Patent Publication Number: US-10317117-B2

Title: Vapor compression system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of, claiming priority and benefit from U.S. application Ser. No. 12/747,286, entitled VAPOR COMPRESSION SYSTEM, having a filing date of Sep. 3, 2010, which is a PCT National Stage Entry of, claiming priority and benefit from PCT/US09/30592, entitled VAPOR COMPRESSION SYSTEM, having a filing date of Jan. 9, 2009, which claims priority and benefit from U.S. Provisional Application No. 61/020,533, entitled FALLING FILM EVAPORATOR SYSTEMS, filed Jan. 11, 2008, all of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     The application relates generally to vapor compression systems in refrigeration, air conditioning and chilled liquid systems. 
     Conventional chilled liquid systems used in heating, ventilation and air conditioning systems include an evaporator to effect a transfer of thermal energy between the refrigerant of the system and another liquid to be cooled. One type of evaporator includes a shell with a plurality of tubes forming a tube bundle, or a plurality of tube bundles, through which the liquid to be cooled is circulated. The refrigerant is brought into contact with the outer or exterior surfaces of the tube bundle inside the shell, resulting in a transfer of thermal energy between the liquid to be cooled and the refrigerant. For example, refrigerant can be deposited onto the exterior surfaces of the tube bundle by spraying or other similar techniques in what is commonly referred to as a “falling film” evaporator. In a further example, the exterior surfaces of the tube bundle can be fully or partially immersed in liquid refrigerant in what is commonly referred to as a “flooded” evaporator. In yet another example, a portion of the tube bundle can have refrigerant deposited on the exterior surfaces and another portion of the tube bundle can be immersed in liquid refrigerant in what is commonly referred to as a “hybrid falling film” evaporator. 
     As a result of the thermal energy transfer with the liquid, the refrigerant is heated and converted to a vapor state, which is then returned to a compressor where the vapor is compressed, to begin another refrigerant cycle. The cooled liquid can be circulated to a plurality of heat exchangers located throughout a building. Warmer air from the building is passed over the heat exchangers where the cooled liquid is warmed, while cooling the air for the building. The liquid warmed by the building air is returned to the evaporator to repeat the process. 
     SUMMARY 
     The present invention relates to a vapor compression system including a compressor, a condenser, an expansion device and an evaporator connected by a refrigerant line. The evaporator includes a shell, a first tube bundle; a hood; a distributor; a first supply line; a second supply line; a valve positioned in the second supply line; and a sensor. The first tube bundle includes a plurality of tubes extending substantially horizontally in the shell. The distributor is positioned above the first tube bundle. The hood covers the first tube bundle. The first supply line is connected to the distributor and an end of the second supply line is positioned near the hood. The sensor is configured and positioned to sense a level of liquid refrigerant in the shell. The valve is configured and positioned to regulate flow in the second supply line in response to a sensed level of liquid refrigerant from the level sensor. 
     The present invention also relates to a vapor compression system includes a compressor, a condenser, an expansion device and an evaporator connected by a refrigerant line. The evaporator includes a shell; a first tube bundle; a hood; a distributor; a supply line; a pump; an expansion device; a sensor; and wherein the first tube bundle comprises a plurality of tubes extending substantially horizontally in the shell. The distributor is positioned above the first tube bundle. The hood covers the first tube bundle. The supply line is connected to the expansion device and the expansion device is connected to a discharge of the pump. The sensor is configured and positioned to sense a level of liquid refrigerant in the shell. The pump is operated in response to a sensed level of liquid refrigerant decreasing below a predetermined level when the expansion device is in an open position. 
     The present invention further relates to an evaporator including a shell; a tube bundle; an enclosure; and a supply line. The tube bundle includes a plurality of tubes extending substantially horizontally in the shell. The enclosure receives refrigerant from the supply line and provides liquid refrigerant for the tube bundle and vapor refrigerant for an outlet connection in the shell. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows an exemplary embodiment for a heating, ventilation and air conditioning system. 
         FIG. 2  shows an isometric view of an exemplary vapor compression system. 
         FIGS. 3 and 4  schematically illustrate exemplary embodiments of the vapor compression system. 
         FIG. 5A  shows an exploded, partial cutaway view of an exemplary evaporator. 
         FIG. 5B  shows a top isometric view of the evaporator of  FIG. 5A . 
         FIG. 5C  shows a cross section of the evaporator taken along line  5 - 5  of  FIG. 5B . 
         FIG. 6A  shows a top isometric view of an exemplary evaporator. 
         FIGS. 6B and 6C  show a cross section of the evaporator taken along line  6 - 6  of  FIG. 6A . 
         FIG. 7A  shows a cross section of another exemplary evaporator having an additional refrigerant distribution supply line. 
         FIG. 7B  shows a cross section of yet another exemplary evaporator having a distributor connected to the additional refrigerant distribution supply line. 
         FIG. 8  shows an exemplary evaporator having a booster pump connected thereto. 
         FIG. 9  shows an exemplary evaporator having a deflector in an internal enclosure for redirecting refrigerant. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
       FIG. 1  shows an exemplary environment for a heating, ventilation and air conditioning (HVAC) system  10  incorporating a chilled liquid system in a building  12  for a typical commercial setting. System  10  can include a vapor compression system  14  that can supply a chilled liquid which may be used to cool building  12 . System  10  can include a boiler  16  to supply heated liquid that may be used to heat building  12 , and an air distribution system which circulates air through building  12 . The air distribution system can also include an air return duct  18 , an air supply duct  20  and an air handler  22 . Air handler  22  can include a heat exchanger that is connected to boiler  16  and vapor compression system  14  by conduits  24 . The heat exchanger in air handler  22  may receive either heated liquid from boiler  16  or chilled liquid from vapor compression system  14 , depending on the mode of operation of system  10 . System  10  is shown with a separate air handler on each floor of building  12 , but it is appreciated that the components may be shared between or among floors. 
       FIGS. 2 and 3  show an exemplary vapor compression system  14  that can be used in an HVAC system, such as HVAC system  10 . Vapor compression system  14  can circulate a refrigerant through a compressor  32  driven by a motor  50 , a condenser  34 , expansion device(s)  36 , and a liquid chiller or evaporator  38 . Vapor compression system  14  can also include a control panel  40  that can include an analog to digital (A/D) converter  42 , a microprocessor  44 , a non-volatile memory  46 , and an interface board  48 . Some examples of fluids that may be used as refrigerants in vapor compression system  14  are hydrofluorocarbon (HFC) based refrigerants, for example, R-410A, R-407, R-134a, hydrofluoro olefin (HFO), “natural” refrigerants like ammonia (NH 3 ), R-717, carbon dioxide (CO 2 ), R-744, or hydrocarbon based refrigerants, water vapor or any other suitable type of refrigerant. In an exemplary embodiment, vapor compression system  14  may use one or more of each of VSDs  52 , motors  50 , compressors  32 , condensers  34  and/or evaporators  38 . 
     Motor  50  used with compressor  32  can be powered by a variable speed drive (VSD)  52  or can be powered directly from an alternating current (AC) or direct current (DC) power source. VSD  52 , if used, receives AC power having a particular fixed line voltage and fixed line frequency from the AC power source and provides power having a variable voltage and frequency to motor  50 . Motor  50  can include any type of electric motor that can be powered by a VSD or directly from an AC or DC power source. For example, motor  50  can be a switched reluctance motor, an induction motor, an electronically commutated permanent magnet motor or any other suitable motor type. In an alternate exemplary embodiment, other drive mechanisms such as steam or gas turbines or engines and associated components can be used to drive compressor  32 . 
     Compressor  32  compresses a refrigerant vapor and delivers the vapor to condenser  34  through a discharge line. Compressor  32  can be a centrifugal compressor, screw compressor, reciprocating compressor, rotary compressor, swing link compressor, scroll compressor, turbine compressor, or any other suitable compressor. The refrigerant vapor delivered by compressor  32  to condenser  34  transfers heat to a fluid, for example, water or air. The refrigerant vapor condenses to a refrigerant liquid in condenser  34  as a result of the heat transfer with the fluid. The liquid refrigerant from condenser  34  flows through expansion device  36  to evaporator  38 . In the exemplary embodiment shown in  FIG. 3 , condenser  34  is water cooled and includes a tube bundle  54  connected to a cooling tower  56 . 
     The liquid refrigerant delivered to evaporator  38  absorbs heat from another fluid, which may or may not be the same type of fluid used for condenser  34 , and undergoes a phase change to a refrigerant vapor. In the exemplary embodiment shown in  FIG. 3 , evaporator  38  includes a tube bundle having a supply line  60 S and a return line  60 R connected to a cooling load  62 . A process fluid, for example, water, ethylene glycol, calcium chloride brine, sodium chloride brine, or any other suitable liquid, enters evaporator  38  via return line  60 R and exits evaporator  38  via supply line  60 S. Evaporator  38  chills the temperature of the process fluid in the tubes. The tube bundle in evaporator  38  can include a plurality of tubes and a plurality of tube bundles. The vapor refrigerant exits evaporator  38  and returns to compressor  32  by a suction line to complete the cycle. 
       FIG. 4 , which is similar to  FIG. 3 , shows the refrigerant circuit with an intermediate circuit  64  that may be incorporated between condenser  34  and expansion device  36  to provide increased cooling capacity, efficiency and performance. Intermediate circuit  64  has an inlet line  68  that can be either connected directly to or can be in fluid communication with condenser  34 . As shown, inlet line  68  includes an expansion device  66  positioned upstream of an intermediate vessel  70 . Intermediate vessel  70  can be a flash tank, also referred to as a flash intercooler, in an exemplary embodiment. In an alternate exemplary embodiment, intermediate vessel  70  can be configured as a heat exchanger or a “surface economizer”. In the flash intercooler arrangement, a first expansion device  66  operates to lower the pressure of the liquid received from condenser  34 . During the expansion process in a flash intercooler, a portion of the liquid is evaporated. Intermediate vessel  70  may be used to separate the evaporated vapor from the liquid received from the condenser. The evaporated liquid may be drawn by compressor  32  to a port at a pressure intermediate between suction and discharge or at an intermediate stage of compression, through a line  74 . The liquid that is not evaporated is cooled by the expansion process, and collects at the bottom of intermediate vessel  70 , where the liquid is recovered to flow to the evaporator  38 , through a line  72  comprising a second expansion device  36 . 
     In the “surface intercooler” arrangement, the implementation is slightly different, as known to those skilled in the art. Intermediate circuit  64  can operate in a similar matter to that described above, except that instead of receiving the entire amount of refrigerant from condenser  34 , as shown in  FIG. 4 , intermediate circuit  64  receives only a portion of the refrigerant from condenser  34  and the remaining refrigerant proceeds directly to expansion device  36 . 
       FIGS. 5A through 5C  show an exemplary embodiment of an evaporator configured as a “hybrid falling film” evaporator. As shown in  FIGS. 5A through 5C , an evaporator  138  includes a substantially cylindrical shell  76  with a plurality of tubes forming a tube bundle  78  extending substantially horizontally along the length of shell  76 . At least one support  116  may be positioned inside shell  76  to support the plurality of tubes in tube bundle  78 . A suitable fluid, such as water, ethylene, ethylene glycol, or calcium chloride brine flows through the tubes of tube bundle  78 . A distributor  80  positioned above tube bundle  78  distributes, deposits or applies refrigerant  110  from a plurality of positions onto the tubes in tube bundle  78 . In one exemplary embodiment, the refrigerant deposited by distributor  80  can be entirely liquid refrigerant, although in another exemplary embodiment, the refrigerant deposited by distributor  80  can include both liquid refrigerant and vapor refrigerant. 
     Liquid refrigerant that flows around the tubes of tube bundle  78  without changing state collects in the lower portion of shell  76 . The collected liquid refrigerant can form a pool or reservoir of liquid refrigerant  82 . The deposition positions from distributor  80  can include any combination of longitudinal or lateral positions with respect to tube bundle  78 . In another exemplary embodiment, deposition positions from distributor  80  are not limited to ones that deposit onto the upper tubes of tube bundle  78 . Distributor  80  may include a plurality of nozzles supplied by a dispersion source of the refrigerant. In an exemplary embodiment, the dispersion source is a tube connecting a source of refrigerant, such as condenser  34 . Nozzles include spraying nozzles, but also include machined openings that can guide or direct refrigerant onto the surfaces of the tubes. The nozzles may apply refrigerant in a predetermined pattern, such as a jet pattern, so that the upper row of tubes of tube bundle  78  are covered. The tubes of tube bundle  78  can be arranged to promote the flow of refrigerant in the form of a film around the tube surfaces, the liquid refrigerant coalescing to form droplets or in some instances, a curtain or sheet of liquid refrigerant at the bottom of the tube surfaces. The resulting sheeting promotes wetting of the tube surfaces which enhances the heat transfer efficiency between the fluid flowing inside the tubes of tube bundle  78  and the refrigerant flowing around the surfaces of the tubes of tube bundle  78 . 
     In the pool of liquid refrigerant  82 , a tube bundle  140  can be immersed or at least partially immersed, to provide additional thermal energy transfer between the refrigerant and the process fluid to evaporate the pool of liquid refrigerant  82 . In an exemplary embodiment, tube bundle  78  can be positioned at least partially above (that is, at least partially overlying) tube bundle  140 . In one exemplary embodiment, evaporator  138  incorporates a two pass system, in which the process fluid that is to be cooled first flows inside the tubes of tube bundle  140  and then is directed to flow inside the tubes of tube bundle  78  in the opposite direction to the flow in tube bundle  140 . In the second pass of the two pass system, the temperature of the fluid flowing in tube bundle  78  is reduced, thus requiring a lesser amount of heat transfer with the refrigerant flowing over the surfaces of tube bundle  78  to obtain a desired temperature of the process fluid. 
     It is to be understood that although a two pass system is described in which the first pass is associated with tube bundle  140  and the second pass is associated with tube bundle  78 , other arrangements are contemplated. For example, evaporator  138  can incorporate a one pass system where the process fluid flows through both tube bundle  140  and tube bundle  78  in the same direction. Alternatively, evaporator  138  can incorporate a three pass system in which two passes are associated with tube bundle  140  and the remaining pass associated with tube bundle  78 , or in which one pass is associated with tube bundle  140  and the remaining two passes are associated with tube bundle  78 . Further, evaporator  138  can incorporate an alternate two pass system in which one pass is associated with both tube bundle  78  and tube bundle  140 , and the second pass is associated with both tube bundle  78  and tube bundle  140 . In one exemplary embodiment, tube bundle  78  is positioned at least partially above tube bundle  140 , with a gap separating tube bundle  78  from tube bundle  140 . In a further exemplary embodiment, hood  86  overlies tube bundle  78 , with hood  86  extending toward and terminating near the gap. In summary, any number of passes in which each pass can be associated with one or both of tube bundle  78  and tube bundle  140  is contemplated. 
     An enclosure or hood  86  is positioned over tube bundle  78  to substantially prevent cross flow, that is, a lateral flow of vapor refrigerant or liquid and vapor refrigerant  106  between the tubes of tube bundle  78 . Hood  86  is positioned over and laterally borders tubes of tube bundle  78 . Hood  86  includes an upper end  88  positioned near the upper portion of shell  76 . Distributor  80  can be positioned between hood  86  and tube bundle  78 . In yet a further exemplary embodiment, distributor  80  may be positioned near, but exterior of, hood  86 , so that distributor  80  is not positioned between hood  86  and tube bundle  78 . However, even though distributor  80  is not positioned between hood  86  and tube bundle  78 , the nozzles of distributor  80  are still configured to direct or apply refrigerant onto surfaces of the tubes. Upper end  88  of hood  86  is configured to substantially prevent the flow of applied refrigerant  110  and partially evaporated refrigerant, that is, liquid and/or vapor refrigerant  106  from flowing directly to outlet  104 . Instead, applied refrigerant  110  and refrigerant  106  are constrained by hood  86 , and, more specifically, are forced to travel downward between walls  92  before the refrigerant can exit through an open end  94  in the hood  86 . Flow of vapor refrigerant  96  around hood  86  also includes evaporated refrigerant flowing away from the pool of liquid refrigerant  82 . 
     It is to be understood that at least the above-identified, relative terms are non-limiting as to other exemplary embodiments in the disclosure. For example, hood  86  may be rotated with respect to the other evaporator components previously discussed, that is, hood  86 , including walls  92 , is not limited to a vertical orientation. Upon sufficient rotation of hood  86  about an axis substantially parallel to the tubes of tube bundle  78 , hood  86  may no longer be considered “positioned over” nor to “laterally border” tubes of tube bundle  78 . Similarly, “upper” end  88  of hood  86  may no longer be near “an upper portion” of shell  76 , and other exemplary embodiments are not limited to such an arrangement between the hood and the shell. In an exemplary embodiment, hood  86  terminates after covering tube bundle  78 , although in another exemplary embodiment, hood  86  further extends after covering tube bundle  78 . 
     After hood  86  forces refrigerant  106  downward between walls  92  and through open end  94 , the vapor refrigerant undergoes an abrupt change in direction before traveling in the space between shell  76  and walls  92  from the lower portion of shell  76  to the upper portion of shell  76 . Combined with the effect of gravity, the abrupt directional change in flow results in a proportion of any entrained droplets of refrigerant colliding with either liquid refrigerant  82  or shell  76 , thereby removing those droplets from the flow of vapor refrigerant  96 . Also, refrigerant mist traveling along the length of hood  86  between walls  92  is coalesced into larger drops that are more easily separated by gravity, or maintained sufficiently near or in contact with tube bundle  78 , to permit evaporation of the refrigerant mist by heat transfer with the tube bundle. As a result of the increased drop size, the efficiency of liquid separation by gravity is improved, permitting an increased upward velocity of vapor refrigerant  96  flowing through the evaporator in the space between walls  92  and shell  76 . Vapor refrigerant  96 , whether flowing from open end  94  or from the pool of liquid refrigerant  82 , flows over a pair of extensions  98  protruding from walls  92  near upper end  88  and into a channel  100 . Vapor refrigerant  96  enters into channel  100  through slots  102 , which is the space between the ends of extensions  98  and shell  76 , before exiting evaporator  138  at an outlet  104 . In another exemplary embodiment, vapor refrigerant  96  can enter into channel  100  through openings or apertures formed in extensions  98 , instead of slots  102 . In yet another exemplary embodiment, slots  102  can be formed by the space between hood  86  and shell  76 , that is, hood  86  does not include extensions  98 . 
     Stated another way, once refrigerant  106  exits from hood  86 , vapor refrigerant  96  then flows from the lower portion of shell  76  to the upper portion of shell  76  along the prescribed passageway. In an exemplary embodiment, the passageways can be substantially symmetric between the surfaces of hood  86  and shell  76  prior to reaching outlet  104 . In an exemplary embodiment, baffles, such as extensions  98  are provided near the evaporator outlet to prevent a direct path of vapor refrigerant  96  to the compressor inlet. 
     In one exemplary embodiment, hood  86  includes opposed substantially parallel walls  92 . In another exemplary embodiment, walls  92  can extend substantially vertically and terminate at open end  94 , that is located substantially opposite upper end  88 . Upper end  88  and walls  92  are closely positioned near the tubes of tube bundle  78 , with walls  92  extending toward the lower portion of shell  76  so as to substantially laterally border the tubes of tube bundle  78 . In an exemplary embodiment, walls  92  may be spaced between about 0.02 inch (0.5 mm) and about 0.8 inch (20 mm) from the tubes in tube bundle  78 . In a further exemplary embodiment, walls  92  may be spaced between about 0.1 inch (3 mm) and about 0.2 inch (5 mm) from the tubes in tube bundle  78 . However, spacing between upper end  88  and the tubes of tube bundle  78  may be significantly greater than 0.2 inch (5 mm), in order to provide sufficient spacing to position distributor  80  between the tubes and the upper end of the hood. In an exemplary embodiment in which walls  92  of hood  86  are substantially parallel and shell  76  is cylindrical, walls  92  may also be symmetric about a central vertical plane of symmetry of the shell bisecting the space separating walls  92 . In other exemplary embodiments, walls  92  need not extend vertically past the lower tubes of tube bundle  78 , nor do walls  92  need to be planar, as walls  92  may be curved or have other non-planar shapes. Regardless of the specific construction, hood  86  is configured to channel refrigerant  106  within the confines of walls  92  through open end  94  of hood  86 . 
       FIGS. 6A through 6C  show an exemplary embodiment of an evaporator configured as a “falling film” evaporator  128 . As shown in  FIGS. 6A through 6C , evaporator  128  is similar to evaporator  138  shown in  FIGS. 5A through 5C , except that evaporator  128  does not include tube bundle  140  in the pool of refrigerant  82  that collects in the lower portion of the shell. In an exemplary embodiment, hood  86  terminates after covering tube bundle  78 , although in another exemplary embodiment, hood  86  further extends toward pool of refrigerant  82  after covering tube bundle  78 . In yet a further exemplary embodiment, hood  86  terminates so that the hood does not totally cover the tube bundle, that is, substantially covers the tube bundle. 
     As shown in  FIGS. 6B and 6C , a pump  84  can be used to recirculate the pool of liquid refrigerant  82  from the lower portion of the shell  76  via line  114  to distributor  80 . As further shown in  FIG. 6B , line  114  can include a regulating device  112  that can be in fluid communication with a condenser (not shown). In another exemplary embodiment, an ejector (not shown) can be employed to draw liquid refrigerant  82  from the lower portion of shell  76  using the pressurized refrigerant from condenser  34 , which operates by virtue of the Bernoulli effect. The ejector combines the functions of a regulating device  112  and a pump  84 . 
     In an exemplary embodiment, one arrangement of tubes or tube bundles may be defined by a plurality of uniformly spaced tubes that are aligned vertically and horizontally, forming an outline that can be substantially rectangular. However, a stacking arrangement of tube bundles can be used where the tubes are neither vertically or horizontally aligned, as well as arrangements that are not uniformly spaced. 
     In another exemplary embodiment, different tube bundle constructions are contemplated. For example, finned tubes (not shown) can be used in a tube bundle, such as along the uppermost horizontal row or uppermost portion of the tube bundle. Besides the possibility of using finned tubes, tubes developed for more efficient operation for pool boiling applications, such as in “flooded” evaporators, may also be employed. Additionally, or in combination with the finned tubes, porous coatings can also be applied to the outer surface of the tubes of the tube bundles. 
     In a further exemplary embodiment, the cross-sectional profile of the evaporator shell may be non-circular. 
     In an exemplary embodiment, a portion of the hood may partially extend into the shell outlet. 
     In addition, it is possible to incorporate the expansion functionality of the expansion devices of system  14  into distributor  80 . In one exemplary embodiment, two expansion devices may be employed. One expansion device is exhibited in the spraying nozzles of distributor  80 . The other expansion device, for example, expansion device  36 , can provide a preliminary partial expansion of refrigerant, before that provided by the spraying nozzles positioned inside the evaporator. In an exemplary embodiment, the other expansion device, that is, the non-spraying nozzle expansion device, can be controlled by the level of liquid refrigerant  82  in the evaporator to account for variations in operating conditions, such as evaporating and condensing pressures, as well as partial cooling loads. In an alternative exemplary embodiment, expansion device can be controlled by the level of liquid refrigerant in the condenser, or in a further exemplary embodiment, a “flash economizer” vessel. In one exemplary embodiment, the majority of the expansion can occur in the nozzles, providing a greater pressure difference, while simultaneously permitting the nozzles to be of reduced size, therefore reducing the size and cost of the nozzles. 
       FIG. 7A  illustrates an exemplary embodiment of evaporator  168 . Evaporator receives refrigerant through supply line  142  and supply line  144 . Supply line  142  and supply line  144  are bifurcated at a control device  122 . Supply line  142  and supply line  144  penetrate hood  86  at upper end  88  to dispense refrigerant over tube bundle  78 . Evaporator  168  includes a downwardly opening hood  86  that substantially surrounds and covers tube bundle  78 .  FIG. 7A  shows expansion device  36  controlled by sensor. Supply line  142  dispenses refrigerant via distributor  80 . Supply line  144  is a an additional supply that provides an additional distribution device to dispense liquid refrigerant over tube bundle  78 . Supply line  144  may be controlled by control device  122 , for example, a control valve. Control device  122  may substantially open fully in response to a drop in the refrigerant level in evaporator  168 , as sensed by a level sensor  150  to provide more refrigerant from condenser. Control device  122  opens when expansion device  36  is open and liquid refrigerant level  82  continues to decrease. Level sensor  150  senses when a predetermined low refrigerant level in evaporator  168  has been reached and then transmits a signal that causes control device  122  to open and supply refrigerant to evaporator  168  through supply line  144 . Level sensor  150  is an exemplary means for determining low refrigerant. Other means may be employed for determining low evaporator refrigerant, including but not limited to, for examples, high refrigerant level in condenser  34 , increased head pressure on system  14 , or a high degree of subcooling. When the refrigerant level in evaporator  168  is above the predetermined level, control device  122  is in a closed position, preventing refrigerant flow in supply line  144 . An alternative embodiment of evaporator  168  is shown in  FIG. 7B . In the alternative embodiment of  FIG. 7B  supply line  144  is connected to a distributor  80   a  to distribute refrigerant over tube bundle  78 . In an exemplary embodiment, distributor  80   a  may include one or more low pressure nozzles. In another exemplary embodiment, supply line  144  may provide refrigerant directly to the reservoir of liquid refrigerant  82 , or to other locations in tube bundles  78 ,  140 . 
       FIG. 8  illustrates an exemplary embodiment of evaporator  178 . Evaporator  178  includes downwardly opening hood  86  that surrounds and covers tube bundle  78 . Tube bundle  78  receives refrigerant from distributor  80 . Tube bundle  140  is located at least partially beneath tube bundle  78 . Tube bundle  140  boils liquid refrigerant that collects at the bottom of evaporator  178  in pool of liquid refrigerant  82 . A booster pump  152  can receive liquid refrigerant from a condenser or from an intermediate vessel such as an intercooler or a flash tank. Booster pump  152  may be actuated in response to sensing a head pressure in system  14 , which is lower than a predetermined head pressure value. Booster pump  152  may be operable at variable speeds. Booster pump  152  may also be actuated on or off in response to a decrease in the refrigerant level in evaporator  178 , as sensed by level sensor  150 , when expansion device  36  is in a fully open position. Each of the evaporator embodiments shown in  FIGS. 7A, 7B and 8  may be arranged with only first tube bundle  78 , that is, in the absence of tube bundle  140 , as shown in  FIGS. 6A and 6B . 
       FIG. 9  illustrates another exemplary embodiment of an evaporator  188 . Evaporator  188  includes a refrigerant inlet line  154  that directs flow of a two-phase refrigerant that is, liquid and vapor refrigerant, through shell  76  and into an internal enclosure  160 . Flow of the two-phase refrigerant into enclosure  160  may be controlled by an expansion device  156 . A baffle or deflector  158  is positioned within enclosure  160  to direct the inward flow of refrigerant downward in enclosure  160 . In an exemplary embodiment, deflector  158  may be, for example, a downwardly curved protrusion extending from a wall of enclosure  160 . Enclosure  160  includes a distributor  162 . Distributor  162  permits liquid refrigerant collected in enclosure  160  to travel from enclosure  160  to tube bundle  78 . Liquid refrigerant  82  may accumulate in enclosure  76 , which is removed via a drain pipe as described above with respect to  FIGS. 6B and 6C . Distributor  162  can be a perforated sheet or other structural element or device that can provide a regulated flow of liquid from enclosure  160 . Upper end  170  of enclosure  160  allows vapor refrigerant  166  in enclosure  160  to flow from enclosure  160  into outlet  104 , while vapor refrigerant  96  generated through heat transfer with tube bundle  78  follows a path around sidewalls of enclosure  160 . In an exemplary embodiment, upper end  170  may be a mesh structure  164 . 
     While only certain features and embodiments of the invention have been shown and described, many modifications and changes may occur to those skilled in the art (for example, variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (for example, temperatures, pressures, etc.), mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. Furthermore, in an effort to provide a concise description of the exemplary embodiments, all features of an actual implementation may not have been described (that is, those unrelated to the presently contemplated best mode of carrying out the invention, or those unrelated to enabling the claimed invention). It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation.