant flows through the spaces between the filler beads. The filler beads move thereby dispersing the refrigerant and dislodging bubbles from the outside of the tubes, resulting in an increase in efficiency of heat exchange.

Description:
[0001]    This application claims the benefit of U.S. provisional application Ser. No. 60/900,139, filed Feb. 8, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to shell and tube flooded evaporators for refrigeration applications. 
       BACKGROUND OF THE INVENTION 
       [0003]    A shell and tube flooded evaporator is an integral part of a refrigeration system. In a typical refrigeration system there is an evaporator that cools the process fluid at the expense of boiling the refrigerant that is at a lower saturation temperature and pressure, a compressor that compresses the boiled off refrigerant to an elevated pressure and temperature, a condenser uses a cooling medium to condense the high pressure refrigerant to liquid phase at the expense of heating the cooling medium, and an expansion device that drops down the pressure of the condensed refrigerant back to the low side which then enters the evaporator to repeat the above cycle again. This cycle is called the reverse Rankine cycle. 
         [0004]    Such refrigeration systems are found in a variety of installations, such as food processing plants. 
         [0005]    A shell and tube flooded evaporator has a shell with tubes extending through the shell. The tubes carry the process fluid. The shell of the evaporator is flooded with the refrigerant. The liquid refrigerant typically enters the bottom of the shell, contacts the tubes, which tubes carry the hot process fluid. The refrigerant vaporizes and exits the shell at the top. 
         [0006]    Refrigerants are typically natural, such as ammonia or propane. Synthetic refrigerants are falling out of favor due to environmental concerns. However, even natural refrigerants have drawbacks; ammonia is toxic and propane is flammable. 
         [0007]    It is desirable to design an evaporator that has a higher efficiency than found in the prior art. A more efficient evaporator would use less refrigerant, thus minimizing any danger from an accidental refrigerant release. In addition, a more efficient evaporator would be physically smaller, taking up a smaller footprint on a factory or plant floor, thus saving money. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a frontal view of an even-pass (four pass) flooded shell and tube evaporator, partially cut away. 
           [0009]      FIG. 2  is a side or end view of the flooded shell and tube evaporator of  FIG. 1 . 
           [0010]      FIG. 3  is a frontal view of an odd-pass (three pass) flooded shell and tube evaporator, partially cut away. 
           [0011]      FIG. 4  is a side or end view of the flooded shell and tube evaporator of  FIG. 3 . 
           [0012]      FIG. 5  is a cross-sectional view of a tube bundle of the evaporator of  FIG. 1  taken at section A-A. 
           [0013]      FIG. 6  is a frontal view of a partially cut away flooded shell and tube evaporator of the present invention. 
           [0014]      FIG. 7  is a detailed cross-sectional view of tubes and filler beads. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]    In  FIGS. 1 and 2 , a shell and tube evaporator is shown with a plurality of parallel tubes  6  in horizontal orientation. The tubes  6  are received at each end by two end plates  3  (round or rectangular in shape) called tube sheets. Each tube sheet  3  has a plurality of parallel holes that are machined at a specific distance with respect to each other according to industry standards, viz., Tubular Exchanger Manufacturers Association, TEMA. The tubes are further supported by baffles or tube supports  7  within the span between the tube sheets  3 . The distance between the adjacent baffles or tube supports  7  is determined according to industry standards, e.g., Tubular Exchanger Manufacturers Association, TEMA. The baffles or tube supports  7  have a hole pattern identical to the tube sheets  3  as shown in  FIG. 5  (larger scale). The combination of tube sheets  3 , the tubes  6 , the baffles or tube supports  7  and the tie-rods  9  is also known as tube bundle and is welded to the shell  4  at each end,  19  and  20 , hence isolating the shell side  16  from the tube side  17 . At the ends, the tube side  19  is confined by front and rear heads  1 ,  2 . The tubes  6  are individually joined to the tube sheets  3  at the corresponding holes in the tube sheets  3  via mechanical means or welding. 
         [0016]    The process fluid such as water or brine or any other fluid to be cooled enters the tube side  17  at the front head  1  (attached to tubes sheets  3  through bolting  5  or welding) via inlet port  10 . Depending upon the nature of the application, the heads  1  and  2  could be arranged for multiple pass or single pass configuration. In the case of multiple pass, the front head  1  and the rear head  2  carry pass partition plates  14  at the corresponding lane  21  on the tube sheets  3  that directs the process fluid in the tubes  6  back and forth through a respective quantity of tubes in each pass until the fluid exits at head  1  via port  11  for even-pass configuration as shown in  FIG. 1  and  FIG. 2  or at head  2  for odd-pass configuration as shown in  FIG. 3  and  FIG. 4  via exit port  11 . The process fluid entering at inlet port  10  is hot, while the process fluid exiting at outlet port  12  is cooled. 
         [0017]    Low temperature and low pressure liquid or liquid-gas mixture of refrigerant enters the shell side  16  via port  12 . As the refrigerant travels upwards it extracts heat from the hot fluid in the tubes  6  and progressively evaporates. The vapor/liquid ratio increases along the height of the tube bundle. The wet vapor exits the shell side  16  via risers  15  and enters the separator  8  and leaves the separator  8  as liquid-free vapor via port  13 . 
         [0018]    From the separator  8 , the refrigerant vapor is routed to the compressor (not shown), where the refrigerant is compressed. From the compressor, the refrigerant, which is hot, is cooled in the condenser. After leaving the condenser, the pressure of the refrigerant is dropped by an expansion device, wherein the refrigerant reenters the shell  4  at port  12 . 
         [0019]    As shown in  FIG. 5 , the tubes  6  are spaced apart from each other, thus creating gaps  31  between the tubes. The refrigerant flows through these gaps  31 . The tubes are grouped into the sections, with each section representing a pass through the shell. Thus, section I is the first pass of the process fluid through the shell, from the inlet port  10  and the front head  1  to the rear head  2 . Section II is the second pass, from the rear head  2  back to the front head  1 . Section III is the third pass, from the front head  1  to the rear head  2 . Section IV is the fourth pass, from the rear head  2  to the front head  1  and the outlet port  11 . The tubes within a section are separated from each other by a relatively small gap  31 . The tubes in adjacent sections are separated from each other by a larger gap, or lane  21 , in order to accommodate the pass partition plates  14 . In a prior art evaporator, these gaps  31 ,  21 , which represent the interior volume of the shell, are filled with refrigerant. 
         [0020]    In the present invention, much of the interior volume of the shell is filled with filler beads  35  (see  FIGS. 6 and 7 ). The filler beads have a neutral buoyancy when immersed in the refrigerant  33 . This minimizes the possibility of the beads accumulating at the bottom of the shell (if negative buoyancy) or at the top (if positive buoyancy) as assures the even distribution of the filler beads throughout the shell. For example, the density of the filler beads  35  could be the same as the density of the refrigerant. Because the refrigerant changes from a liquid state to a vapor state, the density of the refrigerant changes. The filler beads can have a neutral buoyancy relative to the liquid refrigerant. 
         [0021]    The filler beads in the preferred embodiment are made of solid plastic. The filler beads remain solid and do not turn to liquid inside of the shell. In the preferred embodiment, the filler beads  35  are spherical, although the beads could be of any shape. In the preferred embodiment, the filler beads are solid and not hollow. Solid beads are easier to manufacture and easier to match neutral buoyancy with the refrigerant. The filler beads  35  are of different sizes. In the preferred embodiment, there are at least three sizes  35 A,  35 B,  35 C (see  FIG. 7 ). The largest size bead  35 A is small enough to pass through the gaps  31  between adjacent tubes  6  in a section. Thus, the diameter of the largest size bead is less than P-D, where P is the tube pitch and D is the tube outside diameter. As an example, one type of flooded evaporator has gaps between tubes of 3/16 inches. Thus, the largest size filler bead  35 A is less than 3/16 inches. When several of the largest size beads  35 A are located so as to contact one another, there are spaces between the beads. The intermediate size beads  35 B and the smallest size beads  35 C are sized relative to the largest size beads so as to fit within the spaces of the adjacent largest size beads  35 A. 
         [0022]    The filler beads  35  displace refrigerant inside of the shell  4 . The filler beads are located in the gaps  31 ,  21  between the tubes. The filler beads have the same isothermic state as the refrigerant and consequently are thermally inert. The amount of filler beads inside the shell depends on how efficient the evaporator is to be. For example, filler beads can displace 10% of the volume inside of the shell, thus reducing the volume of refrigerant. Higher evaporator efficiencies can be achieved by using more filler beads. It is believed that up to one half to two thirds of the shell volume can be taken up by filler beads  35 . As described below, it is desirable not to overfill the shell with beads to the extent that the beads are immobile. It is desired if the beads can move inside of the shell. 
         [0023]    The filler beads  35  can be put into an evaporator before the evaporator&#39;s initial operation. Alternatively, an evaporator can be retrofitted with the filler beads. If retrofitted, the beads will quickly reach the same temperature as the refrigerant. 
         [0024]    In operation, the refrigerant  33  (see  FIG. 6 ) flows through the spaces  37  between the filler beads  35  and consequently through the gaps  31  between the tubes. The filler beads form a structure similar to sponges, with voids formed by the filler beads. Thus, the filler beads channel the refrigerant through the spaces or gaps between the beads. The filler beads  35  move and disperse the refrigerant resulting in enhanced heat exchange and refrigerant distribution. In addition, the filler beads contact and scrub the outside diameter of the tubes  6 . This is useful in dislodging bubbles  39  that are formed on the outside of tubes  6  as the refrigerant boils. A bubble  39  on a tube decreases the heat exchange at that particular location on the tube. Dislodging the bubble increases the heat exchange. 
         [0025]    An evaporator equipped with the filler beads is more efficient and utilizes less refrigerant than prior art evaporators. As a more efficient heat exchanger, the size of the evaporator can be reduced, saving material costs and also floor space. The evaporator requires a lower charge of refrigerant for the same heat exchange capacity when compared to the prior art. The requirement of less refrigerant results in a savings in startup and maintenance cost. In addition, any accidental release of refrigerant is less dangerous as there is less refrigerant to release. 
         [0026]    The foregoing disclosure and showings made in the drawings are merely illustrative of the principles of this invention and are not to be interpreted in a limiting sense.