Abstract:
The core of a high temperature furnace has an elongated central test cavity for testing/calibration of a temperature-sensitive component, such as the probe of a thermocouple. At least three electrical heating elements as positioned adjacent to the test cavity, including two elements adjacent to the opposite end portions of the test cavity, respectively, and a center element between the end elements. A uniform temperature profile is maintained in the cavity by control circuitry that manages the supply of electrical power to the heating elements, by periodically configuring interconnections of the elements during multiple phases of a duty cycle. The duty cycle includes a first phase in which current is passed through the center element, a second phase in which current is passed through the end elements in series, and a third phase in which current is passed through one of the end elements and also through the center element without passing current through the other end element.

Description:
BACKGROUND 
       [0001]    The present invention relates to heater circuits for high temperature furnaces, and particularly to heaters for calibration furnaces having internal test cavities and multiple heating zones. In such a furnace, it is desirable to achieve a uniform temperature profile throughout the test cavity for reliable testing and/or calibration of temperature-sensitive elements such as thermocouple probes. 
       SUMMARY 
       [0002]    This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. The present invention provides a high temperature calibration furnace having a core with an elongated central test cavity for testing/calibration of a temperature-sensitive component. The core has at least three electrical heating elements adjacent to the test cavity, including a first end heating element adjacent to a first end portion of the test cavity, a second end heating element adjacent to a second end portion of the test cavity opposite the first end portion thereof, and a central heating element between the first and second end heating elements. Control circuitry manages the supply of electrical power to the first, second and third electrical heating elements by periodically configuring interconnections of the heating elements during multiple phases of a duty cycle. The duty cycle includes a first phase in which current is passed through the central heating element, a second phase in which current is passed through the first and second end heating elements in series, and a third phase in which current is passed through one of the first and second end heating elements and also through the central heating element without passing current through the other of the first and second end heating elements. 
         [0003]    The present invention also provides a method of operating a high temperature calibration furnace of the type having: an elongated central test cavity for testing/calibration of a temperature-sensitive component; and three electrical heating elements including a first end heating element adjacent to a first end of the test cavity, a second end heating element adjacent to a second end of the test cavity opposite the first end thereof, and a central heating element between the first and second end heating elements. Such method includes managing the supply of power to the three electrical heating elements by periodically configuring interconnections of the three heating elements during multiple phases of a duty cycle, such duty cycle including: a first phase during which electrical current is passed through the central heating element; a second phase during which electrical current is passed through the first and second end heating elements in series; and a third phase during which electrical current is passed through the central heating element and one of the first and second end heating elements without passing current through the other of the first and second end heating elements. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0004]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0005]      FIG. 1  is a somewhat diagrammatic top perspective of a high temperature furnace in accordance with the present invention; 
           [0006]      FIG. 2  is a top perspective corresponding to  FIG. 1  but with some parts removed and parts broken away; 
           [0007]      FIG. 3  is a section of an interior portion of the furnace of  FIGS. 1 and 2 , taken along line  3 - 3  of  FIG. 2 ; 
           [0008]      FIG. 4  is a simplified top plan corresponding to  FIG. 3  with some parts deleted, parts broken away, and parts shown diagrammatically, illustrating a multiple-zone heater component; 
           [0009]      FIG. 5  is a diagrammatic end elevation of the heater component of  FIG. 4 ; 
           [0010]      FIG. 6  is a simplified block diagram of components of a furnace in accordance with the present invention; and  FIG. 6A  is a corresponding block diagram of an alternative embodiment; 
           [0011]      FIG. 7  is a simplified circuit diagram of components of the control circuitry for a furnace in accordance with  FIG. 6  or  FIG. 6A , and  FIGS. 7A-7C  are corresponding circuit diagrams illustrating operation of such circuitry; and 
           [0012]      FIG. 8  is a simplified circuit diagram of components of alternative control circuitry for a furnace in accordance with  FIG. 6  or  FIG. 6A . 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    With reference to  FIGS. 1 and 2 , a representative furnace  10  in accordance with the present invention can have a thin-walled housing including a vented top and side closure member  12 , front panel  14 , and rear panel  16 . These parts connect to and are supported on a base  18 . The base has a mounting and support structure for the internal core  20  of the furnace. The core is cylindrical and, in the illustrated representative embodiment, oriented horizontally. For example, the mounting structure can include a concave cradle  22  and an upper mounting clamp or saddle  24 . The cradle and saddle hold the heater  26 , which can be formed in upper and lower halves. 
         [0014]    The heater has internal heating elements, including separate elements at each end portion and at the central portion. The elements heat an internal test zone or cavity. In the illustrated embodiment the test cavity is the central portion of the internal bore of a longitudinally extending tube  28 . Power to the heating elements is managed by a controller  30  which can be mounted in the housing base  18 . As described in detail below, the controller operates the separate heating elements to maintain an even temperature along at least a major portion of the test cavity. 
         [0015]    In use, heat sensitive elements to be tested or calibrated, such as the probes of thermocouples, can be positioned in the test zone (tube bore), such as by insertion through one or more apertures  32  of a front end plug  34 . The furnace is designed for operation at very high temperatures, such as in the range of 300° C. to 1200° C. Heat can be dissipated from the housing by one or more fans drawing cooling air through the vents of the housing and exhausting the air through openings in the rear panel  16 . 
         [0016]    In the plan view of  FIG. 3  (horizontal axial section), it can be seen that the interior of the tube  28  is open and unobstructed. The opposite ends of the tube are supported in end plates  40  of the heater component  26 , but an annular space  43  is provided between the outer periphery of the tube  30  and the inner periphery of the cylindrical heater  26  along the length of the test zone. 
         [0017]    Temperature sensors  42 ,  44 , and  46  (such as high precision thermocouples) have probes that extend into the annular space  43  at different longitudinal locations. The sensors can be high precision thermocouples having stems for mounting the probes to the housing end plate  16  or other appropriate support structure. One probe extends a short distance into the space  43 , a second probe extends approximately halfway into the space  43 , and a third probe extends much farther into the space  43 . Tube  28  necessarily is a highly heat conductive, high temperature, structurally stable material, such as a high temperature ceramic or quartz. The temperatures sensed by the probes correspond to the temperature inside the tube, i.e., in the test cavity. 
         [0018]    In the simplified plan view of  FIG. 4 , and the corresponding end elevation of  FIG. 5 , other aspects of the heater  26  are illustrated. The heater is formed of a high temperature insulating material in which electric resistance heating elements are partially embedded. In a representative embodiment, the heater can be a molded fibrous ceramic, and the conductors C of the heating elements are mounted in slots that open radially inward toward the test cavity. Three separate electric resistance heating elements are provided. A first element  50  extends approximately one quarter of the longitudinal length of the heater and encircles a first end portion thereof. A second element  52  extends for about one quarter of the length of the heater at the other (second) end thereof. The third element  54  extends for about one half the length of the heater in the central portion thereof, in between the end elements  50 ,  52 . 
         [0019]    As illustrated in  FIG. 5 , the heat producing conductors C of each heating element are closely spaced around the circumference of the heater.  FIGS. 4 and 5  show the bottom half of the heater, but the top half is the mirror image, and the conductors of each element are connected in series at the top and bottom to form one continuous element that extends  360  degrees around the inner cylindrical space in which the central tube is fitted. 
         [0020]    In order to achieve a uniform axial temperature profile throughout the test cavity, it is preferred that there be control of three aspects of the temperature profile: the average temperature over the region of interest, the slope of the temperature profile through the center of the region (i.e., the difference in temperature from one end of the region to the other), and the “curvature” of the temperature profile (variations along the region of interest in the situation where the slope of the temperature profile is not linear). Corrections can be achieved if heat from each of the three heater sections is adjustable relative to the others. 
         [0021]    In general, a controller is provided to manage the amount of power each heater section receives. Time is divided into equal duration control cycles. Within each control cycle, each heater is switched on for a precise amount of time to supply a precise amount of energy. Over many control cycles, the average amount of electric power that the heating elements receive determines the temperature within the furnace. Ideally, each heating element would be controlled independently of the others, in combination with temperature monitoring at a multitude of locations, so that the temperature profile can be fully adjusted and made uniform. In practice, such individual control and complicated monitoring is difficult to achieve and expensive. For example, the furnace with which the present invention is concerned has a power supply of 240 V AC. A 120 V supply would require too much current to produce the necessary amount of power, particularly at higher temperatures. The longer center heating element is rated for 240 V so it can be driven directly from the 240 V mains supply. However, each of the end heating elements is shorter (one-half the length of the center element), with corresponding lower resistance and rated for only 120 V. 
         [0022]    To allow the outer sections to operate from 240 V, they have been wired in series with their common current switched by one triac, for example. Another triac controls the power to the center section. A problem with this design is that it allows only two independent variables of temperature control. The average temperature in the furnace can be adjusted, and typically the curvature of the temperature profile can also be adjusted, but there is no efficient and effective means to adjust the slope of the temperature profile through the center, because the end heating elements always receive the same amount of power. In practical implementations of a heater of this type, one end tends to be colder than the other, which can be due to a variety of factors including differences in performance of the separate heating elements and/or the overall design of the furnace resulting in greater heat loss toward one end than the other. 
         [0023]    The present invention proposes an improved system and circuitry for reconfiguring the connections of the separate heating elements during a control cycle, using the uniform power (preferably 240 V AC), without subjecting the lower rated end heating elements to the full power voltage, but allowing different amounts of power to the respective end elements to achieve a desired temperature profile. 
         [0024]      FIG. 6  is a simplified block circuit diagram of a representative embodiment. The three temperature sensors (thermocouples)  42 ,  44 ,  46 , each have a dedicated analog-to-digital converter  60 ,  62 ,  64 , respectively, conveying temperature signals to a processor-controller  66 . The processor, in turn, controls a series of solid-state relays  68 ,  70 ,  72  that configure the connections of the heating elements  50 ,  52 ,  54  to the power supply  74 . The arrangement of  FIG. 6A  is essentially identical, the only difference being that a single analog-to-digital converter  76  is used with a switch  78  for polling the temperature signals from the sensors  42 ,  44 ,  46 . The central processing unit  66  still monitors the temperatures and controls the relays  68 ,  70 ,  72  appropriately, adjusting the duty cycle as described in more detail below to achieve the desired uniform temperature profile in the test cavity. 
         [0025]      FIG. 7  is a diagram of the heating element wiring in a first embodiment of the present invention. The system power (preferably 240 V AC) is applied between the top and bottom terminals  80  and  82 . From the top terminal  80 , a branch  84  to the left extends first through one low power rated end heating element  50  and continues to the other of the low power rated end elements  52 , then to a first solid state relay  70  (called the “zone” relay) and from there to the other power terminal  82 . A branch  86  to the right from the top power terminal  80  has a second solid state relay  68  (called the “center” relay) and then extends through the higher power rated center heating element  54  and then to the bottom power terminal  82 . A third solid state relay (referred to as the “balance” relay) is connected between the first two branches, one end extending from a point between the two end heating elements  50 ,  52  and a point between the center relay  68  and the center heating element  54 . 
         [0026]    Using the circuit of  FIG. 7 , heater control can be done in three distinct phases within the control cycle. With reference to  FIG. 7A , during a first phase the center relay  68  is switched on and the balance relay  72  and zone relay  70  are switched off. As indicated by the arrow, current flows only through the center heating element  54 . In the second phase of the control cycle represented in  FIG. 7B , the center relay  68  and the balance relay  72  are turned off and the zone relay  70  is turned on. Current flows through first one end heating element  50 , then the other  52 . With reference to  FIG. 7C , in the third phase within the control cycle, the center relay  68  and the zone relay  70  are turned off, and the balance relay  72  is turned on. Current flows through the first end heating element  50  and through the center heating element  54 . 
         [0027]    The above-described scheme allows three dimensions of heater power control, and thus three way adjustment of the temperature profile. The total average power all heating elements receive determines the average temperature of the furnace. The proportion of time the outer heater sections receive power while the center heater is off determines the amount of curvature there is in the temperature profile. The proportion of time only the balance relay is on determines the slope of the temperature profile through the center region. The duration of each phase within each control cycle can be adjusted to achieve a uniform temperature profile throughout the center region of the furnace. This particular scheme is appropriate when it is known which end of the furnace will tend toward being cooler than the other, such as when the furnace design results in more heat loss at one end, e.g., by greater exposure to the atmosphere or more heat convection at one end. The end tending toward a colder temperature can have its heating element configured in a series circuit with the center heating element during the described third phase of the control cycle. 
         [0028]    In one embodiment, the controller adjusts the relative durations of the distinct phases of the control cycle based on readings of the temperature sensors. In a representative embodiment, the control cycle may be one half second to two seconds long. By way of example, to achieve a desired temperature, it may be necessary for the center heating element to operate for 0.2 seconds, and the end heater elements to operate for approximately the same length of time, but in order to achieve the end-to-end uniform temperature, it may be necessary to operate the balance phase for a period approximately one-half the period of the first and second phases. This can be determined during calibration of the furnace. Thereafter, the controller can operate the cycle with the same proportionate time of the three distinct phases, adjusting them to be proportionately longer for a higher desired temperature, and proportionately lower for a lower desired temperature. Also, the first and second phases can be performed simultaneously by opening the balance relay while closing both of the center and zone relays, in addition to or instead of being performed consecutively. The duration of the “combined” phase one and two operation can be adjusted accordingly so that the simultaneous supply of power to all three heating elements achieves the same amount of heating as a consecutive operation and the desired uniform temperature profile in the test cavity. 
         [0029]    In some situations, it is not known which of the two end portions will tend toward a cooler temperature than the other. The diagram of  FIG. 8  shows a circuit with an additional double pole relay  84  allowing one or the other of the end heaters  50 ,  52  to be powered concurrently with the center heater  54  in the third phase. With the control relay  84  in the position shown, and the center and zone relays  68 ,  70  turned off, and with the balance relay  72  turned on, current will flow through the first end heater  50 , the balance relay  72 , and the center heater  54 . If, however, it is determined that the other end of the heater tends to run cooler, the position of the control relay  84  can be switched whereby, again with the center and zone relays  68 ,  70  open and the balance relay  72  closed, during the third or balance phase of the control cycle, current will flow through both the second end heater element  52  and the center heater element  54 . In the first and second phases, operation is the same as for the embodiment described above. More specifically, the center relay  68  can be closed and the balance and zone relays  70 ,  72  opened such that all current supplied during the first phase passes through the center heating element  54 . Similarly, with the balance and center relays  72 ,  68  open and the zone relay  70  closed, current is passed through both the first and second end heating elements  50 ,  52  during the second phase. In appropriate situations the first and second phases can be performed simultaneously (in part or instead of only consecutive operation) by closing both the zone and center relays and opening the balance relay during part of the duty cycle. 
         [0030]    The described invention allows three-way control of the temperature profile. To work well, the duration of each of the phases within each control cycle must be precisely regulated. The controller drives the gates or triggers of the relays, switching them on and off at precise times necessary to achieve accurate control of the furnace temperature and temperature profile. The controller may, and typically does, use two or more temperature sensors to measure the temperature and the temperature profile, and the controller uses that feedback to make adjustments to the average temperature power and the balance of heater power to correct the temperature and temperature profile. Once the cooler end of the furnace is known, two sensors are effective to achieve adequate feedback, one approximately centered in the test zone (which is centered in the center heating element) and one positioned in the end heating element at the cooler end of the furnace. The controller performs all the mathematical calculations necessary to determine how the relays should be switched on and off during each control cycle to achieve stable and uniform temperature control. While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.