Abstract:
A refrigerated case for storing food product includes a sensor module and a control module. The control module receives input from the sensor module and compares the input to a set point to generate an output indicative of a difference between the input and the set point, and further updates the output based on the input from the sensor module. A heater module controls a heater attached to a control surface of the refrigerated case based on the output to maintain a temperature of air adjacent the sensor module above a dew point temperature of room air.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 11/124,909 filed on May 9, 2005, which claims the benefit of U.S. Provisional Application No. 60/569,581 filed on May 10, 2004. The disclosures of the above applications are incorporated herein by reference. 
     
    
     FIELD  
       [0002]     A system and method for preventing condensation and, more particularly, a system and method for operating anti-condensation heaters.  
       BACKGROUND  
       [0003]     Refrigerated spaces such as refrigerated display cases, walk-in refrigerators, and walk-in freezers commonly include heaters to prevent condensation from forming on certain areas of the device from water vapor present as humidity in the surrounding air. For example, walk-in refrigerators and freezers typically employ a heater to prevent condensation from forming on air vents, personnel doors, drain lines, and observation windows. Similarly, refrigerated display cases such as coffin cases, island cases, and tub cases typically employ a heater to prevent condensation from forming on and around an opening and/or door of the display case.  
         [0004]     For example, glass-door refrigerated display cases are frequently used in supermarkets and convenience stores and often include heaters in the glass doors and the door frames to prevent condensation on the glass from humid air. The glass doors and frames are typically heated to a temperature above the dew-point temperature of the air in the room in which the display cases are located to prevent condensation.  
         [0005]     Prior art control systems apply heat to the glass doors in proportion to a measured dew point in an open-loop system. Manual intervention, in the form of manually adjusting the control scheme, is required to achieve condensation-free doors. The adjustment process is prone to human error, typically resulting in setting the heat too high and losing some of the promised energy savings. Also, such adjustments usually are made at a particular operating condition, and may not work correctly year round where climate changes are more drastic, as dew point and conditions change with the season. Further, the adjustment process is time consuming and does not result in a known door temperature.  
         [0006]     One method of controlling the amount of heat applied to the display case doors includes applying full power (i.e., line voltage, typically) to the door heaters. The applied heat prevents condensation but wastes energy as more heat is applied than is necessary. The excess energy consumed by the door heaters directly increases the cost of operating the refrigeration system. Such costs are further increased as excess energy in the form of heat is dissipated into the refrigerated space and must be removed by the refrigeration system.  
         [0007]     Other control systems modulate the heat applied to the display case doors and, as a result, reduce door heat energy and related costs. Such systems generally control the applied proportion of maximum heat, which is proportional to the square of line voltage to adjust the heat applied to the doors. While such systems adequately reduce the amount of heat applied to the doors, such systems suffer from the disadvantage of being susceptible to variations in line voltage and are therefore not precise.  
         [0008]     For example, as illustrated in  FIG. 1 , a prior art proportional controller has one or more adjustments to allow a user to adjust a door heater between a minimum and a maximum in response to variation of dew point of the room air (i.e., more heat for higher dew point). Some systems permit limiting the upper and lower limits of the heat modulation to values other than zero and one hundred percent, e.g., limiting the heat to a twenty percent minimum and a ninety percent maximum. Others have a simple rotary dial that adjusts a gain or an offset. Still others define limits as endpoints of a line, as illustrated in  FIG. 2 , which shows control over a 3-segment line. Segment  1 , which is at a low dew point, shows modulation held at twenty percent of full heat. In segment  2 , modulation varies with dew points between 25 and fifty degrees F. dew point. In segment  3 , modulation is ninety percent, of full heat, for high dew points.  
       SUMMARY  
       [0009]     A refrigerated case for storing food product including a sensor module and a control module receiving input from the sensor module and comparing the input to a set point to generate an output indicative of a difference between the input and the set point. The control module updates the output based on the input from the sensor module. A heater module controls a heater attached to a control surface of the refrigerated case based on the output to maintain a temperature of air adjacent the sensor module above a dew point temperature of room air.  
         [0010]     A bank of refrigerated cases for storing food product includes a sensor module having a first sensor positioned adjacent a control surface of at least one of the refrigerated cases and a control module that receives an input from the sensor module and compares the input to a set point to generate an output indicative of a difference between the input and the set point. The control module updates the output based on the input from the sensor module. A heater module controls a heater attached to the control surface of the at least one of the refrigerated cases based on the output to maintain a temperature of air adjacent the first sensor above a dew point temperature of room air.  
         [0011]     Further areas of applicability of the present teachings will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the teachings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The present teachings will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0013]      FIG. 1  is a schematic representation of a prior art proportional controller;  
         [0014]      FIG. 2  is a graph showing percentage heat modulation versus temperature for a prior art door heater control system;  
         [0015]      FIG. 3  is a schematic representation of an anti-condensation control scheme in accordance with the present teachings;  
         [0016]      FIG. 4  is a cross-sectional view of a relative humidity sensor incorporating a drip-shielding baffle and disposed within a door casing or door frame;  
         [0017]      FIG. 5  is a cross-sectional view of a relative humidity sensor showing the drip-shielding baffle of  FIG. 4  from another direction;  
         [0018]      FIG. 6  is a perspective view of an air flow path of a relative humidity sensor incorporating a housing having an open bottom portion and an air passage formed in a side wall;  
         [0019]      FIG. 7  is a perspective view of a relative humidity sensor in accordance with the principles of the present teachings incorporating a housing having a pair of air passages formed in a side wall;  
         [0020]      FIG. 8  is a perspective view of a relative humidity sensor in accordance with the principles of the present teachings incorporating a housing having a pair of air passages formed in another side wall;  
         [0021]      FIG. 9  is a psychrometric chart for use with the anti-condensation control scheme of  FIG. 3 ;  
         [0022]      FIG. 10  is another psychrometric chart for use with the anti-condensation control scheme of  FIG. 3 , wherein water vapor is at twice the amount as the psychrometric chart of  FIG. 9 ;  
         [0023]      FIG. 11  is a schematic representation of another anti-condensation control system in accordance with the principles of the present teachings; and  
         [0024]      FIG. 12  is a schematic representation of the control system of  FIG. 11  applied to a plurality of doors. 
     
    
     DETAILED DESCRIPTION  
       [0025]     The following description is merely exemplary in nature and is in no way intended to limit the teachings, its application, or uses.  
         [0026]     The control system and method achieves a temperature slightly higher than the dew point of humid air adjacent a control surface of a component of a refrigeration device to prevent condensation from forming on the control surface. For example, the control system maintains air adjacent a door of a refrigerated display case, or an observation window of a walk-in refrigerator or freezer, slightly above the dew point of humid air adjacent the door or observation window to maintain the respective component free from condensation. Thus, the relative humidity of the humid air adjacent the component (the air which has been cooled to component temperature) is high, but less than one hundred percent. Because humid air has a dew point, or temperature at which relative humidity is one hundred percent, cooling the humid air to a temperature below the dew point causes water vapor to condense.  
         [0027]     If the temperature of the component (i.e., glass door or observation window) is below the dew point of the humid air in the room where the component is located, the cool air of the room will cool the humid air at the component below the dew point, which will cause moisture to condense thereon. But, if the temperature of the component is slightly above the dew point of room air, the humid air touching the component will be cooled, but not to the point of causing condensation.  
         [0028]     The system and method according to the present teachings may be used in a variety of refrigeration and freezer applications such as, but not limited to, display cases, walk-in refrigerators, and walk-in freezers, to control the temperature of any control surface. For example, walk-in refrigerators and freezers could employ the present system to prevent condensation from forming on air vents, personnel doors, drain lines, walls, and observation windows. Similarly, refrigerated display cases such as coffin cases, island cases, and tub cases could employ the present system to prevent condensation from forming on any wall or surface surrounding an opening and/or door of the display case. While the present system is applicable to each of the aforementioned refrigeration and freezer applications, the present system will be described in association with a refrigerated display case having a glass door.  
         [0029]     To achieve the system and method according to the present teachings, a relative humidity sensor  10  may be mounted on a control surface, such as a door  12  or other structure of a refrigerator/refrigerated case  14 , such that the sensor itself, and the air it monitors, are cooled to a control surface temperature. The sensor  10  may be mounted to any portion of the door  12  or structure of the refrigerator/refrigerated case  14  so long as the structure to which the sensor  10  is mounted is indicative of the temperature of the control surface.  
         [0030]     For example, if a glass pane of the door  12  is deemed the control surface (i.e., the portion of the door  12  to maintain free from condensation), the sensor  10  may be mounted directly to the glass pane or, alternatively, to support structure either on the door  12 , such as a door casing  25  generally surrounding the glass pane, or to surrounding support structure, such as a door frame  26  that operably supports the door  12 . The door casing  25  and door frame  26  are schematically represented in  FIGS. 4 and 5 . The sensor  10  may be mounted either on the glass pane, door casing  25 , or door frame  26  or within the glass pane, door casing  25 , or door frame  26 , provided that the respective structure is generally at the same temperature as the control surface. By mounting the sensor  10  in close proximity to the control surface, the sensor  10  is able to accurately measure the relative humidity of air adjacent the control surface.  
         [0031]     Mounting the RH sensor  10  within the door casing  25  or door frame  26  protects the sensor  10  from dust, moisture, or other liquids. For example, the sensor  10  and appropriate drip protection or baffles  30 , may be arranged on a small plate, which is mounted in a hole cut into the door casing  25  or door frame  26 . The casing  25  or frame  26  may be further modified to include air vents  32 , such as screens, louvers or small holes, generally above and below the sensor location. By locating the sensor  10  approximately in the middle portion of a vertical portion of the door casing  25  or door frame  26 , adequate air flow over the sensor  10  may provide a reliable relative humidity measurement. Such arrangements are shown in  FIGS. 4 and 5 .  
         [0032]     The RH sensor  10  may be arranged in a thin vertical tube  27  (represented schematically in  FIGS. 4 and 5 ) having baffles  30  near the sensor  10  to prevent moisture or dust from falling on the sensor  10 , and to prevent water or other cleaning solutions from dripping onto the sensor  10 . The vertical tube  27  may be thermally in contact with the door  12 , so that the tube  27  is at approximately the same temperature as the door  12 . The tube  27  may permit air flow vertically, so that air passes by the relative humidity sensor  10  located inside the tube  27 . The tube  27  should be long enough to cool air passing therethrough to door temperature, or close to door temperature, before passing over the sensor  10 . Furthermore, the tube  27  should have a path long enough for cooling air both above and below the sensor  10  so that the air is cooled before reaching the sensor  10 , regardless of the direction of air flow (i.e., due to air currents in the room can cause flow in either direction).  
         [0033]     While the RH sensor  10  may be mounted within a door casing  25 , door frame  26 , and/or tube  27  including air inlets and outlets  32  at the top and bottom thereof to accommodate air flow, air inlets  32  may also, or alternatively be, located on the front or the sides of the respective assembly (i.e., casing  25 , frame  26 , or tube  27 ), which lessens the opportunity for water to drip into the assembly or dust to collect on the assembly. Such an arrangement may be useful where the RH sensor  10  is not mounted inside the door frame  26  (e.g., when mounted on an external surface of the door frame  26 ). Possible arrangements are shown in  FIGS. 6, 7 , and  8 .  
         [0034]      FIGS. 6 and 7  illustrate air entry and exit holes  32  on a front surface  36  of the door casing  25 , door frame  26 , and tube  27  with the arrangement of  FIG. 7  having an open bottom for air flow.  FIG. 8  illustrates air entry and exit holes  23  on sides  38  of the door casing  25 , door frame  26 , and tube  27 . Note, however, that the air entry and exit holes may be on both sides or, if mounted on the door frame  26 , preferably on the side toward the door glass only. The RH sensor  10  may be made as thin as practical, measuring from front to back, to sense air as close to the door surface as possible, and thus nearly at door temperature. Furthermore, casing  25 , frame  26  and tube  27  may be open or closed at both ends to tailor the flow of air therein. Such arrangements may be particularly appropriate for RH sensors  10  not mounted inside a door frame  26 .  
         [0035]     While the RH sensor  10  is described as being associated with a door of a refrigerator/refrigerated case, it should be understood that the sensor  10  may alternatively be used with an open refrigerator/refrigerated case or a walk-in refrigerator/freezer. In such applications, the sensor  10  can be mounted on any surface to be controlled (i.e., for which prevention of condensation is desired), such as walls, windows, doors, housing rails, or other support structure.  
         [0036]     An anti-condensation control system  13  employing a heater controller  15  having an adder-subtractor  16 , a proportional integral controller (PID)  18 , a limiter  20 , and a heater modulator  22  is illustrated in  FIG. 3 . The RH sensor  10  provides an input to the adder-subtractor  16 , which also receives a RH set point as an input. The set point may be provided at ninety percent, and the adder-subtractor  16  determines an error, which is input to the PID controller  18  to determine an output between zero and one hundred percent. The output may be applied to the limiter  20  having a percent minimum and percent maximum output to be applied to the heater modulator  22 , which controls a door heater  24  as the RH sensor  10  at the door  12  continues to supply an input to the adder-subtractor  16  for comparison to the set point. Thus, the anti-condensation control system  13  provides closed-loop control. While a PID controller is disclosed, other control logic, such as, but not limited to, fuzzy logic, may also be used with the control system  13 , and should be considered within the scope of the present teachings.  
         [0037]     The control system  13  according to the present teachings may have a set point at a relatively high RH value, such as ninety or 95 percent. The RH set point may be adjusted for lack of accuracy in the RH sensor  10  or to account for temperature variations at different areas of the door  12 . For example, if parts of the door  12  are cooler than the air flowing over the RH sensor  10 , a lower RH set point (RHSP) may be appropriate, such as lowering the RHSP to eighty percent. Lowering the RHSP ensures that the entire door  12  remains free from condensation by applying additional heat to cooler areas of the door  12 .  
         [0038]     The set point may never have to be adjusted, particularly if there is a control system for each door  12 . In such systems, it is not necessary to provide accessibility to the system to make adjustments to the set point as user intervention is not required to properly adjust the control system  13 . This feature, in system design, may result in considerable cost savings.  
         [0039]     With reference to  FIG. 9 , operation of the control system  13  can be illustrated by plotting an example on a psychrometric chart. The system control goal is to maintain relative humidity at the RH sensor  10  at ninety percent relative humidity, i.e., RHSP equaling approximately ninety percent. In a room having a dry bulb temperature of +68 degrees F. and relative humidity of 36.9 percent (away from the refrigerated surfaces), the dew point is +40 degrees F., which is determined by extending a line horizontally from the air condition to a point on the one hundred percent RH curve in  FIG. 9 . The control point is where that same horizontal line intersects ninety percent RH, and the heater modulator  22  will apply just enough heat to bring the door temperature to +42.7 degrees F., which is slightly above the dew point. Further, if the RH sensor  10  indicates 95 percent relative humidity, the controller  15  would apply more heat as the door temperature is +41.3 degrees F. If the RH sensor  10  indicates 85 percent relative humidity, the controller  15  would reduce the heat being applied, as the door temperature is +44.2 degrees F. Thus, the controller  15  will adjust the heat applied until the RH sensor  10  achieves ninety percent relative humidity.  
         [0040]     Another psychrometric chart is illustrated in  FIG. 10 , where water vapor (i.e., airborne moisture, humidity) is present at approximately twice the amount as in the example of  FIG. 9 , which is noted on the vertical axis of the psychrometric chart of  FIG. 10 . Where  FIG. 9  included 0.0052 pounds of moisture per pound of dry air,  FIG. 7  illustrates 0.0107 pounds of moisture per pound of dry air. While in  FIG. 9 , DT minus DP equals 2.7 degrees F., the differential in  FIG. 10  is 3.0 degrees F. In both situations, however, controlling the heat to maintain the RH sensor  10  at ninety percent relative humidity causes the door temperature to be about 3 degrees F. above the dew point. This approximate differential of door temperature over dew point is true over a wide variation of airborne moisture or humidity.  
         [0041]     A separate control system  13  may be applied to each door  12  of a refrigerated display case such that one controller  15  may be used to control heaters  24  for a plurality of doors  12 . For example, the RH sensor  10  may be located in the side of one door  12  that is closest to another door  12  sought to be controlled, (i.e., adjacent doors  12 ) to control both doors  12 . The heaters  24  of both doors  12  may be connected in parallel and be driven by the same controller  15 . For three adjacent doors  12 , the RH sensor  10  may be mounted in the middle door  12 . The heaters  24  of all three doors  12  may be connected in parallel and be driven by one controller  15 . The system and method may include three different heater outputs, all modulated in the same way, but each output powered by a different phase of three-phase power.  
         [0042]     In a system incorporating multiple doors  12 , each anti-condensation system  13  may be monitored and tracked separately to diagnose faults associated with each door  12  and/or system  13 . In this manner, each system  13  may be in communication with a main controller  34  that tracks system performance and updates the RH set point, when necessary. The refrigeration controller  34  is preferably an Einstein of E2 Area Controller offered by CPC, Inc. of Atlanta, Ga., or any other type of programmable controller that may be programmed.  
         [0043]     Another apparatus and method for preventing condensation includes controlling component temperature in relation to a measured dew point in order to minimize heater energy use. A closed-loop control system  40  according to the present teachings efficiently prevents condensation and lowers energy use, while providing automated adjustment of the system  40 . Like control system  13 , control system  40  may be used in a variety of refrigeration and freezer application such as, but not limited to, display cases, walk-in refrigerators, and walk-in freezers. For example, the control system  40  may be employed in walk-in refrigerators and freezers to prevent condensation from forming on air vents, personnel doors, drain lines, and observation windows. Similarly, refrigerated display cases such as coffin cases, island cases, and tub cases could employ the control system  40  to prevent condensation from forming on and around an opening and/or door of the display case. While the control system  40  is applicable to each of the aforementioned refrigeration and freezer applications, the control system  40  will be referred to hereinafter and in the drawings as associated with a refrigerated display case having a glass door.  
         [0044]     As shown in  FIG. 11 , the apparatus and method includes measuring and controlling door temperature to a temperature equal to dew-point temperature of the room air, plus a delta temperature offset. Thus, the door temperature is held at slightly above dew-point temperature, which is an optimum door temperature for preventing condensation with minimum heat applied to the doors  12 .  
         [0045]     With reference to  FIG. 11 , the anti-condensation control system  40  is shown including a dew-point sensor  42  and a heater controller  41  having a math block  43 , an adder-subtractor  44 , a proportional integral controller (PID)  48 , a limiter  50 , and a heater modulator  52 . The dew-point sensor  42  provides a temperature measurement to the adder/subtractor  44 , which also receives a delta temperature offset for adjusting the measurement received by the dew-point sensor  42 . Further, the adder/subtractor  44  receives a temperature measurement from a temperature sensor  46  located on the door  12  of the refrigerated display case and determines an error value between the dew-point sensor input plus the delta temperature offset, and the temperature measurement received from the temperature sensor  46 . This error value is applied to the proportional integral derivative (PID) controller  48 , which outputs a percentage to the limiter  50 , which limits the output percentage to a predetermined percentage minimum and/or percentage maximum. The limiter  50  outputs an adjusted demand signal to the heater modulator  52 , which then applies heat to the doors  12  via heater  54  in accordance with the required demand. While a PID controller is disclosed, other control logic, such as, but not limited to, fuzzy logic, may also be used with the control system  40 , and should be considered within the scope of the present teachings.  
         [0046]     In addition to the foregoing, the control system  40  may include a relative humidity sensor  55  and a temperature sensor  57  in place of the dew-point sensor  42 , and a math block  43  in the heater controller  41 . The relative humidity sensor  55  detects door temperature relative humidity and supplies an input indicative thereof to the math block  43  while the temperature sensor  57  measures ambient temperature and provides an input indicative thereof to the math block  43 . The math block  43  computes the dew point based on the inputs from the relative humidity sensor  55  and temperature sensor  57 . Therefore, the control system  40  could employ a stand-alone dew-point sensor  42  or could use a math block  43  in conjunction with a relative humidity sensor  55  and a temperature sensor  57  to compute the dew point. In either event, the dew point is fed to the adder-subtractor  44  for processing, as previously discussed.  
         [0047]     In a system incorporating multiple doors  12 , the performance of each anti-condensation system  40  may be separately monitored and tracked to diagnose faults associated with each door  12  and/or system  40 . In this manner, each system  40  may be in communication with a system controller  59  that tracks system performance and updates system parameters, when necessary.  
         [0048]     In  FIG. 12 , a dew-point sensor  42  for the room provides an input for temperature control of multiple doors  12 , which collectively are subject to a single delta temperature offset. Doors with different heat loads, such as when one is open, are all precisely controlled to a temperature just above the dew point. It should be understood that the arrangement shown in  FIG. 12  may alternatively include a relative humidity sensor  55  and a temperature sensor  57  (with math blocks  43  in the heater controllers  41 ) in place of the dew-point sensor  42 .  
         [0049]     The system and method may also include a temperature sensor  46  on one door  12 , but the system and method controls heaters  54  in all similar doors  12 , for example, a group of doors  12  for a single refrigerated display case or a circuit, based on a single door temperature sensor measurement. While this arrangement provides lower installation cost by eliminating multiple door temperature sensors  46 , it may require a higher delta temperature offset to ensure that other door temperatures remain above the dew point for dependable prevention of condensation on all the doors  12 . Accordingly, the energy cost savings may be less than an arrangement where each door  12  includes its own door temperature sensor  46 .  
         [0050]     A similar arrangement would include a door temperature sensor  46  for each door  12 , but the door temperatures being averaged before being input to the PID controller  48 . A similar variation would include a door temperature sensor  46  for each door  12 , but apply the minimum door temperature to the PID controller  48 . For this arrangement, each door  12  would remain above the dew-point temperature, but may not result in the maximum energy savings because some door temperatures may be relatively high compared to the dew-point temperature.  
         [0051]     As described above for the RH sensors  10 , the door temperature sensors  46  can be arranged on the glass, on the frame  26 , in the frame  26 , or any of the variations discussed above, as well as any reasonable alternatives.  
         [0052]     The description is merely exemplary in nature and, thus, variations are intended to be within the scope of the teachings and are not to be regarded as a departure from the spirit and scope of the teachings.