Abstract:
A tube and shell evaporator operable at near freezing includes a temperature sensor that senses the temperature of chilled water discharging from one or just a few of the very coldest tubes, whereby the sensed temperature is less than the average leaving chiller water temperature (LCWT). The result provides an exceptionally low LCWT, which can be especially desirable in district cooling systems where the chilled water is usually piped a great distance.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention generally pertains to tube and shell heat exchangers and more specifically to an evaporator that provides a chiller water temperature marginally above freezing. 
     2. Description of Related Art 
     Many chiller systems include a closed loop refrigerant circuit comprising a compressor, a condenser, a flow restriction, and an evaporator. Expanded, cold refrigerant in the evaporator cools a secondary closed loop chilled fluid circuit. The chilled fluid, such as water or a water-based solution, is distributed to and circulated through various smaller heat exchangers. The smaller heat exchangers cool various comfort zones, such as rooms or other areas within a building. 
     In many cases, one or more chillers are dedicated to a single building. However, in some cases one large central cooling system, comprising one or more chillers, serve several distinct buildings. The chilled water is typically piped a great distance to reach the various buildings. Such a chiller system is often referred to as a “district cooling system.” 
     As chilled water is conveyed through a relatively long network of pipes, the water takes on heat before reaching its various designated heat exchangers. To ensure that the chilled water is sufficiently cold upon reaching the heat exchangers, it is usually desirable to have the evaporator reduce the temperature of the water as much as possible. However, if the water gets too cold, it may freeze inside the evaporator. Freezing, of course, can destroy the evaporator and/or its associated piping. 
     To avoid freeze up, the chilled water solution may be a glycol and water solution or some other solution having a lower freezing point than pure water. However, with district cooling systems, an appreciable amount of glycol or other solution that may lower the freezing point can be rather costly due to the large volume contained within the chilled water piping that interconnects the evaporator and the remote heat exchangers. Consequently, current district cooling systems use water solutions that consist of primarily water with perhaps small amounts of water treatment chemicals. Since such solutions have a freezing point near 32 degrees Fahrenheit, evaporators are typically operated at a temperature safely above that. 
     To this end, many chillers control the leaving chiller water temperature (LCWT) in response to a temperature sensor installed immediately downstream of the evaporator or situated within an outlet water box of the evaporator (see U.S. Pat. Nos. 5,083,438 and 5,355,691). The outlet water box serves as somewhat of a manifold or collection point into which the numerous heat exchange tubes within the evaporator shell discharge. The temperature sensor, whether in the water box or immediately downstream of the evaporator, usually provides a generally good indication of the LCWT. 
     However, the sensed temperature is only an average of the actual water temperature discharging from each individual tube of the evaporator. In a tube and shell heat exchanger the discharge temperature at each tube often varies from one tube to the next, depending on its location within the shell and the conditions under which the system is operating. Thus, to avoid freeze up at any individual tube, chillers are usually controlled to provide an average LCWT that is well above freezing, typically 37 degrees Fahrenheit or higher. 
     Unfortunately, when leaving the evaporator at 37 degrees, the chiller water temperature may rise to an unacceptable high temperature by the time it reaches the remote heat exchangers of a district cooling system. 
     In some chiller systems, such as the one disclosed in U.S. Pat. No. 5,782,131, a temperature sensor senses the temperature of the refrigerant inside an evaporator, as opposed to directly sensing the temperature of the chilled water. However, with such a system it may be difficult to determine what minimum allowable refrigerant temperature still avoids freezing the water. For example, in some cases, a refrigerant temperature of 30 degrees might only be able to chill the water to 38 degrees Fahrenheit. 
     SUMMARY OF THE INVENTION 
     To minimize the LCWT of a tube and shell evaporator, it is an object of the invention to monitor the temperature of the chiller water discharging from generally one or just a few of the very coldest tubes, as opposed to just sensing the average LCWT. 
     Another object is to control the operation of a chiller system in response to feedback from a temperature sensor that senses the temperature of the chiller water discharging from one or just a few of the very coldest tubes, as opposed to just sensing the average LCWT. 
     Another object is to maintain the temperature of the chiller water discharging from one or just a few of the very coldest tubes to a temperature of no more than 36 degrees Fahrenheit. 
     For chiller systems operating from part load to full load, another object is monitor the chiller water temperature at a location between the coldest tube at part load and the coldest tube at full load. 
     For chiller systems subject to refrigerant loss, another object is to monitor the chiller water temperature near the coldest tube during a normal operating condition as well as during a condition of low refrigerant charge. 
     In some embodiments, another object of the invention is to monitor the chiller water temperature at an elevation within the upper third of the tube bundle, where the refrigerant tends to boil most dramatically. 
     In some embodiments, further object of the invention is to monitor the chiller water temperature just below the top row of tubes to avoid sensing at an elevation where the refrigerant is in a primarily gaseous state. 
     In some embodiments, a still further object is to monitor the chiller water temperature at about the third row of tubes from the top where the refrigerant is a mixture of both liquid and gaseous refrigerant. 
     Another object is to monitor both the average LCWT and the temperature of the chiller water discharging from one or just a few of the very coldest tubes, whereby the average LCWT provides an indicator of the chiller system&#39;s overall operating performance, while the monitoring the coldest water temperature provides feedback that helps in optimizing that performance. 
     Another object is to monitor the refrigerant temperature within the evaporator in addition to monitoring the temperature of the chiller water discharging from one or just a few of the very coldest tubes, whereby the refrigerant temperature can be lowered well below 32 degrees Fahrenheit without significant risk of freezing. 
     These and other objects of the invention are provided by a tube and shell evaporator that includes a temperature sensor that senses the temperature of chiller water discharging from one or just a few of the very coldest tubes, whereby the sensed temperature is less than the average leaving chiller water temperature. 
     The present invention provides an evaporator that uses a refrigerant to chill a water solution. The evaporator comprises a housing defining an inlet water chamber, an outlet water chamber, and a refrigerant chamber; and a plurality of tubes each of which have an exterior surface exposed to the refrigerant chamber and an interior surface adapted to convey said water solution from the inlet water chamber to the outlet water chamber. The plurality of tubes are adapted to transfer heat from the water solution to the refrigerant to provide an average leaving chiller water temperature within said outlet water chamber. A first tube of the plurality of tubes is disposed at a higher elevation than a second tube of the plurality of tubes. The first tube and the second tube are adapted to convey the water solution at a first temperature and a second temperature respectively. The first temperature is less than the second temperature and is less than the average leaving chiller water temperature. A temperature sensor is situated closer to the first tube than the second tube and is adapted to sense a water solution temperature that is less than the second temperature and less than the average leaving chiller water temperature. 
     The present invention also provides a chiller system that uses a refrigerant to chill a water solution. The chiller system comprises a compressor adapted to provide a variable output of the refrigerant; a condenser adapted to receive refrigerant discharged from the compressor; a flow restriction adapted to receive refrigerant discharged from the condenser and adapted to create a pressure and temperature drop upon the refrigerant passing through the flow restriction; an evaporator defining an inlet water chamber, an outlet water chamber, and a refrigerant chamber wherein the refrigerant chamber is adapted to receive refrigerant discharged from the flow restriction and discharge the refrigerant back to the compressor; and a plurality of tubes each of which have an exterior surface exposed to the refrigerant chamber and an interior surface adapted to convey the water solution from the inlet water chamber to the outlet water chamber. The plurality of tubes are adapted to transfer heat from the water solution to the refrigerant to provide an average leaving chiller water temperature within the outlet water chamber. A first tube of the plurality of tubes is disposed at a higher elevation than a second tube of the plurality of tubes. The first tube and the second tube are adapted to convey the water solution at a first temperature and a second temperature respectively, where the first temperature is less than the second temperature and less than the average leaving chiller water temperature. A temperature sensor is situated closer to the first tube than the second tube and is adapted to sense a water solution temperature that is less than the second temperature and less than the average leaving chiller water temperature. 
     The present invention further provides a chiller system that uses a refrigerant to chill a water solution. The chiller system comprises a compressor adapted to compress the refrigerant selectively at a full load condition and a partial load condition; a condenser adapted to receive refrigerant discharged from the compressor; a flow restriction adapted to receive refrigerant discharged from the condenser and adapted to crate a pressure and temperature drop upon the refrigerant passing through said flow restriction; an evaporator defining an inlet water chamber, an outlet water chamber, and a refrigerant chamber, where the refrigerant chamber is adapted to receive refrigerant discharged from the flow restriction and discharge the refrigerant back to the compressor; and a plurality of tubes each having an inlet end exposed to the inlet water chamber, an outlet end exposed to the outlet water chamber, and an exterior surface exposed to the refrigerant chamber. the refrigerant is adapted to cool the water solution upon the water solution passing through the plurality of tubes from the inlet water chamber to the outlet water chamber to create an average leaving chiller water temperature within the outlet water chamber. The chiller system creates a first minimum water temperature at a first outlet end of the plurality of tubes at a full load condition and creates a second minimum water temperature at a second outlet end of the plurality of tubes at a partial load condition. The first outlet end is at a higher elevation than the second outlet end. A temperature sensor disposed at an intermediate elevation between that of the first outlet end and the second outlet end and being sufficiently close the plurality of tubes to sense a water solution temperature that is less than the average leaving chiller water temperature. 
     The present invention additionally provides a method of preventing fluid freeze up in a chiller system. The method comprises the steps of: locating a temperature sensor in an upper third of an evaporator tube bundle; using the temperature sensor to determine the coldest temperature in the evaporator tube bundle; and controlling the operation of the chiller to prevent a fluid being chilled by the chiller from freezing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic view of a refrigerant chiller system in a distinct cooling application. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a refrigerant chiller system  10  in a district cooling application provides chilled water  12  for meeting the cooling demand of several remote buildings  14 . A pump  16  draws chilled water  12  provided by a tube and shell evaporator  18  of chiller  10  and discharges the water solution through a rather long supply line  20 . Supply line  20  could be a single line or a network of pipes extending up to a mile or more to distribute chilled water  12  to several heat exchangers  22  associated with buildings  14 . After circulating through heat exchangers  22  to cool rooms or areas within buildings  14 , water  12  returns to evaporator  18  by way of a return line  24 . Although water solution  12  is primarily water in a preferred embodiment, the term, “water solution” actually encompasses any liquid, including but not limited to pure water, chemically treated water, glycol, and various mixtures thereof. 
     To cool water  12 , chiller system  10  includes a hermetically sealed, closed loop refrigerant circuit comprising a refrigerant compressor  26  (e.g., centrifugal, screw, scroll, or reciprocating), a condenser  28  (preferably a tube and shell heat exchanger), a flow restriction  30  (e.g., one or more orifices, or an expansion valve), and evaporator  18 . Compressor  26  discharges pressurized refrigerant  32  (e.g., R 123 ) into condenser  28 , which cools refrigerant  32  by way of a secondary fluid such as water and/or ambient air. Refrigerant  32  leaves condenser  28  through a line  34  and decreases in pressure and temperature upon passing through restriction  30 . Refrigerant  32 , now cooler, passes through a line  36  to enter evaporator  18 . 
     Although the specific structure of evaporator  18  may vary, in the illustrated exemplary embodiment evaporator  18  comprises a housing  38  that contains an inlet water chamber  40 , an outlet water chamber  42 , and a refrigerant chamber  44 . In this example, refrigerant chamber  44  is defined by a generally cylindrical shell  46  interposed between two tube sheets  48 . Water chambers  40  and  42  are defined by an inlet water box  50  and an outlet water box  52  being bolted to the face of tube sheets  48 . Several heat exchanger tubes  54  are arranged in generally horizontal rows (i.e., each row includes several tubes, one behind the other, as viewed looking into FIG.  1 ). Tubes  54  are collectively referred to as a tube bundle  56 , which extends across a vertical span  58  from a lower most point  60  to upper most point  62 . Each tube  54  has an exterior surface  64  exposed to refrigerant chamber  44 . And each tube  54  has an interior surface  66  extending between an inlet end  68  of the tube and an outlet end  70  to convey water  12  from inlet water chamber  40  to outlet water chamber  42 . Thus, tubes  54  place refrigerant  12  in heat transfer relationship with water  12 . 
     Once refrigerant  32  enters evaporator  18 , refrigerant  32  passes across tubes  54  to absorb heat from water solution  12 . This often causes refrigerant  32  to boil, while water solution  12  cools. Resulting gaseous refrigerant  12  is drawn back into compressor  86  by way of suction line  72 , where a compressing element  74 , such as an impeller, recompresses refrigerant  32  to repeat the closed loop refrigeration cycle. Chilled water  12  passing through tubes  54  (from inlet water chamber  40  to outlet water chamber  42 ) is pumped back to remote heat exchangers  22 . 
     To control and/or monitor the operating performance of chiller system  10 , several temperature sensors are employed. For example, a temperature sensor  76  (refrigerant sensor) senses the refrigerant temperature within evaporator  18 , and a temperature sensor  78  (LCWT sensor) senses the average leaving chiller water temperature or LCWT. To minimize the LCWT while preventing water  12  from freezing, a temperature sensor  80  (tube sensor) is preferably located where it can sense the lowest water temperature at outlet ends  70 . To determine the location at which the water temperature is at a minimum, one might expect that the lowest temperature would be near the bottom of tube bundle  56 , since heat rises and heat transfer across a tube is often better from a liquid to a liquid, as opposed to a liquid to a vapor. 
     However, the surprising and unexpected empirically derived results indicate that the lowest water temperature is often in the upper third of tube bundle  56 . This has been found to be true even when the heat transfer at the lowest row of tubes involves liquid refrigerant  32  absorbing heat from liquid water  12 , while the heat transfer toward the upper portion of tube bundle  56  involves vaporous refrigerant  32  absorbing heat from liquid water  12 . 
     The exact tube row providing the lowest temperature depends on numerous factors including the output capacity at which chiller system  10  is operating. For example, when chiller  10  is at full load, the boiling rate of the refrigerant within evaporator  18  is rather high. The rapidly boiling refrigerant  32  tends to rise near the upper rows of tube bundle  56 , and the lowest water temperature may occur at the highest row. However, under a partial load, the refrigerant boiling rate is lower, and the refrigerant&#39;s liquid to vapor transition point tends to be lower than when at full load. This tends to place the lowest water temperature several tube rows below the top row. 
     For chiller systems operable at varying load, the preferred location for sensor  80  is at an elevation below the tube outlet that provides the lowest water temperature at full load and above the tube outlet that provides the lowest water temperature at a partial load. In some embodiments, the preferred location is one tube diameter below upper most point  62 , and more specifically near the third row of tubes from the top of tube bundle  56 . 
     For some chiller systems subject to refrigerant loss, an alternate preferred location for the temperature sensor is approximately at the vertical center of tube bundle  56 , as shown by temperature sensor  80 ′. In other word, sensor  80 ′ is disposed generally midway between uppermost point  68  and lowermost point  60 , i.e., within the central third to bundle  56 . To illustrate alternate mounting locations, water box  52  is shown having both sensors  80  and  80 ′. However, actually only one sensor at just one of the preferred locations is normally used. The horizontal location of sensor  80 ′ may be centrally located or may be biased to one side of water box  52 . With some chillers, the generally central elevation provides the coolest water temperature during normal operation with a proper amount of refrigerant or charge. That same elevation may also provide the coolest water temperature when there is a loss of refrigerant. With a loss of refrigerant, the level of liquid refrigerant in evaporator  18  drops, which greatly diminishes the refrigerant cooling affect near the top of tube bundle  56 . This increases the water temperature near the top of bundle  56  and decreases the water temperature near the bottom. The water temperature near the center of bundle  4  remains the same or changes the least, and thus provides a good indication of the minimum water temperature, regardless of reasonable amounts of refrigerant loss. 
     To control the operation and various temperatures of chiller system  10 , a control unit  82  is electrically connected to receive feedback signals  84  from sensor  76 , signal  86  from sensor  78 , and signal  88  from sensor  80  or  80 ′. In response to feedback signals  84 ,  86 , and  88 , control unit  82  provides various outputs such as outputs  90  and/or  92 . Output  90  controls the opening of inlet guide vanes  94 , and output  92  controls the speed of a motor  96  that drives compressing element  74 . Varying the output capacity of a chiller by varying the speed of its compressor and/or adjusting the position the compressor&#39;s inlet guide vanes are well known to those skilled in the art. Thus, control unit  82  is schematically illustrated to encompass a myriad of control circuits including but not limited to microcomputers, programmable controllers, integrated circuits, discrete circuitry, and various combinations thereof. It should also be appreciated by those skilled in the art, that the number and type of inputs and outputs might vary, depending on the desired operating features of the specific chiller system being controlled. 
     In a preferred embodiment, control  82  modulates the position of inlet guide vanes  94  to maintain a temperature at tube sensor  80  or  80 ′ that is just marginally above 32 degrees Fahrenheit. This allows the average LCWT, as sensed by sensor  78 , to be safely maintained at 36 degrees or lower. Moreover, sensor  80  or  80 ′ being properly positioned allows the refrigerant temperature, as sensed by refrigerant sensor  76 , to be safely lowered below 29 degrees and perhaps down to 27 degrees or lower. Thus chiller  10  normally operates in response to feedback  88  from tube sensor  80  or  80 ′, as opposed to feedback  86  from LCWT sensor  78 . Also, if the temperature at the tube sensor  80  or  80 ′ drops below 33 degrees or below some other predetermined limit, control  82  shuts down the operation of chiller  10  to prevent feeding the chilled water. In some embodiments, feedback  86  from LCWT sensor  78  is useful in determining the actual output capacity of chiller  10 ; however, feedback  86  is not necessarily relied upon for modulating the position of inlet guide vanes  94 . Although LCWT sensor  78  could shut down the operation of chiller  10  upon sensing a LCWT below a predetermined limit, it is more likely that tube sensor  76  would be first to shut down chiller  10 , as the temperature is normally lower at tube sensor  80  or  80 ′ than at LCWT sensor  78 . 
     Although the invention is described with respect to a preferred embodiment and various modifications thereto will be apparent to those skilled in the art. Therefore, the scope of the invention is to be determined by reference to the claims, which follow.