Abstract:
A heat exchanger for exchanging heat between gasses such as air and a liquid or gaseous coolant has narrow spacing between exchanger surfaces for high efficiency. To avoid undue obstruction of gas flow due to ice buildup on the exchanger surfaces, the heat exchanger is equipped with sensors to monitor the gas flow and an actuator that widens the spacing between exchanger surfaces such that gas flow remains unimpeded. Embodiments provide for defrosting of the exchanger surfaces when an limit on spacing of exchanger surfaces is reached, and for relaxing the spacing to the original narrow spacing when defrosting is completed.

Description:
FIELD 
       [0001]    The present apparatus relates to the field of heat exchangers or evaporators for exchanging heat between a gas, such as air, and a coolant such as a refrigerant or other cold fluid. 
       BACKGROUND 
       [0002]    It is known that a heat exchanger exchanges heat between a gas and a refrigerant more efficiently when the gas flows through spaces between exchanger surfaces that are narrow. In addition, more exchange surface can fit into a given volume if this spacing is narrow. 
         [0003]    It is also known that, when the gas being cooled contains moisture, narrow spaces are far more prone to icing-up than when spaces are wide. Narrow-spaced heat exchangers are therefore often avoided when moisture-containing gasses, such as air, are to be cooled with coolant or refrigerant at, or below, the freezing point of water. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0004]      FIG. 1  illustrates a helical heat exchanger with stretching apparatus. 
           [0005]      FIG. 2  is a flow chart of a method of operating the apparatus of  FIG. 1 . 
           [0006]      FIG. 3  illustrates a heat-exchanger system suitable for use with the heat exchanger of  FIG. 1 . 
           [0007]      FIG. 4  illustrates an embodiment having two facing, interdigitated, multiple-wedge heat exchange surfaces, where a multiple-wedge surface moves to adjust heat-exchanger gap. 
           [0008]      FIG. 5  illustrates an embodiment having a coiled microchannel embodiment in tight-wound condition. 
           [0009]      FIG. 6  illustrates the embodiment of  FIG. 5  in unwound condition. 
           [0010]      FIG. 7  illustrates an embodiment having parallel plates and apparatus to ensure even spreading. 
           [0011]      FIG. 8  illustrates an embodiment resembling that of  FIG. 7 , wherein elastomeric sheets are the apparatus to ensure even spreading. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0012]      FIG. 1  illustrates an embodiment having a helically coiled microchannel refrigerant evaporator or coolant heat exchanger  102 . The coiled microchannel heat exchanger has multiple passages  104  for refrigerant or other cold liquid or gaseous coolant running lengthwise through microchannel tubing  106 . The microchannel tubing  106  is fabricated from a metal, a polymer, an electrically conductive polymer, or composite material that retains some deformability and springiness at low temperatures. The microchannel tubing is coiled such that a small space  108 , typically less than two millimeters and in an embodiment one millimeter wide when not under tension, exists for airflow between the wider surfaces of the turns of the microchannel tubing. In some embodiments, a fiber  109 , such as monofilament fishing line, is wound about the microchannel tubing, or spacers are provided, to maintain a minimum spacing between the coil turns when no tension is on the microchannel tubing. These spacers or fiber do not significantly disturbing the air flow, and are attached in such a way that the spaces  108  can expand when the helical-wound microchannel tubing is under tension. 
         [0013]    In operation, air or other gas enters the evaporator through spaces  108  and exchanges heat with the tubing and coolant confined in passages  104 , and the axis about which the coil is wound (the same axis as that along which air exits) is preferably horizontal so that melt water when the heat exchanger is eventually defrosted can drip downwards and therefore be removed from the exchanger. In an alternative embodiment, the air-flow direction is reversed from that illustrated in  FIG. 1 , entering along the axis and exiting through the spaces between the helical-wound tubing. 
         [0014]    While the evaporator or heat exchanger of  FIG. 1  is more compact and efficient than typical evaporators, prior devices have avoided tightly spaced heat exchangers such as these because they have a strong tendency to accumulate ice in spaces  108 , with result that airflow becomes obstructed. 
         [0015]    Ice accumulation in the present apparatus results in decreased airflow through the spaces  108 , and decreased heat transfer from the coolant in the coolant passages  104 . Hence, ice accumulation is detected by measuring pressure-drop across or/and airflow volume through the coil, or by measuring temperature differences between coolant input to the coil and coolant output from the coil. Ice accumulation may also be detected indirectly, through measurement of variables including fan speed, fan motor current and/or voltage, refrigerant pressure, and refrigerant compressor motor current and/or voltage. 
         [0016]    In an embodiment, and with reference to  FIG. 2  as well as  FIG. 1 , ice accumulation is detected by decreased difference between a temperature at microchannel tubing coil input, as measured by a thermistor  110 , and temperature at coil output, as measured by a second thermistor  112 , or by air pressure or airflow sensors (not shown). Air pressure and airflow sensors provide more direct measurement of airflow obstruction than coolant temperature difference, but it is expected that decreased coolant inlet and outlet temperature difference will result from impaired heat exchange due to airflow obstruction. These temperatures, pressures, or airflow are continually monitored  202  by a controller  114 . When the controller  114  determines  204  that the heat exchanger (e.g. tubing  106 ) is partially iced over, but not already maximally stretched open  206 , it activates an electric motor and reduction gear assembly  116 , which in turn drives a rotary-to-linear motion conversion apparatus  118 , to open the heat exchanger gaps  208 , in the example of  FIG. 1  by stretching the helix. In an embodiment rotary-to-linear motion conversion apparatus  118  is a rack-and-pinion; in another embodiment a rotating nut riding on a stationary screw; in another it has a steel cable that is wound onto a drum rotated by the motor and reduction gear assembly  116 . 
         [0017]    Conversion apparatus  118  is mounted to a rigid frame  122 , and an end  124  of the coil of the helically-wound microchannel tubing  106  is attached by suitable attachment  126  to an opposing side of frame  122 . 
         [0018]    As the controller activates motor and reduction gear assembly  116 , driving the rotary-to-linear motion conversion apparatus  118 , tension is applied to an end  120  of the coil of the helically-wound microchannel tubing  106 , such that the helically-wound tubing  106  is stretched towards conversion apparatus  118 , thereby opening spaces  108  so that airflow can resume. 
         [0019]    When the controller  114  determines that airflow is obstructed, but that the coil of the helically-wound microchannel tubing  106  is already maximally stretched  206  to a predetermined limit, it shuts down any refrigerant or coolant pump in the system for the duration of de-icing; and activates defrosting of the exchanger  210  in ways known in the art. Determination of stretch to the limit may be accomplished by detecting excessive current in the motor  116 , by a limit switch, by an eddy-current proximity sensor, or by a photosensor. When defrosting is completed, controller  114  allows the resumption of coolant flow, and reverses motion of motor  116  to return the heat exchanger to the narrow-gap initial state  212 , in the embodiment of  FIG. 1  by allowing relaxation of stretch of helically-wound tubing  106 , allowing helically wound tubing  106  to return to its unstretched state. 
         [0020]    The apparatus of  FIG. 1  therefore provides the advantage of narrow spacing of tubing in the heat exchanger, while permitting greater intervals between defrosting than those that would otherwise be necessary with narrow spaced heat-exchange surfaces. 
         [0021]    In the heat-exchange and cooling system of  FIG. 3 , a helically coiled microchannel heat exchanger  302  as described with reference to  FIG. 1  is connected to a refrigerant compressor, orifice, and condenser as known in the art of refrigeration, or other source of chilled coolant, and not shown in the figure. The microchannel heat exchanger is coupled to serve as the refrigerant evaporator, or heat exchanger, while providing heat exchange to air or other gas. Air enters through input duct  304 , and exits through a blower  306  and output duct  308 . A rigid member of housing  309  serves as frame  122 . The heat exchanger  302  is mounted within a plenum having a rigid portion  310  and a stretchable bellows portion  311 . A controller  313  monitors the heat exchanger for airflow obstruction as heretofore described, and activates a stretcher  315 , containing motor and reduction gear  116  and rotary to linear motion converter  118  to stretch the heat exchanger  302  to re-open airflow passages as heretofore described. When stretch reaches a limit, a defrosting cycle of the heat exchanger is activated  210 . When defrosting is complete, the stretch of the heat exchanger is relaxed  212  to allow the helical heat exchanger to return to an unstretched state. 
         [0022]    An alternative embodiment, as illustrated in  FIG. 4 , has multiple wedgelike heat-exchange surfaces  402 ,  404 . Some of these heat exchange surfaces  402  form a first multiple-wedge surface, and are fixed to a frame (not shown) of the heat exchanger. A second group of these heat exchange surfaces  404  form a second multiple-wedge surface interdigitated with heat exchange surfaces  402  of the first multiple-wedge surface. Heat-exchange surfaces  404  of the second multiple-wedge surface are fixed to a movable element  406  of the heat exchanger. Heat exchange surfaces  402 ,  404 , are either fabricated from microchannel tubing or are fabricated from thermally conductive fins in thermal contact with coolant tubes  405 . Movable element  406  is attached to an actuator  410 , that typically incorporates a motor, reduction gear, and one or more rotary-to-linear motion converters similar to those previously discussed. Between wedges  402  and  404  are gas passages  406 . A controller  412  has sensors  414  for monitoring for airflow obstruction, and is coupled to drive actuator  410 . 
         [0023]    In operation, the controller  412  initially drives the movable element  406  to a position such that gas passages  406  are narrow. As moisture condenses out of the gas, such that ice accumulates on heat exchange surfaces  402 ,  404 , sensors  414  detect airflow obstruction; in response to the airflow obstruction controller  412  causes the actuator  410  to open gas passages  406  to allow heat exchange to continue. Eventually, at convenient times or when actuator  410  has reached a maximum spacing between surfaces  402 ,  404  and airflow is still obstructed, heat exchange surfaces  402 ,  404  are defrosted as known in the art. 
         [0024]    In an embodiment having a heat exchange surface made from a spiral-wound microchannel tubing  502 ,  FIG. 5 , airflow is along the axis of the spiral. At the axis of the spiral, the microchannel tubing  502  attaches to a fitting  508  on an axle  504 . The opposite end of the microchannel tubing  502  attaches to a fitting  506 . Refrigerant flow through the microchannel tubing is either from fitting  506  to axle fitting  508 , from axle fitting  508  to fitting  506 , or, since microchannel tubing is available with more than one refrigerant or other coolant channel, both to and from the axle fitting  508 , or both to and from fitting  506 . 
         [0025]    As illustrated in  FIG. 6 , with the embodiment of  FIG. 5  rotation of axle  504  can relax the spiral wound microchannel tubing  502  such that a gas space  510  is enlarged, similarly rotation of axle  504  in an opposite direction can tighten the spiral wound microchannel tubing  502  such that gas space  510  is narrowed. As shown, outer end fitting  506  is allowed to move outward in the relaxed state to allow space  510  to be evenly distributed along the tubing  502 . 
         [0026]    In yet another embodiment, illustrated in  FIG. 7 , coolant tubing  706  is formed as part of or attached to cooling fins  702 ,  704 . One or more of cooling fins  702  is anchored to a frame (not shown), and another  702  is attached to an actuator  708  as previously described. A mechanism for keeping even spacing between cooling fins  702 ,  704 , and thereby ensuring an approximate match of spacing between cooling fins  702 ,  704 , has a pair of rods  710 . Rods  710  attach to a first fin  702  at a pivot  712 , and to a second fin  704  at a pair of pivots  714  that are adapted to sliding along fin  704 . Pivots  714  also attach to a pair of rods  716  that are coupled to a pivot  718  on another cooling fin  702 , and to another slideable pivot  714  on another fin  704 . The apparatus of  FIG. 7  is fitted with airflow obstruction sensing devices and a controller as heretofore described, actuator stretches spacing between fins when ice accumulation obstructs airflow, and relaxes spacing between fins when ice is defrosted. 
         [0027]    In an embodiment,  FIG. 8 , resembling that of  FIG. 7 , cooling fins  802  have projections  804  that fit into holes in elastomeric sheets  806 . When actuator  808  pulls a first fin  803  to enlarge a first air/gas space  810  between fins  802 , pressure is applied to elastomeric sheets  806 , which tend to stretch evenly thereby spreading a second air/gas space  812  and ensuring an approximate match of space  812  to space  810 . The embodiment of  FIG. 8  is fitted with a blower to move air, airflow obstruction detection apparatus and controller apparatus as previously discussed. 
         [0028]    While the actuator  808  may attach directly to a cooling fin  803  if that fin is sufficiently thick and rigid, an optional, rigid, force-spreading bar  814  may be provided to spread force across the fin  803 . If used, force-spreading bar  814  is attached, by wires  816 , bolts, rivets, glue, or other methods known in the art, to cooling fin  803  and to the actuator. Similarly, end cooling fin  820  is securely attached, to a rigid wall (not shown) of an enclosure such as is illustrated in  FIG. 3 ; bolts  822  are illustrated for attaching end cooling fin  820  to a wall. 
         [0029]    Airflow may be reversed in any of the illustrated embodiments without departing from the spirit of the invention. 
         [0030]    A system as herein described has potential to permit construction of a high efficiency, compact, heat exchanger where defrosting is delayed until convenient times. For example, an air conditioning system using a heat exchanger as herein described may be able to postpone defrosting until between two and four AM, when most buildings are unoccupied. 
         [0031]    While the forgoing has been particularly shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit hereof. It is to be understood that various changes may be made in adapting the description to different embodiments without departing from the broader concepts disclosed herein and comprehended by the claims that follow.